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Heating technology in thermoforming
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eating technology in H thermoforming
The most popular heating techniques for heating thermoplastic semi-finished products rely on: Radiant heater elements Contact heaters Convection heaters All of the above-cited heaters are operated using electrical energy. There are also some heaters that operate with gas. Radiant heater elements transfer energy in the form of electromagnetic waves, primarily in the infrared range (0.76 mm to 1000 mm). In contact heaters, the energy is transferred from a heating plate or a roller to the heated material. Convection heaters rely on hot air to relay the energy to the semi-finished product.
4.1 Radiant heaters 4.1.1 Heat-transfer concept with infrared radiation Plastics absorb infrared radiation. The degree of absorption will depend upon the semi-finished product’s type and gauge along with the wavelength of the radiation.
Absorption of IR radiation, in %
128 4 Heating technology in thermoforming
Wavelength of radiation, in µm
Figure 4.1 Absorption of IR radiation (in %) as a function of wavelength; for type 475K polystyrene sheets, blue 849, semi-finished product with gauges of 0.04, 0.5 and 4 mm [source: BASF Ludwigshafen]
Standardised radiation
Every semi-finished thermoplastic material (type, colour, etc.) will display its own specific absorption curve – comparable to a fingerprint (Figure 4.2). Increased thickness in the semi-finished product is accompanied by progressively higher absorption levels. When the power emitted by a heater element impacts against the semi-finished product’s surface, some of the irradiated output is reflected, some of it is absorbed by the material, and some penetrates to the other side of the material without affecting it.
0
2
4
6
8
10
Wavelength of radiation, in µm
Figure 4.2 Power output concept of heaters (standardised radiated heat output) a) Ceramic heater, surface temperature of 700 °C b) Quartz heater, filament at 900 °C c) Halogen heater, filament at 2000 °C
4.1 Radiant heaters
There are no objects (in this case, heaters) that emit their radiant energy in a single wavelength, even if the surface temperature is homogeneous. If a thin-gauge plastic, for instance, with a gauge of 0.04 mm displays a high level of absorption at a specific wavelength, then if the power is radiated against a thick pre-cut sheet manufactured in the same material at an identical wavelength, most of the incident radiation impacting at this wavelength will be absorbed before it progresses beyond the material’s surface. The difference between the employed heater types consists in how the radiated heat output is distributed amongst the various wavelengths. The heater’s temperature determines the correlation of maximum power density with a particular wavelength in the radiated output. If the three heater elements – see Figure 4.2 – featured the same connected power rating, then the sizes of the three surfaces below the curves a, b, c would be equal. Heaters with unsatisfactory radiated power outputs relative to wavelength display poor efficiency levels. Their heating power can be increased by raising the connected power rating – as is the case with halogen heaters. Thus heaters equipped with different types of heater elements operating with equal connected power ratings will display varying heating results, depending upon the semi-finished product being heated.
4.1.2 Heat quantity transferred by radiation The radiated heat output of an infrared heater element The radiated heat output emitted by a heater element and employed to heat the semi-finished product P results in: P = e ×s × A ×T 4 (4.1) Radiated heat output from an infrared heater element with the emissions level e of the heater’s surface (for the “ideal” heater, valid for the black body: e = 1), the Boltzmann constant s of the radiating surface A and the temperature T in Kelvin of the heater element’s radiating surface. The following conclusions can be drawn from Formula (4.1): The quantity of heat emitted by the heater element is proportional to the element’s temperature in Kelvin raised to the fourth power. The transferred quantity of heat varies according to the size of the radiating surface. The emission level of the surface projecting the radiation should be as close as possible to the value for a black body (with e = 1)
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Factor for variation of heating time
Effect of distance between heater element and the heated semi-finished product, refer to Figure 4.3.
Heater temperature (ceramic heater), in °C
Figure 4.3 Change in heating time as function of heater-element temperature; for ceramic heat elements with 200 mm distance to semi-finished product, measured values
Effect of deactivated heaters and machine components with rising temperatures Only the heater’s power, and not the temperature, is included in the formula. This means that a deactivated heater will also radiate heat if it is hot. A deactivated heater element within a heater will be heated by contiguous radiant heaters and will then emit heat according to the attained temperature. The temperature differential between the heater array and the deactivated heater element as a function of heater array temperature is provided in Figure 4.4. The data in Figure 4.4 have been determined on an ILLIG UA 100 Ed thermoforming machine, upper heater equipped with ceramic heaters, heater element size 123 × 62 mm, arranged in a grid of 125 × 105 mm. Figure 4.5 illustrates the temperature differences in the semi-finished product resulting from reduction in the power output of a single heater. The data were recorded using an ILLIG UA 100 Ed sheet-processing machine with FSR/2 ceramic heaters manufactured by Elstein.
Temperature reduction of a deactivated heater, in °C
4.1 Radiant heaters
Heater array temperature, in °C
Figure 4.4 Temperatures registered at a deactivated heater as a function of heater array temperature ① Heater in centre of array “Off” ② Heater in the corner “Off”
x-axis: Temperature differential at affected heater y-axis: Temperature differential on the surface of the semi-finished product
Figure 4.5 Possible temperature differential in semi- finished product under a heater with modified temperature Determined for: – HIPS pre-cut sheets with gauge of 5 mm – Semi-finished product temperature of 160 °C – Heater element size of 123 × 62 mm – Heater array grid of 125 × 105 mm – Temperature at the upper heater 600 °C – Temperature at the lower heater 450 °C ① Change in the temperature of a heater in the centre of the upper heater ② Change in the temperature of a heater element in the centre of the upper heater and a heater element in the centre of the lower heater
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Conclusion regarding effect of a deactivated heater element The heating power that a deactivated heater element projects onto the semi-finished product depends upon its temperature. Every component that heats up and can affect the semi-finished product through direct heat radiation, such as parts of the heater deflection panel, the heater cover, clamping frame (if there is heating in the clamping frame), transport system, machine frame, etc. Its radiated heat output corresponds to the data provided in Formula (4.1). This has disadvantages for the reproducibility of the heating process because these parts are cold when the machine starts, and they heat gradually before their temperature stabilises after a minimum period of 45 minutes. For more details refer to Section 4.2.2 “Compensation”. Effect of the heater element’s distance from the semi-finished product on the heat that it absorbs
Factor for variation of heating time
Progressively greater distances between heater element and semi-finished product equate with correspondingly longer heating times, see Figure 4.6.
Clearance between heater and object material, in mm
Figure 4.6 Variation in heating time as a function of heater element’s distance from the semi-finished product
Effect of the distance between the heater elements of a heater panel The transferable quantity of heat rises with the proportion of radiating surface embedded in a heater panel – one can then refer to a higher effective level of efficiency. If heater elements are not installed immediately next to one another, then they should be equipped with reflectors that compensate for any lack of direct heat
4.1 Radiant heaters
radiation with reflected heat radiation. The reflectors redirect the heat that the heater elements project to the sides and rear by deflecting it toward the front and the semi-finished product. Since no reflector can have a perfect, ideal geometry, and since it will also gather contamination as time passes, it represents a compromise relative to a fully equipped, directly radiating surface. Figure 4.7 shows a heater panel with ceramic heaters installed at a distance; Figure 4.8 portrays a heater panel with densely placed ceramic heaters with a distance between edges of roughly 5 to 6 mm. Owing to the overlap in the radiated heat, both types can apply even heating to the semi-finished product in the centre of the heated surface area, provided that the distance between the semi-finished product and the heater element level is not too short. Problems occur when: a) The distance between the semi-finished product and the heater element’s level becomes less than predicted, e. g., as when the material sags during heating, or b) in the edge zone of the tensioned material. If a gap in the heater elements is present above the edge of the semi-finished product, it will not be possible to heat this edge zone evenly, even if the immediately adjacent heater elements are adjusted to a higher radiation intensity or if vertical reflectors are employed in an attempt to project radiated heat into the edge zone. A gap above the edge, mirrored by the reflector on the surface of the clamping frame, will have the same effect as a double gap in the heater elements.
Figure 4.7 Heater panel with ceramic heat elements installed at a distance, flat reflector surface
Figure 4.8 Heater panel with ceramic heat elements installed in close mutual propinquity
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Effect of the emissions factor of the radiating surface of the heater element Ceramic heater elements are available with two coatings – light and black. The ceramic heater elements with black coatings that have been in use since about 2010 can achieve a higher radiated heat output, with commensurate positive effects on most thermoplastic materials. Effect of heater element size on the heating results A heater element does not merely project heat vertically, at a perpendicular angle to its surface, but instead the emissions travel in all directions. Each point on the surface of the heated plastic receives radiated heat from all of the heater elements unless the heat radiation has been specifically shielded. At the same time, deactivation of a single heater element in a heater array will affect the entire heated surface, with the largest influence below the heater element itself (Figure 4.9). Temperature differences on the surface of the semi-finished product reach their maximum below the centre of the deactivated heater element and become pro gressively less pronounced as one moves outward from this position. The practical effects stop after an angle of approximately 30 degrees.
1 side view
top view
Figure 4.9 Sensitive surface on the heated semi-finished product with deactivated heater element as example 1: Zone practically subject to effects j: Beam projection angle T: Temperature of semi-finished product DT: Temperature differential on the surface of the semi-finished product
Heating the plastic panel with infrared heater elements with a pattern ensuring that small surfaces on the semi-finished product deliberately remain colder is theoretically possible. However, the possibilities are subject to severe limits and many users overestimate the available potential. Achieving different temperatures
4.1 Radiant heaters
on the surface of a semi-finished product with targeted shielding of the beam radiated toward parts of the semi-finished product’s surface or blowing cold air against the heated surface of the semi-finished product is substantially more effective than deactivating individual heater elements. The question repeatedly arises regarding the ideal heater element size for optimal heating. Standard commercial sizes of heater elements are 60 × 60 mm, 123 × 60 mm, 123 × 123 mm, 248 × 60 mm, 248 × 123 mm. With these heater- element sizes, heater shields with heater element grids of 62.5, 125 and 250 mm can be implemented with full-surface emitter occupation. Intermediate spaces consisting of just a few mm are necessary to avoid breakage in the heater elements. Small heater elements (e. g., 62 × 62 mm) are advantageous on machines featuring small clearances between the heater element and semi-finished product as is the case with automatic roll-fed machines featuring a distance of between roughly 80 and 125 mm. When the distances between heater element and semi-finished product are greater, over 200 mm – as is the case with flat-sheet-processing machines, this becomes a matter of faith, and actual advantages are practically impossible to demonstrate. When dealing with more extensive forming surfaces, e. g. for machines from roughly 2000 × 1250 mm, selecting heater elements larger than 125 × 125 mm is certainly justifiable. In automatic roll-fed machines, where the plastic is transported below the heater multiple times, the length of the heater element in the direction of transport is not an important factor. At the same time, it is important to consider the ability to heat a sheet-material surface evenly at the advance-feed rate. Beam grids at an angle of 65 mm to the direction of transport have proven to be optimal for automatic roll-fed machines with sheet-material widths extending up to approximately 800 mm.
