"Outdoors, therefore ASA" is the received answer and it is too blunt. PETG is rated good outdoors and ASA excellent, and both of them will still be recognisable years after a PLA part has gone chalky. Ultraviolet resistance is not what separates them in practice.
What separates them is temperature — specifically, the temperature a dark part reaches when the sun is on it, and whether that part is carrying a load at the time.
The sun heats parts far more than it degrades them
Air temperature is a poor guide to what a mounted part experiences. A black plastic fitting in direct summer sun runs dramatically hotter than the air around it, and dark car exterior surfaces are routinely measured well into the seventies of degrees Celsius on a clear day. Light-coloured parts in the same place run substantially cooler, and shaded parts barely rise at all.
Now put the stored deflection figures next to that. PETG softens between 68 and 80 °C; ASA between 90 and 105 °C. A dark PETG part on a south-facing wall in midsummer is not comfortably inside its range. A white one on a north wall never gets close to the edge of it.
This is why the same material can be a five-year success on one house and a sagging disappointment on another, and why so much outdoor-material advice contradicts itself. The variable that decides it is where the part is mounted and what colour it is.
Load is the multiplier
A material's deflection temperature is measured under a defined bending stress, and the reason that matters is that heat and load compound. A PETG bracket at 60 °C carrying nothing will sit there indefinitely. The same bracket at the same temperature holding a camera will slowly, permanently sag — creep, not failure, and it does not announce itself.
So the question is not "will it get hot", it is "will it be hot and loaded at the same time". A hose clip, a cable guide or a sensor housing is barely loaded and PETG is fine hot. A hinge, a mount or anything cantilevered is loaded continuously, and that is where ASA's higher deflection band is being bought.
Creep is also the reason a part that survived its first summer is not proof of anything. It deforms slowly, at a rate that rises steeply with temperature, so the failure arrives in the second or third year rather than the first — by which time whoever printed it has concluded the material was fine and has printed six more.
What ASA costs in machine terms
ASA needs an enclosure — the stored requirement is required, not helpful — and a bed at 95–110 °C. PETG asks for neither: a 70–85 °C bed and an open frame are enough, and an enclosure only makes large parts slightly better behaved.
If you own an open-frame printer, that is the end of the conversation. ASA is not a material you can print reliably without a chamber, and a warped, split ASA bracket is worse outdoors than an intact PETG one.
The same bracket, both ways
A 150 g outdoor bracket, 6 hours of printing. Since the parts this comparison decides tend to be garden and roof fittings, this one is costed at the United Kingdom unit rate of 34 US cents rather than the American average — a deliberately unflattering choice for the machine that draws more power. The shared assumptions:
- Machine time charged at 50 cents for each hour the printer is occupied.
- Everything else — grams, hours, your labour — held identical between the two.
In PETG at $24/kg on an open-frame machine drawing 100 W, with 8% of attempts written off: $7.40 per good part — $3.60 of filament, 20 cents of electricity, $3.00 of machine time, 59 cents against failures.
In ASA at $30/kg on an enclosed machine with an active chamber drawing 170 W, and 15% written off because a large warp-prone part genuinely fails more often: $9.23, of which $4.50 is filament, 35 cents electricity, and $1.38 the failure surcharge. The 34-cent tariff was picked to be unflattering to the enclosed machine, so it is worth undoing in the cost calculator: dropped to the American average, the power penalty roughly halves and the ordering does not change.
Note where the money went. The filament premium is under a dollar. The electricity term did rise — the enclosed machine draws far more, and at a British rate that shows — but it rose by 15 cents. The surcharge for scrapped prints rose by more than five times that, and it is the largest single difference between the two columns. Costing this comparison as a price-per-kilogram question gets the answer wrong; costing it as an electricity question gets it wrong in the other direction.
A rule you can apply at the wall
Walk to where the part will be mounted and answer three questions.
- Does the sun hit it directly for hours? If not, PETG, whatever colour it is.
- Is it dark, or does it have to be? Black outdoor parts in full sun are the case ASA exists for. A white or light-grey PETG part in the same position is a genuinely different thermal problem.
- Is it holding something up? Constant load plus heat is creep, and creep is the failure PETG suffers first.
Two yeses and ASA is worth the enclosure. Two noes and ASA is money spent on a property the part will never call on.
The middle path, if you cannot enclose
Carbon-filled PETG sits between them: better dimensional stability, a slightly higher deflection range than plain PETG, and still no enclosure requirement. It will not reach ASA's numbers, but it moves PETG in the right direction for large flat outdoor parts, and what the fibre changes covers what you give up for it — starting with transparency and impact resistance.