Can a case stack test stop at 24 hours?

Answer: Only as a pre-shipment check. Day one covers 30 percent of deformation and the other 70 percent develops at sea, so cases passing at 24 hours often arrive with 3 mm of new sink.

Is 30 days worth three extra weeks?

Answer: It adds 6 percent, 3.40 mm against 3.62 mm. Unless the contract names 30 days, seven days is enough, since the extra weeks deliver less than the measurement error.

Why do two suppliers differ by a factor of two at the same duration?

Answer: Almost always the loading method. Real stacking, spread weights and single-point pads read 3.4 mm, 4.9 mm and 7.2 mm at day 7, and an unnamed method makes numbers incomparable.

Should temperature be locked at 23 °C?

Answer: Set it by destination. Hot-climate exports test at 40 °C and temperate markets at 23 °C, a 1.7 mm gap at day 7; quoting 23 °C to a tropical customer is a claim you will pay for.

When should permanent set be measured?

Answer: 24 hours after unloading, always. Reading within the hour counts 0.3–0.4 mm of ongoing recovery as set and overstates a 7-day sample by 10–15 percent.

Case Stack Test: 24 Hours vs 7 Days

Quick answer: The same case sinks 1.1 mm in 24 hours and 3.4 mm in 7 days, so a case stack test judged at one duration cannot serve the other.

Why stack duration decides the verdict
Where the 24-hour and 7-day curves diverge
Deformation against temperature
Loading method: real stacks versus weights
Which time point to judge
FAQ

1. Why stack duration decides the verdict

A 600×400×220 mm PP case under 300 kg of spread load sinks 1.1 mm at the base centre in 24 hours, 3.4 mm at day 7 and 3.62 mm at day 30. The number keeps moving, so a threshold means nothing without the clock beside it.

Plastic is not a spring. Elastic response covers 12–15 percent of the total; the rest arrives as chains slide, a process measured in days, so a case stack test fixes a stopping point rather than a time.

Twenty-four hours comes from shipping routine: press overnight, load next morning. Seven days matches a sea leg plus customs. A case specified at 24 hours holds with a 3.0 mm wall; the same load at 7 days needs a 3.6 mm base or two ribs 25 mm tall, adding 8–12 percent to material cost.

2. Where the 24-hour and 7-day curves diverge

Creep runs in three stages: a fast 2-hour stage carrying about half the total, a transition to 72 hours at a quarter of that slope, then a steady stage that flattens without reaching zero.

Case stack test in a warehouse: a worker holds a beige case between shrink-wrapped stacks of beige and black cases on a green floor

Time	Sink	Share of total	Daily gain
1 h	0.42 mm	12%	10.1 mm
8 h	0.86 mm	24%	2.58 mm
24 h	1.10 mm	30%	1.10 mm
72 h	1.85 mm	51%	0.62 mm
168 h	3.40 mm	94%	0.49 mm
720 h	3.62 mm	100%	0.12 mm

Day one covers 30 percent, day seven covers 94 percent. Testing only the first leaves 70 percent of the deformation outside the report; testing past day 7 adds 6 percent, already inside measurement error.

The knee sits between 60 and 80 hours, where temperature lifts the slope again. A workshop at 26 °C by night and 38 °C by day produces 0.2–0.4 mm steps that read like a second collapse and are thermal expansion stacked on accelerated creep.

3. Deformation against temperature

Holding case and load constant, 7-day sink measures 3.4 mm at 23 °C, 5.1 mm at 40 °C, 7.4 mm at 50 °C and 1.2 mm at -10 °C. The 23 to 40 °C step adds 50 percent, and the next 10 °C adds another 45 percent.

The response is not linear. PP sits well above its glass transition near 0 °C, so warming raises chain mobility exponentially; creep rate runs 1.4–1.6 times faster per 10 °C, and past 45 °C by over 1.8 as the material nears its heat deflection range.

Every case stack test report needs a temperature line. A 3.4 mm result at 23 °C becomes 5.1 mm in a 40 °C warehouse, and quoting the cooler figure assumes the customer air-conditions the building. Summer shipments are the trap: interiors reach 60–65 °C, and cases approved at 23 °C arrive with 6–9 mm of set.

4. Loading method: real stacks versus weights

The loading method sets the stress field, and the approaches differ by over 40 percent, so any duration claim must name the method behind it.

Case stack test support review in a mould store: green steel racks holding steel moulds on five levels, blue kanban boards, yellow floor line

Method	Contact	24 h	7 days	Use
Real stack, 3 high	300×400 mm face	1.10 mm	3.40 mm	Pre-shipment
Weight, spread	Full steel plate	1.60 mm	4.90 mm	Design comparison
Weight, single point	100×100 mm pad	2.40 mm	7.20 mm	Limit screening

Real stacking is closest to service but poorly repeatable: ribs rarely align, load shifts to one side, and three samples differ by 0.6 mm over 7 days. It suits batch release, not design work.

Weights give control. A full plate holds eccentricity under 5 percent and reads 4.90 mm at day 7, 44 percent above real stacking, because steel does not shed load as a deforming upper case does. A 100×100 mm pad raises local stress twelvefold and leaves a dimple, so it finds limits rather than accepting batches.

5. Which time point to judge

The judgement point follows the application, not a standard: 7 days for storage under 30 days, 30 days beyond 45 days at sea, 24 hours for internal transfer.

Thresholds scale between 24 hours and 7 days, and only inside that window. With 1.1 mm against 3.4 mm, a 4.0 mm limit at day 7 corresponds to 1.3 mm at hour 24, not the same 4.0 mm carried across.

Recovery is counted separately. A sample that sinks 3.4 mm returns to 2.9 mm after a day of rest, so the verdict reads the recovered figure; judging the loaded peak overstates deformation by 15 percent.

Measuring points belong in the method: base centre, four edge midpoints and each corner, with the maximum reported, since centre and corner differ by 0.8 mm. Write the purchase clause in three parts: real stack 3 high, 40 °C, 7 days, permanent set under 4.0 mm.

Conclusion

Duration and threshold travel together: 1.3 mm at 24 hours, 4.0 mm at 7 days, 4.2 mm at 30 days. Move a limit across durations and the decision flips, releasing early collapse at one end and scrapping sound designs at the other.

Work in three steps. Fix the application, taking 7 days for storage under 30 days and 30 days beyond 45 days at sea. Fix the method, real stacking for batch release, a spread plate for design comparison. Then write temperature, time, measuring points and limit into one clause. A longer test is not safer; it decides where the curve gets cut, and that cut makes the numbers comparable.

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