4.1.3 Homogeneous heating with radiant heaters Regardless of the machine’s type or size, the primary objective remains uniform heating of the semi-finished product extending across its entire surface. With automatic roll-fed machines, heating the sheet material to a single, uniform forming temperature is absolutely essential. On pre-cut-sheet processing machines, even heating should be considered as the starting point for any subsequent modifications to the heating pattern. Figure 4.10 a shows that more radiated heat is arriving at the centre of a heated zone than on the edge. In the vicinity of the clamping frame or the transport profile, “the heat radiation is missing”. Figure 4.10 b illustrates the theoretical ideal case for homogeneous heating of the semi-finished product, which would be obtained with an infinitely large heater and a frame height of zero.
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Even heating is achieved by: Installation of reflectors for IR radiation with which the missing beam component from the outside is replaced by an inside beam reflected by the clamping frame (Figure 4.11). Reduction in heater element temperatures at the centre of the array relative to the heater elements on the edge and above the edge zone of the clamping frame.
a)
b)
Figure 4.10 From practice to theory and finally to the evenly heated material: a) Unsatisfactory conditions in actual practice; clamping frame without reflection, semi-finished product hotter in the centre b) The theoretical ideal case with an infinitely large heater and a clamping frame with a height of 0
Reflection and temperature control of clamping frames in sheet-processing machines and foil-transport systems in automatic roll-fed machines It is possible to achieve optimal reflection properties by applying self-adhesive high-gloss aluminium strips to employ vertical clamping frames for mirroring. At distances of up to 20 mm from the clamping frame, the temperature loss on the surface of the semi-finished product is practically equal to 0 (Figure 4.11). However, because the clamping frame heats up, the aluminium strip expands more than the steel clamping frame. This leads to partial separation of the aluminium strip from the clamping frame’s surface. This solution is therefore only suitable for application over a limited number of hours in actual production. Depending on surface coarseness, aluminium offers good to extremely good reflective properties. At a distance of 20 mm, the temperature loss amounts to 10 °C to 20 °C (Figure 4.11). The hotter the metal, the lower the temperature loss in the edge zone, which means that heating of the semi-finished product remains more uniform all the way to the edge. Should aluminium sheet be used as a reflector, when selecting the mounting arrangement, it is important to remember that aluminium expands more rapidly than steel when heated. Reflectors in aluminium sheet must be able to expand without restriction. As an alternative, small lengths of the reflective material can be bonded to the surface of the clamping frame.
Reflector
4.1 Radiant heaters
Upper clamping frame
Beam from within the clamping frame
Reflected beam from within the clamping frame, as substitute for "obstructed" beam from outside the clamping frame
Beam "obstructed" by the clamping frame emanating from outside the clamping frame
Figure 4.11 Effect of IR mirrors (IR reflectors) on clamping frame or transport profiles
Nickel-plating and galvanising steel surfaces will lead to adequate reflective properties. At a distance a of 20 mm to the clamping frame (Figure 4.11), the temperature loss is approximately 15 °C to 30 °C. As the temperature of the clamping frame or the reflective material increases, the temperature loss decreases, and heating of the semi-finished product into the edge zone is more uniform. Owing to the good heating in the edge zone, machines with temperature control for the upper and lower clamping frames featuring extremely good reflection from the surfaces can be used to produce parts with less distortion. In comparison tests focusing on forming parts with a minimal clearance from the clamping frame and stringent requirements for contour definition in the edge zone, the initial thickness of the semi-finished product in the edge zone could be reduced by up to 10% owing to the higher temperature. Reflective layouts using aluminium spray consisting of 99% aluminium provide relatively good results. Reflection with aluminium spray is primarily employed for the lower clamping frame in sheet-processing machines. The reflection of the clamping frame or of the chain-transport profiles in automatic roll-fed machines is important in all heated stations. A dirty or cold clamping frame results in temperature losses on the heated plastic panel near the frame of up to about 60 °C. A temperature loss of up to 10 °C at roughly 25 mm from the frame is considered to be normal or good. A hot clamping frame in the heating station compensates for the less effective reflection on its surface. Using clamping frames as reflectors in sheet-processing machines is illustrated in Figure 4.12. The selected distance between the heater deflection panel and the upper edge of the upper clamping frame should be as small as possible. The frame
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should project the reflection over the entire height when possible. Owing to the hazard of deformation and separation from the clamping frame, using sheet metal to reflect onto the lower clamping frame should be avoided if possible. This could lead to destruction when the mold is retracted during the forming process. The reflection system for the sheet-material transport system in an automatic roll-fed machine is shown in Figure 4.13. 1 Reflector on heater deflection panel 2 Distance between heater deflection panel and upper edge of upper clamping frame 3 Upper clamping frame, inside mirrored 4 Material 5 Lower clamping frame, inside mirrored 6 Reflector on lower heater deflection panel 7 Distance between upper heater and material 8 Distance between lower heater and material
Figure 4.12 Reflective surfaces with sheet-processing machines (conceptual illustration) 1 Reflector on upper heater deflection panel 2 Distance between heater deflection panel and upper edge of transport profile incl. reflector 3 Reflector, e.g., in aluminium sheet material 4 Material 5 Reflector on lower heater deflection panel 6 Distance between upper heater and material 7 Distance between lower heater and material
Figure 4.13 Reflective surfaces with automatic roll-fed machines (conceptual illustration)
4.1 Radiant heaters
Effect of transport steps under a long heater Every point on the surface of the semi-finished product must have a single temperature in the forming station. To obtain this result, it is necessary to ensure that each point in the advance-feed direction is heated with the same frequency as all others. If this is not the case, it remains possible to shield the surface from the radiant heat or to deactivate transverse heater element rows (Figure 4.14 and Figure 4.15).
2...3 times
2 times
Figure 4.14 Checking the heating through a whole number of advance-feed cycles (2 or 3 times) Case: Machine tables widths wider than the mold 0, 1, 2, 3, 4 steps (countdown) in transporting F: Forming surface (advance feed)
If the machine’s table width is wider than that of the mold, then there will be differences in how the blank is heated before it arrives in the forming station (Figure 4.14 a). If the sector of the heater in front of the forming station is covered, then all points on the blank will be heated exactly two times (Figure 4.14 b).
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a) 2...3 times
b) 2 times
Figure 4.15 Checking the heating with a whole number of advance-feed cycles (2 or 3 times) Case: Machine table widths narrower than the mold 0, 1, 2, 3, 4 steps (countdown) in transporting F: Forming surface (advance feed)
If the machine’s table width is narrower than that of the mold, then there will be differences in how the blank is heated before it arrives in the forming station (Figure 4.15 a). If the sector of the heater in front of the forming station is covered, then all points on the blank will be heated exactly two times (Figure 4.15 b). pper The schematic explanation in Figure 4.14 and Figure 4.15 only applies to the u heater deflection panel. In actual real-world application, there will also be a lower heater deflection panel. The procedure for heating with an upper heater and lower heater is similar, even if two heater deflection panels are not of equal length or are not perfectly aligned above each other in the advance-feed direction. Cross-over effect with radiant heaters When a heater panel travels from its standby position to its heating position at the start of each cycle and then returns to its standby position once the heating time has elapsed, this leads to the cross-over effect, meaning that the semi-finished product is heated for different amounts of time because the heater crosses over it. More rapid heater travel motion corresponds to reduced cross-over effect and vice versa.
4.1 Radiant heaters
Typical travel times for a heater panel with a width of 1 m are roughly 3 to 5 seconds for pneumatic drive systems and about 1.5 to 2.5 seconds for servo-pneumatic and electro-pneumatic drive systems. If shorter heating times are being used (e. g., 8 s), it will be necessary to compensate for the cross-over effect by adapting the radiant heating pattern and possibly by extending the heating time. When heating times are extremely short, it is no longer possible to compensate for the cross-over effect with heater element adjustments! Minimum heating times are needed for even heating of the semi-finished product. Machines for processing thin-gauge semi-finished products and foams in which the heaters travel to their heating positions and back in cycles must have rapid heating motion. Effect of the cooling fan on the heating pattern in a sheet-processing machine Sheet-processing machines with heaters in the forming station are equipped with cooling fans for cooling the molding in the forming station. If the air current cools a portion of the heater element or the radiating surface owing to the absence of shielding, the result will assume the form of distortions in the heating pattern. Even with heater panels featuring heater element temperature control using pilot heater elements, and especially when a heater panel is only equipped with a single pilot heater element, it is impossible to avoid distortion in the heating pattern stemming from air intrusion from cooling fans. This is demonstrated by the following examples: Example 1 A heater deflection panel has only one heater element. The cooling air cools the heater element in the half-section of the heater panel in which the pilot heater element is installed. The pilot heater element registers the temperature reduction, and the temperature control compensates by increasing the pulse-duty factor. However, because all of the heater elements are controlled based on input from the same pilot heater element, those heater elements that are not located in the c urrent of cooling air will overheat. The result will be a distorted radiant heating pattern, which in turn leads to uneven heating of the semi-finished product. Example 2 Air cools the heater elements on the half of the heater panel in which no pilot heater element is installed. The pilot heater element remains at its setpoint temperature, while the pulse-duty factor remains unchanged. As a result, the half of the heater panel with the pilot heater element retains its temperature, but the other half-section cools. The result will be a distorted radiant heating pattern and uneven heating of the semi-finished product.
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Air draught, also resulting from stack effect The heated air produces a stack effect, or chimney effect, with corresponding air infiltration or air draught in every machine. The installation locations of the machines, open doors, open windows, etc. have a similar effect. The practical effects can assume the form of possible variations in sheet temperature or asymmetrical cooling in a preformed bubble. Countermeasures include shielding the machine against air draughts as well as the obvious remedy of adaptable heating. The latter is only logical provided that the air draught remains constant.
4.1.4 Ceramic, quartz and halogen heaters in comparison 6 Electrical connection 5 Thermocouple (connection)
4 Mounting fitting 3 Ceramic housing 2 High-temperature insulation 1 Filament
Figure 4.16 Cross section of a hollow ceramic element with thermocouple (pilot heater element) 5 Electrical connection 4 Mounting threads 3 Metal housing 2 Quartz tube 1 Filament
Figure 4.17 Cross section of a quartz heater 1
2
3 5
Figure 4.18 Halogen heater 1 Filament 2 Quartz tube 3 Mounting socket 4 Electrical connection 5 Length of filament
4
4.1 Radiant heaters
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The essential differences between ceramic, quartz and halogen heaters Table 4.1 Compilation and comparison of different radiant heater types Property
Ceramic heater
Quartz heater
Halogen heater1)
Energy
Electrical
Electrical
Electrical
Energy conversion
Heated filament
Heated filament
Heated filament
Emission source
Ceramic surface
Filament + fused silica surface
Filament + fused silica surface
Heater element temperature
300 to 700 °C maximum 800 °C
Filament < 1100 °C Quartz < 500 °C
Filament < 2400 °C Quartz < 950 °C
Weight ratios
Modern heater elements lighter than quartz2)
Heavy2)
Very light2)
Warm-up time
< 10 min.
