Bending stiffness
Bending stiffness — A sheet's resistance to sagging. For the same material it rises with the cube of thickness, so moving from 3 mm to 4 mm roughly doubles stiffness and 3 mm to 5 mm roughly quadruples it. That is why thickness is chosen as a ladder of stiffness steps per application, not read off a per-millimetre price list.
Nominal versus actual thickness
Nominal versus actual thickness — A sheet sold as 3 mm typically measures nearer 2.7 to 2.8 mm — normal for both cast and extruded stock. Designing peg holes and slots to the nominal number is how parts arrive that either do not fit or fall out. Drawings should carry the measured thickness with a tolerance.
| Lids, small cases, protective plates | 2–3 mm |
|---|---|
| Dividers, trays, small parts displays | 3–4 mm |
| Display stands and box fixtures | 4–5 mm |
| Shelf boards and loaded surfaces | 6 mm and up, more over long spans |
Thickness is a ladder of steps, not a price list
The common mistake is reading thickness off the price column, one millimetre at a time. Because stiffness of the same material rises with the cube of thickness, the steps are not equal: going from 3 mm to 5 mm more than quadruples how hard the sheet is to flex, and in hand it feels like a different material. The other direction wastes money — 8 mm where 4 mm was enough buys weight, freight and material cost for stiffness that never gets used. The input that decides the step is never the price list; it is what the part will carry and how wide the supported surface is.
Designing to the name breaks at assembly
A sheet called 3 mm measuring 2.7–2.8 mm is normal, not a defect. But if the drawing was made against the name, pegs and slots arrive wrong — either too tight to assemble or loose enough for parts to drop. With fixtures built from several pieces, the per-sheet variation accumulates. Put the measured thickness with its tolerance on the drawing, and match your drawing against the supplier’s actual stock before the order, not after delivery.
Two numbers before anything else
Fix the weight the part will carry and the span of the surface carrying it. With those two, the thickness falls out of the ladder and every proposal can be checked against them. Without them, no quote can be argued with. When samples arrive, look at flatness empty, then leave the real load on for a week and look again; buyers who do this before volume ordering almost never reopen the tooling afterwards.
Can I just order the thinnest sheet to save money?
For a lid or a cover, yes. On a shelf or any surface that carries weight, a thin sheet turns sag into a safety problem, and the saving disappears with the panels that warp out of use. Once the load and the span are known, the honest cheapest option is the thinnest grade that survives both — chosen backwards from those two numbers.
How do I tell good stock from reprocessed material?
Look at the cut edge and hold the sheet to the light. Weak material shows a cloudy cut edge and visible streaks or distortion inside, and heavily bowed sheets leave gaps after assembly. Where the price looks too good, order a sample first and check it flat under the real load before committing to volume.