Common plastic injection molding defects

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.

Common plastic injection molding defects
plastic product design possible defects


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.

injection molding warping

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.

Common plastic injection molding defects sink marks

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.

injection molding drag

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.

Common plastic injection molding defects

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.

Common plastic injection molding defects sink marks short shots

Design rules for plastic injection molded parts

Injection molding of plastic parts allows us to form complex shapes. In this way a well designed plastic part serves multiple functions.

Once the mold is manufactured, these complex parts can be reproduced at a very low cost. However,  changes to the mold design at later stages of development can be very expensive. Therefore, achieving the best results on the first time is essential. It is important as a product designer to follow the design guidelines in order to avoid the most common defects in injection molding.