The material you choose for an injection moulded part matters more than almost any other decision on the programme. It sets how the part performs, how it looks, what it costs per unit, how the tool has to be built, and which defects you will spend the next five years chasing. Get it right and the part runs cleanly and does its job. Get it wrong and you inherit warping, brittle failures, a resin that costs ten times what the job needed, or a tool cut from the wrong steel.
This is a practical guide to injection moulding materials for the engineers who specify them and the buyers who have to sign off the quote. We cover the main thermoplastic families you will actually choose between, the criteria that should drive the decision, the material grades that matter in regulated markets like medical and automotive, and how material choice ripples through into tooling, defects and cost. It is written by the toolmakers and project engineers at Sino Manufacturing, a British-owned factory that has been moulding parts for OEMs across automotive, medical, electronics and lighting for over 20 years.
The main injection moulding material families
Almost every moulded part is made from a thermoplastic — a polymer that melts when heated and solidifies when cooled, and can be reheated and remoulded. Within that, materials split into commodity plastics (cheap, high-volume, everyday parts) and engineering plastics (higher performance, higher cost). Here are the families that cover the large majority of real programmes.
ABS — the versatile all-rounder
Acrylonitrile butadiene styrene is the default for rigid, good-looking plastic parts. It is easy to mould, takes a good cosmetic finish, paints and plates well, and is tough at room temperature.
- Typical uses: electronics housings, consumer product enclosures, automotive interior trim, toys, appliance parts.
- Trade-offs: limited heat resistance, poor UV and weathering resistance unless stabilised, and modest chemical resistance. Not for outdoor or high-temperature parts without help.
Polypropylene (PP) — the low-cost workhorse
PP is one of the cheapest and most widely used moulding materials. It is light, chemically resistant, and can flex repeatedly without breaking — which is why it is the go-to for living hinges.
- Typical uses: packaging and closures, living-hinge lids, containers, automotive under-bonnet parts, housewares, medical disposables.
- Trade-offs: low stiffness and strength unless filled, poor UV resistance unstabilised, and it can be harder to bond and paint than ABS.
Polycarbonate (PC) — strength and clarity
PC is a tough engineering plastic with excellent impact strength and optical clarity, and it holds up better to heat than ABS.
- Typical uses: transparent covers and lenses, safety glazing and guards, lighting diffusers, electronic housings that need impact strength, medical components.
- Trade-offs: more expensive than commodity resins, notch-sensitive, prone to scratching without a hard coat, and attacked by some solvents.
Polyamide / nylon (PA) — tough and wear-resistant
Nylon (PA6, PA66 and others) is a strong, tough, abrasion-resistant engineering plastic with good chemical and heat resistance — a favourite for mechanical and under-bonnet parts, especially in glass-filled grades.
- Typical uses: gears, bearings, cable ties, structural clips, automotive engine-bay components, power-tool housings.
- Trade-offs: nylon absorbs moisture, which changes its dimensions and properties — a real design consideration. It needs careful drying before moulding, or you get splay and weak parts.
Acetal / POM — precision and low friction
Polyoxymethylene (POM, often called acetal or by the trade name Delrin) is prized for dimensional stability, stiffness, low friction and good wear resistance — ideal for small, precise moving parts.
- Typical uses: gears, bushings, bearings, fasteners, clips, fuel-system and precision mechanical components.
- Trade-offs: more expensive than commodity plastics, harder to bond and paint, and it has a narrow processing window that demands good process control.
TPE / TPU — the soft, flexible materials
Thermoplastic elastomers (TPE) and thermoplastic polyurethanes (TPU) are the rubber-like materials used for soft grips, seals and flexible parts. They mould like a thermoplastic but flex like rubber, and many grades are formulated to bond to a rigid substrate — which is what makes soft-touch 2K moulding and overmoulding possible.
- Typical uses: soft-touch grips, seals and gaskets, cable strain reliefs, flexible bellows, wearable and consumer product surfaces.
- Trade-offs: grade selection is critical — hardness (Shore value), bonding compatibility and chemical resistance vary enormously between grades. TPU offers higher abrasion resistance; general TPEs are cheaper and easier to process.
PC/ABS — the engineered blend
PC/ABS blends the impact strength and heat resistance of polycarbonate with the easier processing and lower cost of ABS. It is a common choice when ABS is not quite tough or heat-resistant enough but full PC is more than the job needs.
- Typical uses: automotive interior parts, electronics and appliance housings, mobile and IT enclosures.
