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The SPE Extrusion Division

Board of Directors

University of Maryland, College Park, MD

Mixing is a critical component to the characterization of extruded material. Screw configuration is one such element that alters mixing within an extruder. Many extrusion processes utilize a mixing section which contains blades, paddles, or pins [1]. The two mixing sections studied for this research were narrow and wide kneading blocks (KB). Narrow KB induce low magnitudes of shear stress but generate high strain this allows for better distributive mixing. On the other hand, wide KB focus on creating high magnitudes of shear stress, which is best suited for dispersive mixing. Distributive mixing helps create a uniform spread of particles within the extruded final product. While dispersive mixing allows for the capability of breaking agglomerates into smaller particles [4].

The degree of mixing achieved in a twin-screw extruder is one of the most important factors that affect the properties of the final product [5]. Erwin demonstrated that the use of a mixing section reorients the interfaces of the fluid for greater area growth and thus better mixing [6]. In order to successfully determine the degree of mixing that occurs within the extrusion process, accurate modeling of the induced stress for a given mixing section and operating condition is necessary.

Past studies have developed RSD measurement techniques but nothing in real time. Wetzel et al used a method of a single ultrasonic probe and a tracer to identify the time the material spends in the extruder, which can determine the RSD [7]. Curry et al used glass spheres to measure the stress history. However, the method was discontinuous because it required burning the polymer off and manually counting the number of broken glass spheres [8].

Recent studies using stress sensitive polymer beads have been used to measure the stress history in real time [9]. These studies were expanded upon to include multiple stress beads that varied in strength in order to observe the accuracy of the methodology. The study produced similar results to research done with one strength stress bead thus validating the method of experimentation [10].

To further expand the analysis of the stress bead approach this study was conducted using three different strength stress beads. The DOE approach depicted the percentage of stress bead breakup along a range of screw speed (N) and specific throughput (Q/N). The DOE technique also enabled a statistical analysis to determine significant processing parameters and generate predictive equations for percent breakup. The following paper presents the results of the three stress beads.

For the blue stress beads the same dye that is encapsulated in them, Automate Blue 8A, was used to stain the polymer melt as a reference shot. For the red beads, the dye used was Red B Disazo. The reference shots were prepared by dissolving super high impact polystyrene (HIPS) 935E pellets in a heated solvent. The reference dye shots represented the paths taken if 100% of the stress beads were broken. This 100% breakup can also be visualized as Residence Time Distribution (RTD) because it accounts for all the paths taken by all the different fluid elements.

Two different screw geometries were investigated. Following a melting and conveying region that were the same, the first mixing section was comprised of four forward conveying right-handed narrow kneading blocks 6.0 cm of total length, followed by a reverse conveying element of 3.0 cm. This geometry ensured that the mixing section would be filled for all operating conditions. For the second screw geometry, three of the narrow kneading blocks were removed and replaced with one right-handed wide KB of 4.5 cm long. The reverse element was kept as the backup element for both configurations.

Figure 1. 28-mm Coperion Co-Rotating TSE and data acquisition setup

A reflective optical probe was placed above the end of the mixing section where the screws are filled. The probe consisted of a bifurcated optical fiber bundle enclosed in a stainless steel shell contained in a larger stainless steel shell with a sapphire window. The probe was inserted into a barrel through a standard Dynisco pressure transducer port. The probe uses a split fiber optic bundle, where white light is transferred from one bundle and enters the extrusion melt. The light is then scattered by the stained melt and enters back through the other fiber bundle. The light recorded is converted to a voltage signal using a photo diode and a signal amplifier.

Data was collected through the optical probe, and due to the transparent nature of HDPE melt, TiO

Figure 2. Example RTD/RSD for the 1.8 lbs/hr 110 RPM experimental condition

Figure 3. CCD grid of experimental conditions

The average percent breakup for all nine experimental conditions was calculated and the results inserted into the CCD grid as a way to compare and analyze the breakup trends for different bead strengths and screw geometries to operating conditions. To determine the percent breakup of the stress beads, the areas under the RTD and RSD curves were calculated and used in the following equation:

{eq1}

Where Ac is the area under the stress bead, or RSD, curve and Ar is the area under the dye, or RTD, curve. Three different strength stress beads were studied for this research, 92 kPa, 119 kPa, and 158 kPa. Figures 4 through 9 present the percent breakup results for each bead from lowest strength to highest using the CCD grid display.

