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ROTATIONAL MOULDING 6 Table 3: Effects of Degree of Fusion on the Properties of Rotationally-Moulded Specimens. Degree of Fusion Observations Physical Properties Severe Under-fusion Some Under-fusion Optimum Fusion Slight Over-fusion Severe Over-fusion The Cooling Cycle On removal of the mould from the oven, the cooling cycle is commenced. Because of the thermal mass of the mould, heat will continue to be transferred to the polyethylene for a short time to complete the fusion process, even though cooling is being applied. The rate of cooling markedly influences the final properties of the moulding, as well as the degree of distortion and shrinkage that occur. This is particularly so with medium density, more crystalline Alkatuff LLDPE grades than with the lower density grades. Careful control of the cooling cycle must be carried out to obtain the best balance of physical properties and dimensional stability. Rotational mouldings which are air-cooled experience a very slow cooling rate. This reduction in the rate of cooling promotes crystalline growth of the spherulites in the solidifying polyethylene. Formation of larger spherulites in the polymer results in higher density, stiffer mouldings that have greater resistance to distortion and warpage. Conversely, generation of such higher density mouldings are impacted by a reduction in toughness (e.g. impact strength and ESCR). On the other hand, fast cooling, or quenching, limits the crystalline growth in the polyethylene. This produces mouldings with decreased density and improved impact strength and ESCR at the expense of stiffness. Low impact strength Low ESCR Poor tensile properties Physical properties almost at optimum Physical properties at optimum Reduced impact strength Low ESCR Drastic drop-off in impact strength Low ESCR Very rough inside surface High porosity Large number of bubbles Powder residues Smoother surface Some porosity at inner surface Smooth surface Dull finish No porosity Glossy inside surface Sharp odour Sharp acrid odour Very glossy inner surface Yellow discolouration of inner surface Severe warpage may also be experienced because of non-uniform cooling in different parts of the moulding. If the cooling rate is too fast, mould life may deteriorate because of the continued thermal shock to the moulds. Hence a compromise must be made in determining the cooling rate, to maximise the toughness and yet have sufficient stiffness and avoid warpage of the moulding. For some mouldings, it is advantageous to use a two-stage cooling cycle, by subjecting the mould to forced-air cooling followed by a water spray or atomized air-water spray – distortion can often be minimised by following this technique. To increase production rates, the moulding can be stripped from the mould while it is still warm and then cooled further outside the mould. However care must be taken that the moulding maintains its proper shape during this subsequent cooling. Faster cooling cycles can be obtained by using internal cooling, in which air is introduced into the moulding via the venting system; this technique is also advantageous in preventing the moulding prematurely pulling away from the mould wall, leading to excessive warpage and shrinkage. (Care should be taken, however, as to whether the mould can withstand such internal pressures). Qenos Technical Guides 15PDF Image | ROTATIONAL MOULDING Guide
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