What is air trap and how to eliminate this problem?

Air trap is air that is caught inside the mold cavity. It becomes trapped by converging polymer melt fronts or because it failed to escape from the mold vents. Entrapped air will result in voids and bubbles inside the molded part, short shot (incomplete fill), or surface defects, such as blemishes or burn marks. Air trap locations are usually in areas that fill last.

Air trap can be eliminated by modifying the filling pattern by enlarging venting, by placing proper venting in the cavity or reducing injection speed so the air will have sufficient time to escape for the vents. Increasing the back pressure setting will make the melt denser and help remove gases and minimize trapped air volume.