USING INFRARED TEMPERATURE SENSORS TO STUDY TEMPERATURE CHANGES OF PVC DURING FLOW WITH THE INCORPORATION OF MELT ROTATION TECHNOLOGY

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1 USING INFRARED TEMPERATURE SENSORS TO STUDY TEMPERATURE CHANGES OF PVC DURING FLOW WITH THE INCORPORATION OF MELT ROTATION TECHNOLOGY Stacey Johnson and Brad Johnson Penn State Erie, The Behrend College Abstract Infrared temperature sensors were used to study the effect of mold rotation technology on the plastic melt temperature and shear-burning that commonly occurs with PVC. Inserts were designed and built so that areas of high shear could be introduced during flow through a runner, as well as provide for the incorporation of melt rotation technology. A DOE was used to investigate how factors such as melt temperature, residence time, injection rate, and packing rate affected the temperature at various points along the flow path. It was found that the use of melt rotation technology could allow more uniform temperatures after the point of rotation without causing a larger problem with shear-burning. molecular weight [1]. The other element in both the first and last term of the equation is the velocity vector which is directly controlled by the flow front area and injection flow rate during injection molding. For PVC, relatively large runners and low injection velocities are generally employed to minimize shear-heating [1]. The heat build-up from shear heating occurs just underneath the frozen layer as the plastic flows. The distribution of the velocity causes a high shear in this region which in-turn causes the temperature to increase. A schematic of these distributions is shown in Figure 1. Introduction The sensitivity of polyvinyl chloride (PVC) to degradation due the integrated thermal input during injection molding is well known by those in the industry. In addition to the time/temperature issues that can cause problems with some other thermoplastics, the sensitivity to shear-work is notable for PVC [1]. During injection molding, this shear-work or shear-heating can be introduced during plasticizing or injection. To minimize shear-heating, the use of low compression ratio screws is common for facilities that specialize in PVC molding. For flow in round runners in injection molds, the energy equation can be reduced to ρρĉ pp vv zz = kk 1 rr + rr ηη(ddvv zz dddd )2 where ρρ = dddddddddddddd, Ĉ pp = Heat Capacity, kk = ttheeeeeeeeee cccccccccccccccccccccccc, TT = temperature, ηη = Shear-rate dependant viscosity, vv = velocity vector, & rr = radial coordinate [2]. The first and last terms in the equation shows that higher viscosities and higher injection velocities will increase the amount of heat energy formed which could then lead to degradation. Additives, such as plasticizers, can ease the heat build-up for flexible PVC by lowering the viscosity. For rigid PVC, the viscosity can be lowered through the use of PVC homopolymer resins of relatively low Figure 1. Typical velocity, shear rate, and temperature distribution through a runner [3]. It is common for the temperature increase in these outer, high shear laminates to be more than 100 C hotter than the inner laminates that are under low shear [6]. A method to manage these hot laminates, primarily to help balance multi-cavity molds, is through the use of melt rotation technology [11]. Figure 2 shows how a level change can convert a side-to-side distribution into a topto-bottom distribution. Figure 2. Illustration of a basic melt rotation to convert a side-to-side shear distribution over to a top-to-bottom [3]. SPE ANTEC Indianapolis 2016 / 1901

2 Figure 3 shows melt rotation technology that will move the high shear outer layers to the middle of the crosssection. In addition to use in balancing runners, this type of rotation has been used to control filling patterns, shrinkage and warpage and, in some cases, improve weld line flexural strain by over 100% [4]. Figure 3. Illustration of a melt rotation configuration to move the high shear laminates from the outer layers to the inner layers [6]. This study will involve creating high-shear laminates on the top and bottom of a flow channel and then using melt rotation technology to turn those hot, high-shear laminates to the center of the flow channel. Because of the fountain flow which occurs during molding (see Figure 4), this hotter material will then come to the surface further downstream. This may be advantageous to get more uniform shrinkage across a part, reduce hesitation in thin areas, or improve weld lines. Materials & Equipment For this study, a clear, high flow grade of rigid PVC was used. A clear grade was desirable in order to be able to see shear-burning and thermal degradation when it occurred. There is also some evidence that less opacity also allows deeper penetration of the IR temperature measurement [8], which should help pick up shear-heating effects. An overall flow length of over 800 mm dictated that a low flow grade be used so that the molding machine did not become pressure-limited during any of the trials. The parts were