- Email: [email protected]
Variable Mold Temperature Technologies
6
Variable Mold Temperature Technologies Shia-Chung Chen
6.1 Introduction The cooling stage in injection molding accounts for about two-thirds of the total injection molding cycle. Therefore, minimizing the cooling time by using a low mold temperature to achieve high production efficiency is a great concern for in jection molding operations. However, desirable part surface qualities such as high gloss, a lack of weld line or floating marks, and low residual stress require the part to be molded at relatively high mold temperatures. Choosing the mold temperature thus requires a trade-off between part quality and cycle time; thus, this is an important issue in injection molding. In typical convectional injection molding, although coolant is used to achieve a fixed mold wall temperature, the cavity surface temperature that the injected melt encounters has a cyclic, transient nature, as shown in Figure 6.1. When hot melt is injected into the mold cavity, the heat associated with the hot melt is brought into the cavity. When the part is ejected, not all of the heat brought in by the melt is removed. Some heat is left within the mold base between the cavity surface and the cooling channels. The heat accu mulates in the first few cycles until equilibrium is reached. Then the cavity surface and mold temperature become cyclically stable.
Figure 6.1 Cyclic, transient nature of mold surface temperature variation
196 6 Variable Mold Temperature Technologies
For thin-wall molding, stress-free lens molding and micro-/micro-feature molding started in the late 1990s. The use of a high mold temperature became necessary to achieve favorable part quality. To compromise between part quality and production efficiency, dynamic mold temperature control technology was proposed. In dynamic mold temperature control (also named variotherm or variable mold temperature) [1–3], the cavity surface temperature is raised to a high level, close to or higher than the glass transition temperature, in the melt-filling stage to assist with melt filling. Then the cavity surface temperature is lowered quickly via efficient cooling to minimize the cycle time. A schematic is shown in Figure 6.2.
Figure 6.2 Schematic of dynamic mold temperature control; the goal is to achieve a high mold surface temperature in the mold-filling stage, and a low mold surface temperature in the other stages of the injection cycle
In conventional mold cooling processes, the range of mold temperature variation is around 10~20 °C, and the average mold temperature is below the ejection temperature Te (Figure 6.3(a)). In dynamic mold temperature control, the mold temperature is usually raised to above the glass transition temperature (Tg) in the filling stage, and the mold surface temperature may vary with a range of 100 °C or more (Figure 6.3(b)). As a result, the heating rate, cooling rate, and relevant energy consumption are critical factors in determining whether or not it is feasible and cost-effective to implement variotherm technology.
6.2 Various Methods for Dynamic Mold T emperature Control
(a)
(b)
Figure 6.3 (a) The mold temperature varies below the ejection temperature; the fluctuation temperature range is about 10 °C or more. (b) Typical mold: temperature varies from below the ejection temperature to the glass transition temperature; the temperature variation range is around 100 °C
6.2 Various Methods for Dynamic Mold Temperature Control Methods for variotherm control [4, 5] can be divided into two categories depending on the location of the heat sources. In the first category, the heat sources are embedded inside the mold base or the mold interior. Heat sources in the form of hot oil, high-pressure hot water, hot steam, and electric heaters from pipes or plates are frequently used. Water or ice water is the most commonly used coolant. When the heating medium is water or steam, the same channels may be used for coolant circulation and heating. The disadvantage of using heating elements within the mold interior is that a portion of the mold base will be heated during the heating period and these heated regions must be cooled down during the cooling phase. Significant energy and time are therefore wasted during the cycle. Heating sources other than water or steam face another issue of competing for space with the cooling channels. The basic performance data of the aforementioned mold interior-based variotherm methods are listed in Table 6.1. Table 6.1 The Basic Performance of Mold Interior-Based Variotherm Methods Source
Max. Temp. Specific Heat ( °C) (J/kg- °C)
Control Unit/Equipment
Water
100
Typical mold temperature control unit
Water
140
High-pressurized mold temperature control unit
Water
180
SINGLE®-Water Advanced®
Vapor
180
MATSUI®-RHCM-100G Steam Jet
Oil
200
Oil
300
4180
2250
Traditional mold temperature control unit High-power mold temperature control unit
197
198 6 Variable Mold Temperature Technologies
The second category of variable mold temperature control is to use heat sources external to the mold to heat up the cavity surface. These include radiation heating (infrared, laser, or lamp), electromagnetic induction heating, hot gas heating, and electric heating on the cavity surface through coated conduction/insulating layers. Because of the reflection effect on the relatively smooth mold cavity surface, the heat absorption from radiation is not high. This limits the possible applications of radiation heating. With electrical heating on the coated conduction layer with insulation embedded beneath, differences in the thermal expansion of the materials used will usually lead to lifespan issues; moreover, the coated layer may not be strong enough to withstand high molding pressures. Although heating on the cavity surface can minimize the wasted energy on the mold base and achieve rapid heating and cooling rates in principle, it also faces operation and limitation issues. The major exterior mold heating method is electromagnetic induction-based heating. The characteristics of exterior-based mold temperature control via induction are listed in Table 6.2. In addition to electromagnetic induction, heating the mold surface using hot gas flowing through the cavity and using infrared was also proposed. Electromagnetic induction heating can also be implemented within the mold interior via embedded coils or via a proximity effect induced by internal flowing current as listed in Table 6.3. Finally, the efficiency comparison between induction heating and water heating, based on an assumptive heating and cooling temperature profile (from initial 50 °C, raised to 120 °C, and cooled down to 50 °C), is listed in Table 6.4. Table 6.2 Varieties of Exterior Mold Heating Methods Based on Electromagnetic Induction [5–8] Mold Exterior: Induction Heating by Surface- Proximity Coils
Mold Exterior: Induction Heating by Externally Wrapped Coils
Mold Exterior: Hot Gas Heating
Mold Exterior: Infrared Heating
Heating rate ( °C/s)
15~25
8~10
15~20
4~5 (with PTFE coating) 0.5~1 (without coating)
Cooling rate ( °C/s)
7~10
3~7
3~7
3~7
Illustration
6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources
Table 6.3 Interior Mold Heating Based on Electromagnetic Induction [9, 10] Mold Interior: Internal-Coil-Induced Heating
Mold interior: Proximity Effect Induced by Internal Current
Heating rate ( °C/s)
2~3
3~5
Cooling rate ( °C/s)
2~3
2~3
Illustration
6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources 6.3.1 Hot Water Heating/Cold Water Cooling In recent years, mold temperature heating using high-pressurized water has been developed. This permits switching between the hot water temperature control unit and the cold water temperature control unit using the same circulation channels. The initial version of the hot water temperature control unit supplied hot water at about 140 °C. Newly developed units can reach temperatures from 180 °C to nearly 200 °C. This provides for simple, convenient, and dynamic mold temperature control. However, a number of safety issues must be considered in regard to the pipe connections. The water supply flow rate should also be considered in practical molding applications [11].
6.3.2 Oil Heating/Water Cooling In traditional mold heating, hot oil is frequently chosen as the medium. The drawback of hot oil is its low heat transfer coefficient, meaning that its heating efficiency is low. Furthermore, oil vapor may contaminate the cavity surface. Target mold temperatures can reach 200 °C using ordinary oil heating, or 300 °C under high-power oil heating. The high target temperature is the reason why hot oil was commonly used for mold heating in the past, when the achievable hot water tem-
199
200 6 Variable Mold Temperature Technologies
perature was still low. However, the limited space within the mold base for both oil circulation and water circulation is a key problem in oil heating. In addition to the highest oil temperature that can be achieved, the flow rate of the oil supply is also crucial for efficiency considerations in practical applications [12].
6.3.3 Steam Heating/Water Cooling (RHCM) In 2014, Ono Sangyo, a Japanese company, announced its “Rapid Heat Cycle Molding (RHCM)” based on steam heating technology. A schematic of the system is shown in Figure 6.4. A typical case showing mold temperature variation is given in Figure 6.5. In the RHCM process, hot steam at about 150 °C is pumped through the cooling channel to heat the mold after part ejection. When the cavity surface temperature reaches a target temperature, say 120 °C, the mold is closed and melt injection begins. Immediately after the end of melt filling, chilled water at about 20 °C is pumped into the cooling channel. The cooling continues until the mold temperature reaches a target low temperature, such as 60 °C. The part is then ejected and another cycle of steam heating begins. RHCM can provide an initial heating and cooling rate of about 3~6 °C/s. Its major application is the injection molding of TV housings, which require a high-gloss surface free of surface defects without the need for secondary surface decoration or painting. Jeng et al. explored RHCM [13]. They found that, for a simple mold plate, steam heating can increase the mold surface temperature from 35 °C to over 135 °C in 11 s, and then cool it to 35 °C in 58 s, as seen in Figure 6.6. The corresponding heating speed is 9 °C/s, whereas the cooling speed is less than 2 °C/s. For practical TV housing mold applications, the steam system has higher efficiency for both heating and cooling. Compared with water heating, steam heating reduces the heating stage from 18 s to 8 s, and cooling from 16 s to 12 s. To further improve the heating and cooling rates and expand the application to parts with curved surfaces, a conformal cooling channel layout or close-to-cavity cooling channels and other technologies have been proposed (Figure 6.7). Case studies may be found in published reports [13–15].
6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources
Figure 6.4 Schematic of RHCM operation
Figure 6.5 Variation of mold cavity temperature as well as steam and coolant setup temperatures
201
202 6 Variable Mold Temperature Technologies
Figure 6.6 Mold surface temperature profile using steam/water cooling for a TV housing mold [13]
6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources
Figure 6.7 The conformal cooling channel layout, or close-to-cavity cooling channels, as implemented to improve heating and cooling efficiency
6.3.4 Electrical Heater Heating Electric heater heating is commonly implemented in two forms: an electric heating plate or an electric heating tube (rod, pipe) [16, 17], as shown in Figure 6.8 and Figure 6.9, respectively. An electric heating plate provides a larger heating area, resulting in a higher heating rate. However, the completion with cooling channels for space within the mold base sometimes limits the wider application of these systems. An electrical heating tube system can be implemented more flexibly since it and the cooling channel may be designed together. It does have a lower heating rate. In both cases, the thermal contact of the heating device is important in heating efficiency. Generally speaking, the heating rate can reach 8~10 °C/s.
