Extruder Principles and Operation

This book is intended to fill a gap between the theoretical studies and the practical experience of the processor in the extrusion of thermoplastic polymers. The former have provided a basis for numerical design of extruders and their components, but generally give scant attention to the practical performance, especially to the conflict between production rate and product quality. In practice extruders are frequently purchased to perform a range of duties; even so, the operator may have to use a machine designed for another purpose and not necessarily suitable for the polymer, process or product in hand. The operator's experience enables him to make good product in unpromising circumstances, but a large number of variables and interactions often give apparently contradictory results. The hope is that this book will provide a logical background, based on both theory and experience, which will help the industrial processor to obtain the best performance from his equipment, to recognize its limitations, and to face new problems with confidence. Mathematics is used only to the extent that it clarifies effects which cannot easily be expressed in words; ifit is passed over, at least a qualitative understanding should remain. The approximate theory will not satisfy the purist, but this seems to the authors less important than a clear representation of the physical mechanisms on which so much of the polymer processing industry depends. M. J. STEVENS J. A.

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Thoroughly updated since the publication of the first edition, with new chapters on die design for operators, solids conveying and melting and twin-screw extruders, this book bridges the gap between theoretical studies and the practical experience of the processor in the extrusion of thermoplastic polymers and rubbers. Theoretical discussions generally give little attention to practical considerations, such as the conflict between productivity and product quality. Here however the authors give a clear representation of the physical mechanisms on which so much of the polymer processing industry depends, followed by guidance on operational strategies and the likely effects in terms of output, quality, uniformity and stability. Practical information is given on start-up trials, scale-up, product changing, dismantling and cleaning. Supported by over 40 tables and 176 figures, Extruder Principles and Operation will provide commercial processors with a logical background based on theory and experience, which will help them to obtain the best performance from their equipment, to recognize its limitations and to face new problems with confidence.

Contenu
1 Introduction.- 1.1 Scope and limitations.- 1.2 Method.- 2 Practical extrusion processes and their requirements.- 2.1 Shaping processes and their requirements.- 2.2 Other applications and their requirements.- 3 Flow behaviour relevant to extrusion.- 3.1 Viscosity.- 3.2 Shear flow.- 3.3 Extensional flow.- 3.4 Elastic effects.- 3.5 Measurement of viscosity and elasticity.- 4 Thermal and energy properties in processing.- 4.1 Thermal properties.- 4.2 Thermal conduction.- 4.3 Non-isothermal flow and heat transfer.- 4.4 Mixing.- 5 Extrusion dies.- 5.1 Introduction.- 5.2 Factors influencing the performance of an extrusion die.- 5.3 Extrusion dies for some profiles.- 5.4 General principles of die design.- 5.5 Specific aspects of die design.- 5.6 Operational strategies for problem-solving.- 6 Principles of melt flow in single-screw extruders.- 6.1 Functions of the extruder.- 6.2 Derivation of flow equation.- 6.3 Leakage flow.- 6.4 Output equations and longitudinal pressure profiles for common screw types.- 6.5 Graphical representation of output for screw/die combinations, including venting.- 6.6 Output corrections.- 6.7 Pseudoplastic flow.- 6.8 Non-isothermal flow.- 7 Solids conveying and melting in single-screw extruders.- 7.1 The relevance of solids conveying and melting.- 7.2 Phenomenological description of solids conveying and melting.- 7.3 Theoretical analysis.- 8 Principles of energy balance.- 8.1 Energy balance and efficiency.- 8.2 Power consumption in the screw: Newtonian isothermal case.- 8.3 Pseudoplastic isothermal approximation.- 8.4 Power in non-isothermal flow.- 8.5 Effect of variables on energy balance.- 9 Operation of single-screw extruders.- 9.1 Overall performance of the screw.- 9.2 Effects of controlled variables.- 9.3 Polymer properties.- 9.4 Screw design.-9.5 Operational strategies.- 10 Twin-screw extruders.- 10.1 Non-intermeshing screws.- 10.2 Partial intermeshing.- 10.3 Full intermeshing: counterrotation.- 10.4 Full intermeshing: corotation.- 10.5 Comparison of machine types.- 11 Extruder operation as part of a total process.- 11.1 Quality.- 11.2 Stability.- 11.3 Shear history.- 11.4 Control.- 11.5 Scale-up.- 12 Practical extruder operation.- 12.1 Steady operation.- 12.2 Colour and grade changing.- 12.3 Start-up and shut-down.- 12.4 Dismantling and cleaning.- 12.5 Waste recovery.- 13 Application to the individual machine.- Appendices.- A Properties of polymers for heat and flow.- B Derivations of flow and pressure.- B.l Alternative derivation of flow equation.- B.2 Estimation of leakage flows.- B.3 Longitudinal pressure profiles.- B.4 Pressure gradients in a stepped screw.- C Energy consumption and energy balance.- C.l Experimental determination of energy balance.- C.2 Derivation of power absorbed in screw.- C.3 Heat flows in melt pumping section.- C.4 Distribution of shear heating and transverse circulation.- C.5 Temperature variation in the flight clearance.- D Stability of melt pumping section.- E List of tables.- References.