The mold design should be based on the fluidity of the plastic used, choose a reasonable structure. During molding, the material temperature, mold temperature, injection pressure, injection speed and other factors can also be controlled to appropriately adjust the filling conditions to meet the forming requirements.
(c) Crystallinity
Thermoplastics can be classified into crystalline plastics and non-crystalline (also known as amorphous) plastics, as they do not exhibit crystallization when condensed. The so-called crystallization phenomenon is that when the plastic melts from the molten state to the condensation, the molecules move independently, completely in an out-of-order state, and become molecules that stop moving freely, occupying a slightly fixed position, and have a tendency to align molecules into a regular model. This phenomenon.
As a criterion for judging the appearance of these two types of plastics, the visible plastics may be opaque or translucent (such as polyoxymethylene, etc.), and the amorphous material may be transparent (such as plexiglass, etc.). . However, there are exceptions, such as poly (4) methyl decene is a crystalline material but high transparency, ABS is amorphous but not transparent.
In the mold design and selection of injection machine should pay attention to the following requirements for crystallization materials:
(1) The amount of heat required to raise the temperature of the material to the forming temperature is large, and equipment with a large plasticizing capacity must be used.
(2) When condensing, heat is released and sufficient cooling is required.
(3) The difference in the specific gravity between the molten state and the solid state is large, the molding shrinkage is large, and shrinkage pores and pores are likely to occur.
(4) Cooling has low crystallinity, small shrinkage, and high transparency. The degree of crystallinity is related to the wall thickness of the plastic parts. The cooling of the wall thickness results in high crystallinity, shrinkage, and good physical properties. Therefore, the crystalline material should control the mold temperature as required.
(5) Significant anisotropy and high internal stress. The molecules that have not been crystallized after demolding tend to crystallize, are in an energy imbalance state, and are prone to deformation and warpage.
(6) The crystal melting point has a narrow range, and unmelted powder is easily injected into the mold or the feed inlet is blocked.
(D) heat sensitivity and water sensitivity
1. Heat-sensitive plastic means that some plastics are relatively sensitive to heat. When the heating time is long at high temperature or the cross-section of the feed inlet is too small, when the shearing action is large, the temperature of the material is prone to discoloration, depolymerization, and decomposition. Plastics of this nature are called heat-sensitive plastics.
Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, polyoxymethylene, polychlorotrifluoroethylene and so on. Heat-sensitive plastics produce byproducts such as monomers, gases, and solids when they are decomposed. In particular, some of the decomposed gases have irritation, corrosion, or toxicity to the human body, equipment, and molds. Therefore, mold design, selection of injection molding machine and molding should pay attention to, should use screw injection machine, casting system should be large cross-section, mold and cartridge should be chrome plated, there must be no dead material, must strictly control the forming temperature, plastic added Stabilizer, attenuates thermal performance.
2, some plastics (such as polycarbonate) even if there is a small amount of water, but also under high temperature, high pressure decomposition occurs, this property is called water sensitivity, which must be heated before drying. (To be continued)
(c) Crystallinity
Thermoplastics can be classified into crystalline plastics and non-crystalline (also known as amorphous) plastics, as they do not exhibit crystallization when condensed. The so-called crystallization phenomenon is that when the plastic melts from the molten state to the condensation, the molecules move independently, completely in an out-of-order state, and become molecules that stop moving freely, occupying a slightly fixed position, and have a tendency to align molecules into a regular model. This phenomenon.
As a criterion for judging the appearance of these two types of plastics, the visible plastics may be opaque or translucent (such as polyoxymethylene, etc.), and the amorphous material may be transparent (such as plexiglass, etc.). . However, there are exceptions, such as poly (4) methyl decene is a crystalline material but high transparency, ABS is amorphous but not transparent.
In the mold design and selection of injection machine should pay attention to the following requirements for crystallization materials:
(1) The amount of heat required to raise the temperature of the material to the forming temperature is large, and equipment with a large plasticizing capacity must be used.
(2) When condensing, heat is released and sufficient cooling is required.
(3) The difference in the specific gravity between the molten state and the solid state is large, the molding shrinkage is large, and shrinkage pores and pores are likely to occur.
(4) Cooling has low crystallinity, small shrinkage, and high transparency. The degree of crystallinity is related to the wall thickness of the plastic parts. The cooling of the wall thickness results in high crystallinity, shrinkage, and good physical properties. Therefore, the crystalline material should control the mold temperature as required.
(5) Significant anisotropy and high internal stress. The molecules that have not been crystallized after demolding tend to crystallize, are in an energy imbalance state, and are prone to deformation and warpage.
(6) The crystal melting point has a narrow range, and unmelted powder is easily injected into the mold or the feed inlet is blocked.
(D) heat sensitivity and water sensitivity
1. Heat-sensitive plastic means that some plastics are relatively sensitive to heat. When the heating time is long at high temperature or the cross-section of the feed inlet is too small, when the shearing action is large, the temperature of the material is prone to discoloration, depolymerization, and decomposition. Plastics of this nature are called heat-sensitive plastics.
Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, polyoxymethylene, polychlorotrifluoroethylene and so on. Heat-sensitive plastics produce byproducts such as monomers, gases, and solids when they are decomposed. In particular, some of the decomposed gases have irritation, corrosion, or toxicity to the human body, equipment, and molds. Therefore, mold design, selection of injection molding machine and molding should pay attention to, should use screw injection machine, casting system should be large cross-section, mold and cartridge should be chrome plated, there must be no dead material, must strictly control the forming temperature, plastic added Stabilizer, attenuates thermal performance.
2, some plastics (such as polycarbonate) even if there is a small amount of water, but also under high temperature, high pressure decomposition occurs, this property is called water sensitivity, which must be heated before drying. (To be continued)
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