< 10 min.3)
< 1 s (3 min.)4)
Connected power ratings
16.6 . . . 25 kW/m (for 700 °C roughly 38.4 kW/m2)
Energy consumption when heating
Up to roughly 75% of Up to roughly 75% of connected power rating5) connected power rating
Up to roughly 85% of connected power rating
Energy consumption in standby mode
Roughly 25% of connected power rating6)
Roughly 25% of connected power rating7)
0%
Full-surface heat emission
Possible
Possible
Not possible, achieved with special reflector
Change in temperature of heater element
Slow
Filament fast, fused silica mass slow
Filament very fast, fused silica relatively fast, as proportionately light
Adjustment options at heater element
Temperature control in °C, power control in % and temperature control with superimposed % control (from ILLIG)8)
Power control in %9) (control in °C not available)
Power control in %
Reproducibility of heating results Short-term
In version with temperature control at heater element, very good10)
Less good if heater element set at % and no IR unit in the machine11) 13)
Good to very good12) 13) if initial temperature of semi-finished product is constant
Reproducibility Long-term
Very good;14) old emitter ≈ new emitter
Less good;15) old emitter differs substantially from new emitters
Unfavourable because high dependency on reflector16)
2
16.6 . . . 50 kW/m
2
50 . . . 75 kW/m2
Service life
∼ 10,000 h
∼ 5,000 h
∼ 5,000 h
Operational monitoring
Complicated
Simple, visual
Simple, visual
Thermal stress resistance of the machine
High17)
High17)
Very low18)
Beam range (wavelength range)
Wide, max. at 3 to 5 mm
Wide, max. at 2 to 4 mm
Narrow, max. at 1 to 2 mm
Application
Universal
Universal
Partially restricted19)
Effect of plastic colours
Heating times practically identical
Heating times almost identical
Different heating times20)
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Table 4.1 Compilation and comparison of different radiant heater types (continued) Property
Ceramic heater
Quartz heater
Halogen heater1)
Application range by machine
Universal
Universal
Limited to single station panel-processing machines21)
Cover shield
No
(No)
Necessary
Regular periodic cleaning
–
–
of cover shield
1)
Halogen heater, also called flash heater Ceramic: HTS/2 (Elstein), 122 × 62 mm, weight 130 g Quartz: TQS FSK, dimensions 124 × 62 mm, weight ∼ 178 g (ceramic: HTS (Elstein), 122 × 122 mm, weight 230 g). 3) The filament itself quickly becomes red, but the remaining mass, the fused silica, the side ceramic mount (also emitting surface) have time requirements similar to those of ceramic heat elements). 4) The proportional weight of halogen heat elements in a heater panel is up to 6 times less than that of a quartz heater; the halogen heat element radiates heat immediately (after 0.2 s), however the fused silica does become warm later. 5) The heater element’s power is selected to ensure that its temperature can be regulated during the heating period and in response to small voltage drops. 6) If the heater is above a reflector in its standby mode but without any reduction in heater element temperature, then the energy consumption will fall by roughly 25% of the connected power rating, depending upon temperature setting. 7) Similar to 6); the difference here is that there is usually no temperature status request. Here, the power is adjusted. The temperature at the heater element is the product of the difference between the selected and the delivered power and the power radiated onto the semi-finished product and into the environment at an inconstant rate. 8) The temperature of the ceramic heat element is usually regulated with closed-loop control. Heaters with the same power and heat loss (to the semi-finished product or the environment) are compiled in groups and controlled with a heater with an installed thermocouple (pilot heater element) from the individual group. 9) Version 8) is not usual in this context, although pilot heater elements with integral thermocouples have been developed. 10) It is possible to start production without rejects once a heating pattern has been stored, closed-loop control with an adequate number of pilot heater elements is in operation and the heaters have reached their specified temperatures. 11) The amount of energy being delivered to the heater element is indicated in the power setting of radiant heaters. The temperature at which the heater element settles will also depend on the energy being radiated into the environment. If no supplementary means (temperature-level sensor in heater panel, IR units, etc.) are employed to accelerate the stabilisation process, then 30 to 40 min. will elapse before the entire heater’s radiant-emission properties achieve stabilised status. Power control with heater elements characterised by high mass is suboptimal. 12) The low heater element weight (25 g for a 700 W heater element with a 165 mm heating length) allows a very short stabilisation time for the beam temperature. Only the ends and the sockets of the heater elements, which also radiate heat, require more time to stabilise. Without IR sensors, tolerances in the heating results for the first thermoformed parts must be anticipated. 13) The temperature is highly sensitive to air supply and reacts to air draughts with corresponding celerity. 14) Up to a heating period of roughly 200 s, virtually no difference between an old, dirty heater element and a new one can be perceived. 15) When it becomes necessary to replace old heaters, their improved heating power will be perceptible. 16) If a heater element depends on a good reflector, then this has consequences: Reflectors accumulate contamination as time passes. The heater element can be protected by a glass-ceramic panel to prevent contamination. Under these conditions, it must be noted that the glass-ceramic surface becomes a heat-radiation surface, i. e., the system’s stabilisation time is extended because the mass is greater. If heater elements featuring integral reflectors are selected, then the energy yield and efficiency level will be poorer if the reflector does not reflect 100%. 17) This statement applies only to machines in which the heater returns to a standby position and is not needed after each cycle. Since heavy heaters react slowly owing to their mass, they must be maintained at operating temperature while in the standby positions. 18) With sheet-processing machines, the heater is deactivated at the standby position. However, the short-wavelength radiation transmits more heat to the painted parts of the machine while in the heating position. 19) There are plastic semi-finished products, primarily thin-gauge, crystal-clear semi-finished materials, that are very difficult to heat with halogen heater elements. 20) The heating times vary according to the colours. White plastics require substantially longer heating times than black ones. 21) The essential advantage of this heater element is the low thermal stress placed on the machine. It would not be a good idea to apply the halogen heater in a machine with an upstream heating station (automatic roll-fed machine or sheet-processing machine) in consideration of its poor efficiency level. The electrical power consumption during heating is higher than with ceramic and quartz heaters. It is only through deactivation at the standby position with single-station sheet- processing machines that the halogen heat element offsets its higher power consumption through deactivation. 2)
4.2 Reproducibility of heating results in radiant heaters
Conclusion from Table 4.1: Compilation and comparison of different radiant heater types The average efficiency levels of ceramic heat elements lie in the range of 25% to 40%. Efficiency levels of approximately 60% can be achieved with optimal insulation for the heater panel and an ideal combination of heater element and plastic. It is possible and necessary to use testing to determine the correct heater elements for machines built for heating a specific material or material group with similar absorption properties. This achieves high efficiency levels using the same radiated heat output. This means that the specific energy use is minimised. However, setting up a machine for one specific plastic represents the exception. If the intention is to use a thermoforming machine to heat as many types of plastic as possible, then the objective will be to cover the largest possible wavelength range, allowing efficient heating of the highest possible number of plastic types. The longwave ceramic heat element meets these requirements. Its disadvantage is thermal inertia – with corresponding negative effects when heating the machine as well as in obtaining the desired dynamic response in temperature control. Owing to the rapid decline in its radiated heat output in the longwave radiation range, the halogen heater requires a high connected power rating (to raise the emissions curve as a whole) in order to achieve the heating times obtained with ceramic heaters and quartz heater. The halogen heat element’s advantage resides in its low level of thermal inertia. It can be switched on and off in cyclical operation, which compensates for the high energy consumption during the warm-up phase – provided that the “off” phase is of adequate duration. While the factors that determine which heater is selected include the type of thermoforming machine, the semi-finished product being processed, its efficiency level and the price, the range of considerations also extends to embrace the philosophy of the machine’s constructor. These are primarily adjustment of the radiated heat output and the reproducibility of the heating results.
4.2 Reproducibility of heating results in radiant heaters 4.2.1 Assessing reproducibility A heating process is perfectly reproducible when both the surface temperature of the semi-finished product and the temperature-distribution pattern extending throughout the depth of the semi-finished product remain constant from the first to the last production cycle.
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Figure 4.19 provides a schematic representation of the problems encountered in achieving a reproducible temperature profile in a semi-finished product. T0
T0
d
a
a
Termination of heating period when surface temperature is reached
Termination of heating period with closed-loop control of heater temperature once specified time has elapsed
Continuous line: hot machine Dotted line: cold machine
Continuous line: hot machine Broken line: cold machine
Figure 4.19 Temperature profile relative to semi-finished product gauge, schematic a: Temperature differential between semi-finished product surface and core of the semi-finished product thickness To: Surface temperature of the semi-finished product d: Thickness of the semi-finished product
Measurement of the temperature at the surfaces of a semi-finished product during production relies on infrared sensors. (The use of thermostrips with permanent displays is only suitable for tests, at which the surface must subsequently be viewed as having been destroyed.) In most cases, IR sensors are used to measure the temperature at a single point. On sheet-processing machines, the temperature sensor is installed roughly at the centre of the heater panel. In automatic roll-fed machines, it is approximately at the end of the heater, where it can measure the temperature reached by the semi-finished product. (Sensors for monitoring extended surfaces are substantially more expensive, and for this reason, they are only seldom used.) If multiple pilot heater elements are used to control the temperature of a heater, then a temperature measurement at one point on the semi-finished material’s surface will be sufficient. When the output of the heater elements in a heater is at a specific setting and no sensors are present in the heater elements, then no monitoring of the heater element temperature is available (no pilot heater element). This means that the heater’s heating pattern continues to change until it stabilises, despite the consistent power settings for the individual heater elements. In this case, a measurement of the semi-finished product’s surface temperature would be more suitable.
4.2 Reproducibility of heating results in radiant heaters
There is no non-destructive method available for measuring temperatures extending throughout the thickness of the semi-finished product and no measurement options for use during running production. The effects of different temperatures in the material at various depths while surface temperatures remain constant can have different effects on wall-thickness distribution and contour definition in the formed part. Example 1 Sheet-processing machines with power-controlled radiant heaters and 1-point measurement with IR sensor for terminating the heating period once the surface of the semi-finished product reaches the specified temperature. Production starts, while the interstices between the heater elements, the heater covers, frame and parts in direct radiation contact with the semi-finished product are still cold. The heating period is concluded once a specified temperature is reached on the surface of the semi-finished product. During production, the machine heats up in a process that continues until the temperatures of all parts have stabilised. The temperature of the semi-finished product is affected by the temperatures of the interstices between the heater elements, the heater covers, the frame, and all parts in direct radiation contact with the semi-finished product. Depending upon its design configuration, the stabilisation period for an assembly will range between approximately 40 minutes and several hours. Although the power output from the heater remains constant, the heat radiation from the heated parts from the entirety of the irradiated environment of the semi-finished product is added to the radiated heat output from the heater elements. As the machine heats up, the overall heat radiation rises and causes the surface of the semi-finished product to reach the specified temperature more quickly. Although the heating period is terminated once the surface temperature is reached on the semi-finished product, the duration of the heating period is reduced once the machine is warmed up. Since the temperature of the core of the sheet is primarily raised by thermal transfer from the outer layers, the temperature obtained in the core also depends on heating time. As heating times are reduced while the surface temperature remains constant, the core of the sheet will become commensurately cooler (throughout its depth). Different heating times at an identical surface temperature on the semi-finished product will result in different temperature profiles extending through the various depths in the interior of the semi-finished product. As the machine becomes hotter, the contour definition decreases because the core of the sheet remains cooler.
