Common Causes of Cracking in Injection Molded Plastic Parts

Cracking is a common defect in injection molded plastic parts. It may appear as surface cracks, microcracks, whitening, split marks, or complete fracture of the part. Cracking can occur during molding, demolding, assembly, or later use.

To reduce cracking problems, manufacturers should evaluate processing conditions, material selection, mold design, and machine performance.

1. Processing Conditions

Improper molding parameters are one of the main causes of cracking in plastic parts.

• Excessive injection pressure, holding pressure, or filling speed can create high internal stress inside the part.
• Excessively long packing or holding time can increase stress concentration and lead to cracking.
• If the mold opening speed is too fast, the part may crack during demolding.
• Increasing mold temperature can make demolding easier and reduce the risk of cracking.
• Reducing melt temperature when necessary can help prevent material degradation.
• Weld lines and degraded plastic material may weaken the part and increase the chance of cracking.
• Release agents should be used carefully, and mold surface contamination should be avoided.

Residual stress can also be reduced through annealing or post-molding heat treatment, which helps minimize the risk of later cracking.

2. Material Issues

The quality and condition of the plastic material have a major influence on part strength and crack resistance.

• Too much recycled material may reduce the mechanical strength of the part.
• Excess moisture in the material can cause hydrolysis or poor molding quality.
• Using the wrong resin for the application environment may increase brittleness and cracking risk.
• Contaminated or degraded material can weaken the final part.

3. Mold Design Problems

Poor mold design can create stress concentration areas that increase the likelihood of cracking.

• Ejection force should be evenly distributed using sufficient ejector pins and proper ejector layout.
• Draft angles should be large enough to allow smooth demolding.
• The mold cavity surface should be smooth to reduce friction during ejection.
• Thin wall sections, sharp corners, and sudden thickness changes can create stress concentration points.
• Rounded transitions and fillets should be used wherever possible.
• Metal inserts can increase internal stress because of shrinkage differences between metal and plastic.
• Deep cavity parts may require additional venting to prevent vacuum sticking.
• The runner, sprue, and gate system should be designed to allow easy release of the part during demolding.

4. Injection Molding Machine Factors

The injection molding machine itself can also affect cracking problems.

If the plasticizing capacity of the machine is too small, the material may not be fully melted and mixed, which can make the part brittle. If the machine is too large for the shot size, the material may remain in the barrel too long and degrade.

Selecting the proper machine size and maintaining stable molding conditions are important for reducing cracking defects in injection molded parts.

By improving molding parameters, material quality, mold design, and machine selection, manufacturers can significantly reduce cracking and improve the long-term performance of plastic components.