Injection Molding Materials: Polypropylene

PP, abbreviated for Polypropylene, is a type of plastic polymer that is known for its tough and flexible properties. It is thus, one of the most popular material choices for manufacturing in  injection molding.

An interesting anecdote in the manufacturing industry is that polypropylene plastic is often referred to as the ‘steel’ of the plastic industry. It is said so, because it can be customized in various ways to manufacture various products. 

This article provides an in-depth analysis of how polypropylene can be used for injection molding. In this article, we discuss what is polypropylene? Its properties, its applications and whether you should consider polypropylene plastic for your projects?

Table of Contents

What is Polypropylene?

Polypropylene is also known as polypropene and is commonly abbreviated as PP. 

PP plastic has been used to manufacture plastic parts in an array of industries that range from consumer products, and medical devices to plastic components in the automotive industry.

pp chemical structure

Polypropylene plastic is a type of thermoplastic polymer that is produced through chain-growth polymerization of the propylene which is a monomer. It basically is a thermoplastic polymer made by combining several propylene monomers. 

pp structure

PP Categories

Polypropylene plastic is further categorized into 2 different types. These are known as homopolymers and copolymers. The copolymers are also of 2 types: block copolymers and random copolymers. The below diagram illustrates the categorization of PP plastic: 

pp plastic category

It’s important to understand the different types of PP plastics as each of these categories can be applied to one application better than the other. You should also note that polypropylene plastic is highly adaptable and this is usually achieved by introducing special additives to it or by manufacturing it in a very particular way.

Types of Polypropylene

  • Homopolymer polypropylene is the default state of PP plastic. It is also considered to be a general-purpose grade plastic. 
  • Block copolymer polypropylene has co-monomer units arranged in blocks in a regular pattern. It contains ethylene which can be anywhere between 5% to 15%. Ethylene in block copolymer polypropylene improves the impact resistance property of the plastic. 
  • Random copolymer polypropylene has also co-monomer units but they are arranged in irregular or random patterns. Random copolymer polypropylene also contains ethylene and here, it is usually between 1% to 7%. This type of PP plastic is usually  selected for projects that require malleable and clearer product manufacturing.

PP Plastic Properties

Let’s understand them in-depth:

pp properties

Polypropylene Plastic Characteristics

  1. Chemical Resistance: PP Plastic does not readily react with dilute bases and acids, which makes it ideal to manufacture containers of cleaning agents, first-aid products, and more.  
  2. Elasticity and Toughness: PP plastic acts with elasticity over a certain range of deflection but it also experiences plastic deformation early in the deformation which is why it is generally considered a ‘tough’ material in the manufacturing industry. 
  3. Fatigue Resistance: Polypropylene plastic is fatigue-resistant as it tends to retain its shape even, after intense torsions and flexings.
  4. Insulation: PP is very useful for manufacturing electrical components as it has a very high resistance to electricity. It is thus, a very popular plastic for manufacturing in the electrical industry. 
  5. Transmissivity: Polypropylene plastic can be made transparent or opaque thus, it can be used for various applications. These applications can range from manufacturing products of different aesthetic values to where the transmissivity of light needs to be controlled.  

Advantages and Disadvantages of PP Plastic

Advantages of polypropylene

  • PP plastic is a relatively inexpensive material
  • It’s semi-crystalline in nature which is why it has high flexural strength. 
  • It’s resistant to moisture, has a low coefficient of friction, possesses good fatigue resistance and has a great impact strength.  
  • PP plastic exhibits good resistance to chemicals and electricity. 
  • It’s easily repaired from damage

Disadvantages of polypropylene

  • PP plastic is  UV sensitive and is susceptible to oxidation. 
  • Though it’s chemical resistant, it has poor resistance towards chlorinated solvents and aromatics. 
  • PP cannot be subjected to high-temperature applications as it has a high thermal expansion coefficient. 
  • It has high flammability and poor bonding properties.

PP Plastic and Injection Molding

Conditions of the PP Plastic for the Injection Molding Process

  1. Drying: PP Plastic does not need to be dried if stored properly. Unlike ABS plastic, it does not absorb moisture.  
  2. Melting Point: The melting point of PP plastic should be maintained between 220 – 280℃ / 428 – 536℉. The temperature under no circumstances should go higher than 280℃ / 536℉ as that would deteriorate the plastic. 
  3. Plastic Injection Molding Pressure: The Injection Molding Pressure should be as high as 180 MPa. 
  4. Mold Temperature: The ideal mold temperature should be set at 50℃ / 122℉ but it can also vary anywhere between 20 – 80℃ / 68 – 176℉. The mold temperature determines the crystallinity level of the PP plastic. 
  5. Plastic Injection Molding Speed: In the case of PP plastic, it is recommended that the plastic injection molding speed is fast enough so that the internal stresses can be kept at the lowest possible level. 
  6. Runners and Gates: The gate and runner need to be optimally designed according to the plastic. In the case of PP plastic, the following parameters should be paid attention to: 
    • The common diameter for cold runner molds should be between 4 to 7mm. 
    • The diameters of a pin gate can be set between 1 to 1.5mm but it can also be as low as 0.7 mm. 
    • In the case of edge gating, the depth of the gate should be no lower than half of the wall thickness. Instead, it should be at least twice as wide as the thickness.


