Injection moulding cooling rates

The time available for disorientation as the melt cools from Tp to T. This will depend on the value of Tp-T where is the temperature of the environment (the mould temperature in injection moulding) since this will with the specific heat determine the rate of cooling. The time will also depend on Tp-T since this will determine the extent of cooling. 

The crystallisation kinetics of PET suit the process (Fig. 3.26). A glassy preform can be injection moulded with a wall thickness of up to 4 mm. If the mould is kept at 10 °C, the inner layers of the polymer cool fast enough for the crystallinity to be negligible. High molecular weight PET, with Mn — 24 000, is used so that the rate of crystallisation is suitably low. 

Moulds are kept at a constant temperature by re-circulating water or oil through cooling channels. Polyethylene when injected has a heat content of about 700 Jg, of which only half is left in the ejected warm moulding. The mould temperature is set to suit the product and polymer. For some products it is low, to maximise the cooling rate. For other products, such as... 

As moulds have a high capital cost, injection moulding is usually economic, only if more than 10000 products are made. It is preferable to reduce the cycle time than to invest in a second mould to achieve the required production rate. The mould only has a high productivity if the cooling time, which dominates the cycle time, is minimised. Consequently, the product wall thickness is minimised (Eq. 5.4 predicts the effect on the cooling time). Sections 3 and 4 of Chapter 13 consider further suitable product shapes for injection moulding. 

A sp>ecial injection mould has been designed such that samples could be cooled at a known cooling rate and imder a known pressure (Brucato et al., 2002, Brucato et al., 2000, La Carrubba, 2001). This heated mould consists of a conical cavity (the sprue), which is located in the fixed platen of the injection molding machine, coupled to a "diaphragm". The front of the cavity is sealed with a high tensile, high thermal conductivity copp>er-beryllium... 

In conventional injection moulding, a polymer melt is prepared in a heated barrel, using a compression-type extruder screw the melt is injected under high rates of shear into a cooled mould, which is clamped closed by a (high) force, sufficient to resist the hydraulic pressure of the injecting melt. 

Injection moulded polypropylene-PIB-silica samples were studied using TA Instruments DSC 2920. The samples were heated and cooled twice from 0°C to +210°C with scan rate 10°C/min. 

Rapid production rates can be achieved with little limitation on size and shape. Cycle times may be as low as 10 seconds for small components, being dependent on the time required to fill the mould and the cooling time. Filling time is proportional to the pressure drop across the nozzle raised to a power dependent on the type of plastics being injected, and cooling time is roughly proportional to the square of the wall thickness of the moulding. 

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