1、 Shrinkage rate

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

1.1 Plastic Variety Due to factors such as volume changes caused by crystallization, strong internal stress, large residual stress frozen in the plastic, and strong molecular orientation during the molding process of thermoplastics, compared with thermosetting plastics, the shrinkage rate is larger, with a wide range of shrinkage rates and obvious directionality. In addition, the shrinkage rate after molding, annealing, or humidity conditioning treatment is generally larger than that of thermosetting plastics.

1.2 Characteristics of plastic parts During molding, the molten material contacts the surface of the mold cavity and the outer layer immediately cools to form a low density solid shell. Due to the poor thermal conductivity of plastics, the inner layer of the plastic part slowly cools and forms a high-density solid layer with large shrinkage. Therefore, those with thick walls, slow cooling, and thick high-density layers have large shrinkage. 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. Therefore, the characteristics of plastic parts have a significant impact on the size and direction of shrinkage.

1.3 The factors such as the form, size, and distribution of the feed port directly affect the direction of the material flow, density distribution, pressure maintaining and feeding effects, and molding time. The direct feed port and the large cross-section of the feed port (especially those with thick cross-section) have small shrinkage but large directionality, while those with short width and length of the feed port have small directionality. Those close to the feed inlet or parallel to the direction of the material flow have a large shrinkage.

1.4 Molding conditions: High mold temperature, slow cooling of molten materials, high density, and large shrinkage. 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, and directly affects the size and directionality of the shrinkage of each part.

In addition, maintaining pressure and time also has a significant impact on contraction, with high pressure and long time leading to small but directional contraction. 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 reduced appropriately. The material temperature is high, the shrinkage is large, but the directionality is small. Therefore, adjusting factors such as mold temperature, pressure, injection speed, and cooling time during molding can also appropriately change the shrinkage of plastic parts.

During mold design, based on the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the form, size, and distribution of the feed port, the shrinkage rate of each part of the plastic part is determined empirically, and then the cavity size is calculated. For high-precision plastic parts and when it is difficult to master 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 to leave room for correction after mold testing.

② Determine the form, size, and molding conditions of the pouring system through mold testing.

③ The size change of the plastic parts to be post processed shall be determined through post processing (the measurement must be conducted after 24 hours after demolding).

④ Correct the mold according to the actual shrinkage.

⑤ Try the mold again and adjust the process conditions appropriately to slightly correct the shrinkage value to meet the requirements of the plastic part.

2、 Liquidity

The fluidity of thermoplastics can generally be analyzed from a series of indicators such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity, and flow ratio (process length/plastic wall thickness). "Low molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long screw flow length, low apparent viscosity, and high flow ratio indicate good fluidity. For plastics of the same product name, it is necessary to check their instructions to determine whether their fluidity is suitable for injection molding.". According to mold design requirements, the fluidity of commonly used plastics can be roughly divided into three categories:

① Good fluidity: PA, PE, PS, PP, CA, poly (4) methylstilbene;

② Moderate fluidity polystyrene series resins (such as ABS, AS), PMMA, POM, polyphenylene ether;

③ Poor fluidity PC, rigid PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastics.

2.2 The fluidity of various plastics also varies due to various molding factors, and the main influencing factors are as follows:

① When the temperature is high, the fluidity increases, but different plastics also have differences. The fluidity of plastics such as PS (especially those with high impact resistance and MFR values), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA, and so on varies greatly with temperature. For PE and POM, the increase or decrease in temperature has little impact on their fluidity. Therefore, the former should adjust the temperature to control the fluidity during molding.

② When the pressure injection pressure increases, the molten material is subject to greater shear action and increases fluidity, especially PE and POM. Therefore, it is advisable to adjust the injection pressure to control fluidity during molding.

③ Factors such as the form, size, layout, cooling system design, and flow resistance of the molten material (such as mold surface finish, material channel section thickness, cavity shape, and exhaust system) of the pouring system of the mold structure directly affect the actual fluidity of the molten material in the mold cavity. Those factors that promote the lowering of the temperature of the molten material and increase the flow resistance will reduce the fluidity. During mold design, 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 condition to meet molding needs.

3、 Crystallinity

Thermoplastics can be divided into two categories: crystalline plastics and amorphous (also known as amorphous) plastics based on their absence of crystallization during condensation.

The so-called crystallization phenomenon refers to a phenomenon in which the molecules move independently and are completely in a disordered state when plastic undergoes a transition from a melting state to a condensation state, becoming molecules that stop moving freely and move in a slightly fixed position, with a tendency to arrange the molecules into a regular model.

The appearance criteria for these two types of plastics can be determined by the transparency of the thick walled plastic parts of the plastic. Generally, crystalline materials are opaque or translucent (such as POM), and amorphous materials are transparent (such as PMMA). However, there are also exceptions, such as poly (4) methylstilbene, which is a crystalline plastic but has high transparency, and ABS, which is an amorphous material but is not transparent.

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

① The amount of heat required to raise the material temperature to the molding temperature is large, and equipment with high plasticizing capacity should be used.

