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Choosing the right material for your injection-molded part

There are a number of benefits to manufacturing parts using injection molding, principal among them the ability to create high-volume production runs with high degrees of accuracy and precision. However, given that there are more than 85,000 different types of plastic injection molding materials for manufacturers to choose from, knowing which one makes the most sense for your desired part can feel overwhelming.

Choosing the right material relies on a number of factors, most critical being the application of the part. For example, the material used to create an injection-molded kid’s toy would obviously need to be different than the material used for the soles of running shoes, because each application requires different physical characteristics. A toy needs to be able to stand up to repeated blunt impacts from small children, whereas shoe soles should be able to flex and provide cushioning for a runner’s feet and joints. Knowing which material is most advantageous for the part’s intended application can help you produce superior, more durable pieces.

In this article, we’ll touch on some of the materials most commonly used in injection molding, as well as the benefits and ideal applications for each.

The most common injection molding materials

Acrylonitrile Butadiene Styrene (ABS)

This tough, impact-resistant plastic is used across a range of industries, from cosmetics to computers. In addition to being relatively inexpensive, ABS offers high dimensional stability — meaning that it’s resistant to changes in temperature and stability — and low shrinkage post-manufacture compared to other materials, while also providing good chemical resistance.

These physical characteristics make ABS a popular choice for parts in handheld devices, such as phones and electrical tool moldings. There are a couple potential issues to keep in mind when considering ABS. First of all, ABS is prone to knit lines — or the places where two flows of plastic resin meet during the manufacturing process. Additionally, it may produce sinks and voids caused by localized shrinkage during the part’s cooling process. These issues can be avoided by making strategic adjustments to part design, or by using an ABS-polycarbonate blend material.

Polycarbonate (PC)

Strong and extremely impact- and heat-resistant, PC is a transparent plastic promising low shrinkage and good dimensional stability. Highly detailed cosmetic finishes can be applied to PC parts, rendering it a strong contender for applications such as cell phone housings or electrical components.

Additionally, because it can be manufactured to optically clear grades and specifications, PC is often used in lenses, bulletproof glass, as well as in some lighting and medical devices. Like ABS, PC is prone to developing voids and sinks in thicker sections, which is why choosing an ABS-PC blend or acrylic material is often a wise choice for non-translucent parts or pieces featuring thick geometries, respectively. PC parts also tend to have poor chemical resistance.

ABS-PC

Using an ABS-PC blend gives manufactured parts the best of both worlds, combining PC’s strength and heat resistance with ABS’s flexibility. The blend is easier to work with during the actual injection molding process, while also providing better durability and dimensional stability than ABS or PC separately. It offers good impact strength, even in cold temperatures, and is often used for applications like door handles, computers, electronics, telecommunications devices. Unfortunately, a key limitation of the blend is that, despite its strengths, ABS-PC still offers poor chemical resistance.

Aliphatic Polyamides (PA or PPA)

There are a variety of PAs available, each with different physical and chemical properties. Also known as nylons, this family of plastic injection molding materials provides high chemical resistance and strength, even in high temperatures.

Some PAs, like nylon 6/6, create strong, rigid, and abrasion-resistant parts, while others, like nylon 6/12, offer higher impact-resistance. Because of their range of different physical properties, PAs may be used in a broad range of applications — from combs, screws, and cameras, to gears and automotive engine components. Nylons tend to be prone to warping and will absorb moisture, which could potentially lead to structural issues if not properly taken into account.

Polymethyl Methacrylate (PMMA)

Also called acrylic, PMMA is a highly glossy, scratch-resistant material commonly used to create lenses, optical fibers, light shades, and signs. Its low shrinkage and reduced chance of developing sinks — in both thin and thick geometries — makes it a good option for these applications. Like PC, however, acrylic generally offers poor chemical resistance. Further, acrylic is more brittle than other materials and therefore more prone to stress fractures — which can be offset to a degree by adding more draft angles in the part design process.

Polypropylene (PP)

This flexible, tough, and inexpensive resin provides high impact resistance, and can be blended with other polymers to create even stronger parts. PP is also resistant to wear and chemicals, and can provide high elongation (or resistance to strain), which makes it a useful material for hinges, bottle lids with snap closures, and medical tubing. It’s important to keep in mind that PP is prone to shrinkage and warping, and thicker sections often suffer voids or bubbles.

Finding the right material for the job

We’ve only listed a few of the most common injection molding materials, but there are many more that we didn’t touch on — such as polyoxymethylene, high density polyethylene, and others — that engineers and designers should be aware of when designing a part. While a more expensive process, metal injection molding may also be used to create durable parts.

When designing parts, it’s important to align the required characteristics of the part applications with the material properties. If a single material provides certain desired qualities but suffers from limitations, it’s possible that adjusting the manufacturing material through blends or the addition of fillers will produce something better suited to the desired outcome. Injection molding is capable of producing a lot of aesthetic options as well. Many materials can be mixed with colorants to create custom colors.

If you have questions about which material makes the most sense for the part you’re trying to make, contact us today. At Fast Radius, we’ll work alongside you every step of the way — from part design and material choice all the way through to post-production and finishing — to ensure that you’re receiving cost-effective, high-quality parts as soon as possible.

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