Series · Plastics processing
Rotational molding
PE powder tumbles on two axes in an oven until it coats the mould wall
Water tanks, kayaks, wheeled bins and road barriers are large, seamless hollow parts made with gravity and heat rather than pressure. This article covers the cycle, the variables that matter and the defects you see on the shop floor.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 21 October 2026 · about 7 min read
The principle in one minute
A weighed amount of plastic powder, usually finely ground LLDPE, goes into a hollow mould, which is then closed. The mould enters an oven and turns slowly about two axes at once (biaxial rotation). The powder tumbles across the whole inner surface; wherever it touches the hot wall it sticks and fuses layer by layer (sintering) until no loose powder is left and a continuous wall has formed. The mould then leaves the oven and is cooled while still rotating, and finally opened to remove the part.
The rotation is slow. This is not centrifugal casting; its only job is to bring powder into even contact with every part of the wall. The plastic is never squeezed under pressure, so parts have low moulded-in stress, no weld seams, and reasonably thick material at the corners.
The production cycle
- Load Weigh the powder to the part weight (which sets the average wall thickness), charge it into the mould and close it. The mould needs a vent tube so the air inside can expand and contract.
- Heat while rotating Powder sticks to the hot wall and fuses in layers. After all the powder has been picked up, heating continues for a while so the melt consolidates and the air trapped between powder particles gradually disappears.
- Cool while rotating Air and/or water mist outside the mould. Cooling must be gradual and even; if it is too fast or uneven, the part pulls away from the mould wall and warps.
- Demould Open the mould, lift the part out, then drill, cut openings or fit components as required.
Rotational molding cycles are much longer than the other processes in this series, and almost all of that time is heating and cooling: the whole metal mould has to be heated and cooled, and heat has to pass through a plastic wall that conducts poorly. Multi-arm machines therefore run moulds at different stations at once (loading, oven, cooling) to keep the oven busy.
Variables to control
| Variable | What it controls | What goes wrong if it is off |
|---|---|---|
| Internal air temperature of the mould | How well the melt has consolidated; the best single indicator of cure | Too low: under-cure, many bubbles, brittle · Too high: yellow inner surface, degraded material, lower toughness |
| Oven temperature and time | Heat delivered to the mould | Time alone cannot control cure, because moulds of different size and wall thickness absorb heat differently |
| Major-to-minor axis speed ratio | How powder is distributed across the mould | Thick on one side, thin on another, especially in long parts |
| Cooling rate | Shrinkage and stress in the wall | Too fast or uneven: warpage, sink · Too slow: lost capacity |
| Powder quality | Particle size and distribution, dry flow, bulk density | Coarse powder or long tails: poor flow, rough surface, more bubbles |
| Mould condition and vent | Heat transfer, release, internal pressure | Blocked vent: collapsed walls or blow holes · Porous mould surface: pinholes on the outer surface |
Parts and resins
PE dominates rotational molding: it withstands the oven well enough, suits the economics of very large parts, stays tough at low temperature and grinds easily into powder.
- LLDPE / MDPE are the main grades for water tanks, chemical tanks, bins and kayaks. Roto grades usually have a medium MFI, because the melt has to consolidate by heat alone with no shear to help. Too low an MFI and the powder fuses poorly; too high and toughness and stress-crack resistance drop.
- HDPE is stiffer, used where rigidity matters.
- XLPE (crosslinkable PE) is used for chemical and fuel tanks that need higher performance; heating must be controlled so the crosslinking system completes.
- Others: PP, PVC plastisol (a liquid) and nylon are used, but far less.
Typical parts: above-ground and underground water tanks, chemical tanks, kayaks, wheeled bins, road barriers, floats, fuel tanks and large playground equipment.
