Everything You Need To Know About Polypropylene Injection Molding

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In this article, we’ll take a detailed look at polypropylene injection molding: when it can be used, what products and applications it’s most suited for, and what the advantages of using it are. As you read through, remember that with the wide variety of plastic resins available from which to choose, there’s no textbook “best solution” — it always depends on your specific needs. Polypropylene is one of the more versatile and commonly used materials available, which is why we’re focusing on it today, but it’s important to always compare the advantages and disadvantages of any material against the requirements of your project.

As a quick refresher, before we begin our deep dive into polypropylene, just a few of the factors that might drive your material choice include:

  • Physical property requirements: hardness, flexibility and more
  • Appearance
  • Temperature resistance (upper and lower extremes)
  • Food- or medical-grade suitability
  • Density and weight
  • Cost

At the outset, only you understand the different weight that each of these factors has in the calculus of material choice for your specific project. Of course, it’s important to share this information with your manufacturing partner so that it can best assist you in choosing the right material.

Now, we’ll examine the best uses for polypropylene, as well as some advantages of using it.

What Applications Is Polypropylene Best Suited for?

Polypropylene is one of the more commonly used injection molding resins for a reason: It’s extremely versatile and has a number of applications for which it’s suited. In addition, polypropylene injection molding parameters, such as melt temperature and viscosity, lend themselves to relatively easy, cost-effective production in a wide range of uses.

Some of the primary applications for which polypropylene is typically drawn on include:

  • When good impact resistance is required: With a high tensile strength, polypropylene is suitable for moderate- to high-impact applications, making it a good choice for a number of industrial uses. Its molecular makeup gives it good shatter resistance, as well.
  • When low friction is important: Going back to the molecular makeup of polypropylene, its structure makes it more “slippery” than many other resins — in other words, it has less friction, making it suitable for many applications where a smooth, machinable metal might instead have to be used, such as gears. (Bear in mind, this is not to say that polypropylene can’t be machined, but it’s typically seen as a more specialized process.)
  • When moisture resistance is required: Outdoor applications, as well as many industrial applications, including those where the piece may be submerged in a liquid, require good moisture resistance. Polypropylene fulfills this need and will not leak, absorb or retain any moisture. For outdoor applications, note the caveat regarding UV exposure below — polypropylene does not offer ideal structural integrity in UV-intensive applications.
  • When chemical resistance is required: With a highly stable molecular makeup, polypropylene is extremely resistant to damage, degradation or adverse interactions with a broad range of chemical types — whether acid or base. This makes it suitable for a number of harsh-environment industrial uses, as well as laboratory applications such as chemical storage.
  • When a cost-effective solution is needed: While price shouldn’t be the No. 1 driver of your material choice, it is, of course, always a factor. Polypropylene is often preferential in such cases because it provides such a broad range of properties and applications at a cost-effective price point, competitive with any other resin. However, in applications where more extreme or unconventional material requirements are necessary, a more expensive material may be needed.

While polypropylene has many applications, there are some for which it isn’t well-suited. These include:

  • UV-intensive applications
  • Extremely high temperatures
  • Direct exposure to fire
  • Aesthetically driven applications where painting or other modification of the final appearance is required

How Does the Cost of Using Polypropylene Vary Compared With Other Resins?

Polypropylene, as mentioned above, is a cost-effective material on a raw material basis. Although resin prices fluctuate based on supply and demand, polypropylene will almost always be a competitive option — provided that it meets your product requirements. Some secondary cost benefits of polypropylene include:

  • Low density: As a lower-density resin, polypropylene can offer good strength at lower weights than other resins or materials (like metal), resulting in lower handling and transportation costs.
  • Ease of injection molding: With material properties in liquid form that are advantageous for injection molding, polypropylene is well-suited to provide consistent, reliable results — meaning fewer defects and part rejections, faster cycle times and shorter overall production times.
  • The nature of thermoplastics: As a thermoplastic, polypropylene will revert to liquid form when subject to extremely high heat. This means much lower material waste and cost, as defective parts and excess material can be melted and re-incorporated into the base resin.

What Are Some of the Common Products Polypropylene Is Used to Create?

With so many versatile applications, polypropylene is thus well-suited to a broad range of product types, as well. One of the most common uses is the living hinge — a one-piece hinged construction commonly seen in products like caps for consumer products (think of the flip-top on a mustard bottle). The living hinge is one of the most important innovations in plastic injection molding, and polypropylene is probably the most ideal resin for the job. This is mainly on account of its high property of flexibility — ability to bend multiple times — hundreds or thousands — without breaking.

Some other common uses of polypropylene include:

  • Pipes: Thanks to its high strength and moisture resistance, polypropylene is commonly used in both household and industrial piping.
  • Test tubes and beakers: As mentioned above, polypropylene is highly resistant to many chemicals, and thus well-suited to laboratory applications such as test tubes and beakers. In addition to this resistance, its status as a shatter-resistant material make it desirable.
  • Pallets: Re-usable plastic pallets are highly preferential to wooden ones for a number of reasons: more return on investment, longer life, fewer damaged products as a result of damaged pallets. With its high strength and impact resistance, polypropylene is often used in this application.

RevPart is ready to answer any questions you have about the benefits of polypropylene, and any other material selection inquiries. Contact us today.

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