Rotational molding uses gravity inside a rotating mold to attain a hollow form. It is also referred to as rotomoulding.
It is an alternative to blow molding for making large, hollow shapes. Rotomoulding tends to favor more complicated external geometries, larger parts, and lower production quantities than blow molding.
The process consists of the subsequent steps.
A predetermined quantity of polymer powder is loaded into the cavity of a split mold.
The mold is then heated and at the same time rotated on two perpendicular axes, in order that the powder impinges on all internal surfaces of the mold, gradually forming a fused layer of uniform thickness.
While still rotating, the mold is cooled in order that the plastic skin solidifies.
The mold is opened and therefore the part is unloaded. Rotational speeds used in the process are relatively slow. It is gravity not centrifugal force that causes uniform coating of the mold surfaces.
Molds in rotational molding are simple and inexpensive compared to injection molding or blow molding however the production cycle is much longer lasting maybe ten minutes or more.
To balance these benefits and disadvantages in production, rotational molding is usually performed on a multicavity indexing machine such as the three-station machine shown below.
The machine is designed so that three molds are indexed in sequence through three workstations.
Thus all three molds are working simultaneously. The first workstation is an unload-load station where the finished part is unloaded from the mold and the powder for the next part is loaded into the cavity.
The second station consists of a heating chamber where hot-air convection heats the mold while it is at the same time rotated.
Temperatures inside the chamber are around 7000F depending on the polymer and therefore the item being molded.
The third station cool the mold using forced cold air or water spray to cool and solidify the plastic molding inside.
Rotational molding cycle performed on a three station indexing machine.
Unload-load station
Heat and rotate mold
Cool the mold.
The most popular molding material is polyethylene, especially HDPE. Other plastics include polypropylene, ABS, and high-impact polystyrene.
APPLICATIONS :
Hollow toys such as hobby horses, playing balls, boat, sandboxes, small swimming pools, truck body parts, automotive dashboards, fuel tanks, furniture, large industrial barrels, containers, and storage tanks.
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