Choosing a material for functional parts: load, heat and chemical resistance

A part is functional the moment it has a job to do beyond looking right. A bracket that holds weight, a housing that sits in a hot engine bay, a clip that lives outdoors for years — all of these only work if the plastic they're printed in matches what they're asked to survive. Get the geometry right and the material wrong, and the part fails anyway. Just later, and more expensively.

This is the question we ask on almost every quote for a working part, and it usually matters more than the design itself. The same bracket, printed in two different materials, can be the difference between a part that lasts years and one that cracks after a fortnight in the sun.

What a functional part actually has to survive

Three things determine whether a printed part holds up: the load it carries (steady weight, a sudden impact, or repeated flexing), the heat it sees (ambient, engine-adjacent, or somewhere closer to a kettle), and what it's exposed to — sunlight, moisture, oils, cleaning chemicals. Most part failures we're asked to diagnose trace back to one of these being ignored, not to a bad print. A bracket that's perfectly rigid on the bench can soften, warp or turn brittle well within its working life if nobody asked what it would actually sit next to.

The everyday materials, ranked by what they resist

PLA is the default for a first prototype: cheap, easy to print accurately, fine for checking a part fits and looks right. It has almost no place in a finished functional part — it softens in the region of 55–60°C and has no meaningful UV resistance, so a PLA bracket left in a car or a shed is a matter of when, not if, it fails. We use it for rapid prototyping to confirm geometry, then reprint the real part in a material that will actually survive its job.

PETG is the workhorse for functional indoor parts. It's tougher than PLA, shrugs off mild chemical exposure such as cleaning products or light oils, and resists the warping that catches out other materials on a large flat part. If the brief is 'this part lives inside, carries some load, and just needs to not fail', PETG is usually the answer — it's also the more affordable option, so we don't upsell past it when it's genuinely enough.

ASA is the material we reach for the moment a part goes outside. It resists UV degradation and weather in a way PLA and PETG simply don't — the difference shows up as chalking, fading and brittleness within months rather than years. We printed a replacement toolbox cover for a daily-commuter motorbike in ASA for exactly this reason: it lives outdoors, in direct sun, for the life of the bike.

Polycarbonate is what we specify when a part needs real strength, impact resistance or higher heat tolerance — brackets under mechanical load, housings near a heat source, anything that has to survive an impact without shattering. Heat deflection is well above PETG and ASA, into the region where PLA has long since given up. It's harder to print well and costs more, so we only recommend it when the job genuinely needs it.

TPU is the flexible option — gaskets, grips, vibration mounts, anything that needs to bend, compress or absorb an impact rather than resist it outright. It's the wrong choice for anything structural, and the right one for anything that needs to give.

ABS sits close to polycarbonate on heat resistance but warps more readily during printing and lacks ASA's UV stability, so we use it selectively — usually indoors, and rarely as our first recommendation.

When you need more stiffness: carbon-fibre-reinforced filaments

All of the materials above are unfilled plastics. Add chopped carbon fibre and the mechanical picture changes: stiffness goes up sharply, warping tends to go down because carbon fibre lowers thermal expansion, and the surface turns matte and slightly fibrous rather than glossy. We print CF-filled grades routinely for parts where deflection under load matters more than raw impact toughness.

PETG-CF takes PETG's chemical resistance and ease of printing and adds real rigidity — a good fit for jigs, fixtures and brackets that need to stay flat and stiff without stepping up to a harder material.

ASA-CF does the same for outdoor and structural parts: UV stability plus a large jump in flexural stiffness, useful for load-bearing brackets and larger flat panels that would otherwise sag or warp.

PPS-CF is the outlier — a genuinely high-performance engineering plastic, not just a stiffened commodity one. It holds shape and strength close to 200°C continuously, shrugs off fuels, oils and most solvents, and is inherently flame-retardant. It needs a printer capable of much higher nozzle and chamber temperatures than the rest of our range, so lead times and cost are higher — we reach for it when the job is genuinely under-bonnet or chemical-exposure territory, not as a default upgrade.

One practical point across all three: carbon fibre is abrasive. We print CF-filled materials on hardened nozzles and factor that into the quote rather than surprise you with it later.

The same bracket, two different jobs

Take a mounting bracket of identical geometry. Sitting inside a control cabinet at room temperature, PETG is more than adequate and the cheaper, faster option. Move the same bracket outdoors — on a trailer, a piece of garden equipment, a vehicle — and PETG is the wrong call: UV will find and exploit any weakness within a season, and ASA is the only sensible choice. Put it in an engine bay a few centimetres from a heat source and both are wrong; that's a polycarbonate job, sometimes a glass-filled grade if the load is significant. The geometry never changes. The environment decides the material, every time.

What we can't do

It's worth being straightforward about the ceiling. Unfilled FDM thermoplastics don't match metal for continuous structural load or sustained heat much above 100°C, though PPS-CF pushes that ceiling much higher for the jobs that genuinely need it. If a part needs more than that, printing isn't the right process and we'll say so rather than sell you a part that fails on the bench. For very high fatigue-cycle applications — living hinges under constant flex, gears under sustained load — we stock nylon/PA, which handles that kind of repeated flex far better than anything above. And food-contact or medical-grade requirements need certification we don't offer as standard: ask, and we'll tell you plainly whether we can help or point you elsewhere.

Brief the environment, not just the shape

The fastest way to get the right material on your part is to tell us more than the shape. If it needs designing from scratch, start with custom CAD, and alongside the file tell us three things: does it carry load, where does it live, and what's it exposed to. That's usually enough for us to specify a material with confidence — and if we're not sure, we'll ask rather than guess.