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Where the water actually gets in: blue PP hollow sheets in a wash-down plant
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A PP hollow sheet does not let water through its walls. Moisture enters at the two cut edges, where the flutes are open, and from condensation on the inside of a closed container during a 25-day sea passage. So the specification that keeps automotive parts dry is not a coating: it is sealed flute ends, a drain at the lowest corner, and a wall thick enough that the seams do not flex open.
Wash-down containers: 6 mm blue sheet, all four edges sealed and riveted to a galvanised frame, hosed at 60 °C between shifts without swelling at the cut line.
Sea freight liners: 4 mm sheets as a full-height liner inside a returnable stillage, with two 8 mm drain holes at the low corner, so condensate leaves instead of pooling under the parts.
Colour-coded loops: blue masterbatch marks a container that has carried automotive parts and must not enter a food or electronics loop; red, yellow and grey do the same job elsewhere in the plant.
Foreign-object control: a solid blue wall hides nothing, so a dropped bolt or clip turns up at the end-of-shift sweep instead of travelling to the next station.
Water absorption of the sheet itself is below 0.01%, which is why a sealed panel can sit in a wet environment for years. The weak point is the cut edge. An unsealed flute acts as a capillary and draws water 20 to 30 mm into the board within a few wash cycles, and that is where delamination starts. We seal every edge that will see water, by ultrasonic welding or by a folded and stapled return, and we leave the low corner unsealed as a drain rather than closing the whole board. |
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| Blue sheet is not a vapour barrier, and it is not a structural material. A container that must hold 800 kg stacked three high needs a steel or plywood base under the pads whatever the sheet thickness. Prolonged UV exposure outside the building embrittles PP within two or three summers, so outdoor storage needs a UV-stabilised grade at extra cost. Colour has a limit too: blue masterbatch fades first on the sun-facing wall, which is cosmetic but makes the code harder to read after a few years of service. |
 | At the 2025 Indonesia Industrial Exhibition buyers filled these containers with water on the stand to see where it came out. That conversation is why our standard wash-down container now leaves the factory with the drain corner marked on the drawing, instead of as an option a customer has to remember to ask for. |  |
