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Resin choice To obtain the desired end product, the choice of a quality powdered resin is essential in rotational molding. One reason is the high temperatures used risk chemical degradation in a less-than-quality product. Melt Index For rotomolding, a resin must have a good flow when molten. With polyethylene, the flow is measured by melt index. The higher the melt index, the better the flow. Most rotomolding resins have melt indices ranging from 2g/10 minutes to 10g/10 minutes. The term “g/10 minutes” refers to the weight of molten resin moving through an orifice of a predetermined size in 10 minutes. The melt index is also a rough measure of the molecular weight or the chain length of a resin. A resin with a high melt index has shorter chains and a lower molecular weight or smaller molecules. A resin with a low melt index has longer chains and a higher molecular weight or larger molecules. Molecular weight distribution is also important in a rotomolding resin. A narrow distribution is more advantageous, since the narrower the distribution, the more uniform the melt properties. Density Density is a measure of the specific gravity of a resin. The density of polyethylene is classified by types according to the American Society of Testing and Materials (ASTM): • Type I: Low Density Resins (range of 0.925 g/cm3 and below). Generally, low density resins are preferable whenever stiffness is not essential or is undesirable, as for many toys, and only when light loads are to be expected. • Type II: Medium Density Resins (range from 0.926 g/cm3 to 0.940 g/cm3). Most linear low density polyethylene resins fall within this range. Medium density resins are useful for self- supporting items that require the higher heat-distortion resistance or stiffness that low density resins do not provide. • Type III: High Density Resins (range from 0.941 g/cm3 to 0.959 g/cm3). High density resins impart the highest rigidity to the end product, which frequently permits reduction in wall thickness. • Type IV: Very High Density Resins (0.960 g/cm3 and above.) These resins are not currently used in rotomolding. In addition to lowering toughness and increasing stiffness, increasing density raises the melting point, permits higher temperature limits and improves barrier properties in the end product. Table 1. Usage of various resins for rotational molding RESIN PERCENT OF MARKET Polyethylene 84% Polycarbonate Nylon Polyvinyl Chloride 15% Polyesters Polypropylene ABS Acetals Acrylics Cellulosics Epoxy Fluorocarbons 1% Phenolics Polybutylene Polystyrene Polyurethane Silicone Table 2. How increases in melt index and density of polyethylene powders affect processing and end product properties AN INCREASE IN . . . . . . . . . . . . . . . . . . . . . MELT INDEX AFFECTS THESE PROPERTIES Melting Point Flow Impact Strength Stiffness Vicat Softening Temp. Resistance to Low Temp. Brittleness Barrier Properties . . . . DENSITY decreases increases decreases remains the same decreases decreases remain the same increases remains the same decreases increases increases decreases increase Today, approximately 84 percent of all resin used in rotational molding is polyethylene (Table 1). Equistar offers a series of Microthene poly- ethylene powders with a wide range of properties, including melt index and density. While the effects of particle size on end-product properties and processability are less critical, those of melt index and density are considerable. The main effects are shown in Table 2. 5PDF Image | A Guide to Rotational Molding Equistar
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