Where exactly do the 0.8 kg go on a single vs double wall case?

Answer: 0.56 kg in the inner skin, 0.2 kg in 24 ribs and 0.04 kg in hardware. The 12 mm cavity holds 4 g of air.

How much does the double wall cut a 1.2 m drop peak?

Answer: From 68 G to 55 G, 19% lower, with the pulse stretched from 2.4 ms to 3.1 ms. The gain comes from 12 mm of travel.

Why does the double wall cost 38% more?

Answer: Tolerance spreading to ±0.8 mm, 40 s of extra cleaning, and 55% more tooling with a 9 s longer cycle.

Where does the 15 ℃ line come from?

Answer: The dew point. At 25 ℃ and 70% RH it sits near 19 ℃, so a 20 ℃ spread breaches a single wall within 30 min, while 15 ℃ leaves 1.3 ℃.

Single vs Double Wall Case

Q: When is a single wall enough?

Answer: Humidity at or below 60%, trips under 2 h, a spread of 15 ℃ or less and no condensation-sensitive load, all four together.

Quick answer: Two shells, one 0.8 kg gap: a single vs double wall case weighs 2.1 kg against 2.9 kg, and the 12 mm cavity holds only 4 g of air.

Where the 0.8 kg comes from
Thirty minutes in a cold chamber
A 1.2 m drop, 68 G against 55 G
What the second wall costs
Where single stops and double starts
FAQ

1. Where the 0.8 kg comes from

Two shells of 400 mm × 300 mm × 200 mm go on the scale: 2.1 kg single, 2.9 kg double. The single wall is one 3.0 mm skin. The double is a 3.0 mm outer skin plus a 2.0 mm inner skin, held 12 mm apart by 24 ribs.

Air is 4 g of that 0.8 kg. A 12 mm gap over the whole shell holds about 3.2 L, and at 1.2 kg/m³ it weighs nothing. The mass sits in the inner skin: 0.31 m² at 0.91 g/cm³ for PP gives 0.56 kg, the ribs add 0.2 kg, and 0.04 kg is hardware.

Item	Single wall	Double wall
Wall build	one 3.0 mm skin	3.0 mm outer + 2.0 mm inner
Cavity	none	12 mm
Ribs	16	24
Weight per case	2.1 kg	2.9 kg
Moulding cycle	69 s	78 s
Tooling	one two-plate tool	three-plate tool with 2 slides

The gap buys thermal resistance and crush travel. It adds almost nothing on the scale.

2. Thirty minutes in a cold chamber

Both cases enter a chamber at −10 ℃ with a logger inside, starting at 20 ℃. After 30 min the single reads −5.8 ℃, 4.2 ℃ above ambient; the double reads −8.4 ℃, 1.6 ℃ above. Half an hour buys 2.6 ℃ of hold.

Single vs double wall case samples beside a long table, coloured cases on shelves and beige office chairs

Still air conducts at 0.026 W/(m·K), which would give 0.46 m²·K/W across 12 mm. Convection and radiation inside the cavity cut the effective value to about 0.18 m²·K/W. Widening the gap from 12 mm to 20 mm adds only 0.03, because past 15 mm convection takes over.

The cavity is vented, not sealed. Two Ø3 mm holes sit between the skins, or a temperature swing would bulge the inner wall. Those holes also pass moisture, and a wet cavity falls below 0.05 m²·K/W, worse than a single wall.

Temperature only matters through condensation. At 25 ℃ and 70% RH the dew point is about 19 ℃, so a move into a 5 ℃ cold room puts a single wall under it within 30 min, while the double keeps 1.3 ℃ of margin.

3. A 1.2 m drop, 68 G against 55 G

One rig, one 6 kg ballast, 1.2 m free fall, three corner drops: 68 G single, 55 G double, 19% lower. Pulse width runs 2.4 ms against 3.1 ms.

