Time to read: 13 min
Plastic injection molding materials have names that sound like terms from chemistry class. It’s hard enough that polyethylene rhymes with polypropylene, which has an abbreviation (PP) that’s similar to polystyrene (PS). Choosing the right injection molding material also means knowing your application requirements, which can change between prototyping and production.
Fortunately, you don’t need a PhD in chemistry to make the right material choice. There are many factors to consider, of course, but it all starts with the basics: the names and abbreviations of leading materials, their advantages and disadvantages, typical applications, and common brands and grades.
There are ten commonly used plastic injection molding materials:
- acrylic (PMMA)
- acrylonitrile butadiene styrene (ABS)
- nylon polyamide (PA)
- polycarbonate (PC)
- polyethyelene (PE)
- polyoxymethylene (POM)
- polypropylene (PP)
- polystyrene (PS)
- thermoplastic elastomer (TPE)
- thermoplastic polyurethane (TPU)
For each injection molded plastic, the following sections explain what you need to know.
Poly(methyl methacrylate) is better known as PMMA or acrylic. It’s a strong, clear thermoplastic that provides a lighter-weight, shatter-resistant alternative to glass. Because PMMA is transparent and has excellent optical clarity, it allows a high percentage of light to pass through.
Advantages and Disadvantages of Acrylics
Acrylics have a high tensile strength to withstand loads, won’t absorb odors, and can be held to tight tolerances during injection molding. The material also resists sunlight and weather, which makes it suitable for outdoor use. Because it doesn’t degrade when in contact with water, PMMA won’t release Bisphenol A (BPA), a chemical in many plastics that has adverse effects on human health.
Acrylics have a few important limitations. They cost more than glass and scratch more readily. They’re also easily stained by greases and oils. In addition, PMMA has a service temperature that is too low for some industrial applications. Injected molded acrylic lacks strong solvent resistance and can be prone to stress cracking under especially heavy loads.
PMMA Applications, Grades, and Brands
Acrylics are used in windows, greenhouses, solar panels, bathroom enclosures, and other transparent components for architectural and lighting applications.
General-purpose acrylic, the least expensive type of PMMA, is used in commodity products rather than engineering applications. Sign-grade acrylic is up to 10 times stronger than general-purpose acrylic and provides excellent light transmission for outdoor signage. Marine-grade acrylic resists damage from continuous exposure to water and is often used by boat builders. Common brand names for acrylic plastics include Trinseo PLEXIGAS®, which is available with high heat resistance, and DuPont LUCITE®, which has excellent optical clarity and transparency.
Acrylonitrile Butadiene Styrene (ABS)
ABS is an engineering-grade thermoplastic with a relatively low melting point that makes it easy to mold. This opaque polymer supports the use of colorants and is available with different textures and surface finishes. ABS is known for its strength and impact resistance.
Advantages and Disadvantages of ABS
ABS is tough, relatively inexpensive, and provides good resistance to heat, oils, acids, and alkalis. Its styrene content imparts a shiny, attractive finish to injection molded parts while its butadiene content delivers exceptional toughness, even at low temperatures.
Because ABS lacks strong resistance to sunlight or weather, it’s not a good choice for outdoor applications. ABS also generates significant smoke when it’s burned, provides poor resistance to high friction, and lacks resistance to solvents such as water.
ABS Applications, Grades, and Brands
Injection molded ABS is used in electronic components and with keyboard keys, protective headgear, plastic wall plates for electrical outlets, and automotive components such as dashboards, wheels covers, and auto body parts. Additional applications include consumer goods, sport equipment, and industrial fittings.
Well-known brand names for ABS plastics include Samsung Starex®, Toray Toylac®, BASF Terluran®, and Ineos Lustran®. Each of these brands are available in different formulations. ABS can also be combined with glass fillers for added strength, or with polycarbonate for low-temperature impact resistance. Some suppliers make both filled and unfilled ABS plastics.
Nylon Polyamide (PA)
Polyamides (PA) are a large group of materials that can be either natural or synthetic. Nylon, a type of polyimide, is always synthetic, however. Injection molding with nylon can be challenging because this polyamide is prone to shrinkage and subject to inadequate filling.
Advantages and Disadvantages of Nylon
The advantages of nylon PAs include their toughness and high heat resistance. They also have high abrasion resistance, good fatigue resistance, and noise dampening properties. With its low coefficient of friction, injected molded nylon is suitable for high friction and wear applications.
Although nylon does not offer inherent flame resistance and can burn quickly, flame retardant versions are available. Similarly, although nylon can be degraded by sunlight, a UV stabilizer can be added to improve performance. In addition, the material offers poor resistance to strong acids and bases.
