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Thermoplastics vs thermosetting plastics — the divide that decides your part

Every plastic ever moulded answers one question: reheat it — does it melt again, or is it ruined? A thermoplastic melts again, as many times as you like. A thermosetting plastic cures once, into a cross-linked network that no reheating will undo. That single fact decides the process, the machine, the mould’s thermal logic, the economics of scrap, the temperatures your part can live at — and which moulder you should be talking to. We keep a full guide on each family — thermoplastics and thermosetting plastics — this page is the fork in the road between them.

The definitions, as a moulder uses them

Bryce’s SME handbook puts the divide in two lines. A thermoplastic, “when heated, undergoes a physical change. It can be reheated and reformed over and over again” — he compares it to water, solid to liquid and back, without altering its chemistry. A thermoset, “when heated, undergoes a chemical change and ‘cures.’ It cannot be reformed, and reheating only degrades it.” The Arburg guide supplies the mechanism: during curing the molecules “crosslink forming a highly dense network of bonds”, after which the part can no longer be melted — “thermoset materials decompose before they can melt”. And Harper’s handbook states the commercial consequence without sentiment: thermosets “can only be processed into a product once”; they “cannot simply be reground and reprocessed as thermoplastics”.

What curing buys — and what it costs

Why would anyone specify a material that can never be remelted? Duty. Harper describes the cure as “an irreversible and exothermic chemical reaction” driven by heat around 150 °C and real pressure — and the reward is that the cured part “retains its physical, chemical, and electrical properties” across a service range he gives as roughly −50 °C to 150 °C or more. That is why the classic thermoset names — Harper’s list runs phenolics, melamines, ureas, diallyl phthalates, unsaturated polyesters, silicones, epoxies, alkyds — keep their place in hot, electrically stressed and chemically loaded duties. The bill arrives elsewhere: sprues, runners and rejects are dead material. Cured thermoset “can be granulated, with the material being useful as filler” (Rosato) — filler, never feedstock. In thermoplastic country, scrap has value and rules of its own; that is a negotiation we tell buyers to settle in writing.

Infographic: thermoplastics vs thermosetting plastics — melt again vs cure once, what each buys, the inverted mould logic (cooled mould vs heated mould), and four checks before you specify

Different machines — and mould logic that runs backwards

Buyers often assume the two families are options on the same machine. They are not. A general-purpose thermoplastic screw runs a length-to-diameter ratio of about 20:1; the figure drops to “14:1 with thermosets, elastomers and liquid silicone rubber” (Goodship, Troubleshooting Injection Moulding), and Rosato notes thermoset screws typically run without a non-return valve. The thermal logic then inverts completely. Rosato again: thermoplastics reach their maximum temperature in the barrel, before entering the mould; thermosets reach their maximum temperature in the heated mould. Arburg says it plainly: “Thermosets are demoulded from the hot mould when crosslinking is complete. Thermoplastics have to be cooled until the part is rigid enough for demoulding.” A thermoplastic mould is a cooling device — the heart of the moulding cycle; a thermoset mould is closer to an oven. A moulder tooled and disciplined for one family is not automatically equipped for the other.

Which does your part actually need?

Start from the base rate: Rosato puts thermoplastics at “at least 90 wt% of all injection-molded plastics”. The default answer is thermoplastic, because the economics are with it — faster cycles, recoverable scrap, colour freedom and the full engineering palette in our materials guide. You cross into thermoset country when a number says so: a continuous service temperature the thermoplastic candidates cannot hold, an electrical or chemical duty their datasheets will not sign for. The number lives on the datasheet, not in the adjective — here is how to read one.

Where we stand

Kruger moulds thermoplastics only — commodity to engineering grades — and that is deliberate: it is what our machines, dryers and quality system are built and audited for. If your drawing says phenolic or epoxy, we will say so plainly and point you toward the compression- and transfer-moulding trade those materials belong to, exactly as our ISBM guide does for blown bottles. An honest routing costs you nothing. A mis-specified material family costs a tool.

Four checks before you specify

Highest continuous service temperature, as a number with hours attached. If it clears the thermoplastic candidates’ datasheets, the economics say stay thermoplastic.
Electrical and chemical duty, named. “Retains its properties” is Harper’s cured-thermoset promise — check whether your duty actually demands it.
Scrap and end-of-life expectations. Regrind clauses and recyclability exist only on the thermoplastic side of the fence.
Volume and cycle economics. The 90 wt% base rate exists for a reason — make the exotic choice pay its way.

Unsure which side your part falls on? Send the drawing and the duty — an engineer replies within 48 hours on working days, including when the honest answer is “not us”. And for judging any moulder you shortlist, thermoplastic or otherwise, take the supplier-qualification guide.