How much clearance should the internal size of a transport and storage container leave?

10 to 15 mm per side around the largest item, plus 5 mm at corners. Too tight and it will not go in.

Can outer tolerance be held to plus or minus 1 mm?

Rarely. Shrinkage varies from 2.5% to 3.2%, a 20 mm spread on 820 mm.

Does a 6 mm wall mean average or minimum?

Minimum. Reading it as average hides corners at 5.2 mm, where capacity drops by a fifth.

Should load be rated as cargo weight or stacking load?

Both. Cargo weight sizes handle and feet, stacking load sizes wall and ribs.

Why is there no universal tolerance table for rotomoulded parts?

Tool, powder and process all move the result, and one tool runs 1 to 2 mm apart across machines.

Transport and Storage Container Specifications: Size and Load

Quick answer: Set transport and storage container specifications outside in: fix the outer size from pallet module and stacking, subtract wall thickness for the inner size, work back from stacking load to wall and ribs, and write sealing as structure plus test condition rather than an absolute rating.

Four Inputs Behind Transport and Storage Container Specifications
Inside and Outside Dimensions: Where Tolerance Goes
Wall Thickness and Ribs: Why It Is Never Even
Converting Load Rating into Stacking Load
Writing Sealing and Ingress Ratings
FAQ

1. Four Inputs Behind Transport and Storage Container Specifications

Four inputs drive every number on the drawing: what goes inside, the pallet and stacking pattern, the transport duty, and the storage environment. Contents set clearance, stacking the outer module, transport the wall, storage the sealing wording.

Input	Parameter it fixes	Typical figure	Owner
Load shape and weight	Clearance, ribs	15 mm clearance	User
Pallet and stacking	Outer module	1200 x 800 pallet	Logistics
Transport duty	Wall, dynamic factor	4 high, 1.5	Carrier
Storage environment	Seal, pressure valve	From 8 kPa	Warehouse

Internal size is outer size minus two wall thicknesses, so designing from the inside out collides with the pallet module.

2. Inside and Outside Dimensions: Where Tolerance Goes

A mid-size transport and storage container runs 820 x 520 x 360 mm outside on a 6 mm wall, a theoretical inside of 808 x 508 x 348 mm. The drawing states 780 x 480 x 320 mm, the rest going to draft and floor ribs.

Transport and Storage Container: square cavity cut in dense black foam, grey moulded wall at its base, seal groove along the rim

Tolerance runs at plus or minus 3 mm outside and 5 mm inside. The outer size follows the cavity, where shrinkage varies from 2.5% to 3.2%; the inner size adds wall distribution on top.

Dimension	Datum	Tolerance	Main source of error
Outer length and width	Rim plane	Plus or minus 3 mm	Shrinkage
Inner length and width	Thinnest wall	Plus or minus 5 mm	Wall spread
Lid to base gap	Closed rim	1.2 to 2.0 mm	Latch preload

3. Wall Thickness and Ribs: Why It Is Never Even

Rotational moulding sets wall from powder charge and heat distribution. A unit specified at 6 mm usually measures 5.2 to 6.8 mm, thinnest at corners. Specify minimum wall, never average, since the thinnest patch sets strength.

Ribs carry stiffness: 14 mm height, 8 mm width, R3 root radius and 120 mm pitch is a common set. Thicker ribs sink outside, taller ribs tear on ejection. Draft at 1.5 degrees matters too: too little leaves the rim true while the base pulls in 6 to 8 mm.

4. Converting Load Rating into Stacking Load

Load means two things: what one unit carries, and what the bottom unit carries when stacked. The first sizes handles and feet, the second sizes wall and ribs. A transport and storage container specification needs both, or acceptance has nothing to test.

With 45 kg of load, 8.5 kg empty and four units stacked, the bottom unit carries three layers: (45 + 8.5) x 3 = 160.5 kg static. With the 1.5 dynamic factor the figure to specify is 241 kg.

Transport and Storage Container: black base and grey lid open, rows of small grey boxes set in a grey foam frame, dark grey lid lining

Load case	Formula	Example	Checked part
Static stack	Gross x layers	160.5 kg	Floor, ribs
Dynamic stack	Static x 1.5	241 kg	Side walls
Handle pull	Gross weight x 2	107 kg	Handle mounts

Keep a safety factor of 3 to 1, so tested failure reaches three times the rating, and add a lid test of 200 kg for 5 minutes with deflection under 2 mm.

5. Writing Sealing and Ingress Ratings

Sealing belongs in the specification as structure plus test condition: an IP67 structure, no pressure equalisation valve, subject to a third-party report. That beats claiming a waterproof unit.

The pressure valve is often left out: cracking pressure from 8 kPa, vent flow at 0.02 litres per minute per litre of volume. Without it, a 30 °C swing builds over 10 kPa.

Conclusion

Transport and storage container specifications are numbers derived from four inputs, not chosen by feel. Fix the outer size from the pallet module, subtract wall thickness for the inner size, specify minimum wall and ribs, and convert stacking load in static and dynamic terms. On the drawing, those rules make the sheet survive acceptance.

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.