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Polypropylene Injection Molding: Process, Advantages and Disadvantages

Polypropylene (PP) is a thermoplastic widely used in the field of injection molding. Its semi-crystalline structure grants it excellent moisture resistance, alkali resistance, and resistance to certain acids.

Through injection molding technology, the manufacturing process of polypropylene can achieve a high degree of automation and scale. This means that manufacturers can produce thousands of identical parts using just one mold, significantly reducing production costs and increasing production efficiency.

This article will discuss in detail the process of polypropylene injection molding, its advantages and disadvantages, and how polypropylene is utilized in injection molding. Please continue reading for more information.

What is Polypropylene?

what is polypropylene

Polypropylene, abbreviated as PP, is a popular commodity thermoplastic. It is renowned for its semi-crystalline structure, excellent moisture resistance, and outstanding resistance to alkalis and certain acids. This polymer is widely used across various industries due to its toughness and flexibility.

There are two main types of polypropylene: homopolymers and copolymers. Homopolymer polypropylene, known for its superior strength, rigidity, and heat distortion properties, is often used in applications requiring high durability and stability. On the other hand, copolymer polypropylene, with its good impact resistance and certain degree of transparency, is ideal for manufacturing transparent or semi-transparent products. Although each type has its unique characteristics, they share similar parameters and performance in the injection molding process.

Advantages of Polypropylene Injection Molding

The advantages of polypropylene injection molding are evident in several aspects, including:

Fatigue Resistance

Polypropylene exhibits excellent fatigue resistance, maintaining stable performance under prolonged repetitive stress or strain. When properly designed, polypropylene-made living hinges can withstand millions of cycles without failure. Additionally, the low friction coefficient of polypropylene makes it easier to eject molded parts from the mold, helping to shorten production cycles and reduce overall costs.

Low Melt Viscosity

One of the core advantages of polypropylene is its low melt viscosity. This characteristic gives the material a smooth, flowing consistency in its molten state, making the mold-filling process more efficient. Regardless of the complexity of the target product’s shape, polypropylene can easily adapt and accurately replicate the mold’s shape, significantly enhancing production efficiency and product precision.

Moisture Resistance

Polypropylene has excellent moisture resistance, with a 24-hour water absorption rate of less than 0.01%. This property makes polypropylene an ideal choice for applications requiring high moisture resistance, such as food and beverage packaging. Additionally, the moisture resistance of polypropylene means that no additional drying treatment is typically needed before injection molding, further enhancing production efficiency.

Chemical Resistance

While polypropylene’s chemical resistance is not as high as that of polyethylene, it still resists a variety of chemicals, including fats, organic solvents, and some non-oxidizing acids. This makes polypropylene widely applicable in industries such as chemicals, pharmaceuticals, and food packaging.

Low Cost

Polypropylene is one of the most widely used commodity plastics after polyethylene, which keeps its cost relatively low. This makes polypropylene an ideal material for large-scale production, helping to reduce overall manufacturing costs.

Disadvantages of Polypropylene Injection Molding

While polypropylene (PP) injection molding offers many advantages, it also has some drawbacks primarily stemming from the characteristics of the polypropylene material and the limitations of the injection molding process. Here are some of the main disadvantages:

Poor cold resistance and low impact strength at low temperatures

Polypropylene products tend to become brittle at low temperatures, leading to reduced impact strength. This makes it necessary to handle polypropylene products with care in cold environments to prevent cracking or damage.

Poor resistance to light, heat, and oxygen

When exposed to light, heat, and oxygen for extended periods, polypropylene products are prone to aging, resulting in degraded performance. This can shorten the lifespan of the products or affect their appearance.

Shrinkage and dimensional stability issues

Due to the high crystallinity of polypropylene, it has a relatively high shrinkage rate. This can cause instability in the dimensions of the molded products, leading to deformation or dimensional deviations. Additionally, the shrinkage rate of polypropylene is influenced by factors such as temperature, pressure, and molding time, necessitating careful control of these process parameters.