Figure 6.8 Electrical heating in the form of plates [16]
203
204 6 Variable Mold Temperature Technologies
Figure 6.9 Electrical heating in the form of pipes [17]
6.3.5 Pulse Cooling (Alternating Temperature Technology) In pulse cooling, also called “Alternating Temperature Technology” commercially, there is a bypass design for the cooling circuit. During part ejection, mold closing, and melt filling, the coolant circulation is bypassed to the branched circuit, and the coolant originally running in the channel close to the cavity surface becomes stagnant. A schematic is illustrated in Figure 6.10 and Figure 6.11. As a result, the poor heat transfer due to lack of movement of the coolant may lead to a higher cavity surface temperature by about 5~20 °C. This range of cavity temperature variation may be of benefit for some molding cases. For example, in the conventional injection molding of Blu-ray optical discs, to fulfill the micro-groove depth requirement at a specific disc location, the required cavity coolant temperature and core coolant temperature should be kept at 125 °C and 110 °C, respectively. Using pulse cooling, the cavity coolant temperature and the core coolant temperature can be lowered to 107 °C and 102 °C, respectively, without negatively affecting the mold surface temperature. Because the effective mold surface temperature under lower coolant temperature is roughly the same as that of conventional injection molding with high coolant temperature, the micro-groove depth achieves the same value (Figure 6.10 and Figure 6.11). However, the use of lower cavity and core coolant temperatures results in a reduction of the cycle from 4 s to 3.5 s, about a 12.5% improvement in production efficiency. The other case shows that water stagnation helps the heating rate in ICIH.
Figure 6.10 Schematic of pulse cooling [18]
6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources
Figure 6.11 Effect of coolant temperature on the depth of micro-grooves in molding Blu-ray DVD optical discs [18]
6.3.6 Electrical Heating at the Mold Surface Using a Two-Layer Coating Yao et al. [19] proposed mold surface heating by applying a current to a conductive metal layer coated on the cavity surface. To prevent current leakage to the mold base, an electrical and thermal insulation layer also needs to be coated beneath the conduction layer. This insulation layer slows the heat transfer so that the heated conduction layer can maintain its temperature for a short period of time for the melt to fill the cavity. A schematic is shown in Figure 6.12. Achieving interface compatibility for various materials is a challenge. The other disadvantage is that, due to the thermal expansion difference between the conduction and insulation layers, the heating/cooling cycle will easily result in failure at the interface. Moreover, the coated layer may not be strong enough to withstand a high molding pressure.
Figure 6.12 Schematic of electrical heating at the mold surface using two-layer coating [19]
205
206 6 Variable Mold Temperature Technologies
6.4 Mold Heating Based on Electromagnetic Induction Technology 6.4.1 Principle and Characteristics of Induction Heating Induction heating is not a new technology; it has been used in the welding and heat-treatment industries for many years. In molding applications, a high-frequency alternating current (AC) is applied to an induction coil close to the mold surface. The electromagnetic (EM) wave penetrates the mold surface and generates electric currents inside the mold known as eddy currents. When the eddy currents flow through the mold, because of the resistance of the mold, heat is generated. This is known as Joule heating (Figure 6.13). This heats the surface portion of the mold. A practical induction heating system for injection molding applications includes a power supply, a properly designed and movable coil device, a cooling unit for the coil, and a control panel capable of monitoring the temperature.
Figure 6.13 The electromagnetic flux lines penetrate the mold and induce eddy currents on the skin of the mold surface
The EM wave decays exponentially as it enters the mold plate surface due to free electron scattering. Most of the induced eddy currents exist in the thin layer surface region beneath the mold plate surface, a phenomenon called the “skin effect.” The location beneath the mold surface where the eddy current inside the mold base decays to 1/e (about 37%) of its strength at the surface is known as the penetration depth (Figure 6.14).
6.4 Mold Heating Based on Electromagnetic Induction Technology
Figure 6.14 Penetration depth is a measure of how deep light or any EM radiation can penetrate a material
The governing equations used to describe the electromagnetic field during the induction are the Maxwell equations, shown below: (6.1) (6.2) (6.3) (6.4) where H is the magnetic field intensity, J is the current density, E is the electric field intensity, B is the magnetic flux density, D is the electric flux density, and ρ is the electrical resistivity. The penetration depth (skin depth), δ, near the surface can be described as follows: (6.5) where f is the alternating current frequency and μ is the permeability. In addition, the electrical resistance associated with eddy currents dissipates as heat, causing the mold temperature to rise.
207
208 6 Variable Mold Temperature Technologies
The volumetric heat generated, Q, from the dissipated power due to eddy current flow can be calculated as follows: (6.6) Hysteresis loss in magnetic induction may also introduce heat; however, its contribution is much less than that of eddy current heating [20]. For steel molds (P20), the skin depth is about 0.09 mm, while for iron it is less than 0.02 mm. Since the polymer melt contacts the cavity surface, this hot thin surface layer will prevent the melt from freezing, thereby assisting the flow and preventing defects.
6.4.2 Induction Heating from Mold Surface (External Heating) External induction heating temperature control (EIHTC) is combined with water cooling (Figure 6.15) [21]. During operation, the mold is opened to enable the induction coil to move into the mold opening space, usually close to the cavity surface. The coil is positioned on the heating area of the mold surface and the power supply is then activated. A water valve will switch off the cooling water to increase the heating speed during heating. In real-world operation, the induction setting temperature will be 10~15 °C higher than the target temperature to compensate for the temperature drop during the mold-close period. At the end of the induction heating stage, the induction heating is turned off and the induction coil is removed for mold closing and filling. During the holding and cooling stages, the water valve will be switched on to increase the cooling efficiency. Since this system offers high heating speed, the cycle time lengthens only slightly and the introduced heat can be rapidly dissipated.
Figure 6.15 Illustration of the induction heating/water cooling process: (a) mold open for surface heating, (b) mold close followed by filling, (c) part cooling, and (d) part ejection and next cycle of surface heating [22]
6.4 Mold Heating Based on Electromagnetic Induction Technology
6.4.3 Induction Coil Design for Mold and Molding In using induction heating for mold temperature control [23], coil design is critical. Figure 6.16 offers an example of a three-coil design enabling a range of heating speeds on the mold surface. The induction coil is usually made of copper tubing with a circulating cooling fluid inside. The design parameters include the diameter, shape, and number of turns, which affect the magnetic field pattern and the associated heating efficiency. The energy transfer of induction heating is also affected by the distance between the coil and the mold surface.
Figure 6.16 The heating speed results with various coil designs [24]
To evaluate its efficiency, an example plate mold was used to study external induction heating. Hot water, electric heater pipes, and induction heating were used to heat the mold (Figure 6.17). The mold temperature in the heating/cooling cycle was varied from 50 °C to 120 °C and then back to 50 °C using each of the three methods, as shown in the following figures.
Figure 6.17 Schematic of the mold dimensions with the three different heating methods [25]
209
210 6 Variable Mold Temperature Technologies
Figure 6.18 clearly shows that the heating speed is best when induction heating is used. The typical heating speed exceeds 25 °C/s. Moreover, because most of the generated heat exists within the thin layer of the mold surface, the energy consumption is much lower than the other two interior heating methods that heat a portion of the massive mold base. As a result, the cooling speed of EIHTC is also superior. The energy consumption for the induction heating/water cooling method and the water heating/water cooling method in the 50–120–50 °C mold temperature variation cycle is listed in Table 6.4.
Figure 6.18 Measured temperature variations of the three types of mold temperature control [25] Table 6.4 Efficiency Comparison between Induction Heating and Water Heating Induction Heating/ Water Cooling (50 °C →120 °C → 50 °C)
Heater Heating/ Water Cooling (50 °C →120 °C → 50 °C)
Water Heating/ Water Cooling (50 °C →120 °C → 50 °C)
Heating time (s)
5
64
272
Dissipated energy (kJ)
0.21
46.5
46.5
Power (kW)
0.042
0.73
0.17
Coolant temperature ( °C)
12
12
12
Cooling time (s)
53
100
102
Ambient temperature ( °C)
25
25
25
When using an induction coil to heat the mold surface, the coil is usually fixed on an insulated fixture and moved by a robot or a motion device to the designated position. Insulation prevents the coil from touching the mold (Figure 6.19).
6.4 Mold Heating Based on Electromagnetic Induction Technology
Figure 6.19 The induction coil is fixed on a moving device and covered by insulating material; it can enter the mold from different directions when the mold is open
Figure 6.20 shows the measured long-term mold temperature history with the trial mold over a duration of 600 s (about 30 cycles). All initial temperatures for these three control methods were set to 90 °C. After 5–10 injection cycles, the controlled mold temperature reached a steady transient state, varying between 120 °C to 160 °C on a cyclical basis. Under EIHTC, the mold temperature increased from 120 °C to 160 °C in 1.5 s. Although the temperature will decrease during the mold close period, in the filling stage, the mold temperature still remained higher than 140 °C (about the Tg of polycarbonate).
Figure 6.20 Measured long-term mold temperature history by external induction heating of the mold surface [26]
211
212 6 Variable Mold Temperature Technologies
6.4.4 The Challenges Facing EIHTC Applications and Their Possible Solutions One of the major issues associated with induction heating on the mold surface arises from complex cavity geometries such as a step, edge, or corner area. This edge effect [15] hinders the coil’s proximity to low-lying cavity surfaces for heating (Figure 6.21). Around this area, overheating and insufficient heating can easily occur. Generally, to prevent the edge effect, several improvements can be made (Figure 6.22). For example, the coil-covered area can be reduced to avoid heating on undesired regions; however, the heating efficiency may decrease when using a smaller coil. Another solution is to shorten the distance between coil and mold surface, but this kind of design needs an extra coil motion toward the mold surface. In mold design, an extra cooling channel near the edge can also minimize overheating.
