Injection molding machine injection molding process crystallinity
Thermoplastics can be divided into two types: crystalline plastics and amorphous (also known as amorphous) plastics.
The so-called crystallization phenomenon is that when the plastic is from the molten state to the condensation, the molecules move independently, completely in a disordered state, and the molecules stop moving freely, at a slightly fixed position, and have a tendency to make the molecules arranged into a regular model. a phenomenon.
As the standard for judging the appearance of these two types of plastics, it is possible to see the transparency of thick plastic parts of plastics. Generally, the crystalline material is opaque or translucent (such as POM), and the amorphous material is transparent (such as PMMA). However, there are exceptions. For example, poly(4) methyl decene is a crystalline plastic with high transparency, and ABS is amorphous but not transparent.
In the mold design and selection of injection molding machines, attention should be paid to the following requirements and precautions for crystalline plastics:
1 The amount of heat required for the temperature of the material to rise to the molding temperature is high, and equipment with a large plasticizing capacity is used.
2 When the cooling is turned back, the heat is released and it needs to be cooled sufficiently.
3 The difference in specific gravity between the molten state and the solid state is large, the molding shrinkage is large, and shrinkage cavities and pores are likely to occur.
4 fast cooling, low crystallinity, small shrinkage and high transparency. The crystallinity is related to the wall thickness of the plastic part, the wall thickness is slow to cool, the crystallinity is high, the shrinkage is large, and the physical properties are good. Therefore, the crystalline material should control the mold temperature as required.
5 Anisotropy is remarkable and internal stress is large. Molecules that have not crystallized after demolding tend to continue to crystallize, are in an energy-unbalanced state, and are prone to deformation and warpage.
6 The crystallization temperature range is narrow, and it is easy to inject the unmelted material into the mold or block the feed port.