Rotational casting methods

In recent years rotational casting methods have made the slush moulding process virtually obsolete. In these processes an amount of material equal to the weight of the finished product is poured into a mould. The mould is then closed and rotated slowly about two axes so that the paste flows easily over the cavity walls in an oven at about 200-250°C. When the compound has gelled, the moulds are cooled and the moulding removed. Compared with the slush moulding process there is no wastage of material, little flash, and more even wall thickness. Completely enclosed hollow articles such as playballs are most conveniently made. 

The test was conducted in accordance with ASTM C 944-90a Standard of Abrasion Resistance of Concrete by the Rotating-Cutter Method. This method measures the capacity of wear resistance of a concrete surface. The specimens were 5 in. (127 mm) long, 4 1/2 in. (114 mm) wide and 1 1/2 in. (38 mm) thick and were cut from the tops of the beams cast for the deicing scaling test. The rotating-cutter was applied to the interior cut surface and not to the tool-finished surface of the sample. 

In rotational casting a predetermined amount of liquid plastisol is placed in a heated, closed, two-piece mold. The liquid is uniformly distributed against the walls of the mold in a thin uniform layer (Figure 2.43c) by rotating the mold in two planes. The solidified plastisol in the mold is cured in an oven the mold is then opened, and the part is removed. The method is used to make completely enclosed hollow objects. Doll parts, plastic fruits, squeeze bulbs, toilet floats, etc. can be made by rotational casting of plastisols. 

Die casting is used to make a solid object, but hollow objects cannot be produced by this method. Hollow objects such as balls and dolls are made by rotational casting. The resin and curative mixture is placed in a hollow mould that can rotate simultaneously along the primary and secondary axes. The resin takes the shape of the mould. The shape is fixed due resin curing under rotation at elevated temperature. The mould is then cooled and opened to remove the product. 

An interesting example of rotationally cast black polyethylene by this method is shown in Figure 6. 

Another method for manufacturing polyurethane covers is the moldless rotational casting process (Fig. 6.12). Here the reacting polyurethane mixture is cast as a spiral on the rotating roll, the fast reaction of the polyurethane mixture prevents the material dripping off the roll. Layers of different hardness can be applied easily with this method. 

This form of PVC is called a plastisol or organisol. PVC in the fluid form can be processed by methods such as spread coating, rotational casting, dipping, and spraying. The parts are then dried with heat to remove any solvent and fuse the polymer. Parts such as handles for tools and vinyl gloves are produced by this method. 

The annual production of mouldings required for the CAT indicated that the most effective manufacturing method would be either GRP hand lay-up, thermoforming or rotational casting, but experience had proved that GRP... 

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