Rotational Molding

Rotational molding (or Rotomolding) is a method that involves the slow tumbling, heating, and melting of a thermoplastic powder in a biaxially rotating mold to produce seamless, hollow plastic parts.  This process is typically used to mold hollow parts, especially those with complex and varied shapes not easily obtainable by other hollow-art processes.  It is a virtually shear-free and pressure-free process.  The wall thickness uniformity and part weight can be easily maintained.  There is very little waste of material due to scrap.  Rotomolding molds are often less expensive than other types of molds.

Lower viscosity resins are typically used in this process as they are more readily sintered to ensure a good surface finish.  The material must be capable of being ground into a 35-mesh powder that flows like a liquid. It must also be able to adhere to the hot surface of the cavity and fuse together without pressure.  The material should be stabilized to resist oxidation, or a nitrogen purge must be used during processing.

Process Steps

1. The rotomolding process is begun by placing a pre-measured amount of plastic material (in either liquid or powder form) in a cavity.  The mold is then closed and indexed into an oven where it and its contents are brought up to the molding temperature.  As the mold is heated, it is rotated continuously about its vertical and horizontal axes.  This biaxial rotation brings all surfaces of the mold in contact with the plastic material.
   
2. The mold is rotated within the oven until all the plastic material has been picked up by the hot inside surfaces of the cavity and densifies into a uniform layer.
   
3. While continuing the rotation, the machine moves the mold out of the oven and into the cooling chamber.  Air, or a mixture of air and water, cools the mold and the layers of molten plastic material. This cooling process continues until the part has cooled sufficiently to retain its shape.
   
4. The machine then indexes the mold to the loading and unloading station.  The mold is opened and the part removed.  A new batch of material is then placed in the cavity, the mold is closed and the process is repeated.

Ticonas "Rotomolding" products:

• Celcon® POM
• Hostaform® POM

Celcon® and Hostaform® Acetal Copolymer (POM) has been used to rotomold chemical storage containers, as well as numerous other parts of various shapes and sizes, for a wide variety of applications. Other grades of Celcon and Hostaform POM are being developed for use in small engine fuel tanks, to meet CARB limits on evaporative emissions, due to its low permeability to gasoline.