How many case thermal cycling cycles are enough?

Answer: 100. Gap and squeeze stay repeatable and the curve flattens past 150. At 30 large swings a year that covers three years of transport exposure.

Can the dwell at -40 °C and 70 °C be shortened?

Answer: Not below two hours. A 3 mm wall has a 12 minute constant, so two hours is ten constants; at 30 minutes the gasket sees 18 °C less range.

What changes between a 3 and a 5 °C/min ramp?

Answer: The through-wall gradient, 4 versus 9 °C. At 5 °C/min cracks appear at the body corners and the groove failure is masked.

Does a breather valve stop condensation?

Answer: No. It passes vapour, so a 10 L cavity at 60% RH still collects 3.1 g of wall water over 100 cycles.

Will another gasket material fix cold stiffening?

Answer: Half of it. Silicone at TR10 -55 °C gains 6 points, EPDM at -45 °C gains 14; the rest moves to groove width and corner radius. ---

Case Thermal Cycling: -40 to 70 °C

Quick answer: Case thermal cycling at -40 °C for two hours widens the lid-to-body gap from 0.05 to 0.45 mm, drops gasket squeeze from 25% to 14% and costs 38 N of latch preload.

Four Parameters That Define Case Thermal Cycling
The Gap After Two Hours at -40 °C
Pressure, Lid Bulge and the Relief Valve
Gasket Versus Plastic: Differential Shrinkage
Condensation, Dew Point and the Pass List
FAQ

1. Four Parameters That Define Case Thermal Cycling

A valid case thermal cycling run comes from four numbers: 3 °C/min ramp, two-hour dwell at each end, 100 cycles, ±2 °C uniformity inside the case. Change one and the failure moves to another corner.

Push the ramp to 5 °C/min and the through-wall gradient grows from 4 to 9 °C. The skin contracts while the core still expands, so cracks migrate to the body corners and the groove looks clean when it is not.

The dwell comes from equilibrium. A 3 mm wall carries a 12 minute time constant, so two hours is ten constants and the air-to-wall difference closes to 1 °C. Cut it to 30 minutes and the gasket sees 18 °C less range.

Stage	Temperature (°C)	Ramp	Dwell	Tolerance
Cold	-40	3 °C/min	2 h	±2 °C
Hot	70	3 °C/min	2 h	±2 °C
Transfer	20	within 5 min	none	—
Cycles	100	logged	—	—

2. The Gap After Two Hours at -40 °C

ABS shrinks at 8×10⁻⁵/K, so a 520 mm body loses 2.5 mm across a 60 K swing. The silicone cord beside it runs at 2.5×10⁻⁴/K and loses 7.8 mm. The 5.3 mm difference leaves through the corners, because the groove is only 6 mm wide.

Case thermal cycling sample in black, lid open, egg-crate foam, base foam cut into stepped cavities, two latches and a centre handle

Measured gaps climb faster than arithmetic: 0.05 mm on the first assembly, 0.12 at 25 cycles, 0.45 at 100. Half arrives while elastic recovery lags, the rest is compression set with the cord dropping from 8.00 to 7.42 mm.

Cycles	Gap (mm)	Squeeze	Latch force (N)	First failure
First part	0.05	25%	180	none
25	0.12	23%	172	none
50	0.24	20%	158	groove corners
100	0.45	14%	142	corners, latch bosses
150	0.62	11%	128	hinge side

The latch deforms second. A PA66-GF30 boss shrinks 0.6 mm less than the ABS body over 60 K, so preload falls from 180 to 142 N; below 120 N the lid lifts a 0.1 mm slit and the 60 s leak check jumps from 0.4 to 2.1 kPa.

3. Pressure, Lid Bulge and the Relief Valve

The hot half is the half people underestimate. A sealed 10 L cavity rising from 20 to 70 °C gains 17 kPa; falling to -40 °C turns it into -20.9 kPa of vacuum.

Without a valve, 17 kPa bows a 520 mm lid 0.6 mm at mid-span while the corners stay put. The vacuum side is worse to use: -20.9 kPa holds the lid shut and opening force climbs from 90 to 160 N.

A breather valve removes most of it. Membranes cracking at 3.5 to 5 kPa cap the differential near 5 kPa and bulge drops to 0.15 mm. Cold defeats them: silicone hardens from 50 to 62 Shore A at -40 °C, cracking pressure rises to 12 kPa, and 100 cycles leave 0.3 mm of set, so reverse flow starts only at -25 kPa.

4. Gasket Versus Plastic: Differential Shrinkage

Material choice is the hardness gain at -40 °C plus the set after 100 cycles, not a feel test at room temperature. At 70 Shore A, silicone stiffens 6 points, EPDM 14, and PVC enters crystalline hardening.

Case thermal cycling production line, two workers on red stools assembling case bodies along a long bench, ceiling fans overhead, film-wrapped cases stacked at left

The pass line for set is 25%. Silicone lands at 18% and still holds 25% squeeze; EPDM reaches 26% and squeeze falls to 19%; PVC reaches 41%, the same as leaving the groove empty.

Material	TR10 (°C)	Gain at -40 °C	Set, 100 cycles	Verdict
Silicone 70 Shore A	-55	6 points	18%	usable
EPDM 70 Shore A	-45	14 points	26%	stiff when cold
PVC 65 Shore A	-20	crystalline	41%	not suitable

A 6 mm groove with an 8 mm cord gives 25% squeeze and 2 mm for growth. The silicone loses 1.9 mm more than ABS per 100 mm, and a square corner turns that into tension, thinning the cord 0.15 mm and dropping squeeze to 17%; an R8 corner holds 22%.

Metal follows none of it. A 6 mm stainless pin moves 0.007 mm across 60 K while the ABS moves 0.03 mm, so the bore opens from 6.02 to 6.05 mm and the lid rocks 3 degrees after 100 cycles.

5. Condensation, Dew Point and the Pass List

Condensation never shows in the gap, only in the water left inside. Air at 20 °C and 60% RH has a dew point of 12 °C, so one pass through 12 °C lasting over 8 minutes deposits a film; across 100 cycles the wall water in a 10 L cavity climbs from 0.2 to 3.1 g. The valve does not stop it, since the membrane passes vapour either way; holding 3 °C/min lets cavity air cool against the walls instead of the groove.

Fail the run when two of four items trip: gap above 0.3 mm, squeeze below 18%, latch force down 20%, or more than 1 kPa drop holding -20 kPa for 60 s. Measure within 24 hours of unloading, because reading straight out of the chamber understates the gap by 0.08 mm.

Conclusion

Read the results by cycle count. Inside 50 cycles the groove corners lead, 0.24 mm gap and 20% squeeze, and an R8 corner with 2 mm of allowance still saves it. At 100 cycles the latch bosses lead, 38 N gone and a 0.45 mm gap, and a new cord is too late because the preload travel needs redesign. At 150 the hinge side leads with 3 degrees of rock, which is structure, not sealing.

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