July 19, 2022

Common Plastics for Injection Molding

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) similar to polystyrene (PS). Choosing the right thermoplastic material for the injection molding also means knowing your application requirements, which can change between prototyping and production.

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 thermoplastic polymer materials:

acrylic (PMMA)

acrylonitrile butadiene styrene (ABS)

nylon polyamide (PA)

polycarbonate (PC)

polyethylene (PE)

polyoxymethylene (POM)

polypropylene (PP)

polystyrene (PS)

thermoplastic elastomer (TPE)

thermoplastic polyurethane (TPU)

Acrylic (PMMA)

Poly(methyl methacrylate) is better known as PMMA or acrylic. It’s a strong, clear thermoplastic 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

Transparency – among the clearest plastics available today
Weather Resistance – resists moisture and ultraviolet discoloration
Chemical Resistance – unaffected by water and weak acids and alkaline solutions
Surface Hardness – good impact strength and rigidity
Dimensional Stability – lends to a predictable final product

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. It takes dyes and colorants very well, so you can produce many aesthetic effects. On top of its optical and transparent properties, acrylic is odorless and tasteless and doesn’t contain Bisphenol A (BPA). BPA is a harmful organic compound, so plastic injection molding resins like acrylic are considered safe for food storage.

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, making mold 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

Impact Resistance – rubber-like formulations make ABS resilient to impact
Surface hardness – high rigidity in certain formulations
Uniform Shrinkage – good dimensional stability
Machinability – easy to form after cooling

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, keyboard keys, protective headgear, plastic wall plates for electrical outlets, and automotive features such as dashboards, wheel covers, and auto body parts. Additional applications include consumer goods, sports 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 is available in different formulations. ABS can also be combined with glass fillers for added strength or polycarbonate for low-temperature impact resistance.

Nylon Polyamide (PA)

Polyamides (PA) are a large group of materials that can be either natural or synthetic. Nylon is one of the most versatile materials used in plastic injection molding.

Advantages and Disadvantages of Nylon

Low Friction – doesn’t heat up under high Friction
Chemical Resistance – resistant to corrosion from chemicals
Abrasion Resistance – useful for elastic and clothing applications

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. Although it’s a strong plastic, it tends to absorb water. It’s not the ideal choice for marine applications.

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, 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)

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

Impact Resistance – doesn’t scratch or dent under impact
UV Resistance – doesn’t corrode or discolor in ultraviolet light
Transparency – can be made into windows and eyewear lenses
Chemical Resistance – doesn’t corrode from chemicals
Rigidity – makes for good eyewear and window applications

Polycarbonate is extremely durable and 250 times stronger than glass. This injection molding material offers precise dimensional control with its predictable and uniform mold shrinkage. 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. The mold tool must be highly polished if you’re making optically clear plastic parts.

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 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 (PE)

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 share useful properties such as chemical resistance, but they differ in their respective hardness, flexibility, melting point, and optical transparency.

Advantages of HDPE

Sustainable – made of 100% recyclable material
Chemical Resistance – resistant to corrosion, rust, and other chemicals
Heat Resistance – doesn’t melt or corrode in high temperatures
Food Grade – safe to use as food packaging and equipment
Inexpensive – can be mass-produced at a low price

HDPE, LDPE, and PET are all low-cost plastics that resist 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

Examples of injection molded PE include toolboxes, electronic wire and cable insulators, and medical device components. PE is also the most common injection molding resin for making toys because it’s non-toxic and can take a beating without complaint.

BASF and DuPont are two of the leading suppliers of PE for engineering rather than general-purpose applications.

Polyoxymethylene (POM)

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

Rigidity – stays strong and doesn’t bend easily
Impact Resistance – strong and stable enough to resist strong impacts
Solvent Resistance – doesn’t corrode or rust due to chemicals and solvents
Glossy surface – smooth, reflective properties

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)

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

Chemical Resistance – doesn’t react with most bases and acids
Fatigue Resistance – retains its shape after a lot of bending and flexing
Electrical Resistance – strong insulation properties
Elasticity – can bend, flex and deform without actually breaking

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. It is also recyclable up to four times. In addition, recycled PP can be combined with other plastic materials. This versatile material is sometimes referred to as the “steel” of plastics since PP can be tailored to so many different uses.

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

Examples include children’s toys, storage containers, sporting goods, packaging, appliances, and power tool bodies. PP is safe for use as food containers because it does not leach chemicals into food products. It can be commonly found in household goods such as utensils, athletic apparel, area rugs, and automotive parts such as car batteries.

PP plastics can be filled with glass fibers for improved dimensional stability, rigidity, strength, and resistance to warping.

Polystyrene (PS)

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. HIPS can be printed, glued, bonded, and decorated easily.

Advantages and Disadvantages of Polystyrene

Optical Clarity – transparent resin can be useful for see-through plastic products
FDA Grade – safe to use when packaging food and consumable materials
Stability – rigid material useful for packaging
Inexpensive – can be mass-produced at a low cost

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, 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.

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.

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. Thermoplastic Polyurethane (TPU) is soft and elastic, with great tensile and tear strength. That’s why it’s often used to make parts that demand a rubber – like elasticity.

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.

Trademarked products are available from manufacturers such as Lanxess, Lubrizol, Texin, and Ultralast.

Specialty Plastic Injection Molding Materials

PEI

Polyetherimide (PEI) is a heat-resistant and electrical insulating 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.

PEEK

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, electrical insulation and withstands long-term liquid submersion.

Resources:

https://www.fictiv.com/articles/the-ten-most-common-plastic-injection-molding-materials

https://www.retlawindustries.com/PlasticTypes

https://www.rodongroup.com/blog/top-5-most-common-plastic-resins