tailieunhanh - Plastics Engineering 3E Episode 12

Tham khảo tài liệu 'plastics engineering 3e episode 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Analysis of polymer melt flow 369 So PQ pQ X Cp X AT AT pC p where p is the density of the fluid and Cp is its specific heat. Experimental Methods Used to Obtain Flow Data In Section design examples relating to polymer processing will be illustrated. In these examples the flow data supplied by material manufacturers will be referred to so it is proposed in this section to show how this data may be obtained. The equipment used to obtain flow data on polymer melts may be divided into two main groups. a Rotational Viscometers - these include the cone and plate and the concentric cylinder. b Capillary Viscometer - the main example of this is the ram extruder. Cone and Plate Viscometer In this apparatus the plastic to be analysed is placed between a heated cone and a heated plate. The cone is truncated and is placed above the plate in such a way that the imaginary apex of the cone is in the plane of the plate. The angle between the side of the cone and the plate is small typically 5 . The cone is rotated relative to the plate and the torque. T necessary to do this is measured over a range of rotational rates Ỡ. Referring to Fig. Area of annulus 2nr dr Force 2nrr dr Torque 2irr2T dr R So Total Torque T ị 2nr2T dr 0 T l 7rR3r . X rd 0 Also shear sttain y - - - h ra a . 0 Strain rate y a 370 Analysis of polymer melt flow fixed Fig. Cone and Plate Viscometer And since viscosity ĨỊ -7 ỹ n - 5 I 2tĩRì e The disadvantage of this apparatus is that it is limited to strain rates in the region 10 to 1 s-1 whereas in plastics processing equipment the strain rates are in the order of 10 s3-1 to 104 s-1. Concentric Cylinder Viscometer In this apparatus the polymer melt is sheared between concentric cylinders. The torque required to rotate the inner cylinder over a range of speeds is recorded so that viscosity and strain rates may be calculated. Referring to Fig. Torque T 2nRL R T 2nR Lx du 2nRN Strain rate Y dy H T TH .

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