A Guide to Rotational Molding Figure 1. Various rotomolded toys A Guide to Rotational Molding Introduction Rotational molding – also referred to as rotomolding or rotational cast- ing – is one of the fastest growing plastics processing methods today. It’s easy to see why. Rotational molding techniques – coupled with Microthene® polyolefin powders from LyondellBasell – can be used to make hollow items of any size, open or closed, and of any desired shape. In recent years, rotomolding techniques have been developed extensively. On the production line, this process can now compete with or outperform blow molding, injection molding and thermoform- ing. In many cases, pieces virtually impossible to fabricate by any other processes can be produced by Figure 2. Smaller storage bins and refuse containers are also rotomolded rotomolding. Rotational molding was first developed early in the 20th century. Rotomolding is different But it wasn’t until the early 1960s, The main difference between The plastic particles make contact when replaced vinyl LyondellBasell rotational molding and other and melt on the inner surfaces of plastisol resins with polyolefin resins, plastics molding techniques, such the hot molds and fuse in layers that rotomolding gained wide as blow and injection molding, are until all the powder is fused and the acceptance. as follows: desired end product and wall thick- Within a few years, the develop- • resin powder is used instead of ness is obtained. The wall thickness ment of low and high density pellets; is controlled by the amount of polyethylene specifically designed powder placed in the mold. • the resin melts in the molds for rotomolding enabled rotational instead of being forced under Rotationally molded pieces are molders to enter markets where pressure into the molds in a stress-free except for slight shrink- vinyl parts and processes could not molten state; age forces because the pieces are compete. In the early 1970s, produced without any external pres- crosslinkable and modified polyeth- • the mold has a biaxial rotation; sure. Additionally, there is practically ylene grades made their way into • rotomolding molds are less no scrap in rotational molding. the rotational molding market. expensive because of their These new resins again opened up simplicity; The uniformity of wall thickness more new market areas, especially • and operating pressures are can be maintained to within production of large tanks. relatively low, allowing molds ±10 percent, which is better than that normally possible with the Linear low density polyethylene for to be made from less expensive blow molding process. Wall thick- rotomolding was developed in the materials. nesses can range from 1/32 inch to late 1970s, while the 1980s brought In rotational molding, rigid, resilient 1 inch (0.8mm to 25mm). Most a surge of non-polyethyl-ene resins, hollow bodies are formed by pow- resins used in rotational molding including nylon, polypropylene and dered plastic material in heated are powders ground to 35 mesh polycarbonate for rotomolding. molds, which are rotated simultane- and ranging in diameter from ously in two planes perpendicular to 74 microns to 2000 microns. each other. 1 Figure 3. Rotomolded products in this photo range from 55-gallon drums to billiard balls 2 Typical rotational molding applications Advantages Rotational molding permits produc- • display window mannequins and of rotational tion of a countless number of fully other hollow display figures of all molding or partially closed items. Design sizes, and; Rotational molding offers signifi- versatility of rotationally molded • playground equipment and cant advantages when compared pieces is almost unlimited. sporting equipment such as golf with other molding techniques or carts, footballs, juggling pins, The rigidity or flexibility of an item thermoforming: is controlled by the properties of helmets, dumbbells, golf tee • costs for molds and tooling are the resin used (see section on Resin markers, canoes and kayaks. relatively low; Choice) and by the wall thickness of Rotomolded parts are also used in • the rotomolding technique is the molding. portable outhouses, battery cases, easily adapted to short produc- light globes, vacuum cleaner and Some typical applications for which tion runs, particularly when sets scrubber housings and garbage rotational molding is particularly of multiple-cavity molds are containers. Furniture, game hous- suited include the following: used; ings, surf boards, traffic barricades, • commercial, industrial and display cases and ducting can also • hollow, totally enclosed items as agricultural storage tanks be produced by rotomolding. The well as pieces with openings can ranging in size from 5 gallons list above indicates just