A box that works perfectly on a pallet can fail on a conveyor, and the failure rarely looks like a packaging failure. It looks like a jam, a mis-scan, a box that will not track straight, or a sensor that reads the wrong object. The line gets blamed, then the belt gets blamed, and the actual cause sits in the underside of the container.
On a belt the base is the whole story
On a pallet, a box is supported across its full footprint and the walls help carry the stack above. On a belt, the box rests on a moving surface that contacts only the base, and nothing above it contributes stiffness. Everything the belt asks of the container – tracking, spacing, sensor presentation, the ability to survive a stop and a start without shifting – is decided by the geometry of the underside. That is why a container designed for stacking often needs a second look before it goes near a conveyor, and why the specification that matters most is the one nobody draws.
Flatness is not the same as smooth
A base can be perfectly smooth and still rock, because what matters is whether it is flat within the tolerances the belt cares about. A slight dish in the middle lifts the perimeter and reduces the contact area to a ring. A slight crown does the opposite and puts the load on a small patch in the centre. Either one reduces friction unpredictably, which shows up as boxes that slide on acceleration and stop late on deceleration. The practical test is simple: put the empty box on a surface plate and try to rock it, then do the same with it loaded. Anything that moves is a box that will behave inconsistently at speed.
Ribs, feet and the contact patch
Most reusable containers carry some structure on the underside – reinforcing ribs, stacking feet, or a moulded perimeter. On a pallet these are useful. On a belt they decide how the box tracks, because only the raised parts touch. A rib pattern that runs along the direction of travel tends to track straight and is forgiving of a slightly skewed infeed. A pattern of isolated pads tends to pivot and is much less forgiving. Where a box has to run on a belt, the orientation of the underside features is worth as much attention as the outside dimensions, and it is usually far easier to change at the tooling stage than after the fleet is in service. The jam that results is diagnosed in why boxes jam on a conveyor line.
What happens at the transfers
Most conveyor problems occur at the joins rather than in the middle of a run. Where two belts meet at slightly different heights, where a box crosses from a belt to a roller section, or where it enters a curve, the container is briefly supported at two points instead of a surface. A base that is stiff enough to span that gap passes through; one that is not dips, catches, and either jams or skews. The gap is a property of the equipment, but whether the box can bridge it is a property of the container, and the two have to be specified together.
Friction, and why more is not always better
Belts rely on friction to accelerate a box, and a high-friction base is normally what you want. The exception is the accumulation zone, where boxes are meant to slip against a running belt while they queue. A container with a very grippy base will not release cleanly in that section and will ride up on the boxes in front of it. That is the reason some lines specify different base finishes for different zones, and it is a conversation worth having before the order rather than after the installation.
Wall stiffness still matters, just not for the reason you think
The walls carry no vertical load on a belt, but they determine what happens when boxes touch. At the infeed, boxes are pushed together, and a wall that deflects lets the queue compress unevenly, which produces the gaps a photo-eye needs and then takes them away again. A stiffer wall holds spacing and keeps presentation consistent. This is the same property discussed in box width tolerance against side guides, where the deflection shows up as a tracking problem rather than a spacing one.
What to send the equipment supplier
An empty box, a loaded box, and the base drawing with the rib pattern dimensioned. Photographs of the underside are more useful than photographs of the box assembled, because the underside is what the equipment actually sees. Where the supplier has a test line, running twenty boxes through it tells you more in an hour than a week of correspondence, and it is the cheapest way to find a problem that would otherwise surface during commissioning. Find out what our PP corrugated containers are specified for on automated lines, and we will tell you which base details matter for your belt.