LCR7000 & RHEOSIGHTS P/N: 974194 |
Rev: A
|
ECO: 52468
www.dynisco.com
From lab to production,
providing a window into the process
-69-
Where:
τ
= the true shear stress
P = melt pressure
P
0
= the intercept obtained for a given shear rate from the above
described plot
D = die diameter
L = die length.
For plunger force measuring devices:
F = force on the plunger
F
0
= the intercept force on the Bagley plot described above.
A
B
= the cross sectional area of the barrel.
Devices which measure plunger force must acquire data for a given shear rate (a given line
on the graph) at the same position in the barrel for the various dies used. In this way barrel
pressure drop effects will be removed along with the other stationary pressures in the
system when the Bagley correction is performed.
Note 1
: Pressure transducers mounted just above the die in the barrel still require the
Bagley correction. If the position of the pressure probe is a substantial distance from the
capillary entrance the pressure drop from the pressure probe to the top of the die can be
significant.
Note 2
: When using very long dies there may be non-linear changes in the pressure vs. L/D
plots due to the effects of pressure or viscous heating on the viscosity. In such cases one
should use only the data from shorter capillaries, which do not exhibit the effect.
Users are warned that the Bagley correction is not valid under circumstances where the
straightline conditions are not met.
Extrudate swell (sometimes called “die swell”) is defined as the ratio of the diameter of the
extrudate over that of the extrusion die. Most materials decrease in swell with residence
time in the die. Thus, at higher shear rates (short time in the die) the extrudate swell is
generally larger than at lower rates. Swell also tends to decrease with temperature, as most
materials become less elastic as they are heated.
14.
Extrudate Swell
