Dynamic Cavity Pressure Integral Correlation with Part Dimensions in Injection Moulding
Dynamic cavity pressure curve integration over hold time governs volumetric polymer density and predicts final part dimensions before tool ejection occurs.
Semi-Crystalline Morphology
Meaning
Molecular structure in certain thermoplastic resins features a mix of ordered, crystalline regions and disordered, amorphous domains. This dual-phase structure is known as semi-crystalline morphology, and it develops as the polymer cools from the melt and chains fold into ordered lamellar structures. The balance between these phases determines both the mechanical strength and the chemical resistance of the molded parts.
Cooling Rate
Thermal history during the molding process directly influences the degree of crystallinity and the size of the spherulites. Rapid cooling in cold tools limits chain mobility, resulting in a low degree of crystallinity and a more amorphous state. Conversely, slow cooling in warmer molds allows chains to organize, which maximizes the semi-crystalline morphology and improves the thermal stability of the polymer.
Material Strength
The presence of crystalline regions provides high chemical resistance and stiffness. However, semi-crystalline morphology also makes the polymer more prone to anisotropy and post-moulding shrinkage. Because the crystalline regions pack much more tightly than the amorphous regions, parts experience high volumetric reduction during cooling.
Shrinkage Behavior
Dimensional control requires careful tool design to compensate for the high shrinkage rates associated with crystalline structures. Unlike amorphous resins, semi-crystalline morphology results in high, non-uniform shrinkage that can cause severe warpage if the cooling is not uniform across the part. Unbalanced flow can also align the crystals, further increasing directional differences in shrinkage.
Moulders must run mold-filling simulations to predict these effects and adjust the cooling channel design to ensure uniform thermal distribution.