Using a slot die viscometer connected to a laboratory extruder can generate accurate data on melt viscosity at various shear rates and temperatures, which is a useful QC tool in processing. # Best Practice#dies
From the power law equation shown here, you can determine the consistency index and the power law index at two temperatures. The power law parameters are shown in Table 3.
Melt viscosity data at different shear rates are useful in most extrusion operations. These data provide a valuable quality control tool. Screw and die design and computer simulation of the extrusion process all require viscosity data. The correct choice of extruder screw or extrusion die requires understanding of viscosity as a function of shear rate. In addition, if viscosity data is obtained from a capillary rheometer or a slot die viscometer, instabilities such as melt fracture can be quantified and predicted. This article describes how to use a slit die viscometer connected to a laboratory extruder to obtain accurate data on melt viscosity at different shear rates and temperatures. Slot die viscometers provide an inexpensive way to determine rheological data, especially when compared to capillary rheometers.
The slit mold has a rectangular channel with a width W much greater than a height H (W>>H). The slot die can be directly connected to the laboratory extruder. Alternatively, a gear pump can be placed between the extruder and the slot die. Figure 1 shows a schematic diagram of the slit mold used in this study.
The shear rate at the wall of the slit mold is a function of the flow rate. The apparent shear rate can be determined by the following expression:
(Apparent wall shear rate) = (flow velocity) × (6/H2W)
The actual shear rate of a power law fluid with a power law exponent n is:
(Actual wall shear rate) = (apparent wall shear rate) × (0.667+0.333/n)
The shear stress on the wall can be determined from the pressure gradient of the measured pressure distribution:
(Shear stress) = 0.5H × (pressure gradient)
Through the shear stress and shear rate, we can determine the shear viscosity:
(Shear viscosity)=(shear stress)÷(shear rate)
In order to obtain viscosity data within a certain shear rate range, the extruder must be operated at different screw speeds. Each screw speed corresponds to a certain flow rate and therefore a certain shear rate. In order to determine the flow rate, the extruder output for each screw speed must be measured. When using a gear pump, the flow rate and shear rate can be determined by the speed of the gear pump.
Collect data and calculate viscosity
When a data acquisition system (DAS) is available, data collection is easy. In this study, the data was obtained on 1-in. A single screw extruder at the Graham Engineering Corp. (GEC) laboratory in York, Pennsylvania. The extrusion run using the slot die is part of a seminar organized by Rauwendaal Extrusion Engineering in cooperation with GEC. The data acquisition software is part of the Navigator XC300 control system used by GEC.
DAS data is exported to Excel in CSV (Comma Separated Value) format. Table 1 shows the screw speed, flow rate, and pressure data for the extruder and die temperature of Lab 1 at 400 F. In Excel, data can be processed in many ways. This allows automatic determination of pressure gradient, shear stress, shear rate and shear viscosity.
The polymer is 0.5 MI HDPE 273-83 manufactured by Kazanorgsintez, Kazan, Tatarstan, Russian Federation. The extruder runs at six screw speeds: 5, 10, 20, 40, 60, and 78 rpm. The maximum screw speed on this extruder is 80 rpm. These screw speeds correspond to a shear rate range of approximately 10 sec-1 to 150 sec-1. The extruder operates at two temperatures, 400 F (204.4 C) and 440 F (226.7 C).
At this point, we have enough information to determine the melt viscosity as a function of shear rate. Figure 2 uses a logarithmic-logarithmic graph to show the relationship between viscosity and shear rate at two temperatures. On the logarithmic graph, the data fits very well with a straight line. This means that a power law equation can be used to express viscosity as a function of shear rate.
Figure 2 shows the power curve equation. From these equations, we can determine the consistency index and the power law index at the two temperatures. The power law parameters are shown in Table 3.
We see a small change (approximately 5%) in the value of the power law exponent from 400 F to 440 F. The consistency index dropped significantly from 400 F to 440 F (approximately 30%).
We now have data on viscosity and shear rate at two temperatures. At this point, it is useful to analyze some performance characteristics of the extruder. This will be the focus of Part 2 of this series.
About the author: Dr. Chris Rauwendaal is a well-known writer, lecturer, researcher, entrepreneur and consultant in the field of extrusion. He holds multiple patents and authored more than 200 articles and 7 books related to extrusion, mixing, injection molding and statistical process control. As a member of the Society of Plastics Engineers (SPE), he is the developer of CRD, VIP, and ASM mixing technologies that use powerful elongational flow to improve mixing during extrusion and molding. Rauwendaal also developed the HHT (High Heat Transfer) extruder screw to improve cooling in foam tandem and other extrusion operations. In 1990, he founded Rauwendaal Extrusion Engineering and served as the company's president. Contact: (530) 269-1082; chris@rauwendaal.com; rauwendaal.com.
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