Can a case still be used after three years of case outdoor storage?

Answer: It can, at reduced duty. Month 36 leaves 63% tensile retention and a 1.2 m drop cracks a corner, so stack height drops from 4 to 2.

Why is ΔE 4 the appearance line?

Answer: ΔE 4 is where the eye separates two surfaces at 0.5 m; below that only a side-by-side comparison works. The gasket has hardened 8 Shore A by then.

Will four-high stacking crush the bottom case?

Answer: It will not collapse but it creeps. Each bottom corner carries 90 N and loses 0.6 mm a year, 1.8 mm over three years, and collapse risk appears at 55 ℃.

How should a yard be laid out to extend case outdoor storage?

Answer: Shade the sun, lift the base 100 mm on pallets and turn cases north. The three push the crossing back 8 to 10 months; each 10 K less cuts scission by 45%.

How fast does tensile retention fall after 70%?

Answer: The slope moves from 11 points per 10 MJ/m² to 15, giving 63% at month 36 and about 42% at month 48. Year three is the last window.

Case Outdoor Storage: Outdoor Case Storage Over Three Years

Quick answer: Case outdoor storage for three years ends in a brittle latch snap, with gloss down from 12 GU to 3 GU and tensile retention at 63%.

A three-year order: whitening, embrittlement, crushing
UV dose accounting: 70% tensile at month 26
What ΔE 4 looks like at what distance
Thermal cycling and four-high stacks
Year 1, 2, 3 checks and when to move inside
FAQ

1. A three-year order: whitening, embrittlement, crushing

Year one only whitens. At month 12 the 60° gloss on the lid had fallen from 12 GU to 7 GU, a finger left fine powder and a damp cloth left a pale smear. Latch force rose from 42 N to 46 N and the gasket still recovered 88%.

Year two splits the colours. By month 18 the gap between lid and base crossed ΔE 4 while control cases kept indoors sat at ΔE 1.6. By month 26 tensile retention hit 70%, the specimen falling from 28 MPa to 19.6 MPa as the break turned brittle.

Year three belongs to the latch. At month 36 it snaps audibly, a −5 ℃ drop test cracks it outright and the crack runs 18 mm along the latch seat root. The case still holds gear, but it can no longer carry the original stack height.

Month	Gloss GU	ΔE	Tensile	Latch N	Action
6	10	1.2	96%	43	Normal use
12	7	2.4	91%	46	Log
18	5	4.0	84%	49	Half shade
26	4	5.1	70%	54	2 high
36	3	6.3	63%	62	Indoors

The order cannot be reversed: whitening does not touch sealing, embrittlement decides whether the latch carries load, and colour is only the early warning.

Placement rewrites the timeline inside one yard. A 30° south slope collects 22% more annual dose than an upright case in shade, and turning a case so the latch faces north pulls month-18 colour from ΔE 4.0 to 3.1.

2. UV dose accounting: 70% tensile at month 26

340 nm penetrates a case wall deeper than any other UVA band, and the tertiary carbon bond in polypropylene absorbs exactly there. 0.76 W/m² is the standard UVA-340 setpoint: 1000 h in the chamber equals roughly 14 months of single-face exposure in southern China.

Case outdoor storage check: black hard case open on a grey studio sweep, diced base foam holding a black instrument, twin latches, ringed purge valve

Counting half the days as sun, 0.76 W/m² gives about 12 MJ/m² a year per face. Month 26 lands at 26 MJ/m² with tensile retention at exactly 70%, within 4% of the 1900 h chamber result. The early curve is nearly straight, losing 11 points per 10 MJ/m².

Heat pushes that curve left. A lid measured 62 ℃ at noon in summer, 18 K above air, and every 10 K of surface temperature multiplies chain scission by about 1.8, so a case leaning south crosses the line 6 months before an identical case in shade.

Past 70% the slope steepens. Month 36 reads 63%, and the same gradient puts month 48 near 42%, where a 1.2 m drop already cracks a corner.