Figure 6.21 The edge effect of induction heating (IH) may cause non-uniform heating in edge locations; image of a 10-inch notebook computer frame
The coil design has to take into consideration the size of the mold. For example, with a larger heating area, a greater coil length is needed. However, a greater coil length will lead to a mismatch of coil and power source, thereby reducing the heating efficiency. Thus, a new power source must be designed to match the length of the coil design. For a specific power supply, only a small range of adjustment is allowed. For a multiple-turn coil with a ring-shaped loop, the central magnetic flux will not be as strong because the current runs in opposite directions as it flows
6.4 Mold Heating Based on Electromagnetic Induction Technology
through the coil, leading to significantly lower heating near the central region. Figure 6.23 shows the relationship between the magnetic field lines and the current direction. Increasing the number of turns at the coil’s inner center and combining them with the flux concentrators would be a possible solution. F urthermore, for the heating requirements of a mold with a 3D curved surface (non-planar), it is also difficult to control temperature uniformity due to the bending preparation issues of the hollowed coil.
Figure 6.22 Possible improvements when overheating occurs near a corner or edge area
Figure 6.23 The currents flow in opposite directions near the central coil area, leading to a lower magnetic field strength and lower associated heating
One method to improve magnetic induction around the coil center is to use magnetic shielding as reported in various studies [27–29]. An example of using an inserted block capable of magnetic shielding and improved temperature control is shown in Figure 6.24.
213
214 6 Variable Mold Temperature Technologies
a)
b)
c)
Figure 6.24 (a) Coil with and without rod-like magnetic inductor assistance device (IAD), (b) the corresponding temperature distribution after 5 s of induction heating, and (c) the temperature distribution after heat transfer (5 s) [27]
6.4 Mold Heating Based on Electromagnetic Induction Technology
6.4.5 The Real Application of EIHTC – Mold Exterior Induction Heating 6.4.5.1 Elimination of a Weld Line and Floating Fiber Marks Several molding trials using EIHTC were conducted. Though the appearance of a weld line is quite obvious without EIHTC, the weld line can easily be eliminated with the application of EIHTC [30] (Figure 6.25). Moreover, the quality of the surface gloss may allow parts to remain paint-free. When parts need sputtering treatment to achieve a metallic appearance, the parts must be molded with ultrahigh surface quality; otherwise, the sputtering process may magnify the surface defects, enabling them to be seen more clearly. The tensile strength of the weld-linefree part will also be increased. In molding parts with fiber reinforcement, floating fiber marks on the part surface are a common and concerning defect. Using EIHTC, high surface quality without marks can be achieved easily, as seen in this frame for an electronic photo album (Figure 6.26).
Figure 6.25 The weld line mark disappears when induction heating is used to heat the mold surface, as clearly shown by surface roughness measurement results (×400)
Figure 6.26 Using EIHTC, floating surface fibers were eliminated and a high-gloss surface finish could be achieved
215
216 6 Variable Mold Temperature Technologies
6.4.5.2 Micro-Features Molding EIHTC can be used for improving the dimensional accuracy of micro-features, especially in high AR (aspect ratio) micro-feature molding (Figure 6.27). As shown in the figure, the sharp tips of the micro-features can be molded accurately even at a high AR up to 12. In the case of an LGP (light guide plate), the residual stress can be reduced as well (Figure 6.28), and the associated luminance values can be increased [31].
Figure 6.27 3D laser microscope images of micro-feature cross sections of the front and back sides of an injection molded mold insert [31]
(a)
b)
Figure 6.28 Residual stress illumination distribution of LGP: (a) without EIHTC, and (b) with EIHTC
6.4 Mold Heating Based on Electromagnetic Induction Technology
6.4.6 Mold Exterior/Induction Heating by an Externally Wrapped Coil The principle of induction heating via an externally wrapped coil is illustrated in Figure 6.29. In the example of a rectangular tube, when the conductor is bent to form a ring-like loop, its current will be redistributed. Most of the current will flow within the thin inner surface layer of the conductor, and the induced magnetic field will be more concentrated on the inner side of the coil, as indicated by the red color portion of Figure 6.29. As a result, the wrapped coil will induce a heating effect on the work piece located inside the wrapped coil due to the combination of both the skin and the proximity effect. The injection mold consists of two mold inserts facing each other and forming a mold cavity between them. The mold plates are commonly made of high-strength tool steel with high electrical resistivity. The mold plates are thus subjected to induction heating under the magnetic field induced by the coil that wraps the mold externally. Because of the magnetic effect, an eddy current will be induced and appear at the surface of the core and cavity plates generating electric heating on the surfaces.
Figure 6.29 Principle of externally wrapped coil-induction heating [6]
In a case study example [6], the mold and the wrapped coil, as well as the cross section view, are shown in Figure 6.30 and Figure 6.31, respectively. Figure 6.32 shows that the wrapped coil can be applied with currents in series or parallel. The resulting heating profiles are depicted in Figure 6.33. The effect of various turns of coil on the heating was also investigated, and is shown in Figure 6.34. The heating rate ranged from about 2 °C to 10 °C. This method has been commercialized by ROCTOOL and applied to many molded parts. Due to the complexity in coil design and mold configuration, the implementation is usually carried out in a project-based (case by case) fashion.
217
218 6 Variable Mold Temperature Technologies
Figure 6.30 Case molding of ROCTOOL®
6.4 Mold Heating Based on Electromagnetic Induction Technology
Figure 6.31 The dimension of the mold plate: (a) measured position and (b) multi-turn coil position [6]
Figure 6.32 Simulation model and mesh model of (a) a serial coil and (b) a parallel coil [6]
219
220 6 Variable Mold Temperature Technologies
Figure 6.33 Variations of surface temperature at point T2 with regard to time for various coil designs [6]
Figure 6.34 Comparison of temperature at point T2 with regard to time given a different number of turns [6]
6.4.7 Induction Heating from the Mold Interior Using Embedded Coils In the internal coil induction heating (ICIH) method, water cooling is integrated into the injection mold base; hence, the overall heating and cooling effects should be considered together. Figure 6.35 shows schematically the configuration of the
6.4 Mold Heating Based on Electromagnetic Induction Technology
ICIH. When the induction coil is embedded inside the mold, the heat will transfer from the bottom portion of the mold plate to the top surface after heating. One feasible practical configuration is depicted in Figure 6.36.
Figure 6.35 Configuration of internal induction heating; the induced heat energy will transfer from the heating surface on the bottom side to the upper cavity surface
Figure 6.36 Mold structure with embedded internal coil and cooling channels [9]
For internal induction heating, the distance from the induction coil to the cavity surface is one of the most important design parameters. For a top mold plate of 15 mm and 20 mm in thickness, the heating speed was evaluated [32–35] and can be seen in Figure 6.37 under specific operating conditions.
221
222 6 Variable Mold Temperature Technologies
Figure 6.37 Comparison of heating speed results with a mold thicknesses of 15 mm and 20 mm [9]
In the general design of ICIH, if the cooling channels are laid out between the coils and mold surface, control associated with water cooling has a strong influence on the temperature distribution. When the water flows continuously during the heating period, it will lower the heating efficiency and temperature distribution. In contrast, when the water stops running or is drained from the channel, the resulting mold surface temperature will be higher and the temperature distribution will improve as well, as indicated in Figure 6.38. Cooling Water Control System
Water Running
Water Stagnant
Water Drained out
1.3
1.5
1.8
(15mm) (oC) oC/s)
Figure 6.38 Mold plate surface temperature distribution with different methods of water switchover control [32]
For real-world applications of ICIH in the injection molding process, an embedded induction coil is used as the heating source to increase the mold surface temperature. In general, injection molding has three main stages. The mold is first heated
6.4 Mold Heating Based on Electromagnetic Induction Technology
by running the coil until the target temperature is reached. Then the polymer melt is injected. During the packing/cooling stages, the coil induction is turned off and the mold is cooled and opened to eject the parts. Unlike external induction heating of the mold surface, the embedded coils do not move. Instead, the power is just switched on and off. However, because of the skin effect and mold plate strength considerations, the coils cannot be embedded too close to the cavity surface. This requires some portion of the mold base to be heated. Furthermore, the mold must be cooled for part ejection. This creates competition in the available space for cooling channels and coils. If the coils are laid out just beneath the cavity surface, and the cooling channels are located below the coils, heating will be more rapid, but cooling will be slower. Conversely, if the cooling channels are laid out above the coils, cooling will be faster and heating will be slower. Chen et al. proposed a compromise in which the coils and cooling channels exist co-axially [33]. Another possibility is to lay the cooling channels and coils on the same level (Figure 6.39).
Figure 6.39 Four designs with cooling channels and a coil. Design 1: Cooling channels beneath the cavity surface and the coil were placed below the cooling channels. Design 2: Coil beneath the cavity surface and the cooling channels were placed below the coil. Design 3: Cooling channels and the coil were on the same level. Design 4: Coil and cooling channels exist co-axially [34]
The temperature measurement results on the mold surface based on four different designs for coil–channel configurations are shown in Figure 6.39. The results show that the total cycle time of Designs 3 and 4 are significantly better than those of Designs 1 and 2. The heating speeds of Designs 3 and 4 are very close. The cooling speed of Design 4 is better than that of Design 3, while the heating speed of Design 3 is slightly higher than that of Design 4 (Figure 6.40). However, the temperature uniformity is better in Design 4 (Figure 6.41). If one considers the temperature uniformity as one of the design criteria, Design 4 is the optimal design.
223
224 6 Variable Mold Temperature Technologies
Figure 6.40 Temperature history with different designs [34]
Figure 6.41 Temperature distribution comparisons between Design 4 (left) and Design 3 (right) [34]
6.4.8 Mold Interior/Proximity Effect Induced by Internal Current The principle of high-frequency proximity heating can be described and shown by Figure 6.42. When two metallic blocks are brought to face each other in parallel, with a small gap, and carry high-frequency electric current in opposite directions, the carrying currents tend to flow along the two facing inner surfaces, forming a strong magnetic field in the space between the two blocks. This phenomenon is known as the proximity effect [25]. An injection mold consists of two mold plates facing each other and forming a mold cavity in between. The mold plates are usually made of high-strength tool steel with high electrical resistivity (Figure 6.43). When the two mold plates are brought to a small intervening distance during the mold closing stage, the injection mold is thus subjected to the proximity effect if a high-frequency current (HFC) is applied to each mold plate.