all molded in a 125 kn clamp molding machine with a 25 mm diameter general-purpose screw with a 34 cm 3 shot capacity, 235 MPa maximum injection pressure, and 245 cm 3 /sec maximum injection flow rate. The machine had closed loop injection velocity control and the ability to use an external signal for V/P (velocity to pressure control) transfer. All in-mold sensor signals were read by an external data collection system which sent a signal to the molding machine for V/P transfer. An existing mold was retrofitted with inserts to make what amounts to an 830 mm long, 5 mm square runner attached to a 115 mm long standard sprue with a 4.8 mm O dimension. It is actually a trapezoidal runner due to the 2 draft angle on the sides for ejection purposes. There are three inserted areas in the layout that is shown in Figure 5. The two high shear areas ( A & C ) are insertable for different thickness that have the same 5 mm width. In addition to being able to run the same 5 mm thickness as the rest of the runner, these areas could be inserted to run with a 2 mm thick or 3 mm thick flow channel. The 2 or 3 mm thickness would extend for 60 mm and then taper for 20 mm on each end back to the 5 mm thick section. Figure 4. Illustration of fountain flow, where the inner laminates flow perpendicular to the flow direction once they reach the flow front. Infrared (IR) temperature measurement was used to obtain the temperature of the plastic during fill. IR temperature sensors have been shown to be capable of measuring subsurface temperature and give an indication of bulk temperature during fill [8]. How deeply the plastic penetrates the surface depends on the material characteristics and the temperature profile through the cross-section of the melt [9, 10]. A value of 7 mm for the penetration depth of PVC has been published [10]. This is deeper than the flow channel used in this study. Sprue Figure 5. Schematic of runner layout showing location of inserts and sensors. SPE ANTEC Indianapolis 2016 / 1902

3 pressure sensors were 4 mm diameter, flush mounted with the cavity surface, and could be installed in any of the 4 positions shown in Figure 5. Figure 6. Moldflow images of the shear rate in the 5 mm thick (left) cross-section and the 2 mm thick (right) crosssection at a flow rate of 40 cm 3 /sec. For both thicknesses the dark blue is zero shear. For the 5 mm thickness the dark red is a shear rate of 2500 sec -1 and for the 2 mm thickness the dark red is sec -1. Figure 6 shows how the thinner inserts would increase the shear in areas A and C in Figure 5. These areas were intended to make the highest temperature occur on the top and bottom of the runner. Two types of sensors were used and placed in positions 1 thru 4 (shown in Figure 5) for different parts of this study, infrared temperature sensors and piezoelectric cavity pressure sensors. The temperature sensors had an infrared detection method using optical fiber and had an 8 ms response time. Both the temperature sensors and the Figure 6. Schematic showing the position of the high shear laminates when flowing from position a to position d for a 0 (straight) angle and a 45 angle in the melt rotation insert. The insert area B shown in Figure 5 is where the adjustable melt rotation insert was installed. As the melt entered and exited the insert there were level changes similar to Figure 2 that would rotate the melt 90. In the center of area B, the melt could rotate the high shear laminates from the outside to the center of the flow crosssection if angled 45 as shown in Figure 3. For this study the angle was either at 45 or 0 (straight) as shown in Figure 6. Figure 6 also shows how the melt was rotated. At position a, the high shear laminates were top and bottom after going through the high shear area A and ended up in the same orientation at position d if the 0 (straight) angle was employed or horizontally between the top and bottom at position d if the 45 angle was used. Experimental Procedure Three studies will be discussed. They will be referred to as the pre-doe study, the DOE, and the post-doe study. The pre-doe study was done to confirm that the melt rotation was being achieved as anticipated by purposely shear-burning the PVC and visually observing where the burnt high shear laminates were rotated to. For the pre- DOE there were temperature sensors at positions 1 & 2 and no cavity pressure sensors. The flow was stopped at the end of the second high shear area for the pre-doe. For the DOE, the temperature sensors were in positions 1 & 3 and there were cavity pressure transducers in positions 2 & 4. The post-doe study was done to further investigate the most interesting findings of the DOE. In the post-doe, the cavity pressure sensor in position 2 was replaced with a temperature sensor. A 16-run Placket-Burman, resolution IV design of experiments (DOE) was carried out to investigate the effect of several factors on the temperature change through the runner. The effect of these factors on pressure drop in the runner was also looked at. A list of the factors and the levels used for each run is shown in Table 1. The factors were chosen that were thought to have an effect on the melt temperature or degradation due to viscous flow. Some preexperimenting with the factors was done to find levels that didn t create too much degradation. The high level of barrel temperatures (200 C) was where the material started to show a slight yellow tint due to thermal degradation in the barrel. The combination of the low level of shear thickness (2 mm), which had the highest shear rate, and the fastest flow rates (20 cm 3 /sec) were the only runs which had a slight amount of injection shear-burning. The cycle time was run at either 60 seconds or 90 seconds in order to compare the effect of different residence times in the barrel. SPE ANTEC Indianapolis 2016 / 1903

