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PPT02-notes_probs

# PPT02-notes_probs - ME4 Polymer Processing Technology 2...

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2 Melt rheology and rheometry Two main groups of material property data are needed to analyse and simulate flow in the injection moulding process: 1 Properties of resistance to flow under stress ( rheology ): expressed by shear stress vs . shear strain rate data; 2 Properties of static compressibility and thermal expansion: expressed using PVT data. These notes deal with the first group: rheological properties and their measurement ( rheometry ). Rheometric methods have to be simple and analysable, so they provide a clear introduction to the analysis of melt flow in injection moulding and a warning about possible problems. Learning outcomes After completing this section you should be able to… 1 Demonstrate an understanding of the theoretical basis and practical application of a Capillary Rheometer and a Cone and Plate rheometer. 2 Manipulate power-law, Cross, Cross-Arrhenius and Cross-WLF constitutive equations for polymer melts, and use them to represent data. 3 Assess whether Bagley or Rabinowitsch corrections are needed in calculations for polymer melt flow in channels (e.g. rheometer capillaries, mould runners and gates), and apply them if necessary. 4 Demonstrate understanding and make appropriate use of the following terminology: apparent viscosity, Bagley correction, consistency, Cross model, degradation, die, extensional flow, extensional viscosity, flow curve, melt, melt slip, MFI, newtonian viscosity, power-law flow, pseudoplasticity, Rabinowitsch correction, reference viscosity, reference shear rate, jetting, rheometer (capillary, cone-plate), shear rate (= shear strain rate), temperature coefficient of viscosity, viscometer = rheometer, zero-shear viscosity. 2.1 Revision SPAP introduced: 1 The MFI test : a widely-used industrial test which provides a single-point ‘index’ of melt viscosity. ME4 Polymer Processing Technology PSL 19 October 2009

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Barrel of diameter 9.57 mm Capillary die length = 8.0 mm diameter = 2.09 mm Melt flow index (MFI) = weight (in grams) extruded in 10 min Barrel heaters Melt at specified temperature Total mass M 2 The cone-and-plate viscometer : an easily understood but very limited method for measuring melt viscosity at a controlled range of shear rates. 3 The capillary rheometer : a rheometric method which is much more powerful, but more difficult to analyse. 4 Flow curves , representing viscosity data and showing that melts are very viscous, non-Newtonian (pseudoplastic) and temperature sensitive. 2.2 Melt ﬂow processes During injection moulding, a polymer melt meets a very wide range of processing variables (shear strain rate, temperature, pressure etc.). Finding methods to measure and represent melt rheology across this range is a serious challenge. There are two basic classes of high-viscosity flow. Most real flow processes are mixed, but one form usually dominates.
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PPT02-notes_probs - ME4 Polymer Processing Technology 2...

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