Hundreds of different kinds of plastic composites can be used for injection molding. Plastic types have diverse properties, making some better for certain applications than others. These properties include heat and impact resistance, along with tensile strength and how well it withstands molding pressure. These properties also affect how they react during the injection molding process. Plastics also differ by grade, with resin types that act as fillers like fiberglass or glass. To choose the best one for the application, it’s important to understand the properties of the most commonly used types of plastics used in injection molding.

Injection Molding: Plastic Types Most Commonly Used

There are a wide variety of materials used for injection molding. Plastic types that are utilized should thus not only be capable of going through this molding process but also suit the purpose for which they’re being fabricated.

Some of the most common injection molding plastic types include:

• Nylon (polyamide, PA)
• Polyethylene (PE)
• Polybutylene terephthalate (PBT)
• Polycarbonate (PC)
• Acrylonitrile butadiene styrene (ABS)
• Thermoplastic polyurethane (TPU)
• Thermoplastic elastomer (TPE)

Now, let’s briefly evaluate each one of these plastics by comparing them to other polymers and providing examples of each plastic type’s applications, along with their trade names and how they’re graded.

Thermoplastic Polyurethane

Thermoplastic polyurethane (TPU) is a widely used rubber-like material, a type of thermoplastic elastomer often used to replace hard rubber due to its ability to resist deformation. Plastic types like TPU can withstand ozone exposure and bear loads well for injection molding. When compared to other thermoplastic elastomers, TPU better resists chemicals and extreme temperatures. However, for creating certain kinds of molded components with injection molding, other plastic types are comparatively inexpensive and require less drying time.

When it comes to injection molding, plastic types like TPU offer the following properties:

• Maximum temperature: between 71°C to 121°C (160°F to 250°F)
• Chemical resistance: Fair to good
• Tensile strength: 6960 to 12,000 psi (pounds per square inch) (490 to 844 kg/cm²)
• Flexural strength: 260,000 to 340,000 psi (18,280 to 23,904 kg/cm²)
• Impact strength: 0.8 to 10.1 foot-pounds per inch (ft-lb/in) (42.7 to 539.1 joules per meter (J/m))

Regarding TPUs, injection molding plastic types are used for items like caster wheels, electronic enclosures, footwear, gaskets, and sports equipment. They’re also used for medical devices, with TPU sometimes replacing polyvinyl chloride (PVC), as it can irritate skin. The primary kinds of TPU materials are polycaprolactone, polyester, and polyether, with trademarked products from manufacturers that include Ultralast, Texin, Lubrizol, and Lanxess.

Thermoplastic Elastomer

A thermoplastic elastomer (TPE) is a cross between rubber and plastic. Though it can be processed via injection molding, TPE plastic types perform like other elastomers. Also referred to as thermoplastic rubber (TPR), like rubber, it returns to its original shape after stretching. TPE also can be recycled over and over again. When compared to materials like liquid silicone rubber (LSR), it’s inexpensive and easy to create forms via injection molding. However, plastic types like TPE tend to lose their elastic properties when exposed to heat and warp for good when exposed to prolonged pressure. They’re also much more expensive than other plastics used in injection molding.

When it comes to injection molding, plastic types like TPE offer the following properties:

• Maximum temperature: 170°C (338°F)
• Chemical resistance: Fair to good
• Tensile strength: 1000 to 7000 psi (70 to 492 kg/cm²)
• Flexural strength: 5000 to 800,000 psi (352 to 56,246 kg/cm²)
• Impact strength: 2.5 ft-lb/in (133 J/m)

Used for footwear and pet products, TPE has automotive and medical uses as well. Auto components like shock dust boots and weather seals are often made from TPE, while breathing tubes, catheters, valves, and ventilation masks are made with medical grades of plastic. Manufacturers include Avient, Dynaflex™, Kraiburg HIPEX®, and Teknor Apex Telcar®.