4 Table 1. DOE factors, factor levels and runs. Table 2. Constant molding conditions for all studies. Parameter Setpoint Mold Cooling 90 C Water Temperature Shot Size 32 cm 3 Screw Speed 100 RPM Back Pressure 0.2 MPa Hold Pressure 0 Hold Time 1 sec Results The pre-doe did confirm that the melt was being rotated as anticipated. Figure 7 shows the shear-burn that occurred at the two different rotations of the adjustable insert and figure 8 shows the burnt layer that was rotated to the center of the melt stream using the 45 rotation. The injection rate was changed to look at the effect of different shear rates. The peak temperatures typically occurred at the transfer from the fast injection rate to the slow pack rate when the runner was almost full. The part was then packed to a set cavity pressure at sensor location 4. The pack rate, velocity switch (between injection and pack) position, and transfer cavity pressure were all included as sensitivity checks but were not believed to be significant prior to the DOE. The last factor, the 0 and 45 angles of the melt rotation insert insert, was the main comparison of interest in this study. For the DOE and post-doe studies, the process that was used was basically that of a 3-stage set-up with cavity pressure transfer where most of fill is accomplished with a relatively fast injection speed which is then transferred to a slow pack speed just prior to the cavity being filled. Previous studies [5,7] have shown that using a sensor at the last place to fill gives very repeatable parts when using this process and using cavity pressure V/P transfer. The pre- DOE did not include a cavity pressure sensor, so V/P transfer was done via screw position. Table 2 shows some of the other process parameters that were held constant for all studies. Figure 7. Shear-burning pattern for the 0 and 45 rotations at an injection rate of 49 cm 3 /sec. Figure 8. Shear-burning horizontally in the center of the flow resulting from the 45 rotation at 49 cm 3 /sec. Data was collected for three different flow rates during the pre-doe: 37, 49, & 61 cm 3 /sec. Figure 9 shows graphs of the temperature at positions 1 and 2 for a flow rate of 49 cm 3 /sec. It can be seen that the position 1 curve was close to the same for both angles of the melt rotation insert, whereas the temperature did not rise as high at position 2 when the 45 angle was employed in the adjustable insert. The difference in the peak temperatures for all three flow rates at both positions is shown in Figure 10. It can be seen that the temperatures increase with flow rate and that the temperature at position 2 is significantly lower when the 45 angle was used. Also of note was the fact that there were shear-burn streaks present at both position 1 SPE ANTEC Indianapolis 2016 / 1904

5 and position 2 for both of the two higher flow rates in Figure 10, but not at the lower flow rate. inserts then, continues to rise until the injection velocity is transferred to a slower pack velocity. Figure 11 shows that the rate of this temperature rise was directly proportional to the injection rate. inserts Figure 9. Graphs contrasting position 1 & 2 temperature for the pre-doe study. Figure 11. Graph showing temperature rise at position 1 for the pre-doe study. The data was overlaid so that the arrival of the melt at the sensor occurred at 0.93 seconds on the x-axis. The estimated effects of the factors used in the DOE on the difference in peak temperature between the end of fill (position 3 ) and before the melt was rotated (position 1 ) are shown in Figure 12. It can be seen that the injection rate (H), melt rotation insert angle (D), and high shear area thickness (B) had the most significant effect on the temperature difference. The average of the difference at the slower injection rate (10 cm 3 /sec) caused about 8 C more temperature difference than the higher injection rate (20 cm 3 /sec), the 45 melt rotation insert angle caused about a 4 C higher temperature difference than the straight inserts, and the 2 mm thick high shear areas caused about a 4 C higher temperature difference than the 3 mm thick. The other factors and interactions (blue bars in Figure 12) were not significant enough to warrant mention here. Figure 10. Graphs showing 95% confidence intervals of peak temperatures for the pre-doe study. In Figure 9, it can also be seen that the temperature rises sharply when the flow front gets to the sensor and, Figure 12. Estimated effect of the DOE factors and interactions on the peak temperature at position 3 minus the peak temperature at position 1. SPE ANTEC Indianapolis 2016 / 1905

