Process selection
Rotational Molding vs Blow Molding
Rotational molding tumbles polyethylene powder inside a heated biaxially rotating mold until it coats the walls, then cools it slowly. Blow molding inflates a molten parison against a cooled cavity in seconds. The core trade: rotomolding gives cheap tooling and stress-free large hollow parts; blow molding gives 20 to 40x the throughput.
| Rotational Molding | Blow Molding | |
|---|---|---|
| Tooling cost | $10,000 to $35,000 cast aluminum | $40,000 to $150,000 machined, cooled |
| Cycle time | 45 to 90 min full oven cycle | 60 to 180 s large industrial parts |
| Economic volume | 50 to 5,000 units per year | 5,000+ units per year |
| Wall thickness | 3 to 12 mm, corners thicker, near stress-free | 1 to 6 mm, thins at corners, blow-up limited |
| Max part size | Tanks to 50,000 L | IBC and drum class, ~1,000 to 3,000 L practical |
| Materials | Mostly PE, some PP and nylon | HDPE, PP, PET, PVC, multi-layer |
| Machine cost | $150,000 to $600,000 carousel | $500,000 to $2M accumulator head |
Choose Rotational Molding when
- Runs of 50 to 5,000 units where a $15,000 mold decides the project
- Very large tanks, kayaks, and bins beyond blow molding size limits
- Uniform, stress-free walls with molded-in inserts and thick corners
Choose Blow Molding when
- Volumes above about 5,000 units per year, where 60 to 180 s cycles win
- Lighter parts: blow molding holds thinner walls at 1 to 3 mm
- Multi-layer structures such as barrier fuel tanks and drums
The verdict
Volume and size make this call. Under roughly 3,000 to 5,000 units per year, or for anything bigger than about 1,000 L, rotomold it and pocket the tooling savings. Above that volume in a blow-moldable size, the 45 to 90 minute roto cycle can never compete with a 2 to 3 minute blow cycle.
Cost comparison
A cast aluminum rotomold for a 500 L tank costs $10,000 to $35,000 and yields 4 to 8 parts per hour per arm across a 45 to 90 minute oven cycle. An industrial accumulator-head blow molding machine runs $500,000 to $2 million with molds at $40,000 to $150,000, but cycles in 60 to 180 s. Below roughly 3,000 to 5,000 units per year, rotomolding's cheap tooling wins; past that, blow molding's 20 to 40x throughput amortizes everything and takes over.
Common questions
Why are rotomolded tanks tougher at the corners?
Powder naturally builds thicker at outside corners during rotation, exactly where impact loads concentrate, and the zero-shear process leaves almost no molded-in stress. Blow molding does the opposite: the parison stretches thinnest at corners and pinch-offs, which is why blown tanks need generous radii and careful parison programming.
Can rotomolding scale to higher volumes?
Somewhat. A 4-arm carousel with multiple cavities per arm can push out 20 to 60 parts per hour, and shuttle machines add capacity cheaply. But cycle physics cap it: a 45 to 90 minute heat and cool loop cannot approach blow molding's 60 to 180 s, so past about 5,000 units per year per SKU the math flips.