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The machine operator corrects the machine’s settings by raising the setpoint temperature for the IR measurement unit as a means of restoring the contour definition, but the disadvantage is that the surface temperature of the semi- finished product is also increased. In this case, the heating time is not set by the operator, but instead it is terminated when the semi-finished product reaches its surface temperature. At this equipment level, the following applies: The reproducibility during heating worsens as the sheet becomes thicker. Example 2 Sheet-processing machines featuring heaters with closed-loop temperature control based on pilot heater elements and with static heating times. Although the heat-emitter array is maintained at a constant temperature level, the lateral heat radiation emanating from machine components as they warm up impacts against the semi-finished product as in Example 1. If a machine with this equipment is set to a specified heating time, then pro gressively higher semi-finished product temperatures will be reached – both on the surface and in the interior of the sheet as the machine becomes hotter. In this case, the quality of the contour definition does not decrease as the machine becomes hotter – it actually becomes better. The problem in this case is preforming with semi-finished products exhibiting various levels of heat penetration. During preblow or pre-suction, the height of a bubble will increase as the machine becomes hotter, which will inevitably lead to a different wall-thickness distribution in the formed part. This is amplified by the changing coefficients of friction between the semi-finished product and mold. No perfect heating reproducibility is achieved with this equipment version, either. Example 3 Automatic roll-fed machines with closed-loop temperature control of radiant heaters, automatic roll-fed machines with power-controlled radiant heaters. For many customers, all possibilities for changing the machine settings are locked out or password protected – with the exception of access to the heater element temperature. With automatic roll-fed machines, it is also the case that the machine becomes hotter as time elapses. In power-controlled heaters, this is the change in heat- emitter temperature. During forming of complicated parts, the operator must intervene in the molding process if no closed-loop control of molding temperature using sheet temperature measurement is being used. The heater element temperatures are reduced
4.2 Reproducibility of heating results in radiant heaters
to prevent the sheet material from overheating. Owing to the limited cooling time, it is only in rare cases that the option of reducing the forming station’s cycling time, and thus shortening the sheet material’s time beneath the heater, will be available. Perfect reproducibility without intervention from the machine’s operator is not available. These examples lead to the conclusion that perfectly reproducible heating results can only be achieved if the temperature progression can be reproduced throughout the depth of the semi-finished material. This is of particular importance with heavy-gauge semi-finished products, meaning with sheet-processing machines. Given the same starting temperature for the semi-finished product, this means maintaining a constant total radiation against the semi-finished product and a constant heating time. Since the total heat radiation consists of the sum of the heat radiated by the IR heater elements and the incident radiation from all machine parts capable of radiating heat onto the semi-finished product, it will be necessary to compensate for the latter factor by modifying the heat radiation from the IR heater. This means that the hotter the machine becomes, the greater the need to reduce the heat radiation from the radiant heaters. If the entry temperature of the semi-finished product changes, then compensation must be provided using the heating time and the radiated heat output.
4.2.2 Compensation for uncontrollable external influences on the heating process Machine equipment to compensate for uncontrollable external influences on the heating process must include: Compensation for fluctuations in the electrical power grid supplying the machine. Compensation for machine heating through reduction in the radiated heat output while heating time remains constant. Compensation for semi-finished product’s entry temperature through change of heating time; cooler semi-finished products must be heated longer, warmer semi-finished products must be heated for shorter periods. Controlling this process entails equipping the machine with sensors. The registered data can be employed for continuous adaptation of the parameters governing the warm-up process, the objective being to maintain constant temperatures on the surface of the semi-finished product and a consistent temperature profile throughout its depth, regardless of machine warming and other external influences. Implementation of corrections is fully-automatic (option on ILLIG machines).
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4.2.3 Power control and temperature control in heaters Two solutions have achieved popularity as a means for influencing the radiated heat output in the heating process with current thermoforming machines: Power control and closed-loop control of heater temperature Power control simply means that the radiated heat output is reduced by lowering the power rating of the radiant heaters. Although various analogies are available, the result is largely comparable to dimming lighting elements. With temperature control of heater elements, the power is adjusted by controlling the temperatures of the heater elements. However, this entails using heater elements that can be equipped with a thermocouple as is the case with commercially available ceramic heater elements. Quartz and halogen heaters with thermocouples are not commercially available. For this reason, the following comparison is based on ceramic heat elements. Concept behind temperature control A radiant heater with closed-loop temperature control is operated at its full rated power until the thermocouple signals that the setpoint temperature has been reached – regardless of whether the surrounding environment is hot or cold and without being affected by any air draughts that may be present. Thus, assuming a consistent initial temperature for the semi-finished product, it is only the heating time that must be calculated and set to a specific value for uniform heating of the plastic to its forming temperature, and this is possible with standard machine equipment. Concept of power control With power control, the end temperature that can be achieved by the heater element is determined by the selected power in percent of the rated power along with the heat radiated by the heater element to heat the semi-finished product and its environment. Changes in the ambient influences – for instance, through temperature fluctuations or air draughts in the production hall – will exercise an ineluctable effect on the achievable radiant heater temperature, which will have a direct influence on the time that the semi-finished material needs to reach its forming temperature. For this reason, power-controlled heaters are usually equipped with an infrared measurement unit for registering the temperature of the semi-finished product, allowing the heating time to be terminated once the surface of the semi-finished product reaches the specified temperature. On machines featuring heaters with power control, setting the machine by calculating the heating time is not standard practice because the forming temperatures that could be achieved at the semi-finished product would be too imprecise.
4.3 Contact heaters
Figure 4.20 shows another, serious difference between these two procedures. Temperature control of a high-temperature ceramic heat element (HTS) achieves the setpoint temperature much more quickly than power control at the same heater element.
Figure 4.20 Times t until temperature stabilisation of a heater panel with temperature control (index reg.) and power control (index. control.). Schematic representation.
The time that elapses before temperature stabilises in a power-controlled heater panel is essentially dependent on the selected heating power (set power output of the heater element) and the weight of the total heater, i. e., the heater element and all additional heater-panel elements that radiate heat at which the heater panel cover must also be included.
4.3 Contact heaters Contact heaters transfer the heat from heated plates or cylinders through contact and thermal transfer to the semi-finished product (Figure 4.21 and Figure 4.22). Advantages of contact heaters: Precise regulation of temperature at the semi-finished product is possible. Provided that the heater is set to the correct temperature, it will not be possible to overheat the semi-finished product. Production can be started without start-up rejects. The option of heating to suit specific formats is available, i. e. heating can be limited to strictly delineated surfaces for molding in the forming station. Format-based heating guarantees minimal thickness tolerances in sealing rims during production of sealed packaging. With appropriate shielding, the heat loss can be restricted to extremely low levels. The heating time for preprinted materials is not affected by the print colour.
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Figure 4.21 Heating with contact heater plates (upper contact heater plates with machine stopped tilted up) Image: ILLIG, type HSA machine
Figure 4.22 Heater with two contact rollers Image: ILLIG, type VHW machine
1
Figure 4.23 Contact heaters on two sides 1 Full-surface contact heater plates 2 Format-based contact heater plates
2
4.4 Convection heaters
Disadvantages of contact heaters: Contact-heaters tend to stick at high temperatures. Contact heaters with anti- adhesive coatings reduce the sticking tendency, but in especially critical con ditions, even the plastic sheet material will need to be treated with an anti-ad hesive layer. Not all anti-adhesive coatings on semi-finished product are suitable for thermoforming. Some, such as calcium stearate, form a deposit layer on the heating surface and the mold that can even plug vent holes. Under these conditions, both the contact heater and the mold, and possibly even the format parts such as upper plug, etc. will need to be cleaned on a regular basis. Even and uniform contact extending over the entirety of the semi-finished product’s molding surface is required for effective heat transfer. Since the plastic expands when heated, warping could cause markings to appear on the heated surface. For this reason, contact heaters can only be used for semi-finished product in limited sizes.
4.4 Convection heaters In thermoforming, convection heating is used: To dry hygroscopic semi-finished product: The drying times and temperatures vary according to the plastic and can be found in the table for the thermoformer. As preheaters for automatic roll-fed machines. In rare cases, convection heaters are used as final heaters, for instance, when processing polycarbonates and plexiglas. These semi-finished products are usually reshaped on presses or special equipment. The accuracy of the achieved semi-finished product temperature both on the surface and at the core also depends on the duration of the heating period when convection heating is used. A high level of thermal uniformity can only be achieved with an extended heating time. The peripheral surfaces of the semi-finished products must be rinsed by the hot air. Here, the objective is to obtain homogeneous air speeds.
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4.5 Minimum heating time, effective heating time and residence time The minimum heating time for a particular type and gauge of semi-finished product is the shortest possible period required to heat the semi-finished product to the desired forming temperature. The effective heating time is the heating time that is actually required to heat the semi-finished product to the desired molding temperature. The effective heating time is never shorter than the minimum heating time. The residence time is the time during heating of the semi-finished product during which the temperature within the semi-finished product is higher than its softening temperature.
4.5.1 Effect of heating time on thermoforming response The minimum heating time is dependent on: The installed heating power The effectiveness of the heat radiation, that is: The type of heater element The distance between the heater element and the thermoplastics being heated The effective level of efficiency Reflections, etc. Notice The limit on the installed specific heating power (kW/m2) depends both on the max. thermal stresses placed on machine components and on the maximum temperature resistance of the semi-finished product (burning hazard). As effective heating times increase, so do the potential residence times. Both during heating with IR radiation and when heating with contact heaters, direct heat is applied to the surface of the semi-finished product. The penetration depth that IR radiation achieves in the semi-finished product is usually derisory. Heating at the core, meaning the centre, of the semi-finished product proceeds through thermal transfer from the outer layers. With both types of heater, the core remains colder than the surface of the semi-finished product.
4.5 Minimum heating time, effective heating time and residence time
4.5.2 Positive effect of residence time The simplest way to lengthen the residence time is to increase the duration of the heating period. Better heating consistency, i. e., a smaller temperature differential between the semi-finished product’s surface and its core, can be measured with thicker semi- finished products. Measurements on sheets with a thickness of 10 mm have indicated temperature differences of up to 60 °C. Owing to practical considerations, it is not possible to measure the differences between the surface and core when dealing with thin-gauge semi-finished products, i. e., on sheet material with thicknesses of 0.3 to 1 mm. Although a theoretically inconsequential temperature difference between the surface and core has been calculated for thin sheet materials, a deliberate extension of the residence time makes a demonstrable improvement in thermoforming properties. The improved performance is primarily encountered in ductility, dimensional stability, distortion resistance and temperature resistance in the molding. The explanation for the improved thermoforming ability with longer residence times lies in the morphological change that these semi-finished products undergo during the residence time. Progressively longer residence times correlate with correspondingly more pronounced changes in morphological structures, resulting in improved thermoformability, which is basically mirrored in the visco-elastic response of the thermoplastic materials.
4.5.3 Negative effect of residence time There are semi-finished products, such as APET, CPET, PHA, in which an extended residence time at a single thermoforming temperature will have negative consequences for thermoforming response, regardless of whether the objects are thin sheet material or heavy-gauge sheets. Here, the root cause resides in the temperature-sensitive and time-dependent start of crystallisation and the enhanced tensile and flexural strength that accompanies the higher crystallisation level. The result is a deterioration of the contour definition. In this case, the changes in the sheet material cannot usually be detected through mere observation during the heating period – not even with crystal-clear sheet material. In extreme cases, a mild discolouration in the form of whitening may be detected on crystal-clear sheet material. A phenomenon that seldom appears in real-world applications is depolymerisation during heating. Under the influence of extended residence times below the heater and the accompanying high material temperature, instability in monomer bonds in
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the polymer chains can lead to partial dissolution of the polymer chains. When a relatively thick PETG (for instance, 4 mm) is heated on one side, this can lead to depolymerisation. At this point, minute points can be seen in transparent material, and the material has been damaged by incorrect heating. In such cases, the solution is to heat up the semi-finished product as quickly as possible and to avoid unnecessary extensions in residence times at high temperatures.
eating technology in H thermoforming
The most popular heating techniques for heating thermoplastic semi-finished products rely on: Radiant heater elements Contact heaters Convection heaters All of the above-cited heaters are operated using electrical energy. There are also some heaters that operate with gas. Radiant heater elements transfer energy in the form of electromagnetic waves, primarily in the infrared range (0.76 mm to 1000 mm). In contact heaters, the energy is transferred from a heating plate or a roller to the heated material. Convection heaters rely on hot air to relay the energy to the semi-finished product.
