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Commonly used plastic injection molding processes
Release time:2023-11-16
Commonly used plastic injection molding processes
I. Polypropylene (PP) injection molding process
PP is commonly known as polypropylene, because of its good resistance to fracture, also known as "folding glue". PP is a semi-transparent, semi-crystalline thermoplastic, with high strength, good insulation, low water absorption, high heat distortion temperature, low density, crystallinity and other characteristics. Modified fillers are usually glass fibers, mineral fillers, thermoplastic rubber and so on.
Different uses of PP its fluidity varies greatly, the general use of PP flow rate between ABS and PC.
Pure PP is translucent ivory white, can be dyed into a variety of colors.PP dyeing in the general injection molding machine can only use masterbatch. In some machines, there are independent plasticizing elements to strengthen the role of mixing, and can also be dyed with color powder. For products used outdoors, UV stabilizers and carbon black fillers are generally used.
The proportion of recycled material should not be more than 15%, otherwise it will cause strength loss and decomposition and discoloration.PP generally does not need special drying treatment before injection molding process.
PP injection mold process points:
There is no special requirement for the selection of injection molding machine. Because PP has high crystallinity. Need to use high injection pressure and can be multi-stage control of the computer injection molding machine. Clamping force is generally determined by 3800t/m2, the injection volume of 20% -85% can be.
Mold temperature 50-90 ℃, for higher size requirements with high mold temperature. Core temperature is lower than the cavity temperature of more than 5 ℃, runner diameter 4-7mm, needle gate length 1-1.5mm, diameter can be as small as 0.7mm. edge gate length is as short as possible, about 0.7mm, the depth of half of the wall thickness, the width of the wall thickness is twice as long as the length of the melt flow with the mold cavity increased by Ken. Mold must have good exhaust, exhaust hole depth 0.025mm-0.038mm, 1.5mm thick, to avoid shrinkage marks, we must use a large and round injection and round runners, the thickness of the reinforcement should be small (for example, is the wall thickness of 50-60%).
Homopolymerized PP manufactured products, the thickness can not exceed 3mm, otherwise there will be bubbles (thick-walled products can only be copolymerized PP).
PP melting point of 160-175 ℃, decomposition temperature of 350 ℃, but in the injection process the temperature setting can not exceed 275 ℃, the temperature of the molten section of the best 240 ℃.
In order to reduce the internal stress and deformation, should choose high-speed injection, but some grades of PP and molds are not suitable (bubbles, gas lines). If the patterned surface shows light and dark streaks spreading from the gate, then low speed injection and higher mold temperature should be used.
Use a back pressure of 5 bar for melts and a higher back pressure for color powders.
Use high injection pressure (1500-1800 bar) and holding pressure (about 80% of injection pressure). Turn the 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 should be soaked in hot water.
II. Polyethylene (PE) injection molding process
PE is crystalline raw materials, moisture absorption is very small, not more than 0.01%, so no need for drying before processing.PE molecular chain flexibility, small inter-bond force, melt viscosity is low, excellent fluidity, so molding without too high pressure can be molded out of thin-walled long flow products.
PE shrinkage range, shrinkage value is large, the direction is obvious, LDPE shrinkage rate of 1.22% or so, HDPE shrinkage rate of 1.5% or so. Therefore, it is easy to deformation warping, mold cooling conditions have a great impact on the shrinkage rate, so it should control the mold temperature, keep the cooling uniform and stable.
PE's crystallization ability is high, the temperature of the mold has a very large impact on the crystallization of the plastic parts. Mold temperature is high, the melt cools slowly, the crystallinity of the plastic parts is high, and the strength is also high.
PE's melting point is not high, but the specific heat capacity is larger, so the plasticization still need to consume more heat, so the plasticizing device is required to have a larger heating power, in order to improve the production efficiency.PE's softening temperature range is small, and the melt is easy to oxidize, so in the molding process should be avoided as far as possible to avoid contact between the melt and the oxygen occurs in order not to reduce the quality of the plastic parts.
PE parts are soft and easy to demold, so when the molded part has shallow side grooves can be strong demolding. non-Newtonian PE melt is not obvious, the change of shear rate on the viscosity of the impact of small, PE melt viscosity by the effect of temperature is also small. the cooling rate of the PE melt is slower, and therefore must be sufficiently cooled. The mold should have a better cooling system.
If PE melt is injected with direct feed port feeding, it should increase the stress and produce uneven shrinkage and direction of the obvious increase in deformation, so attention should be paid to the choice of feed port parameters.PE molding temperature is wider, in the flow state, the temperature of the few fluctuations on the injection molding has no effect.PE thermal stability is better, generally below 300 degrees without significant decomposition, there is little effect on the quality.
