The Bagley correction is needed to calculate the inlet pressure drop when round hole capillaries are used. Near the inlet area of round hole capillaries a pressure drop is resulting from convergent flow in the capillary inlet. In the same way a part of the pressure is also used for the outlet. By the help of the Bagley correction it is possible to separate the viscose pressure drop and the inlet/outlet pressure drop. To determine the inlet/outlet pressure drop the pressure drop is being applied with different capillaries of the same diameter but different lengths and extrapolated to zero (Bagley plot).
The 3-parameter extension, especially suitable to describe the viscosity run of linear thermoplastic materials.
Elongational viscosity after Cogswell
After previous determination of entrance pressure loss using Bagley correction (see Bagley correction with Bagley plot) elongational viscosity can be calculated. Beside the shear viscosity the elongational viscosity delivers important information about the handling behavior. While the shear viscosity describes the behavior about a pipe flow the elongational viscosity shows information about the behavior in cross-section changes like i.e. in extrusion dies or in injection molds.
For correction of the elastic pressure losses at the inlet and outlet of the die by means of the “Gleissle-Correction” the total pressure PV measured in front of the die will be separated into its viscous and elastic components. This separation is done iteratively using the data of the flow curve. To perform the iteration two material dependent parameters must be set beforehand by the operator. Here setting are given for PE materials. The software the calculate normal stress and normal stress coefficient.
The Hagenbach correction has its application with low viscous media like dispersion paints, varnish or others. The correction corrects the pressure loss which occurs by the change of velocity at the entrance of the die. Because of the flow contraction the material is accelerated strongly which causes a change in kinetic energy. The viscosity has to be reduced by the increase of kinetic energy with increasing flow velocity. Elastic effects are not considered by this method. The example shows the effect of Hagenbach correction compared to uncorrected data for a paint used in paper coating. For most plastics Hagenbach correction usually has no effect because change in kinetic energy is low compared to the pressure change caused by viscosity.
The Mooney correction is used to determine the velocity of the wall slip at materials i.e. HDPE or PVC (wall slip tending materials). Here the model expectation is using the fact that the material glides with a constant wall slip velocity vg.
The law of Muenstedt has no physical background but an approximation of the flow curve with a polynomial fourth order. This law gives very good agreement in the experimental data region.
Normal stress difference
The first normal stress difference describes normal forces vertical to flow direction and is caused by elastic material behaviour. It is calculated from the flow curve. The software allows to make a comparison between 1st normal stress difference and shear stress. If 1st normal stress difference is higher than shear stress elastic properties are dominating the flow, if shear stress is higher than viscous properties are dominating the flow.
The Non-Newtonian Index or NNI is calculated according the Mitsui standard. The value describes the slope of the flow curve between two stress levels, which are defined by the standard. The software plots automatically the graph of the flow curve including the two point in between the NNI is calculated the NNI value and the correlation of the regression.
Ostwald - de Waele
Power law model is the most simple model to describe the pseudoplastic behaviour of thermoplastic materials Especially elastomers can be described very well by this simple law.
PVT measurement is the measurement of a sample volume at different pressures and temperatures. A distinction is made between isobaric and isothermal PVT measurements.
Due to shear thinning of polymer melts with pseudoplastic flow behaviour a strong flection of the velocity profile close to the wall occurs. Consequently the shear rates near the wall are higher than these from Newtonian fluids, where the calculation of the apparent shear rates is valid for (see part “Apparent values”). This effect is considered by the Rabinowitsch Weissenberg correction and adjusted.The corrected viscosity is calculated via the corrected shear rate and the shear stress close to the wall.
Die swell is measured using the option for die swell measurement (laser). The software calculates die swell value from diameter or surface ratio between die and strand. Using the software the value can be plotted against a free selectable data. The graph shows an example of die swell versus shear rate.
Sabia is a flow law which is suitable to describe the viscosity function in the same way as the Yasuda model from branched polymers.
Yasuda is an extended Carreau model which describes the viscosity run of branched thermoplastic materials. These kind of materials show a stronger change of the viscosity curve in the transient area between zero-shear viscosity and pseudoplastic area, which can be described by this model. The software generates a comparison between measured and model data beside the evaluation of the correlation coefficients as shown in the example.