Velocity Measurements on a Polypropylene Melt During Extrusion Through a Flat Coat-Hanger Die Hans Ju ¨ rgen Grieß, Teodor I. Burghelea, Helmut Mu ¨ nstedt Institute of Polymer Materials, Friedrich-Alexander-University Erlangen-Nu ¨ rnberg, D-91058 Erlangen, Germany An experimental investigation of various flow regimes observed during the extrusion of a polypropylene melt through a flat coat-hanger die by laser-Doppler veloc- imetry (LDV) is presented. LDV measurements of the velocity profiles across the gap of the die at various locations along the die reveal three different extrusion regimes. At small wall shear stresses, the velocity pro- files can be fitted by symmetrical curves with the velocities becoming zero at the die walls. These pro- files are not uniformly distributed along the die. An increase of the wall shear stress reveals a second flow regime characterized by a uniform distribution of the velocity profiles along the die. As the wall shear stress is increased even further, a third flow regime charac- terized by wall slip on the glass windows is observed. This flow regime is systematically characterized by measurements of the slip velocities at various temper- atures and throughputs. The maximum velocities along the die are taken to assess the uniformity of flow which decisively influences the thickness of the extruded film. By measuring velocity profiles, at differ- ent throughput, and temperatures, the conditions for constant velocities along the die were determined. PO- LYM. ENG. SCI., 00:000–000, 2011. ª 2011 Society of Plastics Engineers INTRODUCTION Prefabricated parts of polymer materials are wide- spread today. Especially, the production of thin films is of great interest for many polymer manufacturers. Polymer films have found many applications which include pack- aging, electrical insulation, decoration, etc. There exist two methods to produce thin films at an industrial scale: film blowing and film casting. The film blowing process is the most economical one. The film cast- ing process usually provides films of better thickness uni- formity which is crucial for many practical applications. The capability to continuously produce films which have reproducible properties in terms of thickness, width, and molecular orientation is of enormous practical interest for applications. From a fundamental point of view, under- standing the optimal conditions of a successful film casting process is a challenging task as it involves a nontrivial cou- pling between hydrodynamics, rheology, and heat transfer. Due to the industrial relevance of film casting, there are several studies available in the literature. A large number of numerical simulations can be found in the lit- erature. They are based either on a one-, two- or three- dimensional model. The one-dimensional model (1D) [1] lacks the ability to predict the film thickness along the film width, i.e., the edge bead effect. D’Halewyn et al. [2] applied a two-dimensional model to simulate the neck-in and the edge bead effect of the polymer film. Sakaki et al. [3] presented a three-dimensional simulation of film casting. They used a constant viscosity model and analyzed the velocity and stress distributions over various cross-sections. Most of the models assume the cast film process to be isothermal. The number of experimental studies on film casting is much smaller, however. Seay and Baird [4] investigated the influence of sparse long-chain branches on the film casting behavior of polyethylene. Acierno and Di Maio [5] conducted measurements on a polyethylene terephtha- late (PET). For several experimental conditions, they measured the temperature distribution within the extruded film and its relation to the geometrical properties of the film. For a commercial isotactic polypropylene, the influ- ence of cooling of the film in air on the velocity distribu- tion within the film was investigated experimentally by Lamberti and Titomanlio [6]. Seyfzadeh et al. [7] present velocity fields measured for a polyethylene terephthalate (PET) in machine direction as well as in transverse direc- tion with laser-Doppler velocimetry (LDV). They were not able to draw conclusions from these observations, however, due to the limited resolution of the experiments. The present contribution is concerned with an experi- mental investigation of flow profiles in the die during the cast-film process. Teodor I. Burghelea is currently at Universite ´ de Nantes, Nantes Atlan- tique Universite ´s, CNRS, Laboratoire de Thermocine ´tique de Nantes, UMR 6607, La Chantrerie, Rue Christian Pauc, B.P. 50609, F-44306 Nantes Cedex 3, France Contract grant sponsor: German Research Foundation; contract grant numbers: MU 1336/10-4, MU 1336/6-4. Correspondence to: Helmut Mu ¨nstedt; e-mail: helmut.muenstedt@ ww.uni-erlangen.de DOI 10.1002/pen.22124 Published online in Wiley Online Library (wileyonlinelibrary.com). V V C 2011 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2011