Indoor vs outdoor. Similar plastics. UV makes the difference.

ABS contains butadiene rubber for impact strength. Butadiene has double bonds (C=C) that are attacked by UV photons. UV oxidation: breaks molecular bonds, yellows color, increases brittleness, initiates surface cracks. Within 1-2 years outdoor: visible yellowing. 3-5 years: brittle, cracking. ASA replaces butadiene with acrylic elastomer (saturated, no double bonds) — UV-stable chemistry. Same impact performance, radically different UV behavior.

Paint provides UV protection for ABS underneath. For painted parts, standard ABS is adequate — paint blocks UV from reaching polymer. Many painted automotive interior and consumer products use ABS with UV protection from paint. Problem arises if paint scratches or chips — exposed ABS then degrades rapidly. For unpainted outdoor or for paint reliability concerns: ASA.

ASA filament available for FDM 3D printing. Same print parameters as ABS (enclosed chamber, 230-260°C nozzle, 100°C bed). Better UV resistance makes ASA popular for outdoor 3D printed parts — automotive accessories, garden hardware, outdoor enclosures. Cost premium over ABS filament 30-50%. For outdoor FDM applications, ASA is generally preferred choice.

ASA is the automotive industry standard for exterior trim: grilles, side mirrors, wheel arch flares, bumper trim, fender liners. Both painted and unpainted parts. Premium cost over ABS justified by 10+ year outdoor durability required for warranty. Major automotive producers specify ASA (often by trade names BASF Luran S or Ineos Styrolution Luran). For aftermarket accessories, ASA preferred.

Both ABS and ASA: moderate chemical resistance. Attacked by: aromatic hydrocarbons (toluene, xylene), chlorinated solvents, some alcohols in prolonged contact. Resistant to: water, weak acids, weak bases, oils, alcohols (brief contact). For chemical-contact applications, neither is ideal — consider PP, HDPE, or PPS instead. For general exposure, both acceptable.

ABS outdoor: yellows significantly within 1-2 years. Light colors (white, cream) show yellowing dramatically. Dark colors (black, gray) mask yellowing somewhat but still fade. ASA outdoor: maintains color 10+ years. Pigmented ASA maintains color even in light shades. For color-critical outdoor applications: ASA required. Dark-painted exterior: ABS + paint adequate if paint is UV-stable.