Injection Molding Material Selection: 14 Plastics Compared

1. Four questions that eliminate most materials

Buyers usually start a material conversation with a name they have seen before — ABS, nylon, polycarbonate — and then argue about price. The faster route is to answer four questions first, because they cut the candidate list from forty grades to three before anyone opens a datasheet.

Question one: what does the part actually have to survive? A 60 °C car interior, a −20 °C freezer, a chlorinated swimming pool, a steam autoclave at 134 °C, a UV-exposed rooftop. This single answer removes more materials than any other. PP survives pool chemicals and fails in sunlight without stabiliser. PC survives impact but stress-crazes on contact with many solvents. POM survives fuel and fails in hot water.

Question two: what is the wall thickness? Below 0.8 mm, flow length becomes the limiting factor and you are choosing between PC, ABS, PA and POM — not PP. Above 4 mm, sink marks and internal voids take over and you need a material with low and predictable shrinkage, or a design change.

Question three: how many parts, and over how long? A 500-piece prototype run and a 500,000-piece annual programme should not use the same grade. Glass-filled grades wear a tool faster and cost more per kilogram; unfilled grades are cheaper to run but weaker and shrink more.

Question four: what does the part have to look like? A visible class-A surface, a soft-touch feel, a transparent window, a laser-markable label area — each of these narrows the list hard. PMMA gives you optical clarity and scratches; PC gives you clarity and impact resistance but yellows; PS gives you cheap clarity and shatters.

2. The numbers that decide whether a material is mouldable at all

Four numbers do most of the work. They are also the four numbers most often missing from a buyer's RFQ, which is why quotes come back vague.

• Schrumpfung — how much the part contracts as it cools. This is not a property you "allow for later"; it is baked into the tool cavity dimensions. Get it wrong and the part is out of tolerance on day one.
• Drying requirement — whether the resin absorbs moisture from the air. Hygroscopic resins that are moulded wet produce silver streaks, splay and a measurable drop in impact strength. Non-hygroscopic resins do not care.
• Melt temperature window — the range the barrel has to sit in. Narrow windows (POM, PVC) are unforgiving; wide windows (PP, PE) tolerate a distracted operator.
• Cost index — relative resin cost per kilogram, indexed to PP at 1.0. Absolute prices move with the market; the ratio between grades does not move much.

The table below is the reference we use internally when a customer sends a drawing with no material specified. Values are typical for unfilled, general-purpose grades — glass-filled versions shrink less and cost more.

Typical values for unfilled general-purpose grades. Always confirm against the specific grade datasheet — two ABS grades from different suppliers can differ by 0.2 % shrinkage, which is enough to miss a tolerance.

3. Commodity plastics: PP, HDPE, PS and PVC

PP — polypropylene

The default choice for anything that needs to be cheap, tough and chemically resistant: caps, closures, containers, living hinges, battery housings, automotive trim. It has the best fatigue resistance of any commodity plastic, which is why one-piece living hinges are almost always PP.

Two things catch buyers out. First, shrinkage is high and anisotropic — a long thin PP part will shrink differently along the flow direction than across it, so it warps. Second, PP has poor UV stability without a stabilised grade, and unpainted PP exposed outdoors in strong sun will chalk within a season.

Do not specify PP if the part needs to hold a screw thread under load, take paint without flame treatment, or bond with adhesive. All three are difficult and expensive to solve.

HDPE — high-density polyethylene

Chemically the most inert of the cheap plastics: bleach, acids, detergents, most solvents. Used for chemical drums, cutting boards, food containers, pipe fittings. Shrinkage is the highest on the list, so flat panels warp unless you design in ribs or a slight crown.

HDPE feels waxy and resists printing and labelling. If the part needs a durable printed surface, that has to be designed in — a textured area, a label recess, or a different material.

PS and HIPS

General-purpose PS is cheap, clear and brittle. HIPS is PS with rubber added: opaque, far tougher, still cheap. Both are common in disposable packaging, cosmetic cases, appliance liners and point-of-sale display.

HIPS is a legitimate cost-down from ABS for parts that do not need ABS's impact strength or heat resistance — but only if the part is not going to be dropped, exposed to solvent, or screwed together. It also has a different gloss level, which matters on a visible surface.

