Engineering guide

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. Thermoplastics divide again into amorphous polymers (ABS, polycarbonate, PMMA — low, predictable shrinkage, tighter tolerances) and semi-crystalline polymers (PP, polyethylene, nylons, PET — tougher and more chemically resistant, 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.

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, medical and industrial products today is thermoplastic; thermosets survive where extreme heat or electrical duty demands them. 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 resist chemicals and fatigue better: fuel, acids and detergents that attack amorphous parts leave PP and nylon largely untouched.

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 notch sensitivity. 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. Nylon’s one demand is dryness: it drinks moisture from the air and must be dried (typically around 100 °C for ~4 hours to below 0.15% moisture) or parts emerge brittle with streaked surfaces. 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. 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.

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.