A container retrieved from long storage looks the same as the one that was put away. It may not be. Sustained load, temperature and UV change a polymer slowly and permanently, and the changes are not visible from outside. Three checks, done on a sample at the point of retrieval, separate the fleet that can go back into service from the part that cannot.
Check one: flatness
Put a straight edge across the base and look for a gap. A container that has crept under stack load will rock, and a container that rocks will not track on a conveyor, will not seal for a vacuum lift, and will not stack stably. Where the gap exceeds a couple of millimetres on the bottom containers in a stack, the whole stored population is suspect.
Check two: brittleness
A container that has been exposed to UV loses impact strength. The field test is a firm tap on a corner with a knuckle or a mallet: a sound container gives a dull thud and a brittle one gives a sharper, more brittle sound and may crack. Where a stored fleet has been outdoors, this check matters more than any other, because the loss is invisible.
Check three: the stacking features
Do they still seat? A container whose stacking geometry has deformed will not interlock, and a stack built from them is unstable. That is a functional check that takes seconds – try two together – and it catches deformation that a visual inspection misses entirely. The height and stability consequences are covered in how high to stack empty containers.
What the checks find, typically
That the bottom of each stack is worse than the top, that outdoor stacks are worse than indoor ones, and that the degradation is concentrated in a minority of containers rather than spread evenly. All three point at the same conclusion: sorting on retrieval is necessary, and sorting only the obviously damaged ones is not enough.
How many to check
A sample from the bottom, middle and top of several stacks, plus every container that shows any visible sign. Where the sample passes, the population is probably fine. Where it fails, the whole population has to be sorted – which is expensive but cheaper than discovering the problem in service.
What to do with the failures
Deformed containers are not repairable, and brittle ones are not worth repairing either – the material has degraded and a repair will not restore it. Those are replacement cases, and they are the ones that make long storage expensive. Where a fleet has to be stored, reducing the storage damage is far cheaper than replacing what it destroys.
Preventing it before the next storage period
Store nested or folded rather than stacked, under cover, off the ground, and with the stacks lower than in service. All four are free or cheap, and they address the three mechanisms – load, UV and heat – directly. The storage arrangements are set out in storing containers through the off season.
Recording what happened
How many containers failed on retrieval, from which stacks, and in which storage conditions. That record turns a retrieval exercise into information about how to store next time, and it is the difference between repeating the loss every year and preventing it.