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Polyethylene powders – the workhorse of the industry Table 3. Polyethylene resins: advantages and disadvantages RESIN LDPE LLDPE/LMDPE HDPE Crosslinkables EVA Copolymers ADVANTAGES Flexible Excellent warp resistance Consistent shrinkage Most meet FDA requirements 21CFR177.1520 Excellent impact strength Excellent ESCR Good warp resistance More stiff than LDPE Most meet FDA requirements 21CFR177.1520 Excellent stiffness Good impact strength Higher heat deflection temperature Most meet FDA requirements 21CFR177.1520 Excellent impact strength Excellent ESCR Flexible Excellent cold temperature impact strength Consistent shrinkage Most meet FDA requirements DISADVANTAGES No stiffness No ESCR Less stiff than HDPE Lower heat deflection temperature Low ESCR Warpage and shrinkage not consistent Longer molding cycle Does not meet FDA requirements No stiffness No ESCR Hard to mold 21CFR177.1520 6 The polyethylene pellets that are normally produced in the resin manufacturing process cannot be used for rotational molding; they must be reduced to a much smaller particle size. This reduction is neces- sary to obtain good heat transfer from the mold to the powder. The reduction also improves the flow of the particles during melting so that oxidation does not inhibit the moldability and development of the physical properties of the resin. The size reduction is usually done by the resin supplier, but can be done by the rotational molder who has grinding equipment. In addition to mechanically ground powder, some resins are available as reactor powder or granules. Several linear low density polyethyl- enes 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 with proper stabilization additives; • they can be molded in high- temperature, high-speed rotational molding equipment without excessive oxidation; • they have excellent low tempera- ture physical properties, such as impact strength, allowing their use in a broad temperature range; • they are relatively low in cost, making them a material to consider in all cost-effective applications; • they are available in a wide range of densities and melt indices to fit the needs of simple, nonstressed items as well as extremely large, highly stressed applications; • they can have their UV stability or outdoor life significantly improved by the addition of pigment or UV stabilizer; • they may meet FDA food contact requirements; contact your Equistar sales or technical service representative for more information; • they (especially crosslinkable polyethylene) have excellent chemical resistance making them ideal for numerous large agricultural and industrial chemical tanks; • they have high dielectric strength for use in electrical applications requiring insulation properties, such as the cherry-picker baskets used by power companies. A new technology that produces “micropellets” can also eliminate grinding. This technology provides small-diameter pellets that, in some applications, mold similarly to powder. Types of Polyethylene (See Table 3 for an overview of polyethylene types) • Low Density Polyethylene (LDPE) is flexible and tough, easy to process and has excellent chemi- cal resistance. • Linear Low Density Polyethylene (LLDPE) or Linear Medium Density Polyethylene (LMDPE) has better mechanical properties than LDPE as well as higher stiffness, excel- lent low temperature impact strength and excellent environ- mental stress crack resistance. • High Density Polyethylene (HDPE) is the stiffest resin of the poly- ethylene family. HDPE has excel- lent chemical resistance and good processability.PDF Image | A Guide to Rotational Molding Equistar
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