7 points that must be considered in plastic injection molding

7 factors such as shrinkage, fluidity, crystallinity, heat-sensitive plastics and easily hydrolyzed plastics, stress cracking and melt fracture, thermal performance and cooling rate, and moisture absorption should be considered in the setting of injection molding process. factor.

7 factors such as shrinkage, fluidity, crystallinity, heat-sensitive plastics and easily hydrolyzed plastics, stress cracking and melt fracture, thermal performance and cooling rate, and moisture absorption should be considered in the setting of injection molding process. factor.

1. Shrinkage rate

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

1. Plastics: In the molding process of thermoplastics, due to the volume change caused by crystallization, strong internal stress, large residual stress frozen in the plastic part, strong molecular orientation and other factors, the shrinkage rate is compared with thermoset plastics. Larger, the shrinkage range is wide, and the directionality is obvious. In addition, the shrinkage after molding, annealing or humidity treatment is generally greater than that of thermosetting plastics.

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.

3. Factors such as the form, size and distribution of the feed inlet directly affect the direction of material flow, 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 small shrinkage but large directionality, and short feed ports with short width and length have small directivity. The ones that are close to the feed inlet or parallel to the direction of the material flow will shrink more.

4. Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for the crystalline material, 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 size and direction of the shrinkage of each part.

In addition, holding pressure and time also have a greater impact on contraction, and the contraction is smaller but the directionality is greater when the pressure is high and the time is long. 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 mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of plastic parts.

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:

Take 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 the test mold.

Trial molds determine the form, size and molding conditions of the gating system.

The plastic parts to be post-processed are subjected to post-processing to determine the size change (measurement must be 24 hours after demolding).

Correct the mold according to the actual shrinkage.

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

1. The 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, 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.

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

① The fluidity will increase when the material temperature is high, but different plastics are also different, PS (especially impact-resistant and high 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, temperature increase or decrease has little effect on their fluidity. Therefore, the former should adjust the temperature during molding to control fluidity.

② As the pressure of injection molding increases, the molten material is subject to greater shear and fluidity, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control fluidity during molding.

③ The form, size, layout, cooling system design of the mold structure, the design of the cooling system, 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 inside, if the molten material is caused to reduce the temperature and increase the fluidity resistance, the fluidity will decrease. 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.

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.

The so-called crystallization phenomenon refers to the fact that when the plastic changes from the molten state to the 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 to raise the material temperature to the forming temperature requires a lot of heat, and equipment with large plasticizing capacity is needed.

A large amount of heat is released during cooling and reconversion, so it must be cooled sufficiently.

The specific gravity difference between the molten state and the solid state is large, the molding shrinkage is large, and shrinkage and pores are prone to occur.

Fast cooling, low crystallinity, small shrinkage and high transparency. The crystallinity is related to the wall thickness of the plastic part, and the wall thickness is slow to cool, high crystallinity, large shrinkage, and good physical properties. Therefore, the mold temperature of the crystalline material must be controlled as required.

The anisotropy is significant and the internal stress is large. Molecules that have not crystallized after demolding have a tendency to continue to crystallize, are in an energy imbalanced state, and are prone to deformation and warping.

The crystallization temperature range is narrow, and it is easy to cause unmelted material to be injected into the mold or block the feed port.

4. Heat-sensitive plastics and easily hydrolyzed plastics

1. Heat sensitivity means that some plastics are more sensitive to heat. They will be heated for a long time at high temperature or the feed inlet section is too small. When the shearing effect is large, the material temperature increases and the tendency of discoloration, degradation and decomposition occurs. The characteristic plastic is called heat-sensitive plastic.

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 the 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. The section of the pouring system should be large. The mold and barrel should be chrome-plated. There should be no corners. The molding temperature must be strictly controlled. Add stabilizer to weaken its thermal sensitivity.

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

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.

To this end, in addition to adding additives to the raw materials to improve the crack resistance, the raw materials should be dried and the 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 the mold, the demolding angle should be increased, and a reasonable feed inlet and ejection mechanism should be selected. The material temperature, mold temperature, injection pressure and cooling time should be adjusted appropriately during molding, and try to avoid demolding when the plastic part is too cold and brittle , After molding, the plastic parts should also undergo post-treatment to improve crack resistance, eliminate internal stress and prohibit contact with solvents.

2. When a polymer melt with a certain melt flow rate passes through the nozzle hole at a constant temperature and its flow rate exceeds a certain value, obvious lateral cracks on the melt surface are called melt fractures, which damage the appearance and physical properties of the plastic parts. Therefore, when selecting polymers with high melt flow rate, the cross-section of the nozzle, runner, and feed opening should be increased to reduce the injection speed and increase the material temperature.

6. Thermal performance and cooling rate

1. Various plastics have different specific heat, thermal conductivity, heat distortion temperature and other thermal properties. Plasticizing with high specific heat requires a large amount of heat, so an injection molding machine with large plasticizing capacity should be used. The cooling time of the plastic with high heat distortion temperature can be short and the 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 fully cooled and the cooling effect of the mold must be strengthened. 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. Appropriate injection molding machines and enhanced mold cooling must be selected.

2. Various plastics are required to maintain an appropriate cooling rate according to their types, characteristics and shapes of plastic parts. Therefore, the mold must be equipped with heating and cooling systems 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 demolding, shorten the molding cycle, and reduce the crystallinity.

When the plastic waste heat is not 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 inside and outside of thick-walled plastic parts and increase crystallinity.

For those with good fluidity, large molding area, and uneven material temperature, depending on the molding conditions of plastic parts, sometimes it needs to be heated or cooled alternately or locally heated and cooled. To this end, the 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, moisture adhesion, and non-absorption and non-stick moisture. The water content in the material must be controlled within the allowable range. Otherwise, the water will become gas or hydrolysis under high temperature and high pressure, which will cause the resin to foam, decrease the fluidity, and 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.

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