Why does a custom protective box under 400 mm use two latches, not four?

Answer: Four would sit under 120 mm apart, so a thumb presses the neighbour. Two at 180 N already compress a 5 mm gasket.

Where does the 2.2 mm wall floor come from?

Answer: Flow length against wall thickness plus shrink. Under 2.2 mm, 1.4 percent shrink pulls a 320 mm panel 0.5 mm out of flat.

Is five layers the stacking limit?

Answer: Five layers leave 27.2 kg on the bottom unit, inside the 60 kg design value, which is why it is the usual limit.

How much can the handle carry?

Answer: 15 kg for an hour, 2.5 times the loaded weight. That margin covers the snatch of a lift and an off-centre load.

Does foam thickness count inside the cavity figure?

Answer: No. The cavity is clear space: with 5 mm foam, a 90 mm cavity leaves 80 mm of depth.

Custom protective box specs: size, wall thickness and load

Quick answer: Size a custom protective box from one hand, not from a catalogue. Below a 400 mm cavity the wall floor is 2.2 mm and the working figure 3.0 mm, two latches suffice, five layers stack, and loaded weight stays under 11.5 kg.

Sizing a custom protective box for one hand
Cavity tiers and the clearance around them
The minimum wall and what the ribs do
Latch count and how these boxes stack
Writing weight and load into the sheet
FAQ

1. Sizing a custom protective box for one hand

One hand sets the limit. A 6.8 kg box carried 50 m burns the fingers, and at 11.5 kg most people switch sides before 20 m. So the first line of a custom protective box sheet is loaded weight, not volume.

Three tiers cover the range: 3.2 kg, 6.8 kg and 11.5 kg; past 11.5 kg the box wants two hands.

Tier	Outer L×W×H	Cavity	Side wall	Empty	Loaded
Small, one hand	260×190×90 mm	230×160×70 mm	2.5 mm	0.9 kg	3.2 kg
Standard, one hand	360×250×110 mm	320×210×90 mm	3.0 mm	1.6 kg	6.8 kg
Two-hand threshold	420×310×130 mm	380×270×105 mm	3.5 mm	2.4 kg	11.5 kg

The handle is a load part: an R8 root radius, a 28 mm grip face, and a 15 kg static hold for an hour at 2.5 times the load.

2. Cavity tiers and the clearance around them

A longer cavity lengthens the moment arm: a 320 mm custom protective box at 6.8 kg feels like a 260 mm box at 9 kg, so weight comes down as length grows.

Custom protective box: yellow shell open, foam holding two black probes and a coiled red lead, one front latch half released

Clearance is set per item: 3 mm single unit, 5 mm with foam, 2 mm ribs between items. Cavity tolerance is ±0.3 mm, tighter than ±1.2 mm on a large case.

Height adds up separately: 8 mm of lid foam, a 4 mm stacking lip and 8 mm of base ribs, so a 90 mm cavity sits inside a 110 mm shell.

3. The minimum wall and what the ribs do

The 2.2 mm floor is a processing limit. Below it a panel sinks 0.5 mm after ejection, the latch seats follow, and the lid needs force to close. A 2.5 mm wall on a custom protective box works because the flow path is short.

Walls are zoned: 3.0 mm sides, 2.5 mm lid, 3.5 mm base, 4.0 mm corners, 4.5 mm at the handle root. Ribs run 1.6 mm thick, 8 mm high, 45 mm apart, capped at 0.6 of the wall.

Moulding data sits beside it: 380 tonne press, two cavities, 42 second cycle, 40 °C tool, shrinkage 1.4 to 1.8 percent. Cutting the cavity to the upper figure keeps every box large enough for foam.

4. Latch count and how these boxes stack

Two latches hold a sub-400 mm box. At 180 N each and 160 to 200 mm apart, a thumb seats them one-handed; a third needs two hands, while a large case takes six at 1400 mm.

Custom protective box: orange lower corner, folded handle clipped to the side wall, four vertical ribs forming feet, parting line across the body

Stacking differs: four bosses drop into lid recesses, so walls never press on walls. Shift a boss 10 mm inward and it lands on the lid centre, denting it by the third layer.

Tier	Stack layers	Bottom load	Design value	Boss height	48 h set
Small, one hand	5	16.0 kg	40 kg	3 mm	0.4 mm
Standard, one hand	5	27.2 kg	60 kg	4 mm	0.6 mm
Two-hand threshold	4	34.5 kg	80 kg	5 mm	0.8 mm

5. Writing weight and load into the sheet

Load goes on the sheet as three numbers: loaded weight, handle static load, bottom-of-stack load. One merged figure leaves the line nothing to check.

Drops run at 1.5 m, higher than a large case, because a light box tumbles; at -20 °C the height falls to 1.2 m, where polypropylene keeps about 60 percent of its impact strength.

A living hinge 0.8 mm thick takes 8000 cycles, bought-in catches are nickel plated and clear 72 hours of salt spray. Service range is -20 °C to 60 °C, and above 60 °C latch force drops about 15 percent.

Conclusion

Three numbers hold a small box together: loaded weight, a wall floor that keeps its shape, bosses that spread a stack. A custom protective box is won by calculation, never by material.

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.