Rotational Molding

What is rotational molding?

Rotational molding is a thermoplastic processing method that allows for the creation of hollow bodies of any size and shape. Rotomachinery Group has over 50 years of experience in creating machines for the production of countless products.


What products can be molded?

Rotational molding is a processing method for thermoplastic materials that allows the creation of hollow bodies of any size and shape.

This technology is particularly suitable for small batch production of even complex large items and blank items that cannot be obtained otherwise.

Applications have also multiplied due to the continuous evolution of research applied to polymers used in the process: polyethylene (LLDPE, LDPE, LHDPE, HDPE), cross-linked polyethylene, polypropylene, E.V.A., nylon, PVC, etc.

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Advantages of rotational molding

This is a transformation technology that differs from the more widespread injection and blow molding processes. The advantages of this technology are numerous: 


The most common rotational molding system uses a horizontal rotating unit, commonly called a carousel equipped with 3-4 or more mold-holding arms that are automatically passed through the different work stations (loading, firing, cooling, part extraction).

The ability to equip the same machine with molds of different shapes and sizes combined with the ease and speed of their replacement makes the use of automatic extraction/loading systems very expensive and leads to an inevitable loss of flexibility.

Automation has instead produced excellent results in the weighing and dosing of materials. 

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Phases of the process

The molding cycle is fully automated on all our machines: only loading/unloading operations are manual.

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The formation of the part in the mold, occurs differently depending on the raw material used whether powder or liquid. The material (liquid or powder) is introduced into the mold and rotated simultaneously on two perpendicular axes so that the powder can reach every point on the inner surface of the mold.

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The powder (mostly linear polyethylene), during the heating phase, initially forms a porous film on the inner surface of the mold to which the rest of the material that has gradually become fluid then adheres. This results in the formation of a uniform layer that will then solidify in the subsequent cooling phase. Liquid material (normally PVC, PLASTISOL), on the other hand, flows along the walls of the mold and heats up to the temperature at which the fluid solidifies, taking on the shape of the mold which will later be cooled in a water bath or with blown air. Different types of heating are used although the forced hot air system has been proven to be the cleanest, least expensive and safest.

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The cooling phase, in some circumstances, is crucial because each material requires different cooling times; for example, in the case of polyethylene, it is necessary to avoid product deformation caused by too rapid a temperature drop (thermal shock).
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Once the object is obtained, the last phase of the cycle is proceeded to: opening the mold for the extraction of the obtained product and the introduction of new raw material for the next cycle.

Discover the areas of application

The items that can be obtained with rotational molding technology are continuously increasing due to innovations introduced in equipment and ongoing research on materials.

Types of machinery

The right machine for every need

The range of machines available for rotational molding ranges from small and simple installations for exploratory development and laboratory use with one arm and a molding chamber, to complex systems with independently manageable arms, of high spherical diameter capable of large production volumes and for bulky products.

Recently, several shuttle-type rotational molding machines have been designed and built – Rock'n Roll machine – for molding larger items. One of these machines currently produces containers over 40,000 lt. With this type of machine, the mold and material are mounted on a large independent shuttle that moves on tracks for moving from the oven to the cooling chamber.

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Which molds to choose?

Various types of metals are used in construction. Most practical for the production of small or medium-sized items with multiple cavities are aluminum molds. Sheet steel is usually used for prototype molds and for the production of large items generally cylindrical containers and other simple shapes.

Electroformed nickel mol ds are used for molding small PVC items and have the advantage of good surface reproduction, e.g., molds for dolls.

Their cost varies according to the quality or details required for the finished product, in any case compared to equipment for injection or blow molding they are certainly cheaper.