4.1 Radiant heaters 4.1.1 Heat-transfer concept with infrared radiation Plastics absorb infrared radiation. The degree of absorption will depend upon the semi-finished product’s type and gauge along with the wavelength of the radiation.
Absorption of IR radiation, in %
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Wavelength of radiation, in µm
Figure 4.1 Absorption of IR radiation (in %) as a function of wavelength; for type 475K polystyrene sheets, blue 849, semi-finished product with gauges of 0.04, 0.5 and 4 mm [source: BASF Ludwigshafen]
Standardised radiation
Every semi-finished thermoplastic material (type, colour, etc.) will display its own specific absorption curve – comparable to a fingerprint (Figure 4.2). Increased thickness in the semi-finished product is accompanied by progressively higher absorption levels. When the power emitted by a heater element impacts against the semi-finished product’s surface, some of the irradiated output is reflected, some of it is absorbed by the material, and some penetrates to the other side of the material without affecting it.
0
2
4
6
8
10
Wavelength of radiation, in µm
Figure 4.2 Power output concept of heaters (standardised radiated heat output) a) Ceramic heater, surface temperature of 700 °C b) Quartz heater, filament at 900 °C c) Halogen heater, filament at 2000 °C
4.1 Radiant heaters
There are no objects (in this case, heaters) that emit their radiant energy in a single wavelength, even if the surface temperature is homogeneous. If a thin-gauge plastic, for instance, with a gauge of 0.04 mm displays a high level of absorption at a specific wavelength, then if the power is radiated against a thick pre-cut sheet manufactured in the same material at an identical wavelength, most of the incident radiation impacting at this wavelength will be absorbed before it progresses beyond the material’s surface. The difference between the employed heater types consists in how the radiated heat output is distributed amongst the various wavelengths. The heater’s temperature determines the correlation of maximum power density with a particular wavelength in the radiated output. If the three heater elements – see Figure 4.2 – featured the same connected power rating, then the sizes of the three surfaces below the curves a, b, c would be equal. Heaters with unsatisfactory radiated power outputs relative to wavelength display poor efficiency levels. Their heating power can be increased by raising the connected power rating – as is the case with halogen heaters. Thus heaters equipped with different types of heater elements operating with equal connected power ratings will display varying heating results, depending upon the semi-finished product being heated.
4.1.2 Heat quantity transferred by radiation The radiated heat output of an infrared heater element The radiated heat output emitted by a heater element and employed to heat the semi-finished product P results in: P = e ×s × A ×T 4 (4.1) Radiated heat output from an infrared heater element with the emissions level e of the heater’s surface (for the “ideal” heater, valid for the black body: e = 1), the Boltzmann constant s of the radiating surface A and the temperature T in Kelvin of the heater element’s radiating surface. The following conclusions can be drawn from Formula (4.1): The quantity of heat emitted by the heater element is proportional to the element’s temperature in Kelvin raised to the fourth power. The transferred quantity of heat varies according to the size of the radiating surface. The emission level of the surface projecting the radiation should be as close as possible to the value for a black body (with e = 1)
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Factor for variation of heating time
Effect of distance between heater element and the heated semi-finished product, refer to Figure 4.3.
Heater temperature (ceramic heater), in °C
Figure 4.3 Change in heating time as function of heater-element temperature; for ceramic heat elements with 200 mm distance to semi-finished product, measured values
Effect of deactivated heaters and machine components with rising temperatures Only the heater’s power, and not the temperature, is included in the formula. This means that a deactivated heater will also radiate heat if it is hot. A deactivated heater element within a heater will be heated by contiguous radiant heaters and will then emit heat according to the attained temperature. The temperature differential between the heater array and the deactivated heater element as a function of heater array temperature is provided in Figure 4.4. The data in Figure 4.4 have been determined on an ILLIG UA 100 Ed thermoforming machine, upper heater equipped with ceramic heaters, heater element size 123 × 62 mm, arranged in a grid of 125 × 105 mm. Figure 4.5 illustrates the temperature differences in the semi-finished product resulting from reduction in the power output of a single heater. The data were recorded using an ILLIG UA 100 Ed sheet-processing machine with FSR/2 ceramic heaters manufactured by Elstein.
Temperature reduction of a deactivated heater, in °C
4.1 Radiant heaters
Heater array temperature, in °C
Figure 4.4 Temperatures registered at a deactivated heater as a function of heater array temperature ① Heater in centre of array “Off” ② Heater in the corner “Off”
x-axis: Temperature differential at affected heater y-axis: Temperature differential on the surface of the semi-finished product
Figure 4.5 Possible temperature differential in semi- finished product under a heater with modified temperature Determined for: – HIPS pre-cut sheets with gauge of 5 mm – Semi-finished product temperature of 160 °C – Heater element size of 123 × 62 mm – Heater array grid of 125 × 105 mm – Temperature at the upper heater 600 °C – Temperature at the lower heater 450 °C ① Change in the temperature of a heater in the centre of the upper heater ② Change in the temperature of a heater element in the centre of the upper heater and a heater element in the centre of the lower heater
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Conclusion regarding effect of a deactivated heater element The heating power that a deactivated heater element projects onto the semi-finished product depends upon its temperature. Every component that heats up and can affect the semi-finished product through direct heat radiation, such as parts of the heater deflection panel, the heater cover, clamping frame (if there is heating in the clamping frame), transport system, machine frame, etc. Its radiated heat output corresponds to the data provided in Formula (4.1). This has disadvantages for the reproducibility of the heating process because these parts are cold when the machine starts, and they heat gradually before their temperature stabilises after a minimum period of 45 minutes. For more details refer to Section 4.2.2 “Compensation”. Effect of the heater element’s distance from the semi-finished product on the heat that it absorbs
Factor for variation of heating time
Progressively greater distances between heater element and semi-finished product equate with correspondingly longer heating times, see Figure 4.6.
Clearance between heater and object material, in mm
Figure 4.6 Variation in heating time as a function of heater element’s distance from the semi-finished product
Effect of the distance between the heater elements of a heater panel The transferable quantity of heat rises with the proportion of radiating surface embedded in a heater panel – one can then refer to a higher effective level of efficiency. If heater elements are not installed immediately next to one another, then they should be equipped with reflectors that compensate for any lack of direct heat
4.1 Radiant heaters
radiation with reflected heat radiation. The reflectors redirect the heat that the heater elements project to the sides and rear by deflecting it toward the front and the semi-finished product. Since no reflector can have a perfect, ideal geometry, and since it will also gather contamination as time passes, it represents a compromise relative to a fully equipped, directly radiating surface. Figure 4.7 shows a heater panel with ceramic heaters installed at a distance; Figure 4.8 portrays a heater panel with densely placed ceramic heaters with a distance between edges of roughly 5 to 6 mm. Owing to the overlap in the radiated heat, both types can apply even heating to the semi-finished product in the centre of the heated surface area, provided that the distance between the semi-finished product and the heater element level is not too short. Problems occur when: a) The distance between the semi-finished product and the heater element’s level becomes less than predicted, e. g., as when the material sags during heating, or b) in the edge zone of the tensioned material. If a gap in the heater elements is present above the edge of the semi-finished product, it will not be possible to heat this edge zone evenly, even if the immediately adjacent heater elements are adjusted to a higher radiation intensity or if vertical reflectors are employed in an attempt to project radiated heat into the edge zone. A gap above the edge, mirrored by the reflector on the surface of the clamping frame, will have the same effect as a double gap in the heater elements.
Figure 4.7 Heater panel with ceramic heat elements installed at a distance, flat reflector surface
Figure 4.8 Heater panel with ceramic heat elements installed in close mutual propinquity
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Effect of the emissions factor of the radiating surface of the heater element Ceramic heater elements are available with two coatings – light and black. The ceramic heater elements with black coatings that have been in use since about 2010 can achieve a higher radiated heat output, with commensurate positive effects on most thermoplastic materials. Effect of heater element size on the heating results A heater element does not merely project heat vertically, at a perpendicular angle to its surface, but instead the emissions travel in all directions. Each point on the surface of the heated plastic receives radiated heat from all of the heater elements unless the heat radiation has been specifically shielded. At the same time, deactivation of a single heater element in a heater array will affect the entire heated surface, with the largest influence below the heater element itself (Figure 4.9). Temperature differences on the surface of the semi-finished product reach their maximum below the centre of the deactivated heater element and become pro gressively less pronounced as one moves outward from this position. The practical effects stop after an angle of approximately 30 degrees.
1 side view
top view
Figure 4.9 Sensitive surface on the heated semi-finished product with deactivated heater element as example 1: Zone practically subject to effects j: Beam projection angle T: Temperature of semi-finished product DT: Temperature differential on the surface of the semi-finished product
Heating the plastic panel with infrared heater elements with a pattern ensuring that small surfaces on the semi-finished product deliberately remain colder is theoretically possible. However, the possibilities are subject to severe limits and many users overestimate the available potential. Achieving different temperatures
4.1 Radiant heaters
on the surface of a semi-finished product with targeted shielding of the beam radiated toward parts of the semi-finished product’s surface or blowing cold air against the heated surface of the semi-finished product is substantially more effective than deactivating individual heater elements. The question repeatedly arises regarding the ideal heater element size for optimal heating. Standard commercial sizes of heater elements are 60 × 60 mm, 123 × 60 mm, 123 × 123 mm, 248 × 60 mm, 248 × 123 mm. With these heater- element sizes, heater shields with heater element grids of 62.5, 125 and 250 mm can be implemented with full-surface emitter occupation. Intermediate spaces consisting of just a few mm are necessary to avoid breakage in the heater elements. Small heater elements (e. g., 62 × 62 mm) are advantageous on machines featuring small clearances between the heater element and semi-finished product as is the case with automatic roll-fed machines featuring a distance of between roughly 80 and 125 mm. When the distances between heater element and semi-finished product are greater, over 200 mm – as is the case with flat-sheet-processing machines, this becomes a matter of faith, and actual advantages are practically impossible to demonstrate. When dealing with more extensive forming surfaces, e. g. for machines from roughly 2000 × 1250 mm, selecting heater elements larger than 125 × 125 mm is certainly justifiable. In automatic roll-fed machines, where the plastic is transported below the heater multiple times, the length of the heater element in the direction of transport is not an important factor. At the same time, it is important to consider the ability to heat a sheet-material surface evenly at the advance-feed rate. Beam grids at an angle of 65 mm to the direction of transport have proven to be optimal for automatic roll-fed machines with sheet-material widths extending up to approximately 800 mm.
4.1.3 Homogeneous heating with radiant heaters Regardless of the machine’s type or size, the primary objective remains uniform heating of the semi-finished product extending across its entire surface. With automatic roll-fed machines, heating the sheet material to a single, uniform forming temperature is absolutely essential. On pre-cut-sheet processing machines, even heating should be considered as the starting point for any subsequent modifications to the heating pattern. Figure 4.10 a shows that more radiated heat is arriving at the centre of a heated zone than on the edge. In the vicinity of the clamping frame or the transport profile, “the heat radiation is missing”. Figure 4.10 b illustrates the theoretical ideal case for homogeneous heating of the semi-finished product, which would be obtained with an infinitely large heater and a frame height of zero.
