7 factors that must be considered in plastic injection molding

The form and calculation of thermoplastics molding shrinkage are as mentioned above. The factors that affect the molding shrinkage of thermoplastics are as follows:

During the molding process of thermoplastics, due to factors such as volume changes caused by crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation, etc., compared with thermosetting plastics, the shrinkage rate is larger, the shrinkage range is wide, and the directionality is obvious. In addition, the shrinkage after molding, annealing or humidity control treatment is generally larger than that of thermosetting plastics.

When molding, the molten material contacts the surface of the cavity and the outer layer is immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of the plastic part cools slowly to form a high-density solid layer that shrinks greatly. Therefore, those with thick walls, slow cooling, and thick high-density layers 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 greater impact on shrinkage size and directionality.

Factors such as the form, size and distribution of the feed inlet directly affect the material flow direction, density distribution, pressure-holding and feeding effect, and molding time. Direct feed inlets and feed inlets with large cross-sections (especially those with thicker sections) have smaller shrinkage but greater directionality, while feed inlets with wider and shorter lengths have less directivity. Those close to the feed inlet or parallel to the direction of material flow will shrink more.

The mold temperature is high, the molten material cools slowly, has high density, and shrinks greatly. 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 internal and external cooling and density uniformity of the plastic part, which directly affects the shrinkage and directionality of each part.

In addition, the holding pressure and time also have a greater impact on shrinkage. If the pressure is high and the time is long, the shrinkage will be small but directional. The injection molding pressure is high, the viscosity difference of the molten material is small, the shear stress between layers is small, and the elastic rebound after demoulding is large, so the shrinkage can be appropriately reduced. The material temperature is high, the shrinkage is large, but the directionality is small. Therefore, adjusting various factors such as mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of the plastic part.

When designing the mold, based on the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the size and distribution of the feed inlet, the shrinkage rate of each part of the plastic part is determined based on experience, and then the cavity size is calculated.

(1). The fluidity of thermoplastics can generally be analyzed from a series of indices such as molecular weight, melt index, Archimedean 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. Plastics with the same product name must check their instructions to determine whether their 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, poly(4) methylpentene;
• Medium fluidity polystyrene series resins (such as ABS, AS), PMMA, POM, polyphenylene ether;
• Poor fluidity PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastics.

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

• The fluidity of temperature materials increases when the material temperature is high, but different plastics also have differences. The fluidity of PS (especially impact-resistant and high MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics changes greatly with temperature.
• As the pressure of injection molding increases, the molten material will be subject to greater shearing and the fluidity will also increase. Especially PE and POM are more sensitive.
• The form, size, layout of the mold structure pouring system, cooling system design, molten material flow resistance and other factors directly affect the actual fluidity.

Thermoplastics can be divided into two categories: crystalline plastics and amorphous (also known as amorphous) plastics according to the fact that they don't crystallize when condensed.

When designing molds and selecting injection molding machines, attention should be paid to the following requirements for crystalline plastics:

• It requires a lot of heat to raise the material temperature, so equipment with large plasticizing capacity needs to be used.
• A large amount of heat is released during cooling and recovery, so cooling is required.
• The difference in specific gravity between the molten state and the solid state is large, resulting in large molding shrinkage.
• Fast cooling results in low crystallinity and small shrinkage. The degree of crystallinity is related to the wall thickness.
• Significant anisotropy and large internal stress. Prone to deformation and warping.
• The crystallization temperature range is narrow, easy for feed port to be blocked.

(1). Thermal sensitivity refers to the tendency of some plastics to be more sensitive to heat. When heated at high temperatures for a long time, the material is prone to discoloration, degradation, and decomposition.

Therefore, attention should be paid to mold design, injection molding machine selection and molding. A screw injection molding machine should be selected. The cross-section of the pouring system should be large. The mold and barrel should be chromium-plated.

(2). Even if some plastics (such as PC) contain a small amount of moisture, they will decompose under high temperature and high pressure. This property is called hydrolyzability, and it must be heated and dried in advance.

(1). Some plastics are sensitive to stress. They are prone to internal stress during molding and are brittle and easy to crack. Plastic parts will crack under the action of external force or solvent.

When designing the mold, the draft angle should be increased, and a reasonable feed port and ejection mechanism should be selected. After molding, the plastic parts should be post-processed to improve cracking resistance.

(2). When the polymer melt exceeds a certain value when passing through the nozzle hole, obvious transverse cracks will occur on the melt surface, which is called melt rupture. The cross-sections of the nozzle, runner, and feed inlet should be increased.

(1). Various plastics have different thermal properties such as specific heat, thermal conductivity, and heat distortion temperature. Plasticizing materials with high specific heat require a lot of heat, so an injection molding machine with large capacity should be selected.

(2). Various plastics require an appropriate cooling rate according to their type characteristics and shape of plastic parts. Therefore, the mold must be equipped with a heating and cooling system according to the molding requirements to maintain a certain mold temperature.

Because there are various additives in plastics, they have different degrees of affinity for moisture. The moisture content in the material must be controlled within the allowable range. Otherwise, the moisture will turn into gas or hydrolyze, causing the resin to foam, reduce fluidity, and have poor appearance.

Therefore, hygroscopic plastics must be preheated using appropriate heating methods and specifications as required to prevent re-absorption of moisture during use.