Problems encountered in Injection Molding

There are many details to pay attention in injection molding which affects the physical properties, they may even cause to the failure of the molding. 

Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas, such as the nozzle, runner, or gate, into open, thicker areas, without forming contact with the mold wall. This leads to part weakness, surface blemishes, and a multiplicity of internal defects.

An 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, or mold inserts, which also act as vents.

A short shot is a molded part that is incomplete because insufficient material was injected into the mold. It can be caused by entrapped air, insufficient machine injection pressure (resulting from high melt resistance and a restricted flow path), pre-mature solidification of the polymer melt, and machine defects.

A sink mark is a local surface depression and a void is a vacuum bubble in the core. Sink marks and voids are caused by localized shrinkage of the material at thick sections without sufficient compensation when the part is cooling. 

A weld line (also called a weld mark or a knit line) is formed when separate melt fronts traveling in opposite directions meet. The formation of weld lines can be caused by holes or inserts in the part, multi-gate cavity systems, or variable wall thickness where hesitation or race tracking occurs. The weld lines are undesirable when the strength and the surface quality are important.