7 factors to be considered in injection molding process cannot be ignored!

Time：2021-04-28 14:52:16 / Popularity： / Source：

1 Shrinkage

Form and calculation of thermoplastic molding shrinkage are as mentioned above. Factors that affect thermoplastic molding shrinkage are as follows:
1.1 Plastic Variety. During molding process of thermoplastic plastics, due to volume change caused by crystallization, strong internal stress, large residual stress frozen in plastic part, strong molecular orientation, etc., 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 conditioning is generally greater than that of thermosetting plastics.
1.2 Characteristics of plastic parts. 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. Therefore, characteristics of plastic parts have a greater impact on shrinkage and directionality.
1.3 Factors such as form, size, and distribution of feed inlet directly affect direction of material flow, density distribution, pressure maintaining and shrinking effect, molding time. Direct feed ports and feed ports with large cross-sections (especially thicker cross-sections) have less shrinkage but greater directionality, shorter feed ports with shorter width and length have less directionality. The ones 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 material, shrinkage is greater due to high crystallinity and large volume changes. Mold temperature distribution is also related to internal and external cooling, density uniformity of plastic part, which directly affects size and directionality of shrinkage of each part. In addition, holding pressure and time also have a greater impact on contraction, contraction is smaller but directionality is larger when pressure is high and time is long. 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 appropriately reduced. 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 test mold.
② Mold trial determines form, size and molding conditions of gating system.
③ Plastic parts to be post-processed are subjected to post-processing 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

2.1 Fluidity of thermoplastics 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 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 with same product name, manual must be checked to determine whether its fluidity is suitable for injection molding. According to mold design requirements, fluidity of commonly used plastics can be roughly divided into three categories:
① Good fluidity PA, PE, PS, PP, CA, poly(4) methylpentene;
② Medium fluidity Polystyrene series resin (such as ABS, AS), PMMA, POM, polyphenylene ether;
③ Poor fluidity PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastics.
2.2 Fluidity of various plastics also changes due to various molding factors, and main influencing factors are as follows:
① Fluidity will increase when material temperature is high, but different plastics are also different. Fluidity of PS (especially impact-resistant and higher 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.
② As pressure of injection molding increases, molten material is subject to greater shear and fluidity, especially PE and POM are more sensitive, so injection pressure should be adjusted to control fluidity during molding.
③ Form, size, layout, cooling system design of mold structure, design of cooling system, flow resistance of molten material (such as surface finish, thickness of channel section, shape of cavity, exhaust system) and other factors directly affect actual fluidity of molten material in cavity. Anything that causes molten material to lower temperature and increase resistance to fluidity will decrease 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 is a phenomenon that when plastic changes from a molten state to a condensation state, molecules move independently and are completely in a disordered state, become molecules that stop freely moving, press a slightly fixed position, and have a tendency to make molecular arrangement become 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, etc.), 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, 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:
① A lot of heat is required for material temperature to rise to molding temperature, and equipment with large plasticizing ability is needed.
② It emits a large amount of heat during cooling back and needs to be cooled sufficiently.
③ Specific gravity difference between molten state and solid state is large, molding shrinkage is large, and shrinkage and pores are prone to occur.
④ Fast cooling, low crystallinity, small shrinkage and high transparency. Crystallinity is related to wall thickness of plastic part. Thicker walls result in slow cooling, high crystallinity, large shrinkage, and good physical properties. Therefore, mold temperature of crystalline material must be controlled as required.
⑤Significant anisotropy and large internal stress. Molecules that are not crystallized after demolding have a tendency to continue to crystallize, are in an energy imbalance state, are prone to deformation and warpage.
⑥ Crystallization temperature range is narrow, unmelted material is prone to be injected into mold or block feed port.

