VIRTUAL AND EXPERIMENTAL COMPARISON OF DIFFERENT DYNAMIC MIXING DEVICES FOR SINGLE SCREW EXTRUSION

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1 VIRTUAL AND EXPERIMENTAL COMPARISON OF DIFFERENT DYNAMIC MIXING DEVICES FOR SINGLE SCREW EXTRUSION Bastian Neubrech, Gregor Karrenberg, Johannes Wortberg, University of Duisburg-Essen, Institute of Product Engineering, Engineering Design and Plastics Machinery, Germany Abstract Depending on the application, the demands on mixing devices in extrusion processes vary. The most common tasks can be summarized by providing a thermally and materially homogeneous melt for the downstream processes and mixing in additives or color master batches. Anyhow, the requirements increase due to increasing reachable screw speeds and the processed materials. Besides the named tasks the final plasticizing becomes relevant, as residence times inside the extruder decrease or a higher energy input is needed for the material. The major problem of implementing mixing devices is the increasing shear stress in combination with the rising melt temperature. Higher screw speeds intensify the problem of inadmissible high melt temperatures and material stressing even further. Nevertheless, the use of mixing devices is often indispensable, in order to meet the melt quality requirements. The consequence is the need of improved mixing devices as well as the development of new mixing device geometries. The aims of these improvements have to be an sufficient melt homogeneity and less material stress, leading to lower melt temperatures. For this development, the flow situation and the geometric influences of mixer geometries on the shear rate have to be analyzed. This paper deals with a numerical and an experimental comparison of two variants of an Improved Quality - Dynamic Mixing Ring device. The different designs are verified by CFD simulations and at the end validated by experimental data. Introduction The market of mixing devices offers a numerous variety of mixing elements, which can be divided into static and dynamic mixing elements. For a sufficient melt quality the combination of distributive and dispersive mixing, which can be realized by dynamic mixing devices, is required. Thereby, the dispersive mixing is highly dependent on the actual screw speed and increases with rising screw speeds. Unfortunately, the final plasticizing behaves contrary [1]. An approach to optimize the mixing efficiency is the so-called Cavity-Transfer Mixing Device [2]. A proven design has been developed by Semmekrot and was originally called Twente Mixing Ring (TMR) named after the University of Twente [3]. According to the operating principle it is also called Dynamic Mixing Ring (DMR). Such a device is depicted in Figure 1. The construction of this device consist of a rotor, which is applied to the screw of the extruder and has spherical cavities in circumferential and lengthwise direction. The screw thus actively drives the rotor. It is surrounded by a cylindrical sleeve with bores in the diameter of the cavities of the rotor, which are shifted lengthwise by half of the diameter. The sleeve is set in rotation via drag forces and is passively driven. The use of a combination of dispersive mixing elements like mandrel shear mixer and dynamic mixing devices can be found more frequently. Figure 1. Schematic representation of a Dynamic Mixing Ring The melt transport is induced by the flow of the extruder. A melt element passes from one cavity to the next via the connecting bore. The good mixing results are achieved by the multiple division and reorientation of the melt. Thereby, distributive and dispersive effects overlay as shearing in gaps and melt vortexes in the cavities [1]. Gorczyca examined the mixing efficiency and the material stress caused by dynamic mixing elements for the highspeed extrusion and confirmed the assumptions [4]. However, further potentials for improvement can be found in the consumption and the high melt temperature. Based on these results Karrenberg optimized the DMR leading to a device, which is specifically designed to fit the requirements of high-speed extrusion processes and has been simulated for respective operating points. His approach intends to minimize the metallic surface as the energy, which is dissipated in the clearance between SPE ANTEC Anaheim 2017 / 1194

2 rotor and sleeve is significantly responsible for the heating of the material [5, 6]. Further adjusting factors are the shear gap itself and the diameter of the rotor. The result is a change from spherical cavities to a honeycomb structure in order to obtain the smallest possible metallic surface. The computer-aided investigations have been performed for a nominal rotor diameter of 30 mm and a screw speed of 800 rpm. The speed of the passively driven sleeve is assumed with 125 rpm and the throughput is set to 115 kg/h. For these restrictions, the final geometry achieves a decrease in temperature rise by 44 % and a reduction of the consumption by 25 % [5]. In this paper, the design approach is transferred to a small production scale for the blown film application. Thereby, it is investigated if the concept is effective at moderate screw speeds for the use in conventional single screw extrusion processes. The computer-aided investigation is validated by experimental data. Dynamic Mixing Devices for improved melt quality The approach of [5] is maintained to introduce a Dynamic Mixing device with other than spherical cavities for the application in single screw extrusion. The general design is shown in Figure 2. The honeycomb structure allows to arrange as many cavities