Rotational Moulding Cycle Time Reduction through Exterior Mould Modification

Abstract

Rotational moulding has been heralded as a plastic moulding method with great potential. The process offers virtually stress-free products having no weld lines or material wastage, and utilises relatively inexpensive moulds. Yet, its widespread growth is hindered due to long production cycle times, which are limited by the time required to heat up and cool down a mould and the product within. To address this issue, efforts have been made by the author to enhance heat transfer to and from moulds, ultimately reducing cycle times. The aims of this project are four-fold: . To investigate the effect of convective heat transfer to and from a mould by changing the external mould surface (i.e. adding roughness elements or pins to the mould surface). • To predict cycle time reductions gained through the use of surface enhanced moulds. • To validate the predicted cycle times with experimental trials. . To recommend a combination of moulding conditions, applied together with surface enhanced moulds, for the rotomoulding industry to pursue. Due to the complex rotation of the mould, heating and cooling are most commonly achieved by convection to the external surfaces of the mould using air as the transfer medium. Two methods of heat transfer enhancement were considered, which are applicable to both external forced convection and mould exterior surface modification within the existing rotomoulding process. These methods are rough and extended surfaces. A series of preliminary analytical studies (predictions) and experiments were performed using roughness-enhanced and pin-enhanced plates to verify that these methods work. The results for the average effective heat transfer coefficient, h^, from the parametric studies for the extended surfaces were too high compared with the preliminary experimental data. While a turbulent flow was expected in the rotomoulding machine, the reality was a transitional flow (i.e. a mixture of laminar and turbulent flows). As a result, the correlations used in the parametric studies for the rough surfaces were invalid. Better prediction methods, which involved both analytical and numerical studies, were employed to improve the prediction of heJfec for both rough and extended surfaces. The h values were then applied directly into an existing rotomoulding simulation package, RotoSim, to predict cycle times. Based on the preliminary experimental results and predicted heat transfer coefficients, dimensions and array of roughness elements and pins were decided. Three cubical aluminium moulds (i.e. plain, roughness-enhanced, and pin-enhanced) were fabricated for the experimental purposes. To validate cycle time predictions, a series of experimental trials were carried out. Experimental and predicted results have shown significant cycle time reductions for both surface enhanced moulds. The average experimental cycle time reductions gained through the use of surface enhanced moulds were 18 and 28% for the roughness-enhanced and pin-enhanced moulds respectively. Even though the prediction methods fail to estimate the exact experimental cycle times, they are very useful for predicting relative cycle time reductions between the plain and surface enhanced moulds. The average predicted cycle time reductions were 21 and 32% for the roughness-enhanced and pin-enhanced moulds respectively. These results indicate that the predicted and experimental cycle time reductions are in excellent agreement with each other. This study has demonstrated that a significant reduction in rotomoulding cycle times can be achieved through the use of surface enhanced moulds. This thesis also provides a foundation for performance analysis of surface enhanced moulds. Recommendations have been made to maximise the benefits from such moulds, without depreciating the quality of the end products. Applying the proposed moulding conditions (i.e. a combination of higher oven flow rates, internal mould pressure, and water-cooling), the cycle time reductions were approximately 65 and 70% for roughness-enhanced and pin-enhanced moulds respectively (weighted against the plain mould with 'normal· moulding conditions). Such savings are very significant, inviting the rotomoulding community to incorporate these techniques efficiently in an industrial setting.