Short answer: Heat the acrylic in a forced-air oven to about 180 °F (80 °C), hold it for about 1 hour per millimeter of thickness with a 2-hour minimum, then cool it slowly and don't remove it until the oven is below 140 °F (60 °C). That's ACRYLITE's schedule. Trinseo's Plexiglas manual allows higher temperatures for shorter times, for example about 2 hours at 230 °F (110 °C) for 1/4 in. (6 mm) sheet. Anneal after all cutting, machining, and polishing, and before cementing.
Key facts
- Temperature: 180 °F (80 °C), just below the deflection temperature (ACRYLITE).
- Time: 1 hour per mm of thickness; at least 2 hours for thin sheet.
- Cooling: slowly; for parts over 8 mm, cooling hours = thickness in mm ÷ 4. Remove below 140 °F (60 °C).
- Oven: forced-air circulation, held within ±10 °F (±6 °C).
- When: after laser cutting, machining, flame polishing, buffing, or bending, and before solvent cementing or painting.
- Prep: remove paper masking first; support parts fully so they don't sag.
Why anneal acrylic?
Cutting, machining, polishing, and bending all leave stress locked into acrylic, concentrated at edges and bends. On its own the stress is invisible. Add a solvent (cement, paint, alcohol, glass cleaner) and the stressed areas craze into a web of fine cracks, sometimes days later. Annealing heats the part just enough for the plastic to relax, then cools it slowly so no new stress forms. ACRYLITE notes that annealing reduces crazing from solvents and water absorption and cites studies showing cemented joint strength rising by more than 50%. The Plexiglas manual calls proper annealing one of the most effective single measures for good service life. Background: why laser-cut acrylic cracks later and what is crazing.
ACRYLITE annealing schedule
| Thickness | Hold at 180 °F (80 °C) | Cooling |
|---|---|---|
| 1/16 to 1/4 in. (2 to 6 mm) | 2 to 6 hours (1 hr per mm, 2 hr minimum) | Same time as heating, about 28 °F per hour |
| 3/8 in. (9.5 mm) | about 9.5 hours | about 2.5 hours, about 22 °F per hour |
| Over 8 mm | 1 hr per mm | thickness in mm ÷ 4 hours |
| Surface-machined only (no cemented joints) | at least 2 hours | same as heating, at least 2 hours |
Plexiglas annealing schedule (higher temperature, shorter time)
Trinseo's Plexiglas manual gives heating times in a forced-air oven for Plexiglas G (cast) and MC (extruded). The times include warming the part through. Use the highest temperature that doesn't distort your part; if it distorts, drop to the next column.
| Max thickness | 230 °F (110 °C) | 221 °F (105 °C) | 212 °F (100 °C) | 203 °F (95 °C) |
|---|---|---|---|---|
| 0.060 to 0.177 in. | 1.5 hr | 3.5 hr | 7.5 hr | 24.5 hr |
| 0.236 to 0.354 in. | 2 hr | 4 hr | 8 hr | 25 hr |
| 0.472 to 0.708 in. | 3 hr | 5 hr | 9 hr | 26 hr |
| 0.944 in. | 4 hr | 6 hr | 10 hr | 27 hr |
Cooling rates to 120 °F in the same manual: about 140 °F per hour for sheet up to 0.177 in., 54 °F per hour up to 0.354 in., 25 °F per hour up to 0.708 in., and 18 °F per hour at 0.944 in. Formed parts and extruded sheet are more likely to distort at the high end, so test a sample first. The manual suggests staying about 10 °F below the temperature where a part deforms by 1%.
Step by step
- Finish all fabrication first: cutting, drilling, machining, polishing. Annealing before polishing wastes the bake.
- Clean and dry the parts. Remove paper masking and spray masking; plastic film may stay on per ACRYLITE.
- Support every part flat on a smooth tray or glass, with raised sections supported too. Leave space between parts for air.
- Orient drilled parts so air flows through the holes.
- Bring the oven to temperature and hold for the scheduled time.
- Cool slowly in the oven. Many shops set the oven to ramp down overnight.
- Remove below 140 °F (60 °C). Pulling parts out hot can undo the benefit.
- If you cemented parts, wait at least 6 hours after bonding before annealing so trapped solvent doesn't bubble.
Can I anneal acrylic in a kitchen oven?
ACRYLITE says a restaurant-style oven can work if it has air circulation and accurate temperature control. Most home ovens swing well past their set point and heat unevenly from the element, which can warp or bubble parts. If you try, use an oven thermometer, a convection setting, a sheet of glass or a thick tray between the element and the parts, and never exceed the schedule. Ventilate: the Plexiglas manual warns that acrylic releases monomer vapor if heated above 350 °F without ventilation, and acrylic is combustible. Use an oven you don't cook food in.
What if I can't anneal?
- Choose cast sheet for parts that will be glued; it resists crazing better than extruded.
- Machine glue edges instead of laser cutting them, since routing and sawing leave less stress. See laser-cut vs router-cut edges.
- Use the least solvent that makes the joint, and keep it off flame-polished or laser-cut faces. See how to glue acrylic.
- Avoid solvent-based cleaners on stressed parts entirely.
Planning a bonded box or case? We cut and assemble them; see acrylic fabrication.
Questions
Should I anneal before or after flame polishing?
After. Flame polishing adds stress to the edge, so anneal last, then cement. See how to polish acrylic edges.
Is annealing different for extruded acrylic?
The ACRYLITE schedule applies to its sheet generally, and Plexiglas lists the same table for cast and extruded. Extruded sheet shrinks more if overheated, so stay at the lower temperatures and test a sample.
Can annealing fix crazing that already happened?
No. Annealing prevents crazing; it doesn't close cracks that have formed. Crazed areas need to be sanded and polished out or the part replaced.