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t
(1/s)
Figure 5.6 Simulation results for a pelletizer die
a) Pelletizer Dies
The aim here is to design a die for a given throughput or to calculate the maximum
throughput possible without melt fracture for a given die. These targets can be achieved
by performing simulations on dies of different tube diameters, flow rates, and melt
temperatures. Figure 5.6 shows the results of one such simulation.
b) Blow Molding Dies
Figure 5.7 shows a blow molding parison and the surface distortion that occurs at a specific
shear rate depending on the resin. In order to obtain a smooth product surface, the die
contour has been changed in such a way that the shear rate lies in an appropriate range
(Figure 5.8). In addition, the redesigned die creates lower extrusion pressures, as can be
seen from Figure 5.8 [36].
c) Blown Film Dies
P
(bar)
Flow rate: 350 kg/h
Temperature: 280 0 C
g: Shear rate
t Residence time
LDPE
1414.7/S
0.0005654 s
Die length (mm)
Following the procedure outlined above and using the relationships for the different
shapes of the die channels concerned, a blown film spider die was simulated (Figure 5.9).
On the basis of these results it can be determined whether these values exceed the boundary
conditions at which melt fracture occurs. By repeating the simulations, the die contour
can be changed to such an extent that shear rate, shear stress, and pressure drop are
within a range, in which melt fracture will not occur. Figure 5.10 and 5.11 show the
shear rate and the residence time of the melt along the flow path [37].
The results of simulation of a spiral die are presented in Figure 5.12 as an example. As in
the former case, the die gap and the geometry of the spiral channel can be optimized for
the resin used on the basis of shear rate and pressure drop.
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