some of the be made; to 22,000 gallons; possibilities. Figures 1-4 illustrate • rotational molding eliminates the • containers for packaging and some of these applications. need for secondary tooling; material handling; • there is little or no waste due to • a variety of industrial parts, Design – almost resin scrap; especially covers and housings, endless potential • wall thickness and piece weight water softening tanks, tote bins; With rotomolding, a plastics can be easily controlled; • numerous under-the-hood and product designer can create a huge • rotational molding procedures in-the-cab automotive parts; array of innovative products. Typical assure uniform wall thickness; • hobby horses, pool tables and design concerns that are handled • deviations can be controlled to many other small or large toys routinely by rotational molding within a maximum tolerance of with complex shapes; include the following: ±10 percent; • baby cribs, balls, dolls, and doll • maintaining uniform wall thick- • pieces with intricate contours parts; ness – rotational molding can and undercuts can be easily provide a more consistently molded; uniform wall thickness for a • virtually any size piece can be part compared to other plastic rotationally molded; processing methods; • there is a minimum of cross- • producing double-wall construc- sectional deformation and tion – rotational molding can warpage; provide uniform double-wall construction on parts; • rotational molding yields pieces with excellent surface detail and • molding thicker corners – due finish; to the process, rotomolded parts will have thicker outer corners • rotationally molded items are which help strengthen the parts; virtually stress free; and • identical or similar items or • molding inserts, reinforcing ribs, different sections on one piece kiss-off ribbing and undercuts – can be molded at the same time these are easily included in a in different colors on a single roto-molded part. spindle; • plastic or metal inserts can often Designers interested in acquiring be molded as integral parts of more specific information can con- the item; and tact the Association of Rotational Molders, 2000 Spring Road, • double wall constructions are feasible. Figure 4. Large storage tanks are Oakbrook IL 60521, (630) 571-0611. a major market for rotomolding 3 The Race Car Bed is rotationally molded from tough, scratch-resist- ant, high impact strength linear low density polyethylene. Kids are all over this seven-foot- high Big Climber gym rotomolded from weather-resistant polyethyl- ene. The colors have been molded-in, so the parts will not chip, splinter, rust or peel. This rocking horse comes as either a palomino or a mustang. Both are rotationally molded from polyethylene. The spinal immobilizer board used by emergency medical services has a rotomolded, linear medium density polyethylene, outer skin and a rigid urethane foam core. 4 Resin choice Melt Index • Type I: Low Density Resins (range of 0.925 g/cm3 and below). To obtain the desired end product, For rotomolding, a resin must have Generally, low density resins are the choice of a quality powdered a good flow when molten. With preferable whenever stiffness is resin is essential in rotational polyethylene, the flow is measured not essential or is undesirable, as molding. One reason is the high by melt index. The higher the melt for many toys, and only when temperatures used risk chemical index, the better the flow. Most light loads are to be expected. rotomolding resins have melt degradation in a less-than-quality • Type II: Medium Density Resins indices ranging from 2g/10 minutes product. (range from 0.926 g/cm3 to to 10g/10 minutes. The term 0.940 g/cm3). Most linear low “g/10 minutes” refers to the weight Table 1. Usage of various resins density polyethylene resins fall for rotational molding of molten resin moving through an within this range. Medium orifice of a predetermined size in density resins are useful for self- RESIN PERCENT OF MARKET 10 minutes. supporting items that require the The melt index is also a rough higher heat-distortion resistance Polyethylene 84% measure of the molecular weight or or stiffness that low density the chain length of a resin. A resin resins do not provide. Polycarbonate with a high melt index has shorter • Type III: High Density Resins Nylon chains and a lower molecular (range from 0.941 g/cm3 to Polyvinyl Chloride 15% weight or smaller molecules. A resin 0.959 g/cm3). High density Polyesters with a low melt index has longer resins impart the highest rigidity Polypropylene chains and a higher molecular to the end product, which weight or larger molecules. frequently permits reduction ABS Molecular weight distribution is in wall thickness. Acetals also important in a rotomolding •