injection molding process influencing factors

In the injection molding process of plastics, thermoplastics, due to the volume change caused by crystallization, strong internal stress, large residual stress frozen in the plastic part, strong molecular orientation and other factors, so compared with thermoset plastics, the shrinkage rate is larger. The shrinkage rate is wide and the directionality is obvious. In addition, the shrinkage after molding, annealing or humidity treatment is generally larger than that of thermosetting plastics.

When the plastic part is molded, 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. Therefore, the characteristics of plastic parts have a greater impact on shrinkage and directionality.

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Factors such as the form, size, and distribution of the feed inlet directly affect the direction of material flow, 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 less shrinkage but greater directivity, and shorter feed ports with shorter width and length have less directivity. The ones that are close to the feed inlet or parallel to the direction of the material flow will shrink more.

Molding conditions: high mold temperature, slow cooling of molten material, high density and large shrinkage, especially for crystalline materials, due to high crystallinity and large volume changes, so the shrinkage is greater. 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 melt viscosity difference 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:

①The trial mold determines the form, size and molding conditions of the gating system.

②The plastic parts to be post-processed shall be processed to determine the size change (the measurement must be 24 hours after demolding.

③ Correct the mold according to the actual shrinkage.

④ Retry the mold and appropriately change the process conditions to slightly modify the shrinkage value to meet the requirements of the plastic part.

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:

①Good 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.

The fluidity of various plastics also changes due to various molding factors. The main influencing factors are as follows:

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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.

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② As the pressure of injection molding increases, the molten material is subject to greater shear and fluidity, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control fluidity during molding.

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③The form, size, layout, cooling system design, 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 of the mold structure casting system 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.

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Crystalline 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:

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① A lot of heat is required for the material temperature to rise to the molding temperature, and equipment with large plasticizing ability is needed. To

②It emits a large amount of heat during cooling back and needs to be cooled sufficiently.

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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.

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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, the crystallinity is high, the shrinkage is large, and the physical properties are good. 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 have a tendency to continue to crystallize, are in an energy imbalance state, and are prone to deformation and warpage. ⑥ The crystallization temperature range is narrow, and the unmelted material is prone to be injected into the mold or block the feed port.

Heat sensitivity refers to the tendency of certain plastics to be more sensitive to heat. When heated at high temperatures for a long time or the feed port section is too small, when the shearing effect is large, the material temperature increases and the tendency of discoloration, degradation, and decomposition occurs. 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. There should be no corner stagnation. The molding temperature must be strictly controlled. Stabilizer, weaken its heat-sensitive performance.

Some plastics (such as PC) even contain a small amount of water, but 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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