to be melted. The stages of heating and cooling involve transfer of heat first from the hot medium to the polymer material and next from it to the cooling environment. In both cases, the process of heat transfer occurs in an unsteady regime; therefore, its kinetics attracts the greatest interest in considering these steps. In the heating stage, the heat taken from the hot gas is absorbed both by the mold and the polymer material. The rig for rotational molding usually has a relatively small wall thickness and is manufactured from metals with a high thermal conductivity (aluminum, steel). As a rule, the mold transfers much more heat than plastic can absorb; therefore, the mold temperature must vary linearly. The rotational velocity in rotational molding is rather low (4 to 20 rpm). As a result, in the first stages of the heating cycle, the charged material remains as a powder layer at the bottom of the mold. The most convenient way of changing the cycle is by applying PU sheets in hot rolled forms. Material requirements Owing to the nature of the process, materials selection must take into account the following: Owing to high temperatures within the mold, the plastic must have a high resistance to permanent change in properties caused by heat (high thermal stability). The molten plastic will come into contact with the oxygen inside the mold. This can potentially lead to oxidation of the melted plastic and deterioration of the material's properties. For this reason, the chosen plastic must have a sufficient number of antioxidant molecules to prevent such degradation in its liquid state. Because there is no pressure to push the plastic into the mold, the chosen plastic must be able to flow easily through the cavities of the mold. The part's design must also take into account the flow characteristics of the particular plastic chosen. References Bibliography } Further reading Crawford, R, Throne, James L., Rotational Moulding of Plastics, William Andrew Inc. (2002). Crawford, R, Kearns, M, Practical Guide to Rotational Moulding, Rapra Technology Ltd. (2003). External links Rotational Moulding Technical Guide Rotational Molding process videos Association of Rotational Molders Association of Rotational Moulders Australasia British Plastics Federation - Rotational Moulding Overview Casting (manufacturing) Molding processes