6 The other response that was looked at for the DOE was the pressure drop between position 2, just after the melt rotation was completed, and position 4 at the very end of the flow path. Figure 13 shows an example of the pressure curves from run 1 of the DOE. The pressure drop is the difference in the peaks of the two curves and occurred at the transfer from the controlled pack rate to the hold pressure (set at 0). The estimated effects on the pressure drop are shown in Figure 14. Figure 13. Cavity pressure curves for run 1 of the DOE. It can be seen in Figure 14 that the transfer pressure (K), the high shear area thickness (B), the injection rate (H), the velocity switch position (N), and the pack rate (M) were all significant to the pressure drop. A higher pressure drop was obtained when the 26 MPA transfer cavity pressure, the 2 mm high shear area thickness, the 10 cm 3 /sec injection rate, the 2% left to fill transfer, and the 1.5 cm 3 /sec pack rate were used. It is important to note that the melt rotation insert angle (D) had no effect on pressure drop. that did not show the yellow tint. The other DOE factors were also held constant for this study with the 3 mm thickness in shear area A, the 2 mm thickness in shear area B, the 60 second cycle time, the 26 MPa transfer pressure, the 1.5 cm 3 /second pack rate, and transferring to hold when the runner was about 5% from being full. For the post-doe study, four different flow rates were run with each of the two melt rotation insert angles. The peak temperature was found at each position for each combination of injection flow rate and melt rotation insert angle and is shown in Figure 15. It can be seen that the temperatures prior to the rotation were the same regardless of the melt rotation insert angle. Also, the temperature at the end of fill only changed about 4 C when increasing the injection rate from 10 to 60 cm 3 /second when the angle was at 0. The major observation with this graph is that when the 45 angle was used, the peak temperature at the two positions after the melt rotation, 2 and 3, were much closer than when the 0 angle was used. The temperature was 6 to 9 C lower at position 2 and 3 to 10 C higher at position 3 when the 45 melt rotation insert angle was used. Visually, for the post-doe study, minor shear burning was not observed until the 30 cm 3 /second injection rate. The shear-burns that were observed barely reached sensor position 1 at the 60 cm 3 /second injection rate. Figure 15. Graph showing 95% confidence intervals of peak temperatures for the post-doe study. Solid lines are the 0 melt rotation insert angle and dashed lines the 45 melt rotation insert angle. Figure 14. Estimated effect of the DOE factors and interactions on the pressure drop after melt rotation. The post-doe study was run to investigate the temperature change between positions 1, 2, and 3 for each of the two melt rotation insert angles. The barrel temperature was the only other significant factor and was held constant at the low level of 190 C for this study since Discussion of Results When looking at the data generated with the pre-doe, DOE, and post-doe, changing the melt rotation definitely caused changes to the temperature of the plastic. The first difference that was seen in the results was the substantial lowering of the temperature at position 2 when the high shear laminates were rotated to the center of the cross- SPE ANTEC Indianapolis 2016 / 1906

7 section. This is shown graphically in Figures 10 and 15. This difference is most likely due to the fact that as the high shear laminates were rotated to the center, the low shear laminates were rotated to the outside and were not as hot since position 2 is located only 20 mm after the last rotation. The IR sensor will pick up subsurface radiation, but this result appears to indicate that the penetration depth is less than half the thickness of the flow channel (2.5 mm) or that the portion of the intensity emitted from the center layers do not affect the temperature signal as much as the outer layers. This can be contrasted to the post-doe results shown in Figure 15. For this study, the temperature was higher at position 3 (end of fill) for the runs made with the high shear laminates rotated to the center. This difference is most likely due to the fact that the hotter, high shear laminates that were rotated to the center are what come to the surface after the flow front gets far enough from the point of rotation because of fountain flow. The combined effects of the cooler plastic at position 2 and the warmer plastic at position 3 made the temperature change two to three times less when the high shear laminates were rotated to the center of the flow channel. If this reduced temperature difference across a length of flow can be achieved in a plastic part, it could