Injection moulding cycle

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... 

Injection moulding cycle with cooling the largest component. 

During the Pooling and ejection parts of the injection moulding cycle, the material in the barrel is continulusly homogenised and prepared for flowing into thick section... 

A single injection moulding cycle can be broken down into three distinct stages plastication, mould filling and cooling with solidification. 

A guide to the basic parameters of nozzle temperature, mould temperature, injection pressure, holding pressure and back pressure for selected materials is shown in Table 8.2. It can be seen that injection moulding parameters vary widely from material to material. With this in mind, it is important to imderstand what these injection parameters are and how they affect the injection moulding cycle. The next section will therefore discuss the injection moulding process. 

Both thermoplastics and thermosets can be formed by compression moulding (Fig. 24.5). The polymer, or mixture of resin and hardener, is heated and compressed between dies. The method is well suited to the forming of thermosets (casings for appliances, for instance) and of composites with a thermosetting matrix (car bumpers, for example). Since a thermoset can be removed while it is still hot, the cycle time is as short as 10 seconds for small components, 10 minutes for large tliick-walled mouldings. Pressures are lower than for injection mouldings, so the capital cost of the equipment is much less. 

An example of the effect on production rates is provided by injection moulding. The longer it takes after injection for solidification of the polymer to occur, the longer will be the overall cycle. (Provided the moulding is not distorted on ejection it will only be necessary to form a rigid skin to the moulding.)... 

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... 

The methacrylic polymer remains a useful glazing material. In aircraft applications it is used extensively on aircraft which fly at speeds less than Mach 1.0. They form the familar bubble body of many helicopters. On land, acrylic sheet is useful for coach roof lights, motor cycle windscreens and in do-it yourself cabins for tractors and earth-moving equipment. Injection mouldings are frequently used for plaques on the centre of steering wheels and on some fascia panelling. 

The negligible water absorption avoids the need for predrying granules. The low specific heat (compared with polyethylene) enables the polymer to be rapidly heated in injection cylinders, which therefore have a higher plasticising capacity with polystyrene than with polyethylene. The setting-up rates in the injection moulds are also faster than with the polyolefins so that faster cycles are also possible. 

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

Wall sections in foam moulding are thicker than in solid material. Longer cycle times can therefore be expected due to both the wall thickness and the low thermal conductivity of the cellular material. In contrast, however, the injection pressures in foam moulding are low when compared with conventional injection moulding. This means that less clamping force is needed per unit area of moulding and mould costs are less because lower strength mould materials may be used. 

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). 

The application of instrumentation to the automatic control of a sequence of operations, e.g., injection moulding processes. Once the mould has been loaded with inserts (assuming a rubber to metal bonded part) into the press, the operation of a push-button starts the controller which closes the press, injects the rubber, controls the cure cycle, recharges the injection unit, opens the press, operates the ejectors and presents the mould for cleaning and loading of inserts. 

To facilitate and accelerate folding and crystallization of polymer chains, internal plasticizers are often added to PET to serve as crystallation promoters. Such additives are usually based on poly(ether ester)s. These plasticizers are liquids that are typically added at levels of 2-4 wt%. They reduce cycle time in injection moulding operations by increasing the rate of crystalline formation. They also plasticize the resin and act as processing aids by virtue of their lubricating action in the melt. On a molecular level, these plasticizers reduce the intermolecular... 

The injection moulding process for producing a microcellular crosslinked EVA has several advantages over the conventional compression moulding process reduced cycle time, labour and scrap. The presence of skin adds aesthetic value and ability to engrave logos in one step. However, the injection moulding process has... 

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