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Plastic processing
Technical Report
Plastic Processing This issue is devoted to close-to-form forming methods; it is important not to forget that the primary reason for the attractiveness of plastics in many applications is the ability to achieve near-net-shape in one operation. With many processes available for the production of plastics components, and the interactive relationship between materials properties, processing method and endproduct properties, the choice of forming method has both technical and economical aspects to its solution.
Influence of polymer type The thermoplastic polymers are the most versatile class, for which a wide variety of processing methods exist. A wide variety of melt processes are available because the polymer melts have sufficient strength during flow to remain integral. However, thermoplastics can also be shaped as a solid, just below the Tg temperature, or in the intermediate, 'rubbery' stage. With thermosets, processes are more limited, for two main reasons: the premoulded state is either a powder, a compressed pellet, a dough, or a set of sheets, which first soften under heat, but in this condition have little melt strength; secondly, when the material is heated to make a moulding, an irreversible cross-linking reaction is initiated, demanding that the material must be confined in some manner under pressure. On the other hand, because prepolymers for cross-linking systems are of low molecular weight (and therefore low viscosity), lower forces and lighter equipment can be employed. The major processes for moulding thermosets are thus compression moulding, transfer moulding and, more recently, injection moulding. However, the epoxies and polyesters to be used with a wide range of processing techniques. Elastomers are converted in a similar manner to thermosets, the cross-linking occurring during moulding; however, the thermoplastic elastomers, as their name implies, can be processed as conventional thermoplastics.
Melt Flow Index, (MFI) and the melt 'elasticity'. The MFI is the quantity of polymer extruded under specific load and temperature conditions in a given time; the elastic properties of the melt are a major factor in determining whether residual strain and moulding defects will be present.
Common methods for thermoplastics Extrusion is a continuous process in which a rotating, helicaUy-flighted screw supplies a steady flow of molten thermoplastic through a die. After being shaped by the die, the .molten plastic is cooled and solidified into different final dimensions by means of the cooling fixtures and haul-off conditions. Extruded products tend to be of simple
L
Choice of processing method In an ideal world, with unlimited time and capital, there are numerous ways to consider shaping a polymer melt. Of course, in reality, the designer is sandwiched between many tonnes of capital-intensive equipment and an unsympathetic accountant, whose combined influences tend to restrict the options. Nevertheless, most processes use a mould or an extrusion die, although processes such as calendering (in which a series of heavy rolls converts a plastic mass into sheet form) are also practised. Two important features of polymer melts which are important in processing are shear viscosity, measured by the
566
Fig. 1
Rotationally moulded boat fuel tanks made of crosslinked high density polyethylene (Courtesy Phillips Petroleum).
MATERIALS & DESIGN, Vol. 3, OCTOBER 1982
Table 1
Shape and form of parts suitable for particular processes (Source: Reference 3). lntrictae,
Nonreinforced Materials Compression moulding Transfer moulding Injection moulding Extrusion
Shape
Complicated
Limitations
Shapes
mouldable mouldable mouldable constant
yes yes yes yes
ConUolled Open, Wall Hollow
Thickness
yes yes yes yes
Shapes
Enclosed, Hollow
Shapes
Large, Enclosed Volume
Very Small Items
yes yes yes
Plan Are 10 I ~
yes yes
Factor
Limiting Maximum
Size
Inserts
Molded-in Holes Threads
press press press die
yes yes yes yes
yes yes yes
yes yes yes
available machine mould
yes
yes
yes
CROSS-
Rotational moulding Blow moulding
section hollow
yes
yes
yes
yes
yes
yes
yes
yes
Thermoforming
hollow, thin-wall thin-wall
Casting Forging Foam moulding
mouldable mouldable mouldable
