Rotation Molding
Rotational molding tumbles powdered polymer inside a heated mold on two axes so it fuses into a seamless, stress-free hollow part with a uniform wall.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Plastics and Rubber
- Typical tolerances
- About ±1–2% of the dimension on mold-formed features; wall thickness commonly varies ±10–20% around nominal. Polyethylene shrinkage of 3.0–3.5% must be built into the tool.
- Surface finish
- Reproduces the mold surface on the outside; the inside surface is free-formed and slightly textured. Cast aluminum molds can carry a fine texture; fabricated steel molds are smoother but show weld lines
- Typical volumes
- 10–10,000 parts per year; the sweet spot is large parts at low to medium volume
- Lead time
- 4–10 weeks for tooling depending on size and whether it is fabricated steel or cast aluminum; a 10–60 minute cycle per part thereafter
- Materials
- Plastic
What it is
Rotational molding tumbles a measured charge of powdered polymer inside a heated, closed mold turning slowly on two perpendicular axes. The powder melts and fuses against the cavity wall layer by layer, producing a seamless, stress-free hollow part with a naturally uniform wall. Kayaks, water and chemical tanks, road barriers, agricultural bins, playground equipment, and vehicle fuel tanks are all rotomolded.
Roughly 80–90% of the volume is polyethylene — LLDPE, HDPE, and crosslinked PE — with PP, nylon, and PVC plastisol making up most of the balance. The process runs at essentially atmospheric pressure, so molds are fabricated sheet steel or cast aluminum shells rather than pressure-bearing tools. That makes tooling far cheaper than blow molding or injection molding for a large part.
The trade is speed: a full heat-and-cool cycle runs 10–60 minutes, against seconds for injection molding.
How it works
- Charge. A weighed shot of polymer powder, typically ground to around 35 mesh (roughly 500 µm), is loaded into one half of the open mold. Shot weight, not process pressure, sets wall thickness — double the charge and you double the wall.
- Heat and rotate. The clamped mold enters an oven at 500–700 °F (260–370 °C) while rotating on two axes, usually 4–12 rpm on the major axis with a speed ratio around 4:1 between axes. The rotation is slow: powder tumbles and deposits, it does not fling outward. Centrifugal force plays no meaningful role.
- Fuse and densify. Powder sticks to the hot cavity wall and coalesces. The control variable is peak internal air temperature (PIAT), measured with a wireless probe through the mold vent — around 400–430 °F (204–221 °C) for polyethylene. Under-cook and the part is porous with poor impact strength; over-cook and the polymer oxidizes, going yellow and brittle.
- Cool while still rotating. The mold moves to a cooling station with forced air and often a water mist, still turning so the melt cannot pool. Cooling too fast warps the part and locks in stress; too slow costs cycle time and increases crystallinity in PE, which increases shrinkage.
- Demold. The mold opens and the part is pulled. Polyethylene shrinks about 3.0–3.5%, which is high, and the part shrinks away from external surfaces but onto any male feature projecting into the cavity.
Why is the wall so uniform?
Because deposition is driven by contact with a hot surface rather than by flow. Every square inch of cavity spends the same time in contact with tumbling powder, so material distributes itself without the flow-length and stretch problems that dominate injection and blow molding. This is the process's defining advantage: corners come out at full wall thickness rather than thinned.
Design guidelines
Wall thickness
Practical range is 0.060–0.500 in (1.5–12 mm), with 0.125–0.250 in (3–6 mm) covering most tanks and containers. Wall is set by shot weight and applies to the whole part, so you cannot locally thicken one region — if one area needs more material, the entire part gets it. Expect thickness variation on the order of ±10–20% around the nominal.
Radii — the opposite rule from injection molding
Outside corners want a radius of at least 0.25 in (6 mm), and 0.5 in (13 mm) or more is far better. Inside corners must be even more generous, because powder bridges across a tight internal angle and leaves that corner thin and weak — exactly inverted from injection molding, where inside corners run thick. As a rule, make inside radii roughly twice the outside radius on the same corner.
Draft
1–2° per side on external surfaces is enough, since the part shrinks away from the cavity. Any feature that projects into the cavity — a core forming a recess, a molded-in handle pocket — needs 3–5°, because the part shrinks onto it as it cools and grips hard.
Stiffening
You cannot rib a rotomolded part the way you rib an injection molded one; there is no pressure to force material into a narrow slot. Stiffen with geometry instead: broad corrugations, domed panels, and kiss-offs, where two opposing walls are brought together in the tool so they fuse into an internal web. Kiss-offs are the standard way to add a shear web to a tank or a door and cost nothing but tool geometry.