6.4 Mold Heating Based on Electromagnetic Induction Technology
Figure 6.42 Principle of high-frequency proximity heating [10]
Figure 6.43 Mold design for high-frequency proximity heating [10]
Because of the proximity effect, the HFC will flow along the two inner cavity surfaces and generate electric heating at both surfaces. To make use of the proximity heating, one may design different current-conducting paths via the embedded copper pipe of the various layouts. Examples of different designs can be seen in Figure 6.44. Details can be found elsewhere [10]. The corresponding heating efficiencies are depicted in Figure 6.45. Generally speaking, the heating rate is around 6~10 °C/s. The disadvantage is that insulation is required on the parting surface to prevent the current flow from one side of mold plate to the other side or, in other words, the leakage of electrical current. Insulation of the clamping plate of the injection molding machine is also required. The limited space within the mold base also hinders the design of current conduction channels.
225
226 6 Variable Mold Temperature Technologies
Design 1 1 Channel
Design 2 3 Channels
Design 3 Rectangular Channel
(a)
(b) Figure 6.44 (a) Three types of coil channels designs; (b) illustration of testing the mold (Design 1) [10]
Figure 6.45 Temperature variation vs. time at the center of the cavity surface for the three inductor designs [10]
6.5 Other Mold Surface Heating Technologies
6.5 Other Mold Surface Heating Technologies 6.5.1 Hot Gas-Assisted Heating Gas-assisted mold temperature control (GMTC), proposed by Chen et al. [7, 36–38], is a new technique for mold temperature control. It can heat and cool the cavity surface rapidly during the injection molding process. The GMTC system consists of a menu-driven controller, a hot-gas generator system, and a water temperature control unit, as shown in Figure 6.46. The hot gas generator is composed of an air compressor, an air dryer, a digital volumetric flow controller, and a high-efficiency gas heater. The function of the high-power hot gas generator system is to support a heat source, which is required to provide sufficient hot air with a flow rate up to 500 l/min and an air temperature of 500 °C or higher. For the coolant system, a typical water-cooling-based mold temperature control unit can be used to provide water at a preset temperature to cool the mold after the filling process. It would be better if the water cooling unit could also warm the mold to a higher initial temperature at the beginning of the process. Fluid valves were also used to control the air flow in the heating stage and the water flow for the cooling channels.
Figure 6.46 System of hot gas-assisted mold surface heating in injection molding [38]
For practical operation, the mold was not completely closed prior to melt injection. Instead, a gap was left, allowing the hot gas to flow in through the cavity. The inlet and outlet for the gas must be properly designed. After the cavity surface achieves
227
228 6 Variable Mold Temperature Technologies
the target temperature, the gas valve closes and then the mold completely closes for melt injection to begin. An operation schematic is shown in Figure 6.47.
Figure 6.47 Schematic of GMTC operation [7]
The heating rate is dependent on various parameters; among them, the temperature of the hot gas and the gas flow rate are two crucial factors. The open gap of the mold cavity and the shape of the gate that diverts the gas flow are also important. Since the heat delivered to the mold surface via convective heat transfer will quickly dissipate, the most efficient way to keep the heat and the associated temperature on the surface is to use a nickel-plate-like stamp. The contact resistance between the plate and the mold base will hinder heat loss to the mold base and result in a good replication of micro-featured patterns. The initial heating speed can be as high as 45 °C/s and should stabilize at about 25 °C/s if the hot gas temperature and flow rate are reasonable, as seen in the Figure 6.48 [7]. The two levels of heating speed are due to the temperature difference between the hot gas and the cavity surface temperature. The heating speed slows down with increased cavity surface temperature. GMTC has potential application in micro-molding in which nickel-plated stampers are frequently utilized. A molding example for improving the replication accuracy of micro-features is exhibited in Figure 6.49 and Figure 6.50. Another application to eliminate floating fiber marks resulting in a glossy surface is also shown in Figure 6.51 [38].
6.5 Other Mold Surface Heating T echnologies
Figure 6.48 Comparison of mold temperature between molds designed with and without nickel plating with regard to various heating times [7]
a)
b)
Figure 6.49 (a) Molded product and (b) microstructure [7]
229
230 6 Variable Mold Temperature Technologies
Without gas heating
Heating target temp. = 90°C
Heating target temp. = 100°C
Heating target temp. = 110°C
Heating target temp. = 120°C
Heating target temp. = 130°C
Figure 6.50 3D laser microscope images of a molded micro-feature at the center point. The depth of the molded micro-feature varies with regard to the target mold temperature [7]
6.5 Other Mold Surface Heating Technologies
Figure 6.51 Surface quality and elimination of floating fiber marks (a) with and (b) without gas-assisted mold heating [38]
6.5.2 Infrared Heating Efficiency and feasibility of mold surface heating using infrared was evaluated [8]. An experimental schematic is shown in Figure 6.52. As mentioned earlier, for a regular, smooth mold surface, the high reflection of radiation heating on the mold surface will result in poor heating efficiency. In Chen’s study [8], both a smooth mold surface and a PTFE-coated mold surface were used for comparison. For the PTFE-coated mold surface, the infrared heating achieved a heating rate of about 4~5 °C/s. For the smooth mold surface, the heating rate was below 1 °C/s. The experimental results of temperature variation are illustrated in Figure 6.53. A comparison between mold exterior induction is also shown.
Figure 6.52 System of infrared mold surface heating [8]
231
232 6 Variable Mold Temperature Technologies
Figure 6.53 Comparison of heating rates between induction heating and infrared heating [8]
References [1] Michaeli, W., Spennemann, A., Gartner, R., Microsystem Technologies (2002) 8, pp. 55–57 [2] Michaeli, W., Scharf, M., Annual Technical Conference - ANTEC, Conference Proceedings (2009) 3, pp. 1578–1582 [3] Michaeli, W., Eilbracht, S., Scharf, M., Hartmann, C., Bobzin, K., Bagcivan, N., Thieß, S., Journal of Polymer Engineering (2012) 32, pp. 95–101 [4] Yao, D. G., Chen, S. C., Kim, B. H., Advances in Polymer Technology (2008) 27, pp. 233–255 [5] Chen, S. C., Chang, J. A., Asian Workshop of Polymer Processing (2010) [6] Lin, H. L., Chen, S. C., Jeng, M. C., Minh, P. S., Chang, J. A., Hwang, J. R., International Communications in Heat and Mass Transfer (2012) 39, pp. 514–522 [7] Chen, S. C., Chang, J. A., Hsu, W. Y., Huang, S. W., Microelectronic Engineering (2011) 88, pp. 1594– 1600 [8] Chen. Y. C., Chung Yuan Christian University PhD Thesis (2005) Taiwan [9] Lin, Y. T., Chung Yuan Christian University Master Thesis (2009) Taiwan [10] Chen, S. C., Pham, S. M., Chang, J. A., Huang, S. W., International Communications in Heat and Mass Transfer (2012) 39, pp. 216–223 [11] Wang, G. L., Zhao, G. Q., Guan, Y. J., Materials and Design (2010) 31, pp. 3426–3441 [12] Huang, M. S., Tai, N. S., Journal of Polymer Science (2009) pp. 1345–1354 [13] Jeng, M. C., Chen, S. C., Pham, S. M., Chang, J. A., International Communications in Heat and Mass Transfer (2010) 37, pp. 1295–1304 [14] Wang, G. L., Zhao, G. Q., Guan, Y. J., Expert Systems with Applications (2011) 38, pp. 6705–6719 [15] Wang, G. L., Zhao, G. Q., Guan, Y. J., International Journal of Thermal Sciences (2011) 50, pp. 790– 802 [16] Bürkle, E., Burr, A., Müller, A. K., Kübler, M., Kunststoffe International (2007) 10, pp. 144–148 [17] Xiong, C. L., Huang, H. X.. Applied Thermal Engineering (2014) 73, pp. 712–722
References
[18] Liu, S. C., Chung Yuan Christian University Master Thesis (2007) Taiwan [19] Yao, D. G., Polymer Engineering and Science (2002) 42, pp. 2471–2481 [20] Rudnev, V., Loveless, D., Cook, R. L., Black, M., Handbook of Induction Heating (2003) Marcel Dekker, New York. [21] Chen, S. C., Device for advancing even distribution of high cycle wave magnetism, U. S. Patent No. 6919545 (2005) [22] Chen, S. C., Chang, J. A., Cin, J. C., Annual Technical Conference - ANTEC, Conference Proceedings (2009) 3, pp. 1630–1364 [23] Sung, Y. T., Hwang, S. J., Advances in Mechanical Engineering (2014) Article ID 349078, 10 pages [24] Chen, S. C., Peng, H. S., Chang, J. A., Jong, W. R., International Communications in Heat and Mass Transfer (2004) 31, pp. 971–980 [25] Chen, S. C., Jong, W. R., Chang, Y. P., Annual Technical Conference – ANTEC (2005) 2, pp. 166–170 [26] Chen, S. C., Jong, W. R., Yang, K. Y., Annual Technical Conference – ANTEC (2008) 1, pp. 499–502 [27] Pham, S. M., Chung Yuan Christian University PhD Thesis (2011) Taiwan [28] Nian, S. C., Huang,. M. S., Tsai, T. H., International Communications in Heat and Mass Transfer (2014) 50, pp. 52–60 [29] Shih, S. Y., Nian, S. C., Huang, M. S., Huang, R. C., Chen, C. H., International Communications in Heat and Mass Transfer (2016) 71, pp. 44–55 [30] Chen, S. C., Jong, W. R., Chang, J. A., Chang, Y. J., International Polymer Processing (2006) 5, pp. 1–7 [31] Chen, S. C., Jong, W. R., Chang, J. A., Chang, Y. J., Journal of Micromechanics and Microengineering (2006) 16, pp. 1783–1791 [32] Pham, S. M., Chen, S. C., Y. T. Lin, Annual Technical Conference - ANTEC, Conference Proceedings (2010) 2, pp. 896–899 [33] Chen, S. C., Mold Heating/Cooling Structure, U. S. Patent No. 8162640 (2012) [34] Liu, G. C., Chung Yuan Christian University Master Thesis (2014) Taiwan [35] Sung, Y. T., Hwang, S. J., Tehnički vjesnik (2014) 21, pp. 651–656 [36] Chen, S. C., Chien, R. D., Lin, S. H., Lin, M. C., Chang, J. A., International Communications in Heat and Mass Transfer (2009) 36, pp. 806–812. [37] Chen, S. C., Gas-Assisted Mold Surface Heating System, U. S. Patent No. 8092208 (2012) [38] Chen, S. C., Pham, S. M., Chang, J. A., International Communications in Heat and Mass Transfer (2011) 38, pp. 304–312
233
Variable Mold Temperature Technologies Shia-Chung Chen
6.1 Introduction The cooling stage in injection molding accounts for about two-thirds of the total injection molding cycle. Therefore, minimizing the cooling time by using a low mold temperature to achieve high production efficiency is a great concern for in jection molding operations. However, desirable part surface qualities such as high gloss, a lack of weld line or floating marks, and low residual stress require the part to be molded at relatively high mold temperatures. Choosing the mold temperature thus requires a trade-off between part quality and cycle time; thus, this is an important issue in injection molding. In typical convectional injection molding, although coolant is used to achieve a fixed mold wall temperature, the cavity surface temperature that the injected melt encounters has a cyclic, transient nature, as shown in Figure 6.1. When hot melt is injected into the mold cavity, the heat associated with the hot melt is brought into the cavity. When the part is ejected, not all of the heat brought in by the melt is removed. Some heat is left within the mold base between the cavity surface and the cooling channels. The heat accu mulates in the first few cycles until equilibrium is reached. Then the cavity surface and mold temperature become cyclically stable.