as possible. Figure 2. Dynamic Mixing device for improved melt quality The evaluation comprises two Improved Quality - Dynamic Mixing Ring devices. The difference is an adjustment of the metallic surface by resizing the surface of the cavities. By this means, the influence of the width of the frame structure can be examined. Therefore, the general geometric dimensions are kept identical (compare Table 1). Table 1. Investigated Dynamic Mixing Devices No. Rotor Cavity Version cavities surface structure / bores [mm²] Sleeve surface [mm²] IQ-DMR 1 honeycomb 55 / 10 8,657 14,227 IQ-DMR 2 honeycomb 55 / 10 5,432 10,247 The diameter of the rotor is 35.5 mm, the inner sleeve diameter is 36 mm and the outer sleeve diameter is mm. All dimensions are chosen to fit the experimental investigations with a barrel diameter of 48 mm and guarantee transferability. Compared to state of the art dynamic mixing solutions the total length of the mixing devices is enlarged to 240 mm, which equals 5D in the experimental application. The determining factor for the distinction of the designs is the frame surface of the rotor, respectively sleeve. This adjustment is justified by the influence of the shear rate on the dissipated energy, as shown in equation 1. The shear rate itself is highly dependent on the dimension of the shear gap. In this context, the shear rate has a quadratic influence on the result making its reduction purposeful. The consequence has to be a reduction of the frame surface forming the shear gap. For this reason, the second model has a reduced surface area of the rotor by 38 % and the sleeve by 27 %. That is obtained by reducing the frame width from 6 mm to 4 mm. (1) Restrictions and user-defined temperature inlet profile The process of plasticizing is not considered in the performed numerical flow simulations in ANSYS Fluent The system boundaries are defined by the extruder barrel from right in front of the mixing devices up to the front of the rotor. It is assumed that the polymer is completely plasticized before it enters the system boundaries. Before the simulations are set up, a user-defined function (UDF) for the temperature inlet profile is generated. Commonly, the inlet temperature is defined as the average mass temperature, which does not reflect the actual temperature distribution inside the screw channel. Regarding this, the UDF takes the temperature peak in the circumferential center of the flow profile into consideration. This is shown in figure 3, where the temperature is plotted against the radius. Temperature [K] Radius [m] Figure 3. User-defined temperature inlet profile The user-defined temperature inlet profile qualifies an assessment of the thermal mixing efficiency of the different devices. Besides the resulting shear heating an SPE ANTEC Anaheim 2017 / 1195

3 inhomogeneous temperature distribution of the incoming material is taken into account. The restrictions for the simulations are oriented on the experimental process to guarantee the comparability. Therefore, an initial reference simulation is performed to assure a steady-state operating point. Based on this, the inlet temperature profile for the first mixing device is iteratively adjusted to meet the measured outlet temperature of the experimental process. This procedure can be justified since the actual inlet temperature can not be measured in the process. The temperature inlet for the second device has to be identical to evaluate the mixing performance based on equal melt inlets. An overview of the restrictions is listed in table 2. Those are kept identical for all simulations. the superimposed rotor. The sleeve is blanked out for better overview. Nevertheless, it can be identified by the white spaces between the sleeves cavities. The mass-flow inlet is located on the right side showing the resulting temperature distribution from the user-defined temperature inlet profile. With the help of the vector display, the cavity transfer principle is visualized. According to the expectations, the temperature increases due to shear heating in machine direction. Especially the sleeve gap shows a higher temperature level than the subjacent cavities. Table 2. Restrictions for the CFD simulations Mass flow rate [kg/h] Screw / Sleeve speed [rpm] Outlet Inlet temp. [K] Outlet temp. [K] Barrel temp. [K] / UDF The inlet areas are defined as mass-flow inlets. As it can be differentiated by the inlet of the sleeve and the inlet of the rotor the total mass flow rate is divided by the percentage in accordance to the respective inlet surface area. Thereby, the inlet area of the rotor equals 75 % of the inlet area of the sleeve. This distribution is considered in the definition of the mass flow rate. The outlet area is defined as a outlet with a value of 350 bar. The rotational speed of the sleeve is also determined in an iterative process. The initial value has been set at 10 % of the screw speed in accordance to Semmekrots and Karrenbergs observations. It is stated that the speed of the passively driven sleeve equals 5 20 % of the screw speed [3, 6]. The experimental data shows, that a sleeve speed of 15 rpm ( 15 %) complies with the reality. The material used for the simulations is a PE-HD (Lupolen 4261 AG, LyondellBasell) as the experimental data is also generated with a material consisting of 95 % PE-HD. All simulations are performed stationary as Karrenberg already proved that the rotor position in relation to the sleeve is negligible for the simulation result [4]. Evaluation of the CFD simulations Within the virtual comparison, the increase of temperature and the demand along the mixing devices are