Meaning
Thermoplastic materials containing both disordered regions and organized crystalline structures exhibit high chemical resistance and thermal stability when processed under controlled conditions. These materials differ from amorphous resins because they can form a tightly packed molecular lattice as they cool. This duality allows them to offer the toughness of a disordered state alongside the strength of an ordered one.
Common examples include polyethylene, polypropylene, and nylon. The percentage of the material that becomes crystalline is a major factor in determining the final part properties. During the manufacturing process, these polymers undergo a significant reduction in volume as the crystals form.
This leads to higher shrinkage rates that must be carefully managed in the mold design. They also have a sharp melting point rather than a broad softening range. This characteristic requires precise temperature control in the extrusion or injection barrel.
Crystalline Growth
Formation of ordered regions occurs as the temperature of the melt drops below the crystallization point but remains above the glass transition. During this window, the polymer chains have enough mobility to fold into a lattice. The speed of cooling is the primary tool used by manufacturers to control this process.
Slow cooling encourages the growth of larger and more numerous crystals, which increases the stiffness and density of the part. In contrast, very fast cooling can trap the material in a more amorphous state. This control is vital for achieving the desired balance of properties in the finished product.
Process engineers monitor the mold temperature to ensure that the crystallization is consistent from part to part.
Processing Sensitivity
Response of semi-crystalline polymers to heat and pressure makes them more difficult to mold than their amorphous counterparts. Because they shrink more as they solidify, they are more prone to warping and internal voids. Holding pressure must be applied for a longer duration to feed more material into the mold as the crystals form.
If the cooling is uneven, different parts of the component will have different levels of crystallinity. This variation leads to internal stresses that can cause the part to fail prematurely. The orientation of the polymer chains during flow also affects the way the crystals grow.
This anisotropy means the part may be stronger in one direction than another. Designers must account for these effects when placing gates and cooling lines.
Part Stability
Long-term performance of components made from these materials depends on the stability of their internal structure. Semi-crystalline polymers can continue to crystallize slowly even after they have been ejected from the mold. This post-molding shrinkage can lead to dimensional changes weeks after the part was produced.
To prevent this, some parts are annealed at a high temperature to complete the crystallization process before they reach the customer. This treatment ensures that the dimensions remain stable over the life of the product. The presence of crystals also makes these materials opaque or translucent because the boundaries between regions scatter light.
This opacity is a simple indicator of the structural state of the plastic. High-performance grades are often reinforced with glass fibers to further improve their mechanical and thermal stability.
