Originally published April 1996
Spin-Cast Plastics, Inc., South Bend, IN
Although the term rotational molding is unfamiliar to many, the products produced by the process are visible and familiar in a wide variety of everyday settings. Rotational molding (also called rotomolding) enables manufacturers to produce medium- to large-scale hollow plastic components ranging from 6-in.-diam containers to 20,000-gal tanks.
Recent applications of rotational molding in the medical device field include squeeze-bulb-type ear syringes, dental chairs, cases
for resuscitators, medical carts, foam-filled tubs, and biomedical
Rotational molding begins with a mold, a charge of resin, and a molding machine. A mold is mounted to an arm of a rotomolding machine and charged with raw material, which may be either in liquid form (such as a polyvinyl chloride plastisol) or in a dry, pulverized powder form (such as a polycarbonate).
Equipment. Rotomolding machines are classified into five categories: clamshell, independent cart, rock-'n'-roll, shuttle, and turret. Each of these offers processors an alternative manufacturing technology. Rotomolding machines may use a single arm, or may have three or four arms. The arms serve two functions: to rotate the molds on both axes in order to distribute the resin evenly and consistently along the mold surfaces, and to move the molds into one of the three stations of the machine--the oven, the cooling chamber, or the product loading and unloading station. The latest generation of equipment features onboard computers that control the arm rotation sequence, cycle time, oven temperature, and the internal and external cooling apparatus for the molds.
Molds may be fabricated of aluminum, steel, or stainless steel; electroformed from nickel; or cast from aluminum. Factors that determine what type of mold to use include the product's design, function, and aesthetics as well as the cost/benefit ratio of the mold's anticipated annual usage. Regardless of their material, all rotational molds are female cavities that do not require an inner core, similar to those used in blow molding. Unlike blow molding, however, rotomolding does not generate high pressures, and the molds used for the process therefore require only a minimum of cavity support and structure.
In rotational molding, molds act as vehicles for transferring heat to the resin, establishing the shape to be formed, and providing a cavity in which to cool the material. Compared to the molds used in injection or blow molding, those used in rotomolding are both inexpensive and quick to produce; typical lead time for production of a cast aluminum rotomold is 10 to 12 weeks.
A well-designed and properly specified mold is a key element in the successful rotomolding of a product. In fact, where other processes would require multiple molds, a properly designed rotomold can enable manufacturers to use a single tool for producing multiple parts as well as multiple-wall and multiple-layer products.
Materials. Although the number of materials suitable for rotomolding is limited compared with other molding processes, the resins that can be used still offer the medical device designer a wide range of capabilities. Acceptable resins include low-density polyethylene, high-density polyethylene, polyvinyl chloride, ethylene vinyl acetate copolymer, nylon 6, nylon 12, polycarbonate, and polypropylene. The density and melt index of these materials vary according to type as well as from vendor to vendor, but all are in an acceptable range for use in rotomolding.
Choosing the appropriate resin is frequently a function of the product's design, purpose, and anticipated work environment. For instance, rotomolded polyethylene offers significant impact resistance at low temperatures, while the nylon resins offer high-temperature resistance and stiffness.
Processing Parameters. Rotational molding is a phase-change process: its goal is to induce a physical change in the material from a dry or liquid state to a solid-wall state. This change is accomplished by inducting heat into the material via the mold. The typical temperature range of the rotomolding oven is 500° to 650°F. The precise temperature range is determined by the type of material being used in each mold.
Rotomolding offers manufacturers the capacity to produce multiple products in the same cycle. Typically, more than one mold is mounted to each arm of the rotomolding machine; so long as the time and temperature requirements of the products are similar for all molds on the arm, all can be produced at the same time.
Design Considerations. Rotomolding offers designers the ability to make use of cramped spaces by molding irregular shapes, and to enhance strength and impact resistance through the use of increased thickness in outside corners. Because rotomolding is a low-pressure process, products manufactured with this technique are relatively stress-free.
Dimensional tolerances in rotational molding are similar to those for blow molding, ranging from ±0.020 in./in. for commercial applications to as little as ±0.010 in./in. for precision applications. Most cast aluminum molds offer tolerances of about ±0.015 in./in. Processing tolerances for rotomolding are similar to those found with other processes, and can include the use of CNC-controlled routers for secondary processing of molded products.