Why does a rotomolded case follow ship width rather than payload?

Answer: Freight is charged per box. Three 780 mm cases total 2340 mm inside a 2352 mm width; cross 784 mm and only two columns fit.

Is plus or minus 0.6 mm too loose a wall tolerance?

Answer: Tighter bands need longer cycles and weighing of every unit. The 0.6 mm band with a 12-point spread limit of 0.8 mm covers most transport duty.

Does the 120 kg rating include case weight?

Answer: No, it is evenly distributed payload only, and it sits apart from top load, which tests walls and lid flange.

Does a colour change need a new mould?

Answer: No, only a powder change. Light pigments absorb heat slowly and add about ten percent to dwell time, so re-check shrinkage after the switch.

The spec says four high; can the warehouse stack five?

Answer: Not without rechecking. Four comes from both the ceiling and the 800 kg top load; confirm under 2 mm permanent set after 24 hours first.

How to Set Rotomolded Case Specs: Size, Wall and Load

Quick answer: A rotomolded case spec is written backwards from logistics: the pallet module fixes the envelope, the envelope minus wall fixes the cavity, and only then do thickness, tolerance and rated load become measurable.

Why a Rotomolded Case Spec Starts with the Load Module
Writing Wall Thickness and Tolerances
Turning Strength into a Rated Load
Sea Freight Loading versus Stack Height
Six Spec Lines a Supplier Cannot Guess
FAQ

1. Why a Rotomolded Case Spec Starts with the Load Module

Spec sheets usually start with capacity, which produces a shell 850 mm wide that ships two abreast. A rotomolded case should obey the logistics module first: a 1200 x 800 mm pallet and a 40-foot high-cube with 2352 mm internal width, 2690 mm height and a 2340 mm door.

Three abreast at 780 mm allows an envelope of 1150 x 780 x 620 mm. Subtract a 5 mm wall, the lid flange and a 45 mm foot, and the cavity lands at 1080 x 710 x 540 mm, about 414 L.

Step	Example	What sets it
Outer envelope	1150 x 780 x 620 mm	2352 mm width, three abreast
Cavity	1080 x 710 x 540 mm	Envelope minus wall and foot

Tolerance follows the chain: plus or minus 3 mm outside, 2 mm inside.

2. Writing Wall Thickness and Tolerances

One thickness number cannot be inspected. The drawing needs nominal thickness, zone and one-sided tolerance: wall 5.0 mm with 0.6 mm, base 6.0 mm with 0.7 mm, corners 7.5 mm with 1.0 mm, lid flange 8.0 mm, hinge bosses 9.0 mm.

Rotomolded case: three white shells on a wooden table, the left open showing dark grey foam, a closed unit behind with a top handle

Mould temperature runs 230 to 280 C with dwell near 3 minutes per millimetre. Linear low-density polyethylene shrinks 1.8 to 2.2 percent, so a 1080 mm cavity at 2.0 percent compensation becomes 1102 mm in steel.

Zone	Nominal	Tolerance
Side wall	5.0 mm	plus or minus 0.6 mm
Corner and caster pad	7.5 mm	plus or minus 1.0 mm

Draft is 1.5 to 2 degrees, corner radii R15, ribs at 110 mm pitch and 6 mm deep, so a 5.0 mm ribbed wall matches a 6.5 mm plain wall.

3. Turning Strength into a Rated Load

Load is usually written as an adjective; the usable form is three figures with a safety factor of 1.5: rated load 120 kg, top load 800 kg held 24 hours with no more than 2 mm permanent set, and permitted stacking.

The formula gives 800 divided by (42 x 1.5), about 12.7, so 12 layers. A 2.7 m ceiling with a 620 mm case allows only 4, so space binds before strength. Lifting is separate: a four-leg sling at 60 degrees is proof loaded at twice the rating, and optional forklift pockets are 120 x 60 mm clear.

4. Sea Freight Loading versus Stack Height

A 40HQ gives 12030 mm of length, 2352 mm of width and 2690 mm of height: 10 rows x 3 columns x 4 tiers, or 120 units. A 20GP gives 5 x 3 x 3, or 45 units.

Rotomolded case: close view of a white shell with one latch closed, pebbled texture, a workshop bench behind

Each extra 50 mm of height can drop a 40HQ from four tiers to three, losing 30 units at once. At 24 kg empty and 42 kg loaded, 120 units weigh about 5.0 t; a steel inner frame at 38 kg shifts the limit from volume to weight.

5. Six Spec Lines a Supplier Cannot Guess

A purchase-ready spec has six lines, and each omission returns as rework.

Line	How to write it	Cost of omitting
Dimensions	1150 x 780 x 620 mm, plus or minus 3 mm	Wrong unit count
Wall zones	Five zones with tolerances	No gauge reference
Load	120 kg rated, 800 kg top, 4 high	Collapse in the field
Hardware	304 stainless, M6, 3.5 N·m	Loose hinges
Seal	Gasket, compression, test	Leaks with no trail
Colour	Chip plus texture	Batch mismatch

Torque is the line people skip. An M6 screw into a moulded insert seals at 3.5 N·m; hand-tightened to 6 N·m it splits the insert.

Conclusion

Specifications for a rotomolded case are a chain of constraints, not a list of wishes: envelope follows ship width, cavity follows wall, wall follows load, stacking follows ceiling. Written as tolerances and test conditions, they make quotations comparable and put every millimetre where the load travels, which is the discipline behind a rotomolded case specification.

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.