PP, ABS, HDPE or aluminium: choosing case material before you choose a supplier
Material decides which factories can even quote you. A short decision table with the trade-offs that actually cost money.
Short answer
- Ask what has to go wrong for the shipment to be a loss: a dropped case on a building site, a lid that creeps open in a truck, water ingress on a boat deck, or a crate that cracks in a cold store.
- ABS gives stiff, good-surface injected shells and is the default for instrument and protective cases with moulded-in features; it is brittle in severe cold and attacked by some solvents.
- For volumes under a few thousand units, tooling dominates: an injection tool for a case with latch bosses and slides costs far more than a blow mould or a rotomould tool for the same external size.
- Small instrument case, tight internal features, 5k+ units: injected ABS.
Start from the failure mode, not the material
Ask what has to go wrong for the shipment to be a loss: a dropped case on a building site, a lid that creeps open in a truck, water ingress on a boat deck, or a crate that cracks in a cold store. Each of those points to a different resin and a different forming process. Buyers who start with "we need an ABS case" usually end up paying for a material that solves the wrong problem.
The four families that matter
ABS gives stiff, good-surface injected shells and is the default for instrument and protective cases with moulded-in features; it is brittle in severe cold and attacked by some solvents. PP is tougher against chemicals and cheaper, tolerates blow moulding for tool cases, but shows more flex in large flat lids. HDPE in rotational moulding is the choice for big, thick-walled, near-unbreakable shells with generous tolerances. Aluminium profile plus composite panels is the choice when the interior needs rails, cut-outs, repeat repair and a rectangular form factor.
Where the cost really sits
For volumes under a few thousand units, tooling dominates: an injection tool for a case with latch bosses and slides costs far more than a blow mould or a rotomould tool for the same external size. Above that, cycle time and part weight dominate unit price, and an injected part usually wins. Aluminium cases have almost no tooling cost but the highest labour content per unit.
A decision table you can quote
Small instrument case, tight internal features, 5k+ units: injected ABS. Tool case with trays, 3-10k: blow-moulded PP. Large equipment case over ~50 litres or deep undercuts: rotomoulded HDPE. Rack, flight-deck or repeatedly-repaired case with cut-outs: aluminium profile. Cold store or outdoor winter service: check low-temperature impact data before choosing PP or ABS, and consider HDPE.
Questions buyers ask
Is recycled content ever acceptable in a protective case?
In structural shells, only with a material declaration and impact testing, because regrind changes low-temperature behaviour. In liners and secondary packaging it is usually fine.
Can I switch material on an existing mould?
No — a mould is designed for one material’s shrinkage, wall and gating. Switching material means a new tool, with a new lead time.