Developments in Robotic Rotational Moulding

Rotational moulding is fundamentally a very simple manufacturing process in which a heated polymer powder is tumbled across the internal surface of a heated mould. Most commonly this is achieved by placing a mould partially filled with powder into an oven where it is subjected to gentle movement (generally rotation) about two perpendicular axes. In this way the heated powder gradually coats the internal surface of the mould to create a thin polymer shell. The process is extremely effective and economic for the manufacture of large hollow structures, but it is inflexible and cycle times are extremely long. In its early days the process was operated almost entirely by trial and error but over many years the industry has gradually embraced the use of in-mould instrumentation and some use is now made of process modelling. However, such developments have lagged well behind those of other polymer processing techniques. The simplicity of the process is partly to blame since neither temperature nor mould movement can be easily changed during a production cycle. Latest technological developments in two areas are now changing this. Firstly, the fixed temperature convection oven is being replaced by moulds containing internal heating and cooling elements. Secondly, the simple two axis rotating arm is being replaced by programmable multi-axis manipulators such as robot arms. Together these complementary technologies are creating an entirely new form of rotational moulding that is highly flexible and controllable, and which offers the prospect of a step change in rotational moulding technology. The development of this new robotic-based technology is the subject of this investigation and the prototype system that is under trial at Queen’s University is shown in Fig. 1. Tests are currently being conducted to assess the capabilities of the new system. Robotic system data and in-mould sensors enable continuous control of mould temperature, position, speed and direction of travel. Together these provide the possibility of matching temperatures and movements to suit particular stages of the production cycle and to provide a degree of coordination and precision never before experienced in rotational moulding. In addition, the continuous two-way flow of data, coupled to the fully programmable system offers the prospect of a truly intelligent manufacturing cell where processing variables are adjusted and optimised in-cycle.