There is no honest way to make this a broad comparison, so here is the narrow one: ASA and ABS are close enough in every printing and mechanical respect that if you can run one you can run the other with a small temperature change, and the reason to pay more for ASA is sunlight. If your part never sees any, you are buying nothing.
What the swapped component does
Both plastics are three-part copolymers built around styrene and acrylonitrile. The third component is where they part company. ABS uses butadiene, a rubbery phase that gives it impact toughness and contains carbon–carbon double bonds. Those double bonds are what ultraviolet light attacks: the rubber phase oxidises, the surface chalks, the colour drifts yellow, and the toughness the butadiene was there to provide quietly leaves the part.
ASA replaces butadiene with an acrylate rubber that has no such bonds to break. The stored ratings say it in one word each — ABS is rated poor outdoors, ASA excellent — and the practical version is that an unpainted ABS bracket on a south-facing wall becomes brittle in a season or two while its ASA twin is still serviceable years later.
That is genuinely the whole of it. Everything else below is either identical or a footnote.
What does not change
Set the two side by side and the printing requirements are the same list:
- Bed at 90–110 °C for ABS, 95–110 °C for ASA — the same plate, the same adhesive habits.
- An enclosure is required for both. ASA warps exactly as readily; the acrylate does nothing for contraction.
- Stored contraction is 0.4 to 0.8% for both, so every ABS anti-warping measure transfers unchanged: brim, no draughts, no part cooling to speak of, chamfer the corners.
- Both need drying at 65 °C before a serious print.
- Both emit styrene. ASA is not the answer to a ventilation problem, and it is sometimes marketed as though it were.
If you have a working ABS profile, an ASA profile is that profile with the nozzle moved up.
The one printing difference worth the paragraph
ASA's stored nozzle window is 240–265 °C against ABS's 230–260 °C. Ten degrees is not dramatic, but it lands ASA closer to the ceiling of hotends rated to 260 °C, and on a machine at that limit you lose the top of the useful range — which is where layer adhesion on a large part lives.
The consequence shows up as a specific failure: an ASA part that looks perfect and then splits along a layer line partway up, usually on the tall thin section. If that happens, the chamber is too cool or the nozzle is too low, in that order. Its diagnosis is cracking and delamination, not a material defect.
What the premium actually costs on a real part
ABS retails at $18–30 per kilogram and ASA at $22–38. Applied to an actual job rather than a spool: a 250 g outdoor housing that occupies an enclosed machine for 9 hours at 130 W. Everything except the filament price is held constant between the two runs: the printer at 50 cents an hour, the meter at the 17.5-cent American average, and one job in ten scrapped.
In ABS at the middle of its range, $24/kg: $11.89 per good part — $6.00 of filament, 20 cents of electricity, $4.50 of machine time and a $1.19 failure allowance.
In ASA at the middle of its range, $30/kg: $13.56, of which $7.50 is filament. Everything else is identical.
So the ASA premium on this part is $1.67. Set against the housing failing in daylight and being printed twice, it is not a decision that deserves any thought. Set against an indoor part, it is $1.67 spent on a property the part will never use. Whether $1.67 is money well spent is a question about where the housing lives, not about the plastic, and the cost calculator opens on the ASA side of it.
When ABS outdoors is still defensible
Two cases, and one non-case.
A part that is shaded or enclosed — inside a housing, under an eave, in a wheel arch — is outdoors in the weather sense and indoors in the ultraviolet sense. ABS is fine there, and water alone does not bother it.
A part with a short intended life — a jig for one season, a mock-up, a prototype for a mould — will not be around long enough to chalk.
The non-case is a coated ABS part. A UV-blocking topcoat does work while it is intact, but it is a maintenance commitment: the first chip in the coating starts a local failure you will not see until the surface underneath goes powdery. If the part is going to live in the sun for years, buy the right polymer rather than a paint schedule.
What ends up in ASA in practice
The stored use list is a good guide to where the premium gets spent, and it is narrower than the marketing suggests: outdoor enclosures, garden and roof fittings, car exterior trim, antenna mounts. Every one of those is a part that is fixed in place, exposed all day, and irritating to replace — which is the combination that justifies buying a property you cannot see.
ABS's list runs the other way: interior automotive parts, enclosures, and anything destined for an acetone jar. Note that vapour smoothing works on both, since the styrene backbone is common to them, but the surface it produces on ASA is slightly less glossy for the same exposure.
Choosing, in one line
Outdoors and unpainted, ASA every time. Indoors, ABS, because the acrylate is a property you are paying for and cannot use — and if the real requirement was "tough outdoors without an enclosure", the answer is neither of these and is on PETG against ASA.