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The next steps include calculating apparent wall shear stress and apparent wall shear rate through the capillary, from which viscosity is determined. After this, Bagley and Rabinowitsch cor- rections must be applied. The Bagley correction requires a second capillary rheometer test to account for the fact that pressures are measured prior to the melt entering the small capillary. Once data is collected from a capillary rheometer, it's typically represented on a log-log graph (see Fig. 1). On the "Y" axis is viscosity, with units commonly represented in poise, Pascal-seconds or psi-seconds. On the "X" axis are shear rates expressed as reciprocal seconds (1/sec or sec'). This test method provides detailed knowledge of how the melt's viscosity behaves under a wide range of shear rates and how it reacts to temperature changes. By itself, this information and its units are fairly abstract and rarely used directly by any- one outside of polymer testing labs or polymer compounders. The primary use of this data is for injection molding simulation programs. These programs are designed to take this isothermal test data and couple it with thermal solutions to predict how melt will flow through a mold. Without the simulation, the capillary rheometer data has little use in the process of selecting plastic materials for an application by a designer or processor. It is only through use with simulation software that this data can be used to provide meaningful information.
The second method is melt flow index (MFI), which is by far the most common method used for characterizing plastic materials due to simplicity and low cost. It's interesting that injection molders, practitioners of one of the most complex mainstream manufacturing methods in existence, settle for simplicity and low cost as their primary means of guidance, particularly when the conditions of this isothermal extrusion method are so unlike injection molding.
Unlike a rheometer, which is used to characterize the viscosity behavior of a non-Newtonian melt at multiple shear rates, the MFI test is used to develop a single reference value. Like the rheometer, this is also an isothermal extrusion test; and, as defined by ASTM D1238, it is somewhat similar to a capillary rheometer.
The test begins by hand-loading plastic pellets into a heated chamber. They are compacted and heat soaked until they reach a desired temperature and then a load is applied to a piston to drive the melt through a small orifice. Once the load is applied, the melt will extrude out through the small orifice in the bot- tom, collecting in a sample container. Over a period of 10 minutes, this material is collected and weighed. The result is a single MFI data point expressed in grams per 10 minutes.
In contrast to the capillary rheometer, not only is only one data point collected but the speed (velocity) of the ram injecting the material through the small orifice is not controlled. Therefore each material having a different MFI will have been measured at a different shear rate. A higher-MFI material will be measured at a higher shear rate. This will exaggerate differences in the flow behavior of a non-Newtonian melt.
Though not normally done, the shear rate during a MFI test can be determined. The following values are based on a melt having a melt density of 1 g/cm^:
MFI of 0.2 = shear rate of approximately 0.4 sec' MFI of 1.0 = shear rate of approximately 2.2 sec ' MFI of 10 = shear rate of approximately 22 sec ' MFI of 100 = shear rate of approximately 217 sec ' MFI is most commonly used for evaluating lot-to-lot variations of a given material. The exaggerated effects provided by the uncontrolled shear rate can actually help accentuate the differences in material lots' behavior. This method can also be used to provide an indirect measurement of relative molecular weight of similar materials.
Some of the primary faults of this test for use with injection molding is that it is isothermal and its shear rates are more representative of those found in extrusion than in injection molding. Additionally this test does not offer a real measure of viscosity; nor, as stated before, does MFI allow for velocity control, meaning that any change in a material viscosity will result in the viscosity being measured at different shear rates.
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