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Achieving Precision Injection Molding by Mould Control

Dec-26

 And the temperature of the mould has an effect on the actual shrinkage rate. Therefore, in order to determine the forming conditions conveniently, the arrangement of the cavity must be paid attention to when designing the cavity of the precision injection mould. Because molten plastics bring heat into the die, the temperature gradient distribution of the die is generally around the cavity, which is a concentric circle centered on the main channel.

Therefore, the design measures such as flow channel equalization, cavity arrangement and concentric circular arrangement centered on the main channel are necessary to reduce the shrinkage error between the cavities, expand the allowable range of forming conditions and reduce the cost. The cavity arrangement of precision injection moulds should meet the requirements of flow channel equalization and centering on the main channel, and the cavity arrangement with the main channel as symmetrical line must be adopted.

Because the temperature of the mould has a great influence on the shrinkage rate of the moulding, but also directly affects the mechanical properties of the injection moulded products, and also causes various forming defects such as the surface blooming of the products, it is necessary to keep the mould within the prescribed temperature range, and also to keep the temperature of the mould unchanged with time. The temperature difference between the various cavities of the multi-cavity die must not change. Therefore, the temperature control measures for heating or cooling of the die must be adopted in the design of the die. In order to minimize the temperature difference between the cavity of the die, attention must be paid to the design of the temperature control-cooling circuit. In the temperature control loop of the cavity and core, there are two main connection modes: series cooling and parallel cooling.

In terms of heat exchange efficiency, the flow of cooling water should be turbulent. However, in the parallel cooling circuit, the flow rate in the circuit that becomes the shunt is smaller than that in the series cooling circuit, which may form laminar flow, and the actual flow rate into each circuit is not necessarily the same. Because the temperature of cooling water entering each circuit is the same, the temperature of each cavity should be the same, but in fact, because the flow rate in each circuit is different, and the cooling capacity of each circuit is different, the temperature of each cavity can not be the same. The disadvantage of the series cooling circuit is that the flow resistance of the cooling water is large, and the cooling water temperature at the front inlet of the cavity is obviously different from that at the last inlet of the cavity. The temperature difference of cooling water inlet and outlet varies with the flow rate. For small precision injection moulds, it is generally appropriate to adopt series cooling circuit in order to reduce the cost of the moulds.

Mold cavity and core should have their own cooling water loop system. In the design of cooling circuit, due to the different heat absorbed from the cavity and core, the thermal resistance of the circuit structure is also different, and the water temperature at the inlet of the cavity and core will produce a large temperature difference. If the same system is used, the design of cooling circuit is difficult. In addition, it is also hoped that the temperature difference between the cavity and the core will be maintained when the warpage prevention measures are taken for injection moulded products. Therefore, when designing the cooling circuit of the cavity and core, the temperature should be adjusted and controlled separately.

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