Detailed Explanation of Five General Plastic Injection Molding Processes
1. Polypropylene (PP) injection molding process
PP is commonly known as polypropylene, and it is also known as “100% glue” because of its good fracture resistance. PP is a translucent, semi-crystalline thermoplastic with high strength, good insulation, low water absorption, high heat distortion temperature, low density, and high crystallinity. Modified fillers usually include glass fiber, mineral filler, thermoplastic rubber, etc.
The fluidity of PP for different purposes is quite different, and the flow rate of PP generally used is between that of ABS and PC.
Pure PP is translucent ivory white and can be dyed into various colors. PP dyeing can only use color masterbatch on general injection molding machines. In some machines, there are independent plasticizing elements that strengthen the mixing effect, and can also be dyed with toner. Products for outdoor use are generally filled with UV stabilizers and carbon black.
Do not use more than 15% recycled materials, otherwise it will cause strength decline and discoloration. PP injection molding generally does not require special drying treatment before processing.
Key points of PP injection molding process:
There are no special requirements for the selection of injection molding machines. Because PP has high crystallinity. A computerized injection molding machine with high injection pressure and multi-stage control is required. The clamping force is generally determined as 3800t/m2, and the injection volume is 20%-85%.
The mold temperature is 50-90°C, and high mold temperature is used for higher size requirements. The temperature of the core is 5°C lower than the temperature of the cavity, the diameter of the runner is 4-7mm, the length of the needle gate is 1-1.5mm, and the diameter can be as small as 0.7mm. The shorter the length of the side gate, the better, about 0.7mm, the depth is half of the wall thickness, the width is twice the wall thickness, and it increases gradually with the length of the melt flow in the cavity. The mold must have good air venting, the vent hole is 0.025mm-0.038mm deep, and the thickness is 1.5mm. To avoid shrinkage marks, it is necessary to use a large and round sprue and a circular runner, and the thickness of the ribs should be small ( For example 50-60% of the wall thickness).
For products made of homopolymer PP, the thickness cannot exceed 3mm, otherwise there will be air bubbles (thick-walled products can only use copolymerized PP).
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 preferably 240°C.
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 marks appear). If there are light and dark stripes diffused by the gate on the surface engraved with patterns, low-speed injection and high mold temperature should be used.
The back pressure of 5bar melt adhesive can be used, and the back pressure of toner material can be adjusted appropriately.
Use higher injection pressure (1500-1800bar) and holding pressure (about 80% of injection pressure). Turn to holding pressure at about 95% of the full stroke, and use a longer holding time.
In order to prevent shrinkage and deformation caused by post-crystallization, the product generally needs to be soaked in hot water.
2. Polyethylene (PE) injection molding process
PE is a crystalline raw material with very little hygroscopicity, no more than 0.01%, so it does not need to be dried before processing. PE molecular chains have good flexibility, small inter-bond forces, low melt viscosity, and excellent fluidity, so thin-walled long-flow products can be formed without too much pressure during molding.
The shrinkage range of PE is large, the shrinkage value is large, and the directionality is obvious. 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 the cooling conditions of the mold have a great influence on the shrinkage rate, so the temperature of the mold should be well controlled to keep the cooling uniform and stable.
The crystallization ability of PE is high, and the temperature of the mold has a great influence on the crystallization of plastic parts. The mold temperature is high, the melt cooling is slow, the crystallinity of the plastic part is high, and the strength is also high.
The melting point of PE is not high, but the specific heat capacity is large, so it still needs to consume more heat during plasticization, so the plasticizing device is required to have a larger heating power in order to improve production efficiency. The softening temperature range of PE is small, and the melt is easy to oxidize, so the 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 the plastic part.
PE parts are soft and easy to demould, so when the plastic part has shallow side grooves, it can be demolded strongly. The non-Newtonianity of PE melt is not obvious, the change of shear rate has little effect on viscosity, and the viscosity of PE melt is also less affected by temperature. The cooling rate of PE melt is slow, 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 uneven shrinkage and directional deformation should be increased, so attention should be paid to the selection of the feed port parameters. The molding temperature of PE is wide, and in the flowing state, a small fluctuation of temperature 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:
Cylinder temperature: Cylinder temperature is mainly related to the density of PE and the melt flow rate, and it is also related to the type and performance of the injection molding machine and the shape of the primary plastic part. Since PE is a crystalline polymer, the crystal grains absorb a certain amount of heat during melting, so the temperature of the barrel should be 10 degrees higher than its melting point. For LDPE, the barrel temperature is controlled at 140-200°C, and the barrel temperature of HDPE is controlled at 220°C. The rear of the barrel takes the minimum value and the front end takes the maximum value.
