Why can't the cavity of a custom case copy the size of what goes in?

Answer: Cushioning clearance must be added, while ribs, posts and draft take space away, so publish cavity and clear cavity separately.

Does a thicker wall always carry more load?

Answer: No. Stacking follows section shape; 5 mm to 8 mm adds weight, a 12 mm rib lifts inertia 2.6 times.

How is the layer count worked out?

Answer: Static load divided by full weight, less one layer. A sheet without full weight cannot be solved.

Does ±0.5 mm apply to every dimension?

Answer: No. It covers assembly features such as outline length; the cavity takes ±0.8 mm and fits take a range.

Do hinges and latches belong on the spec sheet?

Answer: Yes, as force and life: 180 N allowable per hinge, 8,000 cycles, 400 N holding force.

Custom Case Specifications: Dimensions, Wall Thickness and Load

Quick answer: A custom case specification is a size chain from cavity to outline, wall thickness graded by zone, and hinges and latches written as forces rather than shapes.

The Custom Case Size Chain from Cavity to Outline
Wall Thickness Grades and Ribs
Hinge and Latch Forces
Stacking Load and Base Structure
Mould, Tolerances and Inspection
FAQ

1. The Custom Case Size Chain from Cavity to Outline

A drawing with one outside dimension cannot be inspected. A usable custom case specification is a four-level chain, each level with its own tolerance.

Level	Definition	Example	Tolerance
Inner cavity	Floor to lid face	760 × 560 × 380 mm	±0.8 mm
Clear cavity	Minus ribs and posts	740 × 540 × 350 mm	±1.0 mm
Outline	Latches and feet included	800 × 600 × 420 mm	±0.5 mm
Shipping unit	Plus pallet clearance	820 × 620 × 445 mm	—

Cavity to outline is 20 mm per side: 5 mm of wall, 6 mm of rib zone, 9 mm of radius and lid fit. Itemise all three, since each moves with a different change.

Pallet modulus closes the outline: 600 × 400 fits four units on a 1200 × 1000 pallet at 96 percent, 620 × 420 fits two at 52 percent.

2. Wall Thickness Grades and Ribs

One body carries five thicknesses, and each is specified on its own: sides, base, lid, corner post and rib crest.

Custom case: closed white body lying on a warehouse steel shelf, a raised rib around the lid edge, one plastic latch and one folding handle

Zone	Thickness	Job	If too thin
Main wall	5.0 mm	Side stiffness	Bulging walls
Base	6.0 mm	Load, abrasion	Base sag
Lid	5.5 mm	Dent resistance	Seal line lifts
Corner post	8.0 mm equivalent	Drop impact	Corner cracking
Rib crest	2.0 mm	Bending	Snapped ribs

Thickness cannot simply be added, because uneven walls shrink unevenly. Past 3 to 1, the thick zone cools late and the boundary tears into sink marks. Ribs do the work instead: 12 mm tall, 3.5 mm at the root, 60 mm apart, lifting bending inertia 2.6 times for 8 percent more weight.

3. Hinge and Latch Forces

Hinges and latches are the only moving parts, and the easiest line to get wrong: two stainless hinges leave the load requirement to the supplier.

A 2.4 kg lid runs on a 6 mm pin, 180 N allowable per hinge, no sag past 1.0 mm after 8,000 cycles. Latches hold 400 N and open at 60 to 80 N on an M5 screw at 2.5 N·m, surviving a 1.2 m drop onto six faces.

The 400 N is not arbitrary: dragged at an angle with 55 kg inside, a latch peaks near 0.7 times the full weight, about 190 N each. Position matters more, since a mid-span latch lets the lid edge lift and the seal open.

4. Stacking Load and Base Structure

Stacking is arithmetic: bottom static load divided by full weight, less an allowance. A 100 L custom case weighs 9.5 kg empty and 55 kg loaded, stands 300 kg static, so 5.4 layers, marked as four.

Custom case: showroom shelves of black and orange cases, a row of black wheeled cases on the cabinet, a man standing at right

The allowance covers uneven floors and creep. Only corner posts and the base frame carry it: an 18 mm frame closed on four sides, with 3 mm locating grooves at each lid corner. Outside that zone the lid dishes and the seal lifts.

5. Mould, Tolerances and Inspection

Tolerances belong per feature: ±0.5 mm on the outline, ±0.8 mm on the cavity, ±1.0 mm on depth, and 1.0 to 1.5 mm on the lid gap.

Two mould numbers decide whether those hold: polypropylene shrinks 1.4 to 1.8 percent, so an 800 mm edge takes 12.8 mm, and the 0.48 m² lid needs 1,440 tonnes on a 1,600 tonne press.

Inspection runs in two levels: a full CMM layout on the first-off sample, then three units per 500 for outline, depth and gap.

Conclusion

A custom case specification is four sets of numbers locking together: the size chain decides whether it ships, the wall grades whether it moulds straight, the hinges how long the moving parts last, and the tolerances whether any of it can be judged. A drawing worth signing puts a tolerance, a test method and a failure criterion behind every figure, which separates a custom case from a brochure.

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.