Why can a rotationally molded case not be ordered by wall thickness alone?

Answer: Thickness follows charge weight and mould heat, so a drawing fixes only a nominal value. Write charge weight and thickness spread together, then verify both.

What causes a blister on the inner wall?

Answer: Usually a blocked vent. Trapped air lifts the half fused layer. Clear the vents at the parting line and in dead corners.

Can a warped rotationally molded case rim be fixed by adding thickness?

Answer: No. Warp comes from uneven cooling; extra gauge only lengthens the cooling stage.

Why must inserts be moulded in?

Answer: A drilled hole cuts the fused layers and leaves a gap that lets moisture in, while a moulded-in insert is wrapped by melt.

How thin can the wall of a rotationally molded case go?

Answer: Around 3 mm is the practical floor. Below that, finer powder and shorter cycles apply, and thin spots appear faster than weight comes off.

Rotationally Molded Case Structure and Process: Five Key Points

Quick answer: A rotationally molded case grows inside its tool, so wall thickness comes from the powder charge rather than from the cavity. Powder mesh, the heat and cooling rhythm, rib geometry and moulded-in inserts decide whether the wall is even and the rim can compress a gasket.

Why a Rotationally Molded Case Is a Seamless Shell
Matching Powder and Mould for Even Wall Thickness
The Four Stages of Biaxial Rotation
Ribs, Radii and the Rim
Inserts and the Gasket Groove
FAQ

1. Why a Rotationally Molded Case Is a Seamless Shell

The shell of a rotationally molded case is grown, not pressed. Powder is loaded into the cavity, the tool turns on two perpendicular axes in an oven, and the charge fuses onto the wall layer by layer. No injection gate, no pinch line: the body leaves the tool as one continuous part.

Continuity pays off twice. Base and side wall share a single wall with no weld, so stack load runs down the side into the corners. The rim forms in the same shot, so the gasket presses on a plane that was never machined.

Item	Typical value	Effect on the shell
Powder density	0.935 g/cm3	Converts volume into charge weight
Melt flow index	4 g/10min	Low values leave unfused granules
Powder mesh	35, about 0.5 mm	Coarse powder will not fully fuse

2. Matching Powder and Mould for Even Wall Thickness

Charge weight can be calculated rather than guessed. A rotationally molded case body with 1.2 square metres of internal surface at a 5 mm wall needs about 5.6 kg of powder at 0.935 g/cm3, plus 3 percent for flanges and rib grooves. That figure belongs on the process card, so a mould change does not restart it.

The tool is the other half, welded from 3 mm steel plate and vented through 6 mm tubes at the parting line. A blocked vent lifts a blister on the inner wall. Mesh must match thickness: 35 mesh at 5 mm, 80 mesh at 3 mm, and fine powder in an 8 mm wall traps bubbles into pinholes.

3. The Four Stages of Biaxial Rotation

One shot has four stages, and temperature and time are linked.

Stage	Temperature	Time	Axis speeds
Charge and clamp	Ambient	3 to 5 min	Stationary
Heat and rotate	Oven 300 to 330 C	12 to 18 min	8 and 2 rpm
Cool	Air to below 60 C	8 to 12 min	8 and 2 rpm
Demould	Ambient	2 min	Stationary

Powder forms a film at about 205 C, and from there the speed ratio decides where it lands: 8 rpm on the main axis against 2 rpm spreads material evenly, while 4 rpm on the minor axis thickens corners.

Cooling is the stage most often cut: demould too warm and rim flatness moves past 1.5 mm, which no gasket can follow.

4. Ribs, Radii and the Rim

Flat walls are held by ribs, not extra gauge. Ribs 8 mm high at 100 mm pitch with radii of R15 let a 4 mm wall behave close to a 6 mm panel while adding about 4 percent in weight, but they must run along the stacking load.

Feature	Value	Purpose
Vertical ribs	8 mm high, 100 mm pitch	Resist side wall bulge
Corner radius	R15 or larger	Spread drop impact
Draft angle	1 to 1.5 degrees	Release without scuffing

The rim needs the same care: parts are pulled by hand, so a rim without 1 to 1.5 degrees of draft whitens on release. Rim width runs about 6 mm beyond the gasket groove: narrower and the cord escapes, wider and the rim lands first in a drop.

5. Inserts and the Gasket Groove

Inserts belong in the mould of a rotationally molded case, never in a drill. Latch and hinge seats use M6 stainless inserts with 3 mm of encapsulation and a pull-out above 1.5 kN, because a drilled hole cuts the fused layers. Hinge pins run 5 mm stainless in holes formed in the tool.

Sealing depends on groove geometry, not latch count. A groove 5.5 mm wide and 4 mm deep takes a 6 mm silicone cord at 25 percent compression, so it fills the groove without rolling out. Too shallow and it escapes; too deep and compression falls short, showing as a drop above 1 kPa at 30 kPa.

Conclusion

Five links end up written on the shell: charge weight sets thickness, axis speed sets distribution, cooling sets warp, ribs hold the wall, groove geometry holds the seal. Fix those five and batch consistency of a rotationally molded case follows.

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.