Thermoplastic TPO can be divided into crystalline and non-crystalline types based on their lack of crystallization during solidification.
The so-called crystallization phenomenon is a phenomenon where the plastic changes from a state of independent molecular movement in the molten state to a state where the molecules stop free movement and are slightly fixed in position, with a tendency to arrange the molecules into regular models during solidification.
The appearance standard for distinguishing between these two types of plastics is the transparency of thick-walled plastic parts. Generally, crystalline plastics are opaque or translucent (such as polyoxymethylene), while amorphous plastics are transparent (such as organic glass). There are exceptions, however; for example, poly(4-methylpentene) is a crystalline plastic with high transparency, and ABS is an amorphous plastic, but is not transparent.
When designing molds and selecting injection molding machines, the following requirements and considerations should be given to crystalline plastics:
More heat is required to raise the temperature of the material to the molding temperature of TPO thermoplastic, so equipment with high plasticizing capacity should be used.
The cooling and solidification process releases a lot of heat, so sufficient cooling is required.
The difference in density between the molten and solid states is large, resulting in significant shrinkage during the molding process, and leading to shrinkage cavities and porosity.
Fast cooling results in low crystallinity, small shrinkage, and high transparency. Crystallinity is related to the wall thickness of the part, with high crystallinity, high shrinkage, and good properties for thicker parts. Therefore, the mold temperature must be controlled as required.
There is significant anisotropy and internal stress, and molecules that have not yet crystallized after demolding tend to continue crystallizing and are in an energy-imbalanced state, making deformation and warping more likely.
The temperature range for crystallization is narrow, making it prone to powder injection into the mold or clogging the feed port.
Thermosensitive refers to the sensitivity of certain plastics to heat; at high temperatures, with prolonged heating or when the cross-sectional area of the feed port is small and the shearing effect is large, the material temperature easily discolors, undergoes degradation, and exhibits a tendency towards decomposition. Plastics with these characteristics are referred to as thermosensitive plastics.
Examples include hard polyvinyl chloride, polyvinyl chloride, vinyl acetate copolymer, polyoxymethylene, polytrifluorochloroethylene, etc. Thermosensitive plastics generate by-products such as monomers, gases, and solids during decomposition, and some decomposition gases are irritating, corrosive, or toxic to humans, equipment, and molds. Therefore, during mold design, selection of injection molding machines, and production, screw-type injection molding machines should be selected, the pouring system cross-sectional area should be large, the mold and barrel should be plated with chrome, there should be no dead corners for stagnant materials, the molding temperature should be strictly controlled, and stabilizers should be added to the plastic to weaken its thermosensitivity.
Some plastics (such as polycarbonate) undergo decomposition even with a small amount of moisture under high temperature and pressure; this property is referred to as hydrolysis, and it requires preheating and drying.
