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A Comprehensive Guide For PP Injection Molding

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Polypropylene PP Injection molding is a process in which a polypropylene material is heated to its melting point and then injected into the mold cavity to get the desired shape of parts. The injection molding process will produce mass and complex consistent polypropylene products. Do you know what is polypropylene? what are polypropylene injection molding conditions? As an injection molding material, what are its properties, advantages, and disadvantages? What are the applications of polypropylene injection molding parts? In this article, we will discuss polypropylene injection molding, and you will find the answer following.

What is Polypropylene?

Polypropylene, also known as PP, is a polymer made from propylene through the polymerization reaction. It is a thermoplastic with excellent performance. PP is colorless, semi-transparent, and famous for its tough and flexible properties. Polypropylene plastic has been used to manufacture plastic parts in various industries, and polypropylene injection molding is the most popular manufacturing process.

pp-plastic-chemical-structure

Polypropylene Types

Polypropylene can be categorized into 2 types, homopolymers and copolymers. It’s important to understand the different types of PP plastics. Each type can be applied to different applications.

Homopolymer Polypropylene

Homopolymer polypropylene also known as PPH, is the common state of PP plastic. It is also considered to be a general-purpose-grade plastic. PPH can be divided into the following three categories.

  • Isotactic PP, it is accounting for about 95% of PP production.

 

  • Intercalated PP: it has low crystallinity. It is a highly elastic thermoplastic or elastomer.

 

  • Irregular PP: its strength is very low, and its use alone is of little value, but it works well as a modifier for polypropylene, etc.

Copolymer Polypropylene

Copolymer PP can be divided into random copolymer and block copolymer.

  • The random copolymer has better optical transparency, flexibility, lower melt finger temperature, and high impact resistance. It is usually used in manufacturing highly transparent film, water, and sewage pipes, heating pipes, and injection molding products.

 

  • Block copolymer has better rigidity and low-temperature toughness.

Polypropylene Injection Molding Process Conditions

Drying:

PP has excellent resistance to moisture absorption. Different from ABS plastic, PP does not need to be dried before injection molding.

Melting Point:

PP plastic melting temperature is between 220 – 280℃ / 428 – 536℉. The Temperature should not be higher than 280℃ / 536℉, or the material will be thermal degradation.

Plastic Injection Molding Pressure:

The injection molding pressure should be as high as 180MPa. It’s important to note that while higher injection pressure produces favorable results, excessively high pressures can result in the part sticking to the mold.

Mold Temperature:

The mold temperature can vary between 20 – 80℃ / 68 – 176℉. The suggested temperature is 50℃ / 122℉. The mold temperature is the factor that determines the PP plastic’s crystallinity level.

The Cooling Temperature:

After injection, the molded part should be cooled to 54°C before ejection from the mold cavity to prevent warping.

Plastic Injection Molding Speed:

For the PP injection molding process, it is recommended that the injection molding speed is fast enough so that to keep the internal stresses at the lowest possible level.

Runners and Gates Parameter:

The full-round sprues and runners are usually recommended. The common diameter should be between 4-7mm for cold runner molds.

Usually, the diameter of a pin gate is between 1-1.5mm. But it can also be as low as 0.7mm.

In the case of edge gating, the depth of the gate should be kept higher than half of the wall thickness, and the wide is at least twice the thickness.

The hot runner molds can be used for the PP injection molding directly.

Shrinkage Range:

The polypropylene’s average shrinkage rate is between 1% and 3%. Filled polypropylene can have lower shrinkage values, the values depend on the fill material that is used.

Advantages of Polypropylene Injection Molding Material

Low Cost

PP is easy to access and not expensive. After polyethylene, polypropylene is the most widely used plastic, so is relatively inexpensive compared to other types of thermoplastics.

High Flexural Strength and Resistance Impact

PP has a high flexural strength by virtue of its semi-crystalline property.

Low Coefficient of Friction

PP has a relatively slippery surface. The slippery surface makes PP has a lower coefficient of friction. That characteristic facilitates the release of injected molded parts from tooling, which helps to speed cycle times and reduce overall costs.

Resistant to Moisture Absorption

Polypropylene has excellent moisture resistance and the water absorbs less than 0.01% over 24 hours.

Good Chemical Resistance

Polypropylene resists a wide range of chemicals, including fats, organic solvents, and some non-oxidizing acids.

Resistant to Fatigue

With excellent fatigue resistance, polypropylene is commonly used in a living hinge. A living hinge is a part that supports bending.

Great Electrical Insulator

Polypropylene does not conduct electricity well. These properties make it an excellent material for manufacturing electronic components.

Low Melt Viscosity

The low melt viscosity polypropylene makes it easier to inject mold and can fill the mold more quickly, thereby reducing cycle times and costs.

Disadvantages of Polypropylene Injection Molding Material

Limits High Temperature

Polypropylene has limited use in high temperatures as it has a high thermal expansion coefficient.

Sensitive to UV

PP plastic is UV-sensitive and susceptible to oxidation.

Not Resistant to Chlorinated Solvents and Aromatics

Though polypropylene has good chemical resistance, but it has poor resistance to chromic acid and aromatic hydrocarbons.

Not Easy to Paint On

Polypropylene has poor bonding properties, so it is hard to paint on polypropylene.

Highly Flammable

Polypropylene is extremely flammable and will melt when exposed to heat. the melt temperature is just 260℃.

Polypropylene Properties

Property

Value

Density (g/cm³)

0.895- 0.92

Shrinkage Rate (%)

1.0 – 3

Pockwell Hardness

R80 – R125

Tensile Strength at Yield (MPa)

20 – 40

Elongation at Break (%)

200 – 600

Flexural Modulus (GPa)

1.0 – 1.5

Flexural Strength (MPa)

30 – 50

Drying Temperature (°C)

70 – 80

Melt Temperature (°C)

220– 280

Mold Temperature (°C)

20 – 80

Applications of Polypropylene Injection Molding

Polypropylene (PP) is a common manufacturing plastic material and is used in a wide range of applications in daily life. As a food-grade plastic, it is widely used in food packaging, such as foam boxes, PP plastic cups, and so on. Except that, It is also used in the following applications. Packaging for consumer products.

  • Plastic parts for the automobile industry.
  • Special devices like living hinges.
  • Medical instruments.
  • Textiles materials.
  • Toys and consumer products.

Conclusion

Polypropylene is a commonly used injection molding plastic, it is not expensive and easy to get. In this post, we introduce polypropylene types, properties, advantages, and disadvantages, and the polypropylene injection molding process conditions. In the PP material selection and injection molding process, it is important to cooperate with experienced manufacturers and to ensure the project smoothly. Zhongde is a professional on-demand manufacturer with 40-year history. Our production and material experts will support your project and get quality injection molded parts. Contact us or request a quote today.

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