Rotational moulding, known also as rotomolding or rotocasting, is a process for manufacturing hollow plastic products. For certain types of liquid vinyls, the term slush molding is also used. Rotational molding has particular advantages in terms of relatively low levels of residual stresses and inexpensive molds. Rotational molding also has few competitors for the production of large (> 2 m3) hollow objects in one piece. Rotational molding is best known for the manufacture of tanks but it can also be used to make complex medical products, toys, leisure craft, and highly aesthetic point-of-sale products.
The process begins with specially developed and uniquely applied resins. They can include plastisols, polyethylene, polycarbonate, acetate butyrate, polyamide, elastomers, polyurethane, polypropylene, ethylene vinyl acetate and fluorocarbons. Some of the most advanced work is being done with cross-linked polyethylene and even nylon.
The principle of rotational molding of plastics is simple. Basically the process consists of introducing a known amount of plastic in powder, granular, or viscous liquid form into a hollow, shell-like mold.7œ9 The mold is rotated and/ or rocked about two principal axes at relatively low speeds as it is heated so that the plastic enclosed in the mold adheres to, and forms a monolithic layer against, the mold surface. The mold rotation continues during the cooling phase so that the plastic retains its desired shape as it solidifies. When the plastic is sufficiently rigid, the cooling and mold rotation is stopped to allow the removal of the plastic product from the mold. At this stage, the cyclic process may be repeated. The basic steps of (a) mold charging, (b) mold heating, (c) mold cooling, and (d) part ejection are shown in Figure 1.1.
Professor Ian Harrison from The Pennsylvania State University has created a 3D interactive model of the entire rotational moulding process (requires Adobe Flash Player). Click here to view.
For more concise information about the rotomoulding process please CLICK HERE
Advantages and Disadvantages
The main attractions of rotational moulding are:
- A hollow part can be made in one piece with no weld lines or joints
- The end product is essentially stress-free
- The moulds are relatively inexpensive
- The lead time for the manufacture of a mould is relatively short
- Short production can be economically viable
- There is no material wastage in that all of the material is normally consumed in making the part
- It is possible to make multilayer products
- Different types of product can be moulded together on the one machine
- Inserts are relatively easy to mould in
- High quality graphics can be moulded in
The main disadvantages of rotational moulding are:
- The manufacturing times are long
- The choice of moulding materials is limited
- The material costs are relatively high due to the need for special additive packages and the fact that the material must be ground to a fine powder
- Some geometrical features (such as ribs) are difficult to mould
Want to see rotational moulding in action? Click here to see arms rotate on carousel machine.
