point to its long cycle times—only one or two cycles an hour can typically occur, as opposed to other processes such as injection molding, where parts can be made in a few seconds. The process does have distinct advantages. Manufacturing large, hollow parts such as oil tanks is much easier by rotational molding than any other method. Rotational molds are much cheaper than other types of mold. Very little material is wasted using this process, and excess material can often be reused, making it a very economically and environmentally viable manufacturing process. The rotational molding process consists of four distinct phases: Loading a measured quantity of polymer (usually in powder form) into the mold. Heating the mold in an oven while it rotates, until all the polymer has melted and adhered to the mold wall. The hollow part should be rotated through two or more axes, rotating at different speeds, in order to avoid the accumulation of polymer powder. The length of time the mold spends in the oven is critical: too long and the polymer will degrade, reducing impact strength. If the mold spends too little time in the oven, the polymer melt may be incomplete. The polymer grains will not have time to fully melt and coalesce on the mold wall, resulting in large bubbles in the polymer. This impairs the mechanical properties of the finished product. Cooling the mold, usually by fan. This stage of the cycle can be quite lengthy. The polymer must be cooled so that it solidifies and can be handled safely by the operator. This typically takes tens of minutes. The part will shrink on cooling, coming away from the mold, and facilitating easy removal of the part. The cooling rate must be kept within a certain range. Very rapid cooling (for example, water spray) would result in cooling and shrinking at an uncontrolled rate, producing a warped part. Removal of the part. Recent improvements Until recently, the process largely relied on both trial and error and the experience of the operator to determine when the part should be removed from the oven and when it was cool enough to be removed from the mold. Technology has improved in recent years, allowing the air temperature in the mold to be monitored and removing much of the guesswork from the process. Much current research is into reducing the cycle time, as well as improving