Figure 6.1 Cyclic, transient nature of mold surface temperature variation
196 6 Variable Mold Temperature Technologies
For thin-wall molding, stress-free lens molding and micro-/micro-feature molding started in the late 1990s. The use of a high mold temperature became necessary to achieve favorable part quality. To compromise between part quality and production efficiency, dynamic mold temperature control technology was proposed. In dynamic mold temperature control (also named variotherm or variable mold temperature) [1–3], the cavity surface temperature is raised to a high level, close to or higher than the glass transition temperature, in the melt-filling stage to assist with melt filling. Then the cavity surface temperature is lowered quickly via efficient cooling to minimize the cycle time. A schematic is shown in Figure 6.2.
Figure 6.2 Schematic of dynamic mold temperature control; the goal is to achieve a high mold surface temperature in the mold-filling stage, and a low mold surface temperature in the other stages of the injection cycle
In conventional mold cooling processes, the range of mold temperature variation is around 10~20 °C, and the average mold temperature is below the ejection temperature Te (Figure 6.3(a)). In dynamic mold temperature control, the mold temperature is usually raised to above the glass transition temperature (Tg) in the filling stage, and the mold surface temperature may vary with a range of 100 °C or more (Figure 6.3(b)). As a result, the heating rate, cooling rate, and relevant energy consumption are critical factors in determining whether or not it is feasible and cost-effective to implement variotherm technology.
6.2 Various Methods for Dynamic Mold T emperature Control
(a)
(b)
Figure 6.3 (a) The mold temperature varies below the ejection temperature; the fluctuation temperature range is about 10 °C or more. (b) Typical mold: temperature varies from below the ejection temperature to the glass transition temperature; the temperature variation range is around 100 °C
6.2 Various Methods for Dynamic Mold Temperature Control Methods for variotherm control [4, 5] can be divided into two categories depending on the location of the heat sources. In the first category, the heat sources are embedded inside the mold base or the mold interior. Heat sources in the form of hot oil, high-pressure hot water, hot steam, and electric heaters from pipes or plates are frequently used. Water or ice water is the most commonly used coolant. When the heating medium is water or steam, the same channels may be used for coolant circulation and heating. The disadvantage of using heating elements within the mold interior is that a portion of the mold base will be heated during the heating period and these heated regions must be cooled down during the cooling phase. Significant energy and time are therefore wasted during the cycle. Heating sources other than water or steam face another issue of competing for space with the cooling channels. The basic performance data of the aforementioned mold interior-based variotherm methods are listed in Table 6.1. Table 6.1 The Basic Performance of Mold Interior-Based Variotherm Methods Source
Max. Temp. Specific Heat ( °C) (J/kg- °C)
Control Unit/Equipment
Water
100
Typical mold temperature control unit
Water
140
High-pressurized mold temperature control unit
Water
180
SINGLE®-Water Advanced®
Vapor
180
MATSUI®-RHCM-100G Steam Jet
Oil
200
Oil
300
4180
2250
Traditional mold temperature control unit High-power mold temperature control unit
197
198 6 Variable Mold Temperature Technologies
The second category of variable mold temperature control is to use heat sources external to the mold to heat up the cavity surface. These include radiation heating (infrared, laser, or lamp), electromagnetic induction heating, hot gas heating, and electric heating on the cavity surface through coated conduction/insulating layers. Because of the reflection effect on the relatively smooth mold cavity surface, the heat absorption from radiation is not high. This limits the possible applications of radiation heating. With electrical heating on the coated conduction layer with insulation embedded beneath, differences in the thermal expansion of the materials used will usually lead to lifespan issues; moreover, the coated layer may not be strong enough to withstand high molding pressures. Although heating on the cavity surface can minimize the wasted energy on the mold base and achieve rapid heating and cooling rates in principle, it also faces operation and limitation issues. The major exterior mold heating method is electromagnetic induction-based heating. The characteristics of exterior-based mold temperature control via induction are listed in Table 6.2. In addition to electromagnetic induction, heating the mold surface using hot gas flowing through the cavity and using infrared was also proposed. Electromagnetic induction heating can also be implemented within the mold interior via embedded coils or via a proximity effect induced by internal flowing current as listed in Table 6.3. Finally, the efficiency comparison between induction heating and water heating, based on an assumptive heating and cooling temperature profile (from initial 50 °C, raised to 120 °C, and cooled down to 50 °C), is listed in Table 6.4. Table 6.2 Varieties of Exterior Mold Heating Methods Based on Electromagnetic Induction [5–8] Mold Exterior: Induction Heating by Surface- Proximity Coils
Mold Exterior: Induction Heating by Externally Wrapped Coils
Mold Exterior: Hot Gas Heating
Mold Exterior: Infrared Heating
Heating rate ( °C/s)
15~25
8~10
15~20
4~5 (with PTFE coating) 0.5~1 (without coating)
Cooling rate ( °C/s)
7~10
3~7
3~7
3~7
Illustration
6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources
Table 6.3 Interior Mold Heating Based on Electromagnetic Induction [9, 10] Mold Interior: Internal-Coil-Induced Heating
Mold interior: Proximity Effect Induced by Internal Current
Heating rate ( °C/s)
2~3
3~5
Cooling rate ( °C/s)
2~3
2~3
Illustration
6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources 6.3.1 Hot Water Heating/Cold Water Cooling In recent years, mold temperature heating using high-pressurized water has been developed. This permits switching between the hot water temperature control unit and the cold water temperature control unit using the same circulation channels. The initial version of the hot water temperature control unit supplied hot water at about 140 °C. Newly developed units can reach temperatures from 180 °C to nearly 200 °C. This provides for simple, convenient, and dynamic mold temperature control. However, a number of safety issues must be considered in regard to the pipe connections. The water supply flow rate should also be considered in practical molding applications [11].
6.3.2 Oil Heating/Water Cooling In traditional mold heating, hot oil is frequently chosen as the medium. The drawback of hot oil is its low heat transfer coefficient, meaning that its heating efficiency is low. Furthermore, oil vapor may contaminate the cavity surface. Target mold temperatures can reach 200 °C using ordinary oil heating, or 300 °C under high-power oil heating. The high target temperature is the reason why hot oil was commonly used for mold heating in the past, when the achievable hot water tem-
199
200 6 Variable Mold Temperature Technologies
perature was still low. However, the limited space within the mold base for both oil circulation and water circulation is a key problem in oil heating. In addition to the highest oil temperature that can be achieved, the flow rate of the oil supply is also crucial for efficiency considerations in practical applications [12].
6.3.3 Steam Heating/Water Cooling (RHCM) In 2014, Ono Sangyo, a Japanese company, announced its “Rapid Heat Cycle Molding (RHCM)” based on steam heating technology. A schematic of the system is shown in Figure 6.4. A typical case showing mold temperature variation is given in Figure 6.5. In the RHCM process, hot steam at about 150 °C is pumped through the cooling channel to heat the mold after part ejection. When the cavity surface temperature reaches a target temperature, say 120 °C, the mold is closed and melt injection begins. Immediately after the end of melt filling, chilled water at about 20 °C is pumped into the cooling channel. The cooling continues until the mold temperature reaches a target low temperature, such as 60 °C. The part is then ejected and another cycle of steam heating begins. RHCM can provide an initial heating and cooling rate of about 3~6 °C/s. Its major application is the injection molding of TV housings, which require a high-gloss surface free of surface defects without the need for secondary surface decoration or painting. Jeng et al. explored RHCM [13]. They found that, for a simple mold plate, steam heating can increase the mold surface temperature from 35 °C to over 135 °C in 11 s, and then cool it to 35 °C in 58 s, as seen in Figure 6.6. The corresponding heating speed is 9 °C/s, whereas the cooling speed is less than 2 °C/s. For practical TV housing mold applications, the steam system has higher efficiency for both heating and cooling. Compared with water heating, steam heating reduces the heating stage from 18 s to 8 s, and cooling from 16 s to 12 s. To further improve the heating and cooling rates and expand the application to parts with curved surfaces, a conformal cooling channel layout or close-to-cavity cooling channels and other technologies have been proposed (Figure 6.7). Case studies may be found in published reports [13–15].
6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources
Figure 6.4 Schematic of RHCM operation
Figure 6.5 Variation of mold cavity temperature as well as steam and coolant setup temperatures
201
202 6 Variable Mold Temperature Technologies
Figure 6.6 Mold surface temperature profile using steam/water cooling for a TV housing mold [13]
6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources
Figure 6.7 The conformal cooling channel layout, or close-to-cavity cooling channels, as implemented to improve heating and cooling efficiency
6.3.4 Electrical Heater Heating Electric heater heating is commonly implemented in two forms: an electric heating plate or an electric heating tube (rod, pipe) [16, 17], as shown in Figure 6.8 and Figure 6.9, respectively. An electric heating plate provides a larger heating area, resulting in a higher heating rate. However, the completion with cooling channels for space within the mold base sometimes limits the wider application of these systems. An electrical heating tube system can be implemented more flexibly since it and the cooling channel may be designed together. It does have a lower heating rate. In both cases, the thermal contact of the heating device is important in heating efficiency. Generally speaking, the heating rate can reach 8~10 °C/s.
