Pick the right filament for the job: heat, sun, load, flex, looks or easy
A decision table that starts from what the part must survive, not from the spool you own. Six questions, a worked example for a car phone mount, and the swaps that fail.
- Ask the job six questions in order: heat, sun, load, flex, looks, ease. The first hard requirement usually decides the material.
- Indoors and under about 50 °C, PLA. Anything tougher or slightly warm, PETG. Hot or sunny, ASA. Rubbery, TPU. Beyond that you are in enclosure territory.
- Your printer limits the choice as much as the job does: no enclosure rules out ABS/ASA for large parts, a Bowden extruder makes TPU painful, a brass nozzle rules out fibre fills.
- Design can compensate for material: thicker walls, more perimeters and a better orientation turn a PETG part into something that would otherwise need nylon.
The question is never “which filament is best”. It is “what does this part have to survive, and what printer is in front of me”. Lesson 5 described each filament; this lesson turns that into six questions and a table you can run through in a minute, before you load a spool. The limits quoted (PLA softening above about 60 °C, PETG usable below about 80 °C, ASA to about 93 °C, PC with a glass transition near 150 °C) come from the Prusa Knowledge Base and the Simplify3D materials guide.
Six questions, in this order
- Heat. Where will the part sit and how hot does that spot get: an air-conditioned room, a sunlit windowsill, a car interior, next to a motor or a hotend?
- Sun. Outdoors or behind glass for months? UV makes PLA brittle and yellows ABS; ASA is the UV material.
- Load. Does it carry weight, take impacts, flex repeatedly or rub against something? Stiff, tough and wear-resistant are three different properties.
- Flex. Must it bend and return, like a gasket, strap or bumper? That is a different family (TPU), not a setting.
- Looks. Fine detail and sharp corners favour PLA; a glossy surface favours PETG; a matte, “engineering” look favours fibre fills; a wood look is wood fill.
- Ease. How much time do you have, and what printer: enclosure or not, direct drive or Bowden, brass or hardened nozzle?
Go down the list and stop at the first question that rules materials out. Heat and sun eliminate the most; ease is the tie-breaker at the end.
The decision table
| The job says | First choice | Fallback | Avoid | Why |
|---|---|---|---|---|
| Indoors, room temperature, decorative or a test fit | PLA | PETG | — | Easiest, sharpest detail, cheapest |
| Indoors, functional, gets handled or clipped | PETG | PLA with more walls | — | Tougher, snap-fits survive |
| Warm spot up to roughly 70 °C (near a window, inside an appliance housing) | PETG | ASA | PLA | PLA softens past about 60 °C |
| Hot: car interior, near a motor, 80-90 °C | ASA | ABS | PLA, PETG | PETG is rated below about 80 °C |
| Hotter than about 100 °C | PC (blend) | fibre-filled nylon | everything else | Only PC and PA stay stiff there |
| Outdoors in sun for years | ASA | PETG (shaded) | PLA, ABS | UV resistance |
| Carries load, must not crack on impact | PETG | nylon, ABS | PLA | Toughness over stiffness |
| Rubs, slides, gear teeth, hinges | nylon | PETG | PLA | Low friction, abrasion resistant |
| Stiff and light, must hold its shape | PLA or CF-filled PETG/PA | PETG | TPU | Stiffness and dimensional stability |
| Must bend and recover | TPU | — | rigid plastics | Only a flexible can do this |
| Must look like wood | wood-filled PLA | PLA painted | — | Cosmetic, weaker |
| Water contact, planters, outdoor box in shade | PETG | ASA | PLA | Water and humidity resistant |
| Fine miniatures, sharp edges, small text | PLA | PETG | TPU, fibre fills | Detail and low stringing |
Food contact and children’s toys are their own topic: the plastic, the pigment, and the layer grooves that trap residue all matter, and no hobby filament comes with a guarantee. Treat those jobs as “wash-only, not for long contact” unless the filament maker states otherwise.
