Forming
The radius is not a preference. Below about eight times the thickness, acrylic will craze, thin or whiten at the bend, and no amount of care in the oven will prevent it.
Line bending is the standard way to form acrylic: a strip heater softens a narrow line, the part is folded against a former and held until it cools. It is fast, cheap and repeatable, within limits. Those limits are set by thickness, radius and the amount of stress the part can carry afterwards.
The working minimum inside radius is about eight times the sheet thickness for a bend that has to look right and not craze in service.
| Sheet thickness | Absolute minimum radius | Recommended radius | What a tighter bend does |
|---|---|---|---|
| 3 mm | 12 mm | 24 mm | Outside surface crazes |
| 5 mm | 20 mm | 40 mm | Visible thinning on the outside wall |
| 8 mm | 32 mm | 64 mm | Crazing and a milky outside face |
| 10 mm | 40 mm | 80 mm | Often needs local heating on both sides |
| 15 mm | 60 mm | 120 mm | Requires controlled local heating, not a strip heater |
| 20 mm | 80 mm | 160 mm | Usually machined or hot-formed, then annealed |
Tighter than eight times the thickness is possible and we do it for enclosures, but the bend must then be annealed, and the outside of the bend will be slightly thinner and slightly less clear. It is a decision, not a default.
| Defect | Appearance | Cause |
|---|---|---|
| Whitening or crazing | Milky band along the bend | Radius too tight, or the material too cold when folded |
| Bubbles inside the bend | Small voids in the material at the bend | Overheating: the sheet has absorbed too much heat |
| Wavy outside surface | Rippled or orange-peel texture | Heating line too wide, or the part held too long at temperature |
| Thinning on the outside | Visible step in wall thickness | Radius too tight; material stretched beyond its limit |
| Distortion next to the bend | The flat area is no longer flat | Heat spread beyond the bend line |
| Crack on cooling | A split along the outside of the bend | Part cooled under stress, or folded while too cold |
If a part is designed as a folded shape, the flat pattern needs a bend allowance, because the material on the inside of the bend compresses and the outside stretches. The neutral axis sits at roughly 40 to 50 percent of the thickness from the inside face for acrylic.
In practice a fabricator works from a developed flat length rather than the finished dimensions, and this is where drawings go wrong: a drawing that gives outside dimensions of a folded part does not define the flat part. Send the finished shape as a STEP file, or state the flange lengths and let us develop the flat.
A bend has stress concentrated on the outside radius. Annealing the part after bending relaxes it and substantially reduces the chance of crazing at the bend if the part is later cleaned with a solvent or sees temperature cycling. For a display piece that lives indoors it is usually unnecessary. For an enclosure, a guard or a laboratory part, it is the difference between one that lasts and one that crazes.
BEND: R40 INSIDE, 90° ±0.5° — the radius, not just the angle.BEND LINE MARKED ON FLAT PATTERN if you supply a DXF, because a flat DXF alone does not define a bend.ANNEAL AFTER BENDING where the part will see solvent or temperature cycling.NO VISIBLE MARK ON FACE A: a former can leave a faint mark, and a masked face avoids it.Bends can also be made on a CNC press brake with a heated tool, which holds the angle more accurately and gives a very consistent radius. For a part with several bends that have to align, this is worth asking about.
About 20 mm inside radius as an absolute minimum, and 40 mm for a bend that has to stay clear and uncrazed. Tighter than 20 mm will craze the outside surface.
The sheet was folded before it was hot enough, or the radius was too tight for the thickness. The whitening is crazing, micro-cracks on the outside of the bend, and it cannot be polished out.
No. Cold bending produces crazing and residual stress and the part will crack. Acrylic has to be heated to around 150 degrees at the bend line to form.
About ±0.5 degrees on a line bender and ±0.2 degrees on a controlled press brake. The radius is less accurate, typically ±10 percent, because the heated zone width varies.
It can be reheated and straightened, but the bend area will show as a faint band and the part will not return to its original flatness. Treat a bend as permanent.
Every bent part is quoted from a flat pattern, and the flat pattern comes from a bend allowance. Get it wrong by a millimetre and a box lid either will not close or rattles. The arithmetic is not complicated once the neutral axis is fixed, and for acrylic the neutral axis sits close enough to mid-thickness that the simple form is accurate to about 0.3 mm on normal work.
For a 90° bend: BA = 0.5 × π × (R + 0.5t), where R is the inside radius and t the thickness. The flat length of a two-leg part is leg A + leg B − 2(R + t) + BA.
| Thickness | Inside radius | Bend allowance | Setback per leg | Note |
|---|---|---|---|---|
| 3 mm | 3 mm | 7.1 mm | 6 mm | Minimum radius for line bending |
| 5 mm | 5 mm | 11.8 mm | 10 mm | The common display-stand bend |
| 8 mm | 8 mm | 18.8 mm | 16 mm | Radius below 1t risks crazing on the outside |
| 10 mm | 10 mm | 23.6 mm | 20 mm | Pre-dry the sheet before forming |
| 15 mm | 20 mm | 44.0 mm | 35 mm | Thick sheet needs R ≈ 1.3t or a wider heat zone |
Worked example. A 5 mm tray, 300 × 200 mm outer, 40 mm walls, inside radius 5 mm. Each wall is a leg of 40 − 5 = 35 mm measured to the tangent, and the bend allowance is 11.8 mm, so the blank for each 300 mm side is 35 + 300 + 35 − 2 × 10 + 2 × 11.8 = 353.6 mm. Cut it at 350 mm and the tray finishes 4 mm short; cut it at 360 and the walls stand 3 mm proud and the lid will not seat. This is why a bent part without a flat pattern in the drawing gets a DFM note back rather than a price.
Two practical corrections. The first is spring-back: acrylic returns about 1–2° after release at normal forming temperature, so a jig is set 1–2° past 90 and the part relaxes into square. The second is that the outside of a bend thins. At R = 1t on 8 mm sheet the outside wall loses roughly 8–10% of thickness, which matters if the bend is a structural corner or a thread sits near it — either thicken the corner or move the feature.
A STEP file or the flange lengths lets us develop the flat pattern with the right bend allowance, so the folded part fits first time.