The rotoforming process involves five steps
1. Loading. 2. Moulding. 3. Cooling. 4. Unloading.5. Finishing.
1. Loading: the mould is charged (or loaded) with required amount of powder or liquid. This can be done by weighing the raw material so that uniform parts are produced. The weight can be calculated from the volume of product and the density of the material used.
2. Moulding: It contains two simultaneous processes: a) rotation and b) heating
a) Rotation: the mould is rotated in two axes. This biaxial rotation can be done by using a mould on the end of an offset arm or using a series of moulds on the end of a centre swing arm, which has an effect of a radius arm powered from a central hub. In each case the ratio 1:4 is maintained between the speed of rotation of the major axis and the minor axis. The typical speed being 3-10 rpm for the major axis and 12-40rpm for the minor axis. However the actual speeds must be determined experimentally and it depends on the part size, geometry, material, heating rate etc.
b) Heating: It is accomplished either by hot air—by placing mould assembly in an oven or in some cases by a hot liquid—either oil or molten salt. The oven temperature can be 200-3700C or even higher for some material. The major requirement of the oven is to furnish the hot air in large quantities at right temperature and with an even air flow having minimum hot or cold spots. The types of heating are:
i) Liquid heating: Though considerably faster has a disadvantage of requiring thicker moulds and more perfect parting line closure. The liquid used is usually molten salt, which may lead to corrosion problems with some materials unless precautions are taken to wash the mould after each use.
ii) High frequency RF Heaters: They are the 3rd heating technique in use (after hot air and liquid heating). It is very efficient but quite costly.
iii) Infrared heating: Though it is fast and efficient method, it has its own distinguished limitations. Its use is confined to simple shapes and single mould because of the “shadowing effect” of one mould on the other which causes very uneven heating.
In general the heating units in the mould should cause the resin to melt. The heating period may range from 1-10 minutes depending on the part, the wall thickness and the type of the resin being used. Here the gravitational force (and not the centrifugal force) distributes the material uniformly over the entire inside surface of the mould.
3. Cooling: After heating period (i.e. the moulding is completed) the mould assembly is moved to the cooling station, where cooling is done by means of a cold water spray. The spray is sometimes combined with cold air for greater flexibility. The moulds must be rotated during the cooling stage to prevent distortion in the moulded part. Sometimes too rapid cooling can cause warpage in the finished parts. However, as the heating time usually determines the time for all other operations, cooling can almost always be adjusted satisfactorily with little difficulty.
4. Unloading: After cooling, the mould assembly is moved to the loading station where loading and unloading takes place. For low volume production, a simple, single-spindle, rotating mould on a reciprocating unit is used for loading, moulding and cooling. For high production, a multiple spindle unit is used where one or more spindles undergo different stage of the process. With such an assembly, there are no interruptions at any time during the cycle, but the times (intervals) must be coordinated so that each step is completed in same time. This can be done by adjusting the size of the oven or cooling chamber, the rate of heat input etc.
5. Finishing: The rotationally moulded parts require very slight finishing only at the mould parting line. For olefins the surface can be treated by flame or electrical discharge method to promote ink and paint adhesion. The use of pigment in resins should be kept to a minimum (0.1-1.0%) in order not to affect the mechanical properties of the finished parts.
Automatic loading device: For small production runs, filling is usually done manually. But for large production runs fillings are usually done on weight basis because the apparent density of a powder may vary with the mesh.
Mould design: Introduction of inert gas like N2 or CO2 to replace air must be made possible by redesigning mould, so that the resin can be heated to the highest temperature and the danger of oxidation, discolouration, odours etc is minimized. The physical properties of the final part can be improved, compared to the parts in an air atmosphere because the thermal and oxidative degradation is reduced. Also the mould must be manifold to allow introduction of coolant like cool air followed by a water spray. Internal cooling can reduce the cooling time and in the case of crystalline material like linear PE can result in improved physical properties by lowering the crystalline content, thus improving the toughness of the final product.