Answer: There are several factors that can contribute to parts sticking in the mold. The first to consider is injection pressure. If the injection pressure is too high, the mold will be excessively saturated, and the plastic resin will fill into other areas of the mold, making the part difficult to demold. 

Temperature is also a factor. If the material barrel temperature is set too high, the temperature of the plastic resin can increase to a point that alters the material properties. This can result in the parts fracturing during the demolding process, thus sticking in the mold. It is also important to note that excessive temperatures result in longer cycle times due to parts needing to cool to the proper temperature before demolding.

Insufficient draft angles in the mold may also contribute to parts sticking. The vertical walls of the mold should be properly drafted to allow the part to eject without sticking. 

Lastly, if the mold filling is unbalanced, the finished part (or parts) will not cool consistently, which may lead to parts sticking in the mold. If sticking occurs, the means for filling the cavity or cavities may need to be modified. For multi-cavity molds, this may mean redesigning the runner system, so each cavity fills simultaneously.