How many thermal cycling cycles are enough?

Answer: 10 for storage, 20 for vehicle use, 50 when a contract asks. Most failures appear within 5 cycles; 10 clean cycles with drift under 0.1 kPa clear the basic grade.

Can a silicone gasket hold at -40 °C?

Answer: Yes. Silicone is rated to -60 °C; hardness rises to 75 Shore A and recovery takes about 5 s, so keep 20 % latch margin and 28 % squeeze.

Pressure decay rose after thermal cycling — is it leaking?

Answer: Rule out temperature. Without a warm-up, 40 K yields a false 2.7 kPa. Above 0.5 kPa after 2 h at 23 °C, check corners and vent seat.

Which denominator belongs in the compression set formula?

Answer: The squeeze. A 10 mm cord pressed to 7.5 mm and measuring 9.7 mm gives 0.3 over 2.5, or 12 %; using 10 gives 3 % and passes a bad cord.

Does 70 °C affect the PP case itself?

Answer: Yes. PP modulus at 70 °C is about 40 % of its room temperature value and preload falls 30 %. Dimensions return on cooling; gasket set does not.

Thermal Cycling -40 to 70 °C: What Happens to the Gasket

Quick answer: Thermal cycling pushes the gasket to two limits: at -40 °C it stiffens and cannot follow the shrinking case, at 70 °C it loses load to relaxation. Pass means 25 % set after 24 h at 70 °C and 0.5 kPa decay after ten cycles.

Thermal Cycling Set Points: Temperature, Dwell, Transfer, Count
Gasket Materials and Their Temperature Edges
Cold Stiffening and the Pressure Decay Recheck
Compression Set After 24 h at 70 °C
Differential Expansion and Latch Preload at 110 K
Condensation, Venting and Pass Criteria
FAQ

1. Thermal Cycling Set Points: Temperature, Dwell, Transfer, Count

The set points are not random. 70 °C matches the air at the top of a closed van in summer, -40 °C matches winter storage in cold regions, and the 110 K gap puts wall, latch and gasket at their limits together.

Dwell is 2 h per set point. Transfer stays inside 5 min. A 3 mm PP wall equilibrates in about 1 min, so the 2 h belongs to the 40 L air volume and to the gasket relaxing its compression stress. Transfer is by hand or by ramping at 3 K/min. The two are not comparable. Cycles run 10 for storage, 20 for vehicle work and 50 on military contracts.

Parameter	Value	Purpose
Upper set point	70 °C	Van heat soak
Lower set point	-40 °C	Cold storage floor
Dwell	2 h	Stress relaxation
Transfer	≤5 min	Thermal shock
Cycles	10	Transport grade

2. Gasket Materials and Their Temperature Edges

Heat ageing sets the top end, the glass transition the bottom; only a material clearing both survives thermal cycling.

Thermal cycling: close-up of a white case rim, grey foam gasket seated in the groove beside a rounded lid lip with fine pebbled texture

Silicone spans -60 to 200 °C with set near 12 % at 70 °C, costs 2.5 times EPDM and tears at a sharp groove corner. EPDM covers -40 to 120 °C, enough for most transport work, landing between 20 % and 24 %. Thermoplastic elastomer overmoulds onto the case and skips fitting, but its ceiling is 90 °C and load at 70 °C pushes set past 30 %.

Material	Range	Set at 70 °C	Recovery force
Silicone	-60 to 200 °C	12 %	4.5 N/cm
EPDM	-40 to 120 °C	22 %	3.8 N/cm
Thermoplastic elastomer	-40 to 90 °C	31 %	3.0 N/cm

At -40 °C silicone still behaves like rubber, EPDM sits on its floor, and thermoplastic elastomer takes over ten seconds to recover from a 30 % squeeze.

3. Cold Stiffening and the Pressure Decay Recheck

The cold failure in thermal cycling is not a crack but hardness. Silicone climbs from 50 to 75 Shore A and recovery stretches from 1 s past 5 s, so the lid latches before the gasket conforms and a gap stays at the corners.

Pressure decay is the main recheck: charge to 20 kPa, hold 60 s, accept 0.5 kPa or less. Temperature is where readings fail. A cold sample gains roughly 2.7 kPa as trapped air warms by 40 K, five times the limit, so every test passes. Give it 2 h at 23 °C ± 2 °C first.

Tooling causes the other false pass: a fixture clamping too hard flattens the gasket an extra 1 mm, hides the corner gap and returns a clean reading. Match the latch force, two at 60 N.

4. Compression Set After 24 h at 70 °C

Compression set turns thermal cycling ageing into a figure. GB/T 7759: squeeze 25 %, hold 24 h at 70 °C, release, wait 30 min, measure.

Thermal cycling: white case lying open with a handheld meter and coiled cable in the foam, grey lid foam above, latch at the front

The denominator is where the arithmetic fails. A 10.0 mm cord limited by a 7.5 mm spacer sees 2.5 mm of squeeze; measured at 9.7 mm, set is 0.3 over 2.5, or 12 %, not over 10.

The line is 25 %. Silicone at 12 % and EPDM at 22 % stay inside; thermoplastic elastomer at 31 % takes a flat spot after one summer, and the next closure rests on the latch crushing it. Every 10 K doubles relaxation, so a cord passing at 70 °C reads above 30 % at 90 °C.

5. Differential Expansion and Latch Preload at 110 K

Under thermal cycling PP expands at about 1.5 × 10⁻⁴/K. Over the 110 K gap a 1200 mm rim grows 19.8 mm while a silicone gasket at 2.2 × 10⁻⁴/K grows 29 mm; the spare 9 mm goes to the corners. A 1.5 mm allowance in groove depth and radiused corners give it room, otherwise the corners lift and the leak starts there.

The latch side runs a different sum. At 70 °C PP flexural modulus falls from 1300 MPa to 550 MPa and preload drops from 120 N to 78 N, then returns on cooling, but the gasket has taken 0.2 mm of set. A 6.0 mm groove with an 8.5 mm cord gives 29 % squeeze cold and 33 % hot, inside the 30 % to 40 % band; start at 38 % and the hot value passes 42 %.

6. Condensation, Venting and Pass Criteria

Air inside the case carries moisture at 70 °C. Cool to -40 °C and it condenses in the groove and freezes; ice takes 9 % more volume than water and pries a 0.1 mm gap open. A vent with a 0.2 μm ePTFE membrane and 3 kPa cracking pressure lets air pass only above that differential. Without one, thermal cycling doubles pressure decay by the third round, from 0.2 kPa to 0.6 kPa, traced to a weld line at the vent seat.

Check	Limit	Method
Visible cracks	None at 10×	Corners, groove floor
Pressure decay	≤0.5 kPa	20 kPa, 60 s hold
Compression set	≤25 %	GB/T 7759, 70 °C × 24 h
Latch force	Change ≤15 %	Gauge, three closures

Log the measured temperature within ±2 K of set point, transfer time and cycle number.

Conclusion

Thermal cycling does not test whether a case survives cold; it tests whether the gasket still carries load across a 110 K gap. Set points define the profile, material sets the ends. Silicone finishes with 12 % set, EPDM at 22 %, thermoplastic elastomer over the line at 70 °C.

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.