What types of plastics are commonly used in injection molding?

The short answer

Three tiers, not one long list

A supplier will happily list twenty resins, which does not help a buyer choose. It is more useful to sort them by what they cost and what they tolerate, because price and capability rise together.

• Commodity resins are produced in the largest volumes and priced lowest: polypropylene, polyethylene, polystyrene and ABS. They are strong enough for the great majority of enclosures, containers, caps and housings, and they are forgiving to mould.
• Engineering resins cost two to four times more and are chosen for a specific property that a commodity grade cannot deliver: the impact and clarity of polycarbonate, the wear and dimensional precision of acetal, the strength and fatigue resistance of nylon, or the electrical and thermal stability of PBT.
• High-performance resins such as PPS, PEEK, PEI and PPSU sit above that, at multiples of engineering prices, and are justified only where continuous temperature, chemical exposure or a regulatory requirement rules everything else out.

The commodity workhorses

• Polypropylene (PP). Density 0.90 g/cm3, tensile 32 to 40 MPa, heat deflection 95 to 105 C and shrinkage 1.5 to 2.5 percent. It has the best chemical resistance of any low-cost resin and survives millions of flex cycles, which is why living hinges and fluid containers are almost always PP. The price is high and anisotropic shrinkage, so flat PP parts need generous radii and careful gating to stay straight.
• High-density polyethylene (PE-HD). Tensile 26 to 33 MPa, very low moisture sensitivity and excellent chemical resistance. It is the material for bottles, caps, closures and chemical tanks. It is soft and low-stiffness, and it releases dimensionally poorly, so it is rarely used for precision parts.
• Polystyrene (PS). Cheap, rigid and optically clear in its general-purpose form, and easily foamed. It is brittle, has poor chemical resistance and burns readily, which limits it to disposables, laboratory ware and packaging.
• ABS. Tensile 40 to 45 MPa, deflection 88 to 100 C, shrinkage 0.4 to 0.7 percent. It is the cosmetic default because it moulds with a clean, gluable, paintable, plateable surface, and it takes impact well at room temperature. Its weaknesses are UV, heat above roughly 90 C and solvents.
• PMMA (acrylic). Tensile 60 to 70 MPa with glass-like clarity and outstanding weathering, but brittle and easily scratched. Lenses, light guides, displays and protective covers are its territory.

The engineering resins

These are specified when one property genuinely has to be better than a commodity grade can deliver, and the price difference is the cost of that single requirement.

• Polycarbonate (PC). Tensile 60 to 70 MPa, notched impact 600 to 800 J/m and deflection 125 to 140 C. Optical clarity around 88 percent light transmission and impact toughness that no commodity resin approaches make it the material for safety equipment, lenses and transparent guards. It is notch sensitive, susceptible to solvent crazing and needs higher melt temperatures than ABS.
• Nylon - PA6 and PA66. Tensile 60 to 80 MPa unfilled, rising to 150 to 180 MPa with 30 percent glass fibre, with excellent wear and fatigue behaviour. Nylons absorb moisture, which changes dimensions in service, so they are specified for gears, bearings and brackets where toughness matters more than dimensional stability.
• Acetal (POM). Tensile 60 to 75 MPa, low friction, high stiffness and very low moisture pickup, which makes it the easiest resin for tight-tolerance moving parts. It is the grade to specify when a plastic part has to do the job of a metal one. Watch for notch sensitivity, centreline porosity in sections over about 6 mm, and formaldehyde release that requires corrosion-resistant tool steel.
• PBT and PET. Dimensionally stable and electrically robust, with low moisture absorption. Automotive connectors, sensor bodies and electrical housings are the classic applications.

High-performance resins and soft-touch grades

At the top of the range sit PPS, PEI, PPSU and PEEK, which hold properties above 150 C continuously and resist fuels, solvents and sterilisation. PEEK costs well over USD 50 per kilogram, requires barrel temperatures of 350 to 400 C and abrasive glass or carbon fillers in many grades, so it is only justified where a regulatory or performance requirement leaves no alternative - typically aerospace brackets, semiconductor parts and medical implants. At the other end of the hardness scale, TPE and TPU are the soft-touch resins used for grips, seals and overmoulded surfaces, chosen for adhesion to a rigid substrate as much as for their own properties.

How to choose: four questions in order

1. What temperature does the part see? Service temperature eliminates more resins than any other single question. Continuous exposure above about 100 C rules out standard ABS, PP and PE; above 150 C only the high-performance tier remains.
2. What is it exposed to? Oils, fuels and solvents drive the choice toward semi-crystalline resins such as PP, PA and POM. Direct sunlight rules out unmodified ABS and PC unless they are UV stabilised or painted.
3. What does it have to do mechanically? Sliding and precision motion points to POM or PA. Drop and impact protection points to PC or a PC/ABS blend. Repeated flexing points to PP. High static load without heat points to a glass-filled grade.
4. Only then look at price and process. Compare resin cost against cycle time, yield and secondary operations together rather than on unit price alone, because a cheaper resin that needs a longer cycle or extra finishing is not cheaper.

Where the choice goes wrong

• Writing the resin family instead of the grade. One family contains dozens of grades with different flow, heat resistance, flame rating and glass content. Naming ABS or PC is only the first step, and it is the step that most often produces a quote for the wrong part.
• Changing resin after the tool is cut. Shrinkage alone changes every cavity dimension. A part dimensioned for ABS at about 0.5 percent shrinkage will be undersized if it is switched to PP at about 1.8 percent without reworking the cavity, and in practice that can mean a replacement cavity.
• Assuming a flame rating is included. General-purpose ABS and PP are flammable. UL 94 V-0 needs an FR grade, which costs more and often moulds differently, so it has to be in the original specification.
• Forgetting moisture. PA, PC, ABS and PBT are hygroscopic. Moulding them wet produces splay, silver streaks and reduced strength, so drying is part of the quoted cycle whether or not it appears on the drawing.
• Over-specifying on a cosmetic part. Putting a high-performance resin on a visible cover that never sees heat or chemicals buys nothing and costs real money, and often processes less attractively than ABS or a PC/ABS blend.

From a resin family to a real grade

The fastest route to a correct quotation is to describe the part's duty rather than naming a resin. Tell us the continuous and peak service temperature, the chemicals or UV exposure it will see, the load and whether it is static, sliding or impact, the wall thickness, whether it must be transparent or plated, and any food-contact, flame or medical requirement. With that we can shortlist two or three grades, flag the shrinkage, drying and tool-steel implications of each, and price the part so the resin choice is visible in the total cost rather than only in the material line. See plastic injection molding for the process, surface finishing for paints, textures and plating, and aluminium die casting where the part should be metal.