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Even heating is achieved by: Installation of reflectors for IR radiation with which the missing beam component from the outside is replaced by an inside beam reflected by the clamping frame (Figure 4.11). Reduction in heater element temperatures at the centre of the array relative to the heater elements on the edge and above the edge zone of the clamping frame.
a)
b)
Figure 4.10 From practice to theory and finally to the evenly heated material: a) Unsatisfactory conditions in actual practice; clamping frame without reflection, semi-finished product hotter in the centre b) The theoretical ideal case with an infinitely large heater and a clamping frame with a height of 0
Reflection and temperature control of clamping frames in sheet-processing machines and foil-transport systems in automatic roll-fed machines It is possible to achieve optimal reflection properties by applying self-adhesive high-gloss aluminium strips to employ vertical clamping frames for mirroring. At distances of up to 20 mm from the clamping frame, the temperature loss on the surface of the semi-finished product is practically equal to 0 (Figure 4.11). However, because the clamping frame heats up, the aluminium strip expands more than the steel clamping frame. This leads to partial separation of the aluminium strip from the clamping frame’s surface. This solution is therefore only suitable for application over a limited number of hours in actual production. Depending on surface coarseness, aluminium offers good to extremely good reflective properties. At a distance of 20 mm, the temperature loss amounts to 10 °C to 20 °C (Figure 4.11). The hotter the metal, the lower the temperature loss in the edge zone, which means that heating of the semi-finished product remains more uniform all the way to the edge. Should aluminium sheet be used as a reflector, when selecting the mounting arrangement, it is important to remember that aluminium expands more rapidly than steel when heated. Reflectors in aluminium sheet must be able to expand without restriction. As an alternative, small lengths of the reflective material can be bonded to the surface of the clamping frame.
Reflector
4.1 Radiant heaters
Upper clamping frame
Beam from within the clamping frame
Reflected beam from within the clamping frame, as substitute for "obstructed" beam from outside the clamping frame
Beam "obstructed" by the clamping frame emanating from outside the clamping frame
Figure 4.11 Effect of IR mirrors (IR reflectors) on clamping frame or transport profiles
Nickel-plating and galvanising steel surfaces will lead to adequate reflective properties. At a distance a of 20 mm to the clamping frame (Figure 4.11), the temperature loss is approximately 15 °C to 30 °C. As the temperature of the clamping frame or the reflective material increases, the temperature loss decreases, and heating of the semi-finished product into the edge zone is more uniform. Owing to the good heating in the edge zone, machines with temperature control for the upper and lower clamping frames featuring extremely good reflection from the surfaces can be used to produce parts with less distortion. In comparison tests focusing on forming parts with a minimal clearance from the clamping frame and stringent requirements for contour definition in the edge zone, the initial thickness of the semi-finished product in the edge zone could be reduced by up to 10% owing to the higher temperature. Reflective layouts using aluminium spray consisting of 99% aluminium provide relatively good results. Reflection with aluminium spray is primarily employed for the lower clamping frame in sheet-processing machines. The reflection of the clamping frame or of the chain-transport profiles in automatic roll-fed machines is important in all heated stations. A dirty or cold clamping frame results in temperature losses on the heated plastic panel near the frame of up to about 60 °C. A temperature loss of up to 10 °C at roughly 25 mm from the frame is considered to be normal or good. A hot clamping frame in the heating station compensates for the less effective reflection on its surface. Using clamping frames as reflectors in sheet-processing machines is illustrated in Figure 4.12. The selected distance between the heater deflection panel and the upper edge of the upper clamping frame should be as small as possible. The frame
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should project the reflection over the entire height when possible. Owing to the hazard of deformation and separation from the clamping frame, using sheet metal to reflect onto the lower clamping frame should be avoided if possible. This could lead to destruction when the mold is retracted during the forming process. The reflection system for the sheet-material transport system in an automatic roll-fed machine is shown in Figure 4.13. 1 Reflector on heater deflection panel 2 Distance between heater deflection panel and upper edge of upper clamping frame 3 Upper clamping frame, inside mirrored 4 Material 5 Lower clamping frame, inside mirrored 6 Reflector on lower heater deflection panel 7 Distance between upper heater and material 8 Distance between lower heater and material
Figure 4.12 Reflective surfaces with sheet-processing machines (conceptual illustration) 1 Reflector on upper heater deflection panel 2 Distance between heater deflection panel and upper edge of transport profile incl. reflector 3 Reflector, e.g., in aluminium sheet material 4 Material 5 Reflector on lower heater deflection panel 6 Distance between upper heater and material 7 Distance between lower heater and material
Figure 4.13 Reflective surfaces with automatic roll-fed machines (conceptual illustration)
4.1 Radiant heaters
Effect of transport steps under a long heater Every point on the surface of the semi-finished product must have a single temperature in the forming station. To obtain this result, it is necessary to ensure that each point in the advance-feed direction is heated with the same frequency as all others. If this is not the case, it remains possible to shield the surface from the radiant heat or to deactivate transverse heater element rows (Figure 4.14 and Figure 4.15).
2...3 times
2 times
Figure 4.14 Checking the heating through a whole number of advance-feed cycles (2 or 3 times) Case: Machine tables widths wider than the mold 0, 1, 2, 3, 4 steps (countdown) in transporting F: Forming surface (advance feed)
If the machine’s table width is wider than that of the mold, then there will be differences in how the blank is heated before it arrives in the forming station (Figure 4.14 a). If the sector of the heater in front of the forming station is covered, then all points on the blank will be heated exactly two times (Figure 4.14 b).
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a) 2...3 times
b) 2 times
Figure 4.15 Checking the heating with a whole number of advance-feed cycles (2 or 3 times) Case: Machine table widths narrower than the mold 0, 1, 2, 3, 4 steps (countdown) in transporting F: Forming surface (advance feed)
If the machine’s table width is narrower than that of the mold, then there will be differences in how the blank is heated before it arrives in the forming station (Figure 4.15 a). If the sector of the heater in front of the forming station is covered, then all points on the blank will be heated exactly two times (Figure 4.15 b). pper The schematic explanation in Figure 4.14 and Figure 4.15 only applies to the u heater deflection panel. In actual real-world application, there will also be a lower heater deflection panel. The procedure for heating with an upper heater and lower heater is similar, even if two heater deflection panels are not of equal length or are not perfectly aligned above each other in the advance-feed direction. Cross-over effect with radiant heaters When a heater panel travels from its standby position to its heating position at the start of each cycle and then returns to its standby position once the heating time has elapsed, this leads to the cross-over effect, meaning that the semi-finished product is heated for different amounts of time because the heater crosses over it. More rapid heater travel motion corresponds to reduced cross-over effect and vice versa.
4.1 Radiant heaters
Typical travel times for a heater panel with a width of 1 m are roughly 3 to 5 seconds for pneumatic drive systems and about 1.5 to 2.5 seconds for servo-pneumatic and electro-pneumatic drive systems. If shorter heating times are being used (e. g., 8 s), it will be necessary to compensate for the cross-over effect by adapting the radiant heating pattern and possibly by extending the heating time. When heating times are extremely short, it is no longer possible to compensate for the cross-over effect with heater element adjustments! Minimum heating times are needed for even heating of the semi-finished product. Machines for processing thin-gauge semi-finished products and foams in which the heaters travel to their heating positions and back in cycles must have rapid heating motion. Effect of the cooling fan on the heating pattern in a sheet-processing machine Sheet-processing machines with heaters in the forming station are equipped with cooling fans for cooling the molding in the forming station. If the air current cools a portion of the heater element or the radiating surface owing to the absence of shielding, the result will assume the form of distortions in the heating pattern. Even with heater panels featuring heater element temperature control using pilot heater elements, and especially when a heater panel is only equipped with a single pilot heater element, it is impossible to avoid distortion in the heating pattern stemming from air intrusion from cooling fans. This is demonstrated by the following examples: Example 1 A heater deflection panel has only one heater element. The cooling air cools the heater element in the half-section of the heater panel in which the pilot heater element is installed. The pilot heater element registers the temperature reduction, and the temperature control compensates by increasing the pulse-duty factor. However, because all of the heater elements are controlled based on input from the same pilot heater element, those heater elements that are not located in the c urrent of cooling air will overheat. The result will be a distorted radiant heating pattern, which in turn leads to uneven heating of the semi-finished product. Example 2 Air cools the heater elements on the half of the heater panel in which no pilot heater element is installed. The pilot heater element remains at its setpoint temperature, while the pulse-duty factor remains unchanged. As a result, the half of the heater panel with the pilot heater element retains its temperature, but the other half-section cools. The result will be a distorted radiant heating pattern and uneven heating of the semi-finished product.
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Air draught, also resulting from stack effect The heated air produces a stack effect, or chimney effect, with corresponding air infiltration or air draught in every machine. The installation locations of the machines, open doors, open windows, etc. have a similar effect. The practical effects can assume the form of possible variations in sheet temperature or asymmetrical cooling in a preformed bubble. Countermeasures include shielding the machine against air draughts as well as the obvious remedy of adaptable heating. The latter is only logical provided that the air draught remains constant.
4.1.4 Ceramic, quartz and halogen heaters in comparison 6 Electrical connection 5 Thermocouple (connection)
4 Mounting fitting 3 Ceramic housing 2 High-temperature insulation 1 Filament
Figure 4.16 Cross section of a hollow ceramic element with thermocouple (pilot heater element) 5 Electrical connection 4 Mounting threads 3 Metal housing 2 Quartz tube 1 Filament
Figure 4.17 Cross section of a quartz heater 1
2
3 5
Figure 4.18 Halogen heater 1 Filament 2 Quartz tube 3 Mounting socket 4 Electrical connection 5 Length of filament
4
4.1 Radiant heaters
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The essential differences between ceramic, quartz and halogen heaters Table 4.1 Compilation and comparison of different radiant heater types Property
Ceramic heater
Quartz heater
Halogen heater1)
Energy
Electrical
Electrical
Electrical
Energy conversion
Heated filament
Heated filament
Heated filament
Emission source
Ceramic surface
Filament + fused silica surface
Filament + fused silica surface
Heater element temperature
300 to 700 °C maximum 800 °C
Filament < 1100 °C Quartz < 500 °C
Filament < 2400 °C Quartz < 950 °C
Weight ratios
Modern heater elements lighter than quartz2)
Heavy2)
Very light2)
Warm-up time
< 10 min.