Polypropylene for Prototype Development on CNC Machines, 3D Printers, & Injection Molding Machines

  • 3D Printing Polypropylene: PP plastic is not available in its filament form for 3D printing. 

  • CNC Machining Polypropylene: In the case of CNC machining, polypropylene is widely used as sheet stock for manufacturing via CNC machine. You should note that typically, CNC machines are used to prototype a small number of polypropylene parts. This is because it is assumed that PP plastic cannot be machined due to its low annealing temperature. Since it’s a soft material, it requires an extremely high skill level to be cut with precision. 

  • Injection Molding Polypropylene: PP is available in the form of resins or pellets for injection molding. It’s easy to mold and it flows well within the mold because of its low melt viscosity. It’s important to also note that PP plastic shrinks at a rate of about 1-2% but this factor can vary and is dependent upon a number of factors such as holding pressure, melt temperature, mold wall thickness, mold temperature, and the type of additives.

Applications of PP molded plastic parts

Food Industry

The next factor that your manufacturer should ensure is the injection molding speed. If the injection molding speed is high, it can cause burning or thermal decomposition of the plastic material. It can also cause poor glossiness, discoloration and weld lines in the plastic material.  

On the other hand, if the injection molding speed is too slow, insufficient mold filling will occur which can result in other types of defects in the injection molded part. 

bottles made with pp
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  • PP plastic in its flexible packaging is transparent and is commonly applied to be used as plastic containers. 
  • Rigid packaging is thicker and opaque. It is commonly used in the food industry to create various types of consumer products such as plates, cups, bottles, pots, etc. 

Medical Supplies

PP Plastic can be applied to various medical applications. PP is generally resistant to strong chemicals and steam sterilization which makes it ideal for manufacturing medical supplies. It is often used to create petri-dishes, syringes, specimen bottles, pans, vials, and pill dispensers. For insights in this industry, take a look at our medical device industry page.

medical syringe pp
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Automotive Parts

Polypropylene plastic is affordable and easy to mold, and is thus, very popularly used to manufacture plastic parts for the automotive industry. These automotive parts can range from car battery cases, dashboards, and instrument panels to door trims, and more. Another common material for automotive parts, is the heat resistant POM (Polyoxymethylene).

car dashboard out of pp
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Consumer Goods

Polypropylene is very popularly used for the production of consumer products. Since PP plastic is durable and resistant to high temperatures, it’s an ideal material for kitchen and home appliances such as washing machines and coffee makers. Apart from home and kitchen appliances, PP plastic can also be used to manufacture toys, furniture, carpets, and mats, to other consumer products. PP products from the consumer industry are the easiest do recognize in your daily life.

toys made with PP
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Plastic Films

PP film is known for its strength and resistance against tears, punctures, and high temperatures. It is thus, one of the most popular plastics for industrial parts. 

packaging material
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Where can you purchase PP Plastic?

PP Plastic is available in various forms that can range from resins, rods, films and sheets. We have listed some of the leading suppliers for PP Plastic:

LyondellBasell is a US-based company and is a global manufacturer for PP plastic. 

Braskem is a leading producer of propylene in the US and other global markets. Along with PP, they also manufacture ethylene, naphtha, polyethylene, etc. 

China Petrochemical is a part of the Sinopec Group and is an oil and energy company. They have a wide array of polypropylene products to choose from. 

Exxon Mobil excels in gas and oil exploration. It’s a global company and is present in 21 countries. Exxon Mobil manufactures PP packaging films and along with that, it is also known for manufacturing olefins, aromatics, fluids, synthetic rubber, and polyethylene. 

GE Plastics cater to global industries. They have various grades of PP resins that the manufacturers can buy to develop their products. 

Cheil is a Korean based company and it produces PP resins that can be used to produce quality plastic parts for various industries. 

Dow is a US-based based company that provides approximately 100 types of PP resins, it is also popular for ABS resins.

Contact Us for PP Injection Molding

VEM Tooling has the expertise for your PP injection molding project. At VEM Tooling, we offer a dependable service with quick quotes. We ensure that you have a great experience with a seamless manufacturing process. 

To better understand how VEM tooling can serve you for your PP injection molding project, contact us or request a quote today.