In a protective case size range, do I pick on cavity or envelope?

Answer: Cavity first, then the envelope against shelves, vehicles and doorways.

Two protective case size range steps differ by 10 mm in cavity. Normal?

Answer: Yes. A 2.5 versus 4.0 mm wall and a 4 versus 6 mm rib differ 2 mm per side; draft adds the rest.

Does the stacking figure include the pallet?

Answer: It should: three 290 mm cases on a 150 mm pallet sit at 1020 mm.

A 560 mm cavity measures 557 mm. Out of tolerance?

Answer: Only the stated range decides: at plus 2 minus 1 the floor is 559 mm, so it fails.

Can shipping size serve as stack height?

Answer: No. Wheels carry no load; the lid plane does.

Protective Case Size Range: Internal, External and Stack Heights

Quick answer: A protective case size range stacks four figures: cavity, outside envelope, shipping size with wheels and handle, and stacking footprint. They differ by wall, rib and draft, so one case can show a 16 to 20 mm gap between inside and outside.

Three Measurements and One Stacking Figure
What Wall and Ribs Take Away
Cavity Volume Is Not Loadable Volume
Steps from Small Case to Pallet Class
Nominal Versus Measured Figures
FAQ

1. Three Measurements and One Stacking Figure

A protective case size range never reduces to one number: a line carries cavity, envelope, shipping size with wheels and handle, and stacking figure. Swapping one for another shifts a quote by tens of millimetres.

Cavity runs wall to wall, clear of the floor pad. Envelope is the largest body, ribs in and latch bosses out. Shipping size runs from the wheel patch to the retracted handle. A 560 x 400 x 250 mm cavity gives a 584 x 424 x 274 mm envelope, and wheels push shipping height to 330 mm.

Figure	What it measures	Where the tape sits
Cavity	Usable internal space	Wall to wall, radius start
Envelope	Largest body dimension	Ribs in, latch bosses out
Shipping size	Footprint in transit	Wheel contact to handle top
Stacking figure	Bearing area, pile height	Lid plane plus wheel height

2. What Wall and Ribs Take Away

In every protective case size range, wall, ribs and draft sit between cavity and envelope. Rotomoulded walls run 2.5 to 4.0 mm and side ribs add 4 to 6 mm, so one side carries 8 to 10 mm and a pair 16 to 20 mm. A 560 mm cavity ending at 580 mm outside is normal practice.

Rib pitch runs 60 to 80 mm at 1.5 to 2 times the wall. Sparse ribs let the wall bulge in a drop; tight ribs trap air, sink marks form at the root and the wall measures 1.8 mm. A 1 to 2 degree draft over 250 mm of depth pulls each side in by 4 to 9 mm, so tall items enter on the floor dimension.

3. Cavity Volume Is Not Loadable Volume

Length times width times depth gives gross volume, not usable volume. Radii, floor pad and rim flange take a share: a 430 x 310 x 200 mm cavity holds 26.7 litres gross, 1.4 litres goes to radii and pad, 0.4 litres to the flange, leaving 24.9 litres.

Foam takes another bite: 20 mm per side and 15 mm on top shrink the cavity to 390 x 270 x 185 mm, so usable volume falls to 19.5 litres. Of two 25 litre cavities, 500 x 300 x 167 and 400 x 400 x 156 mm, only the first takes a 480 mm bar. In a protective case size range, longest edge comes before volume.

4. Steps from Small Case to Pallet Class

A mature protective case size range runs six to eight steps, not proportional: small sizes jump 60 to 80 mm, mid sizes 100 to 150 mm, pallet class 200 mm or more. Jumps follow the carry method, hand to shoulder to two-person lift to wheels.

Weight sets the step in any protective case size range: hand carry stops at 9 kg, a two-person lift at 25 kg, wheels above that. Going from 11.4 to 26.7 litres takes tare from 2.1 to 3.6 kg, right at the limit.

Step	Cavity (mm)	Volume (L)	Envelope (mm)	Tare (kg)
S2	310 x 230 x 160	11.4	330 x 250 x 180	2.1
S3	430 x 310 x 200	26.7	452 x 332 x 222	3.6
M1	560 x 400 x 250	56.0	584 x 424 x 274	6.2

5. Nominal Versus Measured Figures

In a protective case size range, most disputes are about method, not moulding: where cavity length starts, whether latches or wheels count. Leave those open and one case reports figures 30 mm apart.

Rotomoulding shrinks 1.2 to 1.8 percent: a 1000 mm mould loses 12 to 18 mm, the rim pulls in 2 to 3 mm as it cools, and an assembled shell closes with 1 to 2 mm of compression. Quote a range, plus 2 minus 1 mm, so a 560 step accepts 559 to 562 mm.

Item	Instrument	Datum	Tolerance
Cavity length, width	Tape and internal caliper	Straight run, radius excluded	+2 / -1 mm
Cavity depth	Depth gauge	Floor to rim plane	plus or minus 1.5 mm
Envelope	Tape	Ribs in, latch bosses out	plus or minus 2 mm

Conclusion

Cavity, envelope, shipping size and stack height each answer a different question. A protective case size range only works when each figure carries its datum.

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.