4 Heat-sensitive plastics and easily hydrolyzed plastics

4.1 Heat sensitivity refers to tendency of certain plastics to be sensitive to heat. They will be heated for a long time at high temperature or feed port section is too small, shearing effect is large, material temperature increases and tendency of discoloration, degradation, and decomposition occurs. Plastics with this characteristic are called heat-sensitive plastics. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polychlorotrifluoroethylene, etc. Heat-sensitive plastics produce monomers, gases, solids and other by-products during decomposition. In particular, some decomposition gases have irritating, corrosive or toxic effects on human body, equipment, and molds. Therefore, attention should be paid to mold design, injection molding machine selection and molding. Screw injection molding machine should be used. Section of gating system should be large. Mold and barrel should be chrome-plated. Add stabilizer to weaken its thermal sensitivity.
4.2 Even if some plastics (such as PC) contain a small amount of water, they will decompose under high temperature and high pressure. This property is called easy hydrolysis, which must be heated and dried in advance.

5 Stress cracking and melt fracture

5.1 Some plastics are sensitive to stress. They are prone to internal stress during molding, are brittle and easy to crack. Plastic parts will crack under action of external force or solvent. For this reason, in addition to adding additives to raw materials to improve crack resistance, attention should be paid to drying raw materials, molding conditions should be selected reasonably to reduce internal stress and increase crack resistance. And should choose a reasonable shape of plastic parts, it is not appropriate to install inserts and other measures to minimize stress concentration. When designing mold, demolding angle should be increased, a reasonable feed inlet and ejection mechanism should be selected. Material temperature, mold temperature, injection pressure and cooling time should be adjusted appropriately during molding, try to avoid demolding when plastic part is too cold and brittle. After molding, plastic parts should also be subjected to post-treatment to improve crack resistance, eliminate internal stress and prohibit contact with solvents.
5.2 When flow rate of a polymer melt with a certain melt flow rate exceeds a certain value when passing through nozzle hole at a constant temperature, obvious lateral cracks on the surface of melt are called melt fracture, which damages appearance and physical properties of plastic part. Therefore, when selecting polymers with high melt flow rate, cross-section of nozzle, runner, and feed opening should be increased to reduce injection speed and increase material temperature.

6 Thermal performance and cooling rate

6.1 Various plastics have different thermal properties such as specific heat, thermal conductivity, and heat distortion temperature. Plasticizing with a high specific heat requires a large amount of heat, so an injection molding machine with a large plasticizing capacity should be used. Cooling time of plastic with high heat distortion temperature can be short and demolding is early, but after demolding, it is necessary to prevent cooling deformation. Plastics with low thermal conductivity have a slow cooling rate (such as ionic polymers, etc.), so they must be sufficiently cooled to enhance cooling effect of mold. Hot runner molds are suitable for plastics with low specific heat and high thermal conductivity. Plastics with large specific heat, low thermal conductivity, low thermal deformation temperature, and slow cooling rate are not conducive to high-speed molding. Appropriate injection molding machines and enhanced mold cooling must be selected.
6.2 Various plastics are required to maintain an appropriate cooling rate according to their types, characteristics and shapes of plastic parts. Therefore, mold must be equipped with heating and cooling systems according to molding requirements to maintain a certain mold temperature. When material temperature increases mold temperature, it should be cooled to prevent plastic part from deforming after demolding, shorten molding cycle, and reduce crystallinity. When plastic waste heat is not enough to keep mold at a certain temperature, mold should be equipped with a heating system to keep mold at a certain temperature to control cooling rate, ensure fluidity, improve filling conditions or control plastic parts to cool slowly, prevent uneven cooling inside and outside of thick-walled plastic parts, increase crystallinity. For those with good fluidity, large molding area, and uneven material temperature, depending on molding conditions of plastic parts, sometimes it is necessary to alternately use heating or cooling, or use both local heating and cooling. To this end, mold should be equipped with a corresponding cooling or heating system.

7 Hygroscopicity

Due to various additives in plastics, which make them have different degrees of affinity for moisture, plastics can be roughly divided into two types: moisture absorption and moisture adhesion, non-absorption and non-stick moisture. Water content in material must be controlled within allowable range. Otherwise, moisture will become gas or hydrolyze under high temperature and high pressure, which will cause resin to foam, decrease fluidity, have poor appearance and mechanical properties. Therefore, hygroscopic plastics must be preheated with appropriate heating methods and specifications as required to prevent re-absorption of moisture during use.

Gud Mould Industry Co., Ltd