yes yes yes
yes yes yes
yes
mouldable large, thin-wall
yes yes
yes yes
yes yes
mouldable
yes
yes
yes
mouldable mouldable surface of revolution constant
yes yes
yes yes yes
yes yes
yes
yes
Fibre-reinforced Materials Injection moulding Hand lay-up and spray-up Compressiontype moulding (SMC, DMC) Preform moulding Cold-press moulding Filament winding Pultrusion
yes
yes
yes
yes, by joining
yes
yes yes
available machine mould die press
press mold, or transport of parts press
yes
yes
yes yes
yes yes
yes yes
yes
yes
yes
yes
10 6
10 7
press press available machine die
cross-
section
Table 2
Typical economic criteria for various plastic processing methods (Source: Reference 3). Equipment Capital Cost
Typical Production Volume Number of Parts
Production Rate
Tooling Cost
medium medium fast
high high high low low medium low low medium medium
101
10 2
10 3
10 4
10 s
Nonreinforced Materials Machining Compression moulding Transfer moulding Injection moulding Extrusion Rotational moulding Blow moulding Thermo forming Casting Forging Foam moulding
high high high medium low medium low low high high
slow medium slow very slow slow medium
Fibre-Rein forced Materials Injection moulding Hand lay-up Spray-up Compression SMC Compression DMC Preform moulding Cold-press moulding Filament winding Pultrusion
high low high high high high low low
MATERIALS & DESIGN, Vol. 3, OCTOBER 1982
fast slow slow medium medium medium medium
high low low high high medium low low low
567
is fed to a blowing mould, where it is inflated and cooled against the mould walls. Whilst the process is strictly only suitable for dosed hollow objects, ingenious adaptations of the technique can produce complex shapes. Machines and moulds, though expensive, are less so than injection moulds. There is a related process, known as injection blow moulding, in which injection moulded preforms are reheated and the secondary shaping achieved by blow moulding. Advantages are the increased extent of biaxial orientation, (for toughness) and the ability to compensate for changes in wall thickness by preshaping the parison. On the minus side is the need for more complicated moulds. Thermoforming is a secondary process which involves the shaping of sheet or foil, usually be vacuum or compressed air. Tooling costs can be extremely low.
Thermoset forming
ln/ection moulding can be used for thermosets and is becoming increasingly popular, due to the high degree of automation possible. However, the traditional process for thennosets has been compression moulding; in this process a mould is dosed on a partially polymerised thermoset plastic. It is forced to flow and fiU the cavity, where it crosslinks under the influence of heat. Mould costs are relatively high and cycle times are long. With the introduction of injection moulding, transfer moulding can now be thought of as a half-way stage. It is similar to compression moulding, but uses a more complicated mould arrangement, into which the thermoset plastic flows from a transfer pot while the mould is dosed. Fig. 2
Pet¢ol riller pipe for Ford Escort blow-moulded in polypropylene.
shape and constant cross-section. In/ection moulding concerns the softening of granules of plastic in a heated cylinder, within which a screw turns to mix the plastic. By thrusting the screw forward in the cylinder, a known quatity of polymer is rapidly injected into a closed, relatively cool mould. Mould and capital costs are high, so production runs need to be long to amortise costs. For this reason, the majority of IM machines will run fully automatically. Moulds, which may be of virtually any realistic size, may also be complex and require retractable mould parts, or cores, to form features which are not in the opening direction of the mould. In rotational moulding a split, hollow mould is charged with a predetermined amount of powdered plastic and then slowly rotated about two perpendicular axes in an oven. The plastic melts and flows over the inner surface of the mould, which is then cooled prior to part removal. Mould and tooling costs are relatively low, but the process tends to be limited to hollow components of simple shape. The market penetration of this process has been slow, one reason for which is the limited availability of materials in suitable form for the process.
