What shrinkage means in a molded part
Shrinkage is the dimensional change that occurs as a plastic part cools from the mold temperature to room temperature. Some shrinkage is expected and is included in mold design. The production problem starts when shrinkage is uneven, excessive or different from one batch to the next.
Crystalline materials such as PP, HDPE and POM usually show more molding shrinkage than amorphous materials such as ABS or PC. The exact result still depends on grade, flow direction, wall thickness, mold temperature and the time for which the cavity remains packed.
1. Main causes
- Insufficient holding pressure or holding time leaves the cavity under-packed.
- Thick sections cool slowly and continue to contract after ejection.
- Low melt or mold temperature can freeze the gate before packing is complete.
- Variation in moisture, regrind percentage or material grade changes the process response.
2. A practical troubleshooting sequence
First compare the part dimension at a fixed time after molding, because a measurement taken immediately after ejection may not represent the final size. Then check the gate freeze time, cavity pressure if available, cushion, part weight and cooling-water temperature. Increase packing gradually only while the gate remains open and the machine stays within its pressure limits.
Design changes may be needed when a single thick boss or rib is driving the problem. Core out heavy sections, balance wall thickness, improve cooling near the thick area and place gates so the last-to-fill region can be packed effectively. Confirm the final dimensions after the part has reached thermal equilibrium.
3. Prevention checklist
Keep the resin lot, drying history, mold temperature and cooling conditions recorded. Establish a part-weight window together with dimensional limits. This makes it easier to distinguish a packing problem from a mold-design or material-change problem.
How to use this guide
This page is designed to explain the relevant material, process, decision point or failure mode in practical terms. It is meant to support understanding, not replace a current supplier datasheet, process validation, regulatory requirement or engineering sign-off.
Typical material behavior, common process logic, defect patterns, material comparisons and practical decision framing.
Exact product specifications, grade-specific values, regulatory opinions, food-contact validation or factory-specific trials.
Before a production or commercial decision is made, check the exact grade, processing window, standards and local rules that apply to your application.
Production checks before changing the mold
Measure the same feature at a fixed conditioning time on several consecutive parts. Compare part weight, cavity balance and gate freeze behavior before changing steel. If the dimension moves with part weight, packing or material consistency is the first area to investigate.