MOULD CYCLE

One consequence of this is that in a thermoplastic injection moulding the molecular orientations will vary from place to place according to the flow pattern. This may well set up sufficient stresses in the moulding to cause it to distort from its intended shape. Factors affecting the amount of orientation were discussed briefly in the previous chapter and by the author elsewhere. Sometimes the conditions required to minimise distortion may be such that the moulding cycle becomes too long to be economic. In these cases it may be necessary to modify the moulding or at least the position and number of feed... 

Fig. 4.61 illustrates that the mould temperature is quite different from the set oven temperature (330°C) or indeed the actual oven temperature, throughout the moulding cycle. An even more important observation is that in order to control the rotational moulding process it is desirable to monitor the temperature of the air inside the mould. This is possible because there is normally a vent tube through the mould wall in order to ensure equal pressures inside and outside the mould. This vent tube provides an easy access for a thermocouple to measure the internal air temperature. 

The available range of plastics is very wide with a variety of compositions and related properties within any one type description. Even when physical properties and formulation are specified, minor variations in trace additives, release agents, moulding cycles, etc. may have a considerable influence on corrosivity. Table 18.18 can therefore be considered only as a guide and even in this sense much more experimentation is needed to provide the full picture. In the present state of technology precise information is only likely to result from tests carried out on the material of interest various suitable test methods have been described in the literature ... 

Die swell is dependent upon the L/D ratio of the die. The phenomenon is a limiting factor in the drive to reduce moulding cycles, since the conditions which lead to excess swelling lead also to quality deficiencies in appearance, form and properties of the extrudate. In order to control the swelling the temperature of the melt can be increased, which causes a decrease in relaxation time. A long tapered die has also been found to reduce post-swelling. 

When clarity is required PVC, PS, PET, or cellulose acetate may be used. The last of these tends to cost more even though the moulding cycle is approximately 25% faster this does not compensate for the higher material cost (+30%). Impact modified styrene or styrene is more widely used for coffret-type inserts which are invariably coloured or flock coated. Styrene offers a higher yield than PVC (note densities) and a faster forming time. PET is often the preferred material for surgical or medical device systems. 

Both employ a preprinted transfer (up to six colours). In the in-mould process the transfer is placed within the mould prior to bottle blowing or injection moulding. During the moulding cycle the printed transfer becomes fused to the container. The printing can be produced by either screen or gravure and can be line or half tone. 

Figure 7.8 shows a possible cross section for the beam. The 2 mm thick section was chosen so the injection moulding cycle time is short, while the I beam is efficient in bending (Chapter 13). From the linear portion of the isochronous stress-strain curve, the linear viscoelastic compliance is 7(1 year) = 3.3 x 10 m N . Substituting this and the deflection limit in... 

The technique can be used for all cold-setting sand, excluding silicate sand. The regenerated sand can be re-used in the same moulding cycle, with small additions of new sand to level-off quality losses. 

In injection moulding processors often use multi-cavity moulds to make several products in one cycle. The runners required to supply material to each cavity contain a considerable amount of material which, if the product itself is small, may be of the same mass as the parts themselves. This is evidently quite uneconomic. Hot rurmer systems solve this problem by keeping the material in the runners from solidifying between moulding cycles. 

Physical properties were evaluated for all the formulations found in Tables 1.2 and 1.3 with TPR being applied at various times throughout the moulding cycle as previously discussed. Additionally, physical properties were evaluated with no TPR being applied. The 90 second TPR time was chosen to be the minimum time to perform TPR for all formulations. Physical property pads were produced at this TPR time. Extended TPR... 

This point is significant in relation to the moduli of injection mouldings which can be quenched to a greater or lesser extent, in a moulding cycle (Chapter 7), and also to polymer fibres which when spun cool very rapidly because th are of small diameter ( 10 ftm). 

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