Influencing factors of injection molding process
DATE：2022/7/25 9:14:50 / READ： / SOURCE：This station
Influencing factors of injection molding process
1、 Shrinkage
1. Plastic varieties. During the molding process of thermoplastics, due to the volume changes caused by the formation of products, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation and other factors, compared with thermosetting plastics, the shrinkage rate is larger, the shrinkage range is wide, and the directivity is obvious. In addition, the shrinkage rate after molding, annealing or humidity control treatment is generally larger than that of thermosetting plastics. Coating composite production line www.handern.com
2. The structural characteristics of plastic parts, whether plastic parts have inserts, the layout and quantity of inserts directly affect the direction of material flow, density distribution and shrinkage resistance, etc., so the characteristics of plastic parts have a great impact on the size and direction of shrinkage. Coating composite production line www.handern.com
3. The form, size and distribution of the feed inlet directly affect the material flow direction, density distribution, pressure maintaining and feeding effect and forming time. The direct feed port and the feed port with large section (especially the thicker section) have small shrinkage but large directionality, while the feed port with short width and length has small directionality. If it is close to the feed inlet or parallel to the direction of material flow, the shrinkage is large.
4. Injection molding conditions. Injection molding conditions mainly refer to the mold and plasticizing temperature during plastic parts processing, and the pressure, speed, time, etc. during injection and pressure holding. Coating composite production line www.handern.com
High mold temperature, slow cooling of molten material, high density and large shrinkage, especially for crystalline material, due to high crystallinity and large volume change, the shrinkage is greater. The mold temperature distribution is also related to the internal and external cooling of plastic parts. Chapter 2 injection molding process parameter setting and its influencing factors are also related to the density uniformity, which directly affects the shrinkage and direction of each part. Holding pressure and time also have a greater impact on contraction. Those with high pressure and long time have small contraction but large directionality. The injection pressure is high, the viscosity difference of molten material is small, the interlayer shear stress is small, and the elastic rebound after demoulding is large, so the shrinkage can also be reduced appropriately. The material has high temperature and large shrinkage, but the directivity is small. Therefore, the shrinkage of plastic parts can be appropriately changed by adjusting mold temperature, pressure, speed and cooling time during injection and pressure holding.
During mold design, according to the shrinkage range of various plastics, the wall thickness and shape of plastic parts, the form, size and distribution of feed inlet, determine the shrinkage rate of various parts of plastic parts according to experience, and then calculate the cavity size. For high-precision plastic parts and when it is difficult to master the shrinkage rate, the following methods should be generally 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, so as to leave room for correction after mold test; ② Mold test to determine the form, size and forming conditions of the gating system; ③ The size change of plastic parts to be post treated shall be determined after post-treatment (the measurement must be after 24h after demoulding); ④ Correct the mold according to the actual shrinkage; ⑤ Try the mold again and change the process conditions appropriately to slightly modify the shrinkage value to meet the requirements of plastic parts. Coating composite production line www.handern.com
2、 The fluidity of flowable 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 regularity of molecular structure, high melt index, long spiral flow length, small apparent viscosity, and large flow ratio will improve the fluidity; 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 oxide. Poor liquidity: PC, hard PVC, polyphenylene oxide, polysulfone, polysulfone, fluoroplastics. Injection molding process technology and production management the fluidity of various plastics also changes due to various molding factors, and the main influencing factors are as follows: ① the fluidity increases when the temperature is high, but different plastics also have differences. The fluidity of PS (especially those with high impact resistance and MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, as), PC, Ca and other plastics changes greatly with temperature. PE, POM The increase or decrease of temperature has little effect on its fluidity. Therefore, the former should adjust the temperature to control the fluidity during molding ② pressure when the injection pressure increases, the molten material will be sheared and the fluidity will also increase, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control the fluidity during molding. ③ Mold structure, the form, size, layout, cooling system design, molten material flow resistance (such as cavity surface finish, material channel section thickness, cavity shape, exhaust system) and other factors directly affect the actual fluidity of molten material in the cavity. Coating composite production line www.handern.com
3、 Crystalline thermoplastics can be divided into crystalline plastics and amorphous (also known as amorphous) plastics according to whether there is crystallization during condensation. The so-called condensation phenomenon refers to the phenomenon that when the plastic changes from melting state to condensation, the molecules move independently, completely in an disordered state, and become molecules that stop moving freely, according to a slightly fixed position, and there is a tendency for the arrangement of molecules to become a regular model. The appearance standard of these two types of plastics can be judged according to the transparency of thick wall plastic parts. Generally, crystalline materials are opaque or translucent (such as POM, etc.), Amorphous materials are transparent (such as PMMA, etc.). The following requirements and precautions for crystalline plastics should be paid attention to when designing molds and selecting injection molding machines. ① The heat required for the material temperature to rise to the forming temperature is more, so the equipment with large plasticizing capacity should be used. ② The heat released during cooling and shaping is large, so it should be fully cooled. ③ The density difference between molten state and solid state is large, forming shrinkage is large, and shrinkage and porosity are easy to occur. ④ Fast cooling, low crystallinity, small shrinkage and high transparency.
