Injection Molding is one of the most common manufacturing methods for production parts; this also means that there is an abundant list of material options available for molding. These materials are each engineered and include general, common materials, like PC-ABS, as well as specialty materials like injection-molded PEEK.
With every part you manufacture, the material you select should be intentional. In this instance, careful consideration should be given to the part’s utility, efficiency, and cost. It is important to understand that while certain materials may be more popular than others, there’s no such thing as one-size-fits-all in manufacturing. At the end of the day, your material selection will always be directly related to your part’s application. The tips below are helpful for distinguishing between requirements and “desirements” in a manufacturing project, which can be critical for managing project costs and timelines.
Where Do I Start?
To find the right material for your parts, it helps to work backward. Some great starting questions to ask are: What will the part be used for?, What kind of stresses will it endure?, and Where will it be located? The following criteria can help you understand what factors to prioritize for the material you select:
- Location: You must always consider where your part will live, whether that is outside, inside, under the hood, etc. Will it live in sunlight, outdoors, in a dark and damp environment, or somewhere else entirely?
- Temperature: Consider the temperature when you are choosing your material. Will the product end up in a freezer, or somewhere that’s very warm or hot?
- Life Cycle: How long do you expect this part to be used? Do you expect it to expire after 5 years, 25 years, etc?
- Warranty: Consider the warranty. For the automotive business particularly, you have to consider that years from now your part could break. What is the cost of covering its warranty?
- Cost Constraints: There are two different types of materials. You could get a commodity plastic (such as high-density Polyethylene or Polypropylene), which are typically high-density and low-heat, readily available, and comparatively inexpensive. The second category is engineered plastics, including materials like PEEK and PEI. These are very expensive and more difficult to find, but they are high-heat and stiff.
- Cosmetic Requirements: Does the part need to be completely transparent, or a certain color? Color matching may be required if your part needs to match others in an assembly. Finishing standards, such as SPI, provide standard references for surface treatment you should specify for your parts, ranging from optically-polished SPI-A1, through coarse matte SPI-D3. Mold texturing, such as those defined by MoltTech is another option to consider cosmetic faces.
How Do I Narrow My Decision?
Once you have those initial questions answered, you could still be looking at a handful of different material options. Consider this next series of questions to help you further narrow down material for your part:
- Design Functions: Consider the mechanical engineering aspect of the part. Does it need to be flexible, compressible, bondable, etc? Does it need tensile strength (meaning it cannot be pulled apart)? What’s the part’s required toughness against impact, or electrical insulation? Will the material need to bond to another surface, such as multi-material overmolding or insert-molding application? The ideal weight of a part is also an important consideration.
- Environment Factors: What elements will the part be exposed to? Will it be submerged in chemicals that it needs to be compatible with or in contact with open flames that would require certain additives and retardants such as a UV stabilizer?
- Regulatory Compliances: Adding certain compliances can quickly increase the cost of your part, which is why it’s important to evaluate what is needed and what might be superfluous. Does your part need to be food-safe, and does it need to meet FDA requirements? Or does it need to be medical-grade, ISO, electrical compliance, etc?
Armed with the top 4-5 things that define your material requirements, you can then evaluate material offerings to understand what will work best for your parts.
Xometry offers injection molding services without restrictions on materials. Because each project is custom, Xometry’s molding engineers can work with a customer-specified material, or work with the customer to determine the best material options for that project. The list below can help inspire material decisions based on application and properties. A more comprehensive list of molding services and materials can be found here.
ABS (Acrylonitrile Butadiene Styrene)
- ABS is an opaque thermoplastic and amorphous polymer. Known for being impact resistant and tough, ABS has a strong resistance to corrosive chemicals and physical impacts. It has a low melting temperature, making it particularly good to use in injection molding.
- Application examples: Automotive body parts, wheel covers, and enclosures
- POM, also known as acetal or Delrin, is a semicrystalline thermoplastic that is very strong with good chemical resistance and good dimensional stability. It is naturally lubricating, which lends itself to applications such as gears and pulleys. Acetal also has excellent creep resistance, high surface hardness, low moisture absorption, and fatigue resistance.
- Application examples: gears, springs, plates, bushings, pump housings, etc.
LDPE (Low-density Polyethylene)
- LDPE is a thermoplastic made from the monomer ethylene. It has low density and strong resistance to acids, alcohols, bases, and esters. LDPE is often pliable and has excellent electrical properties.
- Application examples: flexible parts, living hinges, and containers.
HDPE (High-density Polyethylene)
- HDPE is the hardest and stiffest of the polyethylene family. It does not have the impact strength of LDPE but is more resilient. It is rigid, with excellent resistance to acids, alcohols, and bases. A disadvantage of HDPE is that it very prone to sink in thick areas.
- Application examples: Food-related products, hinges, and pipe fittings
- PP is a versatile, tough, thermoplastic material that is lightweight, very low density, and very flexible. These characteristics make it good for hinged parts. PP also has good tensile strength and inertness toward acids, alkalis, and solvents. PP parts are often microwave- and dishwasher-safe, making them useful for products with food contact. Due to the low density, PP parts will dent and easily scrape.
