Injection molding materials: how to choose a resin, and what to write on the drawing
The resin decides how the part survives, how the mold is cut, and what the factory can quietly substitute. Settle it on the drawing before anyone cuts steel.
Filed under Engineering and services
Short answer: Injection molding materials are mostly thermoplastics, sold as pellets by resin makers in thousands of grades. Pick the family from what the part has to survive (heat, chemicals, impact, sunlight, food contact, flame), then pick the exact grade. Write that grade, the color and any fillers on the drawing. The mold is cut for one material's shrinkage, so swapping resins later is rarely free.
Choosing injection molding materials is the decision buyers most often leave to the factory, and it is the one that comes back later as warped parts, cracked clips or a failed certification. This note covers how the common resin families differ, what fillers and color change, and what to put in writing so lot ten is made from the same material as the approved sample. For the molding work we take on, see our injection molding page.
What the resin has to do
Start from the part's job, not from a materials list. Before anyone quotes, write down:
- Temperature. The highest and lowest the part will see in use, in shipping and in storage.
- Chemicals. Cleaners, oils, fuels, sunscreen, or whatever else touches it.
- Load and impact. Snap fits, screws, drops, and whether it carries a load for years.
- Sunlight. Outdoor parts need UV stabilization or a resin that tolerates it.
- Rules. Food contact, a flammability rating, or a customer's material specification.
- Look and feel. Clear, glossy, textured, soft-touch, or a matched color.
A supplier's materials guide puts the molding side this way: "the compatibility of a polymer with the injection molding process is defined by its melt flow index, thermal shrinkage, and cooling behavior". In other words, the resin that suits the use also has to suit the process, and the toolmaker needs to know which one it is before cutting steel.
The common families, briefly
Most consumer and industrial parts come from a short list. The notes below draw on RapidDirect's materials guide, which also gives typical shrinkage ranges for each.
- ABS. Tough, rigid and easy to mold, common for enclosures. The guide flags one limit: "Not UV-stable (indoor use primarily)".
- Polycarbonate (PC) and PC/ABS. Higher impact strength and heat resistance, and PC can be clear. Outdoor use needs the right grade. Wikipedia notes that "Standard polycarbonate resins are not suitable for long term exposure to UV radiation." UV-stabilized grades exist for that reason.
- Polypropylene (PP). Cheap, chemical resistant, and the usual choice for living hinges. The trade-off is shrinkage: "its high thermal shrinkage requires perfectly uniform mold cooling to prevent part warpage".
- Polyethylene (HDPE, LDPE). Very chemical resistant and impact tough, with high shrinkage.
- Nylon (PA6, PA66). Strong, tough and wear resistant, often glass filled for stiffness.
- Acetal (POM). Stiff, low friction and dimensionally stable, used for gears, clips and moving parts.
- TPE and TPU. Soft, rubbery materials for grips, seals and overmolds. Our note on two-shot molding vs overmolding covers bonding a soft layer to a rigid part.
If injection molding is not settled as the process, our note on plastic manufacturing processes compares it with extrusion, blow molding and the rest.
Shrinkage: why the mold is cut for one material
Plastic shrinks as it cools, so the cavity is cut larger than the finished part. Wikipedia's article on the process says "The exact amount of shrinkage is a function of the resin being used, and can be relatively predictable." The catch is in "the resin being used". The RapidDirect table lists ABS at 0.4% to 0.8% and polypropylene at 1.5% to 2.5%, and adds: "Exact shrinkage rates and optimal melt temperatures will fluctuate based on specific manufacturer blends, fiber reinforcements like GF30, and mold cooling architectures."
So a mold cut for one grade will not necessarily make good parts in another. Switching from ABS to PP late in a program can mean recutting steel, not just changing bags at the press. The same guide warns that "Relying on basic material definitions without understanding shrinkage rates, abrasive wear, and thermal flow dynamics leads to severe mold mismatch and catastrophic part warpage." Our note on precision injection molding covers how shrinkage feeds into tight tolerances.
Fillers, glass and color
Glass fiber makes nylon and other resins stiffer and stronger, and it changes the mold. The RapidDirect guide states: "Glass-filled grades (e.g., GF30) are highly abrasive and require hardened H13 steel molds to prevent rapid gate erosion." Ask whether your quote assumes a hardened tool when the resin is filled. Our note on what an injection mold really costs explains why steel choice moves the price.
