[Science stickers] Injection molding process of 5 general plastics

Polypropylene (PP)
injection molding process

PP is commonly referred to as polypropylene, because of its good anti-breaking performance, it is also called "100% plastic". PP is a semi-transparent, 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 fibers, mineral fillers, and thermoplastic rubbers.
The fluidity of PP for different purposes is quite different, and the PP flow rate generally used is between ABS and PC.

1. Plastic processing

Pure PP is translucent ivory white and can be dyed in various colors. For PP dyeing, only color masterbatch can be used on general injection molding machines. On some machines, there are independent plasticizing elements that strengthen the mixing effect, and they can also be dyed with toner. Products used outdoors are generally filled with UV stabilizers and carbon black. The use ratio of recycled materials should not exceed 15%, otherwise it will cause strength drop and decomposition and discoloration. Generally, no special drying treatment is required before PP injection processing.

2. Selection of injection molding machine

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

3. Mold and gate design

The mold temperature is 50-90℃, and the high mold temperature is used for the higher 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 length of the edge gate is as short as possible, about 0.7mm, the depth is half of the wall thickness, and the width is twice the wall thickness, and it will gradually increase with the length of the melt flow in the cavity. The mold must have good venting. The vent hole is 0.025mm-0.038mm deep and 1.5mm thick. To avoid shrinkage marks, use large and round nozzles and circular runners, and the thickness of the ribs should be small ( For example, 50-60% of the wall thickness). The thickness of the products made of homopolymer PP should not exceed 3mm, otherwise there will be bubbles (thick wall products can only use copolymer PP).

4. Melting temperature

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 melting temperature is best at 240°C.

5. Injection speed

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 gas lines appear). If the patterned surface appears with light and dark stripes diffused by the gate, low-speed injection and higher mold temperature should be used.

6. Melt back pressure

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

7. Injection and pressure holding

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

8. Post-processing of products

In order to prevent shrinkage and 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 with extremely low hygroscopicity, no more than 0.01% , so there is no need for drying before processing. PE molecular chain has good flexibility, small force between bonds, low melt viscosity, and excellent fluidity . Therefore, thin-walled and long-process products can be formed without too high pressure during molding. The shrinkage rate of PE is wide, 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 mold cooling conditions have a great influence on shrinkage. Therefore, mold temperature should be controlled to maintain uniform and stable cooling.
The crystallization ability of PE is high, and the temperature of the mold has a great influence on the crystallization condition of the 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. Therefore, the plasticizing device is required to 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. Therefore, 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 plastic parts.
PE parts are soft and easy to demold, so when the plastic parts have shallow grooves, they can be demolded strongly.
The non-Newtonian property of PE melt is not obvious, the change of shear rate has little influence on viscosity, and the influence of temperature on PE melt viscosity is also little.
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 the uneven shrinkage and the directionality should be significantly increased. Therefore, attention should be paid to the selection of the feed port parameters.
The molding temperature of PE is relatively 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, and it has no effect on the quality.

1. The main molding conditions of PE
barrel temperature:
barrel temperature is mainly related to the density of PE and the size of the melt flow rate, in addition to the type and performance of the injection molding machine, and the shape of the first-class plastic part. Since PE is a crystalline polymer, the crystal grains have to 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 140-200°C, the HDPE barrel temperature is controlled at 220°C, the rear part of the barrel takes the minimum value, and the front end takes the maximum value.
Mold temperature:
Mold temperature has a greater impact on the crystallization of plastic parts. High mold temperature, high melt crystallinity, high strength, but shrinkage will also increase. Generally, 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 beneficial to the filling of the melt. Because the fluidity of PE is very good, in addition to thin-walled and slender products, a lower injection pressure should be selected carefully. 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.

Polyvinyl chloride (PVC)
injection molding process

1. Typical application range

Water supply pipes, household pipes, house wall panels, commercial machine housings, electronic product packaging, medical equipment, food packaging, etc.

2. Chemical and physical properties

PVC material is a non-crystalline material. In actual use, PVC materials often add stabilizers, lubricants, auxiliary processing agents, pigments, impact resistance 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 contact with aromatic hydrocarbons and chlorinated hydrocarbons.
The melting temperature of PVC during processing is a very important process parameter. If this parameter is not appropriate, it will cause the problem of material decomposition. The flow characteristics of PVC are quite poor, and its process range is very narrow. Especially the high molecular weight PVC material is more difficult to process (this kind of material usually needs to add lubricant to improve the flow characteristics), so the PVC material with small molecular weight is usually used. The shrinkage rate of PVC is quite low, generally 0.2~0.6%.

3. Injection molding process conditions

1. Drying treatment: usually no drying treatment is required.
2. Melting temperature: 185~205℃ Mold temperature: 20~50℃.
3. Injection pressure: up to 1500bar.
4. Holding pressure: up to 1000bar.
5. Injection speed: In order to avoid material degradation, a considerable injection speed is generally used.
6. Runners and gates: all conventional gates can be used. If processing smaller parts, it is best to use needle-point gates or submerged gates; for thicker parts, it is best to use fan gates. The minimum diameter of needle-point gates or submerged gates should be 1mm; the thickness of sector gates should not be less than 1mm.
7. Chemical and physical properties: Rigid PVC is one of the most widely used plastic materials.

Polystyrene (PS)
injection molding process

1. Typical application range

Product packaging, household goods (tableware, trays, etc.), electrical (transparent containers, light source diffusers, insulating films, etc.).

2. Chemical and physical properties

Most commercial PS are transparent, non-crystalline materials. PS has very good geometric stability, thermal stability, optical transmission characteristics, electrical insulation characteristics and a very small tendency to absorb moisture. It can resist water and diluted inorganic acids, but can be corroded by strong oxidizing acids such as concentrated sulfuric acid, and can swell and deform in some organic solvents. The typical shrinkage is between 0.4 and 0.7%.

3. Injection molding process conditions

1. Drying treatment: Unless stored improperly, drying treatment is usually not required. If drying is required, the recommended drying conditions are 80°C for 2 to 3 hours.
2. Melting temperature: 180~280℃. For flame-retardant materials, the upper limit is 250°C.
3. Mold temperature: 40~50℃.
4. Injection pressure: 200~600bar.
4. Injection speed: It is recommended to use a fast injection speed.
5. Runners and gates: all conventional types of gates can be used.

ABS
injection process

1. Typical applications:
Automobiles (instrument panels, tool hatches, wheel covers, mirror boxes, etc.), refrigerators, high-strength tools (hair dryers, blenders, food processors, lawn mowers, etc.), telephones Shells, typewriter keyboards, entertainment vehicles such as golf carts and jet sleds.

2. 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 smoothness and high strength. From the morphological point of view, ABS is an amorphous material.
The polymerization of the three monomers produces a terpolymer with two phases, one is the continuous phase of styrene-acrylonitrile, and the other is the 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 hundreds of different quality ABS materials have been produced on 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 super easy processing, appearance characteristics, low creep and excellent dimensional stability as well as high impact strength.

3. Injection molding process conditions

1. Drying treatment: ABS material is hygroscopic and requires drying treatment before processing. The recommended drying condition is at least 2 hours at 80~90℃. The material temperature should be less than 0.1%.
2. 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).
3. Injection pressure: 500~1000bar.
4. Injection speed: medium to high speed.