ROTATIONAL MOULDING
Description
| Rotational moulding (also known as rotomoulding or rotocasting) is a method for producing hollow plastic articles. It was developed in the 40´s but in the past two decades a very significant increase in their usage. The concept is simple, a charge of cold plastic powder or a puddle of liquid is placed in one half of a cold mould (of sheet steel or cast aluminium), then it is closed and rotated biaxially in a heated oven. The plastic powder melts and coats inside surface of the mould. Then it is transferred to a cooled environment. The rotation continues until the plastic has solidified. |
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| Food container manufactured by rotomoulding of PE and casting of PU |
Contacts
www.gaiker.es; etxabe@gaiker.es; www.cromoduro.com; comercial@cromoduro.com
Characteristics
| Costs (tooling / unit) | Unexpensive moulds. (E.g. 6000 € for a cilindrical mould of 500 mm length, 150 mm
diameter)
Cost per unit (Cilynder above mentioned of 3 mm thickness): Total cost 3€ . Raw material cost: 1€ Lead time for moulds: 2-4 weeks |
| Volume (units / day) | 1 piece each 20 min.
Possible to introduce in the oven more than a mould. (E.g. if 20 moulds are used at the same time, the ratio would be 1 piece per min. The limitation is the volume of the oven. |
| Possible Materials | Easy flowing materials.
Mainly: Polyethylene (90%) and PVC (plastisols). Other TP´s : special rotomouldeable grades of Nylon, ABS, PC, HIPS, acetal, etc.. Reactive resins: Thermosets as polyester, silicone rubber, reactive nylons and polyurethanes. Foaming grades of thermoplastics. |
| Size / Shape | Hollow components, open and closed shapes.
Size: Ilimited. Depends on the size of the oven. Possible to obtain multimaterial or multicolor components. Possible to obtain sandwich structures. Big components stress-free, no weld lines or sprue marks. Good wall thickness uniformity. Bosses and solid ribs cannot be easily moulded. |
| Mechanical Properties | Low stiffness (overcome through geometrical features)
Mechanical properties depends on the material. Intrinsic properties of each material. E.g., for HPDE: Tensile yield: 20.7...27.6 MPa Flexural module: 930.7...1447.7 MPa HDT: 67.2...71.7 ºC Izod Impact (notched): 245.5...389.7 J/m Density: 0.941...0.959 g/cm3 Mold shrinkage: 1.5...3.0 % Melt Index: 5...8 g/10 min |
| Special Remarks | Rotomoulding is suited for economic short production runs and indeed it has an ever increasing in prototype work for other
processes.
Casting process consists of pouring an amount of liquid resin, usually a thermoset resin, in a hollow part, curing inside. This proccess combine very well with rotomoulding. With rotomoulding, hollow parts are produced and then, using casting proccess those ones can be filled. |
Application / Case Study
The application shown in the photograph above is a container for food. The manufacture of this component is by means of two processes:
The external skin is manufacturated by rotomoulding using Polyethylene resin. Then the inside is filled of cast Polyurethane obtaning the final product.
Typical Process conditions:
Time of cycle= 10-15 min. For rotomoulding the external skin
Time of cycle= 7 min. For curing the polyuretane.