Mold temperature: Mold temperature has a great influence on the crystallization status of plastic parts. Higher mold temperature leads to higher melt crystallinity and higher strength, but the shrinkage rate will also increase. Usually the mold temperature of LDPE is controlled at 30°C-45°C, while the temperature of HDPE is correspondingly higher by 10-20°C.
Injection pressure: Increasing the injection pressure is conducive to the filling of the molten material. Since PE has good fluidity, it should be carefully selected for a lower injection pressure except for thin-walled and slender products. 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.
3. Polyvinyl chloride (PVC) injection molding process
Typical application range: water supply pipes, household pipes, house wall panels, commercial machine casings, electronic product packaging, medical equipment, food packaging, etc.
Chemical and physical properties: PVC material is a non-crystalline material. PVC materials are often added with stabilizers, lubricants, auxiliary processing agents, colorants, impact agents and other additives in actual use.
PVC material has non-flammability, high strength, resistance to climate change and excellent geometric stability.
PVC has strong resistance to oxidizing agents, reducing agents and strong acids. However, it can be corroded by concentrated oxidizing acids such as concentrated sulfuric acid and concentrated nitric acid, and it is not suitable for contact with aromatic hydrocarbons and chlorinated hydrocarbons.
The melting temperature of PVC is a very important process parameter during processing. If this parameter is improper, it will lead to the problem of material decomposition. The flow characteristics of PVC are quite poor, and its process range is very narrow.
In particular, PVC materials with large molecular weight are more difficult to process (such materials usually need to add lubricants to improve flow characteristics), so PVC materials with low molecular weight are usually used. The shrinkage rate of PVC is quite low, generally 0.2~0.6%.
Injection mold process conditions:
Drying treatment: Usually no drying treatment is required.
Melting temperature: 185~205℃ Mold temperature: 20~50℃.
Injection pressure: up to 1500bar.
Holding pressure: up to 1000bar.
Injection speed: In order to avoid material degradation, a considerable injection speed is generally used.
Runners and Gates: All conventional gates can be used. For smaller parts, pin gates or submerged gates are best; for thicker parts, fan gates are better. The minimum diameter of the pinpoint gate or submerged gate should be 1mm; the thickness of the fan gate should not be less than 1mm.
4. Polystyrene (PS) injection molding process
Typical applications: product packaging, household items (tableware, trays, etc.), electrical appliances (transparent containers, light source diffusers, insulating films, etc.).
Chemical and Physical Properties: Most commercial PSs are transparent, non-crystalline materials. PS has very good geometric stability, thermal stability, optical transmission properties, electrical insulation properties and very slight hygroscopic tendency. It is resistant to water and diluted inorganic acids, but it can be corroded by strong oxidizing acids such as concentrated sulfuric acid, and it can swell and deform in some organic solvents. Typical shrinkage is between 0.4 and 0.7%.
Injection mold process conditions:
Drying: Drying is usually not required unless improperly stored. If drying is required, the recommended drying condition is 80°C for 2-3 hours.
Melting temperature: 180~280℃. For flame-retardant materials, the upper limit is 250°C.
Mold temperature: 40~50℃.
Injection pressure: 200~600bar.
Injection speed: It is recommended to use a fast injection speed.
Runners and Gates: All conventional types of gates can be used.
Five. ABS injection molding process
Typical applications: automobiles (dashboards, tool hatches, wheel covers, mirror boxes, etc.), refrigerators, high-intensity tools (hair dryers, blenders, food processors, lawn mowers, etc.), telephone housings keyboards, recreational vehicles such as golf carts and jet skis.
Chemical and physical properties: ABS is synthesized from three chemical monomers: acrylonitrile, butadiene and styrene. Each monomer has different characteristics: Acrylonitrile has high strength, thermal stability and chemical stability; Butadiene has toughness and impact resistance; Styrene has easy processing, high finish and high strength.
In terms of morphology, ABS is a non-crystalline material. Here, the polymerization of the three monomers produces a terpolymer with two phases, a continuous phase of styrene-acrylonitrile and a dispersed phase of polybutadiene rubber.
The characteristics of ABS mainly depend on the ratio of the three monomers and the molecular structure in the two phases. This allows great flexibility in product design, and thus produces hundreds of ABS materials of different qualities on the market. These different qualities of material offer different properties, such as medium to high impact resistance, low to high finish, and high temperature distortion characteristics, among others.
ABS material has super easy processing, appearance characteristics, low creep, excellent dimensional stability and high impact strength.
Injection mold process conditions:
Drying treatment: ABS material is hygroscopic and requires drying treatment before processing. The recommended drying condition is at least 2 hours at 80~90°C. Material temperature should be guaranteed to be less than 0.1%.
Melting temperature: 210~280℃; recommended temperature: 245℃.
Mold temperature: 25~70℃. (Mold temperature will affect the finish of plastic parts, lower temperature will result in lower finish).
Injection pressure: 500~1000bar.
Injection speed: medium to high speed.
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