Nylon Applications, Grades, and Brands
Nylon is often used in strong mechanical parts such as bearings, bushings, gears, and slides. It’s also a good choice for casings and snap-fit closures, threaded inserts, kinetic pats, jigs and fixtures. Additional applications range from toothbrushes and wheels to medical implants and electrical connectors.
Although nylon is the name of a specific type of polyamide that was developed by DuPont, this nylon polyamide is also available from other companies. Today, nylon comes in four main grades: nylon 66, 11, 12, and 46. Each has slightly different mechanical properties.
- Nylon 66 has a higher melting point and greater resistance to acids for use in chemical processing applications.
- Nylon 11 had increased resistance to dimensional changes because of its greater moisture resistance. It’s used in outdoor applications.
- Nylon 12 has the lowest melting point and relatively good resistance to water absorption.
- Nylon 46 has a higher operating temperature than the other grades of nylon. Applications include engines and transmissions
Nylon grades can be filled with glass for greater mechanical strength.
Polycarbonate (PC) plastics are strong, tough materials that are naturally transparent. Their properties are similar to PMMA, but polycarbonate maintains its physical properties over a wider temperature range. When pigmented, PC maintains its color over time and retains its strength.
Advantages and Disadvantages of Polycarbonate
Polycarbonate is extremely durable and 250 times stronger than glass. With its predictable and uniform mold shrinkage, this injection molding material offers precise dimensional control. PC plastic is also lightweight and has excellent optical properties to support light transmission.
Because it contains BPA, injected molded polycarbonate is not recommended for food preparation or storage. It also requires high processing temperatures, which can make it more expensive to mold. Polycarbonate is not scratch resistant either.
PC Applications, Grades, and Brands
Polycarbonate is used for machinery guards, clear or tinted windows, diffusers and light pipes for light emitting diodes (LEDs), and in clear tubing. This material can also be used to produce clear molds for urethane and silicone casting.
Polycarbonate can be filled with glass and compounded with ABS for increased impact resistance. Some grades contain a relatively small percentage of stainless steel fiber. Popular brand names include SABIC Lexan®, which is available in flame-resistant versions, and Covestro Makrolon®, which has glass-like transparency.
Polyethylene is the most commonly used plastic in the world. There are three main types:
- high density (HDPE, PEHD)
- low density (LDPE)
- polyethylene terephthalate (PET, PETE)
PE is the only commercial polymer that can be selected according to its density, a measurement that matters because it controls other material properties. HDPE and LDPE both share useful properties such as chemical resistance, but they differ in terms of their respective hardness, flexibility, melting point, and optical transparency. PET plastic is not defined by density.
Advantages and Disadvantages of Polyethylene
HDPE, LDPE, and PET are all low-cost plastics that resist both moisture and chemicals. LDPE is softer and more flexible than HDPE, which is an opaque material. Like LDPE, PET can be as clear as glass. Engineers and designers like these materials because they provide a range of choices in optical clarity and other properties.
PE plastics are limited by poor UV resistance and cannot withstand high service temperatures. HDPE parts are difficult to mold with thicknesses of less than .020-in. With injection molded LDPE, close tolerances are hard to achieve. PET absorbs significant water and requires resin drying, which adds to production costs.
PE Applications, Grades, and Brands
PE plastics are known for their use in milk bottles (HDPE), plastic films (LDPE), and water bottles (PET); however, these products are not injection molded. They are made using a different type of molding called blow molding. Examples of injection molded PE include children’s toys, toolboxes, electronic wire and cable insulators, and medical device components.
PE plastics come in numbered grades where higher numbers generally indicate higher densities. For example, HDPE 500 has a higher density than HDPE 300. Both grades have a higher density than LDPE, which also uses a numbering system for grades. BASF and DuPont are two of the leading suppliers of PE for engineering rather than general-purpose applications.
Polyoxymethylene (POM) is an engineering plastic that is also known as acetal. It can be injection molded into parts that require low friction, high stiffness, and excellent dimensional stability. Because they have a highly crystalline structure, POM plastics are naturally opaque and white in color.
Advantages and Disadvantages of Polyoxymethylene
Injection molded POM combines excellent rigidity with thermal stability and a low coefficient of friction. It also has low water absorption and good chemical resistance. Two main types of POM plastics are available: homopolymers and copolymers. This gives engineers and designers a choice of acetal materials with different properties.
In its pure form, POM has low impact strength and a very high rate of thermal expansion. Mold shrinkage is relatively high and acetals do not resist UV light. POM plastics that are homopolymers provide higher hardness and tensile strength. However, all POM plastics (both homopolymers and copolymers) cannot be fire-rated to a standard such as UL 94.