Blow moulding accounts for most small and medium-sized containers and, more recently, automotive radiator header tanks and petrol tanks have been blow-moulded. In the most common variant, extrusion blow moulding, a hot parison, usually a circular tube, is produced from an extruder with a suitable die. In a variety of ways the parison 568
Economics of processing In general, there will be a most suitable production method for any component, depending upon shape, size and quantity required. The cost of producing a simple product weighing 200 grammes by different production methods (assuming that they are all feasible) as a function of production numbers is shown in Fig. 3. This can only be used as a guide and for more details of the methods which can be used for this type of assessment, the reader should refer to Reference 1, section 2.4. Any such analysis should, of course, take account of design modifications which may need to be made to enhance the suitability of the product to any individual process; this may even include a change of material! 1 0 01 0
~
1O0
°I 110 1
Fig. 3
Note:
~.~
"J":e°ry:i°11n~ :e~;~jlidlmn:nutSing
" 10
102 10= 10" 10s 10s annualproduction--units Typical comparative production costs for 200g component as function of production numbers (Source: Reference 1).
MATERIALS & DESIGN, Vol. 3, OCTOBER 1982
Table 3
Advantagesand limitations of thermoset moulding methods (Source: Reference 2).
Advantages COMPRESSION MOULDING 1. Can mould heavily reinforced materials 2. Best method for mouldings with large surface area related to thickness 3. Greatest uniformity in moulded properties (i.e. isotropy) 4. Most versatile process with relation to quantities and component complexity 5. Relatively simple to understand and control TRANSFER MOULDING 1. Improved labour utilisation compared with compression moulding 2. Shorter cures often achievable compared with compression moulding 3. Good control of part (and flash) thickness 4. Suitable for incorporation of delicate inserts
INJECTION MOULDING 1. Process offers scope for automatic operation 2. Rapid cycle times built around a substantial reduction in cure time 3. Good control of dimensions and flash thickness 4. Good part-to-part consistency 5. Can mould thin sections and (preferably with vertical clamp units) around delicate inserts
Disadvantages 1. Relatively labour intensive, with more material handling involved 2. Material losses due to the need for flash allowances and general wastage 3. Secondary operations for flash removal are generally required 4. Poor control of part thickness amd weight 5. Cannot mould around delicate inserts or pins
l. More skill in terms of process control is required
2. Moulds often more complex and costly 3. Cannot use as wide a range of compounds as with compression moulding 4. Greater risk of distortion and directionality of mechanical properties (i.e. anistropy) 5. Care often required to minimise effect of weld (knit) lines 6. Material losses in central feed system may be incurred
1. Greater skill required in setting up and controlling the process 2. Machines and moulds are more expensive and complex 3. Process is not tolerant to stop-start conditions of operations 4. Injection-grade materials are required, controlled to closer tolerances than other grades 5. Care is often required to minimise effect of weld (knit) lines 6. Irrecoverable loss of materials in sprues and runners*
*Development of warm runner moulds to obviate these losses is covered in Chapter 6 of Reference 2
Some newer processes A variant of injection moulding for thermosets is reaction injection moulding (-RIM)in which liquid reactants are polymerised in the mould. Glass may also be added in the process known as reinforced RIM (RRIM for short!). This process is extremely well established for urethanes, notwithstanding on-going arguments between suppliers as to the relative merits of chopped or hammer-milled glass.
Outsert moulding
Outsert moulding is a process whereby any number of plastic components are moulded in position on a base plate in one operation by injection moulding.
Polyol and isocyanate are mixed and injected into the mould, wherein, within seconds, a chemical reaction is initiated; releasing gases expand the mix and fill the mould. After solidification, a part with a microcellular core results, a part which is relatively free of stress and sink marks, regardless of configuration. Mould cycle times are typically 30s, and the moulded part is removable in 1- 10 minutes. When high grade microcellular elastomers are stiffened by the incorporation of glass and/or other fillers, property improvements of particular interest to the automotive industry result: reduced thermal expansion coefficient, approximating to that of sheet steel; increased stiffness; high temperature stability to facilitate painting by similar methods to those used for steel. The RIM process has already been employed with nylon, and in the future, hybrids of epoxies, polyesters, acrylics, and phenolics look to be suitable systems.