PVC-U — rigid polyvinyl chloride

Used for pipe fittings, electrical conduit, window profiles and chemical-resistant parts. Excellent flame resistance and chemical resistance, low cost. The problem is processing: PVC degrades and releases hydrogen chloride if it is overheated or held in the barrel too long, which attacks the screw and the tool. That means corrosion-resistant steel, tight temperature control and a purging discipline. Many moulders will simply decline PVC work, and the ones that take it price it accordingly.

4. Engineering plastics: ABS, PC, PA, POM, PBT, PMMA

ABS

Acrylonitrile butadiene styrene. The workhorse of consumer product housings: laptops, power tools, appliance panels, toys, automotive interior trim. Good impact strength, good surface finish, easy to paint, plate and laser-mark, and it moulds at a forgiving temperature window.

Its weaknesses are environmental rather than mechanical. UV exposure yellows it and degrades the butadiene phase, so outdoor parts need an ASA or a stabilised grade. Petrol, brake fluid and many solvents attack it. And it stress-crazes around moulded-in metal inserts, which is why insert design matters more in ABS than in PA.

PC — polycarbonate

Transparent, extremely tough, dimensionally stable, usable from −40 °C to about 120 °C. Safety visors, machine guards, LED lenses, medical housings, electrical enclosures. It is the material you choose when the part must not break.

PC is also the material most often ruined by a supplier who skips drying. Molten PC reacts with water and the molecular chains break — the part looks fine, then cracks in the field weeks later. Drying at 120 °C for three to four hours in a desiccant dryer is not optional. High melt viscosity also means high injection pressure and high clamp force, so a PC part needs a bigger machine than the same part in ABS.

PA6 and PA66 — nylon

Strong, stiff, wear-resistant, good at elevated temperature. Gears, bearings, cable ties, automotive under-hood parts, power tool housings, structural brackets. Glass-filled grades reach tensile strengths that compete with die-cast metal.

The catch is moisture. Nylon absorbs water from the air — up to 2.5 % for unfilled PA6 — and that absorption changes dimensions and softens the part. A PA6 part that measures perfectly on the day it is moulded will measure differently three weeks later in a humid warehouse. For tight-tolerance parts, either specify a conditioned dimension, use a moisture-resistant grade, or switch to PBT or POM.

POM — acetal

Low friction, high stiffness, excellent fatigue and creep resistance, and very stable in the presence of fuel, oil and grease. Gears, cams, sliders, clips, fuel-system components, conveyor parts. It is the material for anything that has to slide or rotate a million times.

POM has the narrowest processing window on this list. Above roughly 220 °C it depolymerises and releases formaldehyde, which smells unmistakable and embrittles the part. It also does not bond to anything, which rules out overmoulding and most adhesives. And it is attacked by strong acids and oxidising agents.

PBT and PET

Good dimensional stability, good electrical properties, fast crystallisation and short cycle times. Electrical connectors, coil bobbins, sensor housings, automotive electrical parts. Glass-filled PBT is the standard for connectors that get soldered.

Unfilled PBT is notch-sensitive — a sharp internal corner will crack under impact. Design in radii. PET is harder to mould than PBT and is usually reserved for specific food-contact or high-clarity applications.

PMMA — acrylic

The best optical clarity of any mouldable plastic, with excellent UV resistance — it does not yellow outdoors the way PC does. Light guides, lenses, display windows, illuminated signage, cosmetic packaging.

It is brittle. A PMMA window that takes a direct impact will crack where a PC window would deform. It also scratches easily and is attacked by many solvents, so cleaning instructions matter. In practice the choice is: PMMA if it has to stay clear for ten years outdoors, PC if it has to survive being hit.

5. High-performance: PPS and PEEK

These two exist to solve problems that no engineering plastic can: continuous service above 150 °C, aggressive chemicals, or both.

PPS (polyphenylene sulfide) is dimensionally stable, inherently flame retardant, resistant to most solvents and acids, and holds properties to about 200 °C. It is the standard for under-hood automotive parts, pump housings, and chemical-process components. The tooling cost is the barrier: PPS needs a mould running at 130–150 °C, hot-runner systems rated for high temperature, and often glass or mineral fillers that wear the tool. Expect to pay two to three times the tool cost of an equivalent ABS part.

PEEK (polyetheretherketone) is the top of the pyramid — about 250 °C continuous, excellent chemical resistance, biocompatible grades for implants, and a resin price roughly forty times PP. It is specified for aerospace brackets, semiconductor components, medical instruments and oil-and-gas seals. Anyone proposing PEEK for a consumer product is either solving a very unusual problem or has not done the maths.

