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«Viscoelastic flow effects in multilayer polymer coextrusion Dooley, J. DOI: 10.6100/IR555718 Published: 01/01/2002 Document Version Publisher’s ...»

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Compare that structure with Figure 5-26 which shows a two-layer concentric ring polystyrene structure near the exit of the teardrop channel. This figure shows that the black layer along the walls has thinned considerably while more material is flowing to the “corners” of the teardrop structure. The flow in these figures follows the trends shown earlier for flows of elastic materials in teardrop channels. The recirculation vortices should move material along the channel walls until it reaches the corners where it should move more toward the central axis of the teardrop.

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Figure 5-27 shows a two-layer concentric ring polystyrene structure near the entry of the rectangular channel. This can be compared with Figure 5-28 which shows a two-layer concentric ring polystyrene structure near the exit of the rectangular channel. Note that the major deformation that has occurred near the exit of the channel is the movement of the black layer along the central axis toward the center of the channel. This demonstrates how much stronger the vortex near the center of the channel is compared to the vortex near the ends of the channel.

–  –  –

Figure 5-28 also shows that there was less deformation in the rectangular channel than there was in the teardrop channel. This has been a consistent result in these experiments regardless of the type of layered structure used in the experiment.

Figure 5-29 shows the progression of a two-layer concentric ring polystyrene structure as it flows down a rectangular manifold channel (with the entry shown on the left and the end of the channel on the right). In the first cut on the left in the figure, the intact concentric ring structure can be seen. As the structure moves down the channel, the black and white layers can be observed to flow out of the bottom of the manifold channel. Note that by the time that the structure has flowed to near the end of the manifold channel, only black material that started near the top of the channel at the entry remains.

Figure 5-29. Progression of a two-layer concentric ring polystyrene structure as it flows down a rectangular manifold channel Figure 5-30 is similar to Figures 5-23 and 5-24 since it shows a two-layer concentric ring structure near the entry and exit of the square channel. However, this figure shows the results for the polycarbonate resin rather than the polystyrene resin. This figure shows how little deformation takes place in a less elastic material like polycarbonate when compared to polystyrene. Even though the square channel shows the largest layer deformations in the more elastic materials, very little layer deformation is seen in the polycarbonate material.

Figure 5-30. Two-layer concentric ring polycarbonate structure near the entry and exit of the square channel Figure 5-31 shows a two-layer concentric ring polycarbonate structure near the entry and exit of the teardrop channel. There seems to be very little difference in the structures at the entry and the exit of the teardrop channel when running polycarbonate. Once again, this figure shows how little deformation occurs when running a material like polycarbonate that is lower in elasticity than the other resins.

Figure 5-31. Two-layer concentric ring polycarbonate structure near the entry and exit of the teardrop channel There are several advantages and disadvantages in using a structure with concentric rings compared to planar layers or a matrix of strands to examine secondary flows in different geometries. The main advantage of the ring structure compared to the planar structure is the symmetry of the concentric ring structure. The ring structure also has advantages over the multistrand structure because it allows deformation very near the channel walls to be studied. The main disadvantage of the concentric ring structure is the inability to observe layer deformations near the center of the flow channel. This problem can be overcome by adding multiple concentric rings to the structure so that layer deformation can be studied from the center of the channel to the walls while still maintaining the symmetry of the ringed structure.

Thirteen concentric rings5.5

To take advantage of the benefits provided by studying concentric ringed structures, a new feedblock was developed that produced a structure with 13 concentric rings. This style of feedblock used the same principles as those shown in Figure 5-20 but with more plates added to produce more concentric layers, similar to the technique shown in Figure 1-2. An example of the output from this feedblock is shown in Figure 5-32. This figure shows a thirteen-layer concentric ring polystyrene structure in which alternating layers have been pigmented black and white to allow observation of the interfaces. This structure will allow examination of elastic layer deformation across the entire channel while maintaining the symmetry of the structure.

Figure 5-32. Thirteen-layer concentric ring polystyrene structure near the entry of the circular channel Figure 5-33 shows a thirteen-layer concentric ring polystyrene structure near the entry and exit of the square channel. Note that the structure near the entrance of the square channel (on the left in Figure 5-33) has rings that extend across the entire structure so that layer deformations across the entire structure can be observed. The structure near the exit of the channel is shown on the right side of Figure 5-33. This shows in great detail the flow patterns that occur in each quadrant and how they follow the secondary flow patterns shown in Figure 4This figure shows that the thirteen-layer concentric ring structure allows observation of the deformations that occur across the entire channel as well as showing the symmetry of the deformations in the channel.





Figure 5-33. Thirteen-layer concentric ring polystyrene structure near the entry and exit of the square channel As was shown in the layered and strand structures, the layer deformation continues as the ringed structure flows down the channel. This is shown in Figure 5-34 for a thirteen-layer concentric ring polystyrene structure. These samples were taken at intervals of approximately

10.16 cm from the entry of the channel (shown in the upper left hand corner of the figure) to near the exit of the channel (shown in the lower right hand corner). These images show how uniformly the layers deform as they move down the channel.

