The Complete Guide to Polypropylene Injection Molding

Table of Contents

Injection molding is a highly effective technique for high volume production of plastic parts. This method is preferred for manufacture of parts with intricate and complex designs with high precision. Through automation of processes the injection molding machine can run continuously leading to large volume production and low labor cost per part.

Polypropylene (PP) is a commonly used thermoplastic polymer for various industrial applications in injection molding due to its favorable properties. PP is widely used due to its chemical resistance, mechanical properties and stable thermal properties.

In this article we will look into the benefits of polypropylene in injection molding, its application and the steps followed in the injection molding process.

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Plastic Polypropylene Pellets for Injection Molding (Image Source: Pinterest)

What Is Polypropylene? 

Polypropylene (PP) is a thermoplastic polymer that results from polymerization of propylene monomers. When relatively small molecules called monomers combine chemically to produce a large molecule called polymer, this process is called polymerization.

In polypropylene,  propylene monomers undergo a polymerization process  to form the polymer structure. Propylene monomers consist of three carbon atoms and six hydrogen atoms.

During the polymerization process, propylene monomers are subjected to high temperatures and pressure in the presence of a catalyst. This catalyst facilitates the bonding of propylene molecules. When  PP is heated it softens, becomes elastic, and solidifies upon cooling without undergoing significant chemical change.

Polypropylene is suitable for a wide range of applications across various industries as a result of its combination of flexible and tough properties. Some of the common applications of polypropylene include, in automotive components, medical devices and packaging.

Importance of Polypropylene in Injection Molding:

Polypropylene has very distinct properties making them easy for molding and shape attainment. These properties make PP suitable for various manufacturing processes. Below we are going to look into the benefits of using polypropylene in injection molding.

Low Melt Viscosity 

Polypropylene has a smooth flow consistency due to its low melt viscosity. During the injection molding process this ensures that the mold cavity is filled properly. Parts with complex dimensions can be easily molded, minimizing mold defects. 

Semi Crystalline Nature

Polypropylene mechanical properties are reinforced by the crystalline sections  which enhance stiffness and strength to the material. The dimensional stability of parts is also guaranteed.

Chemical Resistance 

Polypropylene has outstanding resistance to acids, solvents and a wide range of chemicals. In applications where exposure to harsh environments or corrosion substances is common this property makes it suitable.

Thermal resistance

In applications requiring resistance to high temperatures such as in automotive components PP is suitable due to its high melting temperature. 

Comparison of Polypropylene With Polyethylene

Polyethylene and polypropylene are common thermoplastic polymers used in injection molding processes although they display distinct features in their properties.

Polypropylene has slightly higher rigidity and firmness compared to polyethylene. This makes it preferred for applications that require more structural integrity. On the other hand polyethylene is more flexible making it desired for applications requiring flexibility.

PP generally has a high-temperature resistance capability. The melting point ranges from 160 c to 170c,  making it suitable for applications requiring heat resistance. However, polyethylene has a lower melting point compared to PP ranging from 120c to 130c. This limits its use in high-temperature environments.

Uses of Polypropylene Injection Molding

Polypropylene injection molding is used for various applications across different industries. Below are common uses of polypropylene injection molding.


PP is used in the packing industry for manufacture of bottle caps and containers. It is suitable for packaging beverages, cosmetics, food and household products as a result of its good chemical resistance and thermal stability.

Automotive Components 

The properties of PP such as dimensional stability, stiffness and ability to withstand temperature variation make it suitable for automotive applications. Some of the components produced from PP injection molding include automotive interior and exterior such as dashboards, bumpers and interior trims.

Medical Devices

Disposable medical devices such as specimen containers, IV components, medical packaging and vials are produced from polypropylene. Its chemical resistance and recyclability make it suitable for medical application.


The lightweight and durability property of PP makes it preferred for various textile applications. Products produced from polypropylene injection mold include fibers and ropes.


Properties such as excellent electrical insulation and chemical resistance make it suitable for producing electronic components. Some of these components include, casing, enclosure connectors and housing for electronic devices.

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Polypropylene Film Capacitor (image source: Pinterest)

Steps Involved in Polypropylene Injection Molding Process

In the polypropylene injection molding process the following steps are adhered to in producing finished plastic parts.

Step 1: Feeding 

In polypropylene injection molding process feeding is the initial step. Here the polypropylene pellets are introduced into the hopper of the injection molding machine. The hopper is located at the top of the injection molding machine. It is a funnel shaped container.

Step 2: Plasticization

In the polypropylene injection molding process  the melting phase is very important. Here the polypropylene pellets introduced into the machine barrel are heated into a molten state. The barrel is composed of a heating system that provides the necessary temperature to melt the plastic pellets. 

