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Thermoplastics — the family injection moulding runs on, and the divide inside it

A thermoplastic is a plastic that melts when heated, solidifies when cooled, and regains the ability to flow on every subsequent re-heating — which means it “can be reprocessed and hence recycled by re-melting” (Goodship/Arburg). That one property is why 85 to 90 per cent of everything injection moulded in the world is thermoplastic (Rosato): the process is, at heart, controlled melting and re-freezing, repeated every few seconds for years. But “thermoplastic” is a family name, not a material — and the divide that actually decides how your part behaves runs down the middle of the family.

The other family, for completeness: thermosets cure instead of melting — the molecules crosslink into a network that never flows again, which changes everything about how they are processed and specified. They have their own honest guide here: thermosetting plastics. This page is about the family we mould.

The divide: amorphous versus semi-crystalline

When a thermoplastic solidifies, its long chain molecules settle into one of two arrangements (Goodship/Arburg §2.5.1). In amorphous thermoplastics — polystyrene, polycarbonate, acrylics, ABS, PVC — the solid structure is “random and disordered, the long chain molecules being all entangled rather like solidified spaghetti” (Goodship/Arburg §6.1). In semi-crystalline thermoplastics — most nylons, acetal, polypropylene, polyethylene, the thermoplastic polyesters such as PET — a considerable proportion of the chains pack closely in regular alignment, forming ordered crystalline regions. In the melt, both classes are amorphous; the divide appears on cooling, in your mould, on your cycle.

You can see the divide with your own eyes. Most amorphous thermoplastics are transparent in their natural unpigmented form (ABS is a notable exception); most semi-crystalline materials are translucent or opaque white, because their crystalline regions scatter light. Moulders watch this happen during purging: molten natural polypropylene emerges transparent, then clouds over as it cools and its structure rearranges from tangled to ordered (Goodship/Arburg §6.1). If your part must be clear, your material is amorphous — that decision was made for you.

What the divide costs, in numbers

Melting behaviour. Amorphous materials soften progressively over a wide temperature span and solidify equally gradually; semi-crystalline materials change state over a narrow band, with sharp melting points (Goodship/Arburg §6.1). That difference shapes the processing window your moulder has to hold — the working values per material are on our temperature and drying chart.
Shrinkage. Amorphous thermoplastics typically shrink 0.5 to 1 per cent as they solidify; semi-crystalline materials shrink far more — usually 1.5 to 5 per cent, depending on the material — because their chains pack tighter as crystals form, and more in the direction of flow (Goodship/Arburg §6.1, §2.5.1). This is why the tool is cut to a different size from the part: see the shrinkage calculator.
Tolerance. More shrinkage, and shrinkage that varies with processing, means wider honest tolerance bands. If a drawing puts precision-grade limits on a high-shrinkage semi-crystalline part, somebody is going to have an expensive conversation — better to have it before steel, with the tolerances guide open.
Warpage risk. Shrinkage that differs with flow direction and cooling rate is the raw material of warp. The defects guide covers what that looks like when it arrives.

The mould-temperature trade nobody mentions in the datasheet

Here is the part of the lecture that turns into a question you can ask a moulder. For semi-crystalline materials, how much of the material actually crystallises is set by how fast it cools — and that is set largely by mould temperature. Moulded in a hot mould, the chains get time to order themselves: higher crystallinity, “superior mechanical strength and dimensional stability, but with relatively high shrinkage”. Moulded in a cold mould, crystal formation is inhibited: lower shrinkage now, but inferior mechanical properties and “a tendency for dimensional instability and distortion during later service due to aftershrinkage” (Goodship/Arburg §6.1). Faster and cheaper today can literally warp on the shelf next year.

So if your part is nylon, acetal, PP or PET and its dimensions matter, ask your moulder what mould temperature the job runs at and why. A good one will have an answer that mentions the material, not the cycle time. That question costs you nothing and tells you a great deal — it is the same teach-the-buyer test that runs through our supplier-qualification guide.

Commodity and engineering thermoplastics

The trade also divides the family by performance and price. Commodity thermoplastics — the polyethylenes, PVC, polypropylene, polystyrene — “account for at least two-thirds of plastic sales” (Rosato). Engineering thermoplastics earn their premium through “heat resistance, impact strength, and the ability to be molded to high-precision standards” — nylons, polycarbonate, ABS, acetal, polysulphone, PEEK among them (Rosato). The border moves: as production of an engineering grade scales, its cost falls and it drifts toward commodity status. The practical lesson is to buy the property, not the category — a glass-filled polypropylene often does an “engineering” job at a commodity backbone price, which is exactly the trade our composites guide prices out. And when one polymer nearly does the job, the answer is sometimes two: see blends and alloys.

Whichever tier you buy from, remember that with more than 17,000 compounds commercially available (Rosato), “polypropylene” is not a specification — a grade name and supplier is. Why chain length and formulation change a material’s behaviour is the subject of our basics of polymers guide, and how to read what the supplier promises is the subject of how to read a plastics datasheet.

Five questions before you specify a thermoplastic

Amorphous or semi-crystalline — and did you choose it, or inherit it? Transparency, chemical environment and dimensional behaviour usually make the choice; know which made yours.
What tolerance does the shrinkage class honestly support? Ask before the drawing hardens, not after the first article measures long.
What mould temperature will the job run at, and why? The one-question crystallinity test from this page.
Is the grade specified, or just the polymer? Grade, supplier, and the datasheet filed with the order.
Does it need drying? Several of the family’s biggest names are moisture-sensitive; the schedule is a quality document, not a suggestion — see drying plastics before moulding.

What we run, honestly

Kruger moulds thermoplastics only — both sides of the divide, every day: polypropylene and PPCP, 20 per cent glass-filled polypropylene, nylon 6 and 6/6 in unfilled, 15 per cent and 30 per cent glass-filled versions, HDPE and PET on the semi-crystalline side; polycarbonate and ABS on the amorphous side — on nine injection moulding machines from 60 to 1300 tonnes, under ISO 9001:2015, from 10 g to 5.2 kg. The full palette is on the materials guide. Send the drawing and the duty, and an engineer replies with material-and-moulding feedback within 48 hours on working days — including, when it is the honest answer, “this part wants the other side of the divide”.