lead to reduced shrinkage variation across the part. The fact that the pressure drop was not affected by the different melt rotations was somewhat surprising. Prior to the experiment, it was hypothesized that the pressure drop may be higher when the high and low shear laminates were switched using the 45 melt rotation insert because the lower temperature, non-oriented laminates would cause a higher viscosity for some distance after the rotation in the new high shear laminates. It appears that this theory is not valid or that something such as the higher temperature in the center counter-acted this effect. Additional sensors between positions 2 and 3 would be needed to further investigate the pressure drop, as well as the temperature change along the flow path. The gradual temperature rise shown in Figure 11, after the melt front passed the sensor, is thought to be the result of shear-heating. The highest shear is beneath the surface and the IR sensors appear to pick up some of this temperature change. This rise continued until the injection velocity was switched to the lower pack speed. As an aside, when this mold was originally run in a machine that didn t have enough injection pressure available, the temperature would stop rising as soon as the pressure limit was hit and the velocity slowed down. Figure 11 shows that this would be expected based on the energy equation in that, as the velocity increased, the energy put into the plastic was higher as evidenced by the higher temperature. The visual examination of the shear burns revealed that when the temperature reached around C there would be shear-burning just under the surface. It should be noted that the 49 and 61 cm 3 /second samples shown in Figure 10 all show shear-burning at both positions 1 and 2 and that the temperature was as low as 234 C with the 49 cm 3 /second sample molded with the 45 flipper. However, the burn at position 2 was in the center of the runner because of the rotation and not in the layers under the surface. Conclusions For the material, geometry, and process set-up conditions used in these studies, the following conclusions can be made. 1. The measured temperature was lowered as much as 9 C directly after rotating the high shear laminates to the center of the flow cross-section and the inner laminates to the outside. 2. The measured temperature was raised as much as 10 C at the end of the flow channel when the high shear laminates were rotated to the center of the flow cross-section and the inner laminates to the outside. 3. Shear-burning was evident under the surface at the sensor location when the measured temperature reached approximately C. 4. Injection flow rate, the angle of the melt rotation insert, and the thickness in the high shear areas all had a significant effect on the temperature difference between the sensor before the melt was rotated and the end of fill. 5. In-cavity v/p transfer pressure, the thickness of the high shear areas, the injection rate, the velocity switch position, and the pack rate were all significant to the pressure drop from the sensor position directly after the melt rotation to the end of fill. Acknowledgments The authors would like to thank Bohler-Uddeholm Corporation for the donation of stainless steel to make the mold inserts, PolyOne Corporation for the donation of the PVC used in this study, Futaba Corporation for the infrared temperature sensors and calibration equipment, and Beaumont Technologies for designing the melt rotation geometry with patented MeltFlipperTM technology [11]. References 1. Titow, W. (1984). Injection Moulding of PVC. In PVC technology (4th ed., p. 724). London: Elsevier Applied Science. SPE ANTEC Indianapolis 2016 / 1907

8 2. Bird, R. (1977). Material Functions for Polymeric Fluids. In Dynamics of polymeric liquids (Vol. 1, p. 247). New York, New York: Wiley. 3. K.R. Slye, J.P. Coulter, B. Bekisli, and T.J. Skiba, SPE-ANTEC Tech. Papers, 57, (2011). 4. J. Beaumont, C. Stewart, M. Ezzo, SPE-ANTEC Tech. Papers, 51, (2005). 5. B.G. Johnson, and G.A. Horsemanko, SPE-ANTEC Tech. Papers, 47, 445 (2001). 6. Beaumont, J. (2007). Runner and gating design handbook: Tools for successful injection molding (2nd ed.). Munich: Hanser. 7. B. Johnson, Determining Which In-Mold Sensors Should Be Used For V/P Transfer During Injection Molding For Three Different Injection Strategies, SPE-ANTEC Chicago, IL, USA May, [On-Line], Available: 8. G.-Y. Lai and J.X. Rietveld, Role of Polymer Transparency and Temperature Gradients in the Quantitative Measurement of Process Stream Temperatures During Injection Molding via IR Pyrometry, Polymer Engineering and Science, 36, 13, (1996). 9. C. Maier, Infrared Temperature Measurement of Polymers, Polymer Engineering and Science, 36, 11, (1996). 10. W. Obendrauf, G.R. Langecker, W. Friessenbichler, Temperature Measuring in Plastics Processing with Infrared Radiation Thermometers, International Polymer Processing XIII, 1, (1998). 11. John P. Beaumont, Adjustable Melt Rotation Positioning and Method. U.S. Patent 7,780,895, issued August 24, SPE ANTEC Indianapolis 2016 / 1908