Rotomoulding, injection or blow moulding: which process should your case use
The three case-forming processes have different cost curves, tolerance bands and failure modes. A short decision guide to settle before you request a quotation.
Short answer
- All three processes make perfectly good cases.
- Under about 500 units of a small, feature-rich shell: blow moulding or an existing injection tool.
- Injection holds the tightest dimensional tolerance and the best surface.
- Ask for the resin grade and a low-temperature impact figure rather than accepting a generic label.
The decision is about volume and geometry, not quality
All three processes make perfectly good cases. What changes is where the money goes. Injection moulding needs an expensive tooled mould with slides for latch bosses, then produces parts quickly and consistently. Blow moulding uses a much simpler mould for a hollow two-part shell and is the usual route for toolboxes. Rotomoulding needs a moderate tool but a slow oven cycle, and wins when the part is large, deep, or has undercuts that would need expensive slides.
Where each one is the right answer
Under about 500 units of a small, feature-rich shell: blow moulding or an existing injection tool. Thousands of units with tight internal geometry and good surface detail: injection. Above roughly 40 to 60 litres, or with deep undercuts and thick walls: rotomoulding. Repeatedly reconfigured interiors with a rectangular footprint: aluminium profile.
Tolerance and appearance consequences
Injection holds the tightest dimensional tolerance and the best surface. Blow moulding shows parting-line flash and thicker, less even walls, which is fine for a toolbox and annoying for a case with a fitted lid mechanism. Rotomoulding has the loosest tolerance and a matte, slightly textured surface, so plan mounting hardware with mechanical fasteners or moulded inserts rather than relying on precision bores.
Low-temperature behaviour
Ask for the resin grade and a low-temperature impact figure rather than accepting a generic label. Polyethylene used in rotomoulding stays tough in deep cold; some injected PP formulations become brittle; ABS is stiff and good at room temperature but can crack in a cold store. This is the most common field failure that a specification could have prevented.
What a quotation should state
Process; resin grade and whether the content is virgin or regrind; nominal wall thickness; tool cost and tool ownership; cycle time; cavity count; and the sub-suppliers for gasket, latch and hinge. A quotation missing those is not comparable with one that has them, whatever the price says.
Questions buyers ask
Can one mould serve both an injected and a rotomoulded version?
No. The tool is designed for one process and one material shrinkage and wall behaviour. Comparing the two means quoting two tools.
Is rotomoulding always cheaper because the tool is simpler?
At low volume and large size, usually yes. At high volume the slow oven cycle inverts the arithmetic and injection wins on unit cost.