Three Factors That Should Be Considered In Terms Of Materials In Injection Molding

1. Shrinkage rate

The factors affecting the shrinkage of thermoplastic molding are as follows:

1.1 Plastic Variety During the molding process of thermoplastic plastics, there are still volume changes caused by crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation and other factors, so compared with thermoset plastics, the shrinkage rate is higher. Large, wide range of shrinkage rate and obvious directionality. In addition, the shrinkage rate after molding, annealing or humidity treatment is generally greater than that of thermosetting plastics.

1.2 The characteristics of the plastic part. When the molten material is in contact with the surface of the cavity, 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, so the characteristics of plastic parts have a greater impact on shrinkage and directionality.

1.3 The feed inlet form, size, distribution, these factors directly affect the material flow direction, 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. Those that are close to the feed inlet or parallel to the direction of the material flow will shrink more.

1.4 Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for crystalline materials, the shrinkage is greater due to high crystallinity and large volume changes. The mold temperature distribution is also related to the inner and outer 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 if the pressure is high and the time is long, the contraction is small but the directionality is large. 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 appropriately reduced. 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:

① Take a smaller shrinkage rate for the outer diameter of the plastic part, and a larger shrinkage rate for the inner diameter, so as to leave room for correction after mold trial.

②The mold trial determines the form, size and molding conditions of the gating system.

③The plastic parts to be post-treated shall be subjected to post-treatment to determine the size change (measurement must be 24 hours after demolding).

④ Correct the mold according to the actual shrinkage.

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

2. Liquidity

a) The 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, 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:

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

The higher the temperature of the material, the higher the fluidity, but different plastics are also different, PS (especially impact-resistant 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.

When the pressure injection pressure increases, the molten material will be subjected to greater shearing and fluidity, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control fluidity during molding.

The form, size, layout, design of the cooling system of the mold structure, the 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 directly affect the molten material in the cavity The actual fluidity of the molten material is reduced if the temperature of the molten material is reduced, and the fluidity resistance is increased. 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.

Three, crystallinity

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), 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:

The heat required for the material temperature to rise to the molding temperature is much, and equipment with large plasticizing ability is needed.