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Rotational Molding...An Operating Manual

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Rotational Molding...An Operating Manual Figure 1. Various rotomolded toys Rotomolding Is Different In rotational molding, rigid, resilient hollow bodies are formed by The main differences between powdered plastic material in heated rotational molding and other plastics molds which are rotated simultane- molding techniques, such as blow ously in two planes perpendicular to and injection molding, are as each other. follows: The plastic particles make resin powder is used instead of contact and melt on the inner Figure 2. Smaller storage bins and pellets; surfaces of the hot molds and fuse in layers until all the powder is fused refuse containers are also rotomolded. the resin melts in the molds and the desired end product thick- instead of being forced under ness is obtained. The wall thickness pressure into the molds in a molten is controlled by the amount of the Wall thickness uniformity can be state; powder placed in the mold. maintained to within +lo percent, the mold has a biaxial rotation; Rotationally molded pieces are which is better than what is normally rotomolding molds are less expen- stress-free except for slight shrink- possible with blow molding. Pieces sive because of their simplicity; age forces because the pieces are can be molded to wall thicknesses in the range of 1/32 to 1 inch (0.8 to 25 and operating pressures are produced without any external pressure. Additionally, there is mm). Most resins used in rotational relatively low, allowing molds to molding are powders ground to 35 be made from less expensive practically no scrap in rotational molding. mesh and ranging from 74 to 2000 materials. microns in diameter. 13 Typical Rotational Molding maintaining uniform wall thickness Table 1. Usage of Various Resins for Applications - rotational molding can provide a Rotational Molding ~ ~ ~ more consistently uniform wall Rotational molding permits pro- thickness for a part compared to duction of a countless number of other methods; fully or partially closed items. producing double wall construc- Design versatility of rotationally tion - rotational molding can molded pieces is almost unlimited. provide uniform double wall con- The rigidity or flexibility of an item struction on parts; is controlled by the properties of the molding thicker corners - due to resin used (see section on Resin the process, rotomolded parts will Choice) and by the wall thickness of have thicker outer corners which the molding. help strengthen the parts; and Some typical applications for molding inserts, reinforcing ribs, which rotational molding is particu- kiss-off ribbing, and undercuts larly suited include the following: -these are easily included in a * commercial, industrial and agricul- rotomolded part. tural storage tanks ranging in size Designers interested in acquiring from 5 gallons to 22,000 gallons; more specific information can containers for packaging and contact the Association of Rotational material handling; Molders, 435 N. Michigan Ave., a variety of industrial parts, espe- Chicago, Illinois, 6061 1 ; cially covers and housings, water (312) 644-0828. softening tanks, tote bins; numerous under-the-hood and Advantages in-the-cab automotive parts; of Rotational Molding rotationally molded items are hobby horses, pool tables and Rotational molding offers signifi- virtually stress-free; many other small or large toys with cant advantages when compared complex shapes; 0 identical or similar items or differ- with other molding techniques or ent sections of one piece can be baby cribs, balls, dolls and doll themoforming : molded at the same time in differ- parts; costs for molds and tooling are ent colors on a single spindle; display window mannequins and relatively low; plastic or metal inserts can often other hollow display figures of all the rotomolding technique is easily be molded as integral parts of the sizes; and adapted to short production runs, item; and sporting equipment such as golf particularly when sets of multiple- double-wall constructions are carts, footballs, juggling pins, cavity molds are used; feasible. helmets, dumbbells and golf tee hollow, totally enclosed items as markers. well as pieces with openings can Resin Choice Rotomolded parts are also used be made; in portable outhouses, battery To obtain the desired end prod- rotational molding eliminates the uct, the choice of a quality powdered cases, light globes, vacuum cleaner need for secondary tooling; and scrubber housings and garbage resin is essential in rotational containers. Furniture, game hous- there is little or no waste due to molding. One reason is that the ings, surf boards, traffic barricades, resin scrap; high temperatures used result in the display cases and ducting can also wall thickness and piece weight risk of chemical degradation in a be produced by rotomolding. The can be easily controlled; less than quality product. list above indicates just some of the rotational molding procedures Today, approximately 85% of all possibilities. Figs. 1-4 illustrate assure uniform wall thickness