A container on a long sea leg reaches temperatures high enough to soften polypropylene noticeably. Whether a honeycomb panel comes out of the other end flat depends mostly on what was pressing on it while it was hot, because heat alone rarely does anything permanent.
What heat actually does
PP loses stiffness as it warms. Somewhere around sixty to seventy degrees the change becomes visible, and a panel that is perfectly rigid at ambient will flex under a load it would shrug off cold. A container in the sun in a hot climate gets into that range without difficulty.
The important point is that this is reversible while the load is on and largely reversible once it comes off. A panel that bowed under load in a hot container usually recovers when it cools and is unloaded, unless it was held there long enough to creep.
What makes it permanent
Time under load, at temperature. A panel loaded for six weeks in a hot container will take a set; the same panel loaded for two days will spring back. This is creep, and it is the mechanism behind most permanent bowing.
Point loading makes it much worse. A panel supporting a distributed load at temperature behaves reasonably; the same panel with a heavy item resting on a small area will deform locally and stay deformed. Most container bowing we investigate turns out to be a point load rather than temperature.
What it is not
Manufacturing bow. A panel that arrives bowed and has never been in a container has a different cause, and it is worth distinguishing, because the fixes are different. That is covered in why a honeycomb panel bows in the middle.
What to do about it
Reduce the point loads first – spread the load with a layer pad or a larger footprint, which costs little and fixes most of it. Then reduce the time under load, if the routing allows. Then, if it is still happening, go up a thickness or a density.
Going up a grade is the expensive fix and it is the one people reach for first. It usually works, but it often works because the thicker panel resists the point load better rather than because it resists heat better.
Specifying for a hot lane
If you know the routing runs hot, say so at the enquiry stage. It changes what grade makes sense and it costs nothing to mention. A supplier who knows the lane will specify differently from one who assumes ambient warehousing.
Load capacity and how to estimate it is set out in estimating honeycomb panel load capacity.
What to check on arrival
Lay the panel flat on a level floor and look along it, then flip it and look again. A panel that has taken a set will not sit flat either way up. One that is temporarily flexed will settle within a day at ambient temperature.
Check the loaded faces too. Indentation where something small and heavy sat is the clearest evidence of point loading, and it tells you what to fix.
What we can do
We supply honeycomb panels for automotive dunnage and handle hot-lane applications regularly. Tell us the routing, the load and how long it sits – we will specify for the conditions rather than for a catalogue, and tell you honestly when a cheaper grade will cope.