- Trade-offs: costs more than ABS, and the exact balance of properties depends on the blend ratio — specify the grade, not just “PC/ABS”.
Glass-filled grades — stiffness and strength on demand
Adding glass fibre (commonly 10–50%) to nylon, PP, PC or PBT dramatically increases stiffness, strength and heat resistance. Glass-filled nylon in particular replaces metal in many structural parts.
- Typical uses: structural brackets, load-bearing components, under-bonnet automotive parts, power-tool bodies, anything that needs to be stiff and dimensionally stable under load or heat.
- Trade-offs: glass fibre is highly abrasive and wears out tool steel far faster than unfilled resin — which changes the steel you should cut the tool from. Filled grades also shrink differently, can warp, and give a rougher surface finish.
A material comparison table
A shareable at-a-glance comparison of the common families. Treat the cost column as a relative guide, not a quote — actual resin prices move with grade, colour, fillers and market conditions.
| Material | Key strengths | Main weaknesses | Relative cost | Typical applications |
|---|---|---|---|---|
| ABS | Easy to mould, good finish, tough, paints/plates well | Low heat & UV resistance | $ | Enclosures, trim, consumer goods |
| PP | Cheap, light, chemically resistant, living hinges | Low stiffness, poor UV, hard to bond | $ | Packaging, closures, housewares |
| PC | High impact strength, clarity, heat resistance | Costly, scratches, solvent-sensitive | $$$ | Lenses, guards, lighting, safety parts |
| PA / Nylon | Tough, wear- & heat-resistant, strong | Absorbs moisture; needs drying | $$ | Gears, clips, under-bonnet parts |
| POM / Acetal | Stiff, low friction, dimensionally stable | Bonds poorly, narrow process window | $$ | Gears, bearings, precision parts |
| TPE / TPU | Flexible, soft-touch, bonds for overmoulding | Grade selection critical | $$–$$$ | Grips, seals, strain reliefs |
| PC/ABS | Impact + heat of PC, easier processing | Costs more than ABS | $$ | Auto interiors, IT/appliance housings |
| Glass-filled | High stiffness, strength, heat resistance | Abrasive on tools, warps, rough finish | $$–$$$ | Structural & load-bearing parts |
Selection criteria: how to actually choose
The right material is the one that meets every requirement at the lowest total cost — no more, no less. Over-specifying is as expensive a mistake as under-specifying. Work through these criteria against your part:
- Mechanical requirements: how much load, stiffness, impact and fatigue does the part see? Impact strength points toward PC or PC/ABS; stiffness and load point toward glass-filled grades; repeated flexing points toward PP or a suitable elastomer.
- Thermal requirements: the maximum temperature in service, and during any downstream process like painting or sterilisation. This alone rules out commodity resins for hot applications and pushes you toward PC, nylon or filled engineering grades.
- Chemical and environmental resistance: exposure to solvents, fuels, cleaning agents, UV and moisture. PP and POM resist many chemicals well; PC does not like some solvents; unstabilised ABS and PP degrade outdoors.
- Cosmetic requirements: transparency (PC), a paintable or platable surface (ABS), or a specific texture or finish. Filled grades give a rougher surface and are harder to finish cleanly.
- Regulatory requirements: food contact, medical biocompatibility, flame retardancy (UL ratings), or automotive material specs. These narrow the shortlist to approved grades before any other criterion — treat them as a hard filter, not a preference.
- Cost and volume: the resin cost per part multiplies across the whole programme. On a high-volume part, a small change in resin price or a slightly faster-cooling grade moves the unit cost meaningfully.
A useful discipline: start from the single most demanding requirement (often thermal or regulatory), let it define the candidate materials, then use the remaining criteria to choose between them. Do not start from “what did we use last time”.
Medical and automotive grades
In regulated industries, “nylon” or “PC” is not a specification — the grade and its approvals are. The rules differ enough between sectors that they change the whole conversation, which is why we cover them separately in what sets automotive and medical injection moulding apart.
- Medical grades: require biocompatibility (commonly assessed against ISO 10993 or USP Class VI), and often need to withstand sterilisation by autoclave, gamma or ethylene oxide — which not every grade survives. Materials must have documented, traceable, consistent formulations, and suppliers must guarantee that the grade will not be changed without notice. Common choices include specific medical grades of PC, PP, POM, and certain TPEs and nylons.
