Rotational molding techno|ogy.pdf
Rotational molding is the process of producing hollow parts by adding plastic powder to a shell-like mold and rotating the mold about two axes while heating it and the powder. During rotation, the powder fuses against the inner mold surface into a bubble-free liquid layer. The polymer is then cooled to near room temperature, and the resulting hollow part is removed. The cyclical process is then repeated. Although the rotational molding concept is more than 150 years old, the production of hollow plastic parts for such varied applications as outdoor playground equipment, liquid storage tanks, furniture, and transportation products is around 50 years old. With the advent of process controls and improved polymers, the U.S. market in the year 2000 has exceeded one billion pounds or 450,000 kg. Worldwide production is estimated at more than twice the U.S. market. During most of the 1990s, the rotational molding industry was growing at 10% to 15% per year.
With the growth of rotational molding has come an increasing interest in the complex technical aspects of the process. As detailed in this monograph, the heating process involves the slow rotation of relatively fine particulate powders in a metal mold, the heating of these powders until they begin to fuse and adhere to the metal mold, the coalescence of the powder through building of powder-to-powder bridges, the melting of the powder particles into a densified liquid state, and finally, the dissolution of air bubbles. The cooling process involves temperature inversion in the liquid layer against the mold surface, cooling and crystallization of the polymer into a solid, and controlled release of the polymer from the mold surface to minimize part warpage and distortion. Ancillary aspects of the rotational molding process, including grinding, mold making and mold surface preparation, and part finishing are also included. Characteristics of rotationally molded polymers, including standard tests such as melt index and crosslink
density are detailed. Liquid rotational molding, the oldest form of rotational molding, is also discussed.