Meaning
Geometrical distortion happens when different regions of a moulded part contract at different rates during the cooling process. This polypropylene warpage is a common challenge for semi-crystalline resins because their shrinkage is highly sensitive to temperature gradients and molecular orientation. The resulting bow, twist or sink is the physical consequence of internal stresses being released.
Differential Shrinkage
Variations in the cooling rate between the core and the cavity sides of the tool can cause the part to bend toward the hotter surface. Because the polymer chains align in the direction of the melt flow, the polypropylene warpage is often more significant in the transverse direction than in the longitudinal direction. Designers use ribbing and balanced gating to counteract these tendencies.
Material Influence
Controlling the polypropylene warpage requires a precise balance of injection speed, pack pressure, hold time and cooling duration. Even small changes in the ambient temperature of the shop floor can affect the stability of the moulding run. Because the material is so sensitive, minor variations in the crystallization rate can cause a batch to fall outside of its tolerance limits.
Correction Strategy
Adjusting the tool cooling layout is often the most effective way to manage these dimensional errors. This involves balancing the flow of coolant to ensure that both sides of the part reach the same temperature simultaneously. Strategic gate placement also helps by reducing the pressure drop across the cavity.