6. Elastomers and two-shot overmoulding

Soft-touch grips, seals, gaskets and vibration mounts are usually TPE or TPU rather than a thermoset rubber, because they can be injection moulded — and overmoulded directly onto a rigid plastic part in a second shot.

The single most common failure in overmoulding is a bad bond, and the cause is almost always the substrate. TPE bonds well to PP, PE, ABS, PC and PA; it bonds poorly to POM, PBT and most glass-filled grades. If the design needs a hard-soft combination, the substrate material has to be chosen for its bondability, not just its stiffness.

For true rubber properties — high-temperature compression set, fuel resistance, wide temperature range — vulcanised rubber (NBR, EPDM, FKM, VMQ) is still the right answer, but that is a compression or transfer moulding process, not injection moulding, and it has a different cost structure.

7. Shrinkage: the number that quietly breaks your tool

Every cavity is cut oversize so that the part, after cooling, lands on nominal dimensions. The multiplier is the shrinkage rate. Get it wrong and the tool is scrap.

Three things make shrinkage harder to predict than the datasheet suggests:

• It is directional. Molecules align with the flow, so a part shrinks less along the flow direction than across it. In glass-filled grades the difference can be three to one, which is exactly why long glass-filled parts warp into a banana.
• It depends on wall thickness. A thick section cools slowly and shrinks more. A part with a 2 mm wall and a 5 mm boss will have two different effective shrinkages.
• It depends on mould temperature and holding pressure. Higher pack pressure means less shrinkage. This is why the same tool can produce parts at opposite ends of a tolerance band on different machines.

Practically: for a new tool we cut to the mid-point of the supplier's shrinkage range and then adjust by removing steel on the first trial, never by adding it. If the part is a precision fit or a mating pair, we ask for the mating part's actual measured dimensions rather than the drawing nominal — because the tolerance stack, not the shrinkage, is usually what fails.

8. Drying: the cause of surface defects nobody checks

Hygroscopic resins absorb moisture from the air. When that moisture hits the melt, it turns to steam inside the barrel. The result is silver streaks on the surface, splay around the gate, a drop in impact strength, and — in PC and PA — chemical breakdown of the polymer chain that shows up as cracking weeks later.

The materials that must be dried: PC, PA6, PA66, PBT, PET, PMMA, ABS, POM, PPS, PEEK, TPU. The materials that do not: PP, PE, PS, PVC.

Two details that decide whether drying actually works:

• A desiccant dryer, not an oven. A tray oven at 120 °C dries the outside of the pellet and leaves the core wet. Dried air at −20 °C dew point circulating through a hopper actually removes moisture.
• Drying is not a one-off. A dried hopper left standing open re-absorbs moisture within hours. The dryer has to run continuously, right up to the nozzle.

If a supplier cannot tell you the dew point of their dryer, that is a useful signal about everything else in their process.

9. Four substitution traps that look like savings

Most material downgrades are proposed for good reasons and fail for predictable ones. These four come up constantly.

The pattern: a substitution that changes stiffness, heat resistance or moisture behaviour is a redesign, not a cost saving. A substitution that only changes colour, gloss or processing aids usually is a genuine saving.

10. Compliance: food contact, medical, flame retardant, UV

Regulatory requirements narrow the material list faster than any mechanical property, and they have to be settled before the tool is cut — because changing to a compliant grade later often means re-validating the tool.

• Food contact — EU 10/2011, US FDA 21 CFR 177, and in China GB 4806. Most commodity and engineering grades have compliant versions, but the compliant version of a grade is a different product with different additives, sometimes different shrinkage.
• Medizinische — ISO 10993 biocompatibility, USP Class VI, and for devices, ISO 13485 process control at the moulder. The material is only half of it; the moulding environment and the documentation are the other half.
• Flame retardant — UL 94 V-0, V-2, 5VA. Halogen-free FR grades are increasingly specified in Europe. FR additives change flow, shrinkage and often colour consistency, and some grades are not available in light colours at all.
• UV and weathering — for outdoor parts, specify the stabilised grade from the start. Retrofitting UV resistance by painting is possible but adds a process, a cost and a failure mode.

Ask for the grade's certificate of compliance and the migration test report, not just a claim in an email. If the part is going into a regulated product, the moulder should be able to trace the exact lot of resin used in your parts.

11. Frequently asked questions