Meaning
Morphological order within solid macromolecules indicates the fraction of molecular chains arranged into regular, repeating crystalline lattices compared to disordered amorphous regions. Expressed as a percentage, polymer crystallinity governs density, chemical resistance, stiffness, and optical clarity in semi-crystalline thermoplastics like polyamide, polypropylene, and polybutylene terephthalate. Differential scanning calorimetry calculates this value by dividing the measured enthalpy of fusion by the theoretical enthalpy of a hundred-percent crystalline sample.
The property applies strictly to semi-crystalline polymers, ceasing to exist in entirely amorphous materials such as polystyrene and polymethyl methacrylate.
Cooling Influence
Processing temperatures dictate the extent and perfection of crystalline domain development during the moulding cycle. When molten resin contacts a cold cavity wall, rapid quenching arrests molecular motion, locking a low-crystallinity amorphous skin onto the part exterior. In contrast, higher mould temperatures allow polymer chains sufficient thermal mobility to fold into structured lamellae and dense spherulites.
Moulders select mould coolant temperatures to target specific crystallinity percentages, directly influencing tensile modulus and heat deflection temperatures. Extended cooling cycle times raise crystallinity levels, increasing overall part density and reducing post-mould elongation at break.
Shrinkage Impact
Volumetric contraction during solidification is directly proportional to the formation of tightly packed crystalline structures. High polymer crystallinity causes steep volumetric shrinkage, often exceeding two percent in natural polyethylene and polyoxymethylene. As crystalline domains contract, non-uniform cooling across varying wall sections induces severe internal stresses, resulting in post-ejection part warpage.
Regrind materials containing thermal degradation products or nucleating contaminants alter crystallization kinetics, causing unpredictable shrinkage variance between production shifts. Moulders must balance packing pressure and cycle duration to minimize internal voids caused by crystal phase contraction.
Testing Variance
Standard laboratory testing protocols quantify crystal fractions through differential scanning calorimetry, wide-angle X-ray diffraction, or density gradient columns. Each analytical method measures different physical manifestations of lattice order, yielding slight variations in absolute percentage values. Datasheet values represent ideal slow-cooled laboratory specimens, diverging sharply from the fast-cycle conditions experienced inside high-cavitation production moulds.