MATERIALS & DESIGN, Vol. 3, OCTOBER 1982
Fig. 4
Schematic of (Courtesy ICI).
RRIM processing
method
569
Table 4
Comparison of compression, transfer and injection moulding for Polyester DMC (BMC); rating is 1 (best) to 3 (poorest). (Source: Reference 2). Compression
Press cost Operating cost Factory space Tool cost Production speed Suitability for small parts Suitability for multi-cavities Suitability for large parts Delicate inserts Handling and metering DMC Automatic operation Extraction of parts Insert loading Moulding cleanliness Surface finish Minimal flash and finishing Strength Isotropy Production costs (small parts) Production costs (large parts)
1 1 1 1 3 3 3 1 2 3 2 1 3 2 3 1 1 3 1
Very rapid cure (30s on small parts) leads to fast moulding cycle and extrusion speeds and costs which are reported to be about 60% of those experienced in producing the parts from conventional silicone rubbers.
Transfer Injection 2 2 2 2 2 2 2 2 1 2 3 1 2 3 2 2 2 2 2
3 3 3 3 1 1 1 2 1 1 1 1 2 1 1 1 2 2 1 3
The base plate is placed in the parting line of the mould and polymer is injected into the mould to form the required components on the plate. Acetal copolymer has always been a favoured thermoplastic for this process and for this reason much of the development activity has been carried by suppliers of this raw material. However, other thermoplastics, notably PBT, have also been successfully adopted. Outsert moulding allows a reduction of costs when components using all-metal construction require many components to be mounted onto a base plate.
Foams and Idlers Only in a brief technical report could one justify discussing foams and Fillers under the same sub-heading. In both cases, however, a second material (air in the case of foams) is added to modify the properties of the polymer in a desirable manner. Foams have improved stiffness per unit weight (but not per unit thickness), but the effect is not usually significant unless the section size exceeds 6mm. Several methods are available for processing foamed plastics, which may be commodity or engineering ('structural') resins, most of which are cheaper to run than injection moulding machines of conventional design. Reinforcement can be obtained by adding fillers, beads or fibres to a basic polymer. Fibres are frequently added to improve strength and stiffness, whilst other fdlers may improve rigidity, reduce moulding distortion and/or reduce costs. In some cases strength is impaired and melt flow length reduced. Table 5
Comparison of production rates for compression, transfer and injection moulding for Polyester DMC (BMC). (Source: Reference 2). Small moulding (thin section/10g
weight), 12-impression mould Compression Cure time (seconds) Press open time (seconds) Total cycle (seconds) Saving compared with compression (%)
Transfer Injection
30 150 180
30 15 45
25 10 35
-
75
80
Large moulding (thick section/1000g
weight), single-impression mould Compression Cure time (seconds) Press open time (seconds) Total cycle (seconds) Saving compared with compression (%)
Fig. 5
Use of the outsert moulding process is made in the manufacture of this TV tuner.
Liquid silicone polymers (LPS) LPS is a high speed, high volume technique for injection moulding silicone parts and producing supported silicone extrusions from liquid silicone rubber (LSR). The materials are blends of 100% vinyl-functional dimethylsiloxane polymers with additives. To simplify mixing of the A and B components the ratios have been set at 1 : 1.
570
45 15 60 -
Transfer Injection 40 15 55
30 10 40
8
25
The geometry and surface chemistry of Fillers have a marked influence on the effects they induce, making it difficult to generalise even about the effects of specific fillers on processing and performance. Foams and reinforced plastics will be covered further in future technical reports. References 1. A.W. Birley & M.J. Scott. Plastics Materials, Properties and Applications. Leonard Hill 1982. 2. J.F. Monk (Ed). Thermosetting Plastics, Practical Moulding Technology. George Godwin/PRl, 1981. 3. R.L.E. Brown. Design and Manufacture of Plastics Parts. John Wiley, 1980.
