1. Due to volume change caused by crystallization during molding process of plastics, internal stress is strong, residual stress frozen in plastic parts is large, and molecular orientation is strong. Therefore, shrinkage rate is larger than that of thermosetting plastics, shrinkage range is wide, and directionality is obvious. In addition, shrinkage after molding, annealing or humidity adjustment treatment is generally larger than that of thermosetting plastics.
2. Characteristics of plastic parts. When molten material contacts surface of cavity, 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, plastics with thick wall thickness, slow cooling, and high density layer thickness shrink greatly.
In addition, presence or absence of inserts, layout and number of inserts directly affect flow direction, density distribution and shrinkage resistance, etc., so characteristics of plastic parts have a greater impact on shrinkage size and directionality.
3. Form, size, and distribution of feed port directly affect material flow direction, density distribution, pressure holding, shrinkage effect, and molding time. Direct feed ports and large feed port sections(especially those with thicker sections) have less shrinkage but greater directivity, while short feed port widths and lengths result in less directivity. The one close to inlet or parallel to direction of material flow shrinks greatly.
4. Molding conditions. Mold temperature is high, molten material cools slowly, density is high, and shrinkage is large. Especially for crystalline material, due to high crystallinity and large volume change, shrinkage is greater. Mold temperature distribution is also related to internal and external cooling, density uniformity of plastic parts, which directly affects shrinkage and directionality of each part.
In addition, holding pressure and time also have a great influence on contraction. When pressure is large and time is long, contraction is small but directionality is large. Injection pressure is high, viscosity difference of molten material is small, shear stress between layers is small, and elastic rebound is large after demoulding, so shrinkage can also be reduced appropriately. Material temperature is high and 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 parts.
5. According to shrinkage range of various plastics, wall thickness and shape of plastic parts, size and distribution of feed port, shrinkage rate of each part of plastic parts is determined according to experience, and then cavity size is calculated.

 

Trial mold determines form, size and molding conditions of gating system.
Plastic parts to be post-processed are post-processed to determine dimensional change (measuring must be after 24 hours after demolding).
Correct mold according to actual shrinkage.
Try mold again and change process conditions appropriately to slightly modify shrinkage value to meet requirements of plastic parts.

Thermoplastic fluidity 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 wall thickness).
Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, low viscosity, and high flow ratio, good fluidity. For plastics of same name, instruction manual must be checked to determine whether its fluidity is suitable for injection 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, rigid PVC, polyphenylene oxide, polysulfone, polyarylsulfone, fluoroplastic.

1. Temperature. High material temperature increases fluidity, but different plastics also have their own differences, PS (especially impact resistance type and higher MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS ), PC, CA and other plastic fluidity changes greatly with temperature. For PE and POM, increase or decrease of temperature has little effect on its fluidity. Therefore, the former should adjust temperature to control fluidity during molding.
2. Pressure: When injection pressure increases, molten material is subjected to greater shearing and fluidity, especially PE and POM are more sensitive, so injection pressure should be adjusted to control fluidity during molding.
3. Mold structure: form, size, layout, cooling system design, flow resistance of molten material (such as surface finish, channel section thickness, cavity shape, exhaust system) and other factors directly affect actual fluidity of melt in cavity. If temperature of molten material is reduced, fluidity resistance is increased, fluidity will decrease. 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 adjusted to properly adjust filling conditions to meet molding needs.

1. Thermoplastics can be divided into crystalline plastics and non-crystalline (also called amorphous) plastics according to fact that no crystallization occurs during condensation.
So-called crystallization phenomenon is that when plastic moves from molten state to condensation, molecules move independently, completely in an unordered state, molecules stop moving freely, press a slightly fixed position, and have a tendency to make molecular arrangement a regular model.
2. Appearance standard for judging these two types of plastics depends on transparency of thick-walled plastic parts. Generally, crystalline materials are divided into opaque or translucent (such as POM, etc.), and amorphous materials are transparent (such as PMMA, etc.). However, there are exceptions. For example, polytetramethylpentene is a crystalline plastic with high transparency, and ABS is amorphous but not transparent.
3. When designing molds and selecting injection molding machines, attention should be paid to crystalline plastics. When temperature of material rises to molding temperature, more heat is required, and it is necessary to use equipment with large plasticizing ability.