Designing Restrained State Quality Fixtures for Flexible Plastic Assemblies
Restrained state fixtures simulate final assembly fasteners with calibrated clamps on datum targets to measure flexible plastic profiles accurately.
Free State Warpage
Meaning
Unconstrained shape of a part that represents the deviation from nominal after cooling is finished. Free state warpage is the physical result of non-uniform cooling and molecular orientation within the polymer matrix. High-shrinkage resins like polyethylene are particularly prone to this effect when wall thicknesses vary across the geometry.
It represents the true shape of the part before any assembly forces are applied.
Cooling Influence
Thermal gradients across the mould surface create the conditions for this deformation. If one side of a part cools faster than the other, the resulting density differences pull the material out of its intended alignment. The free state warpage is often measured using a non-contact laser scanner to avoid disturbing the part shape with probe pressure.
Correcting this issue usually requires adjusting the cooling circuit or extending the cycle time.
Material Selection
Amorphous resins generally exhibit lower levels of this phenomenon than semi-crystalline types. Because polycarbonate molecules are disorganized, they do not pack into tight structures that drive high shrinkage. Engineers often switch to a filled resin to reduce free state warpage when tight tolerances are required.
Glass fibers or mineral fillers provide a structural skeleton that resists the pulling forces of the cooling resin.
Economic Impact
Excessive distortion leads to high scrap rates.