To understand the molding process it is important to know the most common plastic injection molding defects. Most defects in injection molding are related to either the flow of the melted material or its non-uniform cooling rate during solidification.
Here is an overview of defects to consider while designing a part for injection molding.
Warping
When certain sections cool and shrink faster than others, then the part will permanently bend because of internal stresses.
Parts with non-constant wall thickness are most affected by warping as a injection molding defect.
Sink marks
When the inside of a plastic wall cools down slower than the outside, a recess or sink mark will appear at the outside surface. It means in this area a larger volume of material will shrink more than the area around it with less volume.
Parts with thick walls or poorly designed ribs are most prone to sinking.
Drag marks
As the plastic shrinks, it applies pressure on the mold. During ejection, the walls of the part will slide and scrape against the mold, which can result to drag marks.
Parts with vertical walls (and no draft angle) are most affected to drag marks. Therefore it is better to always apply draft angles.
Knit lines
Among the common plastic injection molding defects we have the forming of knit lines. When 2 flows meet, small hair-like discolorations may develop. These knit lines affect the parts aesthetics, but also they generally decrease the strength of the part.
Parts with abrupt geometry changes or holes are more prone to knit lines.
Short shots
Trapped air in the mold can inhibit the flow of the material during injection, resulting in an incomplete part. Good design can improve the flow ability of the melted plastic.
Parts with very thin walls or poorly designed ribs are more affected by short shots.
