Air traps - 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. Air-trap locations are usually in areas that fill last. Entrapped air will result in voids and bubbles inside the molded part, a short shot (incomplete fill), or surface defects such as blemishes or burn marks. To eliminate air traps, you can modify the filling pattern by reducing the injection speed, enlarging venting, or placing proper venting in the cavity.

Warpage - Warpage occurs as stress builds up between thin and thick part sections. Thin sections cool faster and solidify first. As the thick section shrinks during cooling, fill in material is pulled from the unsolidified areas to compensate the shrinkage, causing stress build-up with the already solidified thin section. The thick sections remain liquid longer and needs to yield to the stress that builds up. This results in warpage, which can become an extremely costly problem to fix once a mold is in a production environment. Analyzing and improving the part design can bring the part warpage within acceptable levels.

Sink marks and voids - A sink mark is a local surface depression that typically occurs in moldings with thicker sections, or at locations above ribs, bosses, and internal fillets. 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. After the material on the outside has cooled and solidified, the core material starts to cool. Its shrinkage pulls the surface of the main wall inward, causing a sink mark.

Shrinkage - Shrinkage is the reduction in the dimensions of a plastic part, compared with the mold dimensions. Shrinkage occurs as the polymer cools and can vary in different directions. Crystalline and semi-crystalline materials are particularly prone to thermal shrinkage; amorphous materials tend to shrink less.

Weld lines and meld lines - A weld line (also called a weld mark or a knit line) is formed when separate melt fronts traveling in opposite directions meet. A meld line occurs if two emerging melt fronts flow parallel to each other and create a bond between them. Weld and meld lines can be caused by holes or inserts in the part, multiple gates, or variable wall thickness where hesitation or race tracking occurs.

Weld lines are generally undesirable when part strength and surface appearance are major concerns. This is especially true with fiber-reinforced materials, because the fibers do not bridge the weld lines and often are oriented parallel to them.

For more information on possible molding problems and how to avoid them during the design process contact your local Moldflow office.