Rotational molding, or rotomolding, involves heating a plastic resin in a hollow mold while rotating the mold slowly so as to melt and distribute the resin over the inside of the mold by gravity. Although articles of any size can be manufactured, rotational molding can fashion large, sturdy objects beyond the abilities of spin molding or blow molding, which employ centrifugal force or pressure to distribute plastic over the mold walls. Complex one-piece shapes with virtually no internal stress and resistance to stress-cracking are possible. Articles produced by rotational molding also possess uniform wall thicknesses with no thinning at extremities, and excellent load-bearing properties. Some undercutting is possible, without requiring draft angles. Metal inserts can be included as integral parts. A variety of finishes and colors can be accommodated. Tooling costs are low, making rotational molding economical for short runs as well as for volume production. Designs are flexible, with short lead time.
For these technical reasons, as well for product-introduction economic advantages, rotational molding is often employed in manufacturing containers, such as storage and fuel tanks. As one example, fuel tanks for industrial and agricultural vehicles commonly have complex shapes to maximize capacity in oddly shaped available spaces. As another example, tanks up to 2500 or so gallons for storing agricultural seed or chemicals benefit from the large sizes and structural strength afforded by rotational molding.
One shortcoming for rotational molding of containers is the lack of a technique for placing a transparent sight line in an otherwise opaque container for gauging the level of the container's contents without the added cost of mechanical or electrical sensors. Sight lines are common in small blow-molded containers for motor oil and other fluids. However scaling such containers up to the size, strength, and shapes required for many applications is technically difficult or not economically feasible.
Techniques are available for producing multiple-section walls in plastic articles by rotational molding. U.S. Pat. No. 6,383,437 to Thomas G. Grieve proposes a mold having interior removable partition members including thin blades barely touching the mold walls to form multiple spaces closed off from each other within the mold. Different resins are simultaneously melted and fully cured within the separate spaces. The mold is opened, and the partitions and blades removed. A second resin is melted and cured in the reclosed mold. Such a process requires high-precision, condition-sensitive parts that must be accurately assembled and disassembled, especially for complex mold shapes. The second layer must bond to the pieces of the first layer in order to hold them together, yet fully curing the first layer weakens the bond. Curing the first layer and taking the mold completely apart between the first and second layers lengthens the time required to produce an article.