Datum Target Placement Mechanics for Compliant Thermoplastic Parts
Datum target placement on compliant plastic parts requires locating pads on rigid structural features and using restrained-state inspection standards.
Thermal Warpage
Meaning
Distortion of a plastic part caused by non-uniform temperature distributions or differential cooling rates across the geometry. Development of thermal warpage occurs when different sections of the part shrink by different amounts as they reach thermal equilibrium. This phenomenon is a major cause of assembly failures where parts no longer align with their mating components.
Cooling Gradient
Thick sections retain heat longer than thin ribs, leading to delayed contraction in the center of the part. Internal stresses resulting from thermal warpage pull the geometry away from the intended shape as the hotter areas continue to shrink after the colder areas have solidified. Balancing the cooling circuit within the mold is the primary method for controlling this effect.
Material Sensitivity
Semi-crystalline polymers are more prone to this defect than amorphous ones because of the volume change associated with crystal formation. High levels of thermal warpage are often seen in glass-filled materials where the fibers restrict shrinkage in one direction but not the other. Selection of a resin with a lower shrinkage rate can mitigate the severity of the distortion.
Process Variable
Cycle time and melt temperature directly influence the magnitude of the temperature differences within the mold. Adjusting the hold time can sometimes reduce thermal warpage by ensuring the part is rigid enough to resist deformation before it is ejected. If the part is removed too early, the lack of support allows the internal thermal gradients to bend the material as it cools in the open air.