the quality determining factors for the evaluation. Before the two designs are compared, the general flow situation is evaluated by corresponding criteria. Figure 4 shows the temperature development for the IQ-DMR1 with the wider framework design. The fluid volume is depicted as sectional view in the XZ-plane with Figure 4. Temperature development of IQ-DMR1 Figure 5 shows the direct comparison of the temperature distribution at the inlet and the outlet from IQ-DMR1. The mean melt temperature is visualized in the upper right corner. With these values the mean temperature rise is calculated to 37 K. To describe the thermal melt homogeneity the standard deviation of the temperature at the outlet area is calculated. For the IQDMR1 it equals 1.74 K. Figure 5. Temperature plot of the inlet (left) and the outlet area (right) of IQ-DMR1 Subsequently, the simulations of IQ-DMR2 are analyzed. Figure 6 shows the temperature development of the melt passing the mixing device. The tighter frame structure of the rotor to reduce the metallic surfaces is directly obvious. As claimed, it verifiably results in reduced shear heating as the mean outlet temperature decreases to 496 K. Comparing the mean inlet and outlet temperature the global temperature rise can be calculated to 30 K. That corresponds to a reduced temperature rise by 18 %. In addition, the local temperature in the sleeve gap and the cavities decreases as well. The standard deviation of the temperature at the outlet area is computed to 1.25 K. As this value represents the thermal SPE ANTEC Anaheim 2017 / 1196

4 homogeneity, a general improvement of the melt quality can be assumed. The dispersion about the mean is reduced, while the absolute melt temperature is lowered. in the upper left zones of the bores increases due to stagnation. The evaluation reveals a mean age of 8.57 s for the material in IQ-DMR1. In contrast, the mean age in IQ-DMR2 is calculated to s resulting in a delta of 1.81 s respectively 17.4 %. That means, that the residence time in IQ-DMR2 is longer, even if the consumption and the temperature rise is minor. Figure 6. Temperature development of IQ-DMR2 In a second step the demand of the mixing devices is analyzed. The increased length results in a higher demand the extruder has to provide. Nevertheless, it is claimed to keep the necessary as low as possible. The at the outlet is defined by the restrictions at the system boundary. On this basis the inlet is calculated recursively. The consumption of the mixing device is the result of the difference between the outlet and inlet. Table 3 shows the demand for both variants. The effect of the extended cavities becomes apparent. The computed inlet decreases for IQ-DMR2 as the flow resistance is reduced due to larger passed surfaces. As a result, the demand decreases from 102 bar to 64 bar. Normalized on the demand of IQ-DMR1 a reduction by 37 % is achieved. Figure 7. Graphical representation of the mean age Based on this, the streamlines of the fluid have to be depicted to explain these results. A streamline represents the path of a particle while passing the fluid domain. To establish a connection between the streamlines and the residence time each streamline is plotted as a function of its velocity. This is shown in Figure 8 for IQ-DMR1 (top) and IQ-DMR2 (bottom). In this figure, only 100 streamlines beginning at the inlet are depicted to maintain clearness. Applying the findings of the mean age on the streamline plot the difference in residence time becomes obvious. Table 3. Pressure demand Version avg. inlet avg. outlet Pressure demand Normalized [%] IQ-DMR1 IQ-DMR Finally, the mean residence time of the fluid is analyzed. To characterize this virtually, the mean age at the outlet area is used. It describes the mean time that a fluid element needs to pass the fluid domain. Therefore, a user-defined scalar is implemented in ANSYS Fluent that is connected to the transport equation of the solver. Within the simulations, the diffusion coefficient will be neglected, meaning the flux appears to be strictly convective. This simplification might lead to unusual high mean age in wall areas, but is accepted as the flow situation is physically distorted in these areas anyhow. ANSYS post processor offers possibilities to read the scalar values to illustrate the age of the melt at any time along the fluid domain. This is depicted for IQ-DMR1 in Figure 7. It is expected, that the mean age in stagnation areas rises slightly. That becomes visual in the detail view of the bores. The mean age of the material, which resides Figure 8. Comparison of streamlines along the mixing devices (top: IQ-DMR1; bottom: IQ-DMR2) The flow pattern in the upper mixing device proceeds organized and can qualitatively be described to be helical. The local velocity of the melt increases while exiting one cavity. A local jet effect takes place. Nevertheless, a wider distribution of the streamlines cannot be observed. The geometric adjustments in IQ-DMR2 lead to a, at first sight, different flow pattern. The streamlines appear to be more diffuse and not as organized as in the first geometry. This is reasoned by the increased fluid volume in the cavities and the bores and consequently the decreased velocity. As the interface area, at which the melt overflows the frame structure of the cavities, increases for the reduced frame width the observed jet effect declines. The local velocity is lowered and extends the residence time additionally. Nevertheless, the helical flow pattern SPE ANTEC Anaheim 2017 / 1197

