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Precautions for automotive plastic parts

Update:08-06-2021
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Seven precautions for injection molding: 1. Shrinkage r […]

Seven precautions for injection molding: 1. Shrinkage rate. 2. Liquidity. 3. Crystallinity. 4. Heat-sensitive plastics and easily hydrolyzed plastics. 5. Stress cracking and melt fracture. 6. Thermal performance and cooling rate. 7. Hygroscopicity.
Detailed analysis to destroy one by one
      1. Shrinkage rate
   The form and calculation of thermoplastic molding shrinkage are as mentioned above. The factors that affect thermoplastic molding shrinkage are as follows:
        1.1 Plastic Varieties During the molding process of thermoplastic plastics, there are still volume changes caused by crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation and other factors, so compared with thermoset plastics, the shrinkage rate is higher. Large, wide range of shrinkage rate and obvious directionality. In addition, the shrinkage rate after molding, annealing or humidity conditioning is generally greater than that of thermosetting plastics.
   1.2 The characteristics of the plastic part during molding, the molten material contacts the surface of the cavity and the outer layer is immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part is slowly cooled to form a high-density solid layer with large shrinkage. Therefore, the wall thickness, slow cooling, and high-density layer thickness will shrink more. In addition, the presence or absence of inserts and the layout and quantity of inserts directly affect the direction of material flow, density distribution and shrinkage resistance, so the characteristics of plastic parts have a greater impact on shrinkage and directionality.
   1.3 The feed inlet form, size, distribution, these factors directly affect the material flow direction, density distribution, pressure maintaining and shrinking effect and molding time. Direct feed ports and feed ports with large cross-sections (especially thicker cross-sections) have small shrinkage but large directionality, and short feed ports with short width and length have small directivity. The ones that are close to the feed inlet or parallel to the direction of the material flow will shrink more.
   1.4 Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for crystalline materials, the shrinkage is greater due to high crystallinity and large volume changes. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic part, which directly affects the size and direction of the shrinkage of each part. In addition, holding pressure and time also have a greater impact on contraction, and the contraction is smaller but the directionality is larger when the pressure is high and the time is long. The injection pressure is high, the viscosity difference of the molten material is small, the interlayer shear stress is small, and the elastic rebound after demolding is large, so the shrinkage can also be reduced by an appropriate amount. The material temperature is high, the shrinkage is large, but the directionality is small. Therefore, adjusting the mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of the plastic part.
   When designing the mold, according to the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the size and distribution of the inlet form, the shrinkage rate of each part of the plastic part is determined according to experience, and then the cavity size is calculated. For high-precision plastic parts and when it is difficult to grasp the shrinkage rate, the following methods should generally be used to design the mold:
   ① Take a smaller shrinkage rate for the outer diameter of the plastic part, and a larger shrinkage rate for the inner diameter to leave room for correction after the test.
   ②The mold trial determines the form, size and molding conditions of the gating system.
   ③The plastic parts to be post-processed shall be subjected to post-processing to determine the size change (measurement must be 24 hours after demolding).
   ④ Correct the mold according to the actual shrinkage.
   ⑤ Retry the mold and appropriately change the process conditions to slightly modify the shrinkage value to meet the requirements of the plastic part.
   2. Liquidity
   2.1 The fluidity of thermoplastics can generally be analyzed from a series of indexes such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (process length/plastic part wall thickness). Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, low apparent viscosity, high flow ratio, good fluidity, plastics with the same product name must check their instructions to determine whether their fluidity is applicable For injection molding. According to mold design requirements, the fluidity of commonly used plastics can be roughly divided into three categories:
   ① Good fluidity PA, PE, PS, PP, CA, poly(4) methylpentene;
   ②Polystyrene series resin with medium fluidity (such as ABS, AS), PMMA, POM, polyphenylene ether;
   ③Poor fluidity PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastics.
