Just like you wouldn’t go into a machining project saying that the material you’re using is “metal,” it’s unlikely that you’d make it far into an injection molding product design process with your material choice identified merely as “plastic.” There is a broad range of plastic material for injection molding available. Each resin has different qualities, strengths and weaknesses. That’s not to say that one material is “better” or “worse” than another, but certain resins are definitely better suited for certain jobs, applications, environments and situations than others. This article is designed to provide a more in-depth look at some of the most common plastic materials for injection molding, the qualities they offer, and their typical applications.
ABS
What is it? ABS (acrylonitrile butadiene styrene) is one of the most common injection molding materials available. It is a thermoplastic material that can be sourced and molded relatively easily, at an accessible price point.
What is it used for? ABS is often talked about as a “general purpose” injection molding material, but it does offer several specific qualities that lend themselves to targeted applications. ABS is a very strong, sturdy material that offers good impact resistance, and can also be easily machined, sanded or colored to meet aesthetic requirements. For this reason, it’s often found in consumer electronic products such as computer keyboards and laptop housings. It’s also often found in automotive components, thanks to its sturdiness, including plastic bumpers and interior components such as dashboard housings and center consoles.
What isn’t it good for? As a thermoplastic, ABS will deform, warp or melt at extremely high temperatures, so it isn’t ideal for environments where it will be subjected to sustained intense heat (above around 200 degrees Fahrenheit). ABS is also not usually the right choice for applications where flexibility or elasticity are required, and where highly abrasive solvents are present.
Polycarbonate
What is it? Polycarbonates are a class of high-grade thermoplastics typically sold under a number of brand names. Usually seen as slightly higher-end than ABS, polycarbonate remains extremely easily acquired and molded, and offers increased shatter resistance. 
What is it used for? As one of the strongest plastic injection molding materials available at a still-accessible price point, polycarbonate’s strength is one of its two defining qualities. The other is its near-complete natural transparency. These two properties combine to make polycarbonate an ideal material for high-strength glasslike applications such as safety goggles (further bolstered by polycarbonate’s natural UV resistance); bulletproof glass; and medical or lab applications including beakers, test tubes and pipettes.
What isn’t it good for? Like ABS, polycarbonate isn’t especially well suited for applications where higher material flexibility is desired, or where heavy solvent exposure is expected. Aesthetic choices for polycarbonate are also more limited, and it isn’t highly receptive to in-process dye or coloring. Special types of polycarbonates, called BPA-free polycarbonates, must also be used for food-grade applications, due to concerns about the integrity of the material when it comes into contact with water.
Glass-Filled Nylon
What is it? Glass-filled nylon is a reinforced thermoplastic, where the base nylon resin has glass fibers added to it for extra tensile strength. Glass-filled nylons will typically state the percentage of glass added (which won’t usually exceed 40 percent) in the name, such as “10 percent GF nylon.”
What is it used for? Glass-filled nylon offers even higher strength and heat resistance than ABS and polycarbonate. It is also much more nonconductive to electricity than other resins, and so is often found in electrical insulation or electrical components where insulation is a requirement. With an extremely high melting point (about 420 degrees Fahrenheit), glass-filled nylon is also well-suited to kitchen appliances and applications. Glass-filled nylon is receptive to dyes and colorants during the molding process.
What isn’t it good for? In spite of its high melting point, glass-filled nylon is more susceptible to burning than some other materials, especially if exposed directly to a flame. Glass-filled nylon’s extremely high tensile strength also goes hand in hand with a very low flexibility, meaning it is somewhat more break- or shatter-prone in high-impact scenarios. You should also be sure not to use glass-filled nylon if its high-strength properties aren’t required. You’ll simply be making a bigger investment into material properties that essentially will go unused.
Polypropylene
What is it? Polypropylene is a highly flexible thermoplastic that’s equally suitable for industrial and consumer applications. The term “polypropylene” refers to a family of compounds that can vary in quality and cost, though the core properties of the material remain similar. 
What is it used for? Polypropylene is well-known for being flexible and able to be “deformed” (i.e., twisted, bent or otherwise manipulated) without breaking, always retaining its original shape. For this reason, it is most commonly found as a “living hinge” material. Living hinges are the basis of the types of bottle caps often found in consumer products that easily “snap” open and closed — like the top of a ketchup bottle, or the lid of a re-usable water bottle. Polypropylene also offers exceptional resistance to solvents, so it’s ideal for containers (and bottle caps) for materials such as cleaners and other chemical formulations.
What isn’t it good for? Polypropylene would not be the material of choice for applications where a high degree of rigidity is necessary. It also offers subpar UV resistance, is difficult to “bond” (meaning, as a finished product, it isn’t easily glued or painted), and should not be used in high-temperature applications.
Acetal
What is it? Acetal is a thermoplastic sold under many brand names, found in a broad range of applications from consumer products to industrial components.
What is it used for? Acetal is well-known as a low-friction compound, making it highly suitable for applications such as gears and ball bearings. As a highly rigid material, Acetal is also ideal for protective housings and casings for consumer products such as electronics.
What isn’t it good for? Acetal offers subpar heat resistance and isn’t suitable for applications where it will be exposed to sustained high heat. While it can be easily colored or dyed in the molding process, it’s extreme friction-resistance means that, like polypropylene, it cannot easily be painted after production.
