Polypropylene Injection Molding: Process, Advantages, And Design Guide

Polypropylene plastic is very popular in the manufacturing world. It presents several advantages that appeal to the manufacturers. Talk about an inexpensive, versatile, lightweight, and readily available thermoplastic that is resistant to many chemicals.

No wonder the material is common in the medical, clothing, packaging, automotive, and many other industries. Its versatility also extends to the way you can manufacture it. Think about any thermoplastic processing technique. Chances are that you can use that technique on polypropylene plastic successfully.

Injection molding is a major processing method for polypropylene plastic. Join us in this brief but insightful coverage of polypropylene injection molding. We will start with the process basics and the advantages, then proceed to share a design guide.

What Is Polypropylene?

Polypropylene injection molding

Polypropylene (PP) is a popular polymer that is becoming more popular every day. Its popularity is based on the material’s capability to replace conventional alternatives such as metal and wood. Its low cost and versatility are some of the biggest reasons manufacturers are opting to switch to it for manufacturing.

PP emanates from propylene gas polymerization. Alongside pressure, temperature and other polymerization conditions, the process of forming this plastic also uses a catalyst.  There are different grades of PP, which are differentiated through variation of the polymerization parameters.

The main polypropylene structures of this plastic are syndiotactic polypropylene, atactic polypropylene, and isotactic polypropylene.

Although polypropylene manufacture involves components that could be toxic, the finished product is generally regarded as safe for human use. Its use in medical supplies and plastic items has not caused any issues so far.

Popular items made from the thermoplastic are bottles, cups, cutlery, and beverage containers.

How oes Polypropylene Injection Molding Work?

injection molding works

The polypropylene plastic injection molding process is straightforward, but follows a series of steps. This process starts with the heating of plastic granules in a barrel. After reaching their melting point, the plastic enters cold runner molds. Upon hardening, the plastic polymer assumes the shape of the mold. After some time of cooling, the mold is opened and the injection molded part ejected for further cooling. That’s the overview of the process, but here now are the specific steps:


When a polymer changes its mechanical and thermal properties, it is said to undergo plasticization. That is if the property changes make it easier to deform or bend the polymer. In polypropylene plastic injection molding, plasticization is the transformation of solid plastic to a plastic melt. The viscous matter has uniform viscosity and density.


After the plasticization process, the molten plastic leaves the injection barrel. It is guided through the runner and sprue to the mold cavity. The screw or plunger first forces the molten plastic to the barrel end. From the runner and sprue, this matter goes through an injection nozzle to the mold cavity.

Pressure Holding

As the material cools in the mold cavity, it will naturally start to shrink. Pressure holding ensures that the structure and texture of the molded part remains complete despite the cooling. The polypropylene plastic injection molding pressure holding process entails the screw or plunger sustaining pressure from the barrel to the mold cavity during the cooling.


Cooling is a critical part of the polypropylene plastic injection molding process. Technically, cooling in this process starts immediately the molten plastic enters the mold cavity. However, for the in-mold cooling, this would refer to the time between the complete cooling to the ejection of the part.


The ejection step may also be called mold release. An ejection mechanism opens the mold, but this is after the allowed time for cooling. Polypropylene injection molding machines are equipped with a mechanism for removing the part from the mold. For the ejection process to be flawless, a release agent may be applied.

Polypropylene Material Properties 

PPH PPC PP-Talc 30% PP GF 30% PP-Carbon Black - UV
Physical Density(g/cm3) 0.932 0.952 1.13 1.14 1.04
Linear mold shrinkage rate(cm/cm) 0.0133 0.0128 0.0111 0.00387 0.0132
Rockwell Hardness (R) 94.7 86.5 77.6 101 90.1
Mechanical Tensile strength at yield (MPa) 32.8 25.8 25.4 77.8 26.7
Elongation at break (%) 159 231 23 4.01 20.3
Flexural modulus (GPa) 1.44 1.39 2.6 5.42 1.92
Flexural Yield Strength MPa) 44.1 87.4 45.5 118 35.6 Polypropylene injection molding Drying temperature (℃) 82.8 77.7 76.4 85.4 95
Melt temperature (℃) 214 215 223 157 223
Mold temperature (℃) 41.3 42.9 49.7 53.1 53.9

Advantages of Polypropylene Injection Molding

Low Coefficient of Friction

The coefficient of friction of pp plastic is low. With this characteristic, the material flows through the injection molding tools easily. Consequently, the cycle times are faster and the cost of manufacturing lower.

