Detailed explanation of injection molding manufacturing process of 5 general-purpose plastics

"Plastic performance is foundation of injection molding?technology". Mastering process performance and characteristics of various plastics is basic professional knowledge that every injection molding?worker must understand. Performance of plastics is basis for setting "injection molding manufacturing process?conditions". One of factors that must be considered when analyzing quality problems and anomalies that occur during injection molding manufacturing process.

Polypropylene (PP) injection molding manufacturing process

PP is called polypropylene, because it has good anti-breaking performance, also known as "100 fold glue". 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 filler usually has glass fibers, mineral fillers, thermoplastic rubber, and so on.

PP of different uses has a large difference in fluidity, and PP flow rate generally used is between ABS and PC.

Pure PP is a translucent ivory white that can be dyed in a variety of colors. Dyeing of PP can only be used with color masterbatch on general injection molding?machines. On some machines, there are separate plasticizing elements that enhance mixing action, and can also be dyed with toner. Products for outdoor use are typically filled with UV stabilizers and carbon black. Proportion of recycled materials should not exceed 15%, otherwise it will cause strength drop and discoloration. PP does not require special drying before injection molding.

There are no special requirements for selection of injection molding?machines. Since PP has high crystallinity. A computer injection molding?machine with high injection pressure and multi-stage control is required. Clamping force is generally determined by 3800t/m2, and injection volume can be 20%-85%.

Mold temperature is 50-90℃, and high mold temperature is required for high dimensional requirements. Core temperature is 5℃?lower than cavity temperature, runner diameter is 4-7 mm, needle gate length is 1-1.5 mm, and diameter can be as small as 0.7 mm. The shorter length of edge gate, the better, about 0.7 mm, depth is half of wall thickness, width is twice wall thickness, and thickness of melt flow in cavity increases. Mold must have good exhaustibility. Vent hole depth is 0.025mm-0.038mm and thickness is 1.5mm. To avoid shrinkage marks, it is necessary to use large round nozzles and circular runners. Thickness of ribs should be small ( For example, 50-60% of wall thickness). Product made of homopolymerized PP should not exceed 3mm in thickness, otherwise there will be bubbles (thick-walled products can only be copolymerized with PP).

Melting point of PP is 160-175℃, decomposition temperature is 350℃, but temperature setting during injection processing cannot exceed 275℃, and temperature of melting section is preferably 240℃.

In order to reduce internal stress and deformation, high-speed injection should be selected, but some grades of PP and mold are not suitable (bubbles, gas lines appear). If surface of pattern has a light and dark streak spread by gate, a low speed injection and a higher mold temperature are used.

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

Use a higher injection pressure (1500-1800 bar) and a holding pressure (about 80% of injection pressure). It is about to maintain pressure at 95% of full stroke and use a longer holding time.

In order to prevent shrinkage deformation caused by post-crystallization, product generally needs to be immersed in hot water.

Polyethylene (PE) injection molding manufacturing process

PE is a crystalline raw material and has extremely low hygroscopicity of not more than 0.01%, so that it is not required to be dried before processing. PE molecular link is flexible, force between keys is small, melt viscosity is low, and fluidity is excellent, so that a thin-walled long process product can be formed without too high pressure during molding. PE has a large shrinkage range, large shrinkage value, and obvious directionality. LDPE shrinkage is about 1.22%, and HDPE shrinkage is about 1.5%. Therefore, it is easy to deform and warp, and mold cooling condition has a great influence on shrinkage rate, so mold temperature should be controlled to keep cooling uniform and stable.

Crystallization ability of PE is high, and temperature of mold has a great influence on crystallization state of plastic part. Mold temperature is high, melt cooling is slow, plastic part has high crystallinity and strength is high.

Melting point of PE is not high, but specific heat capacity is large, so it still needs to consume more heat during plasticization. Therefore, plasticizing device is required to have a large heating power in order to improve production efficiency. Softening temperature range of PE is small, and melt is easy to be oxidized. Therefore, contact between melt and oxygen should be avoided as much as possible in injection molding manufacturing process, so as to avoid quality of plastic part.

PE parts are soft in texture and easy to release, so when plastic parts have shallow side grooves, they can be strongly demolded. Non-Newtonian properties of PE melt are not obvious, change of shear rate has little effect on viscosity, and viscosity of PE melt is less affected by temperature. PE melt has a slower cooling rate and must therefore be cooled sufficiently. Mold should have a better cooling system.

If PE melt is fed with a direct feed port during injection, stress,?uneven shrinkage and?directional deformation should be increased. Therefore, care should be taken to select feed port parameters. PE has a relatively high molding temperature, and in flow state, a slight fluctuation in temperature has no effect on injection molding. PE has good thermal stability, and generally has no obvious decomposition phenomenon below 300 degrees, and has no effect on quality.