POM Applications, Grades and Brands
With its low coefficient of friction, injection molded POM is suitable for bearings, gears, conveyor belts, and pulley wheels. Additional applications include fasteners, eyeglass frames, parts for knives and firearms, lock systems, and high-performance engineering components.
POM comes in grades that are reinforced with fiberglass or minerals for improved strength and stiffness. Compounds that are reinforced with both materials provide an excellent balance of mechanical properties. Popular brand names for POM plastics include DuPont Delrin® and Ensinger TECAFORM®.
Polypropylene (PP) has similar properties to polyethylene (PE) but is slightly harder and more heat resistant. It is the second most commonly used plastic in the world. Although the density of polypropylene changes with fillers, pure PP is the commodity plastic with the lowest density.
Advantages and Disadvantages of Polypropylene
Polypropylene plastics provide good chemical resistance and retain their shape after torsion or bending. PP also has a high melting point and won’t degrade when exposed to moisture or water. Injected molded polypro, as this plastic is sometimes known, is also recyclable up to four times. In addition, recycled PP can be combined with other plastic materials.
Injection molded polypropylene degrades with UV light and is extremely flammable. At temperatures above 100° C (212° F), it dissolves into aromatic hydrocarbons such as benzene and toluene that are harmful to human health. Because it’s difficult to bond, PP is difficult to paint; however, pigments can be added to color the material.
PP Applications, Grades and Brands
There are a variety of consumer, commercial and industrial applications for injected molded polypropylene. Examples include children’s toys, storage containers, sporting goods, packaging, appliances, and power tool bodies.
PP plastics can be filled with glass fibers for improved dimensional stability, rigidity, strength, and resistance to warping. Grades include high crystalline polypropylene (HcPP), which has a higher stiffness than regular polypro. Semitron® from Mitsubishi Chemical Advanced Materials is a leading brand.
Polystyrene (PS) is a commodity plastic that comes in two basic types for plastic injection molding:
- General purpose polystyrene (GPPS)
- High impact polystyrene (HIPS)
GPPS is brittle and has less dimensional stability than HIPS, which is compounded with butadiene rubber to enhance its material properties. GPPS also has a glass-like clarity whereas HIPS is opaque. Both types of polystyrene are well-suited for injection molding because of their predictable and uniform shrinkage.
Advantages and Disadvantages of Polystyrene
PS plastics are inexpensive, lightweight, and resistant to moisture and bacterial growth. They also provide excellent resistance to gamma radiation, which can be used to sterilize medical devices. Injection molded polystyrene also provides good chemical resistance to diluted acids and bases.
Polystyrene is flammable, susceptible to UV degradation, and attacked by hydrocarbon solvents such as benzene and kerosene. Because it is brittle, GPPS is prone to cracking. Although injection molded polystyrene is recyclable, it is not biodegradable.
PS Applications, Grades, and Brands
Polystyrene is used in medical, optical, and electrical and electronic applications. With its higher impact strength, HIPS is often used with appliances and equipment. Injection molded GPPS is used in plastic toys, cases, containers, and trays.
Polystyrene can be filled with glass for added strength or co-polymerized with acrylic for greater clarity and improved chemical and UV stability. Brand names include American Styrenics PolyRenew® and BASF Polystyrol®.
Thermoplastic Elastomer (TPE)
Thermoplastic elastomer (TPE), or thermoplastic rubber (TPR), is a blend of rubber and plastic materials. Although TPE has the properties and performance of rubber, it is processed like plastic. The elasticity of TPR is part of what makes it attractive to designers and engineers. TPE is also recyclable and can be recycled six times. Moreover, other recycled materials can be compounded together to produce TPEs.
Advantages and Disadvantages of Thermoplastic Elastomer
Thermoplastic elastomers can be stretched to moderate elongations. Upon removal of this stress, they then return to nearly their original shape. TPEs also have shorter molding cycles, support two-shot molding options, and are easier and less expensive to mold than liquid silicone rubbers (LSRs).
Under sustained pressures, however, TPE is susceptible to creep, the tendency of a solid material to deform permanently. This material also tends to lose its rubbery properties at higher temperatures and is significantly more expensive than other plastic injection molding materials.
TPE Applications, Grades, and Brands
Injection molded TPEs are used in footwear, medical devices, auto parts, and pet products. Automotive applications include weather seals and shock dust boots. Medical applications include breathing tubes, valves, catheters, and ventilation masks.
ISO 18064 defines generic classes of commercial TPEs. There are many different manufacturers and brand names, including Teknor Apex Telcar®, Kraiburg HIPEX®, and Dynaflex™ from Avient (formerly PolyOne). Healthcare grade TPEs are available.