Polystyrene

Lightweight, inexpensive, and resistant to bacterial growth and moisture, polystyrene (PS) resists diluted bases and acids to a degree, along with gamma radiation often used to sterilize medical devices. With injection molding of plastics, the types used are typically either general-purpose polystyrene (GPPS) or high-impact polystyrene (HIPS). The former is harder, more brittle doesn’t retain its shape, and appears clear, whereas HIPS is considerably less brittle and looks opaque.

When it comes to injection molding, plastic types like PS offer the following properties:

• • Maximum temperature: from 75°C to 82°C (167°F to 180°F)
  • Chemical resistance: Poor to limited
  • Tensile strength: GPPS – 3000 to 3500 psi (211 to 246 kg/cm²) HIPS – 4061 psi (286 kg/cm²)
  • Flexural strength: 6400 to 11,000 psi (450 to 773 kg/cm²)
  • Impact strength: GPPS – .356 ft-lb/in (19 J/m) HIPS – .84 ft-lb/in (45 J/m)

Makers of polystyrene include American Styrenics PolyRenew® and BASF Polystyrol®. Generally, PS is used for electrical, electronic, medical, and optical applications. HIPS is used in appliances and other equipment, while GPPS is used for plastic cases, containers, toys, and trays. PS generally is flammable and degrades in ultraviolet (UV) light, though with acrylic added to the polymer, the plastic gains chemical and UV resistance, while adding glass can improve strength.

Polypropylene

As the second most used plastic globally, polypropylene (PP) offers good chemical and moisture resistance while having a high melting point and retaining form when bent. Also known as injected molded polypro, PPS is easily recycled. Yet PP also burns readily, and above the boiling point of water, the material dissolves into hazardous hydrocarbons like benzene and toluene. Other weaknesses include being difficult to paint or bond and degrading when exposed to UV light.

When it comes to injection molding, plastic types like PP offer the following properties:

• • • Maximum temperature: between 68°C to 148°C (154°F to 298°F)
    • Chemical resistance: Excellent
    • Tensile strength: 4500 to 18,500 psi (316 to 1301 kg/cm²)
    • Flexural strength: 210,000 to 1,500,000 psi (14,764 to 105,460kg/cm²)
    • Impact strength: 1.4 to 5.5 ft-lb/in (74.7 to 293.6 J/m)

Appliances, bodies of power tools, containers, packaging, sporting goods, and toys are often made from this injection molding plastic. Types of PP, like crystalline polypropylene (HcPP) use glass fibers to improve rigidity, though typically, the plastic is low density. One leading PP brand is Semitron® by Mitsubishi Chemical Advanced Materials.

Polyoxymethylene

As one of the injection molding plastic types made specifically for engineering, polyoxymethylene (POM) combines thermal stability with excellent rigidity while also offering a low friction coefficient. A naturally opaque whitish plastic, POM also resists chemicals well and absorbs very little water when exposed to moisture. A type of acetal plastic, POMs can replace metal in many applications.

When it comes to injection molding, plastic types like POM offer the following properties:

• • • Maximum temperature: between 180°C to 230°C (356°F to 446°F)
    • Chemical resistance: Excellent
    • Tensile strength: 6000 to 22,000 psi (422 to 1547 kg/cm²)
    • Flexural strength: 120,000 to 170,000 psi (8437 to 11,952 kg/cm²)
    • Impact strength: 0.8 to 2 ft-lb/in (42.7 to 106.8 J/m)

Its low friction coefficient makes POM particularly useful for bearings, conveyor belts, gears, pulleys, and high-performance parts. This injection molding plastic also makes fasteners, firearm components, frames for eyeglasses, knife handles, and lock systems. Types of POM polymers are reinforced to improve rigidity and strength, along with other mechanical properties, though in their pure form, acetal plastics tend to expand dramatically when exposed to heat. The material also doesn’t have a very high impact strength, while its use for outdoor applications is limited as it’s not UV resistant. Ensinger’s TECAFORM® and DuPont’s Delrin® are two brands of this plastic.

