Rotational molding process control

The Rotosil process employed by Heraeus and Heraeus-Amersil is used for the production of tubular or cylindrical shapes. It permits greater uniformity and dimensional control than the previous process. The quartz is washed in hydrofluoric acid and distilled water to remove impurities which can promote crystallization during manufacture. The sand is then placed in a rotating horizontal steel tube where it is held at the circumference by centrifugal force. A carbon arc is passed slowly down the center of the drum to fuse the sand. A modification, using open rotating molds on a vertical axis, allows formation of cmcibles, beakers, and bowls. 

Rotational molding is a reasonably safe industry. Most resins used are nonhaz-ardous. Some resins may cause mild irritation suppliers provide safety information on their handling and use. Most resins used for rotational molding are ground to 500 /xm (35 mesh), and appropriate procedures should be followed to prevent a dust explosion and fire. A rotomolder is a relatively safe piece of processing equipment fans are enclosed with guards spindles and arms operate at a low speed. The oven is equipped with a temperature control. 

Process analysis of rotational molding has been concerned largely with the heat-transfer melting and sintering aspects of the cycle [15]. Further, it has been indicated [15] that, although flow occurs across the inner mold surface, it is not an important effect as long as mold aspect ratio (kept near unity) and mold position (reasonable when contrasted with other such units) are kept under control. 

The basic rotational molding machine must be able to satisfy these three stages of the rotomolding process (typically simultaneously) while providing controlled rotation of the molds being used. 

Variation of wall thickness has a direct impact on the way in which rotationally molded parts are dimensioned. Since the wall section may vary and in many cases cannot accurately be predicted before a part is made, the part must be dimensioned to an external feature. If an internal dimension is required, then allowances for the upper tolerance of wall thickness variation must be made. Designers used to specifying injection molded parts with controlled surfaces both inside and outside of the part can find this difficult to deal with. However, extmsion blow-molding and twin sheet thermoforming share the same limitations and rotational molding actually maintains more uniform wall thicknesses than either one of these competitive processes. 

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