Troubleshooting dimensional variation problems

Dimensional variation is characterized by the molded part dimension varying from batch to batch, or from shot to shot while the machine settings remain the same.

Causes

• Inconsistent shrinkage. Resulting from:

  1. Material variations such as property variations, varying moisture content, inconsistent melt and pigmentation;
  2. Process conditions variations such as inconsistent packing and varying mold and melt temperatures;
  3. Machine variations such as a damaged check ring and unstable controller.

• Narrow molding window.

Remedies

• Remove excessive moisture. Material suppliers can provide optimum drying conditions for the specific materials.
• Reduce regrind material. Contact the material suppliers to get the recommended levels of regrind to use.
• Optimize the runner system design. Poor design could cause material degradation through shear heating or inconsistent packing.
• Replace the check ring if it is broken or worn out.
• Ensure uniform mold temperature. Make sure the mold temperature is uniform by checking the cooling system.
• Set processing conditions within the molding window.
• Reduce differential shrinkage.

Solving one problem can often introduce other problems to the injection molding process. Thus, each option requires consideration of all relevant aspects of the mold design specification.