Variable Temperature Moulding

The surface is often described as a weak surface layer and in the case of plastics one can include the surface stresses, general contamination, the presence of abhesive ingredients, i.e., process aids which have migrated to the surface. Some high temperature moulding processes may lead to variable and unwanted oxidation and/or reversion (crosslink... 

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. 

Die swell is important in the design of gates for injection moulds too little swell can lead to jetting of the melt stream across the mould cavity instead of a smooth filling of the mould. The primary variables may be taken to be the gate dimensions t, w, and I (see Figure 7.32), melt temperature T, injection volume flow rate Q, and polymer... 

Figure 2.8 (Kumaraswamy 1999) shows how POM can display orientation and structure gradients in injection moulded materials. These images show polypropylene crystallized in a cell designed to probe the different variables (time, temperature, shear rate) found in this process, and the two parts of the figure illustrate the formation of the characteristic skin-core morphology. The oriented near-skin region...

The thermal equilibrium of any HR system is governed by variable and fixed values of all components of the thermal balance of the mould/injection moulding machine/ambient conditions system. The fact that variable factors are always present means that automatic temperature control and regulation of the HR system must be used to keep deviations from the required temperature to a minimum. The following initial conditions must be fulfilled ... 

The TMS rheometer from Negretti is effectively a modification of the Mooney viscometer principle for use with elastomeric materials. The elastomer is introduced into a cavity containing a shearing disc by injection, and measurements of torque under fixed or variable conditions of temperature and rotor speed can be made. Effective simulation of a variety of process conditions can be made. The instrument has found particular use in the investigation of rubber-to-metal adhesion behaviour, a problem of importance to mould fouling and extrusion processing. 

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