Five general plastic injection molding process details

March 30, 2022

By siedery

“The performance of plastics is the basis of injection molding technology”, mastering the process performance and characteristics of various plastics is the basic professional knowledge that every injection molding worker must know. It is also one of the factors that must be considered when analyzing the quality problems and abnormalities that occur during the injection molding process.

  1. Polypropylene (PP) injection molding process
  2. Polyethylene (PE) injection molding process
  3. Polyvinyl chloride (PVC) injection molding process
  4. Polystyrene (PS) injection molding process
  5. ABS injection molding process

1. Polypropylene (PP) injection molding process

PP is a translucent, semi-crystalline thermoplastic with high strength, good insulation, low water absorption, high heat deflection temperature, low density, and high crystallinity. Modified fillers usually include glass fiber, mineral filler, thermoplastic rubber, etc.

Polypropylene
Polypropylene

The fluidity of PP varies greatly based on application, and the general use of PP flow rate is between ABS and PC.

1. Treatment of plastic

Pure PP is translucent ivory white and can be dyed in various colors. The dyeing of PP is only possible with color masterbatches on general injection molding machines. In some machines, there are independent plasticizing elements that strengthen the mixing effect, and can also be dyed with color powder. For products used outdoors, UV stabilizer and carbon black filling are generally used. The use of recycled material should not exceed 15%, otherwise, it will cause strength loss and decomposition, and discoloration. PP injection molding process generally does not require special drying treatment before.

2. Injection molding machine selection

There is no special requirement for the selection of injection molding machine. Because PP has high crystallinity. Need to use higher injection pressure and multi-stage control of computerized injection molding machine. The clamping force is generally determined by 3800t/m2, and the injection volume is 20%-85%.

3. Mould and sprue design

Mold temperature 50-90℃, for high size requirement use high mold temperature. The core temperature is more than 5℃ lower than the cavity temperature, the runner diameter is 4-7mm, the length of needle-shaped gate is 1-1.5mm, the diameter can be as small as 0.7mm, the shorter the length of edge-shaped gate is, the better, about 0.7mm, the depth is half of the wall thickness, the width is twice of the wall thickness, and the length of melt flow in the cavity increases with each time. The mold must have good exhaust, the exhaust hole is 0.025mm-0.038mm deep and 1.5mm thick, to avoid shrinkage marks, we should use large and round injection port and round runner, the thickness of reinforcement should be small (for example, 50-60% of the wall thickness). For products made of homopolymer PP, the thickness cannot exceed 3mm, otherwise there will be bubbles (thick-walled products can only use copolymer PP).

4. Melting temperature

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

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 applicable (bubbles and air patterns appear). If there are bright and dark streaks spread by the gate on the engraved surface, low speed injection and higher mold temperature should be used.

6. Melting back pressure

The back pressure of melt glue can be 5bar, the back pressure of color powder can be adjusted higher.

7. Injection and pressure-holding

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.

8. Post-processing of the product

In order to prevent shrinkage and deformation caused by post-crystallization, products are generally treated with hot water soaking.

2. Polyethylene (PE) injection molding process

PE is a crystalline raw material, moisture absorption is very small, not more than 0.01%, so there is no need to dry before processing. PE molecular chain flexibility, small inter-bonding forces, low viscosity of the melt, excellent fluidity, so the molding without too high pressure can be formed out of thin-walled long flow products. PE shrinkage rate range, shrinkage value, directional obvious, LDPE shrinkage rate of about 1.22%, HDPE shrinkage rate of about 1.5%. HDPE shrinkage rate is around 1.5%. Therefore, it is easy to deform and warp, and the cooling condition of the mold has a great influence on the shrinkage rate, so the temperature of the mold should be controlled well to keep the cooling uniform and stable.

Polyethylene
Polyethylene

The crystallization ability of PE is high, the temperature of the mold has a very large impact on the crystallization condition of the plastic part. Mold temperature is high, the melt cools slowly, the crystallinity of the plastic part is high, the strength is also high.

PE’s melting point is not high, but the specific heat capacity is larger, so the plasticization still needs to consume more heat, so the plasticizing device is required to have a larger heating power, in order to improve production efficiency.