Main molding conditions of PE

Barrel temperature: Barrel temperature is mainly related to density of PE and flow rate of melt. It is also related to type,?performance of injection molding?machine and shape of the first-class plastic part. Since PE is a crystalline polymer, crystal grains absorb a certain amount of heat during melting, so cylinder temperature should be 10 degrees higher than its melting point. For LDPE, barrel temperature is controlled at 140-200℃, HDPE barrel temperature is controlled at 220℃. Minimum is taken at the rear of barrel, and front end takes maximum value.

Mold temperature: Mold temperature?has a great influence on crystallization condition of plastic part. Mold temperature?is high, melt crystallinity is high, and strength is high, but shrinkage rate also increases. Generally, mold temperature?of LDPE is controlled at 30℃?- 45℃, and temperature of HDPE is correspondingly higher by 10-20℃.

Injection pressure: increasing injection pressure is conducive to filling of melt. Since fluidity of PE is very good, in addition to thin-walled slender products, lower injection pressure should be selected well, and general injection pressure is 50-100 MPa. Shape is simple. For plastic parts with a simple shape and a large wall thickness, injection pressure can be lower, and vice versa.

Polyvinyl chloride (PVC) injection molding manufacturing process

Typical applications: 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 materials are non-flammable, high strength, weather resistant 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, concentrated nitric acid, and is also not suitable for contact with aromatic hydrocarbons or chlorinated hydrocarbons.

Melting temperature of PVC during processing is a very important process parameter. If this parameter is improper, it will lead to problem of material decomposition. Flow characteristics of PVC are quite poor and process range is narrow.

In particular, large molecular weight PVC materials are more difficult to process (this material usually incorporates a lubricant to improve flow characteristics), so small molecular weight PVC materials are usually used. Shrinkage rate of PVC is quite low, generally 0.2 to 0.6%.

Injection molding manufacturing process?conditions:

Drying treatment: Drying is usually not required.

Melting temperature: 185~205°C Mold temperature: 20~50°C.

Injection pressure: up to 1500 bar.

Holding pressure: up to 1000 bar.

Injection speed: In order to avoid material degradation, a considerable injection speed is generally used.

Runners and gates: All conventional gates can be used. If you are working with smaller parts, it is best to use pin-point?gates or submerged gates; for thicker parts, it is best to use fan-shaped gates. Minimum diameter of tip or submerged gate shall be 1 mm; thickness of fan gate shall not be less than 1 mm.

Chemical and physical properties: Rigid PVC is one of the most widely used plastic materials.

Polystyrene (PS) injection molding manufacturing process

Typical applications: product packaging, household items (tableware, trays, etc.), electrical (transparent containers, light source diffusers, insulating films, etc.).

Chemical and physical properties: Most commercial PSs are transparent, amorphous materials. PS has very good geometric stability, thermal stability, optical transmission characteristics, electrical insulation properties and a very small tendency to absorb moisture. It is resistant to water, diluted inorganic acids, but can be corroded by strong oxidizing acids such as concentrated sulfuric acid,?can be expanded and deformed in some organic solvents. Typical shrinkage is between 0.4 and 0.7%.

Injection molding manufacturing process?conditions:

Drying treatment: Drying is usually not required unless stored improperly. If drying is required, it is recommended to dry at 80℃?for 2-3 hours.

Melting temperature: 180~280℃. Upper limit for flame retardant material is 250℃.

Mold temperature: 40~50℃.

Injection pressure: 200~600bar.

Injection speed: A fast injection speed is recommended.

Runners and gates: All conventional types of gates can be used.

ABS injection molding manufacturing process

Typical applications: automotive (dashboard, tool door, wheel cover, mirror box, etc.), refrigerator, high-strength tools (hair dryer, blender, food processor, lawn mower, etc.), telephone housing, typewriter keyboard, 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 properties: acrylonitrile has high strength, thermal stability and chemical stability; butadiene has toughness and impact resistance; styrene has easy processing, high finish and high strength. From a morphological point of view, ABS is an amorphous material.

Polymerization of three monomers produces a two-phase terpolymer, one being a continuous phase of styrene-acrylonitrile and the other being a polybutadiene rubber dispersed phase. Properties of ABS mainly depend on the ratio of three monomers and molecular structure in two phases. This allows for great flexibility in product design and results in hundreds of different quality ABS materials on the market. These different quality materials offer different properties such as medium to high impact resistance, low to high finish and high temperature distortion.

ABS materials are superior in processability, appearance characteristics, low creep and excellent dimensional stability and high impact strength.

Injection molding manufacturing process?conditions:

Drying treatment: ABS materials are hygroscopic and require drying treatment before processing. It is recommended to dry for at least 2 hours at 80-90℃. 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 smoothness of plastic part, and lower temperature will result in a lower finish).

Injection pressure: 500~1000bar.

Injection speed: medium to high speed.