injection molding dimensional variation

Definition
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: a) Material variations such as property variations, varying moisture content, inconsistent melt and pigmentation; b) Process conditions variations such as inconsistent packing and varying mold and melt temperatures; c) 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 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. Each option hence requires consideration of all relevant aspects of the mold design

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