PE’s softening temperature range is small, and the melt is easy to oxidation, so in the molding process should be as far as possible to avoid contact between the melt and oxygen, 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 side grooves can be strongly demolded.

PE melt non-Newtonian is not obvious, the change in shear rate has less effect on the viscosity, PE melt viscosity 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 better cooling system.

If PE melt is injected with direct inlet feed, it should increase the stress and produce uneven shrinkage and directional increase in deformation obviously, so care should be taken to choose the inlet parameters.

PE’s molding temperature is wide, in the flow state, the temperature of a little fluctuation on injection molding has no effect.

PE’s thermal stability is better, generally below 300 degrees without obvious decomposition, there is little impact on the quality.

The main molding conditions of PE

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

Mold temperature: mold temperature has a greater impact on the crystallization of plastic parts, high mold temperature, high crystallinity of the melt, 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: improve the injection pressure is conducive to the melt filling mold, due to the good fluidity of PE, so in addition to thin-walled slender products, should be fine to choose a lower injection pressure, the general injection pressure of 50-100MPa. simple shape. After the wall of larger plastic parts, injection pressure can be lower, and vice versa is high.

3. Polyvinyl chloride (PVC) injection molding process

Typical application areas:

Water supply pipes, household pipes, house wall panels, commercial machine housings, electronics packaging, medical devices, food packaging, etc.

Polyvinyl chloride
Polyvinyl chloride

Chemical and physical properties:

PVC material is a non-crystalline material.PVC material is often used in practice with stabilizers, lubricants, auxiliary process agents, colorants, impact agents, and other additives.PVC material has non-flammability, high strength, resistance to atmospheric 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 nitric acid and is not suitable for contact with aromatic and chlorinated hydrocarbons.

The melting temperature of PVC is a very important process parameter during processing, and if this parameter is not appropriate, it will lead to problems of material decomposition. The flow characteristics of PVC are quite poor and the process range is very narrow.

In particular, large molecular weight PVC materials are more difficult to process (such materials usually require the addition of lubricants to improve the flow characteristics), so small molecular weight PVC materials are usually used. The shrinkage rate of PVC is quite low, generally 0.2~0.6%.

Injection molding process conditions:

  • drying treatment: usually no drying treatment is required.
  • Melting temperature: 185~205℃ Mold temperature: 20~50℃.
  • Injection pressure: can be as large as 1500bar. 4.
  • Holding pressure: up to 1000bar. 5.
  • Injection speed: In order to avoid material degradation, the injection speed should be quite high. 6.
  • Runners and gates: All conventional gates can be used. For small parts it is better to use a needle tip gate or a submerged gate; for thicker parts it is better to use a fan gate. The minimum diameter of the pin-tip gate or submerged gate should be 1mm; the thickness of the fan gate should 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 areas:

Product packaging, household products (tableware, trays, etc.), electrical (transparent containers, light scatterers, insulating films, etc.).

Five general plastic injection molding process details

Chemical and physical properties:

Most commercial PS is a transparent, amorphous 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 rates range from 0.4 to 0.7%.

Injection molding process conditions:

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

5. ABS injection molding process

Typical application areas:

Automobiles (dashboards, tool hatches, wheel covers, reflector cases, etc.), refrigerators, high intensity tools (hair dryers, blenders, food processors, lawn mowers, etc.), telephone housings, typewriter keyboards, recreational vehicles such as golf carts and jet skis, etc.

ABS injection molding process
ABS injection molding process

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 and chemical stability; butadiene has toughness and impact resistance; and styrene has easy processing, high finish and high strength. Morphologically, ABS is a non-crystalline material.

The polymerization of the three monomers produces a terpolymer with two phases, a continuous phase of styrene-acrylonitrile and a disperse 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 a hundred 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 distortion properties.

ABS materials offer superb processability, cosmetic properties, low creep and excellent dimensional stability, and high impact strength. Reply to “ABS” for more information

Injection molding process conditions.

  • Drying: ABS materials are hygroscopic and require drying prior to processing. Recommended drying conditions are 80~90℃ for a minimum of 2 hours. The material temperature should 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, lower temperature will lead to lower finish).
  • Injection pressure: 500~1000bar.
  • Injection speed: medium to high speed.

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