Specification guide Thickness tolerance
A nominal thickness is a name, not a measured wall. Cast acrylic is allowed to vary more than extruded acrylic under the ISO sheet standards. That is why a slot designed to a single number binds, and why two “5 mm” faces do not always meet cleanly. Write the route, and design the fit for the tolerance — not for the label.
The short answer
Decision rule. If a slot, a press fit, or a mating wall depends on thickness, start with extruded acrylic and still leave an allowance. If the edge must be polished or the joint must disappear, start with cast and design the fit for the wider cast tolerance. Do not design either part to the nominal alone.
What a tolerance is
Tolerance is the band the sheet is allowed to sit in, within a sheet and from sheet to sheet. It is not the thickness you hoped for. Cast acrylic is poured and cured between plates, so the band is wider. Extruded acrylic is pushed through a die, so the band is tighter. Both are PMMA. The nickname on the email does not state the band.
A mill can sell inside the ISO band, or agree a tighter one. Unless the quote names the mill tolerance, the safe reading is the standard for that route — and only inside the thickness and area the standard covers.
What the standards actually say
These are the published limits, with their scope. They are not a factory guarantee, and they do not apply outside the ranges named.
| Cell-cast (ISO 7823-1:2003) | Extruded (ISO 7823-2:2003) | |
|---|---|---|
| Thickness covered | 1.5 mm to 25 mm | 1.5 mm to 20 mm |
| Area limit in the clause | Sheets up to 6 m² | Stated for the thickness ranges below |
| Tolerance | ±(0.4 mm + 0.1 × h), where h is nominal thickness in mm | ±10% from 1.5 mm to under 3 mm; ±5% from 3 mm to 20 mm |
| Applies | Within a sheet and from sheet to sheet | Within a sheet and from sheet to sheet |
| Outside the range | Agree it | Agree it |
Outside those thicknesses, or on a continuous-cast sheet covered by a different part of the series, do not reuse these numbers. Ask.
What that means on a 5 mm wall
Using only the formulas above, as an illustration — not as this factory’s measured stock:
- Cast, nominal 5 mm: ±(0.4 + 0.1 × 5) = ±0.9 mm. The standard allows about 4.1 mm to 5.9 mm.
- Extruded, nominal 5 mm: ±5%, so about 4.75 mm to 5.25 mm.
A slot drawn at 5.0 mm for a “5 mm” cast tab can be tight or loose depending on which end of the band both pieces landed on. That is a design problem, not a cutting problem. The same nominal on extruded is a narrower argument, and still not zero.
Thinner sheet does not remove the issue. On extruded under 3 mm the standard’s band is ±10%, which on a 2 mm sheet is ±0.2 mm. On cast the formula still adds the 0.4 mm term, so thin cast is a wide band relative to the wall.
Designing the fit
Press-fit or slot
Prefer extruded if the edge is not a show edge. Leave clearance for the band, not for the nominal. State the fit you need: slip, location, or interference.
Solvent-bonded corner
Cast is the usual sheet. Two walls at opposite ends of the cast band will not meet as drawn. A fit allowance, or a joint designed to hide a small step, belongs on the drawing.
Mixed route in one box
Cast show face against an extruded back is normal. Their bands differ. Do not detail the joint as if both were the nominal.
A fit that must be tighter than ISO
That is a purchasing line: name the mill tolerance, or agree a selected-sheet range. It is not implied by “precision acrylic.”
Parts where this shows up: slotted trays, bonded boxes, and interlocking stands.
What to write on the RFQ
- PMMA, cast or extruded, per member.
- Nominal thickness, and whether a fit depends on it.
- The tolerance you need if it is tighter than the ISO band — otherwise say “ISO 7823 for the route.”
- How thickness will be checked: where on the sheet, film removed, and the instrument if you care.
A quote that does not restate route and nominal thickness cannot be checked against either standard.
Sanait will ask which route when a brief says only “5 mm acrylic,” because the allowed band changes with the route. Intake is on the FAQ. Scope is on the about page.
FAQ
Which acrylic has the tighter thickness tolerance?
Extruded, under ISO 7823-2, compared with cell-cast under ISO 7823-1. A named mill can be tighter than either. The quote has to say so.
Is cast always ±10%?
No. ISO 7823-1 states ±(0.4 mm + 0.1 × nominal thickness) for 1.5–25 mm sheet up to 6 m². That is not a flat 10%, and it is not every sheet on the market.
Why did my 5 mm slot not fit?
Because both pieces have a band, and a slot drawn at the nominal has no room for it. Add clearance, or agree a tighter sheet.
Does a tighter tolerance mean better acrylic?
It means a tighter thickness band. Cast is still the usual sheet for a polished edge and a clear joint. Tolerance and route solve different problems.
Can I hold a press fit in cast?
Only if you design for the cast band, or you buy selected sheet to an agreed range. The nominal will not do it.
What if the part is thicker than the standard covers?
The ISO figures above stop at 25 mm cast and 20 mm extruded. Outside that, the tolerance is whatever the parties agree. Write it down.
Sources
Thickness bands below are taken from the sheet standards, including the thickness and area limits in those clauses. A mill’s own sheet can differ.
- ISO 7823-1:2003, Plastics — Poly(methyl methacrylate) sheets — Part 1: Cast sheets. Thickness tolerance ±(0.4 + 0.1 h) mm for 1.5–25 mm sheet up to 6 m². iso.org
- ISO 7823-2:2003, Plastics — Poly(methyl methacrylate) sheets — Part 2: Extruded sheets. Thickness tolerance ±10% (1.5 mm to under 3 mm) and ±5% (3–20 mm). iso.org
- “Poly(methyl methacrylate).” Wikipedia. en.wikipedia.org
English 
Deutsch 
Italiano 
Français 
Español