Why is the cavity in a military rotomolded case bigger than equipment plus two walls?

Answer: The wall serves the case only; the 25 mm EPE still leaves 18 mm of travel, and that travel absorbs the drop.

The drawing says 4.5 mm and the gauge reads 3.9 mm. Reject?

Answer: Below 4.0 mm, yes. A spread above 1.0 mm in a batch is worse: powder is out of control.

Is five high set by strength or by rack height?

Answer: Whichever is lower. Here clearance binds; on 2.7 m racks six fit but the margin narrows.

Why 8 N·m on the inserts rather than tighter?

Answer: The thread grips on the knurled insert body; excess torque crushes the seat and pulls it out sooner.

Can one cavity serve two outer envelopes?

Answer: Yes, if tooling ownership is clear. Cavity comes from the core, ribs and feet from the frame; a new frame takes 25 days.

Military Rotomolded Case Specifications: Size, Wall and Load

Quick answer: Specify a military rotomolded case from the outside in: fix the equipment outline and transport platform, derive the cavity, reverse the envelope, then commit wall zones, insert torque and load figures.

Start the Spec from the Equipment Outline
Cavity Size and Cushion Thickness
Wall Thickness Zones and Metal Inserts
Rated Load and Stacking Height
What the Spec Sheet Must State
FAQ

1. Start the Spec from the Equipment Outline

Enquiries open with a volume in litres, which is where a military rotomolded case project slips. Geometry starts from the outline with every protrusion, from one- or two-person carry, and from air or road transport. A terminal of 660 × 420 × 240 mm had a handle out 40 mm and an antenna mount 25 mm above the lid; both were missed and the foam pocket was pierced.

The platform sets the ceiling: on a 2680 × 2230 mm deck, an outer length above 893 mm drops three rows to two, and nine cases to six.

Input	Parameter fixed	Value here
Equipment 660 × 420 × 240 mm	Cavity clear size	710 × 470 × 320 mm
One-person carry	Length, weight	800 mm, 19 kg
Pallet 2680 × 2230 mm	Length ceiling	893 mm

The envelope adds 18 mm of lid rim and 30 mm of feet to twice the wall; hold ±2.5 mm outside, ±1.5 mm inside.

2. Cavity Size and Cushion Thickness

The cavity is equipment plus twice the cushion, not twice the wall. EPE at 35 kg/m³ laid 25 mm per side compresses to 18 mm, about 28 percent, holding the load and leaving drop travel.

Military rotomolded case: black case open on a wooden pallet, grey foam with tool pockets, two latches, stacked black cases behind, a yellow forklift

Cold eats that margin. At 1.2 × 10⁻⁴ per °C an 800 mm envelope shrinks 6 mm from 23°C to -40°C and a 710 mm cavity follows by 5 mm, leaving 13 mm of foam; add 6 mm to the cavity or fit a cold-flexible layer.

3. Wall Thickness Zones and Metal Inserts

One wall figure cannot control a military rotomolded case: powder thins at corners, bosses and inserts, so each load path gets its own tolerance.

Zone	Nominal	Tolerance	Why it is loaded
Side wall	4.5 mm	± 0.5 mm	Side squeeze, stacking
Base	7.0 mm	± 0.8 mm	85 kg spread load
Lid rim	9.0 mm	± 0.8 mm	Seal face stiffness
Corners, feet	10.0 mm	± 1.0 mm	First contact in a drop

Mould temperature is 245 to 295°C and HDPE shrinks 2.2 to 2.6 percent, so a 710 mm cavity at 2.4 percent needs 727 mm of steel. Stainless M8 inserts hold 1800 N at 8 N·m; past 11 N·m the seat whitens. Ribs on a 90 mm pitch, 8 mm high, carry the stiffness.

4. Rated Load and Stacking Height

The rated load of a military rotomolded case splits into three figures: 85 kg rated, 650 kg top load for 24 hours, five high. The limit may come from strength or from clearance.

Military rotomolded case: light grey case open on a worn bench, grey foam with square pockets, three wrenches, coiled cable and pallets at right

Five is calculated: 390 mm cases on a 150 mm pallet reach 2100 mm inside 2.4 m, and the bottom case carries 340 kg, so 650 kg leaves a factor of 1.9. On 2.7 m racks six fit, but the limit shifts to top load. Four slings stay within 45 degrees, proof-loaded at twice rated load for ten minutes.

5. What the Spec Sheet Must State

A sheet that goes to production carries the essentials, or it comes back as a dispute.

Item	How to write it	Cost of omitting
Sizes	800 × 560 × 390 ± 2.5 mm, cavity ± 1.5 mm	Pallet count wrong
Load	85 kg rated, 650 kg top, five high	Overload or under-stack
Seal, valve	30% compression, opens below 0.25 bar	Lid sucked shut in flight
Hardware	M8 inserts, 8 N·m, colour codes	Colour drift, no reorder

A military rotomolded case needs the pressure valve: at cruise the differential reaches 0.25 bar, about 11 kN across the lid, beyond hand force.

Conclusion

A military rotomolded case is a chain of numbers that restrain each other: cavity follows equipment and cushion travel, envelope follows the platform, wall zones follow load paths, stacking follows top load and clearance. Write those links as tolerances and test conditions. The saving comes from keeping every millimetre on a load path, which is the craft of military rotomolded case specification.

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.