Step 3: Injection

This is the step where the molten plastic is injected into the mold cavity under pressure to form the desired shape of the final product. This step is crucial in determining the quality of the final product.

Step 4: Pressure Holding 

Pressure holding is responsible for maintaining pressure on the molten polypropylene material within the mold after injection. This pressure is responsible for compensating for material shrinkage as the plastic cools and solidifies.

In addition, it helps prevent part defects by ensuring proper filling of the mold cavity. This contributes to the final part being of high quality with consistent properties.

Step 5: Cooling

This is the step where the molten polypropylene material is solidified within the mold cavity transforming it into a rigid plastic part. The molds are composed of cooling channels that extract heat from the molten plastic rapidly. To promote high quality parts uniform cooling should be optimized.

Step 6: Ejection

Ejection of the plastic part from the mold cavity is the final step in the polypropylene injection molding process. An ejector system found within the mold is used to push the part out of the mold cavity.

Gates and Runners for Polypropylene Injection Molding: 

In the polypropylene injection molding process gates and runners are important to facilitate flow of molten polypropylene from the injection unit into the mold cavity. They enhance high quality of parts and ensure cost effectiveness in production of polypropylene parts. 

Gates are characterized by their narrow opening through which molten polypropylene flows into the mold cavity. There are different types and designs of gates which include edge and fan gates.  Gates allow molten material to flow and determine the location to which the part will be attached to the runner. For optimal polypropylene injection molding process proper gate location and design should be considered.

The runner distributes molten polypropylene from the sprue to cavities within the mold. To ensure uniform filling and packing of mold cavities runners should be optimized.

Molten polypropylene is injected from the nozzle of the injection molding machine into the mold cavity through the sprue which is the primary channel. For efficient flow of molten PP the size and location should be properly designed.

Advantages of Polypropylene Injection Molding

Below we are going to look into some of the benefits of polypropylene injection molding.

Low Coefficient of Friction

Polypropylene has a smooth surface and reduces resistance to sliding against other materials due to its low coefficient of friction. This property helps improve the efficiency and longevity of mechanical systems through reducing wear and tear. This is preferred for applications where low friction is required such as in gears and bearing. 

Fatigue Resistance 

PP can withstand repeated cycles of stress without experiencing deformation or failure due to its fatigue resistance property. Parts from polypropylene maintain their structural integrity and performance over long periods enhancing their durability and reliability.

Cost Effectiveness

Polypropylene is an economically viable manufacturing solution due to its inexpensive nature compared to other thermoplastics. The overall production cost is reduced through production of cost effective parts.

Moisture Resistance

Polypropylene does not absorb water easily. Even after exposure to moisture PP parts maintain their dimensional stability and mechanical properties. This reduces the risk of degradation. This property makes it preferred for applications where exposure to a humid environment is common.

Chemical Resistance

Polypropylene has outstanding resistance to acids, solvents and a wide range of chemicals. In applications where exposure to harsh environments or corrosion substances is common this property makes it suitable.

Design Guidelines for Polypropylene Injection Molding

To ensure optimal performance and cost effectiveness when designing polypropylene parts several considerations should be looked into keenly. Some of these  considerations include the following.

Living Hinges 

Select a PP type with high flexibility and fatigue resistance for living hinge applications. Conduct prototyping and testing to validate living hinge design including durability and flexibility.

Wall Thickness

Ensure uniform wall thickness is maintained throughout the part to promote even cooling, reducing part defects. Design thicker sections in areas requiring additional strength.


Incorporate proper filet and radii to distribute stress concentration and reduce the risk of stress cracking.

Draft Angles 

To ensure effective part ejection to prevent damage to part and mold surface draft angles should be incorporated. Use uniform draft angles across all part features to ensure uniform ejection.

Part Tolerances 

When designing part tolerances it is important to consider desired functional requirements. Consider also PP shrinkage rate and dimensional stability when noting part tolerances.

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Polypropylene Plastics Caps (Image Source: Pinterest)


Polypropylene injection molding is an efficient method for producing durable and cost effective parts. Polypropylene is widely used due to its unique properties. Careful consideration of various factors when designing parts should be made when using this technique.

Some of these crucial considerations are thickness, draft angles and part tolerances requirements. Manufacturers can produce polypropylene parts that meet desired functional requirements through following best practices and design guidelines. 

Polypropylene injection molding is used in the manufacture of various parts for different industrial applications such as automotive components and medical devices due to its flexibility. PP’s excellent properties  enhance the production of high quality and efficient parts which promote long lasting performance.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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