Why can't rotomoulding cases hold thickness the way injection moulding does?

Answer: Injection moulding pins the wall with the cavity; rotomoulding builds it grain by grain, so ±0.5 mm is already production grade.

The charge calculates to 5.3 kg but parts come out thin. What comes first?

Answer: Bulk density and moisture. Dropping from 0.55 to 0.50 g/cm³ takes 9 % of the mass out of the same volume and 0.35 mm off the wall.

How is the arm ratio set?

Answer: Start at 6:1 with the main arm near 9 rpm, cut samples, measure six faces. Long sides heavy means 5:1; heavy corners with thin flats means 7:1.

260 ℃ against 300 ℃, what changes?

Answer: Time and degradation. 260 ℃ needs 18 min to fuse through, 300 ℃ needs 11 min, but polyethylene above 300 ℃ for 15 min yellows and loses 15 % of its impact strength.

What does fast cooling cost?

Answer: Warp and locked-in stress. Doubling the rate to 12 ℃/min bows a 600 mm lid by 2.5 mm and lifts decay from 0.5 kPa to 2 kPa.

Every wall is inside tolerance, so why do some boxes crack on a drop?

Answer: Look at the thinnest point. On an R12 corner, drop stress concentrates there and starts the crack; thicken the radius to 5 mm or add a rib.

Rotomoulding Cases: Wall Thickness Control

Quick answer: Rotomoulding cases hold a 4 mm wall to ±0.5 mm only when charge weight, powder melt state and the biaxial arm ratio are all logged; drop the thinnest point below 3.2 mm and the impact margin is gone.

Where the Deviation Starts
Charge Weight: Kilos Into Millimetres
Powder Size and Melt Index
Arm Ratio Against the Oven Window
Oven Time and Cooling Rate
Measuring a 0.5 mm Tolerance
FAQ

1. Where the Deviation Starts

Rotational moulding never injects melt at pressure. The tool turns on two perpendicular axes while powder fuses layer by layer against the cavity wall, so thickness cannot be pinned by the cavity as in injection moulding. Three quantities decide where it lands: charged mass, how far the powder fused this cycle, and the arm ratio.

On one tool and one lot, a 5 % charge error moves a 4 mm wall by 0.2 mm, an arm ratio cut from 6:1 to 4:1 costs 0.6 mm at the short-axis end, and a 20 ℃ oven drop leaves 0.8 mm patches. The errors stack, which makes rotomoulding cases harder to hold than they look.

Variable	Normal setting	Effect of one step off
Charge	developed area × density × 1.05	5 % short, wall 0.2 mm thin
Melt index	3–6 g/10min	low end, 0.5 mm thin at unfused zones
Arm ratio	6:1, main arm 9 rpm	at 4:1, 0.6 mm thin on the short axis
Oven temperature	280 ℃	20 ℃ low, 0.8 mm thin locally

2. Charge Weight: Kilos Into Millimetres

A cavity of 600×400×220 mm develops about 1.35 m² over six faces and ribs. At 0.935 g/cm³ and a 4 mm target the charge is 1.35×0.004×935, or 5.05 kg, and a 5 % allowance for residue puts it at 5.3 kg. Charging 5.05 kg gives 3.9 mm; 5.6 kg lands at 4.3 mm.

Rotomoulding cases in black: a box standing upright on white, an arch handle opening at the top, two red latches on the upper edge

On an R12 corner powder sinks to the low side, so the concave face reads 0.3–0.5 mm thicker than the flat, while the lid centre is the thinnest point. Reverse weighing on rotomoulding cases uses that spread: mass over area gives the average, and a gap above 0.6 mm points at arm ratio rather than charge.

3. Powder Size and Melt Index

Finer powder is not better. Box bodies run 35 mesh, around 500 μm. Below 200 μm the specific surface multiplies, powder skins over on the hot wall and later grains ride on that skin, leaving bubbles and steps; above 800 μm a short cycle leaves unfused cores.

Melt index is the second gate. Rotomoulding grades sit at 3–6 g/10min at 190 ℃ under 2.16 kg. Below 2 g/10min narrow ribs starve, above 8 g/10min powder migrates along the long axis, so hold ±0.5 g/10min lot to lot.

Property	Normal window	What drifts when it moves
Particle size	35 mesh, about 500 μm	below 200 μm bubbles, above 800 μm unfused grains
Melt index	3–6 g/10min	under 2 starves ribs, over 8 skews thickness one way
Moisture	≤0.05 %	past 0.1 %, silver streaks and 0.3 mm blisters

4. Arm Ratio Against the Oven Window

The ratio sets the path powder takes: the main arm turns the tool about a horizontal axis, the secondary about a vertical one, the pair usually falls between 4:1 and 8:1, and 6:1 at 9 rpm on the main arm suits most box tools. A high ratio parks powder on the long sides, a low one piles it into the corners.

Rotomoulding cases in black: a closed box at a slight angle, arched lid, two red latches below the seam on both sides

Cut three 30 mm strips, measure six faces and see where the heavy zones sit: heavy long sides drop to 5:1, heavy corners with thin flats go to 7:1, and two rounds usually bring the spread inside 0.3 mm.

Oven temperature and dwell read together. Polyethylene runs at 260–300 ℃ and box tools sit at 280 ℃, where 14 min brings internal air to 225 ℃; at 260 ℃ that takes 18 min, at 300 ℃ only 11 min. Hot and short fuses the outside and leaves powder inside, the common misjudgement with rotomoulding cases.

5. Oven Time and Cooling Rate

Cooling fails more often than heating. Internal temperature at demould runs 220–240 ℃, so the tool is air cooled to 120 ℃ before water mist goes on, held at 6–8 ℃/min, and stripped below 60 ℃. Water straight onto a hot tool shrinks the outer 3 mm while the core is hot: two hours later a 600 mm lid bows 2.5 mm and a 0.3 kPa decay test fails.

A probe bonded to the cavity wall makes this controllable. Log three curves on a new rotomoulding cases tool: the slope to 225 ℃, the peak, and the knee on the way down. A peak below 215 ℃ says the charge never fused.

6. Measuring a 0.5 mm Tolerance

The tolerance on a 4 mm nominal is ±0.5 mm, so 3.5–4.5 mm, with a 3.2 mm floor at the thinnest point. Ultrasonic gauging is the workhorse: 5 MHz probe, glycerine couplant, 12 readings per box, two per face, lid centre and one bottom corner always included. Repeatability is ±0.05 mm, though curves and rib shadows jump; sectioning a 30 mm strip and reading three points under the microscope is the reference.

Method	Accuracy	Readings	Where it fits
Ultrasonic	±0.05 mm	12 per box	routine sampling
Sectioning	±0.02 mm	3 per strip	first article and disputes
Reverse weighing	±0.1 mm	whole-box average	lot trend monitoring

Any of the 12 points on a rotomoulding cases body outside 3.5–4.5 mm fails, a single outlier with sound neighbours gets one remeasure, and a thinnest point under 3.2 mm is rejected outright, because a 1.2 m corner drop starts its crack there.

Conclusion

Wall uniformity is not a set value but the sum of three things rotomoulding cases cannot separate: charge weight sets the average, powder size and melt index decide whether the section fuses through, and the arm ratio with oven window decides which faces come out heavy. A 4 mm nominal at ±0.5 mm, a 3.2 mm floor and 6–8 ℃/min cooling let a box survive a 1.2 m corner drop and a 0.5 kPa decay test.

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.