Rotational-molding type

What types of products are produced by rotational molding Why Give three examples,... 

With rotational molding, a closed mold is rotated in two or more planes during fusion or solidification of the fluid polymer system. The action of the rotation deposits an even coating of polymer upon the inside of a "clam shell" type mold. After a predetermined time under heat, the molds are removed from the oven and cooled, and the finished product is removed whereupon the cycle is started again. Conventional gas fired ovens or hot salt bath sprays are used for heat transfer, and cooling is usually effected by spraying or immersion in cold water. This technique is well suited to molding completely hollow objects. 

Aromatic and aliphatic hydrocarbons are used as extenders, particularly in the manufacture of PVC plastisols that must maintain as stable a viscosity as possible for relatively long periods of time (dip and rotational molding). The petroleum industry offers imprecisely defined products with an aliphatic-aromatic structure for use as extenders. They are added to plastisols in small amounts as viscosity regulators. Dibenzyl toluene also serves the same purpose. A disadvantage of this type of extenders is the risk of exudation if excessive quantities are used. 

FIGURE 2.78 Basics of continuous-type three-arm rotational molding machine. 

Modern rotational-molding machines enable large parts to be molded (e.g., 500-lb car bodies and 500-gal industrial containers). For producing small parts an arm of a carousel-type machine may hold as many as 96 cavities. 

Practically all resins (TS and TP) are used in RPs, but a few predominate (Tables 7-2 to 7-5), with TS polyesters being the major type. Typical polyesters are summarized in Table 7-4. The polyester RPs are used in all processes, but their principal use is in the low pressure methods (spray-up, hand lay-up, pressure bag molding, casting, pultrusion, rotational molding, filament winding, and compression molding. 

Figure 14.23. Types of arms employed in rotational molding.

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