Meaning
Crystalline expansion defines the speed at which spherical crystal structures grow outward from a nucleus within a cooling polymer melt. The spherulite growth rate is a fundamental property that dictates the final grain size of the solid plastic. This rate depends heavily on the temperature of the mold and the chemical structure of the resin.
Crystallization Velocity
Kinetic energy in the melt must be low enough for the chains to organize into a lattice. A fast spherulite growth rate leads to many small crystals, while a slow rate allows fewer crystals to grow to a larger size. This difference affects how the part will perform under mechanical stress.
Thermal Profile
Cooling speed across the thickness of a part is never uniform. In the center where the heat stays longer, the spherulite growth rate has more time to operate, resulting in larger crystals. At the surface, the rapid chill stops the growth quickly.
Clarity Limitation
Transparency in a part is reduced when crystals grow larger than the wavelength of light. If the spherulite growth rate is too high, the material will become opaque or hazy. Controlling the cooling cycle is the primary way a moulder maintains the optical quality of a semi-crystalline part.