Fatigue Resistance

Fatigue resistance can be defined as the ability of a plastic to withstand external force without breaking. polypropylene pp is excellent in this aspect. Manufacturers find the element useful where fatigue resistance is critical, for instance, in living hinges.

Low Cost of Injection Molding Polypropylene

If you are looking for an affordable thermoplastic polymer for injection molding, pp plastic is an excellent choice. It is so affordable that only polyethylene is more common.

Moisture Resistance

With a slippery surface, pp plastic does not absorb a lot of moisture. Its low moisture absorption rate comes in handy in manufacturing storage containers. This includes beverage and food containers.

Low Melt Viscosity 

Like for fatigue resistance, melt viscosity determines how fast the plastic melt flows. For pp, low melt viscosity is evident in the smooth and fast filling of the mold.

Chemical Resistance

Lastly but not the least, this thermoplastic addition polymer has excellent chemical resistance. Some of the things it is chemically resistant to are oils and organic solvents.

Polypropylene Injection Molding Design Guidelines

Several factors determine the quality of the injection molding polypropylene product. In that regard, consider the following guidelines for the best results:

Wall thickness

injection molding wall thickness

Wall thickness is one of the commonest areas where design mistakes happen. Strive to have uniform wall thickness for polypropylene injection molding. This allows for the uniform flow of molten plastic to every corner.

It is the best way to prevent non-uniform cooling that results in shrinking and warping. Fortunately, you don’t have to undergo trial and error. There is a recommended wall thickness for polypropylene plastic of between 0.025 and 0.150 inches.


Sharp corners in polypropylene injection molding can also be problematic. They are sources of stress concentration, which can trigger component damage. To counter this issue, you want to watch the radii of corners. The ratio of radius to thickness, otherwise known as R/T value, can guide. Aim for R/T values higher than 0.5.

Draft angle

Draft Angle

Mold removal is much easier where there is draft angle. The draft angle for a specific design depends on the planned use of the part and the depth in the mold. Utilize as much draft as possible.

Part tolerances

It is understandable that there is bound to be a difference in dimensions for the molded part and the design. Tolerance, or the level of specification disparities, is essential for proper function of molded parts. In propylene injection molding, tolerance could be in the molded part and in the mold tool. A molded part tolerance of around 0.051mm/mm.

Design Reinforcements

Even if the wall thickness still appears weak according to the design, there is no need to be desperate. Design reinforcements are allowed in pp injection molding. You can use gussets to support thin walls. The only condition is that the design reinforcements adhere to the 40:60 percent requirement.

Sink marks

Be wary of sink marks – the higher shrinkage of material around thicker parts than in thinner parts of the mold. In PP material injection molding, you can minimize this problem. Packing additional molten material and pressure after filling the mold is necessary. Increasing the time for packing may reduce sink marks.

Integral hinges

Integral hinges help minimize the number of parts, and ultimately, the manufacturing costs of pp injection molded products. Pp plastic is undoubtedly one of the best materials for integral hinges. For even higher effectiveness, ensure that the thickness of the hinge is around 0.012 inches. Also, place the gate properly to allow for uniform rate of material flow.


Talking of the gate, pp injection molding is better with proper considerations of various aspects of this design element. Some factors to consider are the gate size, gate location, and number of the gates.

Tips for Polypropylene Injection Molding

Consider the following tips when injecting molding pp plastic:

Injection pressure

The injection pressure for pp plastic injection molding should be in the range of 5.5 MPa to 10 MPa. A back pressure of 0.7 MPa is enough because too much pressure can cause the molten plastic to stick to the mold.

Moisture resistance

The moisture resistance for pp plastic is excellent. However, you want to consult the supplier for propylene products that contain additives for any need for drying.

Temperature control

To avoid thermal degradation of the polymer, you want to keep the temperature at below 280°C. For the mold temperature, higher than 80°C can spoil the results. Before the part can be released, cool it to about 54°C.

Viscosity of Polypropylene Plastic

Provided you maintain the temperature to below 280°C as indicated above, the melt viscosity of polypropylene pp will increase with temperature.


The shrinkage rate of this material ranges from 1% to 3%, but could increase with wall thickness.


The injection molding polypropylene is critical for many industries owing to its many advantages. We have highlighted some of them here, as well as the basics of the process. Guidelines of the process are also available, considering that design errors happen.

At Aria Manufacturing, we have the expertise in pp injection molding. We continue to provide quality polypropylene parts as we serve the larger plastic injection molding market. Ask for a free quote now!


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.