1、 Shrinkage rate

The forms and calculations of shrinkage during thermoplastic molding are as mentioned earlier. The factors that affect the shrinkage during thermoplastic molding are as follows:

1.1 Plastic Varieties During the molding process of thermoplastic plastics, due to the presence of volume changes caused by crystallization, strong internal stress, high residual stress frozen in the plastic, and strong molecular orientation, the shrinkage rate is larger, with a wide range of shrinkage rates and obvious directionality compared to thermosetting plastics. In addition, the shrinkage rate after molding, annealing, or humidity adjustment treatment is generally larger than that of thermosetting plastics.

1.2 Characteristics of plastic parts: When forming, the molten material immediately cools down to form a low-density solid shell in contact with the surface of the mold cavity. Due to the poor thermal conductivity of plastic, the inner layer of the plastic part slowly cools and forms a high-density solid layer with large shrinkage. So those with thick walls, slow cooling, and thick high-density layers tend to contract 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 the size and direction of shrinkage.

1.3 The form, size and distribution of feed inlet directly affect the direction of material flow, density distribution, pressure maintaining and feeding effect and forming time. The direct feeding port and the large cross-section of the feeding port (especially those with thicker cross-section) have smaller shrinkage but greater directionality, while those with shorter width and length of the feeding port have smaller directionality. Those close to the feed inlet or parallel to the direction of the material flow will experience greater shrinkage.

1.4 Molding conditions: High mold temperature, slow cooling of molten material, high density, and large shrinkage, especially for crystalline materials due to high crystallinity and large volume changes, resulting in greater shrinkage. The distribution of mold temperature is also related to the internal and external cooling and density uniformity of plastic parts, directly affecting the size and directionality of shrinkage in 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 moderately reduced. 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 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 feeding port, the shrinkage rate of each part of the plastic part is determined based on experience, and then the cavity size is calculated. When it is difficult to control the shrinkage rate of high-precision plastic parts, 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-treated must be determined after 24 hours of demolding during measurement.

④ Correct the mold according to the actual shrinkage situation.

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

2、 Liquidity

2.1 The flowability of thermoplastic materials can generally be analyzed based on a series of indices such as molecular weight, melt index, Archimedes helix flow length, apparent viscosity, and flow ratio (process length/plastic wall thickness). With small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, low apparent viscosity, and large flow ratio, the fluidity is good. For plastics with the same product name, the instructions must be checked to determine whether the fluidity is suitable for injection molding. According to mold design requirements, the flowability of commonly used plastics can be roughly divided into three categories:

① Good flowability PA, PE, PS, PP, CA, poly (4) methylene;

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

③ Poor fluidity PC, hard 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 of the material is high, the flowability increases, but different plastics also have differences. The flowability of plastics such as PS (especially those with high impact resistance and MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA, etc. varies greatly with temperature. For PE and POM, the increase or decrease in temperature has little effect on their fluidity. So the former should adjust the temperature to control the fluidity during molding.

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

③ The form, size, arrangement, cooling system design, and flow resistance of molten material (such as surface smoothness, material channel cross-sectional thickness, cavity shape, exhaust system) of the mold structure pouring system all directly affect the actual flow of molten material in the cavity. Those factors that promote the temperature reduction and increase flow resistance of molten material will reduce its flow resistance. 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 the 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 the phenomenon where plastic molecules move independently and are completely in a disordered state from a molten state to a condensation state, resulting in the molecules stopping free movement and following a slightly fixed position, with a tendency to arrange the molecules 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 semi transparent (such as POM), while amorphous materials are transparent (such as PMMA). However, there are also exceptions, such as poly (4) methylene, 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, attention should be paid to the following requirements and precautions for crystalline plastics:

① The amount of heat required to raise the material temperature to the forming temperature requires the use of equipment with high plasticizing capacity.

② During cooling and recycling, a large amount of heat is released, and sufficient cooling is necessary.

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

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

⑤ Significant anisotropy and high internal stress. Molecules that have not crystallized after demolding tend to continue to crystallize, remain in an energy imbalance state, and are prone to deformation and warping.

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

4、 Thermosensitive plastics and easily hydrolyzable plastics

4.1 Thermosensitivity refers to the tendency of certain plastics to change color, degrade, and decompose as the material temperature increases due to their sensitivity to heat, prolonged heating time at high temperatures, or small cross-sectional area of the feeding port, and high shear action. Plastics with this characteristic are called thermosensitive plastics. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polytetrafluorochloroethylene, etc. Thermosensitive plastics produce by-products such as monomers, gases, and solids during decomposition, especially some decomposition gases that have irritating, corrosive, or corrosive effects on the human body, 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 used, and the pouring system cross-section should be large. The mold and material 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 weaken its thermal sensitivity.

4.2 Some plastics (such as PC), even if they contain a small amount of water, will decompose under high temperature and pressure. This property is called hydrophilicity, and it must be preheated 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 materials, attention should be paid to dryness and reasonable selection of forming conditions to reduce internal stress and increase cracking resistance. And reasonable plastic shape should be selected, and measures such as setting embedded parts should not be taken to minimize stress concentration as much as possible. When designing the mold, the demolding angle should be increased, and reasonable feeding ports and ejection mechanisms should be selected. During molding, the material temperature, mold temperature, injection pressure, and cooling time should be appropriately adjusted to avoid demolding when the plastic part is too cold and brittle. After molding, the plastic part should also undergo post-treatment to improve its 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, obvious transverse cracks occur on the surface of the melt, which is called melt fracture and 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 rate

6.1 Various plastics have different thermal properties such as specific heat, thermal conductivity, and thermal deformation temperature. When plasticizing with high specific heat, high heat is required, and an injection molding machine with high plasticizing capacity should be selected. The cooling time of plastics with high thermal deformation temperature can be short and demoulded early, but cooling deformation should be prevented after demolding. Plastics with low thermal conductivity have a slow cooling rate (such as ionic polymers, which are extremely slow), so sufficient cooling is necessary to 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. It is necessary to choose appropriate injection molding machines and strengthen mold cooling.

6.2 Various plastics must maintain an appropriate cooling rate according to their type characteristics and shape. So 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 forming cycle, and reduce crystallinity. When the residual heat of plastic is insufficient to maintain a certain temperature in the mold, a heating system should be installed to keep the mold at a certain temperature, in order to control the cooling rate, ensure fluidity, improve filling conditions, or control the slow cooling of the plastic parts, prevent uneven internal and external cooling of thick walled plastic parts, and increase crystallinity. For materials with good fluidity, large forming area, and uneven material temperature, it is sometimes necessary to use heating or cooling alternately or locally according to the molding situation of the plastic part. For this purpose, the mold should be equipped with corresponding cooling or heating systems.

7、 Hygroscopicity

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