Why is the cavity of a Multi-purpose Outdoor Case fixed from the longest item?

Answer: Volume says how much fits, not whether the piece goes in. A 520 mm radio will not close inside a 500 mm cavity.

Is an 8 mm wall a trade habit or a calculation?

Answer: A calculation. A 1.2 m drop with 31 kg gives 12 kN, and at -25℃ a 6 mm wall whitens 0.4 mm while 8 mm survives.

Does every Multi-purpose Outdoor Case need a pressure valve?

Answer: Any unit crossing altitude does. A 60 kPa differential over 0.235 square metres is 14 kN, which latches and rim cannot resist.

Can wheels and handles be added after delivery?

Answer: Not with a screw gun. The housing needs four M6 inserts 8 mm deep and the root a 10 mm wall, both moulded in.

How a 62-liter Multi-purpose Outdoor Case Gets Its Numbers

Quick answer: A Multi-purpose Outdoor Case is dimensioned backwards: cavity from the longest item, wall from a 1.2 m drop, ribs and rim to recover stiffness, then latches and wheels from the numbers already fixed.

Cavity first, from the load list
Drop impact sets the wall, not static load
Ribs and rim: the second line of resistance
Seals and latches: pressure sizes the gasket
Handles, hinges and wheels follow the load path
FAQ

1. Cavity first, from the load list

A survey crew's list: a 520 mm radio, two 190 mm cable reels, six spare boxes 240 mm long, 11 kg in the heaviest item. Length is 520 mm plus 20 mm of cushion per side, so 560 mm; width takes two 380 mm reels with a 20 mm divider, so 420 mm; depth adds 40 mm of pad to the 225 mm thickest item, so 265 mm. That is 62 liters gross, about 54 liters usable.

A 31 kg load against a 40 kg rating leaves 29% margin for damp fabric and later accessories, so a Multi-purpose Outdoor Case gets its volume as an output.

Load item	Outline	Allowance	Result
Portable radio	520 mm	20 mm per side	560 mm internal length
Two cable reels	190 mm	20 mm divider	420 mm internal width
Packed load	31 kg	29% over load	40 kg rating

2. Drop impact sets the wall, not static load

Static load is the usual excuse for a wall thickness and it fails the arithmetic: 31 kg over a 560 × 420 mm floor is 1.3 kPa, which 2 mm carries. A drop is what pushes the wall of a Multi-purpose Outdoor Case to 8 mm.

Hit the base from 1.2 m and the case stops in about 6 milliseconds; a 40 g peak puts the impact at 12 kN. Four bottom edges take 3 kN each, a 1.6 corner factor lifts the peak to 4.8 kN, and thickness follows that peak.

Multi-purpose Outdoor Case: an open olive-green case on forest soil, firewood and a blue rolled mat inside, two steel latches at the front

Copolymer polypropylene holds 45% of its notched impact strength at -25℃, so the wall is sized cold. At 6 mm the corner whitens 0.4 mm; 8 mm shows no crack; 10 mm costs 46 yuan more for one stack level.

Wall	Part weight	1.2 m drop	Cost per unit
6 mm	3.1 kg	0.4 mm whitening	132 yuan
8 mm	4.3 kg	No crack	168 yuan
10 mm	5.7 kg	No crack	214 yuan

3. Ribs and rim: the second line of resistance

Over a 560 mm span stiffness rises with the cube of thickness, so 6 mm to 8 mm is 2.4 times stiffer and 39% heavier. Ribs at 12 mm height and 90 mm pitch bring a Multi-purpose Outdoor Case close to a flat 8 mm wall for 11% more mass. A 2 mm root radius and rib ends 15 mm short of the rim keep the parting line true.

The rim of a Multi-purpose Outdoor Case is the second line. A 6 mm tongue enters a 6.5 mm groove with 0.5 mm clearance, tighter needs two hands and looser squeezes the gasket out. Flatness holds within 0.3 mm; past 0.6 mm the seal never seats.

4. Seals and latches: pressure sizes the gasket

Gasket section follows pressure, not box size. At altitude a hold can put 60 kPa across 0.235 square metres of lid, which is 14 kN, more than four latches can hold, so a Multi-purpose Outdoor Case that crosses altitude carries a valve. The 12 mm bore cracks at 8 kPa.

Multi-purpose Outdoor Case: a dark green case open on sandy ground before a tent, logs and a blue mat inside, two latches on the front

The gasket is hollow EPDM, 8 × 10 mm, compressed 28%. Set after 72 hours at 70℃ must stay at or below 25%; a solid 12 × 12 mm section gives the same rebound but takes closing force from 180 N to 320 N.

Latches follow gasket reaction. A 1960 mm rim at 0.6 N/mm needs 1176 N; four latches at 350 N give 1400 N, a factor of 1.2 against a rule of 1.5, so anything above 560 mm moves to six latches at 2100 N.

5. Handles, hinges and wheels follow the load path

Pull the handle of a Multi-purpose Outdoor Case at twice the rated load: 62 kg on a 31 kg case for 10 minutes with no whitening at a 10 mm root, 2 mm above the wall, where bending moment peaks.

Hinge pins are 6 mm 316 stainless in a 0.2 mm bush, and 20,000 cycles must stay inside 0.3 mm.

Wheels on a Multi-purpose Outdoor Case are 75 mm on four M6 inserts 8 mm deep, sized for the 2.5 kN pull-out a 10 mm threshold demands.

Part	Specification	Test	Pass line
Handle	10 mm root	62 kg static, 10 minutes	No crack, no set
Hinge	6 mm pin, 0.2 mm bush	20,000 cycles	Clearance 0.3 mm or less
Latch	350 N holding force	5,000 cycles	Loss 15% or less

Resin grade belongs on the same sheet: ABS gives 8 kJ/m² at -25℃ against 21 kJ/m² for copolymer polypropylene, which loses 35% of its modulus at 60℃.

Conclusion

Load list to cavity, drop to wall, differential to latch: the numbers run one way, each setting a floor for the next. Put 31 kg, 12 kN and 60 kPa at the top of the drawing and every dimension below earns its place, which is what a Multi-purpose Outdoor Case specification is for.

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.