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Injection moulding materials — thermoplastics, blends, alloys and composites
Injection moulding materials divide first into thermoplastics, which melt and re-solidify reversibly and dominate precision moulding, and thermosets, which cure irreversibly — the fork our thermoplastics-vs-thermosets guide walks in full. Thermoplastics divide again into amorphous polymers (ABS, polycarbonate, PMMA — low, predictable shrinkage, tighter tolerances) and semi-crystalline polymers (PP, polyethylene, nylons, PET — tougher in sustained load and chemical service, but higher shrinkage that demands better tooling and process control). Most engineering choices are then refinements: blends and alloys combine two polymers’ virtues; glass-filled compounds trade surface finish for stiffness and stability.
The materials used in injection moulding, on one table
| Material | Character | Typical duty | Watch for |
|---|---|---|---|
| PP / PPCP | semi-crystalline workhorse | battery containers, living hinges, chemical duty | higher, direction-dependent shrinkage |
| Glass-filled PP | stiffened PP | filter housings, structural covers | weld-line strength, abrasive on screws |
| ABS | amorphous all-rounder | enclosures, equipment housings | outdoor UV without stabilisation |
| Polycarbonate (PC) | transparent + tough | glazing, guards, coach windows | mandatory drying; stress cracks with specific chemicals |
| PC-ABS | the classic alloy | housings needing both parents | grade-specific behaviour — specify the grade |
| PA6 / PA66 (+GF15/GF30) | mechanical-duty family | gears, guides, fan blades | must be dried; notch-sensitive — radius the corners |
| HDPE | tough, chemically stubborn | tanks, caps, low-cost parts | shrinkage and warp on flat walls |
| PET | clarity + stiffness | food-grade containers | drying discipline |
Thermoplastic or thermoset?
A thermoplastic’s polymer chains are held by physical forces: heat softens it, cooling sets it, and the cycle can repeat — which is what makes runners regrindable and the process economical. A thermoset (epoxies, phenolics, vulcanised rubbers) cross-links chemically during moulding and can never be re-melted. Almost every precision component in automotive, electrical and industrial products today is thermoplastic; thermosets survive where extreme heat or electrical duty demands them — the full fork, machines and mould logic included, is in thermoplastics vs thermosetting plastics. Everything below concerns thermoplastics — the materials we run. Choosing between processes instead? Read ISBM vs injection moulding.
Amorphous vs semi-crystalline — the split that drives everything
Amorphous polymers solidify like glass: no crystal structure, so shrinkage is low (roughly 0.4–0.8%) and uniform — good for tight tolerances, flat walls and transparent parts. Semi-crystalline polymers organise into crystals as they cool: shrinkage is higher (often 1–2.5%) and direction-dependent, which is why nylon and PP parts warp when walls are uneven or cooling is careless — and why they reward a moulder who runs mould-flow analysis before cutting steel. The crystals are also why semi-crystalline materials hold up better in sustained load and chemical service: fuel, acids and detergents that attack amorphous parts leave PP and nylon largely untouched. The deeper physics — and the mould-temperature question that decides crystallinity — lives in our thermoplastics guide.
The families we run, and what they’re for
Polypropylene (PP) and PPCP: the workhorse — light, chemically stubborn, fatigue-resistant (living hinges), economical. We mould PPCP battery containers and covers that must resist acid for years. 20% glass-filled PP: the same chemistry, stiffened — our air-filter housings for automotive duty run in it, month after month since 2010.
ABS: the amorphous all-rounder — good impact, clean surfaces, easy processing; appliance and equipment enclosures live here. Polycarbonate (PC): transparency plus toughness — our polycarbonate coach windows replace steel on Indian Railways coaches; the trade-offs are higher melt temperatures, mandatory drying, and a documented sensitivity the datasheets phrase precisely: susceptible to stress cracks with specific chemicals, and to the long-term effects of hot water and steam — so PC’s chemical environment is specified, never assumed. PC-ABS blends split the difference: PC’s toughness at ABS’s processability — a classic alloy choice for housings.
Nylons (PA6, PA66) — unfilled, 15% and 30% glass-filled: the mechanical-duty family — wear resistance, strength at temperature, fatigue life. Our nylon door guides take daily coach-door abuse; glass-filled nylon fan blades hold their geometry at speed in HVAC and refrigeration units. Two disciplines come with the family. Dryness first: nylon drinks moisture from the air and pre-drying is, in the handbooks’ words, absolutely necessary — a few hours at around 80 °C is the literature’s floor, our dryers run it harder (typically ~100 °C for about 4 hours, to below 0.15% moisture), because moisture causes surface waviness and impaired mechanical properties. Geometry second: nylon and acetal parts are among the toughest a moulder can make, yet the standard references note they are notch-sensitive — scoring low on notched impact tests — which is why sharp internal corners are designed out and radii designed in. HDPE: chemical tanks, caps, tough low-cost parts. PET: clarity and stiffness — we mould food-grade PET containers.
Blends, alloys and composites — plain words
A polymer blend or alloy is two thermoplastics compounded into one material so a part gets both characters at once — PC-ABS is the everyday example, and blends and alloys has the full story. A polymer composite in the moulding shop usually means a glass-fibre-filled compound: 15–30% short glass fibre multiplies stiffness, dimensional stability and heat resistance, at the price of a rougher weld-line, more abrasive wear on screws and barrels (we run wear-protected plasticising units for exactly this), and higher clamping demand. If your part must stay flat, stiff and dimensionally honest at temperature — filled nylon or filled PP is usually where the conversation ends up.
Two questions everyone asks
What is the most commonly used injection moulding material? Polypropylene, by most counts of the world’s moulding tonnage — cheap, light, chemically stubborn and endlessly compoundable; that is why it heads the table above and our own machine hours. Which material should MY part use? The one whose datasheet survives your duty — which is why “polypropylene” is not a specification but a family name: with thousands of commercial compounds on the market, the answer is always a named grade from a named supplier, with the datasheet filed with the order. The working values per material — melt, mould and drying — are on our temperature and drying chart.
Choosing: the five questions that decide it
What chemicals and temperatures does the part live with? What loads — constant, impact, or vibrating? What tolerances actually matter (and which are habit)? Does it need clarity, colour or surface finish? And what will the part cost at your annual volume — material price times part weight times scrap discipline? Answer those five and the material shortlist usually writes itself; a drawing plus its duty description is enough for us to recommend and justify one. Material certificates ship with parts on request, and every batch traces back to a resin lot.
Undecided on material?
Kruger Industries moulds PP, PPCP, glass-filled PP, ABS, PC, PA6/PA66 (unfilled, GF15, GF30), HDPE and PET in Bengaluru, India — ISO 9001:2015, batch traceability, parts from 10 g to 5.2 kg. Send your drawing and duty conditions; an engineer replies with a material recommendation and DFM feedback within 48 hours on working days. Sizing a machine instead? Try the shot-size and clamping-force calculators.
