Detailed explanation of the injection molding proc

Detailed explanation of injection molding process of five general plastics

plastic performance is the foundation of injection molding technology ". Mastering the process performance and characteristics of various plastics is the basic professional knowledge that every injection molding worker must understand. Plastic performance is the basis for setting" injection molding process conditions ", and it is also one of the factors that must be considered when analyzing quality problems and abnormal phenomena in the injection molding process

polypropylene (PP) injection molding process

pp is generally known as polypropylene, which is also known as "hundred fold glue" because of its good breaking resistance. PP is a translucent, semi crystalline thermoplastic, which has the characteristics of high strength, good insulation, low water absorption, high thermal deformation temperature, low density and high crystallinity. Modified fillers usually include glass fiber, mineral filler, thermoplastic rubber, etc

the fluidity of PP for different purposes varies greatly. Generally, the flow rate of PP is between ABS and PC

pure PP is translucent ivory white, which can be dyed into various colors. PP dyeing can only use color masterbatch on general injection molding machines. On some machines that are otherwise easy to cause machine failure, personal injury and other dangerous situations, independent plasticizing elements with enhanced mixing effect can also be dyed with toner. Products used outdoors are generally filled with UV stabilizer and carbon black. The use proportion of recycled materials should not exceed 15%, otherwise it will cause strength reduction and decomposition discoloration. PP generally does not need special drying treatment before injection molding

there are no special requirements for the selection of injection molding machines. Because PP has high crystallinity. Computer injection molding machine with high injection pressure and multi-stage control is required. The clamping force is generally determined by 3800t/m2, and the injection amount is 20% - 85%

mold temperature ℃, for those with high dimensional requirements, use high mold temperature. The core temperature is more than 5 ℃ lower than the cavity temperature, the runner diameter is mm, and the needle gate length 5mm, the diameter can be as small as 0.7m, and it has won three consecutive "long 3 corner Development Zone with the most investment value" M. The shorter the length of the side gate, the better, about 0.7mm, the depth is half of the wall thickness, and the width is twice the wall thickness, and increases with the length of the melt flow in the mold cavity. The mold must have good exhaust performance. The exhaust hole is 0.025mm-0.038mm deep and 1.5mm thick. To avoid shrinkage marks, large and round injection ports and circular flow channels should be used, and the thickness of stiffeners should be small (such as% 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)

the melting point of PP is ℃, and the decomposition temperature is 350 ℃, but the temperature setting during injection processing cannot exceed 275 ℃, and the melting section temperature is preferably 240 ℃

in order to reduce internal stress and deformation, high-speed injection should be selected, but some grades of PP and molds are not applicable (bubbles and wrinkles appear). If there are bright and dark stripes diffused by the gate on the engraved surface, low-speed injection and high mold temperature should be used

5bar melt back pressure can be used, and the back pressure of toner can be adjusted appropriately

adopt higher injection pressure (bar) and holding pressure (about 80% of the injection pressure). Turn to pressure maintaining at about 95% of the full stroke, and use a longer pressure maintaining time

in order to prevent shrinkage deformation caused by post crystallization, products generally need to be soaked in hot water

polyethylene (PE) injection molding process

pe is a crystalline raw material, and its moisture absorption is very small, no more than 0.01%, so it does not need to be dried before processing. PE molecular chain has good flexibility, small bond force, low melt viscosity and excellent fluidity, so it can form thin-walled and long process products without too high pressure. PE has a wide range of shrinkage and a large shrinkage value. Their common feature is that they generally adopt new materials, new processes and new technologies. The shrinkage of LDPE is about 1.22% and that of HDPE is about 1.5%. Therefore, it is easy to deform and warp, and the mold cooling conditions have a great impact on the shrinkage. Therefore, the mold temperature should be well controlled to maintain uniform and stable cooling

pe has high crystallization ability, and the mold temperature has a great influence on the crystallization of plastic parts. High mold temperature, slow melt cooling, high crystallinity and high strength of plastic parts

pe has a low melting point but a large specific heat capacity, so it still needs to consume more heat during plasticization, so it is required that the plasticizing device should have a large 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 melt should be avoided from contacting with oxygen as much as possible in the molding process, so as not to reduce the quality of plastic parts

pe parts are soft and easy to demould, so when plastic parts have shallow side grooves, they can be demoulded strongly. The non Newtonian property 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 fully cooled. The mold should have a good cooling system

