First, liquidity
Good fluidity: Pa, PE, PS, PP, Ca,
Medium fluidity: polystyrene series resin, PMMa, POM, polyphenylene ether;
Poor fluidity: PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastic.
The fluidity of various plastics also varies with each molding factor. The main factors affecting are as follows:
1. When the temperature of the material is high, the fluidity increases, but different plastics also have differentities. The fluidity of PS, PP, Pa, PMMa, modified polystyrene, PC, Ca and other plastics varies greatly with temperature. For PE and POM, the temperature increase and decrease has little effect on the fluidity. Therefore, the former should adjust the temperature to control the fluidity during molding.
2. When the pressure injection pressure is increased, the molten material is subjected to large shearing action and fluidity is also increased, especially PE and POM are sensitive, so the injection pressure should be adjusted to control the fluidity during molding.
3. The form, size, arrangement, cooling system design, flow resistance of the melt, etc. of the mold structure casting system directly affect the actual fluidity of the molten material in the cavity. Where the molten material is lowered in temperature and the fluidity resistance is increased. Then the liquidity is reduced.
Second, crystallinity
Thermoplastics can be divided into two types: crystalline plastics and amorphous plastics according to the phenomenon of crystallization when they are condensed.
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 a criterion for judging the appearance of these two types of plastics, it is possible to visually recognize 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, etc.).
However, there are exceptions. For example, polymethyl decene is a crystalline plastic with high transparency, and aBS is amorphous but not transparent.
Third, heat sensitive plastics and easily hydrolyzed plastics
1. Heat sensitivity means that some plastics are sensitive to heat. When the temperature is high at high temperature or the cross section of the feed port is too small, when the shearing effect is large, the temperature of the material is increased, and the tendency of discoloration, degradation and decomposition is easy. The characteristic plastic is called heat sensitive plastic.
Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polychlorotrifluoroethylene and so on. The heat-sensitive plastics produce by-products such as monomers, gases, solids, etc. during decomposition, and in particular, some decomposition gases have irritating, corrosive or toxic effects on the human body, equipment and molds.
2. Some plastics (such as PC) decompose even under high temperature and high pressure even if they contain a small amount of water. This property is called easy hydrolysis and must be preheated and dried.
Fourth, stress cracking and melt fracture
1. Some plastics are sensitive to stress. They are prone to internal stress during forming and are brittle and fragile. The plastic parts are cracked under the action of external force or under the action of solvent.
To this end, in addition to adding additives to the raw materials to improve the crack resistance, the raw materials should be dry, reasonable selection of molding conditions to reduce internal stress and increase crack resistance, and should choose a reasonable plastic shape, not suitable for setting Measures such as parts to minimize stress concentration.
When designing the mold, the draft should be increased, and a reasonable feeding port and ejection mechanism should be selected. The material temperature, mold temperature, injection pressure and cooling time should be properly adjusted during molding to avoid mold release when the plastic parts are too cold and brittle. After molding, the plastic parts should also be post-treated to improve the crack resistance, eliminate the internal stress and prohibit contact with the solvent.
2. When the polymer melt of a certain melt flow rate exceeds a certain value when passing through the nozzle hole at a constant temperature, a significant transverse crack occurs on the surface of the melt, which is called melt fracture, which impairs the appearance and physical properties of the plastic part.
Therefore, in the selection of polymers with high melt flow rate, the nozzle, runner, and feed port sections should be increased to reduce the injection speed and increase the material temperature.
5. Thermal performance and cooling rate
1. Various plastics have different specific heat, thermal conductivity, heat distortion temperature and other thermal properties. When the plasticization is higher than the heat, it requires a large amount of heat, and an injection molding machine with a large plasticizing capacity should be used. High heat distortion temperature The plastic can be cooled for a short period of time, and the mold release is early, but it is prevented from cooling deformation after demolding.
Plastics with low thermal conductivity have a slow cooling rate (such as ionic polymers and other cooling rates are extremely slow), so they must be sufficiently cooled to enhance the mold cooling effect. Hot runner molds are suitable for plastics with lower heat and high thermal conductivity. Plastics with higher specific heat, lower thermal conductivity, lower heat distortion temperature and slower cooling rate are not conducive to high-speed molding, and proper injection molding machines and mold cooling must be used.
2. Various plastics must maintain proper cooling rate according to their type characteristics and shape of plastic parts. Therefore, the mold must be set to the heating and cooling system according to the molding requirements to maintain a certain mold temperature. When the material temperature raises the mold temperature, it should be cooled to prevent deformation of the plastic parts after demolding, shorten the molding cycle, and reduce the crystallinity.
When the residual heat of the plastic is not enough to keep the mold at a certain temperature, the mold should be provided with a heating system to keep the mold at a certain temperature to control the cooling rate, ensure the fluidity, improve the filling conditions or control the plastic parts to cool slowly. Prevent uneven cooling inside and outside of thick-walled plastic parts and improve crystallinity.
6. Hygroscopicity
Plastics have various additives to make them have different degrees of moisture. Therefore, plastics can be roughly divided into two types: moisture absorption, adhesion moisture, non-absorbent water and non-stick moisture. The water content in the material must be controlled within the allowable range. In the case, the water becomes gas or hydrolyzes under high temperature and high pressure, causing foaming of the resin, deterioration of fluidity, and poor appearance and mechanical properties. Therefore, hygroscopic plastics must be preheated as required by appropriate heating methods and specifications to prevent re-absorption during use.
