Meaning
Organized plates of folded polymer chains represent the primary ordered structures in semi-crystalline thermoplastics. These structures, known as crystalline lamellae, dictate the density, chemical resistance, and rigidity of moulded parts. They form during the cooling phase of injection moulding, where polymer chains fold back and forth to create highly ordered crystalline regions separated by amorphous domains.
Insufficient cooling time can limit their growth, which leads to dimensional instability and warp.
Structural Arrangement
Chain folding must occur in a parallel fashion to form these structures during melt solidification. This thickness ranges from ten to twenty nanometers, depending on the cooling rate. Fast cooling rates produce thinner regions, whereas slow cooling yields thicker and more stable domains.
Thermal Transition
Melting temperature is directly tied to the thickness and perfection of these folded domains. Thicker crystalline lamellae require more thermal energy to disrupt, raising the melting point of the processed resin. During post-moulding shrinkage, reorganizations within these domains can cause ongoing dimensional changes if the part is exposed to temperatures above its glass transition temperature.
Annealing processed parts can stabilize these structures.
Mechanical Strength
Tensile modulus and yield strength increase as the volume fraction of these organized plate structures rises. The presence of crystalline lamellae prevents polymer chains from sliding past each other easily under applied loads. However, if the amorphous regions between them lack sufficient tie molecules, the moulded part may suffer from brittle failure under impact.
Controlling the ratio of crystalline and amorphous phases is therefore a requirement for balanced mechanical performance.
