Injection moulding of thermosets

For the same part, injection moulding of thermosets can offer up to 23% production increase and lower part-costs than compression. 

Shen (1990) reviews the flow modelling of injection moulding of thermosets. The kinetic model was the Kamal model (as shown above in Equation (6.31)) and the chemoviscosity model is given by a modified WLF model for low shear mtes (as shown by Equation (6.35)) and a modified Cross model for high shear (as shown by Equation (6.36)), namely... 

Injection moulding of thermosetting materials is achieved in the same way as for thermoplastic materials except that the temperatures used are more critical. The temperature of the heating chamber is... 

Savings in cost most often accrue from the injection moulding of thermosets (such as polyesters), instead of thermoplastics, because thermoset polymer is usually cheaper than thermoplastic and the cycle times are sometimes shorter (the material cross-links in the mould so that cooling is not necessary before the part is removed). The purpose of the screw is to plasticize and homogenize the precursor material (which may contain short fibres) in preparation for injection into a heated mould in which cure takes place. The basic machine is similar to that described above for thermoplastics, but there are important differeiKes in detail (see Figure 7.40) ... 

Savings in cost often accrue from the injection moulding of thermosets (such as polyesters), instead of thermoplastics, because thermoset polymer is usually... 

Cycle times for injection moulding of thermosets can often be reduced dramatically compared with compression moulding, sometimes to as much as a sixth. Times for transfer moulding would probably be somewhere in between. 

The machinery for injection moulding of thermosets, because of the additional built-in facilities, is considerably more expensive than the conventional hydraulic press. However, advantages occur with the more precise and constant temperature controls and operating cycle which can be achieved with this machine. Many of the machines also have the advantage of being able, by the change of a cylinder and other ancillary equipment, to be used for injection moulding of thermoplastic materials. 

The more eeonomical process is the injection moulding of thermosets on screw injection moulding machines. The material is plasticised with a special screw. The optimum preparation of the melt means a plasticising unit with a corresponding feed unit and screw for material specific dosage. 

A guide to the temperature effects of the various stages of injection moulding of thermosets is given in... 

In ntosl respects the process is similar to the injection moulding of thermoplastics and the sequence of operations in a single cycle is as described earlier. For thermosets a special barrel and screw are used. The screw is of approximately constant depth over its whole length and there is no check value which might cause material blockages (see Fig. 4.50). The barrel is only kept warm (80-110°C) rather than very hot as with thermoplastics because the material must not cure in this section of the machine. Also, the increased viscosity of the thermosetting materials means that higher screw torques and injection pressures (up to 200 MN/m are needed). 

ASTM D3419-93, Practice for in-line screw injection moulding of tet specimens from thermosetting compounds, American Society for Testing Materials Publication. Philadelphia, 1993. ASTM 3297-93. Practice for moulding and machining tolerances for PTFE resin parts. American Society for Testing Materials Publication, Philadelphia. 1993. 

ISO 10724-1 1998, Plastics—Injection moulding of test specimens of thermosetting materials— Part 1 General principles and moulding of multipurpose test speeimens... 

Polymer matrix composites (PMC) are used to increase the rather low stiffness or strength of polymers by adding stiffer or stronger fibres. If thermoplastics are used as matrix material, short fibres can be added to the granulate material that is subsequently softened by heating and processed, for example in injection moulding. If thermoset resins (duromers) are used, the fibres can be placed into the liquid resin before curing. 

Plastics - Injection moulding of test specimens of thermosetting powder moulding compounds (PMCs), 1998... 

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