< 10 min.3)
< 1 s (3 min.)4)
Connected power ratings
16.6 . . . 25 kW/m (for 700 °C roughly 38.4 kW/m2)
Energy consumption when heating
Up to roughly 75% of Up to roughly 75% of connected power rating5) connected power rating
Up to roughly 85% of connected power rating
Energy consumption in standby mode
Roughly 25% of connected power rating6)
Roughly 25% of connected power rating7)
0%
Full-surface heat emission
Possible
Possible
Not possible, achieved with special reflector
Change in temperature of heater element
Slow
Filament fast, fused silica mass slow
Filament very fast, fused silica relatively fast, as proportionately light
Adjustment options at heater element
Temperature control in °C, power control in % and temperature control with superimposed % control (from ILLIG)8)
Power control in %9) (control in °C not available)
Power control in %
Reproducibility of heating results Short-term
In version with temperature control at heater element, very good10)
Less good if heater element set at % and no IR unit in the machine11) 13)
Good to very good12) 13) if initial temperature of semi-finished product is constant
Reproducibility Long-term
Very good;14) old emitter ≈ new emitter
Less good;15) old emitter differs substantially from new emitters
Unfavourable because high dependency on reflector16)
2
16.6 . . . 50 kW/m
2
50 . . . 75 kW/m2
Service life
∼ 10,000 h
∼ 5,000 h
∼ 5,000 h
Operational monitoring
Complicated
Simple, visual
Simple, visual
Thermal stress resistance of the machine
High17)
High17)
Very low18)
Beam range (wavelength range)
Wide, max. at 3 to 5 mm
Wide, max. at 2 to 4 mm
Narrow, max. at 1 to 2 mm
Application
Universal
Universal
Partially restricted19)
Effect of plastic colours
Heating times practically identical
Heating times almost identical
Different heating times20)
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Table 4.1 Compilation and comparison of different radiant heater types (continued) Property
Ceramic heater
Quartz heater
Halogen heater1)
Application range by machine
Universal
Universal
Limited to single station panel-processing machines21)
Cover shield
No
(No)
Necessary
Regular periodic cleaning
–
–
of cover shield
1)
Halogen heater, also called flash heater Ceramic: HTS/2 (Elstein), 122 × 62 mm, weight 130 g Quartz: TQS FSK, dimensions 124 × 62 mm, weight ∼ 178 g (ceramic: HTS (Elstein), 122 × 122 mm, weight 230 g). 3) The filament itself quickly becomes red, but the remaining mass, the fused silica, the side ceramic mount (also emitting surface) have time requirements similar to those of ceramic heat elements). 4) The proportional weight of halogen heat elements in a heater panel is up to 6 times less than that of a quartz heater; the halogen heat element radiates heat immediately (after 0.2 s), however the fused silica does become warm later. 5) The heater element’s power is selected to ensure that its temperature can be regulated during the heating period and in response to small voltage drops. 6) If the heater is above a reflector in its standby mode but without any reduction in heater element temperature, then the energy consumption will fall by roughly 25% of the connected power rating, depending upon temperature setting. 7) Similar to 6); the difference here is that there is usually no temperature status request. Here, the power is adjusted. The temperature at the heater element is the product of the difference between the selected and the delivered power and the power radiated onto the semi-finished product and into the environment at an inconstant rate. 8) The temperature of the ceramic heat element is usually regulated with closed-loop control. Heaters with the same power and heat loss (to the semi-finished product or the environment) are compiled in groups and controlled with a heater with an installed thermocouple (pilot heater element) from the individual group. 9) Version 8) is not usual in this context, although pilot heater elements with integral thermocouples have been developed. 10) It is possible to start production without rejects once a heating pattern has been stored, closed-loop control with an adequate number of pilot heater elements is in operation and the heaters have reached their specified temperatures. 11) The amount of energy being delivered to the heater element is indicated in the power setting of radiant heaters. The temperature at which the heater element settles will also depend on the energy being radiated into the environment. If no supplementary means (temperature-level sensor in heater panel, IR units, etc.) are employed to accelerate the stabilisation process, then 30 to 40 min. will elapse before the entire heater’s radiant-emission properties achieve stabilised status. Power control with heater elements characterised by high mass is suboptimal. 12) The low heater element weight (25 g for a 700 W heater element with a 165 mm heating length) allows a very short stabilisation time for the beam temperature. Only the ends and the sockets of the heater elements, which also radiate heat, require more time to stabilise. Without IR sensors, tolerances in the heating results for the first thermoformed parts must be anticipated. 13) The temperature is highly sensitive to air supply and reacts to air draughts with corresponding celerity. 14) Up to a heating period of roughly 200 s, virtually no difference between an old, dirty heater element and a new one can be perceived. 15) When it becomes necessary to replace old heaters, their improved heating power will be perceptible. 16) If a heater element depends on a good reflector, then this has consequences: Reflectors accumulate contamination as time passes. The heater element can be protected by a glass-ceramic panel to prevent contamination. Under these conditions, it must be noted that the glass-ceramic surface becomes a heat-radiation surface, i. e., the system’s stabilisation time is extended because the mass is greater. If heater elements featuring integral reflectors are selected, then the energy yield and efficiency level will be poorer if the reflector does not reflect 100%. 17) This statement applies only to machines in which the heater returns to a standby position and is not needed after each cycle. Since heavy heaters react slowly owing to their mass, they must be maintained at operating temperature while in the standby positions. 18) With sheet-processing machines, the heater is deactivated at the standby position. However, the short-wavelength radiation transmits more heat to the painted parts of the machine while in the heating position. 19) There are plastic semi-finished products, primarily thin-gauge, crystal-clear semi-finished materials, that are very difficult to heat with halogen heater elements. 20) The heating times vary according to the colours. White plastics require substantially longer heating times than black ones. 21) The essential advantage of this heater element is the low thermal stress placed on the machine. It would not be a good idea to apply the halogen heater in a machine with an upstream heating station (automatic roll-fed machine or sheet-processing machine) in consideration of its poor efficiency level. The electrical power consumption during heating is higher than with ceramic and quartz heaters. It is only through deactivation at the standby position with single-station sheet- processing machines that the halogen heat element offsets its higher power consumption through deactivation. 2)
4.2 Reproducibility of heating results in radiant heaters
Conclusion from Table 4.1: Compilation and comparison of different radiant heater types The average efficiency levels of ceramic heat elements lie in the range of 25% to 40%. Efficiency levels of approximately 60% can be achieved with optimal insulation for the heater panel and an ideal combination of heater element and plastic. It is possible and necessary to use testing to determine the correct heater elements for machines built for heating a specific material or material group with similar absorption properties. This achieves high efficiency levels using the same radiated heat output. This means that the specific energy use is minimised. However, setting up a machine for one specific plastic represents the exception. If the intention is to use a thermoforming machine to heat as many types of plastic as possible, then the objective will be to cover the largest possible wavelength range, allowing efficient heating of the highest possible number of plastic types. The longwave ceramic heat element meets these requirements. Its disadvantage is thermal inertia – with corresponding negative effects when heating the machine as well as in obtaining the desired dynamic response in temperature control. Owing to the rapid decline in its radiated heat output in the longwave radiation range, the halogen heater requires a high connected power rating (to raise the emissions curve as a whole) in order to achieve the heating times obtained with ceramic heaters and quartz heater. The halogen heat element’s advantage resides in its low level of thermal inertia. It can be switched on and off in cyclical operation, which compensates for the high energy consumption during the warm-up phase – provided that the “off” phase is of adequate duration. While the factors that determine which heater is selected include the type of thermoforming machine, the semi-finished product being processed, its efficiency level and the price, the range of considerations also extends to embrace the philosophy of the machine’s constructor. These are primarily adjustment of the radiated heat output and the reproducibility of the heating results.
4.2 Reproducibility of heating results in radiant heaters 4.2.1 Assessing reproducibility A heating process is perfectly reproducible when both the surface temperature of the semi-finished product and the temperature-distribution pattern extending throughout the depth of the semi-finished product remain constant from the first to the last production cycle.
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Figure 4.19 provides a schematic representation of the problems encountered in achieving a reproducible temperature profile in a semi-finished product. T0
T0
d
a
a
Termination of heating period when surface temperature is reached
Termination of heating period with closed-loop control of heater temperature once specified time has elapsed
Continuous line: hot machine Dotted line: cold machine
Continuous line: hot machine Broken line: cold machine
Figure 4.19 Temperature profile relative to semi-finished product gauge, schematic a: Temperature differential between semi-finished product surface and core of the semi-finished product thickness To: Surface temperature of the semi-finished product d: Thickness of the semi-finished product
Measurement of the temperature at the surfaces of a semi-finished product during production relies on infrared sensors. (The use of thermostrips with permanent displays is only suitable for tests, at which the surface must subsequently be viewed as having been destroyed.) In most cases, IR sensors are used to measure the temperature at a single point. On sheet-processing machines, the temperature sensor is installed roughly at the centre of the heater panel. In automatic roll-fed machines, it is approximately at the end of the heater, where it can measure the temperature reached by the semi-finished product. (Sensors for monitoring extended surfaces are substantially more expensive, and for this reason, they are only seldom used.) If multiple pilot heater elements are used to control the temperature of a heater, then a temperature measurement at one point on the semi-finished material’s surface will be sufficient. When the output of the heater elements in a heater is at a specific setting and no sensors are present in the heater elements, then no monitoring of the heater element temperature is available (no pilot heater element). This means that the heater’s heating pattern continues to change until it stabilises, despite the consistent power settings for the individual heater elements. In this case, a measurement of the semi-finished product’s surface temperature would be more suitable.
4.2 Reproducibility of heating results in radiant heaters
There is no non-destructive method available for measuring temperatures extending throughout the thickness of the semi-finished product and no measurement options for use during running production. The effects of different temperatures in the material at various depths while surface temperatures remain constant can have different effects on wall-thickness distribution and contour definition in the formed part. Example 1 Sheet-processing machines with power-controlled radiant heaters and 1-point measurement with IR sensor for terminating the heating period once the surface of the semi-finished product reaches the specified temperature. Production starts, while the interstices between the heater elements, the heater covers, frame and parts in direct radiation contact with the semi-finished product are still cold. The heating period is concluded once a specified temperature is reached on the surface of the semi-finished product. During production, the machine heats up in a process that continues until the temperatures of all parts have stabilised. The temperature of the semi-finished product is affected by the temperatures of the interstices between the heater elements, the heater covers, the frame, and all parts in direct radiation contact with the semi-finished product. Depending upon its design configuration, the stabilisation period for an assembly will range between approximately 40 minutes and several hours. Although the power output from the heater remains constant, the heat radiation from the heated parts from the entirety of the irradiated environment of the semi-finished product is added to the radiated heat output from the heater elements. As the machine heats up, the overall heat radiation rises and causes the surface of the semi-finished product to reach the specified temperature more quickly. Although the heating period is terminated once the surface temperature is reached on the semi-finished product, the duration of the heating period is reduced once the machine is warmed up. Since the temperature of the core of the sheet is primarily raised by thermal transfer from the outer layers, the temperature obtained in the core also depends on heating time. As heating times are reduced while the surface temperature remains constant, the core of the sheet will become commensurately cooler (throughout its depth). Different heating times at an identical surface temperature on the semi-finished product will result in different temperature profiles extending through the various depths in the interior of the semi-finished product. As the machine becomes hotter, the contour definition decreases because the core of the sheet remains cooler.
