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The relation of a slit is
H = height of slit
ya = shear rate according to Equation 5.10
5.1.1.6 Adapting Die Design to Avoid Melt Fracture
(5.21)
Melt fracture can be defined as an instability of the melt flow leading to surface or volume
distortions of the extrudate. Surface distortions [34] are usually created from instabilities
near the die exit, while volume distortions [34] originate from the vortex formation at
the die entrance. Melt fracture caused by these phenomena limits the production of articles
manufactured by extrusion processes. The use of processing additives to increase the
output has been dealt with in a number of publications given in [35]. However, processing
aids are not desirable in applications such as pelletizing and blow molding. Therefore,
the effect of die geometry on the onset of melt fracture was examined.
The onset of melt fracture with increasing die pressure is shown for LDPE and HDPE in
Figure 5.3 [38]. As can be seen, the distortions appear differently depending on the resin.
The volume flow rate is plotted in Figure 5.4 [39] first as a function of wall shear stress
and then as a function of pressure drop in the capillary for LDPE and HDPE. The sudden
increase in slope is evident for LDPE only when the flow rate is plotted against pressure,
whereas in the case of HDPE it is the opposite. In addition, for HDPE the occurrence of
melt fracture depends on the ratio of length L to diameter D of the capillary. The effect
of temperature on the onset of melt fracture is shown in Figure 5.5 [36]. With increasing
temperature the onset of instability shifts to higher shear rates. This behavior is used in
practice to increase the output. However, exceeding the optimum processing temperature
can lead to a decrease in the quality of the product in many processing operations. From
these considerations it can be seen that designing a die by taking the resin behavior into
account is the easiest method to obtain quality products at high throughputs.
Design procedure
Using the formulas given in this book and in reference [33], the following design procedure
has been developed to suit the die dimensions to the melt flow properties of the resin to
be processed with the die.
STEP 1: Calculation of the shear rate in the die channel
STEP 2: Expressing the measured viscosity plots by a rheological model
STEP 3: Calculation of the power law exponent
STEP 4: Calculation of the shear viscosity at the shear rate in Step 1
STEP 5: Calculation of the wall shear stress
STEP 6: Calculation of the factor of proportionality
STEP 7: Calculation of die constant