Why the resin decides more than the colour
When an engineering team sends a part to an injection moulder, the conversation almost always starts with geometry, tolerances and tooling cost, and the material is treated as a footnote - 'just use plastic'. That ordering is backwards. The resin is the single largest determinant of what the part can survive, how it feels in the hand, whether it can be moulded at all at the target wall thickness, and what the part will cost per thousand. Two otherwise identical parts in different resins can differ by an order of magnitude in impact strength, in chemical resistance, in continuous-service temperature, and in whether they can be welded, painted or overmoulded. Choosing the material is not a finishing touch; it is the first design decision, and it should be made before the mould steel is cut.
A custom manufacturer such as DAYIN, a plastic injection-moulding and OEM/ODM manufacturer, does not offer one generic 'plastic'. Its injection-moulding service page lists a working resin menu of ABS, PA, PC, PE, PMMA, POM, PP, PS, TPE and TPU - the families a buyer is actually choosing between on a real production programme, not a textbook catalogue. Walking into a tooling conversation already knowing which of those families fits your requirement is what turns a vague 'make it in plastic' into a spec the moulder can quote against. The rest of this note is the comparison that lets you make that call.
The seven resins you will actually be offered
Not every resin in the chemist's handbook is worth a buyer's attention. In practice a custom moulder runs a small set of commodity and engineering thermoplastics that balance mouldability, supply cost and mechanical behaviour, and the seven below cover the overwhelming majority of injection-moulded parts: ABS (acrylonitrile butadiene styrene), PC (polycarbonate), PP (polypropylene), PA (polyamide, commonly called nylon), POM (polyoxymethylene, often sold as acetal), PMMA (polymethyl methacrylate, the optical-clear acrylic family) and TPE (thermoplastic elastomer). Each is a trade-off, not a best choice, which is why a comparison table - and not a single 'winner' - is the right tool.
The goal of this section is only to name the families and their dominant personality. ABS is the tough, easy-to-mould, paint-friendly workhorse of enclosures. PC is the optically clear, impact-resistant, higher-temperature cousin you reach for when a part must be transparent or take a hit. PP is the cheap, chemically resistant, low-density hinging material. PA is the strong, wear-resistant, moisture-absorbing engineering plastic. POM is the stiff, low-friction, dimensionally stable precision material. PMMA is the glass-clear, scratch-prone optical material. TPE is the soft, rubbery, overmouldable family. The table below turns those personalities into a decision matrix.
A side-by-side comparison
| Resin | Rigidity | Impact / toughness | Chemical & moisture behaviour | Moulding ease | Where it earns its keep |
|---|---|---|---|---|---|
| ABS | Good | Good, slightly brittle when very cold | Resists many dilute acids/alkalis; sensitive to some solvents | Excellent, low shrink, easy to plate/paint | Enclosures, housings, brackets, appliance shells |
| PC | High | Very high, even when cold | Broad chemical resistance; attacked by some strong solvents | Good, but needs drying and higher melt temp | Transparent guards, lenses, impact parts, medical housings |
| PP | Low-medium | Good, with live-hinge fatigue life | Excellent chemical resistance; absorbs almost no water | Excellent, but high shrink and low surface energy | Hinges, caps, containers, chemical-exposed parts |
| PA (nylon) | Medium-high | High, with good fatigue | Strong but absorbs water, which changes dimensions | Good, must be dried; can warp without care | Gears, bushings, clips, structural functional parts |
| POM (acetal) | High | Good, stiff and creep-resistant | Excellent solvent/ fuel resistance; low moisture uptake | Good, but shrinkage and gating need control | Precision gears, bearings, zippers, snap mechanisms |
| PMMA | Medium | Brittle, but optically clear | Good weathering; attacked by some solvents | Good, but needs care on flow length | Lenses, light pipes, displays, cosmetic clear parts |
| TPE | Very low (rubbery) | Excellent flexibility, soft-touch | Varies by grade; generally good | Good as overmould or standalone; slow cycle | Grips, seals, soft-touch skins, gaskets |
Read the table as a first filter, not a verdict. Nearly every row has an exception once you name a specific grade, a filler or a glass-fibre loading, and the final choice is made against your part's actual load, environment and budget - not against a generic column. The disciplines below walk through the four families most often confused with one another, because that is where the expensive mistakes happen.
