Crystalline plastics have a distinct melting point, and their molecules are arranged in a regular pattern when solid. Regularly arranged regions are called crystalline regions, and disordered regions are called amorphous regions (or "amorphous areas"). Percentage of crystalline regions is called crystallinity. Polymers with a crystallinity of 80% or higher are generally called crystalline plastics. Common crystalline plastics include: polyethylene (PE), polypropylene (PP), polyoxymethylene (POM), polyamide (PA6), polyamide (PA66), polyethylene terephthalate (PET), and polyethylene terephthalate (PBT), etc.

 

(1) Mechanical Properties: Crystallization makes plastics brittle (reduced impact strength), also reduces toughness and ductility, especially when spherulites are large.
(2) Optical Properties: Crystallization makes plastics opaque because light scattering occurs at interface between crystalline and amorphous regions. However, reducing spherulite size to a certain extent not only improves strength of plastic (reducing intergranular defects) but also increases its transparency (when spherulite size is smaller than wavelength of light, no scattering occurs). This explains why crystallization ≠ opacity!
(3) Thermal Properties: Crystalline plastics do not exhibit a highly elastic state when temperature rises; when temperature rises to melting temperature Tm, they exhibit a viscous flow state. Therefore, service temperature of crystalline plastics increases from Tg (glass transition temperature) to below Tm (melting temperature).
(4) Solvent resistance and permeability are improved because crystallization makes polymer molecules more tightly packed.

(1) Polymer chain structure: Polymers with good symmetry, few or no branches, small side group volume, strong intermolecular forces tend to come together more easily and crystallize more readily.
(2) Temperature: Polymers move from disordered clumps to surface of growing crystals. Higher mold temperatures increase mobility of polymers, thus accelerating crystallization.
(3) Pressure: External forces during cooling can promote polymer crystallization. Therefore, injection pressure and holding pressure can be increased during production to control crystallinity of crystalline plastics.
(4) Nucleating agents: Low temperatures promote rapid nucleation but slow down grain growth. To eliminate this contradiction, nucleating agents are added to molding material, allowing plastic to crystallize rapidly at high mold temperatures. Addition of nucleating agents usually refines crystals. This explanation clarifies true role of nucleating agents.

(1) Crystalline plastics require more energy to break down crystal lattice during melting. Therefore, a large amount of heat is needed to transform from a solid to a molten melt. Thus, plasticizing capacity of injection molding machine must be high, and maximum injection volume must be correspondingly increased.
(2) Crystalline plastics have a narrow melting point range. To prevent material from crystallizing and clogging nozzle when nozzle temperature decreases, nozzle orifice diameter should be appropriately increased, and a heating coil capable of independently controlling nozzle temperature should be installed. Especially for materials with high melting points, such as glass fiber reinforced nylon 6, nozzle orifice can easily freeze if a separate heating coil is not added.
(3) Since mold temperature has a significant impact on crystallinity, mold should have as many water channels as possible to ensure uniform mold temperature during molding.
(4) Crystallinity undergoes significant volume shrinkage during crystallization, resulting in a large molding shrinkage rate. Therefore, molding shrinkage rate must be carefully considered in mold design.
(5) Due to significant anisotropy and high internal stress, location and size of gate, as well as location and size of reinforcing ribs, must be carefully considered in mold design. Otherwise, warping deformation is likely to occur, and it is quite difficult to improve it through molding process.
(6) Crystallinity is related to wall thickness of plastic part. Thicker walls cool more slowly, resulting in higher crystallinity and greater shrinkage, making it prone to shrinkage cavities and air holes. Therefore, wall thickness of plastic part must be carefully controlled in mold design.

(1) High heat release during cooling necessitates thorough cooling. Careful control of cooling time is crucial during high mold temperature molding.
(2) Large density difference between molten and solid states leads to significant shrinkage during molding, increasing risk of shrinkage cavities and air pockets. Proper holding pressure settings are essential.
(3) Low mold temperature results in rapid cooling, low crystallinity, minimal shrinkage, and high transparency. Crystallinity is related to wall thickness of plastic part; thicker parts cool more slowly, resulting in higher crystallinity, greater shrinkage, and better physical properties. Therefore, mold temperature must be controlled according to requirements for crystalline plastics.
(4) Significant anisotropy and high internal stress mean that uncrystallized molecules tend to continue crystallizing after demolding, resulting in an energy imbalance that easily leads to deformation and warping. Appropriate increases in material temperature and mold temperature, along with moderate injection pressure and injection speed, are necessary.