Sinks

A sink is a depression on the surface of a part. Sinks forms during the cooling of the molten plastic, particularly in places where there is an variation in the otherwise uniform thickness of a part. Features such as ribs and bosses often cause this problem because there is a significant increase in material where the features connect to the base, resulting in cooling times in that area relatively longer than the areas around it.

The amount of sink problems referred back for redesign is not a true indication of the total number of sinks that occurred. Of concern to the designer are sinks visible to the customer or sinks which interfere with the optical qualities of lenses. Generally, sinks occurring on housing parts are not referred back for redesign.

Sink interference with the optical qualities of a lens is of great interest. A sink is a flaw that can greatly diminish the optical characteristics of a part. There is already a base of information which can predict possibilities of sinks on the basis of optical treatment, material, and part thickness.