Case deformation only came back halfway after 24 h. Still usable?

Answer: Usable, downgraded. A 60% recovery is mid creep, so light items only, no stacking, and a re-measure at 3 months; a residual growing from 1.9 mm to 2.6 mm retires the case.

Will five layers at 480 kg always crush the bottom case?

Answer: Not always, it depends on time and temperature. At 0.08 MPa the sag reaches 3.2 mm in 30 days, while four layers at 0.064 MPa reach only 1.8 mm, which makes layer count the easiest variable to change.

3.2 mm after 4 h at 60 ℃. Heat or load?

Answer: Check location and recovery. A sag on the heated face, all four edges drawing in, and a 9% recovery point to heat; a sag confined to the lid centre with 85% recovery points to load.

How should case deformation be measured?

Answer: Three readings are enough: warp with a plate and feeler, diagonal difference with a steel rule, lid gap with a feeler. All three go on the record or the verdict does not stand.

How many layers can a case with 1.5 mm residual take?

Answer: One fewer than rated. Five layers becomes four, with a check every 3 months; at 3 mm residual or 4 mm diagonal difference the case leaves rotation.

Case Deformation: Load or Heat

Quick answer: Case deformation reading 3.2 mm should sit empty at 23 ℃ for 24 hours first: rebound inside 1.5 mm means load, no rebound means heat.

First gauge: 24 hours off load
Load side: creep under five layers
Heat side: four hours at 60 ℃
Measuring: plate, feeler, diagonals
Release line: 1.5 mm residual
FAQ

1. First gauge: 24 hours off load

A lid that has sunk 3.2 mm lands on the desk, and the first move is not a call to the supplier. Put the case empty on a 23 ℃ plate for 24 h and measure the same point again. Rebound to within 1.5 mm means the resin only stretched viscoelastically and the cause is load; no change after 24 h means the chains have slipped, and temperature is the line to follow.

Recovery ratio is rebound divided by original sag. Above 85% the case stays in service one layer lower; below 60% another week changes nothing.

After 24 h	Rebound	Recovery	Reading
Full	3.2 mm	100%	Short overload
Most	2.7 mm	85%	Early creep, drop a layer
Part	1.9 mm	60%	Mid creep, review duty
None	0 mm	0%	Heat sag or yielded

Measure one point with one tool: mark a cross at the deepest spot, read a depth gauge three times, take the median; a tape held at an angle adds 0.8 mm of error.

Three preconditions: empty, flat, steady temperature. Anything stacked inside stops the rebound, an uneven plate mixes warp with sag, and a 5 ℃ drift makes readings incomparable.

2. Load side: creep under five layers

Load-driven case deformation has two signatures: the sag sits at the lid centre or the four base corners, and it is a slow sink with no crease. Five layers at 96 kg each, 480 kg in total, put roughly 0.08 MPa on the bottom case, and 30 days at room temperature produces a 3.2 mm sag.

Case deformation inspection shelving, black blue and red open cases in white wooden bays, black and orange foam inserts, white closed cases on top

Polypropylene creeps even at 0.08 MPa, only slowly: 2.1 mm in week 1, 2.8 mm in week 2, 3.2 mm in week 4, then flat. Flat is not safe, because the lid is already resting on the insert and the cushioning travel is gone.

Creep lines are the signature. At 10× the sag shows parallel fine lines across the load direction, while a heat sag looks uniformly glossy and has none.

Time is the variable people forget. The same five layers give 1.2 mm at 7 days, 3.2 mm at 30 days and 3.9 mm at 90 days, so 82% of the sag happens in the first month and no repositioning brings it back.

Dropping to four layers cuts the bottom load from 480 kg to 384 kg and the pressure to 0.064 MPa, which brings the 30-day sag back under 1.8 mm.

3. Heat side: four hours at 60 ℃

Heat-driven case deformation carries its own evidence chain: temperature record, duration, location. Four hours at 60 ℃ sags the lid centre 3.2 mm, while the same case at 23 ℃ for 30 days sags 0.6 mm.