PE's main molding conditions:
Cylinder temperature: cylinder temperature is mainly related to the density of PE and melt flow rate size, but also with the type and performance of the injection molding machine, the shape of the first level of plastic parts. As PE is a crystalline polymer, in the melt grain to absorb a certain amount of heat, so the barrel temperature should be higher than its melting point of 10 degrees. Degree of LDPE, barrel temperature control at 140-200 ℃, HDPE barrel temperature control at 220 ℃, the barrel back to take the minimum value, the front to take the maximum value.
Mold temperature: mold temperature has a greater impact on the crystalline condition of the plastic parts, high mold temperature, melt crystallinity, high strength, but shrinkage will also increase. Usually the mold temperature of LDPE is controlled at 30℃-45℃, while the temperature of HDPE is 10-20℃ higher accordingly.
Injection pressure: increase the injection pressure is conducive to the melt of the mold, due to the mobility of PE is very good, so in addition to thin-walled elongated products, it should be fine selection of lower injection pressure, the general injection pressure of 50-100MPa. shape is simple. After the wall of the larger plastic parts, the injection pressure can be lower, and vice versa is high.
III. Polyvinyl chloride (PVC) injection molding process
Typical applications: water supply pipes, household pipes, house wall panels, commercial machine housings, electronic product packaging, medical devices, food packaging, etc.
Chemical and Physical Properties: PVC material is a non-crystalline material, PVC material is often used in the actual use of stabilizers, lubricants, auxiliary processing agents, color, impact resistance and other additives.
PVC material has non-flammability, high strength, resistance to weather changes and excellent geometric stability.
PVC is highly resistant 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 is not suitable for contact with aromatic and chlorinated hydrocarbons.
The melting temperature is a very important process parameter when processing PVC, and if this parameter is not correct, it will lead to material decomposition problems.The flow characteristics of PVC are quite poor, and its process range is very narrow.
Especially large molecular weight PVC material is more difficult to process (this material usually need to add lubricants to improve the flow characteristics), so usually use small molecular weight PVC material. the shrinkage of PVC is quite low, generally 0.2 ~ 0.6%.
Injection mold process conditions:
Drying treatment: drying treatment is usually not required.
Melting temperature: 185~205℃ Mold temperature: 20~50℃.
Injection pressure: up to 1500 bar.
Holding pressure: up to 1000 bar.
Injection speed: In order to avoid degradation of the material, the injection speed is usually quite high.
Runners and gates: All conventional gates can be used. For smaller parts it is better to use a pin-tip gate or a dive gate; for thicker parts it is better to use a fan gate. The minimum diameter of a pin-tip or submerged gate should be 1 mm; the thickness of a fan gate should not be less than 1 mm.
IV. Polystyrene (PS) injection molding process
Typical applications: product packaging, household goods (cutlery, trays, etc.), electrical (transparent containers, light diffusers, insulating films, etc.).
Chemical and Physical Properties:Most commercially available PS is a transparent, non-crystalline material.PS has very good geometric stability, thermal stability, optical transmission properties, electrical insulation properties and a very slight tendency to absorb moisture. It is resistant to water, dilute inorganic acids, but can be corroded by strong oxidizing acids such as concentrated sulfuric acid and can swell and deform in some organic solvents. Typical shrinkage is between 0.4 and 0.7%.
Injection Molding Process Conditions.
Drying treatment: Unless improperly stored, drying treatment is usually not required. If drying is required, the recommended drying conditions are 80°C and 2~3 hours.
Melting temperature: 180~280℃. For flame retardant materials the upper limit is 250°C.
Mold temperature: 40~50℃.
Injection pressure: 200~600 bar.
Injection speed: Fast injection speed is recommended.
Runner and gate: All conventional types of gates can be used.
V. ABS injection molding process
Typical applications: automobiles (dashboards, tool hatches, wheel covers, reflector cases, etc.), refrigerators, high-strength tools (hair dryers, blenders, food processors, lawnmowers, etc.), telephone housings, typewriter keypads, recreational vehicles such as golf carts, and jet sled vehicles.
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 and chemical stability; butadiene has toughness and impact resistance; styrene has easy processing, high gloss and high strength.
Morphologically, ABS is a non-crystalline material. In this case, 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 properties of ABS depend largely on the ratio of the three monomers and the molecular structure in the two phases. This allows for a great deal of flexibility in product design and has resulted in hundreds of different qualities of ABS materials on the market. These different qualities offer different properties, such as medium to high impact resistance, low to high gloss, and high-temperature twisting properties.
ABS materials offer superb processability, aesthetic properties, low creep and excellent dimensional stability as well as high impact strength.
Injection molding process conditions:
Drying treatment: ABS material is hygroscopic and requires drying treatment before processing. The recommended drying conditions are 80~90℃ for at least 2 hours. Material temperature should be guaranteed 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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