if PE melt is fed directly through the feed port during injection, the stress should be increased and uneven shrinkage and obvious directional deformation should be produced. Therefore, attention should be paid to the selection of feed port parameters. The molding temperature of PE is wide. In the flowing state, a little fluctuation of temperature has no effect on injection molding. PE has good thermal stability. Generally, there is no obvious decomposition phenomenon below 300 degrees, which has no impact on the quality

main molding conditions of PE

barrel temperature: barrel temperature is mainly related to the density and melt flow rate of PE, as well as the type and performance of injection molding machine and the shape of primary plastic parts. Because PE is a crystalline polymer, the grain must absorb a certain amount of heat during melting, so the barrel temperature should be 10 degrees higher than its melting point. For LDPE, the barrel temperature is controlled at ℃, for HDPE, the barrel temperature is controlled at 220 ℃, and the minimum value is taken at the rear of the barrel and the maximum value is taken at the front end

mold temperature: mold temperature has a great impact on the crystallization of plastic parts. High mold temperature, high melt crystallinity and high strength, but the shrinkage will also increase. Generally, the mold temperature of LDPE is controlled at 30 ℃ -45 ℃, while the temperature of HDPE is correspondingly higher

injection pressure: increasing the injection pressure is conducive to the mold filling of molten material. Due to the good fluidity of PE, except for thin-walled and slender products, a low injection pressure should be carefully selected. Generally, the injection pressure is MPa. Simple shape. For larger plastic parts behind the wall, the injection pressure can be lower, otherwise it is higher

polyvinyl chloride (PVC) injection molding process

typical applications: water supply pipes, household pipes, house wallboards, commercial machine shells, electronic product packaging, medical devices, food packaging, etc

chemical and physical properties: PVC material is an amorphous material. PVC materials often add stabilizers, lubricants, auxiliary processing agents, pigments, impact agents and other additives in practical use. PVC material has non flammability, high strength, weather resistance and excellent geometric stability

pvc has strong resistance to oxidants, reductants 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 occasions in contact with aromatic hydrocarbons and chlorinated hydrocarbons

pvc melting temperature is a very important process parameter during processing. If this parameter is inappropriate, 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 (this material usually needs to add lubricant to improve flow characteristics), so PVC materials with small molecular weight are usually used. The shrinkage of PVC is quite low, generally 0.2~0.6%

injection mold process conditions:

drying treatment: generally, drying treatment is not 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, it is generally necessary to use a considerable injection speed

runner and gate: all conventional gates can be used. If processing small parts, it is best to use needle type gate or submerged gate; For thicker parts, it is best to use fan-shaped gates. The minimum diameter of needle type gate or submerged gate shall be 1mm; The thickness of fan-shaped gate shall not be less than 1mm

chemical and physical properties: rigid PVC is one of the most widely used plastic materials

polystyrene (PS) note the company is well aware of the importance of LEGO in the production of creative toy products. Plastic technology

typical applications: product packaging, household goods (tableware, trays, etc.), electrical (transparent containers, light source scatterers, insulating films, etc.)

chemical and physical properties: most commercial PS are transparent and amorphous materials. PS has very good geometric stability, thermal stability, optical transmission characteristics, electrical insulation characteristics and very small moisture absorption tendency. It can resist water and diluted inorganic acids, but can be corroded by strong oxidizing acids such as concentrated sulfuric acid, and can expand and deform in some organic solvents. The typical shrinkage is between 0.4 and 0.7%

injection molding process conditions:

drying treatment: unless stored improperly, drying treatment is usually not required. If drying is required, the recommended drying condition is 80 ℃ for 2~3 hours

melting temperature: 180~280 ℃. For flame retardant materials, the upper limit is 250 ℃

mold temperature: 40~50 ℃

injection pressure: 200~600bar

injection speed: it is recommended to use a fast injection speed

runner and gate:: all conventional types of gates can be used

abs injection molding process

typical application scope: Automobile (dashboard, tool compartment door, wheel cover, reflector box, etc.), refrigerator, high-strength tools (hair dryer, mixer, food processing machine, lawn mower, etc.), shell, typewriter keyboard, entertainment vehicles such as golf cart and jet sledge, etc

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 an amorphous material

abs material has super workability, appearance characteristics, low creep, excellent dimensional stability and high impact strength

injection molding process conditions:

drying treatment: ABS material is hygroscopic and requires drying treatment before processing. The recommended drying condition is to dry at 80~90 ℃ for at least 2 hours. The material temperature shall 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, and low temperature will lead to low finish)

injection pressure: 500~1000bar

injection speed: medium high speed