Thickness is the number buyers fix first and understand last. It is not a quality grade, and a heavier sheet is not automatically the better answer. It is a span decision with a load attached, and the same sheet that is correct on a 300 mm door is wrong on a 700 mm one.
Deflection grows with the cube of the span
Double the unsupported span and the panel sags eight times as much at the same thickness and the same load. That single relationship explains most of the bad acrylic specifications we see. A 3 mm sheet that sits dead flat across 300 mm will visibly dish across 600 mm with nothing added to it but length.
As a working rule we keep unsupported spans on 3 mm under about 350 mm. At 5 mm we are comfortable to roughly 600 mm before a rail or a return flange is needed. Past that, the fix is structure, not a thicker sheet, because a 10 mm panel at 1.2 m still sags and now weighs twice as much.
Material grade sits underneath that. We cut 3 mm cast PMMA for the MERCH-GRID DUO M frame, which is 320 × 260 mm with a 292 × 232 mm window. At that size cast sheet stays flat through machining and takes a clean solvent joint. Extruded sheet of the same nominal 3 mm runs cheaper and flatter out of the box, then stress-cracks at a machined internal corner or a drilled hole set too close to an edge. For a display that lives on a wall, cast is the cheaper material over three years.
What each thickness is actually for
2 mm is a fitting material, not a structural one. The PIN-GRID insert board runs 2–3 mm because the board has to sit inside a bag window without showing the bag's own stitching through it; a 5 mm board in the same window looks like a slab and reads wrong. That thinness is a fit decision. It also means the board needs support on all four edges, and a large 2.5 mm panel held by one corner will bow enough to see.
3 mm is the default panel, print layer and shelf tier. The RetailPod back panel that slides down two channels is 3 mm. The PACK-STAGE tiers are 3 mm, and the SCENE-SWAP layers are 3 mm. Where a panel is captured on four edges and carries a light load, 3 mm is right and 5 mm only adds weight and cost.
5 mm earns its place in bases, plates and cantilevers. SCENE-SWAP uses a 5 mm plate under 3 mm layers, and the mismatch is deliberate: the plate carries the stack and provides the weight that keeps a tall diorama from stepping sideways when someone brushes it. The MerchDock photo dock runs 3–5 mm depending on span, with the top bar at the top of that range because it hangs the backdrop and the light wing off one edge.
| Thickness | Where it belongs | Practical span | What it costs you |
|---|---|---|---|
| 2 mm | Inserts and backed panels | Under 200 mm, supported on all edges | Flexes; needs four-edge capture |
| 3 mm | Frames, shelves, print layers | Up to about 350 mm unsupported | Dishes past 350 mm; thin edge shows less polish |
| 5 mm | Bases, plates, cantilevers, magnet doors | Up to about 600 mm | Weight, freight cost, deeper pockets needed |
The edge, the joint and the pocket decide more than the number
Two cases built from the same sheet can fail in completely different places because of what happens at the perimeter. A polished edge on 5 mm carries a visible band of light that reads as quality; the same polish on 3 mm is a thin bright line you have to look for. If the edge is the design, thickness is doing marketing work as well as structural work.
Solvent-cemented joints need surface area. On 3 mm, a butt joint between two panels is a 3 mm glue line and it will crack if the box is picked up by one corner. On 5 mm the same joint has enough face to hold. This is why small retail cases are often 5 mm on the carcass and 3 mm on the internal shelves, rather than one thickness everywhere.
Then the fittings nobody draws. A magnet needs roughly 2 mm of material behind it to stay put and not print through the face. Put a 3 mm magnet pocket into a 3 mm panel and you have a 1 mm skin, which is a visible dimple within a season. Door frames go to 5 mm for that reason, or the panel gets a bonded doubler.
Threads are the other trap. You cannot tap a reliable thread into 3 mm acrylic; there is not enough engagement and the sheet splits along the grain of the extrusion. Through-bolts with a cap, or a bonded block, are the two options that survive handling.
Where a thin panel fails quietly
Creep is the failure mode that never shows up in a sample. Acrylic under constant load keeps deforming for months, and a 3 mm shelf carrying a row of 1/7 figures at 30–40 °C in a shop window will bow in a way that a two-week sample test never predicts. If the shelf is permanent and loaded, we design for the load over a year, not over a photo shoot.
Lamination adds thickness at the worst moment. A UV film on a 3 mm panel brings it to about 3.1 mm, which is enough to change a slot fit that was drawn tight. Printed layers add roughly 0.2 mm per colour-white-colour pass, so a printed insert that has to drop into a 3 mm groove needs the clearance cut for the finished thickness, not the bare sheet.
Flat-pack shipping is the third. A 3 mm panel 600 mm wide can bow in transit under its own weight if it is not braced in the carton, and it arrives looking like a manufacturing fault. The panels are fine; the packing decided it.
What to send for a thickness call
- The drawing with each panel's span marked, not just the overall case size
- What sits on each shelf, and the weight and contact area of the heaviest item
- Whether the case is indoors, in direct sun, or in an unheated space
- Whether panels ship flat and get assembled on site, or arrive bonded
- Any magnet, hinge or light channel that needs material behind it
MOQ starts at a single sample set, runs 30–50 sets for a small batch and 100+ for mass production, with the final figure confirmed per product after we see the design. Samples typically take 7–14 days, and production dates are committed before the order rather than after. A thickness decision is worth settling on that sample: it is the cheapest place to discover that a shelf needed 5 mm.
The frames and cases this applies to are on the MERCH-GRID page. Standard answers on MOQ, sampling and materials are in the FAQ. If you have a drawing with a span you are unsure about, send it through the enquiry form and we will mark up the panels that need to change.