Plastic Injection Molding Materials: How to Pick the Right Resin

Resin choice decides more about a molded part than almost any other decision. It sets how strong and stiff the part is, how hot it can run, which chemicals it tolerates, how it shrinks in the tool, and what each part costs. The wrong resin can pass a prototype test and fail in the field a year later. The right one often makes the tool easier to build too.

This guide covers the plastic injection molding materials we see specified most often, where each one fits, and the questions that narrow the choice.

Start with how the part will be used

Before comparing resins, write down the conditions the part has to survive:

• Load: static, impact, repeated flexing, or sliding wear
• Temperature: the highest continuous temperature and any short spikes
• Chemicals: oils, fuels, cleaners, disinfectants, and solvents it will touch
• Environment: outdoor UV, humidity, or immersion in water
• Appearance: color, texture, gloss, clarity, paint or plating
• Regulatory: flame rating, food contact, or medical requirements
• Volume and budget: commodity resins cost far less per pound than engineering resins

With that list in hand, most applications narrow to two or three candidates quickly.

ABS

ABS is the default for housings and enclosures. It’s tough, low-shrink, and dimensionally stable, it molds a good cosmetic surface, and it paints and plates well. It’s also forgiving of imperfect design, which makes it a common first choice for new products.

Watch for: heat resistance lower than engineering resins like polycarbonate, yellowing and embrittlement outdoors (ASA is the weatherable alternative), knit lines, and sink in thick sections. Some solvents attack ABS, so check every cleaner the part will see.

Polycarbonate (PC) and PC/ABS

Polycarbonate is strong, highly impact resistant, and available in clear grades. Unfilled PC typically has a heat deflection temperature of about 130 to 140°C, well above ABS. It holds tight dimensions and takes high-cosmetic finishes.

Watch for: poor chemical resistance and environmental stress cracking, especially around molded-in stress and with some cleaners and lubricants. PC is less forgiving of thick sections than ABS. A PC/ABS blend splits the difference, with better heat and impact than ABS and easier molding than straight PC, at a cost below PC.

Nylon (PA6, PA66, and glass-filled grades)

Nylon is strong, tough, and abrasion resistant, with good chemical resistance except to strong acids and bases. Glass-filled nylon adds stiffness and high-temperature strength, which is why it shows up in automotive under-hood parts, brackets, and structural housings.

Watch for: moisture. Nylon is hygroscopic and can absorb up to about 8% water by weight at saturation, settling near 2% at 50% relative humidity. That moisture grows the part by roughly 0.5 to 0.6% and drops stiffness sharply: nylon 6/6’s flexural modulus falls from about 410,000 psi dry to about 175,000 psi at 50% RH. Resin must be dried before molding, and dimensions should be specified for the conditioned state the part lives in. Glass-filled grades absorb less moisture but can warp from uneven shrink.

Acetal (POM)

Acetal is stiff, hard, and slippery, with low creep and excellent fatigue resistance. It’s a common pick for gears, cams, bushings, rollers, clips, and snap fits. Acetal absorbs far less moisture than nylon, about 0.2% at 50% relative humidity, so it holds dimensions better in humid environments. It resists fuels, hydrocarbons, and organic solvents.

Watch for: high shrink that demands uniform wall thickness, poor paint and bond adhesion, and limited high-cosmetic options. Nylon generally handles higher continuous temperatures and abrasive wear better.

Polypropylene (PP)

Polypropylene is low cost, light, and resistant to acids, bases, and many solvents. It flexes without cracking, which makes it the standard material for living hinges. Our living hinge design guide covers that use in detail.

Watch for: high shrink and warp, sink in thick sections, and low surface energy that makes painting and bonding difficult. Homopolymer PP can turn brittle in the cold, so copolymer grades are preferred for impact at low temperature.

Quick comparison

What to put on the drawing

“Nylon” or “ABS” alone leaves too many variables. Specify the resin family and grade (or “grade X or approved equal”), glass or mineral fill percentage, color, flame rating if required, UV stabilization for outdoor parts, and the conditioned state for dimensions on hygroscopic resins. Require written approval for any resin substitution.

Material and design work together. Wall thickness, draft, and gate location change by resin, and our injection molding DFM guide covers the design side.

How PE helps with resin selection

PE’s engineering team reviews resin and design together before a tool is quoted, and our injection molding network runs commodity and engineering resins for industrial, automotive, and commercial product programs. When a part starts in a machined prototype and moves to molding, the same project manager carries it through, and every part is inspected in San Clemente before it ships.