Color is usually added at the press as masterbatch, a concentrated pellet mixed into natural resin. Wikipedia notes that "Several masterbatches (colors and various additives) can be used together." It also notes a practical failure: "Granule processing is sometimes prone to causing adverse effects, such as the separation of the masterbatch and the base material in the hopper." If color matters, give a color standard (a physical chip or a reference such as a Pantone number) and say whether masterbatch or precolored resin is acceptable.
Regrind is runner and scrap ground up and fed back in. Say on the drawing whether it is allowed and at what percentage, especially for parts that carry load.
Drying: the step that hides inside "process"
Some resins pick up water from the air. Wikipedia lists polycarbonate's "tendency to absorb moisture in humid environments", and its process article names the result when the step is skipped, under splay marks: "Moisture in the material, usually when hygroscopic resins are dried improperly." Wet resin can also weaken a part in ways you cannot see. Ask the factory how it dries each resin and how it checks that drying was done. Our note on injection molding defects covers how to tell a material problem from a tool problem.
Ratings and food contact
Flammability. Electronics housings often need a UL 94 rating. V-0, the strictest of the three V ratings, means "burning stops within 10 seconds on a vertical specimen". The rating belongs to a material at a given thickness: tests use a "specimen of the minimum approved thickness". A part molded thinner than the rated thickness is not covered by that rating, so check the wall against the resin's listing.
Food contact. In the US, FDA regulations set out which polymers may be used in contact with food and under what conditions. For olefins such as polypropylene and polyethylene, 21 CFR 177.1520 says the listed polymers "may be safely used as articles or components of articles intended for use in contact with food", subject to that section's conditions. A resin that meets the rule in natural form still has to be checked with its colorant and additives, so ask the resin supplier for its compliance letter for the exact grade.
Automotive parts carry their own material specifications and approvals; see our note on automotive injection molding.
What to write on the drawing
Most material problems start with a vague drawing. "ABS, black" leaves the factory free to buy whatever ABS is cheapest that month. A tighter callout covers:
- Resin maker and grade, for example a named brand and grade number, not just the family.
- Equivalents. Whether "or equivalent" is allowed, and who approves an equivalent in writing before it is used.
- Fillers and additives. Glass content, UV stabilizer, flame retardant.
- Color. The color standard and the method (masterbatch or precolored).
- Regrind. Allowed or not, and the maximum percentage.
- Paperwork. A certificate of analysis or compliance for each resin lot, kept with the production records.
Then hold the approved sample to that callout. A first article inspection checks dimensions against the drawing, and a golden sample gives later lots something to be compared with. Neither helps if the resin quietly changed between them, which is why the change-approval line in item 2 matters.
Where Jack fits
JCH works as a US point of contact for tooling and molding, built at partner factories. Partner shops are not interchangeable, so the first question is which one suits your part and your resin. JCH itself holds no certification. Where a buyer requires ISO, UL or ISTA, Jack matches the program to partner facilities and accredited labs that hold them. You can see how a project moves from drawing to production on our molding steps page.
If you have a part in mind, send Jack your drawing, the resin you have in mind (or the conditions the part must survive) and the quantity you expect. He will tell you what the callout is missing before a factory quotes it.
Straight answers
What materials are used in injection molding?
Mostly thermoplastics, supplied as pellets. Common choices are ABS, polycarbonate and PC/ABS, polypropylene, polyethylene, nylon, acetal (POM), and soft TPE or TPU for grips and seals. Each family comes in many grades with different fillers, additives and ratings.
Can I change the resin after the mold is built?
Sometimes, but it is rarely free. The cavity is cut larger than the part to allow for one material's shrinkage, and shrinkage differs between resins and even between grades. A switch can mean adjusting or recutting steel, new samples and a new approval.
Why does glass-filled plastic affect the mold?
Glass fibers make the part stiffer and stronger, but they are abrasive. Suppliers commonly recommend hardened steel for glass-filled grades so gates and cavities do not wear quickly, which affects the tooling quote.
Does a UL 94 V-0 resin make my part V-0?
Not automatically. UL 94 ratings are tied to the material at a tested thickness. If your wall is thinner than the thickness in the resin's listing, the rating does not cover it, so check the wall against the listing.
What should a drawing say about the material?
The resin maker and grade, whether equivalents are allowed and who approves them, fillers and additives, the color standard and method, whether regrind is allowed and how much, and the resin certificates you expect with each lot.
How this note was made: researched and drafted with AI assistance, checked against the sources listed below and edited at JCH Design & Manufacturing.