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What is the most common injection moulding plastic?
For general-purpose moulded parts, Komaspec's materials guide names ABS and polypropylene as the most common choices: ABS for enclosures and cosmetic housings, polypropylene for containers, living hinges and low-cost chemical resistance.
Other publishers cast the net a little wider. Formlabs lists ABS, polystyrene, polyethylene, polycarbonate, polypropylene and TPU among the most popular injection moulding materials. For a first product, though, the real decision is usually ABS or polypropylene, with polycarbonate, nylon and acetal as the step up when the part needs more strength, heat resistance or wear performance.
How ABS and polypropylene compare
| Factor | ABS | Polypropylene | Source |
|---|---|---|---|
| Polymer type | Amorphous | Semi-crystalline | Komaspec |
| Typical mould shrinkage | about 0.4 to 0.7 % | about 1.5 to 2.5 % | Komaspec materials guide |
| Dimensional stability | Low shrink, high stability | Shrink and warp can occur | Protolabs |
| Typical uses | Cosmetic parts, handheld devices, housings | Living hinges, fans, snap-over lids | Protolabs |
| Chemical resistance | not highlighted | Resists acids and bases | Protolabs |
| Watch out for | Knit lines, sink and voids in thick areas | Voids in thick sections; homopolymer can be brittle in the cold | Protolabs |
| Density | not given | around 0.90 g/cm³ | Komaspec |
The summary is simple. ABS is the safer choice where the part must look good and fit closely; polypropylene is the safer choice where it must flex, seal against chemicals, or be as light and cheap as possible.
Why ABS is the default for housings
Protolabs describes ABS as tough and impact-resistant, with low shrink and high dimensional stability, and lists housings, handheld devices and cosmetic parts as typical uses. Komaspec gives impact strength, dimensional stability and finish quality as the reasons enclosures are usually ABS. Because it is amorphous, ABS shrinks fairly evenly, which helps snap fits, screw bosses and mating halves line up.
Its weak points, per Protolabs, are visible knit lines and sink or voids in thick areas. Protolabs suggests a PC/ABS blend can reduce sink, and describes PC/ABS as adding toughness, dimensional stability and heat resistance over plain ABS.
Why polypropylene is the default for containers and hinges
Protolabs calls polypropylene wear-resistant and flexible, with very high elongation and resistance to acids and bases. Its living hinge article says polyethylene and polypropylene are ideal for living hinges because of their high melt flow, and gives a starting hinge thickness of 0.012 in (about 0.3 mm). Protolabs lists integral or living hinges and snap-over lids among polypropylene's typical uses.
The trade-off is dimensional. Komaspec explains that semi-crystalline resins like PP shrink more and less evenly, with more part-to-part scatter, and it attributes most PP warpage to that uneven shrinkage. Protolabs also notes that polypropylene homopolymer can be brittle in cold temperatures, while copolymer grades resist impact better.
What it costs
Published resin prices are broad and change with the market, so use them only as orders of magnitude and get current prices from your moulder.
| Publisher | Figure | Notes |
|---|---|---|
| Formlabs | about $1 to $5 per kg | thermoplastic pellets generally |
| Xometry | about $0.90 to $2+ per pound | plastic resins generally |
The cheapest pellet does not always make the cheapest part. Komaspec makes three points:
- Density. Polypropylene is around 0.90 g/cm³ and polycarbonate about 1.20, so the same part in PC needs about a third more material by mass.
- Cycle time. Semi-crystalline resins need more cooling time to crystallise. Komaspec's example is two extra seconds on a 25-second cycle, which adds 8 % to machine cost on every part.
- Scrap. A grade that runs at 1 % scrap can cost less overall than a cheaper resin that runs at 5 %.
Ask for a cost per good part, not just a resin price per kilogram.
How to choose
- List what the part must do: fit, finish, flex, chemical exposure, temperature, impact, and any regulatory requirement.
- If it needs a tight fit or a cosmetic surface, start with an amorphous resin such as ABS; Komaspec says amorphous resins are generally better for tolerances that cannot move.
- If it needs chemical resistance or fatigue life, such as a hinge, start with a semi-crystalline resin such as polypropylene, as Komaspec advises.
- If neither fits, look at the step-up resins. Protolabs describes polycarbonate as strong and highly impact-resistant but with poor chemical resistance, acetal as stiff and tough with good lubricity, and nylon as strong but prone to warp and moisture absorption.
- Pick an exact grade and get its data sheet; for food contact, medical or flammability needs, confirm the grade's compliance with the supplier.
- Fix the resin before the mould is cut, because the cavity is sized for that resin's shrinkage.
These are planning comparisons from published guides. The resin data sheet and your moulder's experience with your part decide the final choice.
Questions buyers ask
Is ABS or polypropylene cheaper?
Resin prices move, so ask for current figures. Komaspec notes PP's low density means fewer kilograms per part, which works in its favour. The finished part price also depends on cycle time, scrap and tooling, so ask the moulder to quote both if you are undecided.
Can I change from ABS to polypropylene after the tool is made?
Komaspec says generally not without modifying the tool, because PP shrinks considerably more than ABS. Switching between grades of the same family is sometimes possible, but measure trial parts before approving it.
What should I use for a box with a flip-top lid?
A one-piece box with an integral hinge points to polypropylene, which Protolabs names as an ideal living hinge material along with polyethylene. If the lid must also look highly cosmetic, discuss texture and colour with the moulder early.
When should I move beyond ABS and polypropylene?
When the part needs more heat resistance, clarity, stiffness, strength or wear performance than either offers. Protolabs' resin descriptions point to polycarbonate for impact and clarity, acetal for gears and sliding parts, and nylon for strength, especially when glass-filled.
Sources
- Komaspec, Injection molding materials: https://www.komaspec.com/about-us/blog/injection-molding-materials/
- Protolabs, Selecting a thermoplastic for your injection-molded part: https://www.protolabs.com/resources/design-tips/selecting-the-right-plastic-for-your-parts/
- Protolabs, Living hinge basics for injection molding: https://www.protolabs.com/resources/blog/living-hinge-basics-for-injection-molding/
- Formlabs, Injection molding cost: https://formlabs.com/blog/injection-molding-cost/
- Xometry, Injection molding cost: https://www.xometry.com/resources/injection-molding/injection-molding-cost/