Why this matters
ABS, PC, PP, PA66, POM and TPU are the resins most injection molding quotes name, and each wins on a different property. A side-by-side view of shrinkage, strength, finish and molding risk shows which resin to put in the RFQ and what to check on the datasheet.
Start from the function, not the resin name
Every resin trades something for something else. ABS gives an easy-to-finish housing, PC gives impact strength and clarity, PP gives chemical resistance and living hinges at low cost, PA66 and POM give strength and wear resistance, and TPU gives flexibility. Naming one in the RFQ without saying what the part has to do forces the molder to guess, so list the load, temperature, appearance and chemical exposure first and let the resin follow from them. The table below is a starting point for that conversation, not a substitute for the datasheet of the exact grade.
Shrinkage decides cavity size and fit
Shrinkage is the amount a part contracts as it cools, and it differs a great deal between resins. Amorphous resins such as ABS and PC usually shrink less, in the range of roughly half a percent, while semi-crystalline resins such as PP, PA66 and POM shrink more, often one to two percent or above, and glass fiber lowers it and makes it directional. A resin with high shrinkage needs a larger cavity, is more likely to warp on flat faces and is harder to hold to a tight tolerance, so the ranges in the table should be confirmed for the exact grade and the wall thickness.
Moisture and drying change the molding risk
PC, PA66, TPU and PMMA absorb moisture, and molding them wet causes splay marks, brittle parts or lower strength. They need to be dried before molding and sometimes conditioned afterwards, since PA66 also takes up moisture in use and changes size. PP and POM absorb very little. Ask the molder how each resin is dried and how the moisture is controlled during a run, because this is a process point that does not show up on a drawing.
Finish and appearance favor different resins
ABS takes paint, plating and texture well and is the default for visible housings. PC and PMMA can be transparent but scratch or stress-crack more easily, and PMMA in particular is hard and brittle. PP is difficult to paint without treatment and tends to show flow marks and sink on thick sections, while glass-filled PA66 can show fiber on the surface. If the appearance matters, state the finish, the cosmetic faces and the inspection method in the RFQ.
Heat, chemicals and wear narrow the choice quickly
For a part that sees heat, solvents or sliding contact, the list shortens quickly. PA66 and POM handle wear and moderate heat, PC handles higher heat than ABS, and PP resists many chemicals but softens early. Check the temperature the part will really see and the chemicals it will meet, including cleaning agents, since some blends stress-crack under common solvents. Where the product needs a flame rating or a food or medical contact grade, name it at the quote stage because it limits the grades available.
Ask for the grade, not only the family
A resin family contains many grades with different flow, strength, color options and price. The RFQ should name the grade, or an approved equivalent, and ask the molder to confirm the shrinkage value they will use for the cavity. A quote built on a general resin can look cheaper than one built on the grade the part actually needs, which is why the resin line should be matched before comparing prices.
Resin comparison at a glance
| Resin | What it is good at and typical shrinkage | Typical parts and what to watch |
|---|---|---|
| ABS | Moderate strength, about 0.4-0.7% shrinkage, easy to paint and texture | Housings, covers and consumer parts; weak against some solvents and sunlight unless stabilized |
| PC (polycarbonate) | High impact strength, about 0.5-0.7% shrinkage, can be clear, needs drying | Covers, lenses and rugged housings; prone to stress cracking near sharp corners and solvents |
| PP (polypropylene) | Chemical resistant and low cost, about 1-2.5% shrinkage, flexible hinges | Caps, closures and packaging; warps on large flat faces and is hard to paint |
| PA66 (nylon 66) | Strong, tough and wear resistant, about 1-2% shrinkage unfilled and lower when glass filled, absorbs moisture | Brackets, clips and gears; size and strength change with moisture, so condition and check samples |
| POM (acetal) | Stiff and low friction, about 1.8-2.5% shrinkage, very little moisture uptake | Gears, latches and sliding parts; gate location and knit lines affect strength |
| TPU / TPE | Flexible, shrinkage varies with hardness, bonds to some substrates | Grips, seals and overmolded areas; specify Shore hardness and confirm bonding on real samples |