Plastic varieties: During the molding process of thermoplastics, there are volume changes caused by crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation and other factors.
Therefore, compared with thermosetting plastics, their shrinkage rate is larger, the shrinkage rate range is wider, and the directionality is obvious. In addition, the shrinkage after molding, annealing or humidity adjustment treatment is generally larger than that of thermosetting plastics.
When the molten material contacts the surface of the cavity during molding, the outer layer immediately cools to form a low-density solid shell. Due to the poor thermal conductivity of plastics, the inner layer of the plastic part cools slowly to form a high-density solid layer with large shrinkage.
Therefore, the thicker the wall, the slower the cooling, and the thicker the high-density layer, the greater the shrinkage.
In addition, the presence or absence of inserts and the layout and number 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 the size and direction of shrinkage.
The feed port form, size, and distribution directly affect the material flow direction, density distribution, pressure holding and shrinkage compensation, and molding time.
Direct feed ports and feed ports with large cross-sections (especially thicker cross-sections) have small shrinkage but large directionality, while feed ports with wide widths and short lengths have small directionality.
Feed ports that are close to the feed port or parallel to the material flow direction have large shrinkage.
Molding conditions: The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for crystallized materials, the shrinkage is greater due to the high crystallinity and large volume change.
The mold temperature distribution is also related to the cooling and density uniformity of the plastic parts, which directly affects the shrinkage amount and directionality of each part.
In addition, the pressure and time of holding also have a great influence on the shrinkage. The shrinkage is small but the directionality is large 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 appropriately.
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 parts.
When designing the mold, according to the shrinkage range of various plastics, the wall thickness and shape of the plastic parts, the size and distribution of the feed port, 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 the shrinkage rate is difficult to grasp, it is generally advisable to design the mold using the following method:
- Test the mold to determine the form, size and molding conditions of the pouring system.
- Determine the dimensional change of the plastic parts to be post-processed after post-processing (the measurement must be made 24 hours after demolding.
- Correct the mold according to the actual shrinkage.
- Test the mold again and appropriately change the process conditions to slightly correct the shrinkage value to meet the requirements of the plastic parts.
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 (flow length/plastic part wall thickness).
Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, small apparent viscosity, and large flow ratio have good fluidity.
The instructions of a brand-name plastic must be checked to determine whether its fluidity is suitable for injection molding. According to the mold design requirements, the fluidity of commonly used plastics can be roughly divided into three categories:
- Good fluidity PA, PE, PS, PP, CA:
- Medium fluidity polystyrene series resins (such as ABS, AS), PMMA, POM, polyphenylene ether;
- Poor fluidity PC, hard PVC, polyphenylene ether, polysulfone, polyaryl sulfone, fluoroplastics.
The fluidity of various plastics also changes due to various molding factors. The main influencing factors are as follows:
① The higher the temperature of the material, the greater the fluidity, but different plastics also have different fluidity. There are differences. The fluidity of plastics such as PS (especially impact-resistant and high MFR), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA, etc.
Varies greatly with temperature. For PE and POM, the temperature increase or decrease has little effect on their fluidity. Therefore, the former should be adjusted to control fluidity during molding.
② When the pressure of pressure injection molding increases, the molten material is subjected to greater shearing and fluidity, especially PE and POM are more sensitive, so the injection molding pressure should be adjusted to control fluidity during molding.
③ The form, size, layout of the mold structure casting system, cooling system design, molten material flow resistance (such as surface finish) Cleanliness, channel section thickness, cavity shape, exhaust system) and other factors directly affect the actual fluidity of the molten material in the cavity.
Any factors that cause the molten material to lower its temperature and increase fluidity resistance will reduce its fluidity.
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 properly adjust the filling situation to meet the molding needs.
Crystalline thermoplastics can be divided into two categories: crystalline plastics and non-crystalline (also known as amorphous) plastics according to the absence of crystallization during condensation.
The so-called crystallization phenomenon is that when the plastic changes from the molten state to the condensation, the molecules stop moving freely and are in a slightly fixed position, and there is a tendency for the molecules to be arranged in a regular model.
As the appearance standard for distinguishing these two types of plastics, the transparency of the thick-walled plastic parts of the plastic can be determined. Generally, crystalline materials are opaque or translucent (such as POM, etc.), and amorphous materials are transparent (such as PMMA, etc.).
When designing molds and selecting injection molding machines, the following requirements and precautions should be taken for crystalline plastics:
- The heat required to raise the material temperature to the molding temperature is large, so equipment with large plasticizing capacity should be used.
- The heat released during cooling and re-melting is large, so it is necessary to cool it fully.
- The difference in specific gravity between the molten state and the solid state is large, the molding shrinkage is large, and shrinkage holes and pores are prone to occur.
- Fast cooling, low crystallinity, small shrinkage, and high transparency. Crystallinity is related to the wall thickness of the plastic part. Thick wall cooling is slow, high crystallinity, large shrinkage, and good physical properties. Therefore, the mold temperature of crystalline materials must be controlled as required.
- Significant anisotropy and large internal stress. After demolding, the molecules that have not been crystallized have a tendency to continue to crystallize, are in an energy imbalance state, and are prone to deformation and warping.
- The crystallization temperature range is narrow, and it is easy for unmelted material to be injected into the mold or block the feed port.
Thermosensitivity refers to the fact that some plastics are sensitive to heat. When the material temperature rises, it is easy to discolor, degrade, and decompose when the temperature is too high or the feed port is too small. Plastics with this characteristic are called thermosensitive plastics.
Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polytrifluorochloroethylene, etc. Thermosensitive plastics produce monomers, gases, solids and other by-products when they decompose.
In particular, some decomposition gases are irritating, corrosive or toxic to the human body, equipment, and molds. Therefore, attention should be paid to mold design, injection molding machine selection and molding.
Screw injection molding machines should be selected, the pouring system section should be large, the mold and barrel should be chrome-plated, and there should be no corner stagnation.
The molding temperature must be strictly controlled, and stabilizers must be added to the plastic to weaken its thermosensitive properties.
Some plastics (such as PC) will decompose under high temperature and high pressure even if they contain a small amount of water. This property is called hydrolysis, and they must be heated and dried in advance.
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