Common defects
| Symptom | Likely causes | How to check | Does an additive help? |
|---|---|---|---|
| Bubbles in the wall, surface pinholes | Under-cure, damp powder or powder with tails, porous or damp mould surface | Examine a wall cross-section under magnification; inspect the mould surface where pinholes appear | Not the main fix. Fix heating, powder quality and the mould |
| Under-cure | Too little heat or too short in the oven, thick or shielded moulds | Grainy inner surface, many bubbles, low results in a low-temperature drop-dart impact test | No. Fix heating, using internal air temperature as the control |
| Over-cure | Too much heat or time; the inner surface is degraded by hot air | Yellow, smelly inner surface; low toughness even without bubbles | Partly: antioxidants in roto grades widen the safe heating window, but fix the heating first |
| Warpage, sink | Cooling too fast or uneven, part releasing from the wall too early, blocked vent | Look for where the part pulled away (dull patches); check the vent tube | No. Fix cooling and the mould |
| Thin walls at corners or narrow ribs | Inside corners and narrow channels get less heat and less powder; powder can bridge across a gap; unsuitable rotation ratio | Section the part and compare corner thickness with flat walls | No. Use larger corner radii, wide-enough channels and a better rotation ratio |
| Outdoor tank fades, chalks and cracks | UV breaks PE chains on the sun-facing surface; too little or no UV protection in the powder | Compare the sun-facing and shaded sides; check whether cracks start at the outer surface | Yes: a UV stabiliser compounded into the resin before grinding, see UV solutions |
A low-temperature drop-dart impact test is the rotomoulding industry's usual check on cure, because both under-cure and over-cure lower the result. Follow the standard or customer agreement that applies to you for the procedure and pass criteria.
Additives in this process
Rotational molding has no screw and no shear to mix anything, so dosing masterbatch pellets at the hopper, as in injection or blow molding, does not work. Additives must already be in the powder before it enters the mould. There are two routes:
- Melt compounding, then grinding: masterbatch or additives are compounded into PE pellets on an extruder and then ground to powder. Dispersion is thorough and part properties are best. This is the route to use for UV stabilisers and antioxidants.
- Dry blending: pigment powder is tumbled with PE powder before loading. Colour changes are quick and cheap, but pigment sitting between powder particles usually reduces toughness. Use it only when that trade-off is acceptable.
- UV stabiliser is essential for water tanks, boats, barriers and any part used outdoors. Choose the level for the target service life and the part colour.
- Antioxidant / heat stabiliser matters more here than in other processes, because the material stays in a hot oven for a long time and the inner surface is exposed to hot air throughout the cycle.
- Colour: some tanks are multi-layer, with a second powder shot added during the cycle; the outer layer carries colour and UV protection, the inner layer is chosen for the application.
If you buy ready-made roto powder, ask the supplier in writing what UV and antioxidant system it contains, what outdoor life it is designed for, and under which conditions the impact values in the TDS were tested.
FAQ
Rotational molding questions we hear often
PE is already chemically resistant. Why does an outdoor water tank still need a UV stabiliser?
Chemical resistance is not sunlight resistance. UV light breaks PE chains through photo-oxidation: the surface fades, chalks and then develops fine cracks that grow deeper. A thick-walled tank may degrade more slowly than a thin part, but it always starts at the sun-facing surface. The UV system has to be in the powder from the start; it cannot be added afterwards.
How do I know a part is correctly cured?
Use several signals together: the internal air temperature of the mould, measured by a data logger that rotates with the mould; the appearance of the inner surface; the bubble count in a wall cross-section; and a low-temperature impact test. Under-cured parts have a rough inner surface and many bubbles; over-cured parts have a yellow, smelly inner surface. Both have lower impact strength than a correctly cured part.
Can masterbatch pellets be mixed directly with PE powder?
Not recommended. Rotational molding has no shear to mix anything, so large pellets segregate from the powder and either do not melt in time or disperse poorly. The usual routes are melt compounding the additives into the resin and then grinding it to powder, or dry blending pigment powder, which is quick but usually reduces toughness.
Why is rotational molding suited to large parts in modest volumes?
The mould carries no pressure, so it can be fabricated from sheet metal or cast aluminium, and tooling cost is low relative to part size. Cycles are long, though, because the whole mould must be heated and cooled. At very high volumes, blow molding is usually more economical for parts that can be made either way.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