   2.2 The fluidity of various plastics also changes due to various molding factors, and the main influencing factors are as follows:
①Higher material temperature increases fluidity, but different plastics have their own differences, such as PS (especially those with high impact resistance and higher MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS) The fluidity of, PC, CA and other plastics varies greatly with temperature. For PE and POM, the temperature increase or decrease has little effect on their fluidity. Therefore, the former should adjust the temperature during molding to control fluidity.
   ② As the pressure of injection molding increases, the molten material is subjected to greater shearing and fluidity, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control fluidity during molding.
③The form, size, layout, cooling system design of the mold structure, the flow resistance of the molten material (such as the surface finish, the thickness of the channel section, the shape of the cavity, the exhaust system) and other factors directly affect the molten material in the cavity The actual fluidity inside, if the molten material is promoted to lower the temperature and increase the fluidity resistance, the fluidity will decrease. When designing the mold, a reasonable structure should be selected according to the fluidity of the plastic used. During molding, the material temperature, mold temperature, injection pressure, injection speed and other factors can also be controlled to appropriately adjust the filling condition to meet the molding needs.
   3.crystallinity
   Thermoplastics can be divided into crystalline plastics and non-crystalline (also known as amorphous) plastics according to their absence of crystallization during condensation.
The so-called crystallization phenomenon refers to the fact that when the plastic changes from a molten state to a condensation state, the molecules move independently and are completely in a disordered state. The molecules stop moving freely, press a slightly fixed position, and have a tendency to make the molecular arrangement a regular model. This phenomenon.
   The appearance criteria for judging these two types of plastics can be determined by the transparency of the thick-walled plastic parts. Generally, crystalline materials are opaque or translucent (such as POM, etc.), and amorphous materials are transparent (such as PMMA, etc.). But there are exceptions. For example, poly(4) methylpentene is a crystalline plastic but has high transparency, and ABS is an amorphous material but not transparent.
   When designing molds and selecting injection molding machines, pay attention to the following requirements and precautions for crystalline plastics:
   ① It takes a lot of heat to raise the material temperature to the molding temperature, and equipment with large plasticizing ability is needed.
   ②When cooling back, it emits a lot of heat, so it should be cooled sufficiently.
   ③The specific gravity difference between the molten state and the solid state is large, the molding shrinkage is large, and shrinkage and pores are prone to occur.
   ④ Fast cooling, low crystallinity, small shrinkage and high transparency. The crystallinity is related to the wall thickness of the plastic part, and the wall thickness is slow to cool, high crystallinity, large shrinkage, and good physical properties. Therefore, the mold temperature of the crystalline material must be controlled as required.
   ⑤Significant anisotropy and large internal stress. Molecules that are not crystallized after demolding tend to continue to crystallize, are in an energy imbalanced state, and are prone to deformation and warping.
   ⑥ The crystallization temperature range is narrow, and it is easy to cause unmelted material to be injected into the mold or to block the feed port.
   4. Heat-sensitive plastics and easily hydrolyzed plastics
4.1 Heat sensitivity refers to the tendency of certain plastics to be sensitive to heat. They will be heated for a long time at high temperature or the feed inlet section is too small. When the shearing effect is large, the material temperature will increase and the material will be easily discolored, degraded and decomposed. It has this characteristic The plastic is called heat-sensitive plastic. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polychlorotrifluoroethylene, etc. Heat-sensitive plastics produce monomers, gases, solids and other by-products during decomposition. In particular, some decomposition gases have irritating, corrosive or toxic effects on the human body, equipment, and molds. Therefore, attention should be paid to mold design, injection molding machine selection and molding. Screw injection molding machine should be used. The section of the pouring system should be large. The mold and barrel should be chrome-plated. Add stabilizer to weaken its thermal sensitivity.
   4.2 Even if some plastics (such as PC) contain a small amount of water, they will decompose under high temperature and high pressure. This property is called easy hydrolysis, which must be heated and dried in advance.

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