Flow of polyethylene melts within and into rectangular ducts investigated by laser-Doppler velocimetry
von Daniela HertelPolymer processing has grown in the past decades into a multi-billion dollar industry with
production facilities and development laboratories all over the world. The primary reason for this immense growth compared to other materials is the relative ease of manufacturing and processing. Numerous methods have been developed to process polymeric materials. Most of these techniques are carried out with polymers in the molten state. The most common processing methods are extrusion and injection moulding. For all these methods the flow through channels with varying cross sections is important. The deformation history which is imposed on the melt in the die is a decisive factor for the processing behaviour as well as for the properties of the manufactured items. Features deteriorating the product performances are secondary flow phenomena occurring at Reynolds numbers far below the critical value for turbulences. This secondary flow is characterized by vortices in the entrance region of a die. The material residence time in these vortices can become excessive and lead to a degradation of the polymer resulting in gels and“black specks” which are unwanted for processed items. The origin of the vortices has attracted much interest and was the subject of extensive research. However, up to now a comprehensive understanding of the entry flow phenomena has not been achieved, yet. As vortices are far away from being a trivial flow pattern, they are subject of numerical simulations, which have to be compared with quantitative measurements in order to assess their validity. The flow behaviour of a fluid is characterized by the local stresses and velocities. A visualization of the flow velocities of polymer melts, however, is not so easy, because high temperatures are necessary and high pressures arise during processing. Therefore, only a few experimental studies exist in literature. Additionally, in the majority of cases, they only give a qualitative insight into the flow processes within the die. Thus, they are not able to supply
data for a quantitative check of theoretical models. Sophisticated methods are needed to obtain precise, quantitative information about the flow behaviour of polymer melts. In the case of transparent melts optical methods are especially suited for this purpose, as they are non-intrusive, i. e. the flow is not influenced by the measurement. A proved method for such measurements is the laser-Doppler velocimetry (LDV), which is based on the optical Doppler effect. It enables the accurate determination of the velocity with a high spatial and temporal resolution. In the present work, this method will be applied to determine velocity measurements of highaccuracy so that numerical and theoretical groups may use this precise information for testing and improving their models.
From an experimental and theoretical point of view, a slit with an abrupt contraction is of
great interest. The flow of a viscoelastic fluid through such a planar abrupt contraction is acomplex one containing regions of strong shear near the walls and non-stationary extension along the centreline of the flow channel. The main objective of the present work is to experimentally determine velocity profiles of a low density polyethylene (LDPE) and a linear low density polyethylene (LLDPE) flowing through such a slit die. The velocity fields of both materials will be investigated not only at the symmetry plane, but in a three-dimensional way, too. The dependence of the flow behaviour on outer processing parameters like temperature and flow rate will be investigated. As tools for polymer processing often possess very complex geometries, the influence of the die geometry like the contraction ratio and the entrance angle will be studied, as well.
A careful characterization of the rheological properties in shear and elongation of both
polyethylenes will be carried out. These data will be used in order to find an explanation for the differences in the flow behaviour of the two polymers and to establish a correlation of the formation of secondary flow with rheological properties.
The quantitative measurements are undertaken to gain a deeper insight into the flow
behaviour of polymer melts through slit contractions. This kind of investigations contributes not only to a fundamental understanding of the flow behaviour of polymer melts, but can also help to improve the manufacturing processes in industry.
production facilities and development laboratories all over the world. The primary reason for this immense growth compared to other materials is the relative ease of manufacturing and processing. Numerous methods have been developed to process polymeric materials. Most of these techniques are carried out with polymers in the molten state. The most common processing methods are extrusion and injection moulding. For all these methods the flow through channels with varying cross sections is important. The deformation history which is imposed on the melt in the die is a decisive factor for the processing behaviour as well as for the properties of the manufactured items. Features deteriorating the product performances are secondary flow phenomena occurring at Reynolds numbers far below the critical value for turbulences. This secondary flow is characterized by vortices in the entrance region of a die. The material residence time in these vortices can become excessive and lead to a degradation of the polymer resulting in gels and“black specks” which are unwanted for processed items. The origin of the vortices has attracted much interest and was the subject of extensive research. However, up to now a comprehensive understanding of the entry flow phenomena has not been achieved, yet. As vortices are far away from being a trivial flow pattern, they are subject of numerical simulations, which have to be compared with quantitative measurements in order to assess their validity. The flow behaviour of a fluid is characterized by the local stresses and velocities. A visualization of the flow velocities of polymer melts, however, is not so easy, because high temperatures are necessary and high pressures arise during processing. Therefore, only a few experimental studies exist in literature. Additionally, in the majority of cases, they only give a qualitative insight into the flow processes within the die. Thus, they are not able to supply
data for a quantitative check of theoretical models. Sophisticated methods are needed to obtain precise, quantitative information about the flow behaviour of polymer melts. In the case of transparent melts optical methods are especially suited for this purpose, as they are non-intrusive, i. e. the flow is not influenced by the measurement. A proved method for such measurements is the laser-Doppler velocimetry (LDV), which is based on the optical Doppler effect. It enables the accurate determination of the velocity with a high spatial and temporal resolution. In the present work, this method will be applied to determine velocity measurements of highaccuracy so that numerical and theoretical groups may use this precise information for testing and improving their models.
From an experimental and theoretical point of view, a slit with an abrupt contraction is of
great interest. The flow of a viscoelastic fluid through such a planar abrupt contraction is acomplex one containing regions of strong shear near the walls and non-stationary extension along the centreline of the flow channel. The main objective of the present work is to experimentally determine velocity profiles of a low density polyethylene (LDPE) and a linear low density polyethylene (LLDPE) flowing through such a slit die. The velocity fields of both materials will be investigated not only at the symmetry plane, but in a three-dimensional way, too. The dependence of the flow behaviour on outer processing parameters like temperature and flow rate will be investigated. As tools for polymer processing often possess very complex geometries, the influence of the die geometry like the contraction ratio and the entrance angle will be studied, as well.
A careful characterization of the rheological properties in shear and elongation of both
polyethylenes will be carried out. These data will be used in order to find an explanation for the differences in the flow behaviour of the two polymers and to establish a correlation of the formation of secondary flow with rheological properties.
The quantitative measurements are undertaken to gain a deeper insight into the flow
behaviour of polymer melts through slit contractions. This kind of investigations contributes not only to a fundamental understanding of the flow behaviour of polymer melts, but can also help to improve the manufacturing processes in industry.