- Automotive grades: must meet OEM material specifications, often including flammability, fogging, UV and heat-ageing requirements. Glass-filled nylons and PBT, PP compounds, PC/ABS and specialist high-temperature grades dominate, with the exact grade dictated by the OEM’s approved materials list.
The practical point for buyers: specify the approved grade and demand documentation. A part moulded in the right family but the wrong grade can fail qualification just as surely as the wrong material entirely.
How material choice drives tooling, defects and cost
Material is not a decision you make in isolation after the tool is designed — it feeds directly back into how the tool is built, which defects you will fight, and what the part costs. Three connections every buyer should understand:
- Material drives the tool steel. Abrasive materials — glass- and mineral-filled grades above all — wear soft tool steel quickly, so they demand a harder, more wear-resistant steel and sometimes surface coatings. That raises the tooling cost, but a tool cut from the wrong steel for a filled resin will wear out early and cost far more over the programme. This is one of the biggest hidden drivers behind injection mould tooling cost.
- Material drives the defects you see. Each resin brings its own failure modes: nylon splays if it is not dried properly; semi-crystalline materials like POM and PP shrink and warp more than amorphous ones like ABS and PC; filled grades warp and show fibre on the surface. Choosing the material is partly choosing which defects you will have to design and process around — our field guide to injection moulding defects maps the causes and fixes.
- Material drives unit cost more than most buyers expect. Engineering and glass-filled grades can cost several times commodity polypropylene, and that difference multiplies across every part you make. Resin choice also affects cycle time — how fast the part cools and ejects — which is itself a major unit-cost lever at volume.
This is exactly why material selection belongs in the DFM conversation at the start of a programme, not as an afterthought once the design is frozen. Choosing the resin, the tool steel and the process together is how you avoid paying for the same mistake twice.
How Sino approaches material selection
Sino has moulded parts across every one of these material families for over 20 years, out of our British-owned, ISO 9001:2015-certified factory in Shenzhen, and now with a mould facility in Querétaro, Mexico for China+1 buyers nearshoring out of Asia. Because we design tools and mould parts under one roof, we look at material, tool steel and process together — and we will tell you honestly when a cheaper resin would do the job, or when the grade you have specified is going to cause you trouble downstream. Where a part crosses into overmoulding or 2K, material compatibility becomes even more critical; we cover that in our overmoulding design and sourcing guide. Material selection also sits inside the broader picture in our complete guide to injection moulding, and it is one of the first things we review against our DFM checklist on any new project.
Frequently asked questions
What is the most common injection moulding material?
Polypropylene (PP) and ABS are the two most widely used. PP dominates high-volume, low-cost parts like packaging and closures; ABS is the default for rigid parts that need a good cosmetic finish, such as enclosures and trim. For higher-performance parts, polycarbonate, nylon and glass-filled grades take over.
How do I choose the right injection moulding material?
Start from your most demanding requirement — usually thermal performance or a regulatory rule such as food contact or medical biocompatibility — and let it define the candidate materials. Then narrow down using mechanical loads, chemical and UV exposure, cosmetic needs, and cost at your volume. The goal is the material that meets every requirement at the lowest total cost, without over-specifying.
Which injection moulding materials are used for medical parts?
Medical parts use grades certified for biocompatibility (commonly against ISO 10993 or USP Class VI) and, where needed, able to survive sterilisation by autoclave, gamma or ethylene oxide. Common choices include medical grades of polycarbonate, polypropylene, POM and certain TPEs and nylons. The specific approved grade and its documentation matter as much as the material family.
Why does material choice affect tooling cost?
Abrasive materials — especially glass- and mineral-filled grades — wear tool steel quickly, so they require a harder, more wear-resistant steel and sometimes coatings, which raises the tooling cost. Choosing the wrong steel for an abrasive resin leads to early tool wear and higher cost over the programme, so material and tool steel should be decided together.
The bottom line
Material selection is the decision that quietly sets the performance, cost and defect profile of an injection moulded part for its entire production life. There is no single best material — only the best fit for your specific mechanical, thermal, chemical, cosmetic and regulatory requirements at your volume. Start from your hardest requirement, resist the urge to over-specify, and decide material, tool steel and process together rather than in sequence. If you would like a second opinion on the right resin and grade for a specific part, talk to our team — send us your drawings and we will give you a straight recommendation.
Need help with a project?
Choosing the right material is crucial. Whether you need a tough structural part, a transparent housing, a medical-grade component, or a soft-touch grip — Sino’s team will help you get it right from the start.
We’ll complete an NDA and provide expert advice tailored to your requirements, timescale and budget.
