7 Important Factors to Consider in Injection Molding

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

• Plastic types: During the molding process of thermoplastic plastics, 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.
• Characteristics of plastic parts: 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.

(3). 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.

(4). Molding conditions: 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.

For high-precision plastic parts and when it is difficult to control the shrinkage rate, the following methods are generally used to design the mold:

• Set a smaller shrinkage rate for the outer diameter of the plastic part and a larger shrinkage rate for the inner diameter to leave room for correction after mold testing.
• Trial mold to determine the form, size and molding conditions of the pouring system.
• The dimensional changes of the plastic parts to be post-processed must be determined after post-processing (measurement must be done after 24 hours after demoulding).
• Correct the mold according to actual shrinkage.
• Try the mold again and change the process conditions appropriately to slightly correct the shrinkage value to meet the plastic part requirements.

(1). The fluidity of thermoplastic plastics 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:

① 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. For PE and POM, the temperature increase or decrease has little impact on their fluidity. Therefore, the former should adjust the temperature to control fluidity during molding.

② As the pressure of pressure 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, so the injection molding pressure should be adjusted to control fluidity during molding.

③ The form, size, layout of the mold structure pouring system, cooling system design, molten material flow resistance (such as surface finish, feed channel section thickness, cavity shape, exhaust system) and other factors directly affect the actual fluidity of the molten material in the cavity. If the temperature of the molten material is reduced and the fluidity resistance is increased, the fluidity will be reduced. When designing the mold, a reasonable structure should be selected based on the fluidity of the plastic used.

During molding, factors such as material temperature, mold temperature, injection pressure and injection speed can also be controlled to appropriately adjust the filling situation to meet molding needs.

Thermoplastic plastics can be divided into two categories: crystalline plastics and amorphous (also known as amorphous) plastics according to the fact that they do not crystallize when condensed.

The so-called crystallization phenomenon is a phenomenon in which when the plastic changes from a molten state to a condensed state, the molecules move independently and are completely disordered, and the molecules stop moving freely and settle into a slightly fixed position, and there is a tendency for the molecules to be arranged into a regular model.

The appearance standard for distinguishing these two types of plastics depends on the transparency of thick-walled plastic parts. Generally, crystalline materials are opaque or translucent (such as POM, etc.), and amorphous materials are transparent (such as PMMA, etc.). However, there are exceptions. For example, poly(4) methylpentene is a crystalline plastic but has high transparency, and ABS is an amorphous material but isn’t transparent.

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

• It requires a lot of heat to raise the material temperature to the molding 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 and prone to shrinkage and pores.
• Fast cooling, low crystallinity, small shrinkage and high transparency. The degree of crystallinity is related to the wall thickness of the plastic part. The wall thickness means slower cooling, higher crystallinity, greater shrinkage, and better physical properties. Therefore, the mold temperature of crystalline materials must be controlled as required.
• Significant anisotropy and large internal stress. Uncrystallized molecules after demoulding tend to continue to crystallize, are in a state of energy imbalance, 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 the 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 or the cross-section of the feed opening is too small or the shearing force is large, the material temperature increases and is prone to discoloration, degradation, and decomposition. Plastics with this characteristic are called heat-sensitive plastics.

Such as rigid PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polychlorotrifluoroethylene, etc. When heat-sensitive plastics decompose, they produce monomers, gases, solids and other by-products. 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. 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. There should be no stagnant material at corners. The molding temperature must be strictly controlled and stabilizers should be added to the plastic to weaken its heat-sensitive properties.

(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.

For this reason, in addition to adding additives to the raw materials to improve crack resistance, attention should be paid to drying the raw materials and reasonable selection of molding conditions to reduce internal stress and increase crack resistance. A reasonable plastic part shape should be selected, and inserts and other measures shouldn’t be installed to minimize stress concentration.

When designing the mold, the demoulding slope should be increased, and a reasonable feed port and ejection mechanism should be selected. During molding, the material temperature, mold temperature, injection pressure and cooling time should be appropriately adjusted to avoid demoulding when the plastic parts are too cold and brittle. After molding, the plastic parts should be post-processed to improve cracking resistance, eliminate internal stress and prohibit contact with solvents.

(2). When the polymer melt with a certain melt flow rate exceeds a certain value when passing through the nozzle hole at a constant temperature, obvious transverse cracks will occur on the melt surface, which is called melt rupture, which will damage the appearance and physical properties of the plastic part. Therefore, when selecting polymers with high melt flow rates, the cross-sections of the nozzle, runner, and feed inlet should be increased, the injection speed should be reduced, and the material temperature 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 plasticizing capacity should be selected. Plastics with high heat distortion temperatures can have a short cooling time and early demoulding, but cooling deformation must be prevented after demoulding.

Plastics with low thermal conductivity have a slow cooling rate (such as ionic polymers, etc., which have an extremely slow cooling rate), so they must be fully cooled and the mold cooling effect must be enhanced. Hot runner molds are suitable for plastics with low specific heat and high thermal conductivity. Plastics with high specific heat, low thermal conductivity, low thermal deformation temperature, and slow cooling rate are not conducive to high-speed molding. An appropriate injection molding machine must be selected and mold cooling must be strengthened.

(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. When the material temperature increases the mold temperature, it should be cooled to prevent deformation of the plastic part after demoulding, shorten the molding cycle, and reduce the crystallinity.

When the waste heat of the plastic isn’t enough to keep the mold at a certain temperature, the mold should be equipped with a heating system to keep the mold at a certain temperature to control the cooling rate, ensure fluidity, improve filling conditions or control the plastic parts to cool slowly, prevent uneven cooling of the inside and outside of thick-walled plastic parts and increase crystallinity, etc.

For those with good fluidity, large molding area and uneven material temperature, heating or cooling may need to be used alternately or both local heating and cooling may be used depending on the molding conditions of the plastic parts. For this purpose, the mold should be equipped with a corresponding cooling or heating system.

Because there are various additives in plastics, they have different degrees of affinity for moisture. Therefore, plastics can be roughly divided into two types: those that absorb moisture, those that adhere to moisture, and those that don’t absorb water and are not easy to adhere to moisture. The moisture content in the material must be controlled within the allowable range. Otherwise, under high temperature and high pressure, the moisture will turn into gas or hydrolyze, causing the resin to foam, reduce fluidity, and have poor appearance and mechanical properties.

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