A. Polypropylene (PP) injection molding process
The fluidity of PP for different purposes is quite different, and the PP flow rate generally used is between ABS and PC.
1. Plastic processing
Pure PP is translucent ivory white and can be dyed in various colors. For PP dyeing, only color masterbatch can be used on general injection molding machines. On some machines, there are independent plasticizing elements that strengthen the mixing effect, and they can also be dyed with toner. Products used outdoors are generally filled with UV stabilizers and carbon black. The use ratio of recycled materials should not exceed 15%, otherwise it will cause strength drop and decomposition and discoloration. Generally, no special drying treatment is required before PP injection molding.
2. Selection of injection molding machine
There are no special requirements for the selection of injection molding machines. Because PP has high crystallinity. A computer injection molding machine with higher injection pressure and multi-stage control is required. The clamping force is generally determined at 3800t/m2, and the injection volume is 20%-85%.
3. Mould and gate design
The mold temperature is 50-90℃, and the high mold temperature is used for the higher size requirements. The core temperature is more than 5℃ lower than the cavity temperature, the runner diameter is 4-7mm, the needle gate length is 1-1.5mm, and the diameter can be as small as 0.7mm.
The length of the edge gate is as short as possible, about 0.7mm, the depth is half of the wall thickness, and the width is twice the wall thickness, and it will gradually increase with the length of the melt flow in the cavity. The mold must have good venting. The vent hole is 0.025mm-0.038mm deep and 1.5mm thick. To avoid shrinkage marks, use large and round nozzles and circular runners, and the thickness of the ribs should be small ( For example, 50-60% of the wall thickness).
The thickness of products made of homopolymer PP should not exceed 3mm, otherwise there will be bubbles (thick wall products can only use copolymer PP).
4. Melting temperature: The melting point of PP is 160-175°C, and the decomposition temperature is 350°C, but the temperature setting during injection processing cannot exceed 275°C, and the temperature of the melting section is best 240°C.
5. Injection speed: In order to reduce internal stress and deformation, high-speed injection should be selected, but some grades of PP and molds are not suitable (bubbles and air lines appear). If the patterned surface appears with light and dark stripes diffused by the gate, low-speed injection and higher mold temperature are required.
6. Melt adhesive back pressure: 5bar melt adhesive back pressure can be used, and the back pressure of the toner material can be appropriately increased.
7. Injection and holding pressure: Use higher injection pressure (1500-1800bar) and holding pressure (about 80% of the injection pressure). Switch to holding pressure at about 95% of the full stroke and use a longer holding time.
8. Post-treatment of the product: In order to prevent the shrinkage and deformation caused by the post-crystallization, the product generally needs to be soaked in hot water.
B. Polyethylene (PE) injection molding process
PE is a crystalline raw material with extremely low hygroscopicity, no more than 0.01%, so there is no need for drying before processing. PE molecular chain has good flexibility, small force between bonds, low melt viscosity, and excellent fluidity. Therefore, thin-walled and long-process products can be formed without too high pressure during molding.
△ PE has a wide range of shrinkage rate, large shrinkage value, and obvious directionality. The shrinkage rate of LDPE is about 1.22%, and the shrinkage rate of HDPE is about 1.5%. Therefore, it is easy to deform and warp, and mold cooling conditions have a great influence on shrinkage. Therefore, the mold temperature should be controlled to maintain uniform and stable cooling.
△ PE has high crystallization ability, and the temperature of the mold has a great influence on the crystallization condition of plastic parts. High mold temperature, slow melt cooling, high crystallinity of plastic parts, and high strength.
△ PE's melting point is not high, but its specific heat capacity is large, so it still needs to consume more heat during plasticization. Therefore, the plasticizing device is required to have a large heating power to improve production efficiency.
△ The softening temperature range of PE is small, and the melt is easy to oxidize. Therefore, contact between the melt and oxygen should be avoided as much as possible during the molding process, so as not to reduce the quality of plastic parts.
△ PE parts are soft and easy to demold, so when the plastic parts have shallow grooves, they can be demolded strongly.
△ The non-Newtonian property of PE melt is not obvious, the change of shear rate has little influence on viscosity, and the influence of temperature on PE melt viscosity is also little.
