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Polymer Processing 643

requiredtoheattheblanktothedesiredprocessingtemper- is relatively slow taking in the range of an hour or more to
atureandtheselectionoftheappropriateheatingtechnique make a part. However, extremely large hollow objects can
(radiation heating, forced convection, etc.). It is necessary be made this way (e.g., a storage tank having a length of
to make sure that the center reaches the desired processing 20 ft and diameter of 8 ft). HDPE is most frequently used,
temperature without the surface being held at too high of a but there are reports of nylons, ABS, and polycarbonate
temperature for too much time. The third is the prediction being used.
of the force required to fill the mold. Finally, the tempera-
ture of the mold must be determined, keeping in mind that
one wants to cool the part down as rapidly as possible, but SEE ALSO THE FOLLOWING ARTICLES
too rapid of a cooling rate will prevent the polymer from
filling the mold. ELASTICITY,RUBBERLIKE • PLASTICIZERS • POLYMERS,
MECHANICAL BEHAVIOR • POLYMERS,RECYCLING •
POLYMERS,THERMALLY STABLE • RHEOLOGY OF POLY-
E. Rotomolding
MERIC LIQUIDS
Rotomolding or rotational molding is a process for making
hollow plastic articles that cannot be easily produced by
techniques such as blow molding. Examples of objects BIBLIOGRAPHY
produced by this method include underground storage
tanks for fluids, playground equipment, and recreational Baird, D. G., and Collias, D. I. (1998). “Polymer Processing: Principles
and Design,” Wiley, New York.
items such as coolers and kyaks. The objects are usually
Bird, R. B., Armstrong, R. C., and Hassager, O. (1987). “Dyna-
larger and have thicker walls than can be handled in blow mics of Polymeric Liquids. Volume I: Fluid Mechanics,” Wiley,
molding. The process consists of placing polymer powder New York.
(typically 40 mesh or 500 µm in size) into a mold and then Dealy, J. M., and Wissbrun, K. F. (1990). “Melt Rheology and Its Role
placing the mold into a forced convection oven where it is in Plastics Processing,” Van Nostrand Rheinhold, New York.
Green, E., ed. (2000). “Modern Plastics Encyclopedia,” McGraw-Hill,
rotated around two axes to distribute the powder over the
New York.
mold walls. The polymer particles are fused into a solid Michaeli, W. (1984). “Extrusion Dies,” Hanser, M¨unich.
mass by the process of sintering, which is governed by Morton-Jones, D. H. (1989). “Polymer Processing,” Chapman and Hall,
surface tension and the polymer melt viscosity. High MI London.
polymers (i.e., low molar mass) are typically used because Rauwendall, C. (1986). “Polymer Extrusion,” Hanser, M¨unich.
Rodriguez, F. (1996). “Principles of Polymer Systems,” Taylor and
it is the viscosity that leads to the resistance of fusion of
Francis, Washington, DC.
the particles. The part must be cooled slowly to ensure Van Krevelen, D. W. (1990). “Properties of Polymers,” Elsevier,
dimensional stability and minimize warpage. The process Amsterdam.