Common Injection Molding Challenges and How to Solve Them
Effective troubleshooting in injection molding helps reduce scrap by identifying defects quickly and correcting the root cause. Most rejected molded parts fall into three categories: dimensional variation, warpage, or appearance issues. The first step is correctly identifying the defect, then determining what is causing it.
Troubleshooting Size Problems
A common issue in molding is when a part measures correctly after production, but changes size later during storage. Features that initially pass inspection may no longer fit or assemble properly.
This is usually caused by shrinkage, a natural behavior of polymers as material continues to relax after molding. Excess heat in the molding process gives molecules energy to keep moving and settling after ejection.
To reduce shrinkage, review process settings related to heat:
- Mold temperature
- Melt temperature
- Shear heat created during processing
The more heat introduced into the process, the more cooling is required to stabilize the part. Increasing cooling time in the mold before ejection can also help parts hold their final dimensions.
Molded Flat, Staying Flat
Warpage occurs when a part bends or twists after molding instead of matching the tool geometry. This is caused by internal stresses created during molding.
Thermal stress often comes from uneven mold temperatures. Both mold halves should cool the part as evenly as possible. If one side runs hotter, balancing temperatures can improve flatness. Parts typically warp toward the hotter side of the mold.
Flow-induced stress can also cause warpage, especially on long-flow parts or when temperature drops too much from gate to fill end. Increasing injection speed or optimizing mold temperature may help.
Surface Defects
Sink marks are one of the most common cosmetic issues in injection molding. They usually appear in thicker sections where material cools more slowly.
As the inner material cools and contracts, it can pull the outer surface inward, creating visible depressions. The best solution is often to pack more material into these areas before the gate freezes.
Helpful adjustments include:
- Increase packing pressure
- Use multi-stage pack/hold profiles
- Add pressure early in the pack cycle
Avoid excessive hold pressure, as it can create internal stress and lead to other quality issues.
TPS welcomes opportunities to help customers solve complex molding and manufacturing challenges with practical, cost-effective solutions. Partnering with the right supplier is key! For more information contact Jay Haverstraw via email.
