ASA: the plastic that lives outside
ASA is ABS that has made its peace with the sun: same toughness, same heat, one letter of chemistry different — and that letter is the whole reason to pay for it.
The short answer
Anything that lives outdoors and has to stay the shape and the colour it arrived in.
What "UV-stable" actually means
ABS is acrylonitrile butadiene styrene. The butadiene is a rubber phase, and it is what gives ABS its impact toughness — it is also what ultraviolet light attacks first. Once it breaks down, the colour goes, the surface chalks, and the toughness goes with it.
ASA is acrylonitrile styrene acrylate: the same recipe with the rubber swapped for an acrylic one that UV does not pull apart. You keep the toughness and lose the fading. That is the entire difference, and it is why the two print almost identically.
How long "outdoors" actually lasts
"ABS fades" deserves a timescale. Weathering is measured in accelerated cabinets — xenon-arc or fluorescent UV lamps that compress a year of sun into a few hundred hours — and the pattern across published testing is consistent rather than dramatic. ABS begins yellowing and shedding impact strength within the equivalent of a season outdoors. ASA comes out the far end with its colour and most of its toughness intact, which is why weathering-grade ASA, not ABS, is what car mirror housings, radiator grilles and outdoor fittings are moulded from.
There is a trade on day one, and it is smaller than it sounds. BigRep's figures put ASA's impact strength about 15 % below ABS, with tensile strength comparable between the two. Read those two facts together: ASA starts about a seventh less tough, and ABS loses far more than a seventh of its toughness after a summer in the light. The part that was stronger on the bench is the weaker one by September.
What you get
- Colour that stays put, and a surface that does not go chalky
- Impact toughness at ABS level — it dents and bends rather than cracking
- The same ~90 °C of heat resistance
- Rain, frost and temperature swings, without the part caring
What you give up
- A little more money than ABS: think of it as roughly a tenth on the quote
- A narrower choice of colours — worth checking before you commit to a shade
- The same printing constraints as ABS: enclosed chamber, warm, slow, and a lead time to match
Tip from the workshop. Check the colour before the design. ASA comes in fewer shades than ABS, so ask us what is on the shelf before you commit a brand colour to a part that lives outside.
Where it earns the difference
Plant labels and garden hardware. Sensor and camera housings on a wall. Mailbox and gate parts. Exterior parts on a car, a bike or an RC model. Anything screwed to a balcony rail. If you can see the sky from where the part will live, this is the material.
What it takes to print
The published windows sit close together, which is the practical half of "one letter of chemistry different":
| Setting | ABS | ASA |
|---|---|---|
| Nozzle | 220–250 °C | 240–260 °C |
| Bed | 90–110 °C | 90–110 °C |
| Chamber | warm | 45–60 °C ambient |
| Part cooling | little | near zero |
| Speed | moderate | 35–70 mm/s |
| First layer | brim | brim, 5–10 mm |
The chamber is the row that is not a preference. Both plastics shrink as they cool, and a part that cools unevenly pulls its own corners off the plate — so the air around the print has to stay warm enough that the layer below is still soft when the next one lands. ASA warps a little less than ABS, but "a little less" is not "not at all", and neither runs well on an open machine.
Part cooling near zero follows from the same thing: a fan aimed at the part is the fastest way to make it shrink unevenly, and it costs layer adhesion on a material chosen for toughness.
And both give off styrene while printing, which is the other reason they live in a closed, vented machine rather than on a desk.
Designing for it
Everything from the ABS side applies unchanged: about half a percent of shrink, warping that starts at wide flat bottoms, splits bonded with acetone, inserts instead of printed threads.
One thing is specific to outdoors, and it is not a UV problem at all: a dark part in direct sun gets genuinely hot — well past air temperature, and potentially past its own comfortable working range. ASA will not fade there, but a black bracket loaded in August can still creep. If the part carries a load in full sun, a lighter colour is free engineering.
When ASA is the wrong answer
If the part never leaves the house, ABS does the same job for less. If it has to flex rather than hold, that is TPU. If it has to survive a hundred degrees, that is PC. And if it is a display piece that will sit on a shelf, PLA prints sharper and costs a fraction.
Common questions
Does ASA need an enclosure? Yes, in practice. It shrinks as it cools like ABS does, and without warm still air around the print the corners lift off the plate and the layers split. It warps slightly less than ABS, which makes small parts forgiving and large flat ones no easier.
How long will an ASA part last outside? Longer than the reason you will eventually replace it. Weathering-grade ASA is specified for years of exterior service, and in practice a printed part outdoors is far more likely to fail at a screw hole or a thin wall than to fade.
Is ASA stronger than ABS? Not on day one — its impact strength is about 15 % lower, tensile about the same. Outdoors that reverses within a season, because ABS loses much more than that to the sun.
Can I just paint an ABS part instead? A UV-stable paint or clear coat does protect it, and for a one-off it can work out cheaper. But the coating becomes the part that ages: it chips, it needs redoing, and any scratch puts bare ABS back in the light. If the part will be handled, or you would rather not maintain it, buy the colour in the plastic.
Does ASA smell like ABS? Yes — both release styrene while printing. It belongs in a closed machine with the air going somewhere, not on a desk beside you.