Polypropylene Uses

Polypropylene (PP) is a highly versatile thermoplastic polymer whose excellent physical properties and cost-effectiveness have led to its widespread use in many fields. Here are some of the primary applications of polypropylene:

  • Automotive parts
  • Children’s toys
  • Sports equipment
  • Household goods and appliances
  • Appliance housings
  • Container closures (such as bottle caps)
  • Food trays
polypropylene injection molding parts

Importance Of Use Of Polypropylene For Injection Molding

PP is a crystalline polymer and the lightest commonly used plastic, with a density of only 0.91 g/cm³ (less than water). Among general-purpose plastics, PP has the best heat resistance, with a heat distortion temperature of 80-100°C, and it can be boiled in water. This makes it the highest quality plastic for injection molding.

Compared to polyethylene, although it has similar properties, polypropylene is slightly harder and has better heat resistance. This allows polypropylene to maintain good performance in high-temperature environments, broadening its range of applications. Whether used to manufacture containers for high-temperature environments or mechanical components that need to operate under high temperatures, polypropylene offers excellent performance and stability.

From an economic perspective, polypropylene is easily obtainable and low-cost. This makes using polypropylene for injection molding a highly competitive manufacturing option.

Polypropylene Injection Molding Process

The injection molding process for polypropylene (PP) involves several key steps. Here is a simplified outline of the main steps in the polypropylene injection molding process:


Plastic pellets (polypropylene PP pellets) are introduced into the hopper of the injection molding machine either through an automated feeding system or manually. The pellets roll into the barrel of the injection molding machine by gravity or with mechanical assistance, such as a screw conveyor.


A rotating screw in the barrel pushes the plastic pellets forward. The pellets are heated by the heating bands surrounding the barrel, while the rotation and forward movement of the screw gradually melt the pellets. The molten plastic continues to be pushed forward by the screw until it is fully melted and reaches a uniform state suitable for injection. The plasticizing temperature for polypropylene typically ranges from 450 to 500°F (approximately 232 to 260°C).


Once a sufficient amount of molten plastic has accumulated in front of the screw, the injection mechanism of the machine is activated. The screw or piston injects the molten plastic at high speed through the sprue and runner system into the mold cavity.

Holding Pressure

After the molten plastic fills the mold cavity, the screw continues to apply pressure (holding pressure) to ensure that the plastic in the mold cavity is adequately compacted and to compensate for shrinkage that occurs as the plastic cools. The mold temperature is controlled by a cooling system (such as circulating water or oil) to ensure uniform cooling of the plastic within the mold.


The molten plastic gradually cools and solidifies into the desired shape within the mold. The cooling process can take a few seconds to several minutes, depending on the type of plastic, mold design, and product size.

Mold Opening and Ejection

When the plastic in the mold has completely solidified and reached sufficient strength, the mold opens. An ejection mechanism (such as ejector rods and pins) pushes the molded plastic product out of the mold. The product and the runner system (if present) are collected for subsequent processing. The ejected plastic product and any runner system components (such as sprues and cold wells) are collected and prepared for further processing (e.g., removing sprues, trimming, and packaging).

Cycle Repetition

Once the mold is emptied and ready, the injection molding machine repeats the above steps to produce more plastic products.


Polypropylene (PP) injection molding is widely used due to its excellent fatigue resistance, moisture resistance, and (to some extent) chemical resistance, making it favored across various industries. This thermoplastic is not only easy to process but also produces stable performance products suitable for diverse applications.

Despite the high cost of large industrial polypropylene injection molding machines, there’s no need for you to worry. BOYI offers professional polypropylene injection molding services, helping you obtain high-quality polypropylene injection molded parts at a reasonable cost. We have the skills and expertise to cover the entire process, from part design and prototyping to production, to meet your various needs.

Whether you need small batch trial production or large-scale manufacturing, we can provide you with flexible and efficient services. We pay attention to detail and pursue quality, ensuring that every part meets your expectations.

Request a quote for injection molding from BOYI today!


Can polypropylene be injection molded?

Yes, polypropylene can be injection molded, and it is commonly used in various applications including consumer packaging and automotive parts. Injection molding is a popular manufacturing process for polypropylene parts, allowing for efficient and cost-effective production of high-quality parts.

Is polypropylene better than ABS for injection molding?

ABS typically exhibits less shrinkage during molding than PP, allowing for tighter tolerances. It is also less prone to warping. However, PP offers better heat resistance and is generally less expensive than ABS.

Catalog Guide: Injection Molding Guide

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