Rotational Molding what is it?
They often ask us for more information about the technique of Rotational Molding and its differences with other plastic forming techniques. We will try to cover the basic questions and the basic points concerning the production of plastic objects by rotomolding, as is often called.
Rotational Molding (BrE casting) includes a heated hollow mold filled with powdered polymer, a material that you can obtain from our company.
It then rotates slowly (usually around two vertical axes), causing the softened material to disperse and stick to the mold walls. In order to maintain a uniform thickness throughout the section, the mold continues to rotate continuously during the heating phase. This continuous rotation in combination with the correct temperature control serves and aims at avoiding deformations of the product during the cooling phase.
The process was applied to plastics in the 1940s, but in the early years it was not used much. The reason was because it was a process that was limited mainly to special bulky objects. But in the last two decades the machines have evolved considerably. Also, improvements in process control and plastic powder developments have led to a significant increase in use in many areas.
The process of rotary casting is a process of high temperature and low pressure plastics. It uses heat and biaxial rotation (ie, rotation in two axes) for the production of hollow objects. Critics of the process discuss the long cycle times, as only one or two cycles per hour can typically be made. This is unlike other processes, where parts can be built in a few seconds. The process of Rotational Molding has some very distinct advantages.
Rotational Molding ADVANTAGES
1. Making large hollow components, such as water, oil tanks, is much easier
2. Can not accept competition from abroad (eg China, Turkey, etc.) due to the high cost of transportation, if it concerns rolling objects.
3. Rotary molds are significantly cheaper than other types of molds.
4. Very little material is lost using this procedure.
5. Surplus material can often be reused, thus making a very cost-effective and environmentally sustainable manufacturing process.
Rotational Molding – The 4 stages of production
Rotary casting process
The rotary casting process consists of four distinct phases:
1. Placing a measured amount of polymer
(usually in powder form) in the mold. Material you can buy from our company that produces polyethylene powder and not just sells.
2. Heat the mold in an oven while rotating until all the polymer melts and adheres to the die wall
The hollow section must be rotated by two or more axes. Axes rotating at different speeds to avoid the accumulation of polymer powder. The length of time that the mold spends in the oven is critical. Long time and the polymer will be degraded, reducing impact resistance. A short period of time and the polymer melt may be incomplete. The polymer will not have time to melt and completely adhere to the mold wall. Therefore, it is likely that bubbles will occur in the polymer. This has an adverse effect on the mechanical properties of the finished product.
3. Cooling the mold with controlled cooling
This stage of the cycle can be quite large. The polymer must be cooled so that it can be solidified and can be handled safely by the operator. This usually takes a few minutes. The section will shrink on cooling so that it can be separated and facilitate easy removal of the article from the mold. The cooling rate must be maintained within a certain range. Very fast cooling (for example, water spraying) will result in cooling and shrinkage at an uncontrolled rate, creating a warped section.