A tolerance that looks generous on a single box can be unworkable across a pallet, because tolerances add. Where boxes are stacked directly on each other, the variation across twenty layers is twenty times the variation in one box, and it all has to go somewhere.
Where it accumulates
In the height, most visibly – the stack ends up taller or shorter than the plan, which decides whether the top layer fits in the vehicle. In the footprint, less visibly – the columns drift out of alignment, which is what produces the lean that people blame on handling.
The height case
Twenty layers at +/-2 mm per box is a possible 40 mm of variation. Where the load plan has 30 mm of headroom, half the pallets will not fit. This is the most common reason a load plan that works on paper fails on the loading bay.
The footprint case
Horizontal variation is absorbed by the clearances in the pattern, which is why patterns need gaps. A pattern with zero clearance works only if every box is exactly nominal, which no production process delivers.
How to budget it
Take the per-box tolerance, multiply by the number of layers or boxes across, and make sure the result is smaller than the clearance the vehicle or pattern provides. Where it is not, either the tolerance has to come down or the pattern has to allow more room. There is no third option.
Where the tolerance can be looser
Internal dimensions, dunnage fit, lid clearance, anything that does not affect how boxes sit against each other. Specifying tight tolerances everywhere is a common way to pay for precision that does nothing.
What to check on delivery
Assembled outer dimensions and the two diagonals of the mouth, on a sample from each pallet. That is the whole inspection, and it catches the two defects that cause stacking problems. The wider question of what to accept is covered in accepting a delivery of boxes.
Where the stack-up shows up first
Height, usually, and it shows up at the loading bay rather than in the warehouse. A plan that assumed nominal box heights produces a stack that is nominally correct and actually 30 mm too tall, and the top layer does not go. Where the plan has no allowance, this is arithmetic that was never going to work.
The second place is the column alignment. Boxes at the top of a tolerance band sit slightly proud of the ones below, and over several layers the offset accumulates into a visible lean. It is not a handling problem, and no amount of additional wrap will correct it.
Budgeting for it
Take the per-box tolerance, multiply by the number of layers or boxes across, and compare the result with the clearance the pattern and the vehicle actually provide. Where it exceeds the clearance, either the tolerance comes down or the pattern opens up. There is no third option, and discovering it at the design stage is free.
What to do when the stack-up will not fit
Either tighten the per-box tolerance or open up the pattern. Which is right depends on volume: on a large fleet, tighter tolerances amortise across many trips; on a small one, a roomier pattern is free. Where the load runs automatically, tighten – the equipment has less tolerance than a person does.
What we can do
We build PP corrugated turnover boxes with outer dimensions and squareness both stated on the drawing, so a load plan and a delivery can be checked against each other. Send us the pattern you are working to.