Meaning
Radial growth of lamellar crystals from a central nucleus creates the primary microstructure of semi-crystalline thermoplastics. Spherulitic crystallization determines the mechanical and optical properties of materials like polypropylene or acetal. These structures grow until they impinge upon one another, forming a complex grain boundary network.
The size of these spheres is a result of the cooling rate and the presence of nucleating agents.
Nucleation Rate
High densities of nuclei lead to the formation of many small spheres rather than a few large ones. This fine-grained structure generally improves the toughness and clarity of the finished part.
Optical Clarity
Large crystalline structures scatter light and make a polymer appear opaque or cloudy. Rapid cooling in a cold tool prevents the growth of these large structures, resulting in a more transparent part. This is why thin sections of a part might be clearer than the thick ribs or bosses.
Controlling the temperature of the mould allows for the adjustment of the aesthetic properties of the plastic. Additives like clarifiers work by providing more nucleation sites to keep the spheres smaller than the wavelength of visible light.
Impact Strength
Grain boundaries between the spheres are often the points where cracks initiate or propagate through the material. A part with smaller, more uniform crystals will resist impact better than one with coarse structures.
