WIP containers are the hardest-working boxes in a plant and the least specified. A shipping box might be handled six times in its life. A WIP box on a three-shift operation is lifted, set down, stacked and slid maybe forty times a day, by people who are not thinking about the box, for years. Specifying it like a shipping box is the standard mistake and it shows up as a replacement budget nobody can explain.
Count the touches
Before specifying anything, count how many times a WIP box changes hands in one shift: out of the rack, onto the bench, off the bench, onto a trolley, off the trolley, into the next rack. Each touch is a chance to drop, drag or overload. A box seeing forty touches a day is seeing roughly ten thousand a year, and at that rate the failure mode is not material fatigue – it is accumulated impact damage at the corners and handles.
Overload is normal, design for it
Every WIP container gets overloaded. Someone finishes a job and puts the last few parts in the box that is closest, and the box that was specified for 15 kg now carries 22 kg. Design to the overloaded figure, not the nominal one: pick a wall thickness and a base construction that tolerates forty per cent over, because the alternative is a container that is correct on paper and split in practice. If the overload is genuinely unbounded, fit a base reinforcement rather than thickening every wall.
Stacking while hot, wet or oily
WIP often means parts that have just been machined, washed or painted, so the box interior sees coolant, oil or swarf. PP corrugated board is indifferent to all three, which is one reason it is used here, but the box design has to let the contaminant out rather than hold it in: drainage slots at the base corners, and a base that does not form a sealed tray. Otherwise the box carries coolant from station to station and leaves a trail.
Identification that survives the environment
A paper label on a WIP box in a machining area lasts a day. A printed or embossed ID on the box body lasts the life of the box, and a colour-coded end panel is readable across an aisle. If the WIP carries a traveller or a job card, put a document pocket on the end rather than taping it to the side – taped cards go missing, and a missing traveller stops a job. The label-adhesion problem is the same one described in whether barcode labels stay on PP boxes.
Standardise the box, vary the insert
The temptation with WIP containers is a different box for every part family, and it produces a fleet nobody can manage: forty sizes, no interchangeability, and a store that cannot rebalance between lines. The better structure is one or two box sizes with interchangeable inserts – dividers, pads, trays – that adapt the standard box to the specific part.
Inserts are cheaper than boxes, they are replaceable when worn, and they can be changed when the product mix changes without touching the fleet. A standard box with a bespoke insert costs less than a bespoke box and survives a product change, which in a plant with a changing mix is worth a great deal. The exceptions are the genuinely awkward parts – very large, very long, or very heavy – where a standard box wastes too much volume to be worth it. Everything else should be a standard box with an insert, and the insert should be the thing that gets redesigned when the part changes.
What we can do
We supply PP corrugated WIP boxes specified against a duty cycle rather than a nominal load. Tell us the touches per shift, the realistic overload and what is inside – coolant, oil, swarf, sharp edges – and we will set wall thickness, base build and corner reinforcement to it.