What are the most common defects when plastic injection molding?

To understand the injection molding process it is important to know what are the most common defects when plastic injection molding. Most defects in injection molding are related to either the flow of the melted material or its non-uniform cooling rate during solidification. In addition, it is important to follow some design rules for plastic injection in when developing new plastic components.

Here is an overview of defects to consider while designing a part for injection molding.

Design and manufacturing of plastic parts
Common plastic injection molding 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

2.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 likely to show to sink marks. Therefore, it is good to check the design and avoid one of the most common defects when plastic injection molding

Common plastic injection molding defects sink marks

3.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

4.Knit lines

Among the most common defects when plastic injection molding, 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

5.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.

How to avoid plastic injection problems?

It is certainly good to have a lot of knowledge and experience in plastic injection molding. Very often, there are small problems to solve after the first molding test. These are the issues to learn from. Therefore, it might be a goo solution to have your products designed by professionals or broaden your experience in the field of injection molding and mold manufacturing.