Figure 6.8 Electrical heating in the form of plates [16]
203
204 6 Variable Mold Temperature Technologies
Figure 6.9 Electrical heating in the form of pipes [17]
6.3.5 Pulse Cooling (Alternating Temperature Technology) In pulse cooling, also called “Alternating Temperature Technology” commercially, there is a bypass design for the cooling circuit. During part ejection, mold closing, and melt filling, the coolant circulation is bypassed to the branched circuit, and the coolant originally running in the channel close to the cavity surface becomes stagnant. A schematic is illustrated in Figure 6.10 and Figure 6.11. As a result, the poor heat transfer due to lack of movement of the coolant may lead to a higher cavity surface temperature by about 5~20 °C. This range of cavity temperature variation may be of benefit for some molding cases. For example, in the conventional injection molding of Blu-ray optical discs, to fulfill the micro-groove depth requirement at a specific disc location, the required cavity coolant temperature and core coolant temperature should be kept at 125 °C and 110 °C, respectively. Using pulse cooling, the cavity coolant temperature and the core coolant temperature can be lowered to 107 °C and 102 °C, respectively, without negatively affecting the mold surface temperature. Because the effective mold surface temperature under lower coolant temperature is roughly the same as that of conventional injection molding with high coolant temperature, the micro-groove depth achieves the same value (Figure 6.10 and Figure 6.11). However, the use of lower cavity and core coolant temperatures results in a reduction of the cycle from 4 s to 3.5 s, about a 12.5% improvement in production efficiency. The other case shows that water stagnation helps the heating rate in ICIH.
Figure 6.10 Schematic of pulse cooling [18]
6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources
Figure 6.11 Effect of coolant temperature on the depth of micro-grooves in molding Blu-ray DVD optical discs [18]
6.3.6 Electrical Heating at the Mold Surface Using a Two-Layer Coating Yao et al. [19] proposed mold surface heating by applying a current to a conductive metal layer coated on the cavity surface. To prevent current leakage to the mold base, an electrical and thermal insulation layer also needs to be coated beneath the conduction layer. This insulation layer slows the heat transfer so that the heated conduction layer can maintain its temperature for a short period of time for the melt to fill the cavity. A schematic is shown in Figure 6.12. Achieving interface compatibility for various materials is a challenge. The other disadvantage is that, due to the thermal expansion difference between the conduction and insulation layers, the heating/cooling cycle will easily result in failure at the interface. Moreover, the coated layer may not be strong enough to withstand a high molding pressure.
Figure 6.12 Schematic of electrical heating at the mold surface using two-layer coating [19]
205
206 6 Variable Mold Temperature Technologies
6.4 Mold Heating Based on Electromagnetic Induction Technology 6.4.1 Principle and Characteristics of Induction Heating Induction heating is not a new technology; it has been used in the welding and heat-treatment industries for many years. In molding applications, a high-frequency alternating current (AC) is applied to an induction coil close to the mold surface. The electromagnetic (EM) wave penetrates the mold surface and generates electric currents inside the mold known as eddy currents. When the eddy currents flow through the mold, because of the resistance of the mold, heat is generated. This is known as Joule heating (Figure 6.13). This heats the surface portion of the mold. A practical induction heating system for injection molding applications includes a power supply, a properly designed and movable coil device, a cooling unit for the coil, and a control panel capable of monitoring the temperature.
Figure 6.13 The electromagnetic flux lines penetrate the mold and induce eddy currents on the skin of the mold surface
The EM wave decays exponentially as it enters the mold plate surface due to free electron scattering. Most of the induced eddy currents exist in the thin layer surface region beneath the mold plate surface, a phenomenon called the “skin effect.” The location beneath the mold surface where the eddy current inside the mold base decays to 1/e (about 37%) of its strength at the surface is known as the penetration depth (Figure 6.14).
6.4 Mold Heating Based on Electromagnetic Induction Technology
Figure 6.14 Penetration depth is a measure of how deep light or any EM radiation can penetrate a material
The governing equations used to describe the electromagnetic field during the induction are the Maxwell equations, shown below: (6.1) (6.2) (6.3) (6.4) where H is the magnetic field intensity, J is the current density, E is the electric field intensity, B is the magnetic flux density, D is the electric flux density, and ρ is the electrical resistivity. The penetration depth (skin depth), δ, near the surface can be described as follows: (6.5) where f is the alternating current frequency and μ is the permeability. In addition, the electrical resistance associated with eddy currents dissipates as heat, causing the mold temperature to rise.
207
208 6 Variable Mold Temperature Technologies
The volumetric heat generated, Q, from the dissipated power due to eddy current flow can be calculated as follows: (6.6) Hysteresis loss in magnetic induction may also introduce heat; however, its contribution is much less than that of eddy current heating [20]. For steel molds (P20), the skin depth is about 0.09 mm, while for iron it is less than 0.02 mm. Since the polymer melt contacts the cavity surface, this hot thin surface layer will prevent the melt from freezing, thereby assisting the flow and preventing defects.
6.4.2 Induction Heating from Mold Surface (External Heating) External induction heating temperature control (EIHTC) is combined with water cooling (Figure 6.15) [21]. During operation, the mold is opened to enable the induction coil to move into the mold opening space, usually close to the cavity surface. The coil is positioned on the heating area of the mold surface and the power supply is then activated. A water valve will switch off the cooling water to increase the heating speed during heating. In real-world operation, the induction setting temperature will be 10~15 °C higher than the target temperature to compensate for the temperature drop during the mold-close period. At the end of the induction heating stage, the induction heating is turned off and the induction coil is removed for mold closing and filling. During the holding and cooling stages, the water valve will be switched on to increase the cooling efficiency. Since this system offers high heating speed, the cycle time lengthens only slightly and the introduced heat can be rapidly dissipated.
Figure 6.15 Illustration of the induction heating/water cooling process: (a) mold open for surface heating, (b) mold close followed by filling, (c) part cooling, and (d) part ejection and next cycle of surface heating [22]
6.4 Mold Heating Based on Electromagnetic Induction Technology
6.4.3 Induction Coil Design for Mold and Molding In using induction heating for mold temperature control [23], coil design is critical. Figure 6.16 offers an example of a three-coil design enabling a range of heating speeds on the mold surface. The induction coil is usually made of copper tubing with a circulating cooling fluid inside. The design parameters include the diameter, shape, and number of turns, which affect the magnetic field pattern and the associated heating efficiency. The energy transfer of induction heating is also affected by the distance between the coil and the mold surface.
Figure 6.16 The heating speed results with various coil designs [24]
To evaluate its efficiency, an example plate mold was used to study external induction heating. Hot water, electric heater pipes, and induction heating were used to heat the mold (Figure 6.17). The mold temperature in the heating/cooling cycle was varied from 50 °C to 120 °C and then back to 50 °C using each of the three methods, as shown in the following figures.
Figure 6.17 Schematic of the mold dimensions with the three different heating methods [25]
209
210 6 Variable Mold Temperature Technologies
Figure 6.18 clearly shows that the heating speed is best when induction heating is used. The typical heating speed exceeds 25 °C/s. Moreover, because most of the generated heat exists within the thin layer of the mold surface, the energy consumption is much lower than the other two interior heating methods that heat a portion of the massive mold base. As a result, the cooling speed of EIHTC is also superior. The energy consumption for the induction heating/water cooling method and the water heating/water cooling method in the 50–120–50 °C mold temperature variation cycle is listed in Table 6.4.
Figure 6.18 Measured temperature variations of the three types of mold temperature control [25] Table 6.4 Efficiency Comparison between Induction Heating and Water Heating Induction Heating/ Water Cooling (50 °C →120 °C → 50 °C)
Heater Heating/ Water Cooling (50 °C →120 °C → 50 °C)
Water Heating/ Water Cooling (50 °C →120 °C → 50 °C)
Heating time (s)
5
64
272
Dissipated energy (kJ)
0.21
46.5
46.5
Power (kW)
0.042
0.73
0.17
Coolant temperature ( °C)
12
12
12
Cooling time (s)
53
100
102
Ambient temperature ( °C)
25
25
25
When using an induction coil to heat the mold surface, the coil is usually fixed on an insulated fixture and moved by a robot or a motion device to the designated position. Insulation prevents the coil from touching the mold (Figure 6.19).
6.4 Mold Heating Based on Electromagnetic Induction Technology
Figure 6.19 The induction coil is fixed on a moving device and covered by insulating material; it can enter the mold from different directions when the mold is open
Figure 6.20 shows the measured long-term mold temperature history with the trial mold over a duration of 600 s (about 30 cycles). All initial temperatures for these three control methods were set to 90 °C. After 5–10 injection cycles, the controlled mold temperature reached a steady transient state, varying between 120 °C to 160 °C on a cyclical basis. Under EIHTC, the mold temperature increased from 120 °C to 160 °C in 1.5 s. Although the temperature will decrease during the mold close period, in the filling stage, the mold temperature still remained higher than 140 °C (about the Tg of polycarbonate).
Figure 6.20 Measured long-term mold temperature history by external induction heating of the mold surface [26]
211
212 6 Variable Mold Temperature Technologies
6.4.4 The Challenges Facing EIHTC Applications and Their Possible Solutions One of the major issues associated with induction heating on the mold surface arises from complex cavity geometries such as a step, edge, or corner area. This edge effect [15] hinders the coil’s proximity to low-lying cavity surfaces for heating (Figure 6.21). Around this area, overheating and insufficient heating can easily occur. Generally, to prevent the edge effect, several improvements can be made (Figure 6.22). For example, the coil-covered area can be reduced to avoid heating on undesired regions; however, the heating efficiency may decrease when using a smaller coil. Another solution is to shorten the distance between coil and mold surface, but this kind of design needs an extra coil motion toward the mold surface. In mold design, an extra cooling channel near the edge can also minimize overheating.
Figure 6.21 The edge effect of induction heating (IH) may cause non-uniform heating in edge locations; image of a 10-inch notebook computer frame
The coil design has to take into consideration the size of the mold. For example, with a larger heating area, a greater coil length is needed. However, a greater coil length will lead to a mismatch of coil and power source, thereby reducing the heating efficiency. Thus, a new power source must be designed to match the length of the coil design. For a specific power supply, only a small range of adjustment is allowed. For a multiple-turn coil with a ring-shaped loop, the central magnetic flux will not be as strong because the current runs in opposite directions as it flows
6.4 Mold Heating Based on Electromagnetic Induction Technology
through the coil, leading to significantly lower heating near the central region. Figure 6.23 shows the relationship between the magnetic field lines and the current direction. Increasing the number of turns at the coil’s inner center and combining them with the flux concentrators would be a possible solution. F urthermore, for the heating requirements of a mold with a 3D curved surface (non-planar), it is also difficult to control temperature uniformity due to the bending preparation issues of the hollowed coil.