–  –  –

Figure 5-35 shows a thirteen-layer concentric ring polystyrene structure near the entry and exit of the rectangular channel. Note that in the sample taken near the exit, the layers near the center of the structure are still parallel and have not been substantially deformed. However, there is more deformation of the layers near the edges of the channel. The layer deformations in the channel show that material is pushed from the edges towards the middle of the channel along the main axis which results in the oval layers that were present near the entry of the channel becoming more rectangular shaped near the end of the channel. This figure also shows, however, that the smaller secondary flow areas in the corners of the channel are also trying to move material from the center of the channel towards the edge of the channel along the main axis. This results in the small projections seen along the centerline of the main axis near the edges of the channel. The projections are formed from a portion of a ring near the wall that was stretched in opposite directions by the influence of the secondary flows near the wall. This flow pattern can also be observed in Figure 5-5, which shows a 165-layer structure in a rectangular channel, but it is less obvious in Figure 5-28 that shows the two-layer concentric ring structure.

This once again shows the details that the multiple concentric ring structure can show compared to the other structures.

Figure 5-35. Thirteen-layer concentric ring polystyrene structure near the entry and exit of the rectangular channel As was shown in Figure 5-35 for the rectangular channel, Figure 5-36 shows a thirteenlayer concentric ring polystyrene structure near the entry and exit of the teardrop channel. The structure near the entry is shown on the left and the structure near the exit is shown on the right.

The structure near the entry of the teardrop channel shows uniform layers that change from a circular shape near the center of the channel to more teardrop shaped layers near the walls.

These layers are deformed significantly in the structure shown near the exit of the channel on the right. The structure is very symmetric about the centerline.

This figure shows how the secondary flow fields can move material from near the center of the channel to near the walls, and vice versa. For example, the central white circular area shown in the middle of the channel near the entry is stretched vertically into a line until the ends of the line are near the walls of the channel. In contrast, the outermost white ring near the entry has had the part that was near the tip of the teardrop deformed to the point that it is nearer the center of the channel near the exit.

Figure 5-36. Thirteen-layer concentric ring polystyrene structure near the entry and exit of the teardrop channel Figure 5-37 shows the progression of a thirteen-layer concentric ring polystyrene structure as it flows down a rectangular manifold channel. This figure is very instructive in showing how the material flows down and out of this manifold channel. Recall that this channel was designed to mimic the flow in a coat-hanger style film or sheet die and so material not only flows down the manifold but also out the bottom of the channel. This can be seen by counting the number of black rings that are visible as the flow progresses down the channel from the entry (left) to the exit (right). Near the entry, all six black rings are visible since the flow out of the bottom of the channel has just begun. In the next cut, only the top halves of the six rings are visible since the bottom portions of the rings have all flowed out of the bottom of the channel.

As the flow progresses down the channel, the number of rings decreases from six near the entry to only one near the exit. Once again as was seen with the layered and strand structures, the material that started at the top of the entry channel becomes the majority of the entire channel near the exit.

This figure also shows some similarities to the flow patterns seen in the rectangular channel with the thirteen-ring structure shown in Figure 5-35. The projections of the black layer observed near the ends of the rectangular channel also show up as a projection in the topmost black layer in the rectangular manifold channel. This is understandable since the aspect ratios of the rectangular channel and the rectangular manifold channel are both four to one. The only difference between the two channels is the addition of the flow out of the bottom of the rectangular manifold channel that affects the final flow pattern. This final flow pattern is a combination of the flow out of the bottom of the channel superimposed on the secondary flow patterns seen in the rectangular channel.

Figure 5-37. Progression of a thirteen-layer concentric ring polystyrene structure as it flows down a rectangular manifold channel

5.6 Concentric ring feedblock with tapered square channel Since the thirteen-layer concentric ring structure proved to be very useful for showing layer deformations across an entire die channel, it was decided to maintain the use of this structure to look at the affect of modifying the square channel geometry. The square channel geometry used for all previous studies maintained constant dimensions (0.95 cm by 0.95 cm) down the entire length (61 cm) of the flow channel. In order to magnify the secondary flow effects, a new channel was designed and fabricated in which the square channel tapered from

0.95 cm on a side to 0.475 cm on a side, effectively reducing the flow area at the channel exit by a factor of four compared to the entry. This should cause an increase in the flow velocity and shear rate as the flow progresses down the channel, which should increase the secondary flow effects. This flow now becomes more three dimensional since the channel dimensions are now changing in all three dimensions.

Using the same processing conditions as were used previously for the straight square channel, an experiment was run using the tapered square channel. Figure 5-38 shows the results for the thirteen-layer concentric ring polystyrene structure near the exit of the straight and tapered square channels. The two images are shown in appropriate sizes to indicate difference in final dimensions between the straight and tapered channel. Note that more extensive deformation of the layers has occurred in the tapered channel as compared to the straight channel. This indicates that the secondary flows are stronger in the tapered channel than in the straight channel.

Figure 5-38. Thirteen-layer concentric ring polystyrene structure near the exit of the straight square channel and the tapered square channel In order to show a better comparison, Figure 5-39 shows the thirteen-layer concentric ring polystyrene structure near the exit of the straight and tapered square channels with the tapered channel expanded to the same size as the straight channel. This figure makes it easier to observe how much more deformation occurs in the tapered square sample. For example, note how much farther the white material has moved down from the corners along the 45 degree diagonals toward the center of the channel in the tapered square sample compared to the straight square sample.

Figure 5-39. Thirteen-layer concentric ring polystyrene structure near the exit of the straight and tapered square channels with the tapered channel expanded to the same size as the straight channel

5.7 Conclusions This chapter has discussed the phenomenon of elastic layer rearrangement. This effect is produced in non-radially symmetric geometries when processing elastic materials due to secondary motions that are produced by an imbalance in the second normal stress difference.

The next chapter will deal with measurement of the layer rearrangement velocities due to elastic as well as viscous forces.

Chapter 6 Comparison of layer rearrangement velocities 6 Comparison of layer rearrangement velocities



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