MATERIALS & DESIGN, Vol. 3, OCTOBER 1982
Plastic Processing This issue is devoted to close-to-form forming methods; it is important not to forget that the primary reason for the attractiveness of plastics in many applications is the ability to achieve near-net-shape in one operation. With many processes available for the production of plastics components, and the interactive relationship between materials properties, processing method and endproduct properties, the choice of forming method has both technical and economical aspects to its solution.
Influence of polymer type The thermoplastic polymers are the most versatile class, for which a wide variety of processing methods exist. A wide variety of melt processes are available because the polymer melts have sufficient strength during flow to remain integral. However, thermoplastics can also be shaped as a solid, just below the Tg temperature, or in the intermediate, 'rubbery' stage. With thermosets, processes are more limited, for two main reasons: the premoulded state is either a powder, a compressed pellet, a dough, or a set of sheets, which first soften under heat, but in this condition have little melt strength; secondly, when the material is heated to make a moulding, an irreversible cross-linking reaction is initiated, demanding that the material must be confined in some manner under pressure. On the other hand, because prepolymers for cross-linking systems are of low molecular weight (and therefore low viscosity), lower forces and lighter equipment can be employed. The major processes for moulding thermosets are thus compression moulding, transfer moulding and, more recently, injection moulding. However, the epoxies and polyesters to be used with a wide range of processing techniques. Elastomers are converted in a similar manner to thermosets, the cross-linking occurring during moulding; however, the thermoplastic elastomers, as their name implies, can be processed as conventional thermoplastics.
Melt Flow Index, (MFI) and the melt 'elasticity'. The MFI is the quantity of polymer extruded under specific load and temperature conditions in a given time; the elastic properties of the melt are a major factor in determining whether residual strain and moulding defects will be present.
Common methods for thermoplastics Extrusion is a continuous process in which a rotating, helicaUy-flighted screw supplies a steady flow of molten thermoplastic through a die. After being shaped by the die, the .molten plastic is cooled and solidified into different final dimensions by means of the cooling fixtures and haul-off conditions. Extruded products tend to be of simple
L
Choice of processing method In an ideal world, with unlimited time and capital, there are numerous ways to consider shaping a polymer melt. Of course, in reality, the designer is sandwiched between many tonnes of capital-intensive equipment and an unsympathetic accountant, whose combined influences tend to restrict the options. Nevertheless, most processes use a mould or an extrusion die, although processes such as calendering (in which a series of heavy rolls converts a plastic mass into sheet form) are also practised. Two important features of polymer melts which are important in processing are shear viscosity, measured by the
566
Fig. 1
Rotationally moulded boat fuel tanks made of crosslinked high density polyethylene (Courtesy Phillips Petroleum).
MATERIALS & DESIGN, Vol. 3, OCTOBER 1982
Table 1
Shape and form of parts suitable for particular processes (Source: Reference 3). lntrictae,
Nonreinforced Materials Compression moulding Transfer moulding Injection moulding Extrusion
Shape
Complicated
Limitations
Shapes
mouldable mouldable mouldable constant
yes yes yes yes
ConUolled Open, Wall Hollow
Thickness
yes yes yes yes
Shapes
Enclosed, Hollow
Shapes
Large, Enclosed Volume
Very Small Items
yes yes yes
Plan Are 10 I ~
yes yes
Factor
Limiting Maximum
Size
Inserts
Molded-in Holes Threads
press press press die
yes yes yes yes
yes yes yes
yes yes yes
available machine mould
yes
yes
yes
CROSS-
Rotational moulding Blow moulding
section hollow
yes
yes
yes
yes
yes
yes
yes
yes
Thermoforming
hollow, thin-wall thin-wall
Casting Forging Foam moulding
mouldable mouldable mouldable
yes yes yes
yes yes yes
yes
