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The most important factors affecting distortion are the rate and uniformity of cooling. Rapid cooling can cause severe distortion in most mouldings because the temperature gradient between the inner and outer layers of the moulded wall section leads to non-uniform cooling rates. Slow cooling rates result in more uniform cooling of the moulding and smaller temperature differences across the moulded section. However slow cooling is not always desirable because it results in higher densities and a proneness to brittle mouldings. The choice of the grade of powder with the best balance of MFI and density is therefore important, so that the rate of cooling necessary to avoid distortion can be achieved without making the moulding brittle. Adequate venting, a pressurised mould during the cooling process, reduction in the amount of mould release agent used, and the introduction of stiffening ribs to the moulding will also help to minimise distortion. Chemical Resistance Although polyethylene resins are used in a large range of end applications, careful consideration needs to be made of the choice of polymer that will meet the demands of the finished product and the environment(s) that it will be exposed to (e.g. oils, fats, alkalis, acids and temperature, etc.). To make the best resin selection, customers are advised to discuss their specific end product requirements with their Qenos Technical Service Representative. HANDLING PRECAUTIONS Although Alkatuff LLDPE powders are inert and present no toxic hazard, care should be taken when handling the powders to minimise atmospheric dust levels and avoid any risk of a dust explosion. This dust hazard is less than with materials such as flour and starch, but precautions should be taken. The prime requirement is that no spilt powder should be allowed to accumulate, such that when agitated it can create a dust cloud. Heat sources of more than 350°C can cause auto-ignition of polyethylene. Attention should be given also to the siting and selection of electrical equipment used in the powder handling areas. Working areas should be well ventilated. In large scale operations, additional precautions are required, since hazards may be introduced by such items as bulk containers, conveying systems and intermediate hoppers. Special attention should be paid to the possibility of electrostatic charge accumulation which could cause spark formation. This could happen with unearthed metal parts or when non-conducting parts are used in the wrong situation. ROTATIONAL MOULDING 6 Qenos Technical Guides 17PDF Image | ROTATIONAL MOULDING Guide
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