TABLE OF CONTENTS In common with other plastics components, it is necessary to apply a shrinkage allowance to the cavity and core sizes. This ensures that after volumetric shrinkage of the polymer has taken place, the moulded thread form will be correct.
If a shrinkage allowance is not applied, this can result in the moulded thread failing to mate correctly with other parts. This is particularly true when semicrystalline materials are being used. The relatively high level of volumetric shrinkage that takes place with these materials makes it essential to compensate for this in the mould tool.
In particular, high-performance engineering materials like acetal and Nylon have large shrinkage values. Threads moulded in these materials are often used for more precise applications; hence, special attention must be given to shrinkage. In addition, such applications require extra attention to ensure moulding consistency so as to minimise dimensional variation.
Great care must be taken with this class of component to make sure that the unscrewing operation does not distort the thread form. This can often be the cause of precision components failing to meet quality standards or failing to mate satisfactorily with other threads.
The overriding requirement in any injection moulding enterprise, however, is to be competitive and profitable. In view of this, where applications call for a simple noncritical fit, there is no point in applying expensive shrinkage compensation techniques or control devices.
Also, on very small thread forms or when using materials with low shrinkage values, it...