Figure 6.22 Possible improvements when overheating occurs near a corner or edge area
Figure 6.23 The currents flow in opposite directions near the central coil area, leading to a lower magnetic field strength and lower associated heating
One method to improve magnetic induction around the coil center is to use magnetic shielding as reported in various studies [27–29]. An example of using an inserted block capable of magnetic shielding and improved temperature control is shown in Figure 6.24.
213
214 6 Variable Mold Temperature Technologies
a)
b)
c)
Figure 6.24 (a) Coil with and without rod-like magnetic inductor assistance device (IAD), (b) the corresponding temperature distribution after 5 s of induction heating, and (c) the temperature distribution after heat transfer (5 s) [27]
6.4 Mold Heating Based on Electromagnetic Induction Technology
6.4.5 The Real Application of EIHTC – Mold Exterior Induction Heating 6.4.5.1 Elimination of a Weld Line and Floating Fiber Marks Several molding trials using EIHTC were conducted. Though the appearance of a weld line is quite obvious without EIHTC, the weld line can easily be eliminated with the application of EIHTC [30] (Figure 6.25). Moreover, the quality of the surface gloss may allow parts to remain paint-free. When parts need sputtering treatment to achieve a metallic appearance, the parts must be molded with ultrahigh surface quality; otherwise, the sputtering process may magnify the surface defects, enabling them to be seen more clearly. The tensile strength of the weld-linefree part will also be increased. In molding parts with fiber reinforcement, floating fiber marks on the part surface are a common and concerning defect. Using EIHTC, high surface quality without marks can be achieved easily, as seen in this frame for an electronic photo album (Figure 6.26).
Figure 6.25 The weld line mark disappears when induction heating is used to heat the mold surface, as clearly shown by surface roughness measurement results (×400)
Figure 6.26 Using EIHTC, floating surface fibers were eliminated and a high-gloss surface finish could be achieved
215
216 6 Variable Mold Temperature Technologies
6.4.5.2 Micro-Features Molding EIHTC can be used for improving the dimensional accuracy of micro-features, especially in high AR (aspect ratio) micro-feature molding (Figure 6.27). As shown in the figure, the sharp tips of the micro-features can be molded accurately even at a high AR up to 12. In the case of an LGP (light guide plate), the residual stress can be reduced as well (Figure 6.28), and the associated luminance values can be increased [31].
Figure 6.27 3D laser microscope images of micro-feature cross sections of the front and back sides of an injection molded mold insert [31]
(a)
b)
Figure 6.28 Residual stress illumination distribution of LGP: (a) without EIHTC, and (b) with EIHTC
6.4 Mold Heating Based on Electromagnetic Induction Technology
6.4.6 Mold Exterior/Induction Heating by an Externally Wrapped Coil The principle of induction heating via an externally wrapped coil is illustrated in Figure 6.29. In the example of a rectangular tube, when the conductor is bent to form a ring-like loop, its current will be redistributed. Most of the current will flow within the thin inner surface layer of the conductor, and the induced magnetic field will be more concentrated on the inner side of the coil, as indicated by the red color portion of Figure 6.29. As a result, the wrapped coil will induce a heating effect on the work piece located inside the wrapped coil due to the combination of both the skin and the proximity effect. The injection mold consists of two mold inserts facing each other and forming a mold cavity between them. The mold plates are commonly made of high-strength tool steel with high electrical resistivity. The mold plates are thus subjected to induction heating under the magnetic field induced by the coil that wraps the mold externally. Because of the magnetic effect, an eddy current will be induced and appear at the surface of the core and cavity plates generating electric heating on the surfaces.
Figure 6.29 Principle of externally wrapped coil-induction heating [6]
In a case study example [6], the mold and the wrapped coil, as well as the cross section view, are shown in Figure 6.30 and Figure 6.31, respectively. Figure 6.32 shows that the wrapped coil can be applied with currents in series or parallel. The resulting heating profiles are depicted in Figure 6.33. The effect of various turns of coil on the heating was also investigated, and is shown in Figure 6.34. The heating rate ranged from about 2 °C to 10 °C. This method has been commercialized by ROCTOOL and applied to many molded parts. Due to the complexity in coil design and mold configuration, the implementation is usually carried out in a project-based (case by case) fashion.
217
218 6 Variable Mold Temperature Technologies
Figure 6.30 Case molding of ROCTOOL®
6.4 Mold Heating Based on Electromagnetic Induction Technology
Figure 6.31 The dimension of the mold plate: (a) measured position and (b) multi-turn coil position [6]
Figure 6.32 Simulation model and mesh model of (a) a serial coil and (b) a parallel coil [6]
219
220 6 Variable Mold Temperature Technologies
Figure 6.33 Variations of surface temperature at point T2 with regard to time for various coil designs [6]
Figure 6.34 Comparison of temperature at point T2 with regard to time given a different number of turns [6]
6.4.7 Induction Heating from the Mold Interior Using Embedded Coils In the internal coil induction heating (ICIH) method, water cooling is integrated into the injection mold base; hence, the overall heating and cooling effects should be considered together. Figure 6.35 shows schematically the configuration of the
6.4 Mold Heating Based on Electromagnetic Induction Technology
ICIH. When the induction coil is embedded inside the mold, the heat will transfer from the bottom portion of the mold plate to the top surface after heating. One feasible practical configuration is depicted in Figure 6.36.
Figure 6.35 Configuration of internal induction heating; the induced heat energy will transfer from the heating surface on the bottom side to the upper cavity surface
Figure 6.36 Mold structure with embedded internal coil and cooling channels [9]
For internal induction heating, the distance from the induction coil to the cavity surface is one of the most important design parameters. For a top mold plate of 15 mm and 20 mm in thickness, the heating speed was evaluated [32–35] and can be seen in Figure 6.37 under specific operating conditions.
221
222 6 Variable Mold Temperature Technologies
Figure 6.37 Comparison of heating speed results with a mold thicknesses of 15 mm and 20 mm [9]
In the general design of ICIH, if the cooling channels are laid out between the coils and mold surface, control associated with water cooling has a strong influence on the temperature distribution. When the water flows continuously during the heating period, it will lower the heating efficiency and temperature distribution. In contrast, when the water stops running or is drained from the channel, the resulting mold surface temperature will be higher and the temperature distribution will improve as well, as indicated in Figure 6.38. Cooling Water Control System
Water Running
Water Stagnant
Water Drained out
1.3
1.5
1.8
(15mm) (oC) oC/s)
Figure 6.38 Mold plate surface temperature distribution with different methods of water switchover control [32]
For real-world applications of ICIH in the injection molding process, an embedded induction coil is used as the heating source to increase the mold surface temperature. In general, injection molding has three main stages. The mold is first heated
6.4 Mold Heating Based on Electromagnetic Induction Technology
by running the coil until the target temperature is reached. Then the polymer melt is injected. During the packing/cooling stages, the coil induction is turned off and the mold is cooled and opened to eject the parts. Unlike external induction heating of the mold surface, the embedded coils do not move. Instead, the power is just switched on and off. However, because of the skin effect and mold plate strength considerations, the coils cannot be embedded too close to the cavity surface. This requires some portion of the mold base to be heated. Furthermore, the mold must be cooled for part ejection. This creates competition in the available space for cooling channels and coils. If the coils are laid out just beneath the cavity surface, and the cooling channels are located below the coils, heating will be more rapid, but cooling will be slower. Conversely, if the cooling channels are laid out above the coils, cooling will be faster and heating will be slower. Chen et al. proposed a compromise in which the coils and cooling channels exist co-axially [33]. Another possibility is to lay the cooling channels and coils on the same level (Figure 6.39).
Figure 6.39 Four designs with cooling channels and a coil. Design 1: Cooling channels beneath the cavity surface and the coil were placed below the cooling channels. Design 2: Coil beneath the cavity surface and the cooling channels were placed below the coil. Design 3: Cooling channels and the coil were on the same level. Design 4: Coil and cooling channels exist co-axially [34]
The temperature measurement results on the mold surface based on four different designs for coil–channel configurations are shown in Figure 6.39. The results show that the total cycle time of Designs 3 and 4 are significantly better than those of Designs 1 and 2. The heating speeds of Designs 3 and 4 are very close. The cooling speed of Design 4 is better than that of Design 3, while the heating speed of Design 3 is slightly higher than that of Design 4 (Figure 6.40). However, the temperature uniformity is better in Design 4 (Figure 6.41). If one considers the temperature uniformity as one of the design criteria, Design 4 is the optimal design.
223
224 6 Variable Mold Temperature Technologies
Figure 6.40 Temperature history with different designs [34]
Figure 6.41 Temperature distribution comparisons between Design 4 (left) and Design 3 (right) [34]
6.4.8 Mold Interior/Proximity Effect Induced by Internal Current The principle of high-frequency proximity heating can be described and shown by Figure 6.42. When two metallic blocks are brought to face each other in parallel, with a small gap, and carry high-frequency electric current in opposite directions, the carrying currents tend to flow along the two facing inner surfaces, forming a strong magnetic field in the space between the two blocks. This phenomenon is known as the proximity effect [25]. An injection mold consists of two mold plates facing each other and forming a mold cavity in between. The mold plates are usually made of high-strength tool steel with high electrical resistivity (Figure 6.43). When the two mold plates are brought to a small intervening distance during the mold closing stage, the injection mold is thus subjected to the proximity effect if a high-frequency current (HFC) is applied to each mold plate.
