Rotational Molding – Working Principle , Application , Advantages, Disadvantages
The rotational molding is a high temperature and low pressure plastic forming process. The powder is inserted into the closed split mold and rotated in biaxial to produce a hollow part. The schematic of rotational molding process is shown in figure
The plastic material in the form of powder is inserted into the mold. The mold is heated in an oven and rotated biaxial until the powder has melted. The molten powder is adhered to the mold wall and makes a thin layer. The mold is opened and finished part is removed.
The mold is rotated at different speeds in order to avoid the accumulation of molten powder. The amount of time required to heat the powder at molten temperature in to the oven is important. If the oven is heated for long time, powder will degrade and it will affect the mechanical properties. If the mold is heated short time, the powder may not be completely melted, resulting in large bubbles in the powder. The formed part must be cooled through the air so that it solidifies slowly. The formed part will shrink on cooling, and facilitating easy removal of part. The cooling rate must be kept within a certain range. The water cooling should be avoided, because formed part may shrink and warped after cooling.
A mold release agent should be used to quickly remove the formed part. Mold release can reduce cycle times and defects of finished product. Three different types of mold release agent may be used i.e. sacrificial coating (silicones), semi-permanent coating, (polysiloxane), and permanent coating (polytetrafluoroethylene).
The amount of powder
Heating temperature and time
Thermosets and thermoplastic material can be used, for example: low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene, and polyvinyl chloride (PVC).
Additives for weather resistance, flame retardation can be incorporated. Products that can be manufactured include storage tanks, bins and refuse containers, airplane parts, road cones, footballs, helmets, and rowing boats.
Allows complex mold split lines.
Allows molded threads and mold-in inserts.
Allows a wide range of surface finishes (textured, smooth, or polished).
Very little waste.
Low residual stresses
Tooling is less expensive
Suitable for both low-volume prototypes and high-volume production runs
Slow production speed. It usually takes about one hour to complete the process