Short answer: Acrylic cold-curved into a frame without heat needs a minimum radius of about 180 times the sheet thickness for cast Plexiglas G and Optix, 300 times for extruded Plexiglas MC, and 330 times for Röhm's PLEXIGLAS GS and XT; for 1/4 in. (6 mm) sheet that's roughly 42 to 78 in. A heated line bend can be much tighter, but Röhm recommends an inside radius of at least twice the sheet thickness.
What's the difference between cold and hot bending?
- Cold curving: flexing a flat sheet into a gentle arc and holding it there with a frame, channel, or fasteners. No heat. The sheet stays under stress for as long as it's installed.
- Hot bending: heating the sheet until it's soft so it takes a new shape and holds it with little stress once it cools. This is how sharp folds and tight curves are made.
Röhm is explicit that cold line-bending (folding to an angle without heat) isn't possible with acrylic. Only gentle cold curves are.
Key facts
- Plexiglas G (cast): cold radius at least 180 × thickness (Plexiglas forming manual).
- Plexiglas MC (extruded): at least 300 × thickness.
- Optix-L (continuous cast): at least 180 × thickness (another passage of the same guide says 250 ×).
- Röhm PLEXIGLAS GS and XT: at least 330 × thickness.
- Hot line bend: inside radius at least 2 × thickness (Röhm); 2 to 3 × is good structural practice (Plexiglas).
- Coated (mar-resistant) sheet: can be cold-curved but not heat-bent; the coating cracks.
What is the minimum cold bend radius for acrylic?
It's a multiple of the thickness, and the multiple depends on the maker and grade. Here's what the published multipliers work out to:
| Nominal thickness | Actual (approx.) | At 180× (Plexiglas G, Optix) | At 300× (extruded Plexiglas MC) | At 330× (Röhm GS/XT) |
|---|---|---|---|---|
| 1/16 in. | 0.060 in. | about 11 in. | about 18 in. | about 20 in. |
| 1/8 in. | 0.118 in. | about 21 in. | about 35 in. | about 39 in. |
| 3/16 in. | 0.177 in. | about 32 in. | about 53 in. | about 58 in. |
| 1/4 in. | 0.236 in. | about 42 in. | about 71 in. | about 78 in. |
| 3/8 in. | 0.354 in. | about 64 in. | about 106 in. | about 117 in. |
| 1/2 in. | 0.472 in. | about 85 in. | about 142 in. | about 156 in. |
The 180× column matches Plexiglas's own table (21.2 in. at 0.118 in., 42.5 in. at 0.236 in., 85.0 in. at 0.472 in.). Treat these as minimums, not targets. If you don't know the exact grade, use the 300 to 330× figures, and give outdoor or warm installations more margin.
Why does the limit exist?
A cold-curved sheet is stretched on the outside face and compressed on the inside face, and that stress never goes away. Plexiglas says curving beyond the limits can craze the sheet because the stress exceeds what's recommended for a continuous load; Acrylite puts that continuous design limit at 1,500 psi for cast sheet and 750 psi for extruded. Above it, especially with time, heat, or chemicals, acrylic develops crazing: fine surface cracks that can grow into real cracks.
Things that make crazing more likely on a curved sheet:
- A tighter radius than recommended.
- Cleaning with window sprays, alcohol, or solvents.
- Holes, notches, or rough cut edges in the curved area.
- Sun and heat; strength drops as temperature rises.
- Extruded sheet, which makers rate for a larger radius than cast.
If a curve needs to be tighter than the cold limit, heat-form it instead.
What is the minimum radius for a hot bend?
Röhm's forming guide says a line bend's radius should be at least twice the sheet thickness; smaller radii cause excessive stress or creasing on the inside. It also recommends heating a zone 3 to 5 times the thickness wide; too narrow a zone over-stretches and thins the bend. For 1/4 in. sheet, that means an inside radius of about 1/2 in. or more and a heated strip around 3/4 to 1 1/4 in. wide.
For a crisp outside corner, fabricators cut a 90° V-groove along the inside of the line before heating. The Plexiglas manual notes this gives a sharp radius that isn't good structural practice, so save it for display parts. For large, smooth curves, the whole sheet is heated in an oven and draped over a form; see how to bend acrylic and thermoforming.
Need a tighter cold bend? Consider polycarbonate
Polycarbonate is far more flexible than acrylic and its makers allow much tighter cold bends, so for a curved window, guard, or cover bent into place without heat it's often the better material. Acrylic is clearer, more scratch-resistant, and easier to polish. Acrylic vs polycarbonate.
How we can help
We line-bend acrylic up to 1/4 in. thick for stands, holders, and guards; see acrylic bending. For a cold-curved panel, we can cut the flat sheet to size with holes placed outside the tightest part of the curve. Get an instant quote.
Questions
Can I curve acrylic into a frame without heating it?
Yes, if the radius is large enough: at least 180 times the thickness for cast Plexiglas G or Optix, and 300 to 330 times for extruded or Röhm sheet. Support it evenly along the curve, use a channel rather than bolts, and keep holes out of the most curved area.
Does thinner acrylic bend tighter?
Yes. The minimum cold radius scales with thickness, so 1/8 in. sheet can curve about twice as tightly as 1/4 in. sheet.
Can flat acrylic be rolled into a tube?
Not cold, except in very large diameters. Small tubes are made by extrusion or casting, or by heat-forming sheet around a mandrel. Buying acrylic tube is usually easier.
How tight can a curved acrylic sneeze guard or desk shield be?
A cold-curved 1/8 in. shield needs a radius of at least about 21 in. (cast, 180×) to 39 in. (330×). For a tighter wrap, the curve has to be heat-formed or the shield made from polycarbonate.