Coring refers to the elimination of plastic material in thick sections that would otherwise cause issues like long cooling times, uneven shrinkage and molded-in stresses. To prevent that, steel is added to the mold, which results in a pocket or opening in the part (see Figure 1).
From a tool manufacturing and cost point of view, cores should ideally be placed parallel to the line of draw. Cores in other directions require the use of some form of side action (cam operated or hydraulic) to be actuated, thus increasing tooling costs (see Figure 2). Sometimes this can be avoided – by clever designing, the same functionality may be achieved at lower cost.
Figure 1 - Coring design
Figure 2 - Construction B could cost as much as 60% less than A