Board is bought on two numbers – thickness and gram weight – and both can move toward the low end of their tolerance bands without any visible change. Since material is the cost, that drift is predictable, and the only thing that stops it is occasionally checking.
The two checks
Thickness. A calliper at several points on several sheets. The reading will vary – that is normal – but it should sit in a consistent band, and the band should include the nominal figure rather than sitting entirely below it.
Gram weight. Cut a piece of known area, weigh it, and divide. A 100 mm by 100 mm square is 0.01 square metres, so the arithmetic is easy. Any kitchen scale that resolves grams is adequate for a 300 to 800 gsm board.
If both check out, you have confirmed the two things that determine what you were sold. If gram weight is low while thickness matches, the board has thinner skins or lighter flute walls, and it will be weaker than expected in service.
Why the check is worth doing
Not because suppliers are dishonest, but because “in specification” covers a range and the range is wider than most buyers assume. A board quoted at 400 gsm that arrives at 375 is in specification if the specification was written as a range, and the buyer who only ever sees a single number has no way to know.
Checking periodically creates a record. If a supply drifts, you find out at goods-in rather than in a customer complaint.
How to make the record useful
- Sample from the middle of the stack, not the top – edges and top sheets are the ones most affected by handling and moisture.
- Note the batch or delivery reference so a trend can be spotted.
- Record a range, not a single figure. “3.8 to 4.1 mm across ten readings” is far more informative than “4 mm”.
- Keep one retained sample per delivery if the board is used in anything visible, so colour and surface can be compared later.
What to write into the specification
Write both numbers with a tolerance, and state which one governs if they conflict. For board that has to fit a slot or a machine, thickness governs and gram weight is secondary. For board that carries load, gram weight is the better proxy for strength and thickness is the fit dimension.
Where the application is critical in either direction, ask for a measured figure on the delivery note rather than a nominal one. It is a small ask and it makes the check trivial.
Where the check is not enough
Verifying thickness and gram weight does not verify strength. Two boards with the same gram weight can have different flute geometry and therefore different stiffness, particularly across the flutes as covered in our article on flute direction.
If strength is what you are buying, ask for a bending or compression figure as well and check that on a sample rather than relying on weight as a proxy.
For the wider list of what is worth checking on arrival, see our article on incoming inspection of PP honeycomb panels.
What we can do
We supply corrugated PP sheet with thickness and gram weight both stated and toleranced, and we will tell you which of the two you should be controlling for your application. If your existing supply is drifting, send us a sample and we will measure it so you have a baseline.