mouldable large, thin-wall
yes yes
yes yes
yes yes
mouldable
yes
yes
yes
mouldable mouldable surface of revolution constant
yes yes
yes yes yes
yes yes
yes
yes
Fibre-reinforced Materials Injection moulding Hand lay-up and spray-up Compressiontype moulding (SMC, DMC) Preform moulding Cold-press moulding Filament winding Pultrusion
yes
yes
yes
yes, by joining
yes
yes yes
available machine mould die press
press mold, or transport of parts press
yes
yes
yes yes
yes yes
yes yes
yes
yes
yes
yes
10 6
10 7
press press available machine die
cross-
section
Table 2
Typical economic criteria for various plastic processing methods (Source: Reference 3). Equipment Capital Cost
Typical Production Volume Number of Parts
Production Rate
Tooling Cost
medium medium fast
high high high low low medium low low medium medium
101
10 2
10 3
10 4
10 s
Nonreinforced Materials Machining Compression moulding Transfer moulding Injection moulding Extrusion Rotational moulding Blow moulding Thermo forming Casting Forging Foam moulding
high high high medium low medium low low high high
slow medium slow very slow slow medium
Fibre-Rein forced Materials Injection moulding Hand lay-up Spray-up Compression SMC Compression DMC Preform moulding Cold-press moulding Filament winding Pultrusion
high low high high high high low low
MATERIALS & DESIGN, Vol. 3, OCTOBER 1982
fast slow slow medium medium medium medium
high low low high high medium low low low
567
is fed to a blowing mould, where it is inflated and cooled against the mould walls. Whilst the process is strictly only suitable for dosed hollow objects, ingenious adaptations of the technique can produce complex shapes. Machines and moulds, though expensive, are less so than injection moulds. There is a related process, known as injection blow moulding, in which injection moulded preforms are reheated and the secondary shaping achieved by blow moulding. Advantages are the increased extent of biaxial orientation, (for toughness) and the ability to compensate for changes in wall thickness by preshaping the parison. On the minus side is the need for more complicated moulds. Thermoforming is a secondary process which involves the shaping of sheet or foil, usually be vacuum or compressed air. Tooling costs can be extremely low.
Thermoset forming
ln/ection moulding can be used for thermosets and is becoming increasingly popular, due to the high degree of automation possible. However, the traditional process for thennosets has been compression moulding; in this process a mould is dosed on a partially polymerised thermoset plastic. It is forced to flow and fiU the cavity, where it crosslinks under the influence of heat. Mould costs are relatively high and cycle times are long. With the introduction of injection moulding, transfer moulding can now be thought of as a half-way stage. It is similar to compression moulding, but uses a more complicated mould arrangement, into which the thermoset plastic flows from a transfer pot while the mould is dosed. Fig. 2
Pet¢ol riller pipe for Ford Escort blow-moulded in polypropylene.
shape and constant cross-section. In/ection moulding concerns the softening of granules of plastic in a heated cylinder, within which a screw turns to mix the plastic. By thrusting the screw forward in the cylinder, a known quatity of polymer is rapidly injected into a closed, relatively cool mould. Mould and capital costs are high, so production runs need to be long to amortise costs. For this reason, the majority of IM machines will run fully automatically. Moulds, which may be of virtually any realistic size, may also be complex and require retractable mould parts, or cores, to form features which are not in the opening direction of the mould. In rotational moulding a split, hollow mould is charged with a predetermined amount of powdered plastic and then slowly rotated about two perpendicular axes in an oven. The plastic melts and flows over the inner surface of the mould, which is then cooled prior to part removal. Mould and tooling costs are relatively low, but the process tends to be limited to hollow components of simple shape. The market penetration of this process has been slow, one reason for which is the limited availability of materials in suitable form for the process.