The peak falls because of the cavity, not a harder wall. The impact compresses the 12 mm gap first, stretching the pulse and cutting the peak. In this role the inner skin is a bumper, not a stiffer plate.

That margin only matters for some loads. A part rated to 100 G passes either way. A part rated to 60 G fails the single and just passes the double, and those 13 G are the argument.

Ballast changes it: at 12 kg the single climbs to 82 G and the double to 64 G, an 18 G gap. Heavier loads bottom out the travel sooner, so the benefit rises then fades, and past 15 kg the two converge.

Read peak with pulse width. Peak alone confuses damping with stiffness, and a median of three drops says more about consistency than the worst single hit.

4. What the second wall costs

The first cost is stacked tolerance. The 3.0 mm skin runs ±0.15 mm, the 2.0 mm skin the same, and the cavity is set by the parting line at ±0.5 mm. Total thickness spreads to ±0.8 mm and seating flatness loosens from 0.3 mm to 0.6 mm, enough to leave one corner hanging on a bench.

The second is a cleaning trap. The joint between skins is a groove 12 mm deep and 2.5 mm wide, too narrow for a brush, so it needs air and a flush. Cleaning goes from 90 s to 130 s, 40 s more, or 5.5 h across 500 pieces.

The third is tooling. A second skin needs 2 slides and 1 lifter, tooling rises 55%, and the cycle stretches from 69 s to 78 s.

Single vs double wall case open on a table, egg-crate foam in the lid and coloured cases on wooden shelves behind

Cost	What it looks like	Measured
Stacked tolerance	three tolerances add up	±0.8 mm total, 0.6 mm flatness
Cleaning trap	ring groove at the skin joint	12 mm deep, 2.5 mm wide, 40 s more
Tooling complexity	extra slides and a lifter	+55% tooling, +9 s cycle

Add the three and the double is 38% dearer per case. At USD 300 a unit that is 114, or 114,000 across 1000 pieces. On 100 pieces the tooling premium alone works out at 90 each.

5. Where single stops and double starts

A single wall is enough on four checkable conditions: humidity at or below 60%, round trips under 2 h, a spread of 15 ℃ or less, and no condensation-sensitive load. When all four hold, 2.6 ℃ and 13 G buy nothing.

Double is required on four too: a spread above 15 ℃, cavity dew-point margin under 3 ℃, an overnight outdoors or a long stay in a car boot, and a one-way run over 4 h with a temperature step. Two hits make the cavity necessary; one says try a divider first.

The borderline band is 10 ℃ to 15 ℃. Measure it: a single wall with one 3 mm divider, three round trips, logging the lowest cavity temperature and the dew point. Two breaches and you move up, none and you stay.

Inspection needs no cut-open sample. A 0.5 mm ultrasonic gauge at five points on the base, minimum value taken as cavity thickness, settles it: two points under 11 mm means a thick inner skin or a shifted rib, and that batch's thermal claim is discounted 8%.

The dividing line is not price. It is spread and dew point, both measurable before an order is placed.

Conclusion

The single vs double wall case question usually ends in a hand-feel argument, when three numbers settle it: weight, 30 min spread and 1.2 m drop peak. The 0.8 kg touches handling and freight only, and at USD 6 per kg across 2000 shipments a year that is 12,000, a rounding error against 114,000 on a 1000-piece order. What decides whether a load survives is the 2.6 ℃ and the 13 G, and those map onto dew-point margin and component shock rating, both measurable before an order exists.

Metric	Single wall	Double wall	Delta	What it decides
Weight per case	2.1 kg	2.9 kg	+0.8 kg	freight only
Spread after 30 min	4.2 ℃	1.6 ℃	−2.6 ℃	dew-point margin
1.2 m drop peak	68 G	55 G	−13 G	component rating
Cost index	100	138	+38%	order quantity

One line decides it: go double only when the spread exceeds 15 ℃ or the cavity dew-point margin falls under 3 ℃. When neither holds, a single wall with one 3 mm divider is the cheaper answer.

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