Quick answer: A stacking test holds 1 t on the lid for 24 h and passes a case that sinks no more than 3 mm under load and springs back to 1.5 mm within an hour of unloading.
Table of contents
- Where the 1 Tonne Load Comes From
- The Rig: Platen, Base Plate, Level
- Reading Deflection and Permanent Set
- Ribs, Corners and Wall Thickness
- FAQ
1. Where the 1 Tonne Load Comes From
A tonne is not a stack anyone builds in a warehouse. A 600 × 400 mm footprint carries 0.24 m², so 1 t works out to 41.7 kPa, the same as twelve cases of that size stacked up, while racking limits real storage to 4 or 5 layers. The stacking test uses 1 t to pull three years of creep into 24 h; it is an accelerated check, not a copy of the floor.
What matters in service is the bottom case. At 20 kg gross, four per layer and four layers, the lowest one carries 3 × 4 × 20 divided by 4, or 60 kg; five layers gives 80 kg, and a 1.5 factor sets the rating at 90 kg and 120 kg.
| Layers | Case weight | Load on bottom case | Rated (×1.5) |
|---|---|---|---|
| 3 | 20 kg | 40 kg | 60 kg |
| 4 | 20 kg | 60 kg | 90 kg |
| 5 | 20 kg | 80 kg | 120 kg |
Pressure travels better than tonnage. The same tonne over 400 × 300 mm is 83 kPa, so a small case deforms before a large one. Load the stacking test a grade above the rated column: a 90 kg case at 1 t is probed at its limit and judged at 2 mm residual, a 120 kg case at 1 t is a normal verification.
2. The Rig: Platen, Base Plate, Level
Platen stiffness decides how load spreads. A 20 mm steel plate deflects about 0.6 mm at mid-span under 1 t; at 10 mm it reaches 2.4 mm, the lid centre takes everything, the edges carry nothing, and readings run 0.7 mm high. Thickness runs at 1/30 of span, 20 mm across 600 mm.
Load the stacking test at 10 mm/min to 1 t and hold 24 h with variation inside ±2 %. A rig that cannot hold pressure measures elastic recovery, not creep.
Under the case sits a hard plate the size of the base, level to 2 mm/m. An uneven floor tilts the case before it compresses and the wall bulges on one side.
Conditioning counts. 23 °C ± 2 °C and 50 % ± 5 % RH is the standard state; at 40 °C the creep rate of PP runs about three times the room value, so 24 h there equals three days at 23 °C. Give the case 4 h in the chamber first.
3. Reading Deflection and Permanent Set
A stacking test reads deflection with the load on and set after it comes off.
The pass line for the top face is 3 mm or 2 % of height, whichever is smaller. A 300 mm case would allow 6 mm by percentage, so 3 mm governs; a 200 mm case allows 4 mm and 3 mm still governs. The taller the case, the tighter that fixed cap becomes.
Set is read one hour after unloading, limited to 1.5 mm. That hour lets elastic recovery finish: under 1 t, PP gives back seventy percent and the rest over a day.
| Check | Limit | Method |
|---|---|---|
| Top face deflection | ≤3 mm or 2 % of height | Gauges, four edge midpoints |
| Permanent set | ≤1.5 mm | Re-measure 1 h after unloading |
| Side bulge | ≤2 mm | Dial gauge at mid-height |
| Lid dishing | No visible hollow | Visual plus feeler gauge |
| Latch condition | No shear, no release | Open and close three times |
The curve informs too. Seventy percent of deflection lands in the first two hours, then creep flattens; a trace still climbing at hour ten says the wall is short. Gauges at 0.01 mm sit on the four edge midpoints, averaged, with the worst point logged separately.
4. Ribs, Corners and Wall Thickness
Wall thickness is the bluntest lever in a stacking test. Going from 2.5 mm to 3.0 mm cuts deflection to 58 %, since stiffness scales with the cube of thickness, and adds twenty percent in weight. Most of the spread between suppliers sits in that half millimetre.
Ribs cover what thickness cannot. Vertical ribs every 80 mm, 6 mm deep with a 1 mm root radius, raise compressive resistance about 35 % for 8 % more weight, and must run from rim to base.
Set thickness and spacing together. A 3.0 mm wall on 80 mm spacing deflects 2.1 mm at 1 t; the same spacing at 2.5 mm reaches 3.4 mm and fails. Halving spacing to 40 mm pulls 2.5 mm back to 2.8 mm but adds 14 % in weight, which is worse than restoring the wall.
Corners decide where a case gives way. An 8 mm radius with an internal gusset cuts corner stress about thirty percent under 1 t; without it, failure starts as a white stress mark at a corner, moves to a wall bulge and ends with the lid sinking.
Latches are the other weak point. They carry closing force, not stack load, so a wall bulging 1 mm pulls the latch 1 mm and puts shear on a plastic hook, which is where cases pop open after 24 h at 1 t. Keep the latch seat out of the zone of maximum wall movement.
Results belong in the storage rule book. With racking at 1.8 m, a 200 mm case stacks five high. The sixth layer exceeds the rating. Four layers per pallet with a 12 mm board between spreads point load into surface load and cuts deflection 0.8 mm. Run the stacking test to the pressure method of GB/T 4857.4. Report platen size, rate, hold time and chamber temperature.
Conclusion
A stacking test is not about the tonne of steel on the lid but three numbers: the load matching a real stack, a 3 mm cap on deflection, a 1.5 mm cap on set. Set the load from layer count with a 1.5 factor, size the platen at 1/30 of span. Half a millimetre of wall and an 8 mm corner radius decide whether a case sinks slowly or folds.
This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.
Related Reading
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- Choosing a stackable plastic box: layers, load, wall
- Stacking, marking and pallet rules for export packaging
- Checking capacity and samples with a waterproof case supplier
- Where impact-resistant case standards begin
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