Polyethylene

Polyethylene (PE) is the most frequently used injection molding plastic. Types of this commercial polymer are often chosen based on their density. Low-density polyethylene (LDPE) and high-density polyethylene (HDPE) differ in flexibility, hardness, melting point, and opaqueness, though both are chemical-resistant. Another PE polymer, polyethylene terephthalate (PET), isn’t defined by density but rather by its transparency. All three of these PE plastics resist moisture as well as chemicals, though HDPE is less flexible and opaquer.

When it comes to injection molding, plastic types like PE offer the following properties:

• • • Maximum temperature: HDPE – between 71°C to 121°C (160°F to 250°F); LDPE 65°C to 90°C (149°F to 194°F)
    • Chemical resistance: Good to excellent
    • Tensile strength: HDPE – 3200 to 4500 psi (225 to 316 kg/cm²); LDPE – 1200 to 4000 psi (84 to 281 kg/cm²)
    • Flexural strength: HDPE – 145,000 to 225,000 psi (10,195 to 15819 kg/cm²); LDPE – 35,000 to 48,000 psi (2461 to 3375 kg/cm²)
    • Impact strength: HDPE – 0.4 to 4 ft-lb/in (21.4 to 213.5 J/m); LDPE – 56 to 150 ft-lb/in (2989.2 to 8006.8 J/m)

Due to poor resistance to UV light, these plastic types aren’t used for outdoor applications and as their operational temperatures are limited, they’re not used for certain processes that would expose them to high heat. Generally, PE plastics tend to be used for auto components, containers, housewares, and toys. Unlike other injection molding plastic, types are graded by density, with higher densities associated with higher numbers. So HDPE 400 would have a higher density than HDPE 300, though both would be denser than any grade of PE. Leading distributors for engineering applications include DuPont and BASF.

Polycarbonate

Polycarbonate (PC) is lightweight, tough, and transparent, with its ability to support light transmission, making it useful for optical applications. Additionally, when added pigments, PC keeps its color and retains strength. Though not resistant to scratches, PC offers significant durability and much better strength than glass. PC often replaces other plastic types like acrylic due to its ability to maintain physical properties over a wide temperature range, making it useful as an injection molding plastic. Types of PC are more expensive since these polymers require high temperatures to process. Yet this plastic’s constant and predictable reduction rates allow for tighter tolerances and more precise control when molded.

When it comes to injection molding, plastic types like PC offer the following properties:

• • • Maximum temperature: between 288°C to 316°C (550°F to 601°F)
    • Chemical resistance: Poor to fail
    • Tensile strength: 8500 psi (598 kg/cm²)
    • Flexural strength: 13,500 psi (949 kg/cm²)
    • Impact strength: 2 to 18 ft-lb/in (106.8 to 960.8 J/m)

PC is used for clear and tinted windows, clear tubing, diffusers, light emitting diodes (LEDs), and machinery guards while also being used to make clear casting molds for silicone and urethane. As it contains bisphenol A (BPA), a chemical linked to in-utero effects on fetuses, PC isn’t recommended for use with food. PC plastic grades may contain glass or, more rarely, stainless steel fibers. Two popular brands include Covestro Makrolon®, which is transparent, and SABIC Lexan®, which has flame-resistant grades.

Nylon

A synthetic polyamide (PA) nylon offers a combination of superior abrasion, fatigue, and heat resistance with toughness and capabilities to dampen noise. Though it’s not especially flame resistant, certain grades are flame retardant; similarly, though it degrades in sunlight, PA with a UV stabilizer allows its use outdoors. Compared with other injection molding plastics, types of nylon have lower resistance to bases and acids. It also isn’t as impact-resistant as PC. Additionally, PA is challenging to work with during the injection molding process due to its tendency to shrink so as not to fill the mold adequately.

When it comes to injection molding, plastic types like PA offer the following properties:

• • • Maximum temperature: between 93°C to 177°C (199°F to 351°F)
    • Chemical resistance: Good to excellent
    • Tensile strength: 6000 to 24,000 psi (422 to 1687 kg/cm²)
    • Flexural strength: 390,000 to 1,100,000 psi (27,420 to 77,338 kg/cm²)
    • Impact strength: 2 to 8 ft-lb/in (106.8 to 427 J/m)

Mechanical components like slides, gears, bushings, and bearings are often made from nylon, as are threaded inserts, snap-fit closures, moving parts, jigs, fixtures, and casings. Additional uses include electrical connectors, medical implants, toothbrushes, and wheels. Injection molding plastic types of PA are also used in abrasive and grinding applications due to their low friction coefficient, with each grade having similar yet distinct mechanical properties.

