Interior Core-Mold Design
The design criteria for the interior, or core, of any functional part differs significantly from the design criteria used for the exterior cavity. The interior of this type of part is formed by a mold core, which must fit within the exterior element or mold cavity. The highest point on the core forms the deepest depression inside the part. As the mold halves close on the parison, the core element presses against the parison and forces it into the cavity element until the pinch-off is sealed around the perimeter of the part.

The half of the parison that is draping over the mold core is already beginning to set as the air is injected into the parison. As in the cavity, the plastic begins to stretch to fit the mold contour. Almost no flow occurs. Unlike the cavity, some different rules apply.