Injection moulding screw machine

Fig. 4.31 Typical cycle in reciprocating screw injection moulding machine...

There are a number of important features in reciprocating screw injection moulding machines and these will now be considered in turn. 

Foamed plastic articles may be produced with good results using normal screw-type injection moulding machines (see Fig. 4.43(a)). However, the limitations on shot size, injection speed and platen area imposed by conventional... 

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). 

A practical difficulty which arises during injection moulding of reinforced plastics is the increased wear of the moulding machine and mould due to the abrasive nature of the fibres. However, if hardened tool steels are used in the manufacture of screws, barrels and mould cavities then the problem may be negligible. 

Simple ram machines are cheaper than screw machines having equivalent shot volumes. Although the simple ram injection moulding machine was once quite widely used it was generally obsolescent by 1977. Plasticisation by screw is quicker and more controllable than by heat conduction in simple ram machines. A higher and more uniform compound temperature and thus a more uniform viscosity is achieved. 

Although the above are the major moulding processes for thermosets, injection moulding can now be used for the selected thermoset moulding operations. The major use of thermosets still remains with screw closures. These are made on either a multicavity platen tool (up to 96 cavities) or a rotary type press which resembles a rotary tableting machine. 

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. 

Computer controlled injection-moulding machines store the optimum process parameters for each mould/polymer combination. However, a number of machine settings affect each polymer variable the melt temperature prior to injection is influenced by the screw speed, the back pressure on the screw, and the barrel and nozzle heater temperatures. There is no simple way of measuring the melt temperature at the nozzle any thermocouple projecting into the melt would be sheared off by the high flow stresses. Therefore, there is open-loop control on the melt temperature, with the value fluctuating both during and between cycles. 

An injection moulding machine with mould screw preplasticator injection unit (after ICl pic). 

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