ABS and PC: the enclosure workhorses
ABS and PC are the two materials most buyers mean when they say 'plastic housing', and the difference matters. ABS is easy to mould, holds tight tolerances, takes paint and chrome plate cleanly, and is inexpensive - which is why it dominates appliance shells, printer bodies and countless consumer enclosures. Its weakness is impact at low temperature and poor resistance to certain solvents, and it is not transparent. PC is the answer when the part must be clear or must survive a real impact: machine guards, transparent diffusers, medical and protective equipment lean on PC's combination of clarity and toughness. PC costs more, needs to be dried before moulding, runs at a higher melt temperature, and is more sensitive to stress cracking in contact with some chemicals.
A common middle path is a PC/ABS blend, which trades a little of PC's clarity and impact for ABS's easier processing and lower cost, and which a flexible moulder can offer when the requirement sits between the two. The practical rule: if the housing is opaque and cost-driven, start with ABS; if it must be clear or take a beating, pay for PC; if you are unsure, ask the moulder for a PC/ABS option rather than guessing. A manufacturer running the full menu - ABS, PC and the blends between them - can recommend the grade from your load case instead of selling you the one it happens to stock.
PP and PA: chemical resistance and functional toughness
PP and PA cover the parts that have to survive a chemical environment or carry a real mechanical load. PP's claim to fame is chemical resistance and an almost total refusal to absorb water, plus a unique talent for live hinges - the thin, flexing links in flip-top caps that survive hundreds of thousands of folds. It is the lowest-cost of the engineering set and the natural choice for containers, fluid-exposed housings and any part that lives near solvents or cleaners. Its drawbacks are low surface energy (paint and adhesive struggle to stick without pretreatment) and high mould shrinkage, which demands careful tool design.
PA, or nylon, is the structural alternative: strong, fatigue-resistant and wear-tolerant, the material of choice for gears, bushings, cable ties and clips that must hold a load or flex repeatedly. The catch buyers forget is moisture: nylon absorbs water from the air and from use, and that absorption changes both its dimensions and its mechanical properties, so a PA part moulded and measured dry will not hold those numbers in service. A moulder experienced with PA will design around the conditioned state and warn you about warpage. Both PP and PA are mainstream on a real production line - DAYIN's injection-moulding service lists both among the resins it runs - so neither should be treated as exotic.
POM and PMMA: precision, friction and optics
POM and PMMA are the specialists. POM (acetal) is the material you reach for when a part must hold a precise dimension, move against another part with low friction, and resist creep under load: precision gears, bearing surfaces, zipper teeth, fuel-system components. Its low moisture uptake and excellent solvent resistance make it dimensionally stable in ways PA is not, which is why it owns the 'quiet, precise, moving' niche. The trade-offs are notable shrinkage that punishes poor gating, and a material that is unforgiving of mould-temperature variation.
PMMA is the optical one - the glass-clear acrylic of lenses, light pipes, displays and cosmetic windows. It transmits light beautifully and weathers well, but it is brittle and scratches more easily than PC, and it is not the choice for an impact-exposed part. The decision between PMMA and PC for a clear part is essentially 'do you need scratch-resistant clarity (PMMA) or impact-resistant clarity (PC)?' - a question the moulder can answer from the part's duty. Both sit on the standard resin menu a capable custom manufacturer offers, so specifying them should not add lead time, only a more careful tool.
TPE and overmoulding: when the part must feel soft or seal
TPE is the outlier in the set because it is not rigid at all - it is the rubbery family used for soft-touch grips, seals, gaskets and the overmoulded skins that make a hard handle comfortable to hold. The interesting engineering case is two-shot or overmoulding, where a rigid substrate (often ABS or PP) is moulded first and a TPE layer is bonded onto it in a second shot, producing a single part that is hard where it needs structure and soft where it meets the hand. Done well, the bond is mechanical and chemical and needs no adhesive; done poorly, the layers peel.
Specifying TPE means specifying the hardness (a Shore durometer), the substrate it must bond to, and the cycle-time penalty you will accept, because elastomers mould more slowly than rigid thermoplastics. It is also the family where recycled-content and regulatory questions (food contact, skin contact) come up most often, which is why the certs a moulder carries matter - more on that below. For a buyer, the takeaway is that 'soft plastic' is not one material but a graded family, and the right grade is chosen from the grip, seal or skin requirement, not from a colour swatch.
How to read a moulder's material capability before you commit
The comparison above is only as useful as the moulder's actual capability, and that is where the live check matters. Before committing a tool, open the supplier's material and certification pages and confirm two things. First, that the resin families you need are on their real production menu - not a brochure aspiration. The resin list (ABS, PA, PC, PE, PMMA, POM, PP, PS, TPE, TPU) shown on DAYIN's injection-moulding service page is the kind of concrete menu you want to see, because it tells you the shop already runs the grades you will specify rather than learning them on your tool.