5 can be identified for the second mixing device as well. Concerning the mixing performance, the enlarged fluid volume for IQ-DMR2 and related to that the unorganized flow pattern will lead to a positive effect on the distributive mixing phenomena. Experimental investigation For the virtual and the experimental comparison, the contemplated process is described beforehand. The experimental data is gathered at a laboratory blown film line with a small-scale production size. The blown film process is most appropriate for the declaration of the homogeneity because of the minimal product thickness and the biaxial stretching. According to the virtual testing the Dynamic Mixing device has a total diameter of mm and a length of 5D related to the used screw. The barrier-screw has a nominal diameter of 48 mm and a length of 24D. After passing the spiral mandrel system the melt flows through a die with a gap of 3.2 mm. The produced film has a thickness of 150 µm at a lay-flat width of 315 mm. The general process conditions are summed up in Table 5. Table 4. Process conditions for the experimental process Blow up ratio [-] Mass flow rate [kg/h] Screw speed [rpm] Haul-Off speed [m/min] Cooling air volume flow [m³/h] For the material, a coextruded recyclate with a 95 % share PE-HD is used. Due to the inhomogeneous recyclate the mixing performance can be qualitatively evaluated by the visual impression. Machine-caused the significant data to analyze is the in front and at the end of the mixing device. The melt temperature is only detected behind the devices and was used to calibrate the operating point for the simulations. Figure 9 compares the demand of the investigated devices for the virtual and experimental investigations. Additionally, the temperature rise is shown. Figure 9. Comparison with experimental data The figure shows that the actual demand is significantly higher than the virtually determined. A delta of up to 60 bar can be seen. Due to the location of the sensors a deviation of the absolute values has been expected beforehand. It is not possible to generate the data at the exact same position as in the simulation environment. Nevertheless, the percentagewise decrease of the demand between simulation and experiment is transferable. The mixing performance, visualized by the produced blown film, can be rated in a first subjective step, as inhomogeneities are clearly visible in the thin film. In this context, the virtually evaluated improvements by IQ-DMR2, can be confirmed by the visual impression and will be evaluated in future works. Conclusions In this paper, the approach of a dynamic mixing device for improved melt quality has been successfully transferred from the high-speed application to small-scale blown film production lines. The length of the mixing devices has been increased to improve the mixing performance. Thereby, the temperature rise is a factor, which has to be taken into account. It is known that a major influencing variable on the temperature is the shear rate. Regarding this, the geometrical design has been successfully adjusted to minimize the metallic surfaces involved in the shear heating. For the realistic simulation, a user-defined temperature inlet profile has been defined to evaluate the thermal mixing effect. The operating principle has been proven virtually and the results of geometric adjustments at the honeycomb structure have been verified. It could be stated, that the reduction of the frame structure results in a reduced temperature rise by 18 % and demand by 37 %. With the help of a streamline analysis the residence time of the material has been interpreted and additional conclusions about the mixing effects have been drawn. The enlarged fluid volume leads to a less directed flow pattern and increases the distributive mixing performance. Finally, first validations with experimental data have been performed on a laboratory blown film line. Machinecaused not all the data of the simulations could be interpreted, but the demand and the melt temperature at the outlet of the mixing devices have been analyzed. Here, a qualitatively similar development has been validated. Initial subjective assessments of the film quality confirm the results of the virtual analysis. The blown film is suitable for a visual evaluation of the melt quality and shows an improved product quality for IQ-DMR2. This impression has to be confirmed by further investigations about the melt homogeneity and compared to the virtual outcome. Suitable following steps are the characterization of the thermal and material melt quality with the help of thermography and the evaluation of film samples. By these means, the mixing performance will be rated. SPE ANTEC Anaheim 2017 / 1198

6 Acknowledgment This publication was created during the processing of the research project Funktionsintegriertes Extruderkonzept mit frei rotierender Schneckenhülse und dynamischem Mischer zur Gewährleistung einer hohen Schmelzequalität im High-Speed-Betrieb (Project-No. WO 302/58-1). Our special thanks go to the Deutsche Forschungsgemeinschaft e. V. (DFG) for supporting this project. Moreover we apply our thanks to the WEMA GmbH and ETA Kunststofftechnologie GmbH for supporting the development of the HSST prototype and the mixing devices. References 1. R. Michels, Mischen und Homogenisieren, VDI- Wissensforum Der Einschneckenextruder, Düsseldorf, Germany, C. Rauwendaal, Polymer extrusion, 5th edition. Munich, Germany, G. Semmkrot, Distributive Mixer Device, US Patent US , P. Gorczyca, PhD Thesis, University of Duisburg- Essen, Germany, G. Karrenberg, PPS 31, Jeju Island, Korea, G. Karrenberg, PhD Thesis, University of Duisburg- Essen, Germany, SPE ANTEC Anaheim 2017 / 1199