Figure 4. Percent breakup of 92 kPa beads using narrow kneading block

Figure 5. Percent breakup of 92 kPa beads using wide kneading block

Figure 6. Percent breakup of 119 kPa beads using narrow kneading block

Figure 7. Percent breakup of 119 kPa beads using wide kneading block

Figure 8. Percent breakup of 158 kPa beads using narrow kneading block

Figure 9. Percent breakup of 158 kPa beads using wide kneading block

For all different strength stress beads certain patterns reappeared within the CCD grids. The wide KB geometry for every operating condition had a higher percent breakup than the narrow KB configuration. Additionally, the average breakup between grids decreased linearly with the strength of the stress bead, thus the lowest breakup results were seen with the strongest bead, the 158 kPa bead. Also the diagonals from the upper right to the lower left direction for all the grids, which approximately have the same mass flow, showed similar percent breakups. The diagonals from the lower left to the upper right direction showed an increase in breakup, displaying the significance of N and Q/N.

Each CCD grid was entered into JMP ® 9.0.0 statistical software for additional quantitative analysis. The results of the statistical report were organized in Table 1. The most notable result the statistical report indicated was which operating parameters proved significant on a 95% confidence interval. In all strength beads only two parameters were found to be significant, N and Q/N. The magnitude of the coefficients of N and Q/N are presented in Table 1. The last third of Table 1 provides statistical output for the concatenated wide and narrow geometry grids, which contributed a factor of change between the overall average and a specified screw configuration.

Equation 2 provides the general form of the predictive equation obtained. The predictive equation shows that percent breakup is only a function of screw speed and specific throughput. Coefficient A specifies the influence of screw speed on the breakup and coefficient B specifies the influence of specific throughput on the breakup. The constant C represents the average breakup for a specific grid or the intercept. The coefficients are also displayed in Table 1.

Table 1. Percent Breakup Predictive Equation Coefficients

The kneading block rows labeled “Combined” represents the average values of the narrow and wide KB equations. The “[Narrow]” column represents the percent shift required to bring the average breakup down to the breakup of the narrow KB grid. The negative sign indicates that the narrow KB breakups were all less than the average percent breakup.

Closer inspection of the comparison between grids brought about additional observations. When comparing the 92 and 119 kPa strength beads against each other there is a clear decrease in breakup from the 92 kPa to the 119 kPa beads. This result concludes that more of the fluid experiences the lower critical stress level of 92 kPa. For the narrow KB configuration the percent breakup increased by 10% when changing from the 119 kPa beads to the 92 kPa beads. Similarly, for the wide KB configuration the increase was 6%. An explanation for this, is that for the wide KB there was already a higher percent breakup, thus less of a chance for increase. Change in configuration between the 158 and the 92 kPa beads showed a similar difference. For the narrow KB there was a difference of 12% and for wide KB a change of 10%. Again the narrow KB showed more sensitivity to the change in bead strength. However, between the 158 and 119 kPa beads the difference for the wide KB was 5%, but the narrow KB only a 2% variation.

Examination of Table 1 yields a deeper understanding into the causes of breakup and the stress bead approach. The magnitude of the coefficients in Table 1 represent the sensitivity of the predictive equation to the changes in input parameters (N, Q/N). The coefficients of 92 kPa beads, compared to the other two beads, has the largest difference in magnitude between the N and Q/N coefficients for each screw geometry, with Q/N always playing a much greater role in percent breakup. The 119 kPa beads have the coefficients with the greatest magnitude, with values of approximately 9 and 10 for both N and Q/N for the wide and narrow KB configurations, which are about double those for the other two stress beads. This spike in magnitude for the 119 kPa beads shows that they are the most sensitive with respect to operating conditions. The last observation from Table 1 comes from the 158 kPa bead data. Although the 92 kPa beads have the largest difference between N and Q/N for each respective screw configuration, the coefficients for both N and Q/N for the 158 kPa beads nearly double when moving from the narrow KB results to the wide KB results. The other two bead strengths have approximately the same magnitude for both types of mixing sections. This indicates that the strongest bead, the 158 kPa, were most sensitive to the geometry change.

