3 Injection Molding Defects and their Solutions
Mould tool making

Mold tool making or injection molded prototype is an art as well as science. High level of attention and technical expertise are needed to avert small mistakes from costing companies a large sum of money; especially when we talk about bulk production.

To prevent mistakes, the plastics mold supplier requires a highly competent design. Following are a few molding defects which can occur in any part during the process of injection molding. This article also includes how to fix those defects.

Below are the shortcomings:

Sink Marks

Illustration: Sink marks are tiny depressions or craters which is developed in thicker parts of the injection-molded prototype when shrinkage occurs. The effect is similar to sinkholes present in topography. However, sinkholes are caused by reduction instead of erosion.

Cause: Sink marks occur during the cooling time for the plastic to completely cure and cold while in a mold. The objective of sink marks can also be due to an excessive temperature at the gate.


  • Minimizing the thickness of the thick wall sections will ensure quick cooling and will aid to reduce the chances of sink marks.
  • Temperatures of mold should be lowered, holding pressure should be increased, and holding time should be extended to allow more cooling and curing.

Surface Delamination

Illustration: It is a condition in which thin layers of the surface appear on the part because of contaminant material. These layers look like coatings and can be peeled off.

Cause: External materials which find their way into the molten plastic segregate out of the finished product due to contaminant and hence plastic is unable to bond. The inability of bonding has an impact not only on the prototype’s appearance but also on its strength. Delamination also occurs due to the over-dependence on mold release agent.


  • Smooth out the sharp turns and corners in the mold prototype to avert sudden changes in the flow of the melt.
  • Mold temperature should be increased.
  • Before molding, pre-dry the plastic.
  • Focus on the mechanism of ejection in the mold design to eliminate or minimize the reliance on mold release agents.


Jetting refers to a condition where molten plastics fail to remain glued to the mold surface because of the injection speed.

Cause: Jetting occurs during mould tool making when the melt temperature is on the lower side. Low temperature aggravates the viscosity. Consequently, resistance is increased during flow through the mold.