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- Rigdahl, M, et al.
(författare)
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The clustering mechanism underlying a kinetic model related to Bose-Einstein statistics
- 2004
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Ingår i: Mechanics of time-dependant materials. - 1385-2000 .- 1573-2738. ; 8:2, s. 95-103
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Tidskriftsartikel (refereegranskat)abstract
- The present paper discusses the clustering of elementary events in a kinetic relaxation model based on a time-domain analog of Bose-Einstein statistics. The clustering results in a spectrum of relaxation times which are integer-valued fractions of the relaxation time of single events, thus providing a stretching of the overall process towards shorter times. The predictions of the model are combined with an expression describing the 'universal' behaviour of solids of various structure and composition in a stress relaxation experiment. This yields the remarkable result that the average cluster size relating to the entire process is six. Correspondingly, the average relaxation time is found to be 1/6 of the relaxation time of the single events.
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- Pisciotti, F, et al.
(författare)
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Evaluation of a model describing the advancing flow front in injection moulding
- 2002
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Ingår i: International polymer processing. - 0930-777X .- 2195-8602. ; 17:2, s. 133-145
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Tidskriftsartikel (refereegranskat)abstract
- The simulation of the advancing flow front during mould filling in the injection moulding cycle is important as a way of anticipating manufacturing defects, particularly different types of welds, air traps, cold spots and hot spots. The purpose of the present work was to evaluate a model (the distance model) simulating the advancing flow front and predicting potential issues related to the progression of the flow front such as welds and air traps. The distance model is based on a mathematical theory of Hele-Shaw flow for strongly shear-thinning fluids, i.e. fluids with a power-law index, n, equal to 0,3 or preferably less. Two grades of general-purpose polystyrene were selected according to their abilities to shear thin (weakly or moderately strong) at the selected processing temperature and in the typical shear rate range of the injection moulding process. The two grades were injection moulded into twelve mould configurations derived from two similar single-gated moulds. The flow length from the gate to the weld was measured in the mouldings obtained at three distinct rates of filling (conventional, slower and faster) and compared to the flow lengths obtained by the distance model. Good agreement was found between the predicted flow lengths and the experimental flow lengths at the conventional and faster filling conditions. Larger deviations between simulation and experiment were found at the slower filling rate, particularly for the weakly shear-thinning polystyrene grade. Some comparisons were also made with the predictions obtained using the commercial simulation code Moldflow. A comparison between the advancing front predictions of the distance model and experimental short shots of a commercial polypropylene grade in a lure-box type of mould geometry showed that the distance model, despite its simplicity, could probably be used to detect welds and air traps in more complex and practice-related mouldings.
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