Lids that fitted perfectly when new stop fitting after a season, and the complaint is usually phrased as the lid having shrunk. Polypropylene does not shrink in service under normal conditions. What has happened is deformation somewhere, and there are three places to look.
The lid has taken a set
Lids are often stored separately, frequently stacked on their own under load, and they are the most flexible part of any assembly. A lid stored flat under weight for months will bow, and a bowed lid will not seat. Check it first, on a flat surface, by rocking it – the test from checking flatness without specialist equipment.
The fix is storage, not redesign: lids stored on their side or on edge rather than flat under a stack.
The box mouth has gone out of square
A box that has been overloaded, dropped or stored badly will have a top opening that is no longer rectangular. A perfectly good lid will not fit a parallelogram. Measure both diagonals of the mouth; if they differ appreciably, the box is the problem and the lid is innocent.
This is common in boxes that get stacked high when loaded, because that is when the mouth is under the most load.
Wear at the seating surfaces
The surfaces the lid actually bears on wear progressively. A few tenths of a millimetre is enough to change how a lid feels and whether it stays on. Look at the contact surfaces rather than the overall dimensions – dimension checks pass while the lid still does not seat.
Reinforcing the seating surface, or designing the lid to bear somewhere other than the most-handled edge, costs little when specified early.
Cold changes the answer temporarily
Both lid and box contract when cold, and not necessarily identically if they are different grades or thicknesses. A lid that fits at ambient and binds in a chilled area is a real and common case. Test at your actual service temperature before concluding anything is deformed.
When to stop matching them
Once lids and boxes have aged differently and been replaced at different times, the fleet is no longer matched. Rather than sorting, consider whether the lid needs to be a precise fit at all – a lid that overlaps generously tolerates variation that a close-fitting one cannot. Loose tolerances are the cheapest long-term answer where product protection allows.
The mixed-fleet trap
Once boxes and lids are replaced at different times, the fleet contains items of different ages that were never matched to each other. Sorting them is expensive and the improvement is temporary, because the mismatch reappears with the next replacement round.
The durable answer is to buy boxes and lids together in matched sets and to identify set membership, so a replacement box comes with its lid. Where that is impractical, an overlapping lid tolerant of variation is better than a precise one, because close fits are the ones that stop working when the fleet ages unevenly.
Check before the busy season
Lid fit failures cluster at peak, because that is when stores are deepest and lids have sat longest under load. A fit check two weeks before peak, with the worst ten per cent retired, prevents the mid-season shortage that no amount of expediting solves.
What we can do
We supply matched picking boxes and lids and can build in generous overlap where the fleet will age unevenly. Tell us your storage method for lids and your service temperature, and we will set the fit accordingly – sometimes looser than you would expect to ask for.