6.4 Mold Heating Based on Electromagnetic Induction Technology
Figure 6.42 Principle of high-frequency proximity heating [10]
Figure 6.43 Mold design for high-frequency proximity heating [10]
Because of the proximity effect, the HFC will flow along the two inner cavity surfaces and generate electric heating at both surfaces. To make use of the proximity heating, one may design different current-conducting paths via the embedded copper pipe of the various layouts. Examples of different designs can be seen in Figure 6.44. Details can be found elsewhere [10]. The corresponding heating efficiencies are depicted in Figure 6.45. Generally speaking, the heating rate is around 6~10 °C/s. The disadvantage is that insulation is required on the parting surface to prevent the current flow from one side of mold plate to the other side or, in other words, the leakage of electrical current. Insulation of the clamping plate of the injection molding machine is also required. The limited space within the mold base also hinders the design of current conduction channels.
225
226 6 Variable Mold Temperature Technologies
Design 1 1 Channel
Design 2 3 Channels
Design 3 Rectangular Channel
(a)
(b) Figure 6.44 (a) Three types of coil channels designs; (b) illustration of testing the mold (Design 1) [10]
Figure 6.45 Temperature variation vs. time at the center of the cavity surface for the three inductor designs [10]
6.5 Other Mold Surface Heating Technologies
6.5 Other Mold Surface Heating Technologies 6.5.1 Hot Gas-Assisted Heating Gas-assisted mold temperature control (GMTC), proposed by Chen et al. [7, 36–38], is a new technique for mold temperature control. It can heat and cool the cavity surface rapidly during the injection molding process. The GMTC system consists of a menu-driven controller, a hot-gas generator system, and a water temperature control unit, as shown in Figure 6.46. The hot gas generator is composed of an air compressor, an air dryer, a digital volumetric flow controller, and a high-efficiency gas heater. The function of the high-power hot gas generator system is to support a heat source, which is required to provide sufficient hot air with a flow rate up to 500 l/min and an air temperature of 500 °C or higher. For the coolant system, a typical water-cooling-based mold temperature control unit can be used to provide water at a preset temperature to cool the mold after the filling process. It would be better if the water cooling unit could also warm the mold to a higher initial temperature at the beginning of the process. Fluid valves were also used to control the air flow in the heating stage and the water flow for the cooling channels.
Figure 6.46 System of hot gas-assisted mold surface heating in injection molding [38]
For practical operation, the mold was not completely closed prior to melt injection. Instead, a gap was left, allowing the hot gas to flow in through the cavity. The inlet and outlet for the gas must be properly designed. After the cavity surface achieves
227
228 6 Variable Mold Temperature Technologies
the target temperature, the gas valve closes and then the mold completely closes for melt injection to begin. An operation schematic is shown in Figure 6.47.
Figure 6.47 Schematic of GMTC operation [7]
The heating rate is dependent on various parameters; among them, the temperature of the hot gas and the gas flow rate are two crucial factors. The open gap of the mold cavity and the shape of the gate that diverts the gas flow are also important. Since the heat delivered to the mold surface via convective heat transfer will quickly dissipate, the most efficient way to keep the heat and the associated temperature on the surface is to use a nickel-plate-like stamp. The contact resistance between the plate and the mold base will hinder heat loss to the mold base and result in a good replication of micro-featured patterns. The initial heating speed can be as high as 45 °C/s and should stabilize at about 25 °C/s if the hot gas temperature and flow rate are reasonable, as seen in the Figure 6.48 [7]. The two levels of heating speed are due to the temperature difference between the hot gas and the cavity surface temperature. The heating speed slows down with increased cavity surface temperature. GMTC has potential application in micro-molding in which nickel-plated stampers are frequently utilized. A molding example for improving the replication accuracy of micro-features is exhibited in Figure 6.49 and Figure 6.50. Another application to eliminate floating fiber marks resulting in a glossy surface is also shown in Figure 6.51 [38].
6.5 Other Mold Surface Heating T echnologies
Figure 6.48 Comparison of mold temperature between molds designed with and without nickel plating with regard to various heating times [7]
a)
b)
Figure 6.49 (a) Molded product and (b) microstructure [7]
229
230 6 Variable Mold Temperature Technologies
Without gas heating
Heating target temp. = 90°C
Heating target temp. = 100°C
Heating target temp. = 110°C
Heating target temp. = 120°C
Heating target temp. = 130°C
Figure 6.50 3D laser microscope images of a molded micro-feature at the center point. The depth of the molded micro-feature varies with regard to the target mold temperature [7]
6.5 Other Mold Surface Heating Technologies
Figure 6.51 Surface quality and elimination of floating fiber marks (a) with and (b) without gas-assisted mold heating [38]
6.5.2 Infrared Heating Efficiency and feasibility of mold surface heating using infrared was evaluated [8]. An experimental schematic is shown in Figure 6.52. As mentioned earlier, for a regular, smooth mold surface, the high reflection of radiation heating on the mold surface will result in poor heating efficiency. In Chen’s study [8], both a smooth mold surface and a PTFE-coated mold surface were used for comparison. For the PTFE-coated mold surface, the infrared heating achieved a heating rate of about 4~5 °C/s. For the smooth mold surface, the heating rate was below 1 °C/s. The experimental results of temperature variation are illustrated in Figure 6.53. A comparison between mold exterior induction is also shown.
Figure 6.52 System of infrared mold surface heating [8]
231
232 6 Variable Mold Temperature Technologies
Figure 6.53 Comparison of heating rates between induction heating and infrared heating [8]
References [1] Michaeli, W., Spennemann, A., Gartner, R., Microsystem Technologies (2002) 8, pp. 55–57 [2] Michaeli, W., Scharf, M., Annual Technical Conference - ANTEC, Conference Proceedings (2009) 3, pp. 1578–1582 [3] Michaeli, W., Eilbracht, S., Scharf, M., Hartmann, C., Bobzin, K., Bagcivan, N., Thieß, S., Journal of Polymer Engineering (2012) 32, pp. 95–101 [4] Yao, D. G., Chen, S. C., Kim, B. H., Advances in Polymer Technology (2008) 27, pp. 233–255 [5] Chen, S. C., Chang, J. A., Asian Workshop of Polymer Processing (2010) [6] Lin, H. L., Chen, S. C., Jeng, M. C., Minh, P. S., Chang, J. A., Hwang, J. R., International Communications in Heat and Mass Transfer (2012) 39, pp. 514–522 [7] Chen, S. C., Chang, J. A., Hsu, W. Y., Huang, S. W., Microelectronic Engineering (2011) 88, pp. 1594– 1600 [8] Chen. Y. C., Chung Yuan Christian University PhD Thesis (2005) Taiwan [9] Lin, Y. T., Chung Yuan Christian University Master Thesis (2009) Taiwan [10] Chen, S. C., Pham, S. M., Chang, J. A., Huang, S. W., International Communications in Heat and Mass Transfer (2012) 39, pp. 216–223 [11] Wang, G. L., Zhao, G. Q., Guan, Y. J., Materials and Design (2010) 31, pp. 3426–3441 [12] Huang, M. S., Tai, N. S., Journal of Polymer Science (2009) pp. 1345–1354 [13] Jeng, M. C., Chen, S. C., Pham, S. M., Chang, J. A., International Communications in Heat and Mass Transfer (2010) 37, pp. 1295–1304 [14] Wang, G. L., Zhao, G. Q., Guan, Y. J., Expert Systems with Applications (2011) 38, pp. 6705–6719 [15] Wang, G. L., Zhao, G. Q., Guan, Y. J., International Journal of Thermal Sciences (2011) 50, pp. 790– 802 [16] Bürkle, E., Burr, A., Müller, A. K., Kübler, M., Kunststoffe International (2007) 10, pp. 144–148 [17] Xiong, C. L., Huang, H. X.. Applied Thermal Engineering (2014) 73, pp. 712–722
References
[18] Liu, S. C., Chung Yuan Christian University Master Thesis (2007) Taiwan [19] Yao, D. G., Polymer Engineering and Science (2002) 42, pp. 2471–2481 [20] Rudnev, V., Loveless, D., Cook, R. L., Black, M., Handbook of Induction Heating (2003) Marcel Dekker, New York. [21] Chen, S. C., Device for advancing even distribution of high cycle wave magnetism, U. S. Patent No. 6919545 (2005) [22] Chen, S. C., Chang, J. A., Cin, J. C., Annual Technical Conference - ANTEC, Conference Proceedings (2009) 3, pp. 1630–1364 [23] Sung, Y. T., Hwang, S. J., Advances in Mechanical Engineering (2014) Article ID 349078, 10 pages [24] Chen, S. C., Peng, H. S., Chang, J. A., Jong, W. R., International Communications in Heat and Mass Transfer (2004) 31, pp. 971–980 [25] Chen, S. C., Jong, W. R., Chang, Y. P., Annual Technical Conference – ANTEC (2005) 2, pp. 166–170 [26] Chen, S. C., Jong, W. R., Yang, K. Y., Annual Technical Conference – ANTEC (2008) 1, pp. 499–502 [27] Pham, S. M., Chung Yuan Christian University PhD Thesis (2011) Taiwan [28] Nian, S. C., Huang,. M. S., Tsai, T. H., International Communications in Heat and Mass Transfer (2014) 50, pp. 52–60 [29] Shih, S. Y., Nian, S. C., Huang, M. S., Huang, R. C., Chen, C. H., International Communications in Heat and Mass Transfer (2016) 71, pp. 44–55 [30] Chen, S. C., Jong, W. R., Chang, J. A., Chang, Y. J., International Polymer Processing (2006) 5, pp. 1–7 [31] Chen, S. C., Jong, W. R., Chang, J. A., Chang, Y. J., Journal of Micromechanics and Microengineering (2006) 16, pp. 1783–1791 [32] Pham, S. M., Chen, S. C., Y. T. Lin, Annual Technical Conference - ANTEC, Conference Proceedings (2010) 2, pp. 896–899 [33] Chen, S. C., Mold Heating/Cooling Structure, U. S. Patent No. 8162640 (2012) [34] Liu, G. C., Chung Yuan Christian University Master Thesis (2014) Taiwan [35] Sung, Y. T., Hwang, S. J., Tehnički vjesnik (2014) 21, pp. 651–656 [36] Chen, S. C., Chien, R. D., Lin, S. H., Lin, M. C., Chang, J. A., International Communications in Heat and Mass Transfer (2009) 36, pp. 806–812. [37] Chen, S. C., Gas-Assisted Mold Surface Heating System, U. S. Patent No. 8092208 (2012) [38] Chen, S. C., Pham, S. M., Chang, J. A., International Communications in Heat and Mass Transfer (2011) 38, pp. 304–312
233