② During cooling and recycling, large heat is released, and sufficient cooling is required.

③ The specific gravity difference between the molten state and the solid state is large, resulting in large molding shrinkage, and prone to shrinkage and porosity.

④ Fast cooling, low crystallinity, small shrinkage, and high transparency. Crystallinity is related to the wall thickness of plastic parts, which leads to slow cooling, high crystallinity, large shrinkage, and good physical properties. Therefore, mold temperature must be controlled for crystalline materials as required.

⑤ Significant anisotropy and high internal stress. The molecules that have not crystallized after demolding have a tendency to continue to crystallize, are in an energy imbalance state, and are prone to deformation and warpage.

⑥ The crystallization temperature range is narrow, and it is easy to inject unmelted material into the mold or block the feed port.

4、 Thermosensitive plastics and easily hydrolyzable plastics

4.1 Thermal sensitivity refers to the tendency of certain plastics to become sensitive to heat, which can cause discoloration, degradation, and decomposition when heated for a long time at high temperatures, or when the feed inlet section is too small, or the shear action is large. Plastics with this characteristic are referred to as thermal sensitive plastics. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polytrifluorochloroethylene, etc.

During decomposition, thermosensitive plastics produce by-products such as monomers, gases, and solids. In particular, some decomposition gases have irritating, corrosive, or corrosive effects on human bodies, equipment, and molds. Therefore, attention should be paid to mold design, selection of injection molding machines, and molding. A screw type injection molding machine should be selected. The pouring system should have a large cross-section. The mold and barrel should be chrome plated, and there should be no corner lag. The molding temperature must be strictly controlled, and stabilizers must be added to the plastic to reduce its thermal sensitivity.

4.2 Some plastics (such as PC), even if they contain a small amount of water, can decompose under high temperatures and pressures. This property is called hydrophilicity, and it must be heated and dried in advance.

5、 Stress cracking and melt fracture

5.1 Some plastics are sensitive to stress and are prone to internal stress and cracking during molding. Plastic parts can crack under external forces or solvents. Therefore, in addition to adding additives to improve the cracking resistance of the raw material, attention should be paid to drying the raw material and selecting reasonable molding conditions to reduce internal stress and increase cracking resistance.

Reasonable plastic shape should be selected, and measures such as installing inserts should not be taken to minimize stress concentration. During mold design, the mold release angle should be increased, and a reasonable feed inlet and ejection mechanism should be selected. During molding, the material temperature, mold temperature, injection pressure, and cooling time should be appropriately adjusted to avoid mold release when the plastic part is too cold and brittle. After molding, it is also advisable to conduct post processing to improve cracking resistance, eliminate internal stress, and prohibit contact with solvents.

5.2 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, significant transverse cracks occur on the surface of the melt, which is called melt fracture, which can damage the appearance and physical properties of the plastic part. Therefore, when selecting polymers with high melt flow rates, it is necessary to increase the cross-section of nozzles, runners, and feed ports, reduce injection molding speed, and increase material temperature.

6、 Thermal performance and cooling speed

6.1 Various plastics have different thermal properties such as specific heat, thermal conductivity, and thermal deformation temperature. Plasticizing with high specific heat requires large amounts of heat, and an injection molding machine with large plasticizing capacity should be selected. The cooling time of plastics with high thermal deformation temperature can be short and demoulding is early, but cooling deformation should be prevented after demoulding. Plastics with low heat conductivity have a slow cooling rate (such as ionic polymers, which are extremely slow), so it is necessary to fully cool them and enhance the cooling effect of the mold. 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 speed are not conducive to high-speed molding. Therefore, appropriate injection molding machines must be selected and mold cooling must be strengthened.

6.2 Various plastics must maintain an appropriate cooling rate according to their type, characteristics, and shape. 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 parts after demolding, shorten the molding cycle, and reduce crystallinity.

When the residual heat of plastic is insufficient to maintain a certain temperature in the mold, the mold should be equipped with a heating system to maintain the mold at a certain temperature to control the cooling speed, ensure fluidity, improve filling conditions, or control the slow cooling of the plastic parts, prevent uneven cooling inside and outside of the thick wall plastic parts, and increase crystallinity. For those with good fluidity, large molding area, and uneven material temperature, it is sometimes necessary to alternately use heating or cooling or partially use heating and cooling according to the molding situation of the plastic part. Therefore, the mold should be equipped with a corresponding cooling or heating system.

7、 Hygroscopicity

Due to various additives in plastics, they have different degrees of affinity and hydrophobicity to water. Therefore, plastics can be roughly divided into two types: hygroscopic, adherent, and non hygroscopic. The water content in the material must be controlled within the allowable range, otherwise, under high temperature and pressure, water can become gas or undergo hydrolysis, resulting in resin foaming, decreased fluidity, and poor appearance and mechanical properties. Therefore, hygroscopic plastics must be preheated using appropriate heating methods and specifications as required to prevent re moisture absorption during use.