Injection molding processing considerations (b)

(3), the crystalline thermoplastics no crystallization phenomenon can be divided into crystalline and non-crystalline plastics (also known as amorphous) plastic according condensation occurs when two categories. The so-called phenomenon of crystallization from the molten state to the plastic is condensed, the molecules move independent, completely unordered state, stops the free movement of molecules into, according to a fixed position slightly, and there is a tendency to make the molecular arrangement becomes a formal model phenomenon. As discriminating these two types of standard visual appearance of plastic transparent plastic thick plastic parts, generally crystalline material is opaque or translucent (such as POM, etc.), the amorphous material is transparent (such as PMMA, etc.). But there are exceptions, such as poly 

(4) methyl pentene to crystalline plastics there is a high transparency, ABS is an amorphous material but not transparent. Design and selection of the injection molding machine mold should pay attention to crystalline plastics have the following requirements and considerations:

① temperature rises to more heat molding temperature required, use the plasticizing capacity of the device.

② release when cooled back of the heat, to cool sufficiently.

③ molten and solid big difference in specific gravity, forming large shrinkage, prone to shrinkage, porosity.

④ cooling fast, low crystallinity, shrinkage, high transparency. The degree of crystallinity and related plastic wall thickness, thickness is slow cooling, high crystallinity, shrink large, good physical properties. So the crystalline material should be required to mold temperature must be controlled.

⑤ significant anisotropy, internal stress. After the release of the molecule has not crystallized continued tendency to crystallize, in energy imbalance, prone to distortion, warping.

⑥ crystallization temperature range is narrow, prone to not melt the end of the injection mold or clogged inlet. Fourth, the heat-sensitive plastic and susceptible to hydrolysis plastic

4.1 heat-sensitive means some plastics are more sensitive to heat, long time at a high temperature heating or inlet cross-section is too small, large shear, solid color temperature increased prone to degradation, decomposition tendency, with this feature plastic called heat-sensitive plastic. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polychlorotrifluoroethylene and the like. Heat-sensitive plastic decomposition by-products generated when monomers, gaseous, or solid, in particular, some decomposition of the gas on the human body, equipment, molds are stimulated, corrosive or toxic effects. Therefore, mold design, injection molding machines and molding the choice should be noted, should use screw injection molding machine, injection system should be large cross-section, molds and chrome barrel should not be there * angle hysteresis material, must strictly control the molding temperature, plastic the addition of stabilizers, weaken its thermal performance.

4.2 Some plastic (e.g. PC) containing a small amount of water, even, but at a high temperature, high pressure will be decomposed, this performance is called easily hydrolyzable, which must be pre-heated and dried. Fifth, stress cracking and melt fracture

5.1 Some plastic to stress sensitivity, easy to produce internal stress during molding and crisp and easy to crack, plastic parts in the external force or cracking which occurred in a solvent. To this end, in addition to improving additives in the feed opening crack resistance, the raw material should note dried, reasonable choice molding conditions to reduce stress and increase resistance to cracking. And should choose the right shape of plastic parts, inserts and other measures should not be set to minimize the stress concentration. Mold design should increase the stripping slope, choose a reasonable inlet and ejector mechanism, molding material temperature should be appropriately adjusted, mold temperature, injection pressure and cooling time, try to avoid too cold brittle plastic stripping , molded plastic parts also should be treated after improving crack resistance, eliminate stress and prohibiting contact with the solvent.

5.2 When some melting of the polymer melt flow rate at constant flow rate through the nozzle orifice exceeds a certain value, the apparent melt surface transverse cracks called melt fracture, undermine appearance and physical properties of plastic parts. Therefore, in the selection of high melt flow rate polymers and the like, should be increased nozzle, runner, inlet cross-section to reduce the injection speed, material temperature increase. Sixth, thermal properties and cooling rate

6.1 There are different kinds of plastic specific heat, thermal conductivity, heat distortion temperature thermal properties. High specific heat of plastics requires heat, should be used plasticizing capacity of the injection molding machine. High heat distortion temperature plastic cooling time can be as short, releasing early, but after stripping to prevent deformation during cooling. Low thermal conductivity of plastic cooling rate (such as ionic polymers slow cooling rate), it must be cooled sufficiently to strengthen the mold cooling effect. Hot runner mold for low specific heat, high thermal conductivity of plastics. Large specific heat, thermal conductivity, low thermal deformation temperature is low, slow cooling is not conducive to high-speed plastic molding, injection molding machine must use the appropriate and strengthen the mold cooling.

6.2 All kinds of plastic and plastic parts according to their shape characteristics of the types, requirements must maintain proper cooling rate. So mold heating and cooling systems must be set according to molding requirements to maintain a certain mold temperature. When the material temperature should be raised when the mold temperature is cooled to prevent deformation of plastic parts after stripping, reduce cycle time, reduce the crystallinity. When the plastic is not sufficient to heat the mold to maintain a certain temperature, the mold should be provided with a heating system, the mold is maintained at a constant temperature to control the cooling rate, ensure liquidity, improve conditions or to control the filling plastic allowed to cool slowly, thick-walled plastic parts inside and outside to prevent uneven cooling and improve the crystallinity degree. Good liquidity, large forming area, press the uneven temperature materials molding plastic case is sometimes used interchangeably to be heated or cooled or local heating and cooling and use. For this reason the mold should be provided with appropriate cooling or heating system.

7, hygroscopic

Plastics additives due to different degrees of water affinities, so the plastic can be divided into moisture, water and non-absorbent adhesion is not easy to adhere to the two water and feed water content must be controlled within the allowable range within, or in high temperature, high pressure water into the gas or the occurrence of hydrolysis, the resin foaming, poor decreased mobility, appearance and mechanical properties. So hygroscopic plastic must require appropriate heating methods and specifications for preheating, when using to prevent further moisture.

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