Meaning
Post-moulding shrinkage describes the slow change in part geometry that occurs after ejection from the tool. This progressive instability is called dimensional drift and occurs as the polymer chains continue to relax and crystallize over hours or days. Semi-crystalline materials like polyamide are particularly prone to this post-moulding movement as they absorb ambient moisture.
Environmental Influence
Moisture absorption and thermal cycles accelerate the physical movement of the polymer chains. This exposure causes dimensional drift, which can push a part outside its specified tolerance limits long after it has been inspected. Storing parts in sealed moisture-barrier bags represents a common method for preventing early exposure to ambient humidity.
Moulding Parameters
Cooling time and tool temperature directly dictate the level of internal stress locked within the solidifying plastic. Insufficient cooling time during the cycle intensifies dimensional drift because the molecules are ejected before they can achieve a stable state. Raising the mould temperature helps by encouraging crystallization to occur inside the tool rather than after ejection.
Lower injection speeds can also reduce orientation and subsequent relaxation.
Measurement Strategy
Metrology protocols must specify the exact time elapsed between ejection and part measurement to ensure repeatability. Tracking dimensional drift over a seventy-two hour period allows engineers to establish the true final dimensions of the production run. A structured window of observation is necessary before releasing high-precision components for assembly or assembly validation.