Blow moulding accounts for most small and medium-sized containers and, more recently, automotive radiator header tanks and petrol tanks have been blow-moulded. In the most common variant, extrusion blow moulding, a hot parison, usually a circular tube, is produced from an extruder with a suitable die. In a variety of ways the parison 568
Economics of processing In general, there will be a most suitable production method for any component, depending upon shape, size and quantity required. The cost of producing a simple product weighing 200 grammes by different production methods (assuming that they are all feasible) as a function of production numbers is shown in Fig. 3. This can only be used as a guide and for more details of the methods which can be used for this type of assessment, the reader should refer to Reference 1, section 2.4. Any such analysis should, of course, take account of design modifications which may need to be made to enhance the suitability of the product to any individual process; this may even include a change of material! 1 0 01 0
~
1O0
°I 110 1
Fig. 3
Note:
~.~
"J":e°ry:i°11n~ :e~;~jlidlmn:nutSing
" 10
102 10= 10" 10s 10s annualproduction--units Typical comparative production costs for 200g component as function of production numbers (Source: Reference 1).
MATERIALS & DESIGN, Vol. 3, OCTOBER 1982
Table 3
Advantagesand limitations of thermoset moulding methods (Source: Reference 2).
Advantages COMPRESSION MOULDING 1. Can mould heavily reinforced materials 2. Best method for mouldings with large surface area related to thickness 3. Greatest uniformity in moulded properties (i.e. isotropy) 4. Most versatile process with relation to quantities and component complexity 5. Relatively simple to understand and control TRANSFER MOULDING 1. Improved labour utilisation compared with compression moulding 2. Shorter cures often achievable compared with compression moulding 3. Good control of part (and flash) thickness 4. Suitable for incorporation of delicate inserts
INJECTION MOULDING 1. Process offers scope for automatic operation 2. Rapid cycle times built around a substantial reduction in cure time 3. Good control of dimensions and flash thickness 4. Good part-to-part consistency 5. Can mould thin sections and (preferably with vertical clamp units) around delicate inserts
Disadvantages 1. Relatively labour intensive, with more material handling involved 2. Material losses due to the need for flash allowances and general wastage 3. Secondary operations for flash removal are generally required 4. Poor control of part thickness amd weight 5. Cannot mould around delicate inserts or pins
l. More skill in terms of process control is required
2. Moulds often more complex and costly 3. Cannot use as wide a range of compounds as with compression moulding 4. Greater risk of distortion and directionality of mechanical properties (i.e. anistropy) 5. Care often required to minimise effect of weld (knit) lines 6. Material losses in central feed system may be incurred
1. Greater skill required in setting up and controlling the process 2. Machines and moulds are more expensive and complex 3. Process is not tolerant to stop-start conditions of operations 4. Injection-grade materials are required, controlled to closer tolerances than other grades 5. Care is often required to minimise effect of weld (knit) lines 6. Irrecoverable loss of materials in sprues and runners*
*Development of warm runner moulds to obviate these losses is covered in Chapter 6 of Reference 2
Some newer processes A variant of injection moulding for thermosets is reaction injection moulding (-RIM)in which liquid reactants are polymerised in the mould. Glass may also be added in the process known as reinforced RIM (RRIM for short!). This process is extremely well established for urethanes, notwithstanding on-going arguments between suppliers as to the relative merits of chopped or hammer-milled glass.
Outsert moulding
Outsert moulding is a process whereby any number of plastic components are moulded in position on a base plate in one operation by injection moulding.
Polyol and isocyanate are mixed and injected into the mould, wherein, within seconds, a chemical reaction is initiated; releasing gases expand the mix and fill the mould. After solidification, a part with a microcellular core results, a part which is relatively free of stress and sink marks, regardless of configuration. Mould cycle times are typically 30s, and the moulded part is removable in 1- 10 minutes. When high grade microcellular elastomers are stiffened by the incorporation of glass and/or other fillers, property improvements of particular interest to the automotive industry result: reduced thermal expansion coefficient, approximating to that of sheet steel; increased stiffness; high temperature stability to facilitate painting by similar methods to those used for steel. The RIM process has already been employed with nylon, and in the future, hybrids of epoxies, polyesters, acrylics, and phenolics look to be suitable systems.
