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6 ROTATIONAL MOULDING Mould Venting Most rotational moulds require a venting system to maintain atmospheric pressure inside the mould during the process. This is to prevent pressure buildup in the mould during the heating cycle and formation of a partial vacuum during the cooling cycle. The size of the vent will depend on the mould size. Since vents leave a hole in the moulded article, correct placement is essential, eg. in an area which may be cut from the product or where it does not impair appearance or performance. Vents should also be located or shaped to prevent cooling water from entering the moulding; this may cause a water-track mark on the inside of the hot article which in some products may be undesirable. Modified Moulds Ideally a moulding of uniform wall thickness is required. However, if it is desirable to have areas of thinner section, then this can be achieved by either increasing the mould wall thickness for these areas or partially shielding the outside of the mould with an insulating material such as “Teflon” to reduce the heat transfer at these points. For parts designed with large flat surfaces, stiffening ribs should be used for additional structural support and internal air pressure maintained in the mould during cooling to prevent warping. These stiffening ribs should have generous radii for an acceptable moulding; deep or narrow ribs will lead to under filling or bridging, respectively. Inserts for attaching to other items can be moulded into the rotational moulding. These inserts should be made either of a material with a high conductivity such as aluminium, or of a high-melting point plastic such as nylon, and their surface should be knurled to give better anchorage of the polyethylene. Integrally moulded-in threads are also possible (see Figure 10). Figure 10: Large Rotomoulded Storage Container Showing Ribbing to Increase Stiffness and Reduce Warpage Mould Release Agents Since many complex rotational mouldings are not easily removed from the mould, it is usually necessary to use some form of release system. Release agents are often used to treat the mould surface. Mould release agents for polyethylene should be selected with care, to ensure that no serious environmental stress crack hazard is introduced. The most effective release agents are silicone based, and these can be applied to the clean inner surface of the mould by spraying, brushing or wiping with a fine cloth. It is essential that only air-curing and bake-on silicone resins are used; free silicone liquids (oils and emulsions) migrate into the moulding and cause severe stress cracking in a very short time (the order of minutes). The coating of release agent can be force-dried or cured by passing the mould through the heated oven for a short cycle. Effective mould release can also be achieved by coating the mould surface with a thin layer of “Fluon” or “Teflon” PTFE. A correctly-applied release coating should retain effective release properties for fifty to several hundred mouldings, although use of high temperatures or long cycles will reduce the number of effective releases. The amount of release agent used depends on the type of polyethylene being moulded; low density polyethylenes require more effective release properties than medium density polyethylenes. The coating should be evenly applied to prevent unacceptable warpage and distortion in the moulding. Insufficient release agent will cause sticking of the moulding to the surface of the mould; too much release agent may lead to warping. Secondary Finishing Operations Once the moulding process has been completed, secondary finishing operations may be required to obtain the marketable product. These operations are generally performed manually, are time-consuming and hence expensive. Such operations include trimming, drilling, cutting to provide openings in the wall section, welding of sections by hot bar or hot gas techniques, attaching fittings, or flame or chemical pre-treatment before painting or decorating. 12 Qenos Technical GuidesPDF Image | ROTATIONAL MOULDING Guide
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