6 Common Plastic Resins For Injection Molding

There are hundreds of plastic resins on the market. Each one with unique properties, advantages, and disadvantages. And, considering the importance of achieving the precise fit, form, and function for your part designs, material selection can be a bit overwhelming.

Never fear. Xcentric’s team of application engineers and material experts are here to help! We have extensive experience working with plastic resins and have a deep understanding of how the different strengths and limitations could impact your products—during and after production.

In this blog we provide details for 6 of the most common plastic resins used in the injection molding process. You can also download the Quick Guide for easy access to the bullet points, advantages, and disadvantages.

Quick Guide: 6 Common Plastic Resins

Injection Molding

ABS: Acrylonitrile Butadiene Styrene

As far as plastic resins go, ABS is by far one of the most popular and versatile. It’s an affordable, highly moldable engineering plastic with mechanical and electrical properties that make it one of the most widely used thermoplastics in the world.

With ABS you get the collective benefits and properties of three monomers. It combines the strength and rigidity of acrylonitrile and styrene polymers with the toughness of polybutadiene to deliver superior hardness and toughness.

In addition, ABS provides a colorfast, high-gloss surface finish. And finally, this resin has excellent stress, impact, creep, and heat resistance properties. And, it can be heated, cooled, and re-heated without compromising its characteristics.

Common applications include small appliances, medical devices, enclosures for electronics and electronic assemblies, office equipment, and toys.

Design Considerations

Parts molded with ABS are susceptible to forming knit lines, which are visible lines on the surface of the part. One solution is to ensure the appropriate wall thickness to help slow the resin cooling. Recommended wall thickness for ABS is 0.045 in. – 0.140 in.

Medical part molded using ABS plastic resin.

Advantages and Disadvantages of ABS Plastic Resins

Understanding the advantages and disadvantages will help you make an informed decision for your product design.

Advantages

  • Structural/Dimensional stability
  • High impact resistance
  • High rigidity and strength
  • Heat and chemical resistance
  • Abrasion and stain resistance
  • Surface brightness and high-gloss finish

Disadvantages

  • Maximum continuous use temperature approx. 70 °C ( 160 °F )
  • Poor solvent and fatigue resistance
  • Poor UV resistance unless protected
  • Poor bearing properties (high friction and wear)
  • High smoke evolution

PC: Polycarbonate

PC polymers are amorphous engineering thermoplastics with a toughness down to -20°C. They are naturally transparent and can transmit over 90% of light as good as glass.

Because of these properties, polycarbonate resins are often used as a light-weight alternative to glass and when high-impact resistance is a requirement.

Some applications include bullet-proof glass, medical device components, light fixtures, and green houses. In addition, PC is a natural UV filter which makes it an optimal material for eyewear.

Another feature of PC is its pliability. In fact, it can often be processed at room temperature without breaking or cracking. This property makes it a popular material for prototyping—especially when transparency is required.

Design Considerations

Though it provides outstanding impact resistance, polycarbonate is susceptible to scratching. Therefore, for applications where this will be an issue, consider adding a scratch-resistant coating.

Advantages and Disadvantages of Polycarbonate Plastic Resins

PC is a great light-weight alternative to glass. It is a very popular, well-known plastic that offers product designers opportunities for design freedom and cosmetic enhancements.

Still, there are some disadvantages that may eliminate PC from your material selection process. Here are some advantages and disadvantages of PC to consider.

Advantages

  • High impact strength and toughness down to -20°C
  • Naturally transparent. Can transmit over 90% of light as good as glass
  • Can be designed to 100% protection from harmful UV rays
  • High dimensional stability
  • Pliable at room temperature without breaking or cracking. Good for prototyping
  • Good Heat resistance and thermally stable up to 135°C

Disadvantages

  • Though resistant to high impact, PC plastics are susceptible to scratching
  • Considered hazardous for food the release of Bishphenol A (BPA)

PC/ABS: Polycarbonate-ABS

PC-ABS is one of the most widely used industrial-grade thermoplastics. It is a high-impact engineering polymer that combines the strength and heat-resistance of polycarbonate with the flexibility and high-quality surface finish of ABS.

In addition, PC/ABS plastic provides improved processing during the injection molding process. Product designers and engineers often choose it for functional prototyping, tooling, and low-volume manufacturing. This because PC/ABS is likely to provide stronger parts and prototypes that mimic the material properties of the final product.

Advantages and Disadvantages of PC/ABS Plastic Resins

Though PC/ABS can provide the best properties of two polymers, it is critical to understand key pros and cons to ensure it will achieve your fit, form, and function.

