Polypropylene under a steady load does not behave like steel. It deforms slowly and continuously, a behaviour called creep, and the deformation does not recover completely when the load is removed. For a box at the bottom of a stack that sits in a warehouse for weeks, the load it has to survive is not the one applied on day one – it is the accumulated effect of the whole period.
Why static ratings mislead
A load rating measured over twenty-four hours tells you almost nothing about a stack that stands for a month. The board continues to deform, the walls bow outwards slightly, and the stack settles. Eventually the geometry changes enough that the load path through the walls is no longer vertical, at which point the deflection accelerates. The box does not break; it slowly stops working.
What the bottom box actually sees
Everything above it, plus the dynamic allowance for any handling while it is loaded, plus whatever point loads come from the feet or lid features of the box directly above. That last item is usually the largest and the least considered. A lid with a raised rib concentrates the entire stack load into a line, and the box below sees a line load rather than a distributed one.
Temperature moves the number a lot
Creep rates in polypropylene are strongly temperature dependent. A stack that is stable in a temperate warehouse can deform noticeably in a hot container or an unshaded yard in summer. This is the single most common reason a specification that worked in one season fails in another, and it is almost never recorded on the drawing.
Design answers that work
Vertical reinforcement at the corners, a base that distributes point loads into the walls, and a lid that registers on the box below rather than sitting on a single rib. All three are cheap at tooling stage and all three are structural rather than cosmetic. Increasing board thickness is the expensive way to solve the same problem.
What to ask a supplier to state
The load rating, the duration it applies to, the temperature it was measured at, and the deflection limit used. Four numbers. A supplier who has tested the box will have them; one who has not will quote a single figure and change the subject when asked what it means.
Rotating stock to limit the effect
Where a stack must stand for long periods, rotating which boxes sit at the bottom spreads the accumulated deformation across the fleet rather than concentrating it in the boxes that happened to be lowest. It is an operational fix rather than a design one, and it costs nothing but discipline.
Time is the variable people forget
A stack rating is not one number. It is a number at a duration and a temperature, and all three belong on the specification. Where they do not appear, the figure being quoted is a short-term test result being used as a long-term guarantee.
Testing for it without waiting
Creep tests run for weeks, which is why they rarely get done. A shorter test at a higher temperature gives a usable indication far sooner, because the two are linked. Where a box has to survive six months in a warehouse, a two-week test at elevated temperature will usually separate the designs that work from the ones that do not.
The measurement is the same in either case: height before, height after, and whether the bottom box returns to shape. Permanent set is the signal, and it is visible long before the box actually fails.
What we can do
We build PP corrugated turnover boxes with corner reinforcement and load-spreading bases, and we quote the rating with its duration and temperature attached. Send us the stack height and the storage conditions and we will tell you what the bottom box will do over six months. The temperature side of that behaviour is covered in how heat and humidity change stack strength.