- Application examples: Packaging, parts for electronic and electrical appliances, food containers, food dishes, hinged parts, toys, and household goods
- PA, better known as nylon, is strong, rigid, and has a very high melting point. It is impact and abrasion resistant, and also self-lubricating. Glass-filled nylon provides the benefits of nylon with stiffer mechanical properties and a lower risk of warping. A weakness of nylon is that it has high water absorption and poor chemical resistance to strong acids and bases.
- Application examples: Bearings, bushings, rugged housings, soldier systems, outboard motor covers, valves, office chairs, rollers, and wear surfaces
- PC is a transparent amorphous polymer that exhibits outstanding physical properties such as impact resistance, heat resistance, and excellent clarity. PC is often used to replace glass or metal in demanding applications when the temperature does not exceed 125°C (257°F).
- Application examples: Power tools, small appliances, Blu-ray discs, battery cell carriers, light pipes, safety glasses, and toys
PC-ABS (Polycarbonate-Acrylonitrile Butadiene Styrene)
- PC-ABS is a blended material of ABS with PC, and offers a good combination of mechanical and thermal properties. This material offers good impact strength—even in cold temperatures—as well as rigidity, dimensionality, and thermal and color stability. A limitation of PC-ABS is its poor chemical resistance.
- Application examples: Door handles, electronics housing, computers, and printers
PBT (Polybutylene Teraphthalate)
- PBT, also known as Valox, is an excellent candidate for molding thin cross-sections due to its low melt viscosity. PBT has a very low coefficient of friction both against metal and against itself, and the moisture absorption of PBT is also very low. This compares very favorably to other crystalline materials. PBT has a high chemical resistance and performs well under varying temperature ranges, making it a common substitute to ABS parts for harsh environment applications.
- Application examples: Under-the-hood automotive parts and plastic components for the electrical utility industry
PET (Polyethylene Terephthalate)
- PET polyesters are semicrystalline and have outstanding electrical properties. PET is abrasion resistant and has a higher modulus than PBT. Due to its different properties in different directions, PET is difficult to mold to extremely close tolerances. When blow molded, PET exhibits unique durability and flexibility.
- Application examples: Reusable bottles, soda bottles, medical devices, replacing metals in motor housings, switches, sensors, and other electrical applications.
PMMA (Polymethyl Methacrylate)
- PMMA, better known as acrylic or plexiglass, is a transparent thermoplastic homopolymer. The material is similar to polycarbonate in that it is suitable as an impact-resistant alternative to glass (particularly when high impact strength is not required).
- Application examples: Lenses, acrylic nails, security barriers, medical devices, LCD screens, and furniture
- PS is a naturally transparent, rigid, and relatively inexpensive thermoplastic. It is available as both a solid plastic as well in the form of a rigid foam material. It has good impact resistance, and food contact is acceptable.
- Application examples: Packaging, containers, and toys
PS-PPE (Polystrene-Polyphenyl Ethers)
- PS-PPE, also called Noryl, consists of blends of PPE and PS. Due to its chemical structure, it displays abnormally low moisture absorption, providing excellent insulating properties and dimensional stability over a wide range of humidity and temperature.
- Application examples: Electronics, electrical equipment, machinery, and aerospace
PPS (Polyphenylene Sulfide)
- PPS, also known as Ryton, is inherently flame retardant and is a good choice for thin-walled plastic injection moldings. It has good mechanical properties and excellent chemical resistance at elevated temperatures. Extremely tight tolerances may be more difficult to achieve when selecting PPS.
- Application examples: electrical applications involving high temperatures, electrical connectors, instrumentation housings, telecommunication devices, and military applications
- PSU, also known as Udel, is a transparent, semicrystalline thermoplastic material. It is known for its high-service temperature, low creep, dimensional stability, and stiffness. It is strong, with a tensile strength between that of polyphenylene oxide (PPO) and acetal. The high-temperature creep resistance of PSU is among the best of the non-reinforced thermoplastics.
- Application examples: appliance parts, electronic parts, automotive parts, medical components, aerospace, and insulators
TPE (Thermoplastic Elastomers)
- TPE, commonly referred to as Santoprene, has similar properties and a feel close to natural rubber, but with longer life. TPE materials are able to be sourced in a variety of Shore A durometers and are therefore are extremely versatile. TPEs are good in both hot and cold environments and are often preferred over rubber. They have good tear strength and are used for overmolding and living hinges.
- Application examples: Consumer goods, medical instruments, and industrial tools like knobs, handles, gaskets, seals, and grips
LSR (Liquid Silicone Rubber)
- LSR, commonly referred to as silicone, has high elongation, temperature resistance, and chemical resistance. Silicone is chemically inert, making LSR options ideal for food- and skin-contact applications and liquid environments. LSR can have superior light transmittance compared to molded TPE.
- Application examples: Consumer goods, skin-contact devices, medical devices, seals, soft enclosures, and grommets
Whether it is a thermoplastic, thermoset, elastomer, synthetic rubber, or silicone rubber material, Xometry can provide instant quotes and fulfillment of your injection molded custom parts.
Want more info on this topic? Our injection molding experts Greg Paulsen and Travis Minyard hosted a 40-minute deep-dive on selecting IM materials, with 4 real-world examples to walk you through the process. Watch the full on-demand webinar here!
This blog post was originally posted on August 24, 2018 and was updated on January 21, 2020.