Advantages

  • High impact strength even at low temperatures
  • Heat resistance
  • High stiffness
  • Easy processing
  • Low overall shrinkage and high dimensional accuracy

Disadvantages

  • Poor solvent resistance
  • Low dielectric strength (not a good insulator)
  • Low continuous service temp. (melts easily)

|Download: Quick Guide: 6 Common Plastics for Injection Molding

PP: Polypropylene

Polypropylene is a crystalline thermoplastic that is tough and flexible with outstanding heat and chemical resistance properties. It’s one of the cheapest plastic resins available and is used both as a plastic and a fiber in a variety of applications across industries.

For instance, PP is used in automotive manufacturing, packaging, furniture assembly, textiles, and aerospace. In addition, PP is also a successful material option for special parts like living hinges—a flexible plastic bridge that connects two hard pieces of plastic.

Design Consideration

Polypropylene may be the cheapest option, but that doesn’t mean it’s the best option for achieving your fit, form, and function. For example, if your part requires impact resistance properties, consider PC/ABS instead.

Medical fluid delivery component molded with PP.

“I worked with a customer recently who wanted to use polypropylene because it was the most expensive option. But after reviewing the part, I realized it was multi-part assembly that would be at risk of being dropped. So I suggested PC/ABS instead because it will provide more structural integrity and impact resistance than PP.” 

Drew Davis, Application Engineer at Xcentric

Advantages and Disadvantages of Polypropylene Plastic Resins

Polypropylene may be the cheapest option, but that doesn’t mean it is the best choice for your injection molding project. In fact, selecting the wrong material can add to your production costs in the end.

Instead, consider these pros and cons of using polypropylene before making a material selection.

Advantages

  • High impact strength even at low temperatures
  • Heat resistance
  • High stiffness
  • Easy processing
  • Low overall shrinkage and high dimensional accuracy

Disadvantages

  • Poor solvent resistance
  • Low dielectric strength (not a good insulator)
  • Low continuous service temp. (melts easily)

Nylon

Nylon is a semi-crystalline polyamide with low density and high thermal stability. Polyamides are a group of technical thermoplastics with properties that range from the hard and tough PA 66 to the soft and flexible PA 12, for example.

In generally, nylon plastic resins provide fantastic wear resistance, good coefficient of friction, and very good temperature and impact properties. Further, nylon also provides chemical resistance and has proven to be a good oil resistant plastic.

This balance of properties make nylon a good candidate for metal replacement in some applications that require toughness and weight reduction. For instance, automotive parts and industrial components.

Design Considerations:

Nylon absorbs water. So if your part will be exposed to moisture, nylon may not be the ideal plastic because it could damage the dimensional and structural integrity.

Advantages and Disadvantages of Nylon

Nylon is easy to process and can be a cost-effective solution for your injection molding project. Like all of the plastic resins on our list, it’s important to understand how the properties will impact your design intent.

Advantages

  • Excellent abrasion & wear resistance
  • High tensile and compressive strength
  • Low coefficient of friction.
  • Lightweight option that’s 1/7th the weight of conventional materials
  • Easy to machine

Disadvantages

  • High shrinkage in molded sections
  • Lack of stability
  • Does not absorb water well

POM: Polyoxymethylene (Acetal)

POM, also known as acetal, is a naturally white semi-crystalline engineering thermoplastic and is characterized by its high rigidity to −40 °C. It is used in the injection molding process to produce parts that require high precision, stiffness, and low friction. It has excellent dimensional stability and resistance to abrasion, heat, water absorption, and creep.

Because of its low coefficient of friction, POM is good material choice for producing high-performance engineering components, medical equipment, and electrical housings. By nature, POM is a slippery material, which makes it a good fit for products that require sliding mechanisms and gears.

It is important to note that POM has a very high shrink rate of about 2%.

Design Consideration

If you are designing parts that have large variations in wall thicknesses, POM can be dimensionally unstable. Large parts molded with POM are more likely to experience warping than smaller parts. Therefore, consider adding fillets or strengthening ribs.

Advantages and Disadvantages of POM Plastic Resins

POM is one of the most common plastics used in the injection molding process. Here are some advantages and disadvantages to help determine whether POM is right for your achieving your design intent.

Advantages

  • High strength
  • Rigidity to -40C
  • High resistance to impact, creep, abrasion, and solvent
  • Good  fatigue endurance and low coefficient of friction

Disadvantages

  • High shrinkage of about 2%
  • Very poor resistance to UV radiation
  • Poor resistance to acids/alkalies
  • Burns easily, is not available with flame retardants
  • Does not absorb water well

Conclusion

Don’t be discouraged when choosing a plastic resin for your part design. Xcentric can  help guide you through the process to help bring your concepts to life faster and on budget. Get connected with an Xcentric team member today: sales@xcentricmold.com or 586-598-4636. If you you’re ready to get started with a project, simply upload your part to request a custom quote