Then check the printer
The table assumes the printer can do it. Cross off anything your machine cannot:
| Printer has | Rules out, or makes hard |
|---|---|
| No enclosure | ABS and ASA beyond small parts, nylon, PC. Warping and split layers |
| Bowden extruder | Soft TPU (below about 90A). Partially flexible grades sometimes work |
| Brass nozzle only | Carbon, glass and metal fills. The orifice widens in a few hundred grams |
| 0.4 mm nozzle only | Wood fill clogs; it wants 0.5-0.6 mm |
| PTFE-lined hotend (not all-metal) | Sustained printing above about 250 °C, which covers PC and most nylon |
| No filament dryer or airtight storage | Nylon and PC in practice, since both print badly once damp |
If the job and the printer disagree, you have three honest options: change the design, change the printer, or send the one part out. Printing ABS open and hoping is not one of them.
Worked example: a phone mount for a car
The job: a clip that holds a phone in an air vent, in a car parked outside in a tropical climate, used daily for a year.
- Heat. A closed car in the sun gets hot inside, and the dashboard area hotter still. We cannot measure every car, so we design for the high end: assume the mount will see 70-80 °C regularly. PLA (soft above about 60 °C) is out. PETG (rated below about 80 °C) is marginal: it will creep under the phone’s weight on the worst days.
- Sun. Direct sunlight through the windscreen for a year. ABS yellows and goes brittle; PLA likewise. ASA stays.
- Load. A phone weighs around 200 g, plus road vibration and the clip being pulled open daily. Needs toughness, not just stiffness.
- Flex. The clip arm flexes slightly each time. That is a design matter (arm thickness), not a TPU job; the body stays rigid.
- Looks. Matte black is fine; detail is coarse.
- Ease. The printer on the bench is an enclosed CoreXY with a hardened nozzle, so ASA is available.
Result: ASA, nozzle around 250-260 °C, bed 100-110 °C, enclosure closed, room ventilated, no draft. Walls at 3-4 perimeters on the clip arm so the flexing part has solid material rather than infill.
Now the same job on an open bed-slinger with a Bowden extruder. ASA on an open printer is a gamble for a part this size. The honest fallback is PETG with the design adjusted: thicker arm (say 3 mm instead of 2 mm), the mount kept away from the windscreen, and the owner told it may sag on the hottest afternoons. Or print the small clip in ASA on someone’s enclosed machine and the rest in PETG. The material did not change the job; the printer changed the design.
A second, quick one
A snap-on battery cover for an indoor ESP32 sensor box: room temperature, no sun, clipped on and off a few dozen times. Heat and sun say nothing. Load says the snap tabs must flex a little without cracking. PETG, 2-3 perimeters, tabs oriented so layers run along their length. PLA would work until the tenth snap.
Common mistakes
- Starting from the spool you have and talking yourself into the job fitting it.
- Confusing stiff with tough. PLA is stiffer than PETG and still cracks first when dropped.
- Trusting “heat resistant” on the box without a number. Ask for the glass transition or a tested service temperature.
- Choosing nylon for strength, then printing it damp. The result is weaker than PETG.
- Forgetting the printer. ABS on an open machine, TPU through a long Bowden tube, carbon fibre through brass. Each works just long enough to waste a day.
- Fixing a design problem with a material. A 1 mm wall will snap in any plastic. Thicken it first, then choose.
With the material decided, the next step is making a new spool actually print well: temperature tower, flow, retraction and a saved profile, in lesson 7.
Sources
FAQ
Can I just print everything in PETG?
For a lot of hobby work, yes, and it is a sensible default once you have tamed stringing. It fails you in a hot car, in direct sun for years, when a part must bend like rubber, and when you need crisp fine detail, where PLA is sharper.
The job needs ASA but my printer is open. What now?
For small parts, try ASA with a draft shield in a still room. For large parts, redesign: PETG with thicker walls kept out of direct sun, or split the hot part off so only a small piece needs ASA. Or print it on an enclosed machine.
Is nylon always stronger than PETG?
Nylon is tougher and wears better, but a wet spool prints weaker than good PETG. If you cannot dry and store nylon properly, a well-printed PETG part with more walls is often the stronger real-world result.
Circuits and code were tested on sample boards. Boards differ in pin layout, so check your board's pinout before you power anything.