The four main grades of PA are:

• • • Nylon 11: Resists dimensional changes better and is used outdoors.
    • Nylon 12: Resists absorbing water and has the lowest melting point.
    • Nylon 46: Operable at the highest temperatures.
    • Nylon 66: Resists acids often used to process chemicals and features a high melting point.

In addition to these four nylon grades, better mechanical strength can be achieved through adding glass fibers.

Acrylonitrile Butadiene Styrene

With a comparatively low melting point, acrylonitrile butadiene styrene (ABS) works well as an injection molding plastic. Types of this tough engineering-grade polymer allow for various colorants, finishes, and textures due to its styrene content, enabling attractive and shiny finishes for components made via injection molding. Plastic types in the ABS family are also strong and resistant to impact, though they tend to weaken more quickly than acrylic when exposed to outdoor environments, particularly rain and sunlight. ABS also smokes significantly when it catches fire and does poorly when exposed to high amounts of friction.

When it comes to injection molding, plastic types like ABS offer the following properties:

• • • Maximum temperature: between 60°C to 93°C (140°F to 199°F)
    • Chemical resistance: Poor to fair
    • Tensile strength: 6600 psi (464 kg/cm²)
    • Flexural strength: 270,000 to 380,000 psi (18983 to 26717 kg/cm²)
    • Impact strength: 3 to 7.5 ft-lb/in (160.1 to 400.3 J/m)

ABS is used in auto dashboards, computer keyboards, head protection, vehicle bodies, wall plates for electrical outlets, wheel covers, sporting gear, industrial accessories, and consumer goods. BASF Terluran®, Ineos Lustran®, Samsung Starex®, and Toray Toylac® are all recognized ABS brands.

Acrylic

Known by its chemical name polymethyl methacrylate (PMMA), acrylic is a clear, strong thermoplastic used in injection molding. Unlike many other polymers, plastic types of PMMA typically offer much better resistance to UV, water, and weather resistance. Acrylic also works as a shatter-resistant and lightweight replacement for glass while also enabling tight tolerances. However, PMMA tends to stain easily when exposed to oil or grease and lacks resistance to solvents. Additionally, it is more likely to crack when bearing heavy loads, though it has a high tensile strength.

When it comes to injection molding, plastic types like PMMA offer the following properties:

• • • Maximum temperature: 130°C (266°F)
    • Chemical resistance: Excellent
    • Tensile strength: 2800 to 10,900 psi (197 to 766 kg/cm²)
    • Flexural strength: 13,000 psi (914 to kg/cm²)
    • Impact strength: .22 to 1.1 ft-lb/in (11.7 to 58.7 J/m)

Acrylics are widely used for their transparency within outdoor, lighting, and architectural applications, including for bathroom showers, greenhouses, solar panels, and windows. PMMA plastics are available in both general and specialty grades. Along with the basic acrylic, PMMA plastics include a stronger sign-grade polymer that transmits light well and is stronger for signage exposed to the weather and a marine-grade polymer that’s water-resistant. Two common brand-name PMMA plastics are Trinseo’s PLEXIGAS®, which can be made heat-resistant, and DuPont’s LUCITE®, which offers excellent transparency and optical properties.

Spaulding Plastic Injection Molding

Spaulding Composites Inc. has many decades of experience in injection molding plastic. The types of markets we serve range from consumer products to extremely complex processes to fabricate precision components for the healthcare and defense industries. With domestic and overseas partnerships, our injection mold production is augmented by our 3D printing design capabilities for developing prototype molds. With proven experience and in-depth knowledge of the plastics industry, Spaulding’s engineering team can provide the injection molding solutions your business needs. Contact us today to learn more about what Spaulding Composites can do for you.