Threads, inserts, and flat surfaces
Molded-in threaded inserts, bosses, and metal fittings are held on posts in the mold and are captured as the polymer fuses around them. Avoid large flat panels — polyethylene shrinks 3% or more and a big unsupported flat will warp or oil-can. Crown it slightly or break it up with corrugations. For selecting between PE grades and alternatives on stiffness and service temperature, see /charts/material-properties.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Wall thickness | 0.125–0.250 in (3–6 mm) | 0.060–0.500 in (1.5–12 mm) | Set globally by shot weight, not locally |
| Outside radius | 0.5 in (13 mm) or more | 0.25 in (6 mm) | Tight corners thin and lose impact strength |
| Inside radius | About 2 × the outside radius | — | Powder bridges tight internal angles |
| Draft, external | 1–2° per side | 1° | Part shrinks away from the cavity |
| Draft, over a core | 3–5° per side | 3° | Part shrinks onto anything projecting inward |
| Flat panel | Crown or corrugate it | — | 3%+ shrinkage warps large flats |
| Stiffening | Kiss-offs and corrugations | No narrow ribs | No pressure available to fill a slot |
Cost drivers
Tooling is the cheapest of any hollow-part process at size, because the mold is an unpressurized shell — fabricated steel for large simple shapes, cast aluminum where detail matters. A rotomold for a 200-gallon tank costs a fraction of a blow mold for the same part and a small fraction of an injection mold. That is why rotational molding owns the large-part, low-to-medium-volume territory.
The recurring cost is time. A 10–60 minute cycle means each mold produces a handful of parts per shift, so throughput comes from running many molds on a carousel or rock-and-roll machine simultaneously. Powder also costs more per pound than pellets because of the grinding step.
Volume breakpoints:
- Under about 100 parts a year: fabricated sheet steel tooling, single mold, manual machine.
- 100–5,000: cast aluminum tooling with better detail and faster heat transfer, multiple molds on a carousel arm.
- Above roughly 10,000 parts a year for a small part, blow molding or injection molding usually takes over on unit cost.
Cost reduction:
- Take wall out globally. Shot weight is the direct material cost. If only one region needs 6 mm, consider whether a kiss-off or corrugation can deliver the same stiffness at 4 mm everywhere.
- Fit more molds on the arm. Cycle time is dominated by oven and cooling residence, not by part count, so a second and third mold on the same arm are nearly free in machine time.
- Use cast aluminum where the part warrants it. It conducts heat much better than fabricated steel, which shortens the cycle and pays back over a long program.
- Design out secondary operations. Molded-in inserts, threads, and fittings placed during the charge step avoid drilling, tapping, and welding after demold.
- Avoid deep, narrow recesses. They trap heat, slow the cycle, and need extra draft to release.
Questions
6 questionsHow is wall thickness controlled in rotational molding?
By shot weight alone. The powder charge you load into the mold determines the wall, and it applies uniformly to the whole part — you cannot thicken one region without thickening everything. Practical walls run 0.060–0.500 in (1.5–12 mm), with 0.125–0.250 in (3–6 mm) covering most tanks and containers.
What corner radius does a rotomolded part need?
At least 0.25 in (6 mm) on outside corners, and 0.5 in (13 mm) or more is much better. Inside corners need roughly twice the outside radius, because powder bridges across a tight internal angle and leaves that corner thin — the reverse of injection molding, where inside corners come out thick.
Why does rotational molding give a uniform wall?
Deposition is driven by contact with a hot mold surface rather than by pressurized flow. Every part of the cavity spends the same time in contact with the tumbling powder, so material distributes itself evenly and corners arrive at full thickness instead of thinned, which is the opposite of blow molding and thermoforming.
Can you put ribs on a rotationally molded part?
Not narrow ones — the process runs at atmospheric pressure and nothing forces material into a slot. Stiffen with broad corrugations, domed panels, and kiss-offs, where two opposing walls are brought together in the tool and fuse into an internal web. Kiss-offs add a structural shear web at no material cost.
How long is a rotational molding cycle?
Typically 10–60 minutes for the full heat-and-cool sequence, versus seconds for injection molding. Throughput therefore comes from running several molds simultaneously on a carousel or rock-and-roll machine rather than from shortening the cycle.
When is rotational molding better than blow molding?
For large, thick-walled, low-to-medium-volume hollow parts. Rotomold tooling is an unpressurized shell that costs a fraction of a blow mold at the same size, the wall comes out uniform including in the corners, and there is no pinch-off seam. Blow molding wins above roughly 10,000 parts a year, where the much shorter cycle dominates.