The injection molding process of the five general plastics is detailed and worth collecting!

“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. The performance of plastics is the basis for setting the “injection molding process conditions”, and is also one of the factors that must be considered when analyzing the quality problems and anomalies in the injection molding process.

1. Injection molding process of polypropylene (PP)

PP is generally called polypropylene, also known as “100 fold rubber” because of its good anti breaking performance. PP is a semi transparent and semi crystalline thermoplastic with high strength, good insulation, low water absorption, high thermal deformation temperature, low density and high crystallinity. The modified fillers usually include glass fiber, mineral filler, thermoplastic rubber, etc.

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

Pure PP is translucent ivory white and can be dyed into various colors. PP can only be dyed with color masterbatch on general injection molding machines. On some machines, there are independent plasticizing elements to enhance mixing, which can also be dyed with toner. Products used outdoors are generally filled with UV stabilizer and carbon black. The proportion of recycled material used shall not exceed 15%, or it will cause strength reduction and decomposition discoloration. PP 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 multistage 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 ℃, and high mold temperature is used for those with high 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 shorter the edge gate length is, the better, about 0.7mm. The depth is half of the wall thickness, and the width is twice of the wall thickness. It increases with the melt flow length 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 orifices and circular flow channels should be used. The thickness of stiffeners should be small (for example, 50-60% of the wall thickness). The thickness of products made of homopolymer PP shall not exceed 3mm, otherwise there will be bubbles (thick wall products can only use copolymer PP).

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

In order to reduce internal stress and deformation, high-speed injection should be selected, but some grades of PP and mold are not applicable (bubbles and air wrinkles). If there are light and dark stripes diffused by the gate on the surface with patterns, low-speed injection and higher mold temperature shall be used.

The back pressure of 5bar fusible adhesive can be used, and the back pressure of toner can be appropriately increased.

The higher injection pressure (1500-1800 bar) and the pressure holding pressure (about 80% of the injection pressure) are used. 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.

2. Polyethylene (PE) injection molding process

PE is a crystalline raw material, and its hygroscopicity is very small, not 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 wall long process products without too high pressure during molding. The shrinkage range of PE is large, the shrinkage value is large, and the directivity is obvious. 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 cooling conditions of the mold have a great influence on the shrinkage rate. Therefore, the mold temperature should be controlled well to maintain uniform and stable cooling.

The crystallization ability of PE is high, 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.

The melting point of PE is not high, but the specific heat capacity is large. Therefore, more heat is still consumed during plasticizing. Therefore, the plasticizing device is required to have a larger 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 molding to avoid reducing the quality of plastic parts.

PE parts are soft and easy to demould, so when the plastic parts have shallow grooves, they can be demoulded forcefully. The non Newtonian property of PE melt is not obvious, the change of shear rate has little effect on the 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 the PE melt is fed through the direct feed port during injection, the stress should be increased, and uneven shrinkage and obvious directional deformation should be generated. Therefore, attention should be paid to the selection of feed port parameters. The molding temperature of PE is wide, and a little fluctuation of temperature has no effect on the injection molding in the flow state. The thermal stability of PE is good. Generally, there is no obvious decomposition phenomenon below 300 ℃, which has no impact on the quality.

Main molding conditions of PE

Barrel temperature: the barrel temperature is mainly related to the density of PE and the melt flow rate, as well as the type and performance of the injection molding machine and the shape of the first grade plastic parts. As PE is a crystalline polymer, the grains will absorb a certain amount of heat when melting, so the barrel temperature should be 10 degrees higher than its melting point. For LDPE, the barrel temperature shall be controlled at 140-200 ℃, and the HDPE barrel temperature shall be controlled at 220 ℃. The minimum value shall be taken at the rear of the barrel, and the maximum value shall be taken at the front.

Mold temperature: The mold temperature has a great influence on the crystallization of plastic parts. The mold temperature is high, the melt crystallinity is high, and the strength is high, but the shrinkage will also increase. Generally, the mold temperature of LDPE is controlled at 30 ℃ – 45 ℃, while that of HDPE is 10-20 ℃ higher.

Injection pressure: increasing the injection pressure is conducive to the filling of molten materials. Due to the good fluidity of PE, the lower injection pressure should be selected, generally 50-100MPa, except for thin and long products. Simple shape. For the plastic part with large wall, the injection pressure can be lower, and vice versa.

3. Injection molding process of polyvinyl chloride (PVC)

Typical application scope: water supply pipeline, household pipeline, housing wallboard, commercial machine shell, electronic product packaging, medical equipment, food packaging, etc.

Chemical and physical properties: PVC material is an amorphous material. In actual use, PVC materials are often added with stabilizers, lubricants, auxiliary processing agents, pigments, impact agents and other additives. PVC material has non flammability, high strength, weather resistance and excellent geometric stability.

PVC has strong resistance to oxidants, 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 contacting aromatic hydrocarbons and chlorinated hydrocarbons.

The melting temperature of PVC during processing is a very important process parameter. 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 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.

·Pressure holding pressure: up to 1000bar.

·Injection speed: In order to avoid material degradation, the injection speed should be equivalent.

·Runners and Gates: All regular gates can be used. If machining smaller parts, it is better to use a pin type gate or a submerged gate; For thicker assemblies, it is best to use a sector gate. The minimum diameter of pin type gate or submerged gate shall be 1mm; The thickness of sector gate shall not be less than 1mm.

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

4. Polystyrene (PS) injection molding process

Typical application scope: product packaging, household products (tableware, tray, etc.), electrical (transparent containers, light scattering devices, insulating films, etc.).

Chemical and physical properties: Most commercial PS are transparent, 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 acid, but can be corroded by strong oxidizing acid such as concentrated sulfuric acid, and can expand and deform in some organic solvents. The typical shrinkage is 0.4~0.7%.

Injection mold process conditions:

·Drying: Drying is usually not required unless storage is improper. If drying is required, the recommended drying condition is 80 ℃ for 2-3 hours.

·Melting temperature: 180~280 ℃. The upper limit for flame retardant materials is 250 ℃.

·Mold temperature: 40~50 ℃.

·Injection pressure: 200~600bar.

·Injection speed: fast injection speed is recommended.

·Runners and Gates: All generic types of gates can be used.

5. ABS injection molding process

Typical application scope: automobile (dashboard, tool cabin door, wheel cover, reflector box, etc.), refrigerator, high-intensity tools (hair dryer, mixer, food processor, lawn mower, etc.), telephone shell, typewriter keyboard, entertainment vehicles such as golf cart and jet sled, 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 is easy to process, high finish and high strength. In terms of morphology, ABS is an amorphous material.

The polymerization of three monomers produces a ternary copolymer with two phases, one is the continuous phase of styrene acrylonitrile, and the other is the dispersed phase of polybutadiene rubber. The properties of ABS mainly depend on the ratio of three monomers and the molecular structure of two phases. This allows great flexibility in product design, and leads to hundreds of different quality ABS materials in the market. These different quality materials provide different characteristics, such as medium to high impact resistance, low to high finish and high temperature distortion characteristics.

ABS material has excellent processability, 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 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 lower temperature will lead to lower finish).

·Injection pressure: 500~1000bar.