Three factors that should be considered in terms of materials in injection molding

Time:2022-06-28 08:51:51 / Popularity: / Source:

1 Shrinkage rate

Factors affecting shrinkage of thermoplastic molding are as follows:

1.1 Plastic Variety

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

1.2 Characteristics of plastic part

During molding, molten material contacts surface of cavity and outer layer is immediately cooled to form a low-density solid shell. Due to poor thermal conductivity of plastic, inner layer of plastic part is slowly cooled to form a high-density solid layer with large shrinkage. Therefore, wall thickness, slow cooling, and high-density layer thickness will shrink more. In addition, presence or absence of inserts, layout and quantity of inserts directly affect direction of material flow, density distribution and shrinkage resistance, so characteristics of plastic parts have a greater impact on shrinkage and directionality.

1.3 Factors that affect molding

Factors such as feed inlet form, size, distribution, directly affect material flow direction, density distribution, pressure maintaining and shrinking effect, molding time. Direct feed ports, feed port(especially thicker cross-sections) have a large cross-section, shrinkage is small but directionality is large, short feed port width and length have small directionality. Those that are close to feed inlet or parallel to direction of material flow will shrink more.

1.4 Molding conditions

Mold temperature is high, molten material cools slowly, density is high, and shrinkage is large. Especially for crystalline materials, shrinkage is greater due to high crystallinity and large volume changes. Mold temperature distribution is also related to inner and outer cooling, density uniformity of plastic part, which directly affects size and direction of shrinkage of each part. In addition, holding pressure and time also have a greater impact on contraction, if pressure is high and time is long, contraction is small but directionality is large. Injection pressure is high, viscosity difference of molten material is small, interlayer shear stress is small, and elastic rebound after demolding is large, so shrinkage can also be reduced by an appropriate amount. Material temperature is high, shrinkage is large, but directionality is small. Therefore, adjusting mold temperature, pressure, injection speed and cooling time during molding can also appropriately change shrinkage of plastic part.
When designing mold, according to shrinkage range of various plastics, wall thickness and shape of plastic part, size and distribution of inlet form, shrinkage rate of each part of plastic part is determined according to experience, then cavity size is calculated. For high-precision plastic parts and when it is difficult to grasp shrinkage rate, following methods should generally be used to design mold:
① Take a smaller shrinkage rate for outer diameter of plastic part, and a larger shrinkage rate for inner diameter, so as to leave room for correction after mold trial.
② Mold trial determines form, size and molding conditions of gating system.
③ Plastic parts to be post-treated shall be subjected to post-treatment to determine size change (measurement must be 24 hours after demolding).
④ Correct mold according to actual shrinkage.
⑤ Retry mold and appropriately change process conditions to slightly modify shrinkage value to meet requirements of plastic part.

2. Liquidity

a) Fluidity of thermoplastics can generally be analyzed from a series of indices 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. For plastics of same product name, manual must be checked to determine whether fluidity is suitable for injection molding. According to mold design requirements, fluidity of commonly used plastics can be roughly divided into three categories:
thermoplastic molding 
Good fluidity PA, PE, PS, PP, CA, poly(4) methylpentene;
Medium fluidity polystyrene series resin (such as ABS, AS), PMMA, POM, polyphenylene ether;
Poor fluidity: PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastics.
b) Fluidity of various plastics also changes due to various molding factors. Main influencing factors are as follows:
Fluidity increases when material temperature is high, but different plastics are also different. Fluidity of PS (especially impact-resistant and high MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics varies greatly with temperature. For PE and POM, temperature increase or decrease has little effect on their fluidity. Therefore, the former should adjust temperature during molding to control fluidity.
When pressure injection pressure increases, molten material will be subjected to greater shearing and fluidity, especially PE and POM are more sensitive, so injection pressure should be adjusted to control fluidity during molding.
Form, size, layout, cooling system design, flow resistance of molten material (such as surface finish, thickness of channel section, shape of cavity, exhaust system) and other factors of mold structure directly affect actual fluidity of the molten material in the cavity. Anything that causes melt to reduce temperature and increase flow resistance will reduce fluidity. When designing mold, a reasonable structure should be selected according to fluidity of plastic used. During molding, material temperature, mold temperature, injection pressure, injection speed and other factors can also be controlled to appropriately adjust filling condition to meet 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. So-called crystallization phenomenon refers to fact that when plastic changes from a molten state to a condensation state, molecules move independently and are completely in a disordered state. Molecules stop moving freely, press a slightly fixed position, and have a tendency to make molecular arrangement a regular model. Appearance criteria for judging these two types of plastics can be determined by transparency of thick-walled plastic parts. Generally, crystalline materials are opaque or translucent (such as POM), 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, ABS is an amorphous material but not transparent. When designing molds and selecting injection molding machines, pay attention to following requirements and precautions for crystalline plastics:
Heat required for material temperature to rise to molding temperature is much, and equipment with large plasticizing ability is needed.

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