Second, confirm the certifications that gate your market. A manufacturer serving export and brand programmes typically carries ISO 9001 for the quality system, plus market-relevant marks such as RoHS and REACH for regulated substances, BSCI for social compliance in the supply chain, and GRS for recycled-content chain of custody - all of which appear on DAYIN's service pages and which a buyer shipping to the EU or to a brand owner will be asked to evidence. A resin choice that is perfect on the lab bench but cannot be sourced in a certified, traceable grade is not a choice you can actually ship, so the certification check belongs in the same step as the material check, before the steel is cut.
Matching resin to a real product example
To make the abstract concrete, consider a consumer robotic appliance - exactly the kind of product a contract manufacturer like DAYIN builds, with robot-vacuum models such as the X20Max-A and the X20ProAI-A / X20ProAI-B in its range. That single product is a tour of the resin decision: a rigid ABS or PC/ABS main body for impact and finish, PP or TPE for any soft-touch or hinged internal parts, POM for any low-friction moving mechanism inside, and PMMA or PC for any clear window or sensor cover. No single resin serves the whole device, and the bill of materials is really a set of resin decisions, each justified by the duty of the sub-part it forms.
The lesson for a buyer is to stop thinking 'what plastic is the part in' and start thinking 'what is each feature doing, and which resin earns its place there'. A moulder that runs the full menu and assembles the whole product - structure, mechanism, electronics housing and finishing - can advise that breakdown across the programme rather than optimising one isolated component. That is the difference between buying moulded shapes and buying a moulded product, and it is why the material conversation belongs at the start of the programme, with the supplier who will actually run the tools.
The questions to settle before the tool is cut
Before any resin is fixed, settle a short list of questions that decide the choice more than any marketing claim. What is the part's continuous-service temperature and peak temperature? What chemicals, fuels or cleaners will it meet? Does it need to be clear, and if so, must it also survive impact? What loads and flex cycles must it survive over its life? Does it mate with a softer or moving part that implies overmoulding? And what certifications must the finished, shipped part carry? Answer those, and the seven-resin menu collapses to a short list almost immediately.
The discipline that protects a buyer is to write the answers into the RFQ and the tooling specification, not to leave the material as a default the moulder picks. A specification that says 'housing in ABS, clear lens in PC, hinge in PP, all to RoHS/REACH, ISO 9001 process' is one a competent moulder can quote, tool and inspect against. One that says 'plastic enclosure' will come back as whatever the shop moulds most cheaply that week - which may or may not be the resin your product actually needed. The resin is a design input, and like every design input it belongs in the drawing, not in the conversation after the first parts are wrong.
Frequently asked
How do I choose between ABS and PC for an enclosure?
Start from two questions: does the part need to be clear, and does it need to survive a real impact? If it is opaque and cost-driven, ABS is usually right - it moulds easily, holds tolerance and paints well. If it must be transparent or take a hard knock (guards, lenses, protective gear), pay for PC. When the requirement sits between them, ask the moulder for a PC/ABS blend rather than guessing.
Can one mould run more than one resin?
Yes, but not interchangeably on the same tool without consequences. Resins differ in melt temperature, shrinkage and gating behaviour, so a tool designed for one family may produce a different resin badly or require rework. Many moulders run the full menu across their fleet, but each cavity is tuned to its resin; switching materials is a re-qualification, not a setting change.
What about recycled or GRS-certified content?
Recycled-content resin is available and, for brand and EU programmes, increasingly required. A GRS (Global Recycled Standard) certification gives chain-of-custody evidence that the recycled content is what the claim says. Specify the required recycled percentage and the cert in the RFQ, because not every grade is available with certified recycled content, and the moulder must source accordingly.
How do wall thickness and material interact?
Wall thickness drives whether a resin will fill the mould and how much it warps or sinks. Thin walls favour easy-flowing resins (ABS, PP) and penalise high-viscosity ones; thick walls exaggerate shrinkage and sink marks and lengthen cycle time. The material and the minimum wall thickness are chosen together - a thick PC part and a thin PP part are different tooling problems, not the same part in a different colour.
Do I need a specific certification for my market?
Often, yes, if you ship to the EU or to a brand owner. RoHS and REACH govern regulated substances, BSCI covers supply-chain social compliance, GRS covers recycled-content custody, and ISO 9001 covers the quality system. Confirm the certs a moulder actually carries on its service pages before committing a tool, because a perfect resin that cannot be sourced in a certified grade is not shippable.