Plastics - Rotationally Molded and Vacuum Formed
Rotational molding of plastic is accomplished with a powdered resin, a metal mold (cast or fabricated aluminum or stainless steel), and an oven that turns biaxially on an X-Y axis at a very slow rate of 11 rpm x 9 rpm. Material sticks to the wall surface of the mold.
The mold moves around the puddle of material creating layer on layer to generate even wall thicknesses. The buildup makes heavier corners on the part being molded. This phase, the turning cycle, takes 12 to 15 minutes, depending on the desired wall thickness.
The parts are then cooled in a chamber by fans that also turn biaxially. Some cooling methods include a water spray. Three- or four-arm turrets are employed in this process. As parts are cooled, another "spider" (turret) of parts is being heated.
This three-phase process is completed in a 45-minute cycle, 15 minutes each for loading/unloading, heating and cooling. Rotational molding is used to manufacture hollow plastic parts. The finished surface of the part is that of the mold surface. Greater wall thicknesses can be adjusted in rotational molding by adding or decreasing resins.
Thermal vacuum and pressure forming are achieved by flat sheet material being stretched over a vacuum mold. This process does not provide an even wall thickness. Wall thickness can be achieved by increasing the thickness of the starting material.
The materials used are UV stabilized or flame-retardant polyethylene, polypropylene, some nylon and polycarbonates for lighting enclosures.
To make hollow parts from pressure or vacuum methods, it is necessary to make two parts and bond together. In vacuum and pressure applications colored sheets can be used or the parts can be painted.
The rotational molding method produces anything from a ping-pong ball to 20,000-gal agricultural tanks or larger. A maximum size produced by vacuum forming is 48" x 48". Pressure forming capacity is a 36" square maximum size.
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