Answer: An insufficiently filled mold cavity is sometimes called a “short shot.” It can be caused by poor injection molding processes, not using enough plastic resin, or a need to redesign the runner system. Many injection molding defects are caused by an excessive drop in pressure or temperature between the gate and the cavity. However, it may also be necessary to rework the runner system to allow each cavity to fill properly. 

Insufficient filling can also be caused by improperly designed parts or tooling. Some examples include poorly designed gate openings, locating the gate too far from the cavities, designing the part with uneven wall thicknesses, or a poorly designed exhaust system.