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Melt disturbance onset from increasing line speed
April 21, 2008
One of the best things you can do to improve the productivity of a film (or any line) is to increase the output of a product. For film products this means increasing the line speed. Several things happen which affects the potential for a melt disturbance of the third kind when we do this. First and foremost the melt temperatures are increasing with increased line speed as the screw speeds are increased. Depending on the polymers you are using and the temperature sensitivity of the polymer this can impact the relative melt viscosity of the skin and core.
A second change that occurs is that the shear rates increase as the output increases. This is easily seen from the shear rate expression for a power law fluid in a slit
Shear rate = [(2n+1)/n]*(2Q/WH2)
Where:
Q is the volumetric flow rate
n the power law index
W is the die land width and
H the die land gap
If the polymers in the coextrusion are similar in their shear thinning behavior (value of n) (a PP and EP copolymer combination or a combination of 2 different LLDPE’s) then the increase in shear rate will not likely cause a problem. But, if you combine two polymers with different shear thinning behaviors then the increase in line speed can cause a change in the relative viscosity and therefore cause a melt disturbance. This is shown in Figure 1 which displays the relative flow behavior of PP and LLDPE. The PP shows a typical shear thinning behavior while the LLDPE has a much more Newtonian flow behavior.
Figure 1
So if we choose to make a PP/LLDPE coextrusion with the LLDPE as a skin, at low line speeds (shear rates) the LLDPE is lower viscosity than the PP which is what we would want. However, as line speed in increased (increasing shear rate) the PP viscosity drops at a much faster rate than the LLDPE and at some point the viscosity will “invert” and now the LLDPE is higher viscosity than the core and a melt disturbance is a distinct possibility. Of course to know if this might occur in advance, you have to have measured the viscosity of the polymers. As you might suspect, I think melt viscosity is so important that I measure it myself!
Posted by Eldridge M. Mount on April 21, 2008 | Comments (0)