1、 Shrinkage
1. Plastic varieties. During the molding process of thermoplastics, due to the volume changes caused by the formation of products, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation and other factors, compared with thermosetting plastics, the shrinkage rate is larger, the shrinkage range is wide, and the directivity is obvious. In addition, the shrinkage rate after molding, annealing or humidity control treatment is generally larger than that of thermosetting plastics. Coating composite production line www.handern.com
2. The structural characteristics of plastic parts, whether plastic parts have inserts, the layout and quantity of inserts directly affect the direction of material flow, density distribution and shrinkage resistance, etc., so the characteristics of plastic parts have a great impact on the size and direction of shrinkage. Coating composite production line www.handern.com
3. The form, size and distribution of the feed inlet directly affect the material flow direction, density distribution, pressure maintaining and feeding effect and forming time. The direct feed port and the feed port with large section (especially the thicker section) have small shrinkage but large directionality, while the feed port with short width and length has small directionality. If it is close to the feed inlet or parallel to the direction of material flow, the shrinkage is large.
4. Injection molding conditions. Injection molding conditions mainly refer to the mold and plasticizing temperature during plastic parts processing, and the pressure, speed, time, etc. during injection and pressure holding. Coating composite production line www.handern.com
High mold temperature, slow cooling of molten material, high density and large shrinkage, especially for crystalline material, due to high crystallinity and large volume change, the shrinkage is greater. The mold temperature distribution is also related to the internal and external cooling of plastic parts. Chapter 2 injection molding process parameter setting and its influencing factors are also related to the density uniformity, which directly affects the shrinkage and direction of each part. Holding pressure and time also have a greater impact on contraction. Those with high pressure and long time have small contraction but large directionality. The injection pressure is high, the viscosity difference of molten material is small, the interlayer shear stress is small, and the elastic rebound after demoulding is large, so the shrinkage can also be reduced appropriately. The material has high temperature and large shrinkage, but the directivity is small. Therefore, the shrinkage of plastic parts can be appropriately changed by adjusting mold temperature, pressure, speed and cooling time during injection and pressure holding.
During mold design, according to the shrinkage range of various plastics, the wall thickness and shape of plastic parts, the form, size and distribution of feed inlet, determine the shrinkage rate of various parts of plastic parts according to experience, and then calculate the cavity size. For high-precision plastic parts and when it is difficult to master the shrinkage rate, the following methods should be generally 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, so as to leave room for correction after mold test; ② Mold test to determine the form, size and forming conditions of the gating system; ③ The size change of plastic parts to be post treated shall be determined after post-treatment (the measurement must be after 24h after demoulding); ④ Correct the mold according to the actual shrinkage; ⑤ Try the mold again and change the process conditions appropriately to slightly modify the shrinkage value to meet the requirements of plastic parts. Coating composite production line www.handern.com
2、 The fluidity of flowable 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 regularity of molecular structure, high melt index, long spiral flow length, small apparent viscosity, and large flow ratio will improve the fluidity; 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 oxide. Poor liquidity: PC, hard PVC, polyphenylene oxide, polysulfone, polysulfone, fluoroplastics. Injection molding process technology and production management the fluidity of various plastics also changes due to various molding factors, and the main influencing factors are as follows: ① the fluidity increases when the temperature is high, but different plastics also have differences. The fluidity of PS (especially those with high impact resistance and MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, as), PC, Ca and other plastics changes greatly with temperature. PE, POM The increase or decrease of temperature has little effect on its fluidity. Therefore, the former should adjust the temperature to control the fluidity during molding ② pressure when the injection pressure increases, the molten material will be sheared and the fluidity will also increase, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control the fluidity during molding. ③ Mold structure, the form, size, layout, cooling system design, molten material flow resistance (such as cavity surface finish, material channel section thickness, cavity shape, exhaust system) and other factors directly affect the actual fluidity of molten material in the cavity. Coating composite production line www.handern.com
3、 Crystalline thermoplastics can be divided into crystalline plastics and amorphous (also known as amorphous) plastics according to whether there is crystallization during condensation. The so-called condensation phenomenon refers to the phenomenon that when the plastic changes from melting state to condensation, the molecules move independently, completely in an disordered state, and become molecules that stop moving freely, according to a slightly fixed position, and there is a tendency for the arrangement of molecules to become a regular model. The appearance standard of these two types of plastics can be judged according to the transparency of thick wall plastic parts. Generally, crystalline materials are opaque or translucent (such as POM, etc.), Amorphous materials are transparent (such as PMMA, etc.). The following requirements and precautions for crystalline plastics should be paid attention to when designing molds and selecting injection molding machines. ① The heat required for the material temperature to rise to the forming temperature is more, so the equipment with large plasticizing capacity should be used. ② The heat released during cooling and shaping is large, so it should be fully cooled. ③ The density difference between molten state and solid state is large, forming shrinkage is large, and shrinkage and porosity are easy to occur. ④ Fast cooling, low crystallinity, small shrinkage and high transparency.
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