MATERIALS & DESIGN, Vol. 3, OCTOBER 1982
Fig. 4
Schematic of (Courtesy ICI).
RRIM processing
method
569
Table 4
Comparison of compression, transfer and injection moulding for Polyester DMC (BMC); rating is 1 (best) to 3 (poorest). (Source: Reference 2). Compression
Press cost Operating cost Factory space Tool cost Production speed Suitability for small parts Suitability for multi-cavities Suitability for large parts Delicate inserts Handling and metering DMC Automatic operation Extraction of parts Insert loading Moulding cleanliness Surface finish Minimal flash and finishing Strength Isotropy Production costs (small parts) Production costs (large parts)
1 1 1 1 3 3 3 1 2 3 2 1 3 2 3 1 1 3 1
Very rapid cure (30s on small parts) leads to fast moulding cycle and extrusion speeds and costs which are reported to be about 60% of those experienced in producing the parts from conventional silicone rubbers.
Transfer Injection 2 2 2 2 2 2 2 2 1 2 3 1 2 3 2 2 2 2 2
3 3 3 3 1 1 1 2 1 1 1 1 2 1 1 1 2 2 1 3
The base plate is placed in the parting line of the mould and polymer is injected into the mould to form the required components on the plate. Acetal copolymer has always been a favoured thermoplastic for this process and for this reason much of the development activity has been carried by suppliers of this raw material. However, other thermoplastics, notably PBT, have also been successfully adopted. Outsert moulding allows a reduction of costs when components using all-metal construction require many components to be mounted onto a base plate.
Foams and Idlers Only in a brief technical report could one justify discussing foams and Fillers under the same sub-heading. In both cases, however, a second material (air in the case of foams) is added to modify the properties of the polymer in a desirable manner. Foams have improved stiffness per unit weight (but not per unit thickness), but the effect is not usually significant unless the section size exceeds 6mm. Several methods are available for processing foamed plastics, which may be commodity or engineering ('structural') resins, most of which are cheaper to run than injection moulding machines of conventional design. Reinforcement can be obtained by adding fillers, beads or fibres to a basic polymer. Fibres are frequently added to improve strength and stiffness, whilst other fdlers may improve rigidity, reduce moulding distortion and/or reduce costs. In some cases strength is impaired and melt flow length reduced. Table 5
Comparison of production rates for compression, transfer and injection moulding for Polyester DMC (BMC). (Source: Reference 2). Small moulding (thin section/10g
weight), 12-impression mould Compression Cure time (seconds) Press open time (seconds) Total cycle (seconds) Saving compared with compression (%)
Transfer Injection
30 150 180
30 15 45
25 10 35
-
75
80
Large moulding (thick section/1000g
weight), single-impression mould Compression Cure time (seconds) Press open time (seconds) Total cycle (seconds) Saving compared with compression (%)
Fig. 5
Use of the outsert moulding process is made in the manufacture of this TV tuner.
Liquid silicone polymers (LPS) LPS is a high speed, high volume technique for injection moulding silicone parts and producing supported silicone extrusions from liquid silicone rubber (LSR). The materials are blends of 100% vinyl-functional dimethylsiloxane polymers with additives. To simplify mixing of the A and B components the ratios have been set at 1 : 1.
570
45 15 60 -
Transfer Injection 40 15 55
30 10 40
8
25
The geometry and surface chemistry of Fillers have a marked influence on the effects they induce, making it difficult to generalise even about the effects of specific fillers on processing and performance. Foams and reinforced plastics will be covered further in future technical reports. References 1. A.W. Birley & M.J. Scott. Plastics Materials, Properties and Applications. Leonard Hill 1982. 2. J.F. Monk (Ed). Thermosetting Plastics, Practical Moulding Technology. George Godwin/PRl, 1981. 3. R.L.E. Brown. Design and Manufacture of Plastics Parts. John Wiley, 1980.
MATERIALS & DESIGN, Vol. 3, OCTOBER 1982