Quality
Flatness and bow: how flat acrylic actually is, and why it moves
Acrylic is never perfectly flat, and a panel that was flat on delivery can be curved a month later. Both are normal, until the part is optical or a door, where they are not.
Quality
Acrylic is never perfectly flat, and a panel that was flat on delivery can be curved a month later. Both are normal, until the part is optical or a door, where they are not.
Every flatness complaint we receive falls into one of four categories: the sheet was never flat to the customer expectation, the part bowed after machining, the installation does not support it, or heat has deformed it. They have different answers, and mistaking one for another wastes everyone's time.
| Material | Typical flatness | What it means in practice |
|---|---|---|
| Cast sheet, 3–6 mm | 1–2 mm per metre | A 1,200 mm panel may have a 2 mm wave; this is within normal supply |
| Cast sheet, 8–15 mm | 1–1.5 mm per metre | Flatter than thin sheet, but still not a surface plate |
| Cast sheet, 20 mm and above | 0.5–1 mm per metre | Thick sheet holds its shape better |
| Extruded sheet | Often flatter as supplied, less stable after machining | Good on day one, may move after cutting |
| Selected or optical grade | Quoted per part, often 0.2–0.3 mm across an aperture | Costs more and must be specified |
These are working figures. If a part has a flatness requirement, state the value and the measurement method on the drawing. "Must be flat" is not a specification and will be interpreted as normal sheet flatness, which may not be what was meant.
This is the most common and most misunderstood cause. Cast sheet is polymerised with stress locked in, balanced between the two faces. Machining a pocket, a slot or a large opening from one face removes stressed material from that side only, and the panel curves towards the machined face.
The effect is proportional to how much material is removed and how large the area is. A 5 mm panel with a 300 by 300 mm window routed out of it will visibly bow; the same window in a 10 mm panel with 2 mm walls will not.
| Cause | Pattern | Fix |
|---|---|---|
| Unsupported storage | Bows in the direction of the support gaps | Store flat and fully supported |
| Supports only at the ends | Sags in the middle under its own weight | Support at least at thirds |
| Heat from a light or a window | Bows towards the heat source | Move the light, shade the panel, or switch to LED |
| One-sided lighting | Bows towards the light | Ventilate both faces |
| Rigid fixing in a frame | Distorts at the fixings, waves along the edge | Slot the holes, use a compressible gasket |
| Sustained load (a shelf) | Creeps progressively and permanently | Reduce span, increase thickness, add a centre support |
Acrylic creeps. A shelf that is flat when loaded will sag over months under a sustained load, and the sag does not recover when the load is removed. A 10 mm shelf at 800 mm span carrying 15 kg will bow visibly within a year.
Design the span and thickness for the sustained load with a generous factor, or add a centre support, or use a different material for the shelf and acrylic only for the visible front. This is a design decision, not a fabrication defect.
FLATNESS: 0.3 mm MAX ACROSS 600 × 600 mm APERTURE, measured on a granite plateTHICKNESS: ±5% (selected sheet), because thickness variation makes a panel read as a lens.NO PRINT OR ETCH IN THE OPTICAL PATHMOUNT ON A COMPRESSIBLE GASKET, NOT CLAMPED, because clamping a flat panel into a non-flat frame reproduces the frame shape.A panel that will not sit flat on a granite plate is not flat. For a part in production we measure with a dial indicator on a surface plate and record the maximum deviation across the specified area. That measurement, on the inspection sheet, is what converts a vague requirement into an accepted part.
Yes. Standard cast sheet is flat to about 1–2 mm per metre, and a visible wave across a large panel is within normal supply. If the part needs better, it has to be specified and measured.
It can be re-flattened by even heating and cooling under weight, but it will bow again unless the cause, whether support, heat or machining stress, is removed first.
Drilling removes material from one face and may release stress unevenly, particularly if the holes are large or numerous. Annealing after machining reduces this.
Yes, considerably. Thick sheet is stiffer and holds flatness better; thin sheet follows whatever supports it and is much more sensitive to heat.
Support it on a compressible gasket, fix with slotted holes so it can move, and make sure the frame itself is flat. A panel clamped rigidly into a twisted frame becomes twisted.
The flatness a panel holds under its own weight is a function of thickness, span and support spacing, and it can be estimated before anything is cut. The numbers below are for cast acrylic sheet at 23 °C, supported on two edges, with no load other than its own weight. They are working figures for scoping a design, not a substitute for a measurement on the actual part.
| Thickness | 600 mm span | 900 mm span | 1200 mm span | Practical note |
|---|---|---|---|---|
| 5 mm | 1.5 mm | 5 mm | 12 mm | Fine as a vertical panel, poor as a horizontal shelf |
| 8 mm | 0.6 mm | 2 mm | 5 mm | The usual choice for a 900 mm shelf with light goods |
| 10 mm | 0.3 mm | 1.2 mm | 3 mm | 1200 mm needs a centre support or a thicker sheet |
| 15 mm | 0.1 mm | 0.4 mm | 1.3 mm | Comfortable at 1200 mm; watch the cost step |
| 20 mm | <0.1 mm | 0.2 mm | 0.6 mm | Where flatness is specified rather than assumed |
Two corrections apply in practice. Load matters more than the table suggests: a shelf carrying distributed weight deflects roughly in proportion to that weight, so 8 mm at 900 mm that sags 2 mm empty will sag about 4 mm with 8 kg spread across it. And temperature matters more than most drawings admit — acrylic expands about 70 µm per metre per °C, so a panel that is flat at 23 °C in the plant is 1.4 mm longer at 43 °C in a shop window, and if the frame does not allow that movement the panel bows instead.
Where the table says the sheet is marginal, the cheaper answer is usually a support rather than thicker sheet: a 900 mm span in 8 mm with a mid-rail performs better than 12 mm unsupported, and costs less. Where the part must be unsupported and visually thin, that is the point to specify flatness on the drawing with the support condition stated, because flatness measured free-standing and flatness measured in the frame are different numbers.
A number plus how it will be measured turns a vague requirement into an accepted part. We will quote selected sheet where it is needed.