PP is rated near 100 ℃ heat deflection, which sounds far from 60 ℃, but that figure belongs to a 0.45 MPa load. An empty case with no insert support starts softening at 55 ℃, and 70 ℃ inside a closed van in summer is routine.

Recovery is the second marker. After 4 h at 60 ℃ and cooling to 23 ℃, the residual returns from 3.2 mm to only 2.9 mm, a 9% recovery, because heat let the chains relax and they set in a new position.

Indicator	Load type	Heat type
Sag location	Lid centre or base corners	Heated face, edges inward
Surface	Parallel creep lines	Glossy, paler
24 h recovery	Above 85%	Below 10%
Corroboration	Worse near the bottom	Several cases in one bay
Reproduction	Two extra layers, 7 days	4 h at 60 ℃

Supporting signs matter as much: the heated wall goes glossy, paler and softer, and holds a fingerprint longer. Three of them together settle the question.

Inside a van at 70 ℃ the lid sags 1.8 mm within 2 h. The fix sits in storage and transport: shade the van, unload out of direct sun, keep the store under 35 ℃.

4. Measuring: plate, feeler, diagonals

Three tools cover case deformation: a grade 1 plate, a 0.02 mm feeler gauge and a 600 mm steel rule. Set the case down empty, check that it does not rock, then slide the feeler under the base edge, where the largest blade that enters is the warp.

Case deformation measurement with three open cases on a pale wood counter, egg-crate lid foam, blue foam in six pockets, green foam with round holes

Diagonal difference is the second measure. More than 2 mm between the two diagonals means the case has become a rhombus, the corners load unevenly and the next stack sags faster; a new case stays within 1 mm.

Check the lid joint as well. With the lid closed, a 0.3 mm blade that enters anywhere marks a spot the gasket cannot press, and that deformation brings water and dust with it.

Record three numbers on one sheet: deepest sag, diagonal difference, lid gap. Read indoors after 2 h, because a case moved from a 5 ℃ store to a 23 ℃ shop keeps rebounding 0.4 mm for those two hours.

5. Release line: 1.5 mm residual

The release line is 1.5 mm of residual sag. Under 1.5 mm sag, 2 mm diagonal difference and 0.3 mm lid gap together, the case stays in service one layer lower.

Between 1.5 and 3 mm the case is downgraded: light items, no stacking, no sealing duty, re-measured every 3 months to see whether the residual is still growing.

Above 3 mm, or 4 mm diagonal difference, it leaves rotation for static storage. That case deformation has gone plastic, and another stack will crack the corners.

Write the verdict on the case itself: date, layer limit, next check, in marker pen. Whoever lifts the case will read it; nobody opens the ledger.

Conclusion

The order for case deformation is fixed: 24 h off load, then three numbers, then the argument about who pays. Rebound inside 1.5 mm with 85% recovery puts the cause on load, where dropping from five layers to four takes 0.08 MPa down to 0.064 MPa; a residual stuck at 3.2 mm with 9% recovery puts it on heat, where shading the van beats changing the case. The middle band is a downgrade: light items, no stacking, re-measure in 3 months. Every verdict has to rest on the three readings, or a recoverable case gets scrapped and a yielded one goes back on the line.

Verdict	Recovery	Residual	Action
Recoverable	≥85%	≤1.5 mm	One layer lower
Downgraded	60–85%	1.5–3 mm	Light items, no stack
Not recoverable	<60%	>3 mm	Static storage only
Failed	n/a	Diagonal >4 mm	Scrap

Stacking improvements, in order:

Four layers instead of five, bottom pressure at 0.064 MPa
12 mm plywood on the pallet to spread point loads
Store below 35 ℃ with a 300 mm aisle at the wall
One card per stack: layers, date, next check
Cases under 2 years old at the bottom, older ones moved up

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.