Technology Notebook: Rotational molding: An active competitorTechnology Notebook: Rotational molding: An active competitor

Rotational molding is noted for big parts such as a 21-ft boat hull and a 21,000-gal tank.

By Glenn Beall, Glenn Beall Plastics Ltd.

Editor?s note: This article is a sequel to ?Rotational Molding, Then & Now,? published in the July/August 2004 issue. That story reviewed the evolution of this industry.

Rotational molding is very different from other plastics molding techniques, and those differences generate its advantages. Rotational molding is a unique, open-molding, low-pressure, high-temperature process that uses biaxial rotation and heat to produce hollow one-piece plastic parts.

The secret to molding large parts is that the material is not stretched as it is by the blowmolding and thermoforming processes. The material does not flow through restrictive cavities, as is required by injection and compression molding. As the mold rotates, the plastic adheres to and coats the cavity. With this process the machine brings the cavity to the material instead of requiring the plastic to flow from some central point to the periphery of the cavity. The technique produces virtually any size part.

A look inside a mold at the actual molding technique reveals the source for some of this process?s impressive advantages. Playballs, for example, are a major market for rotational molding. A cross section of a ball mold is shown in Figure 1 (above). To produce a hollow ball, the molder manually or automatically places a measured amount of plastic in a powder or liquid form into the cavity of a mold. The mold is mounted on a machine for biaxial rotation. Rotational molding is not to be confused with centrifugal casting, which uses high rpm and centrifugal force to push the plastic to the periphery of the cavity. Rotational molding uses slow-speed rotation that allows the material to remain as a pool in the bottom of the cavity as shown in the figure.

The machine moves the rotating mold into an oven where the mold is heated. When the mold becomes hot enough, the plastic will begin to adhere to the cavity and be pulled up out of the pool of plastic material. As the mold rotates, this thin layer of material increases in temperature and fuses together, creating a solid wall. As the mold continues to rotate, additional material is picked up from the pool and the thickness of the material adhered to the cavity increases. With the mold rotating in two directions, all of the surfaces of the cavity repeatedly pass through the pool of material until all of the plastic is adhered to the cavity in a uniform wall thickness.

While continuing to rotate, the mold is moved out of the oven and into the cooling chamber, where the plastic is cooled to the point that the formed part regains strength enough to retain its shape.

The machine then moves the mold to the open station where the molded part is manually or automatically removed from the cavity. The mold can then be recharged with plastic material and the process can be repeated.

All competitive thermoplastic molding processes use heat to soften the material enough to allow it to be forced into the desired shape. The formed shape is then cooled and the plastic part retains its new shape. Rotational molding differs from the other plastic molding processes in that it heats and cools the mold and the plastic material during each cycle. Heating and cooling both the plastic and the mold results in molding cycles that are longer than those required for processes that only heat and cool the plastic material.

The necessity of heating and cooling the mold has resulted in the development of what are called shell molds. A shell mold is one in which the outside surfaces of the mold follow the shape of the cavity. This produces a thin-walled mold that can be quickly heated and cooled. A cast aluminum shell mold is illustrated in Figure 1. Molds of this type are provided with mounting pillars, or posts, which provide space for the heating and cooling medium to reach all surfaces on the mold.

These longer cycle times are the primary disadvantage of the rotational molding process. This disadvantage is, however, offset by some impressive advantages. For example, as the machine rotates the mold through two axes, all surfaces of the cavity repeatedly pass through the pool of material. Molding is actually achieved by the plastic material adhering to the cavity. There are no forces pushing or pulling the material into contact with the cavity. The slow heating and cooling?and the lack of force or shearing of the material?results in relatively low levels of molded-in stress. This reduction in molded-in residual stress results in improved impact strength and chemical resistance with a reduction in postmold warpage.

This low-pressure process allows the use of light-duty molding machines and quick delivery, low-cost molds. This is important in these times of niche marketing, which requires the production of small quantities of many different parts. It is economically feasible to produce small quantities of very large parts.The cavity coating nature of this process allows the molding of parts with walls that can be extremely thin in relation to their overall size. These thin walls reduce the amount of plastic material required and minimize molding cycle time. The end result is a lower cost part.

This coating process allows the formation of parts with wall thicknesses that are more uniform than comparable parts made by the competitive hollow-part processes, which are based on the stretching of a preform.

In summary, rotational molding is a good process for producing large, hollow parts having uniform, relatively thin walls with low levels of molded-in stress. This process has other advantages and disadvantages, but those are topics for another day.