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4 Rotational Molding Technology Table 1.1. The process is distinguished from spin casting or centrifugal cast- ing by its low rotational speeds, typically 4 ñ 20 revs/min. The primary compe- titors to rotational molding are structural blow molding and twin-sheet thermoforming. As with most manufacturing methods for plastic products, rotational molding evolved from other technologies. A British patent issued to Peters in 1855 (before synthetic polymers were available) cites a rotational molding machine containing two-axis rotation through a pair of bevel gears. It refers to the use of a split mold having a vent pipe for gas escape, water for cooling the mold, and the use of a fluid or semifluid material in the mold to produce a hollow part. In the original patent application this was a cast white metal artillery shell. In Switzerland in the 1600s, the formation of hollow objects such as eggs quickly followed the development of chocolate from cocoa. The ceramic pottery process known today as ìslip castingî is depicted in Egyptian and Grecian art, and probably predates history. 1.2 The Early Days Rotational molding of polymers is said to have begun in the late 1930s with the development of highly plasticized liquid polyvinyl chloride, the thermo- plastic competitor to latex rubber.9ñ14 In addition to the ubiquitous beach balls and squeezable toys, syringe bulbs, squeezable bottles and bladders and air- filled cushions were developed during World War II. Until polyethylene pow- ders were produced in the late 1950s, most rigid articles were manufactured from cellulosics. The early equipment was usually very crude. Generally it consisted of a hollow metal mold rotating over an open flame. Sometimes a type of slush molding would be used. In this method, the mold would be com- pletely filled with liquid or powdered plastic and after a period of heating to form a molten skin against the mold, the excess plastic would be poured out. The molten skin was then allowed to consolidate before being cooled and re- moved from the mold.15 In the 1950s the two major developments were the introduction of grades of powdered polyethylene that were specially tailored for rotomolding,16, 17 and the hot air oven. With the new material and equipment it was possible to rapidly advance the types of hollow plastic products that could be manufac- tured. In North America the toy industry took to the process in a big way and, as shown in Figure 1.2, today this sector still represents over 40% of the consumption in that part of the world.PDF Image | INTRODUCTION TO ROTATIONAL MOLDING 1
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