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Factors affecting injection molding processing

Release time:2021-08-30

Factors affecting injection molding process:

The factors that affect the shrinkage of thermoplastic include: the volume change caused by plastic crystallization during the molding process, strong internal stress, large residual stress frozen in the plastic parts, and strong molecular orientation. Compared with thermosetting plastics, the shrinkage is larger, the shrinkage range is wide, and the directionality is obvious. In addition, the shrinkage rate after molding, annealing or humidification is generally larger than that of thermosetting plastics.

Plastic part characteristics when the molten material contacts the surface of the cavity, the outer layer immediately cools to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of plastic parts cools slowly, forming a solid layer with high density and 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 material flow direction, density distribution and shrinkage resistance. Therefore, the characteristics of plastic parts have a great influence on the shrinkage and directivity.

The shape, size, distribution and other factors of the feed inlet directly affect the material flow direction, density distribution, pressure retaining shrinkage effect and molding time. The direct feed inlet and the feed inlet with large cross-section (especially the thick cross-section) have smaller shrinkage but larger directionality, while the shorter feed inlet has shorter width and length and smaller directionality. The shrinkage near the feed inlet or parallel to the direction of material flow is greater.

Molding conditions: high mold temperature, slow melt cooling, high density and large shrinkage. Especially for crystalline materials, due to high crystallinity, large volume change and larger shrinkage. The mold temperature distribution is also related to the internal and external cooling of plastic parts and the uniformity of density, which directly affects the size and direction of shrinkage of plastic parts. In addition, pressure holding and time also have a greater impact on shrinkage. When the pressure is high and the time is long, the shrinkage is small but the directionality is large.

The injection pressure is high, the melt viscosity difference is small, the interlayer shear stress is small, and the elastic rebound after demoulding is large, so the shrinkage can also be appropriately reduced. The material temperature is high, the shrinkage is large, but the directivity is small. Therefore, adjusting the mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of plastic parts. When designing the mold, according to the shrinkage range of various plastics, the wall thickness and shape of plastic parts, the size and distribution of inlet form, the shrinkage rate of each part of plastic parts is determined according to experience, and then the cavity size is calculated.

For high-precision plastic parts and when it is difficult to master the shrinkage rate, the following methods should generally be used to design the mold:

① The outer diameter of the plastic part is taken as the smaller shrinkage rate, and the inner diameter is taken as the larger shrinkage rate, so that there is room for correction after the test.

② The mold test determines the form, size and forming conditions of the gating system.

③ The plastic parts to be post treated shall be post treated to determine the dimensional change (it must be measured 24 hours after demoulding).

④ Correct the mold according to the actual shrinkage.

⑤ Retry the mold, change the process conditions appropriately, and slightly modify the shrinkage value to meet the requirements of plastic parts.

The fluidity of thermoplastics can generally be analyzed from a series of indicators such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (process length / plastic wall thickness). Small molecular weight, wide molecular weight distribution, poor regularity of molecular structure, high melt index, long spiral flow length, low apparent viscosity, high flow ratio and good fluidity. Plastics with the same product name must check their instructions to determine whether their fluidity is qualified and suitable for injection molding.

According to the 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;

② Medium fluidity, polystyrene series resins (such as ABS, as), PMMA, POM, polyphenylene oxide;

③ Poor liquidity, PC, hard PVC, polyphenylene oxide, polysulfone, polysulfone, fluoroplastics.

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

① Higher material temperature increases the fluidity, but different plastics have their own differences, such as PS (especially those with high impact resistance and high MFR value), PP, PA, PMMA, modified polystyrene (such as ABS and as), PC fluidity, Ca and other plastics change greatly with temperature. For PE and POM, the increase or decrease of temperature has little effect on their fluidity. Therefore, the former should adjust the temperature during molding to control the fluidity.

② With the increase of injection pressure, the melt is subject to greater shear and fluidity, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control the fluidity during molding.

③ The form, size, layout, cooling system design of the mold structure, the flow resistance of the melt (such as surface finish, channel section thickness, cavity shape, exhaust system) and other factors directly affect the actual fluidity of the melt inside the cavity. If the melt temperature is reduced and the flow resistance is increased, the fluidity will be reduced. 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, and the filling conditions can be adjusted appropriately to meet the molding needs.

Crystalline thermoplastics can be divided into crystalline plastics and amorphous (also known as amorphous) plastics according to their lack of crystallization in the condensation process. The so-called crystallization phenomenon refers to that when plastic changes from molten state to condensed state, molecules move independently and are in a disordered state. The molecules stop moving freely and press at a slightly fixed position, and there is a trend to make the molecular arrangement become a regular model. This phenomenon. The appearance standard of these two plastics can be determined by the transparency of thick wall 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 with high transparency, while ABS is an amorphous material with opacity.

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