△ PE melt has a slow cooling rate, so it must be cooled sufficiently. The mold should have a better cooling system.
If the PE melt is fed directly from the feed port during injection, the stress should be increased and the uneven shrinkage and the directionality of the obvious increase and deformation should be increased, so attention should be paid to the selection of the feed port parameters.
△ The molding temperature of PE is relatively wide. In the fluid state, a little temperature fluctuation has no effect on injection molding.
△ PE has good thermal stability, generally there is no obvious decomposition phenomenon below 300 degrees, and it has no effect on the quality.
The main molding conditions of PE
Barrel temperature: The barrel temperature is mainly related to the density of PE and the size of the melt flow rate. It is also related to the type and performance of the injection molding machine and the shape of the first-class plastic part. Since PE is a crystalline polymer, the crystal grains have to absorb a certain amount of heat during melting, so the barrel temperature should be 10 degrees higher than its melting point. For LDPE, the temperature of the barrel is controlled at 140-200°C, the temperature of the HDPE barrel is controlled at 220°C, the minimum value at the rear of the barrel and the maximum at the front end.
Mold temperature: Mold temperature has a great influence on the crystallization condition of plastic parts. High mold temperature, high melt crystallinity and high strength, but the shrinkage rate will also increase. Usually the mold temperature of LDPE is controlled at 30℃-45℃, while the temperature of HDPE is correspondingly higher by 10-20℃.
Injection pressure: Increasing the injection pressure is beneficial to the filling of the melt. Because the fluidity of PE is very good, in addition to thin-walled and slender products, a lower injection pressure should be selected carefully. The general injection pressure is 50-100MPa. The shape is simple. For larger plastic parts behind the wall, the injection pressure can be lower, and vice versa
C. Polyvinyl chloride (PVC) injection molding process
The melting temperature of PVC during processing is a very important process parameter. If this parameter is not appropriate, it will cause material decomposition. The flow characteristics of PVC are quite poor, and its process range is very narrow.
Especially the high molecular weight PVC material is more difficult to process (this kind of material usually needs to be added with lubricant to improve the flow characteristics), so PVC materials with small molecular weight are usually used. The shrinkage rate of PVC is quite low, generally 0.2~0.6%.
Injection mould process conditions:
· 1. Drying treatment: usually no drying treatment is required.
· 2. Melting temperature: 185~205℃ Mold temperature: 20~50℃.
· 3. Injection pressure: up to 1500bar.
· 4. Holding pressure: up to 1000 bar.
· 5. Injection speed: In order to avoid material degradation, a considerable injection speed is generally used.
· 6. Runner and gate: all conventional gates can be used. If processing smaller parts, it is best to use needle-point gates or submerged gates; for thicker parts, it is best to use fan gates. The minimum diameter of the needle-point gate or the submerged gate should be 1mm; the thickness of the fan gate should not be less than 1mm.
· 7. Chemical and physical properties: Rigid PVC is one of the most widely used plastic materials.
D. Polystyrene (PS) injection molding process
Injection mould process conditions:
1. Drying treatment: Unless stored improperly, drying treatment is usually not required. If drying is required, the recommended drying conditions are 80°C for 2 to 3 hours.
2. Melting temperature: 180~280℃. For flame-retardant materials, the upper limit is 250°C.
3. Mold temperature: 40~50℃.
4. Injection pressure: 200~600bar.
5. Injection speed: It is recommended to use a fast injection speed.
6. Runner and gate: All conventional types of gates can be used.
E. ABS injection molding process
ABS material has super easy processing, appearance characteristics, low creep and excellent dimensional stability and high impact strength.
Injection mould process conditions:
1. Drying treatment: ABS material is hygroscopic and requires drying treatment before processing. The recommended drying condition is at least 2 hours at 80~90℃. The material temperature should be less than 0.1%.
2. Melting temperature: 210~280℃; recommended temperature: 245℃.
3. Mold temperature: 25~70℃. (Mold temperature will affect the finish of plastic parts, lower temperature will lead to lower finish).
4. Injection pressure: 500~1000bar.
5. Injection speed: medium to high speed.
