How long should a frozen sorting case soak at -25 °C?

Answer: 24 h. A 2.5 mm wall cools in 4 h, but a 4.5 mm rib root needs over 9 h, and a short soak reports cracks 6 to 8 mm short.

How far apart are homopolymer and copolymer PP?

Answer: At -25 °C under a 1 kg ball from 1.0 m, homopolymer splits 42 mm and copolymer 26 mm, a 38% gap from the ethylene segments.

Is R25 worth the tooling change?

Answer: Yes. The stress factor drops from 2.4 to 1.6 and the crack from 30 mm to 9 mm with no added weight, for 8% more tooling cost.

Can a different latch material fix the cold drag?

Answer: Only partly. A cold-grade copolymer takes 95 N down to 78 N, but clearance and condensation dominate, so a 0.3 mm drain gap and a removable hinge pin help more.

Frozen Sorting Case at -25 °C

Quick answer: A frozen sorting case rarely fails by load, it fails by knocks: a 1 kg ball from 1.0 m on a corner chilled for 24 h split homopolymer PP by 42 mm, copolymer by 26 mm and HDPE by only 11 mm, and a corner radius of R25 cuts a 30 mm crack to 9 mm.

Why the crack starts at the corner
Ball drop after 24 hours at -25 °C
PP and PE embrittlement in the cold
Corner radius and wall thickness
Latch drag and the dock temperature shock
FAQ

1. Why the crack starts at the corner

A frozen sorting case left a -25 °C cold store, caught the dock step and split 30 mm along a corner. The shell showed no bulge, and the fracture face was flat and bright with no drawing, a brittle break.

The same drop at 18 °C left a 2 mm scuff, because impact toughness falls and the energy has nowhere to go but a crack.

Corners gather three faces, the sharpest section change and usually the weld line, which holds about half the strength of the parent wall in the cold.

So for a frozen sorting case, low temperature is a structural condition rather than an environment.

2. Ball drop after 24 hours at -25 °C

Fix the parameters: 1 kg steel ball, 1.0 m drop, impact on the three-face corner, within 30 s of 24 h at -25 °C. That is 9.8 J, close to a 25 kg loaded case landing on its corner from 0.4 m.

Frozen sorting case in navy blue opened on steel, lid foam with ten vials and four blue ice packs, bandages and a blood bag

The 24 h soak is not padding. With ribs at 4.5 mm at the root, the core needs over 9 h to equalise from -25 °C, and a 4 h soak reports cracks 6 to 8 mm short.

Whole-case drops over six faces and three edges do not always agree: the case that split 30 mm also released its latches twice, while the 9 mm case passed. The ball screens, the drop decides.

Material	Crack at 23 °C	Crack at -18 °C	Crack at -25 °C
Homopolymer PP	12 mm	30 mm	42 mm
Copolymer PP	6 mm	18 mm	26 mm
HDPE	4 mm	8 mm	11 mm

3. PP and PE embrittlement in the cold

Homopolymer PP has its glass transition near 0 °C and embrittles around -10 °C, so -25 °C sits deep inside the brittle zone. Copolymer PP adds ethylene segments that push embrittlement to about -25 °C and shortens the crack 38%.

HDPE embrittles below -60 °C and splits only 11 mm at -25 °C, but it flexes: at 2.5 mm walls a three-high stack sags 4.0 mm, against 1.5 mm for PP.

A frozen sorting case therefore trades impact against stiffness: dropping favours HDPE, stacking favours copolymer PP.

Recyclate shifts the answer again: 30% regrind moves copolymer PP from 26 mm to 34 mm because the weld line weakens.

4. Corner radius and wall thickness

The corner radius sets the stress concentration factor: R10 near 2.4, R25 near 1.6, a 33% lower peak that shortens the crack from 30 mm to 9 mm at no weight cost.

Frozen sorting case showroom with white shelving full of black, yellow and orange hard cases, one man in a white polo taking a selfie

Wall thickness comes second. From 2.5 mm to 3.0 mm the crack falls from 30 mm to 14 mm, but the case grows from 9.5 kg to 10.1 kg, another 480 kg a day across 800 handling cycles, and past 3.2 mm the gain flattens.

Corner	Stress factor	Crack at -25 °C	Added weight	Tooling
R10, 2.5 mm wall	2.4	30 mm	9.5 kg base	None
R15, 2.5 mm wall	1.9	17 mm	0	Slider tweak
R25, 2.5 mm wall	1.6	9 mm	0	New cavity, +8%
R25, 3.0 mm wall	1.6	6 mm	0.6 kg	New cavity, +8%

Rib layout matters more: below R2 at the root the crack jumps there instead and grows 5 mm, so hold the root at R2.

5. Latch drag and the dock temperature shock

Latch force rises from 42 N at room temperature to 95 N at -25 °C. A gloved hand manages about 120 N, so the limit holds, but each latch takes 3.5 s instead of 1.2 s, and 600 cases an hour lose 8% of pace.

The hinge pin is the second snag: POM shrinks more than its PP seat, clearance opens from 0.15 mm to 0.28 mm and the lid shifts 1.5 mm, which a matching pin cuts to 0.5 mm.

Temperature shock is a dimensional sum: from -25 °C to an 18 °C dock the surface swings 43 °C, 600 mm at 1.5 × 10⁻⁴/K moves 3.9 mm, and lid and base differ by 0.8 mm of misalignment at the latch.

Condensation adds to it: a 0.4 mm film forms within 6 min, and after 20 dock cycles the grease washes out and force rises 12 N, so fit a 0.3 mm drain gap.

Check	Condition	Criterion	Measured
Corner ball drop	24 h at -25 °C, 1 kg, 1.0 m	Crack 5 mm or less, no penetration	6 mm on R25
Whole-case drop	-25 °C, 1.0 m, six faces and three edges	No latch release, no hinge crack	All six faces pass
Latch force	-25 °C, one gloved hand	120 N or less	95 N
Cycle life	-25 °C, 20 cycles	No seizure, shift 1 mm or less	0.5 mm shift

Conclusion

Put the low-temperature verdict for a frozen sorting case on an acceptance table, not in the phrase cold resistant: 24 h at -25 °C, a 1 kg ball from 1.0 m on the corner with a crack of 5 mm or less, a full drop at 1.0 m over six faces with no latch release, latch force at 120 N or below, shift under 1 mm across 20 cycles. Then map crack length to one structural move: 26 to 42 mm means change the compound and open the radius, 14 to 26 mm means R25 alone, and anything above 14 mm means 2.5 mm to 3.0 mm with the handling cost checked. The order is not reversible, because thickening before rounding the corner still leaves a 20 mm crack. Three callouts, R25, the copolymer grade and R2 at the rib root, usually clear the gate first time.

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