Use ASA for anything that sits in direct sun and has to keep its shape and color for years. Use PETG if you have an open-frame printer and the part stays below about 60 °C under load. ABS is the odd one out: it needs the same enclosure as ASA, handles similar heat, and its own makers say it yellows and turns brittle outdoors.
Heat: compare at the same load or not at all
Heat deflection temperature (HDT) is where a test bar bends under a fixed load, and it is reported at 0.45 MPa (light load) or 1.8 MPa (heavy load). A figure without its load is half a figure. These are the ones we hold with both stated.
| Filament | HDT at 0.45 MPa | HDT at 1.8 MPa |
|---|---|---|
| Bambu Lab PETG HF | 69 °C | 62 °C |
| Prusament PETG | 68 °C | 68 °C |
| Bambu Lab ABS | 87 °C | 84 °C |
| Polymaker PolyLite ABS | 100 °C | 98 °C |
| Bambu Lab ASA | 100 °C | 92 °C |
| Prusament ASA | 93 °C | not stated |
Three things fall out of that table.
PETG gives you 62 to 69 °C. ASA gives you 92 to 100 °C. The gap is about 30 °C, and that gap is the main reason to put up with ASA’s printing demands.
ABS and ASA overlap. Bambu’s consumer ABS at 87 °C is below both ASA entries, but Polymaker’s PolyLite ABS matches Bambu’s ASA at 100 °C. “ASA handles more heat than ABS” is true for some pairs of spools and false for others. If heat is your deciding factor, read the data sheet for the exact product. Our ABS and ASA pages list each brand’s row.
Then there is a number I would not lean on. Prusa’s PETG material guide says PETG parts suit “most exterior use (with temperatures below 80 °C)”. Prusament’s own PETG data sheet puts HDT at 68 °C. A part that only has to exist, like a plant label, may survive above 68 °C. A part carrying any load will not hold its shape there. Take the data sheet figure as your ceiling and treat the 80 °C as the point where things have already gone wrong.
A worked choice: the same bracket in two places
Say you are printing a bracket that holds a 2 kg sensor housing on a fence post, and another copy for inside a south-facing greenhouse. Put a cheap thermometer where the part will live on the hottest afternoon you can find before you choose. We have no temperature figures of our own to offer, and guesses about “how hot things get in the sun” are where most bad material advice starts.
If the fence post location peaks at 45 °C, PETG has 17 °C of margin against Bambu’s 62 °C heavy-load figure. That is comfortable, and PETG prints on anything. If the greenhouse reads 65 °C, PETG HF is already past its 1.8 MPa figure and within 4 °C of its light-load one. The bracket will creep. ASA at 92 °C under heavy load still has 27 °C in hand. Same part, same printer, different answer, and the thermometer made the decision.
Notice also how much the load matters for a given spool. Bambu’s PETG HF loses 7 °C between light and heavy load and its ASA loses 8 °C. If your part is a cover or a cap, read the 0.45 MPa column. If it is a bracket, hook or clamp, read the 1.8 MPa one.
UV and weather, in the makers’ words
This is where evidence gets thinner than forum posts suggest. We have not tested anything, and most data sheets carry no weathering figures at all. Here is what the makers themselves put in writing.
Prusa’s ASA guide calls ASA “suitable for outdoor use, due to its high UV light and temperature resistance”, lists high UV resistance as its first advantage, and says that compared with ABS it is more UV resistant, warps less and smells less.
Polymaker’s PolyLite ASA page is the only one I found with numbers. It reports a two-year natural aging test in which the ASA showed a color change (delta E) below 2.5, and an accelerated test to 8,000 kJ/m² of irradiation with delta E below 5 and “minimal tensile strength loss”, which it describes as better than ABS. It states that parts exposed to sun, rain and temperature swings “maintain integrity for a minimum of two years”. That is a maker testing its own product, with no method detail published alongside, so weigh it accordingly. It is still more than anyone else offers.
Prusa’s ABS guide lists “lacks UV resistance (unlike ASA)” among the drawbacks and adds that ABS parts used outdoors turn yellowish and more brittle over time.
For PETG, Prusa says it is suitable for mechanical parts indoors and outdoors and notes that it prints its own printer parts from it. It makes no UV claim in either direction. Nor does any PETG data sheet in our database. So the fair summary is: ASA has a maker-published UV case, ABS has a maker-published UV warning, and PETG has silence. PETG may well last outside for years. I just cannot point you to a manufacturer figure that says so.
What each one asks of your printer
| PETG | ABS | ASA | |
|---|---|---|---|
| Nozzle | 230 to 260 °C | 210 to 270 °C | 240 to 270 °C |
| Bed | 65 to 90 °C | 80 to 100 °C | 70 to 115 °C |
| Enclosure | not needed | recommended to required | required for anything large |
Those ranges come from the data sheets behind our temperature table. On enclosures: eSUN’s PETG page says it needs no heated enclosure. Bambu Lab’s filament guide puts ABS and ASA in the group that needs higher bed and chamber temperatures to suppress warping and layer separation, and lists an enclosed printer for them. It lists PETG as fine on open-frame or enclosed machines, with a note that an enclosure is still recommended so that a cold room does not weaken layer bonding. Bambu’s ABS data sheet gives a chamber target of 45 to 60 °C.
Polymaker is the most specific about ASA. An open printer with a 90 °C bed can manage models smaller than a fist. Bigger prints need an enclosure at roughly 40 to 50 °C, and anything beyond 15 to 20 cm in any direction wants an actively heated chamber at 70 °C or more. Prusa asks for a bed of at least 100 °C for both ABS and ASA, and suggests the slicer’s draft shield as a stopgap if you have no enclosure.
The bed requirement rules out some machines before you start. The Bambu Lab A1 mini tops out at an 80 °C bed and is open-frame, so it is a PETG machine for outdoor work. An enclosed printer such as the Bambu Lab P1S or Prusa CORE One+ opens up ASA. Filter the printer database by enclosure if you are shopping with this in mind.
Both Prusa guides list potentially dangerous styrene fumes as a drawback of ABS and ASA and tell you to ventilate; check the filament maker’s safety data sheet before printing either indoors.
Which I would print
Small ASA parts are less frightening than their reputation, going by Polymaker’s fist-size rule. If you own an enclosed printer, ASA is the default for outdoor parts and there is little reason to reach for ABS at all: similar heat figures, same printing hassle, and a UV warning from the people who sell it.
If your printer is open-frame, print PETG, keep the part out of hot enclosed spaces, and design it a little thicker than you think you need. Dry the spool first either way: the drying chart has each maker’s temperature for PETG and ASA. If the choice is between PETG and PLA instead, the PLA vs PETG guide covers that pair.
