Why a mold is usually designed to have multiple cavities

Update:19-06-2020
Summary:

A molding consists of a sprue to introduce molten resin […]

A molding consists of a sprue to introduce molten resin, a runner to lead it to cavities, and products. Since obtaining only one product by one shot is very inefficient, a mold is usually designed to have multiple cavities connected with a runner so that many products can be made by one shot.

If the length of the runner to each cavity is different in this case, the cavities may not be filled simultaneously, so that dimensions, appearances or properties of the moldings are often different cavity by cavity. Therefore the runner is usually designed so as to have the same length from the sprue to each cavity.

Sprues and runners among moldings are not products. These portions are sometimes discarded, but in other cases they are finely reground and reused as materials for molding. These materials are called reprocessed materials.

Reprocessed materials are not solely used as materials for molding but usually used after blending with virgin pellets, since there is possibility of deterioration in various characteristics of the plastics because of the initial molding process. The maximum allowable limit for the ratio of reprocessed materials is about 30 %, because too high ratio of reprocessed materials may spoil the original properties of the plastics used.

For the properties when reprocessed materials are used, please refer to "reprocessing capability" in the plastic data base.