resin used in rotational molding is some of these applications. polyethylene (Table 1). Quantum deviations can be controlled to offers a series of Microthene poly- Design - within a maximum tolerance of ethylene powders with a wide range Almost Endless Potential *io%; of properties, including melt index With rotomolding, a plastics pieces with intricate contours and and density (Table 2). While the product designer can create a huge undercuts can be easily molded; effects of particle size on end array of innovative products. virtually any size piece can be product properties and processabil- Typical design concerns that are rotationally molded; ity are less critical, those of melt index and density are considerable. handled routinely by rotational there’s a minimum of cross sec- The main effects are shown in molding include the following: tional deformation and warpage; Table 3. rotational molding yields pieces with excellent surface detail and 4 Figure 4. Rotomolded products in this photo range from 55-gallon drums to finish; billard balls. 15 Table 2. MicrothenecRiPolyolefins for Rotational Molding Table 3. How Increases in Melt Index and Density of Polyethylene Powders Affect In addition to mechanically Processing and End Product Properties ground powder, some resins are available as reactor powder or granules. Several linear low density polyethylenes come in powder or granular form. Some other resins such as nylon, due to its high melt flow and small pellet size, can be molded without grinding. Polyethylenes have the following characteristics that have made them the most widely used powders for rotational molding: they are easily ground to 35 mesh at high rates; they can be made thermally stable Melt Index Type II: Medium Density Resins with the proper stabilization addi- For rotomolding, a resin must (range from 0.926 - 0.940 g/cm3). tives; have good flow when molten. With Most linear low density polyethyl- they can be molded in high- polyethylene, the flow is measured ene resins fall within this range. temperature, high-speed rotational by melt index. The higher the melt Medium density resins are useful molding equipment without exces- index, the better the flow. Most for self-supporting items which sive oxidation; rotomolding resins have melt indices require the higher heat-distortion they have excellent low tempera- in the range of 3 to 20 g/lO minutes. resistance or stiffness which low ture physical properties, such as The term, “g/lO minutes,” refers to density resins do not provide. impact strength, allowing their use the weight of molten resin moving Type Ill: High Density Resins in a broad temperature range; through an orifice of a predeter- (range from 0.941 - 0.959 g/cm3). they are relatively low in cost, mined size in 10 minutes. High density resins impart the making them a material to consider The melt index is also a rough highest rigidity to the end product, in all cost-effective applications; measure of the molecular weight or which frequently permits reduction they are available in a wide range the chain length of a resin. A resin of the wall thickness. with a high melt index has shorter of densities and melt indices to fit Type IV: Very High Density chains and a lower molecular weight the needs of simple, nonstressed Resins (0.960 g/cm3 and above). or smaller molecules. A resin with a items as well as extremely large, These resins are not currently low melt index has longer chains highly stressed applications; used in rotomolding. and a higher molecular weight or they can have their UV stability or larger molecules. In addition to lowering toughness outdoor life significantly improved Molecular weight distribution is and increasing stiffness, increasing by the addition of pigment or UV also important in a rotomolding density raises the melting point, stabilizer; permits higher temperature limits resin. A narrow distribution is more they may meet FDA food contact and improves barrier properties in advantageous, since the narrower requirements; contact your the end product. the distribution, the more uniform Quantum polyolefins sales repre- the melt properties. Polyethylene Powders - sentative for more information; Density The Workhorse of the due to their excellent chemical Density is a measure of the specific resistance, they (especially Industry crosslinkable polyethylenes) have gravity of a resin. The density of been used for numerous large polyethylene is classified by types The polyethylene pellets that are agricultural and industrial chemical according to ASTM, the American normally produced in the resin tanks; Society of Testing and Materials: manufacturing process cannot be Type I: Low Density Resins used for rotational molding; they polyethylene resins have a high dielectric strength making parts (range of 0.925 g/cm3 and below). must be reduced to a much smaller Generally, low density resins are particle size. This reduction is acceptable for electrical applica- preferable whenever stiffness is necessary to obtain good heat trans- tions requiring insulation proper- not essential or is undesirable, as fer from the mold to the powder.
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