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The machine operator corrects the machine’s settings by raising the setpoint temperature for the IR measurement unit as a means of restoring the contour definition, but the disadvantage is that the surface temperature of the semi- finished product is also increased. In this case, the heating time is not set by the operator, but instead it is terminated when the semi-finished product reaches its surface temperature. At this equipment level, the following applies: The reproducibility during heating worsens as the sheet becomes thicker. Example 2 Sheet-processing machines featuring heaters with closed-loop temperature control based on pilot heater elements and with static heating times. Although the heat-emitter array is maintained at a constant temperature level, the lateral heat radiation emanating from machine components as they warm up impacts against the semi-finished product as in Example 1. If a machine with this equipment is set to a specified heating time, then pro gressively higher semi-finished product temperatures will be reached – both on the surface and in the interior of the sheet as the machine becomes hotter. In this case, the quality of the contour definition does not decrease as the machine becomes hotter – it actually becomes better. The problem in this case is preforming with semi-finished products exhibiting various levels of heat penetration. During preblow or pre-suction, the height of a bubble will increase as the machine becomes hotter, which will inevitably lead to a different wall-thickness distribution in the formed part. This is amplified by the changing coefficients of friction between the semi-finished product and mold. No perfect heating reproducibility is achieved with this equipment version, either. Example 3 Automatic roll-fed machines with closed-loop temperature control of radiant heaters, automatic roll-fed machines with power-controlled radiant heaters. For many customers, all possibilities for changing the machine settings are locked out or password protected – with the exception of access to the heater element temperature. With automatic roll-fed machines, it is also the case that the machine becomes hotter as time elapses. In power-controlled heaters, this is the change in heat- emitter temperature. During forming of complicated parts, the operator must intervene in the molding process if no closed-loop control of molding temperature using sheet temperature measurement is being used. The heater element temperatures are reduced
4.2 Reproducibility of heating results in radiant heaters
to prevent the sheet material from overheating. Owing to the limited cooling time, it is only in rare cases that the option of reducing the forming station’s cycling time, and thus shortening the sheet material’s time beneath the heater, will be available. Perfect reproducibility without intervention from the machine’s operator is not available. These examples lead to the conclusion that perfectly reproducible heating results can only be achieved if the temperature progression can be reproduced throughout the depth of the semi-finished material. This is of particular importance with heavy-gauge semi-finished products, meaning with sheet-processing machines. Given the same starting temperature for the semi-finished product, this means maintaining a constant total radiation against the semi-finished product and a constant heating time. Since the total heat radiation consists of the sum of the heat radiated by the IR heater elements and the incident radiation from all machine parts capable of radiating heat onto the semi-finished product, it will be necessary to compensate for the latter factor by modifying the heat radiation from the IR heater. This means that the hotter the machine becomes, the greater the need to reduce the heat radiation from the radiant heaters. If the entry temperature of the semi-finished product changes, then compensation must be provided using the heating time and the radiated heat output.
4.2.2 Compensation for uncontrollable external influences on the heating process Machine equipment to compensate for uncontrollable external influences on the heating process must include: Compensation for fluctuations in the electrical power grid supplying the machine. Compensation for machine heating through reduction in the radiated heat output while heating time remains constant. Compensation for semi-finished product’s entry temperature through change of heating time; cooler semi-finished products must be heated longer, warmer semi-finished products must be heated for shorter periods. Controlling this process entails equipping the machine with sensors. The registered data can be employed for continuous adaptation of the parameters governing the warm-up process, the objective being to maintain constant temperatures on the surface of the semi-finished product and a consistent temperature profile throughout its depth, regardless of machine warming and other external influences. Implementation of corrections is fully-automatic (option on ILLIG machines).
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4.2.3 Power control and temperature control in heaters Two solutions have achieved popularity as a means for influencing the radiated heat output in the heating process with current thermoforming machines: Power control and closed-loop control of heater temperature Power control simply means that the radiated heat output is reduced by lowering the power rating of the radiant heaters. Although various analogies are available, the result is largely comparable to dimming lighting elements. With temperature control of heater elements, the power is adjusted by controlling the temperatures of the heater elements. However, this entails using heater elements that can be equipped with a thermocouple as is the case with commercially available ceramic heater elements. Quartz and halogen heaters with thermocouples are not commercially available. For this reason, the following comparison is based on ceramic heat elements. Concept behind temperature control A radiant heater with closed-loop temperature control is operated at its full rated power until the thermocouple signals that the setpoint temperature has been reached – regardless of whether the surrounding environment is hot or cold and without being affected by any air draughts that may be present. Thus, assuming a consistent initial temperature for the semi-finished product, it is only the heating time that must be calculated and set to a specific value for uniform heating of the plastic to its forming temperature, and this is possible with standard machine equipment. Concept of power control With power control, the end temperature that can be achieved by the heater element is determined by the selected power in percent of the rated power along with the heat radiated by the heater element to heat the semi-finished product and its environment. Changes in the ambient influences – for instance, through temperature fluctuations or air draughts in the production hall – will exercise an ineluctable effect on the achievable radiant heater temperature, which will have a direct influence on the time that the semi-finished material needs to reach its forming temperature. For this reason, power-controlled heaters are usually equipped with an infrared measurement unit for registering the temperature of the semi-finished product, allowing the heating time to be terminated once the surface of the semi-finished product reaches the specified temperature. On machines featuring heaters with power control, setting the machine by calculating the heating time is not standard practice because the forming temperatures that could be achieved at the semi-finished product would be too imprecise.
4.3 Contact heaters
Figure 4.20 shows another, serious difference between these two procedures. Temperature control of a high-temperature ceramic heat element (HTS) achieves the setpoint temperature much more quickly than power control at the same heater element.
Figure 4.20 Times t until temperature stabilisation of a heater panel with temperature control (index reg.) and power control (index. control.). Schematic representation.
The time that elapses before temperature stabilises in a power-controlled heater panel is essentially dependent on the selected heating power (set power output of the heater element) and the weight of the total heater, i. e., the heater element and all additional heater-panel elements that radiate heat at which the heater panel cover must also be included.
4.3 Contact heaters Contact heaters transfer the heat from heated plates or cylinders through contact and thermal transfer to the semi-finished product (Figure 4.21 and Figure 4.22). Advantages of contact heaters: Precise regulation of temperature at the semi-finished product is possible. Provided that the heater is set to the correct temperature, it will not be possible to overheat the semi-finished product. Production can be started without start-up rejects. The option of heating to suit specific formats is available, i. e. heating can be limited to strictly delineated surfaces for molding in the forming station. Format-based heating guarantees minimal thickness tolerances in sealing rims during production of sealed packaging. With appropriate shielding, the heat loss can be restricted to extremely low levels. The heating time for preprinted materials is not affected by the print colour.
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Figure 4.21 Heating with contact heater plates (upper contact heater plates with machine stopped tilted up) Image: ILLIG, type HSA machine
Figure 4.22 Heater with two contact rollers Image: ILLIG, type VHW machine
1
Figure 4.23 Contact heaters on two sides 1 Full-surface contact heater plates 2 Format-based contact heater plates
2
4.4 Convection heaters
Disadvantages of contact heaters: Contact-heaters tend to stick at high temperatures. Contact heaters with anti- adhesive coatings reduce the sticking tendency, but in especially critical con ditions, even the plastic sheet material will need to be treated with an anti-ad hesive layer. Not all anti-adhesive coatings on semi-finished product are suitable for thermoforming. Some, such as calcium stearate, form a deposit layer on the heating surface and the mold that can even plug vent holes. Under these conditions, both the contact heater and the mold, and possibly even the format parts such as upper plug, etc. will need to be cleaned on a regular basis. Even and uniform contact extending over the entirety of the semi-finished product’s molding surface is required for effective heat transfer. Since the plastic expands when heated, warping could cause markings to appear on the heated surface. For this reason, contact heaters can only be used for semi-finished product in limited sizes.
4.4 Convection heaters In thermoforming, convection heating is used: To dry hygroscopic semi-finished product: The drying times and temperatures vary according to the plastic and can be found in the table for the thermoformer. As preheaters for automatic roll-fed machines. In rare cases, convection heaters are used as final heaters, for instance, when processing polycarbonates and plexiglas. These semi-finished products are usually reshaped on presses or special equipment. The accuracy of the achieved semi-finished product temperature both on the surface and at the core also depends on the duration of the heating period when convection heating is used. A high level of thermal uniformity can only be achieved with an extended heating time. The peripheral surfaces of the semi-finished products must be rinsed by the hot air. Here, the objective is to obtain homogeneous air speeds.
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4.5 Minimum heating time, effective heating time and residence time The minimum heating time for a particular type and gauge of semi-finished product is the shortest possible period required to heat the semi-finished product to the desired forming temperature. The effective heating time is the heating time that is actually required to heat the semi-finished product to the desired molding temperature. The effective heating time is never shorter than the minimum heating time. The residence time is the time during heating of the semi-finished product during which the temperature within the semi-finished product is higher than its softening temperature.
4.5.1 Effect of heating time on thermoforming response The minimum heating time is dependent on: The installed heating power The effectiveness of the heat radiation, that is: The type of heater element The distance between the heater element and the thermoplastics being heated The effective level of efficiency Reflections, etc. Notice The limit on the installed specific heating power (kW/m2) depends both on the max. thermal stresses placed on machine components and on the maximum temperature resistance of the semi-finished product (burning hazard). As effective heating times increase, so do the potential residence times. Both during heating with IR radiation and when heating with contact heaters, direct heat is applied to the surface of the semi-finished product. The penetration depth that IR radiation achieves in the semi-finished product is usually derisory. Heating at the core, meaning the centre, of the semi-finished product proceeds through thermal transfer from the outer layers. With both types of heater, the core remains colder than the surface of the semi-finished product.
4.5 Minimum heating time, effective heating time and residence time
4.5.2 Positive effect of residence time The simplest way to lengthen the residence time is to increase the duration of the heating period. Better heating consistency, i. e., a smaller temperature differential between the semi-finished product’s surface and its core, can be measured with thicker semi- finished products. Measurements on sheets with a thickness of 10 mm have indicated temperature differences of up to 60 °C. Owing to practical considerations, it is not possible to measure the differences between the surface and core when dealing with thin-gauge semi-finished products, i. e., on sheet material with thicknesses of 0.3 to 1 mm. Although a theoretically inconsequential temperature difference between the surface and core has been calculated for thin sheet materials, a deliberate extension of the residence time makes a demonstrable improvement in thermoforming properties. The improved performance is primarily encountered in ductility, dimensional stability, distortion resistance and temperature resistance in the molding. The explanation for the improved thermoforming ability with longer residence times lies in the morphological change that these semi-finished products undergo during the residence time. Progressively longer residence times correlate with correspondingly more pronounced changes in morphological structures, resulting in improved thermoformability, which is basically mirrored in the visco-elastic response of the thermoplastic materials.
4.5.3 Negative effect of residence time There are semi-finished products, such as APET, CPET, PHA, in which an extended residence time at a single thermoforming temperature will have negative consequences for thermoforming response, regardless of whether the objects are thin sheet material or heavy-gauge sheets. Here, the root cause resides in the temperature-sensitive and time-dependent start of crystallisation and the enhanced tensile and flexural strength that accompanies the higher crystallisation level. The result is a deterioration of the contour definition. In this case, the changes in the sheet material cannot usually be detected through mere observation during the heating period – not even with crystal-clear sheet material. In extreme cases, a mild discolouration in the form of whitening may be detected on crystal-clear sheet material. A phenomenon that seldom appears in real-world applications is depolymerisation during heating. Under the influence of extended residence times below the heater and the accompanying high material temperature, instability in monomer bonds in
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156 4 Heating technology in thermoforming
the polymer chains can lead to partial dissolution of the polymer chains. When a relatively thick PETG (for instance, 4 mm) is heated on one side, this can lead to depolymerisation. At this point, minute points can be seen in transparent material, and the material has been damaged by incorrect heating. In such cases, the solution is to heat up the semi-finished product as quickly as possible and to avoid unnecessary extensions in residence times at high temperatures.