Heat transfer polymer melt processing

Denn, Polymer Melt Processing Foundations in Fluid Mechanics and Heat Transfer Duncan and Reimer, Chemical Engineering Design and Analysis An Introduction Fan and Zhu, Principles of Gas-Solid Flows Fox, Computational Models for Turbulent Reacting Flows... 

Denn, M.M. 2008. Polymer melt processing Foundations in fluid mechanics and heat transfer. Cambridge Cambridge University Press. 

Park SJ, Kwon TH (1998) Thermal and design Sensitivity analysis cooling system of injection mold. Part 1 thermal analysis. J Manufact Sci Eng 120 287-295 Patankar SV (1980) Numerical heat transfer and fluid flow. McGraw-Hill, New York Pearson JRA (1966) Mechanical Principles of Polymer Melt Processing. Pergamon Press, Oxford Pearson JRA, Petrie CIS (1968) On the melt flow instability of extruded polymers. In Wetton RE, Whorlow RW (eds) Polymer systems deformation and Flow. Macmillan, London, pp 163-187... 

Middleman. S. (1977). Fundamentals oj Polymer Processing. McGraw-Hill. New York. A thorough and quantitative account of the subject, covering heat transfer and melt flow in detail. (Relevant to Chapter 7.)... 

Most polymer processes involve heat transfer. Polymers must usually be heated above their melting points before shaping and then cooled to maintain the desired shape. It is during the cooling phase of the process that the physical properties of the polymer can drastically be altered. Because the thermodynamic and thermal properties of most polymers are rather similar to other materials, it is not necessary to develop any new laws as it was for the flow of polymers. Hence, this chapter serves mostly as a review of heat transfer with emphasis on those topics pertinent to polymer processing. The main aspects that require additional discussion and that set polymers apart from other materials are their crystallization behavior and the ability to control molecular orientation during processing. 

Dimensionless analysis is a powerfiil tool in analyzing the transient heat transfer and flow processes accompanying melt flow in an injection mold or cooling in blown film,to quote a couple of examples. However, because of the nature of non-Newtonian polymer melt flow the dimensionless mmibers used to describe flow and heat transfer processes of Newtonian flnids have to be modified for polymer melts. This paper describes how an easily applicable equation for the cooling of melt in a spiral flow in injection molds has been derived on the basis of modified dimensionless numbers and verified by experiments. Analyzing the air gap dynamics in extrusion coating is another application of dimensional analysis. 

Most polymer processing methods involve heating and cooling of the polymer melt. So far the effect of the surroundings on the melt has been assumed to be small and experience in the situations analysed has proved this to be a reasonable assumption. However, in most polymer flow studies it is preferable to consider the effect of heat transfer between the melt and its surroundings. It is not proposed to do a detailed analysis of heat transfer techniques here, since these are dealt with in many standard texts on this subject. Instead some simple methods which may be used for heat flow calculations involving plastics are demonstrated. 

One of the common problems associated with underwater pelletizers is the tendency of the die holes to freeze off. This results in nonuniform polymer melt flow, increased pressure drop, and irregular extrudate shape. A detailed engineering analysis of pelletizers is performed which accounts for the complex interaction between the fluid mechanics and heat transfer processes in a single die hole. The pelletizer model is solved numerically to obtain velocity, temperature, and pressure profiles. Effect of operating conditions, and polymer rheology on die performance is evaluated and discussed. 

The mechanisms described above tell us how heat travels in systems, but we are also interested in its rate of transfer. The most common way to describe the heat transfer rate is through the use of thermal conductivity coefficients, which define how quickly heat will travel per unit length (or area for convection processes). Every material has a characteristic thermal conductivity coefficient. Metals have high thermal conductivities, while polymers generally exhibit low thermal conductivities. One interesting application of thermal conductivity is the utilization of calcium carbonate in blown film processing. Calcium carbonate is added to a polyethylene resin to increase the heat transfer rate from the melt to the air surrounding the bubble. Without the calcium carbonate, the resin cools much more slowly and production rates are decreased. 

The most important boundary condition in heat transfer problems encountered in polymer processing is the constant surface temperature. This can be generalized to a prescribed surface temperature condition, that is, the surface temperature may be an arbitrary function of time T 0, t). Such a boundary condition can be obtained by direct contact with an external temperature-controlled surface, or with a fluid having a large heat transfer coefficient. The former occurs frequently in the heating or melting step in most... 

The Polymer-Engineering process is very similar to the Thermofuel system design, except that the main chamber contains a heavy thermal oil with a high boiling point. The waste plastic is continuously added as flake and it quickly melts in the thermal oil and pyrolyzes. The heavy oil is held at 390°C and the plastics quickly pyrolyze since it is an excellent heat transfer medium. 

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