3. What ΔE 4 looks like at what distance

ΔE 4 is visible at 0.5 m; ΔE 2 needs two cases side by side before anyone can split them. Measurement runs an 8 mm aperture spectrophotometer, D65 light and 45/0 geometry, averaging 3 points on the lid centre where single readings scatter within 0.3.

On white cases ΔE 4 reads as yellowing, b* climbing from 1.8 to 6.2. On dark cases the same number reads as greying, L* rising 3.5.

Colour does not affect sealing, but it is the best proxy available. By the time ΔE reaches 4 the gasket has hardened 8 Shore A and taken 31% permanent set, the edge of leaking.

4. Thermal cycling and four-high stacks

The daily cycle outdoors runs −15 to 55 ℃, 365 times a year. Case and lid shrink 0.8 mm apart, widening the gasket groove fit from 0.4 mm to 0.9 mm. That gap does not leak at once, but it lets the lid chatter in wind and adds one micro impact per day at the latch.

Case outdoor storage comparison rack: three wood grain shelves of small closed coloured cases in blue, orange, purple and green, lower row wrapped in film

Stacked four high, the bottom case carries three cases: 36 kg of static load over 4 feet, about 90 N per corner. Crushing runs 0.6 mm a year, 1.8 mm by year three, and the body diagonal grows from 512 mm to 514 mm.

Load and temperature compound. At 55 ℃ the flexural modulus of PP drops about 40%, so the same 36 kg presses twice as deep on a July afternoon.

Stack	Load per corner N	Crush mm/yr	3-year mm	Guidance
2 high	30	0.2	0.6	Long term
3 high	60	0.4	1.2	Retest yearly
4 high	90	0.6	1.8	Drop a tier in year 3
5 high	120	0.9	2.7	Not advised

The bottom crushes three times faster than the top because it also carries wind load and vibration.

5. Year 1, 2, 3 checks and when to move inside

Year one covers 4 items: lid gloss, colour between lid and base, latch force and gasket recovery. All four fit in 30 min without opening the case, using a gloss meter and a force gauge.

Year two adds 2 more: corner cracks and rib whitening. Ribs are the thinnest section at 3.0 mm walls, so whitening means local yielding, and a fingernail dragged across it catches a step.

Year three adds 4: body diagonal, lid-to-base fit gap, purge valve operation and a stack height review, measured with every case empty.

Three verdict lines trigger a move indoors, any one sufficient: tensile retention below 70%, colour past ΔE 4 together with latch force above 60 N, or corner crush beyond 1.5 mm. A case passing all three still works, but only at two high.

Mitigation is three moves: a canopy to block direct sun, pallets lifting the base 100 mm, and turning cases from a south slope to upright in shade. They delay the crossing by 8 to 10 months, and only delay.

Conclusion

Case outdoor storage is settled by three numbers logged over three years, not by how a lid feels: gloss, ΔE and tensile retention. Log for 12 months, shade from month 18, cut the stack to two high at month 26, and move the case indoors at month 36. The schedule follows the 340 nm, 0.76 W/m² dose curve, which costs 11 points per 10 MJ/m² before 70% and 15 points after, so delay gets dearer every month. The operational step is one sheet at the yard gate: work down the row matching the year, tick each item, and treat a missing entry as a failed check. If ΔE 4 and a 60 N latch arrive together in year two, jump to the year-three action instead of waiting for the anniversary.

Year	Checks	Verdict line	Action
Year 1	Gloss, colour, latch force, gasket recovery	ΔE ≤2.4, latch ≤46 N	Log, keep 4 high
Year 2	Add corner cracks and rib whitening	ΔE ≤4.0, tensile ≥84%	Half shade, 3 high
Year 3	Add diagonal, fit gap, valve, stack review	Tensile ≥70%, crush ≤1.5 mm	Move indoors or 2 high

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