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6 ROTATIONAL MOULDING If long oven residence times and/or high temperatures are unavoidable, then the air within the mould can be replaced with nitrogen, but this is a costly practice. The Optimum Heating Phase The optimum oven residence time can be determined by trial and error, by producing mouldings at a fixed oven temperature for various oven times. Visual inspection of the internal surface of the mouldings will give a good indication of the state of fusion (e.g. absence of powder residues, smoothness of surface, gloss level, lack of porosity, minimum odour and colour development). This can be backed up, if necessary, by performing mechanical tests on the walls of the mouldings (for example, impact tests), measuring the drop-off in MFI, or using Infra-red Spectroscopy or Thermal Analysis to follow the chemical degradation. Impact tests at low temperature are probably the quickest and most sensitive for checking properties. Figure 13 indicates typically the effect of oven residence time on the impact strength of a rotationally-moulded polyethylene. Figure 13: Effect of Oven Residence time on Impact Strength of Rotationally-moulded Polyethylene without Bubbles The maximum impact strength can be achieved at shorter oven times by increasing the oven temperature, however very high temperatures may be detrimental to properties. Alkatuff type LLDPEs are characterised by having a broad latitude in oven residence time and oven temperature conditions for achieving optimum fusion. In large or complex-shaped mouldings, there may be unequal fusion and hence thickness or property variations in different parts of the moulding. An optimum oven residence time may be difficult to determine in this situation, and the design of the moulding and the mould construction may require modification. In summary, the effects of fusion time on the properties of the moulding are listed in Table 3 (on next page). 14 Qenos Technical GuidesPDF Image | ROTATIONAL MOULDING Guide
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