Rotational molding, also known as rotomolding or rotocasting, is a small part of the plastics industry. It began with polyvinyl chloride (PVC) plastisol molding in the late 1950s and progressed into polyethylene, where it has largely remained. This niche within the plastics world reaches almost every market. It offers much in the way of design flexibility and scale of products, with products ranging from simple bulk storage containers to sophisticated automotive, medical, and aerospace applications. Rotational molding lends itself to hollow, complex forms like no other process. Small parts, such as medical pipette bulbs can be made in essentially the same manner as large boats. Intricate parts, such as fuel tanks and components for aircraft ducting are becoming more common as rotational molding is recognized by a broader group of designers and engineers. rotomolding companies
Rotational molding (RM) is also called rotomolding, rotational casting, centrifugal casting, or co-rotational molding. This method, like blow molding (Chapter 6) and thermoforming (Chapter 7), is used to make hollow thermoplastic one-piece products (Table 13.1). Products include many different types such as furniture, light shades, marine accessories, material handling bins, shipping drums, storage tanks and receptacles, surf boards, toys, and so on. Sizes range fromsmall balls to at least 22,000 gallon tanks (83 m3) that weigh at least 2½ tons (5500 lb). Process is based on the heating and cooling of an axially or biaxially rotating split hollow cavity mold that defines the outside shape of the required product. No pressure is applied other than the relatively low-contact pressure developed during rotation of the heated melt. The most common is the multi-arm turret machine that has a three-stage operation.265, 268 269 270 271 272 273 274 275 477
A measured amount of powder or liquid thermoplastic (TP) is placed in the cavity that is mounted on a turret arm capable of rotating the mold. The mold in the oven spins biaxially with rotational speeds being infinitely variable, usually ranging up to 50 rpm on the minor axes and 12 rpm on the major axes. A 4:1 rotation ratio generally is used for symmetrically shaped parts. A wide variety of ratios are necessary for molding unusual and complex shapes.
This mold action permits uniform distribution of the plastics that is forced against the inside surface of the cavity. Following a prescribed cycle, the heat of the oven fuses or sinters the plastic and goes into the cooling chamber. The solidified product is removed from the mold and the cycle is repeated. This process permits molding very small to very large products. To improve product properties, hasten product densification, reduce air voids, reduce cure time, etc.
The cycle times typically range from 3 to 15 minutes. However they can be at least 30 minutes for large and thick products. The wall thickness of the parts affects cycle times, but not in a direct ratio. As an example, with polyethylene plastic the cycle time increases by about 30 s for every 25 mils of added thickness up to ¼ in. thickness. Beyond ¼ in. the heat insulating effect of the walls increases cycle times disproportional for any further increase in thickness; cycle times usually have to be determined experimentally and/or with prior experience.
Venting molds are often used to maintain atmospheric pressure inside the closed mold during the entire molding cycle. A vent will reduce flash and prevent mold distortion as well as lowering the pressure needed in the mold to keep the mold closed. It will prevent blowouts caused by pressure and permit use of thinner molds. As an example the vent can be a thinwalled plastic tube of PTFE that extends to near the center of the cavity. It enters the mold at a point where the opening it leaves will not effect the parts’ appearance, etc. The vent can be filled with glass wool to keep the powder charge from entering the vent during rotation.