Why Do Injection Molded Part Dimensions Keep Changing?

Part dimensions are controlled by material shrinkage. Any factor that changes shrinkage will eventually change part dimensions.

For precision electronic parts, automotive components, and engineering plastic housings, even a 0.2% shrinkage change may push dimensions outside tolerance.

Packing pressure matters, but it is only one part of the system. Dimensional drift is usually caused by several variables moving together.

Many factories can produce acceptable parts, but only at the edge of the material processing window.

This means the process works only when temperature, pressure, drying, cooling, and machine response are all nearly ideal. When ambient conditions or equipment state changes slightly, dimensions begin to drift.

A robust process should run near the center of the processing window, not on the edge. DOE testing and mold flow review can help identify stable ranges for packing pressure, melt temperature, mold temperature, and injection speed.

Materials such as PC, PBT, PA, and reinforced compounds can be highly sensitive to temperature changes.

If the theoretical mold temperature tolerance is +/-3 degC but actual equipment control is +/-5 degC, the difference may be enough to change melt viscosity, packing efficiency, and cooling rate.

For a 100 mm precision part, a shrinkage variation of 0.2% can create dimensional movement of around 0.2 mm. That is often enough to cause assembly failure.

The mold temperature setpoint is not the same as actual part temperature at ejection.

If one region of the part is 10 degC hotter than another at demolding, local shrinkage can continue after ejection. This leads to dimension drift, warpage, and unstable measurement results.

Common causes include: - Uneven cooling channel layout - Scale or blockage inside water lines - Hot spots near thick sections - Core and cavity temperature imbalance - Insufficient cooling near inserts

Even when the material grade is the same, lot-to-lot variation can affect dimensions.

Important variables include: - Melt flow index variation - Moisture content - Filler content - Glass fiber length distribution - Recycled content ratio - Additive dispersion

For precision parts, incoming material checks and drying control are part of dimensional control. The molding machine cannot fully compensate for unstable material behavior.

Increasing packing pressure may reduce shrinkage in some areas, but it can also create new problems: - Flash - Higher residual stress - Warpage - Gate-area overpacking - Difficult demolding

If the gate has frozen, additional packing pressure cannot enter the cavity effectively. If cooling is uneven, pressure adjustment may only move the dimensional problem from one area to another.

Dimensional drift can also come from machine behavior.

Check: - Cushion stability - Screw recovery consistency - Check ring leakage - Barrel temperature fluctuation - Hydraulic or servo response - Clamp force consistency - Actual transfer position

If shot volume or transfer timing changes, the part may show dimensional variation even when the screen settings appear unchanged.

A practical dimensional stability plan should include: - DOE testing to define a robust process window - Actual mold surface temperature measurement - Part weight monitoring - Material moisture and lot control - Cooling circuit maintenance - Gate freeze study - Measurement after full cooling, not only immediately after ejection

The goal is not constant adjustment. The goal is a process that stays stable under normal production variation.