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where c and m are empirical constants and T is the shear stress. These constants can be
determined from the well-known Bagley-curves as shown in Figure 1.7. The values of
these constants are given in Table 1.1 for some thermoplastic materials. Shear stress and
entrance loss are measured in Pa in the calculation of c and m.
1.3.3 True Shear Stress
The flow curves of a particular LDPE measured with a capillary rheometer are given in
Figure 1.6. The plot shows the apparent shear rate J^ as a function of the true shear stress
T at the capillary wall with the melt temperature as a parameter. The entrance loss pc was
obtained from the Bagley plot shown in Figure 1.7.
Shear rate f
S" 1
Figure 1.6 Flow curves of a LDPE [8]
#=1mm
^= 0,6 mm
Pressure p
Shear stress T
T-- 23O 0 C
19O 0 C
15O 0 C
Capilary geometry L/R
Figure 1.7 Bagley plots of a polystyrene with the capillary length L and radius R [3]
bar
Pa