Thermoplastic Polyurethane (TPU)
Thermoplastic polyurethane (TPU) is a type of thermoplastic elastomer (TPE) that is often called out separately because of its popularity. Like other TPEs, TPU has a rubber-like elasticity; however, injection molded TPU has a higher durometer and can be used as a replacement for hard rubber. TPU is often chosen over TPE because TPU is better for extreme temperatures and chemical resistance. TPE, on the other hand, is softer and more flexible.
Advantages and Disadvantages of Thermoplastic Polyurethane
Compared to other TPEs, TPU provides greater resistance to abrasion, chemicals, greases, oils, and high temperatures. Injection molded thermoplastic polyurethane also has good load-bearing capabilities and is suitable for use in environments where the material is exposed to ozone.
TPUs may be too hard for some injection molded parts or lack the required flexibility. Thermoplastic polyurethanes also tend to cost more and drying is required before processing. Because some grades have a short shelf life, injection molders may have to dispose of them before they can be used.
TPU Applications, Grades and Brands
Injection molded TPUs are used in footwear, gaskets, caster wheels, and sporting goods. Applications also include cases or enclosures for electronics and medical devices. In medical applications, TPUs are sometimes used instead of polyvinyl chloride (PVC), a plastic that may cause skin irritation.
Thermoplastic polyurethanes come in commercial, medical, and industrial grades. There are three main classes of TPU materials: polyester, polyether and polycaprolactone. Trademarked products are available from manufacturers such as Lanxess, Lubrizol, Texin, and Ultralast.
Blended Plastic Injection Molding Materials
Although they’re the most common types, the materials listed above aren’t the only choices that you have. Depending on your specific requirements, you can also choose blended plastics that combine the best properties of two different polymers. Various blends are available, but the most common types are PC/ABS, PC/PBT, and PPE/PS.
PC/ABS is a blend of polycarbonate (PC) plastic and acrylonitrile butadiene styrene (ABS). It is strong, stiff, heat resistant, and has low shrinkage for fewer injection molding defects. This blend is used in the automotive industry for glove boxes and the electronics industry for laptop monitor enclosures.
PC/PBT is a blend of PC plastic and polybutylene terephthalate (PBT), an engineering polymer that is used for insulation in the electrical and electronics industries. PC/PBT offers a good combination of chemical resistance, temperature resistance, and mechanical strength. Its properties can be further enhanced by additives that provide flame or UV resistance.
PPE/PS is a blend of polyphenylene ether (PPE) and polystyrene (PS). It provides excellent dimensional stability, low mold shrinkage, low water absorption, and outstanding stability in both hot and cold water. Injection molded PPE/PS is used in fluid engineering, environmental engineering, and potable water applications.
Specialty Plastic Injection Molding Materials
In addition to blended materials, plastic injection molding also uses various specialty materials. The most common types are PEI, PBT, PEEK, LCP, and PVC.
Polyetherimide (PEI) is a heat-resistant thermoplastic that combines stiffness and stability with low flammability and low smoke production. PEI’s environmental resistance and high impact strength make it a good choice for applications such as medical components and parts for automotive transmissions and lighting systems.
With its low melt viscosity, polybutylene terephthalate (PBT) is an excellent material for molding parts with thin cross sections. PBT also has excellent resistance to creep, a property that describes the tendency of a material to slowly deform over a long period of exposure to high levels of stress. Applications for PBT include keyboards, switches, and distribution boxes.
Polyether ether ketone (PEEK) is an engineering plastic that is often used in medical applications because it resists gamma radiation and autoclaving, both of which are used for sterilization. PEEK has excellent mechanical properties, resists chemicals and thermal degradation, and withstands long-term liquid submersion.
Liquid crystal polymers (LCPs) are used for injection molding thin-walled parts and very small components. LCPs also resist gamma radiation, steam autoclaving, and most chemical-sterilization methods. In addition to medical devices, these specialty plastics are used for injection molded sockets for integrated circuits (ICs).
Polyvinyl chloride (PVC) is a lightweight, recyclable, and relatively inexpensive plastic that combines environmental resistance with excellent tensile strength. This strong, hard specialty plastic is used in automotive parts, plumbing, windows, and wire spools. PVC also offers excellent dimensional stability and good color retention.
More Plastic Injection Molding Materials
Because there are so many options, it helps to have an expert’s advice before choosing the material for your next injection molding project. Fictiv has many such experts who can help you make the right choice, so contact us today and get started.
And, to get more useful engineering articles like this one every month, subscribe to our newsletter below!