TABLE OF CONTENTS It is beyond the scope of this book to embark on a comprehensive discussion on injection moulding technique. However, it is very relevant to discuss the effect of certain injection moulding conditions and techniques on the production of screw threads.
It is pointless to have carefully designed mould tools constructed if the moulding conditions vary widely or are wrongly set. Difficulties with the production of screw threads are often encountered because of such problems.
The main causes of difficulty are briefly discussed next.
High pressures are necessary for the production of precision components, particularly those in crystalline materials. However, if this pressure is set too high it can lead to extreme difficulty in unscrewing thread forms from the impression. Material is packed far too tightly into the cavity, making heavier-duty unscrewing motors necessary and promoting the distinct possibility of the occurrence of flash.
Too high an injection speed has a similar effect to too high a pressure in promoting unnecessary flash, again making unscrewing difficult.
While a reasonable unscrewing speed it necessary for an economic cycle, too high a speed can be counterproductive. This can often lead to inaccurate thread forms and can leave a trailer of thread at the bottom of the thread form as the component clears the cavity or core.
Many unscrewing systems have lead screws that move a stripper plate forward at the same rate at which...