Figure 6. RSD curves 2.4/75 for all three strength beads under a single RTD

Figure 6 above displays the visual representation of the variation in bead strength with respect to stress distribution. The selected operating condition was 2.4 lbs/hr and 75 RPM because the curves in this range are mainly noise free. There are three RSD curves depicted, one for each stress bead studied. The RSD curve with the highest magnitude is the 92 kPa bead due to its low critical stress level. The 119 and 158 kPa beads follow in that order. This agrees with the quantitative results seen in the CCD grids. The delay times of all three RSD curves also exhibits an interesting trend. As the bead strength increases the delay time becomes longer. This phenomenon is attributed to more beads passing through unbroken. It has been shown that the material that comes through the fastest, travels through the center of the channel where the stresses are the lowest [11]. The lower critical breaking stress occurs closer to the center of the channel and will have a shorter delay time. The higher critical breaking stress will be experienced closer to the wall resulting in a longer delay time.

Results from the CCD grids and the table of coefficients provided additional information to characterize the stress history. The lowest strength bead, the 92 kPa, showed the greatest difference between magnitudes of coefficients for each respective geometry proving they were affected by specific throughput more than the other two beads relatively. The strongest critical stress bead, the 158 kPa, had an almost doubling of coefficient magnitude when moving from the narrow to wide KB geometry, making the 158 kPa bead the most vulnerable to a screw configuration change. However, the most interesting of all the results were with the 119 kPa coefficients. The coefficients for the 119 kPa beads was far greater than the both the other two beads. The high magnitudes indicate the 119 kPa beads are the most sensitive to operating conditions, and thus provide the greatest controllability for manufacturers. It is postulated that the average stress in the channel and the critical stress of the 119 kPa are closely matched. As one changes conditions one strongly changes the amount of fluid that experience the critical stress. The ability to categorize stress history in real time enables manufacturers to better understand how flow path truly affects their machine and ultimately their product. Future work will continue to broaden the range of beads studied along with a greater variation of screw geometry, material, and operating conditions.

2. Kluter, R. Kline, L. et al. A Systems Analysis of Meal, Ready-to-Eat (MRE) Packaging Materials, Report to Strategic Environment Research and Development Program, 2-4 (2002).

3. Y. Li, “Effect of addition of carbon nanofibers and carbon nanotubes on properties of thermoplastic biopolymers,” Polymer, vol. 52, 2310-2318 (2011).

4. P. J. Cullen, Food Mixing: Principles and Applications. John Wiley and Sons, (2009).

5. G. C. Papanicolaou, A. F. Koutsomitopoulou, and A. Sfakianakis, “Effect of thermal fatigue on the mechanical properties of epoxy matrix composites reinforced with olive pits powder,” Journal of Applied Polymer Science, vol. 124, 67-76 (2012).

6. L. Erwin, “New fundamental considerations on mixing in laminar flow,” SPE-ANTEC Tech. Papers, vol. 24, 488 (1978).

7. M. Wetzel, C. Shih, and U. Sundararaj, “Determination of Residence Time Distribution During Twin Screw Extrusion of Model Fluids,” SPEANTEC Tech. Papers, No. 827 (1997).

8. J. Curry and A. Kiani, “Measurement of Stress Level in Continuous Melt Compounders,” SPE-ANTEC Tech. Papers, vol. 36, 1599-1602 (1990).

9. D. Bigio, W. Pappas, H. Brown II, B. Debebe and W. Dunham, “Residence stress distributions in a twin screw extruder”, SPE ANTEC Tech. Papers, 1382- 1386, (2011).

10. D. Bigio, W. Pappas, H. Brown II, G. Fukuda, and R. Adnew, “Variable strength stress bead analysis in a twin screw extruder,” SPE ANTEC Tech. Papers, (2012).

11. W. Pappas, G. Fukuda, H. Brown II, and D. Bigio, Characterization and Comparison of Stress History in a Twin-Screw Extruder Using Residence Stress Distributions, Polymer Engineering and Science, (2012).

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