Hollow parts can be made three ways, and the choice is usually made on the quoted piece price when it should be made on volume, wall control and tooling cost. A programme that buys a thousand tanks gets a different answer from a programme that buys a million bottles, and the process that wins at one volume loses badly at the other. This note compares the three on the terms that decide the programme.
The three processes in one paragraph each
Rotational moulding charges a hollow mould with powder or liquid resin, rotates it on two axes while heat is applied, and builds a uniform wall from the inside out; the tool is cheap, the cycle is slow, and the wall is even and stress-free. Blow moulding extrudes or injection-moulds a parison or preform and inflates it against the mould; the tool is moderate, the cycle is fast, and the wall follows the stretch pattern, thick at the base and controlled at the neck. Injection moulding melts resin into a closed tool and ejects a solid part, which becomes hollow only by design, two shells or a moulded cavity with a welded or inserted opening.
| Factor | Rotomolding | Blow molding | Injection molding |
|---|---|---|---|
| Tooling cost | Low | Moderate | High |
| Cycle time | Slow, tens of minutes | Fast, seconds | Fast, seconds |
| Wall control | Uniform, easy to vary | Follows preform design | Designed, uniform per feature |
| Part size | Very large, tanks, kayaks | Small to medium, bottles | Any, with tool cost |
| Stress in part | Low, near zero | Moderate, oriented | Low, but knit and gate marks |
| Best volume | Low, hundreds to thousands | High, millions | High, any with amortisation |
Where each process wins
Rotational moulding wins on size and stress. A tank, a buoy, a kayak or an oversized pallet is a rotomoulded part because no other process makes it economically at that size with that wall uniformity, and the low tooling cost matters at the volumes those parts usually carry. Blow moulding wins on volume and weight: a bottle is a blow-moulded part because the process puts material exactly where the stretch needs it and the cycle is seconds, and no other process makes a million containers at that piece price. Injection moulding wins on precision and integration: where the hollow part needs threads, inserts, living features or assembly interfaces, the moulded part is more precise than either hollow process can deliver.
The decisions that are usually made wrong
Two mistakes recur. The first is quoting a rotomoulded part on a blow-moulded piece price: the rotomoulded tool is cheap and the cycle is slow, so the piece price carries the cycle, and a programme that commits to blow-moulding volumes on a rotomoulded quote finds the price was never available. The second is specifying injection moulding for a part that only needs to hold fluid: the tool cost and the two-shell assembly are justified by precision features, not by the hollow itself, and a blow-moulded or rotomoulded alternative is usually half the programme cost.
Choosing with the volume in hand
The sequence that works is: fix the annual volume first, because it eliminates one process immediately; check the wall requirement second, because stress-sensitive and large thin-wall parts point to rotational; then check the feature requirement, because threads, inserts and assemblies point to injection. Programmes that bring all three answers to the RFQ get quotes that can be compared, and programmes that bring a drawing get three quotes for three different parts.
Related reading
The blow moulding process comparison covers the blow family in detail, the preform category lists the injection-blow route, and the semi-finished supply note covers why programmes split moulding and assembly.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
