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25 MICRO-LAYER BLOWN FILM COEXTRUSION DIE

by Henry G. Schirmer BBS Corporation 2066 Pecan Drive

Spartanburg, SC 29307 Tel:(864) 579-3058

E-Mail: hschirmer@att.net

Abstract: An 18” modular disk blown film die was designed from a 2x scale up of a smaller laboratory die using a client’s specifications to make a 25 micro-layer film containing alternating micro-layers of one material of no more than 25% of the total layered material. The finished 18” modular disk die was assembled in the client’s plant and attached to 2-65 mm extruders delivering the 75% material and a third 50 mm extruder to deliver the 25% material. After initial alignment of the .090” exit die gap, the first melt issued uniformly around the die annulus and a bubble was blown. There were no issues such as melt leaks and film thickness uniformity. The 25 micro-layer film product that was made tested satisfactorily in all respects. Production of the film began shortly at 640 lbs/hr mainly in 12 hour shifts but also around the clock in some cases for about 150 days while product and variations of it were made for large market evaluations. After this production run and with other product development experiments completed, a series of die evaluation tests were conducted with the die making quality film up to 800 lbs/hr.

Introduction: For the first time, a blown film co-extrusion die has produced micro-layer film in a production mode. Up to now micro-layer films have been made using a layer multiplier and a flat film-casting die. Dow sponsored research at Case Western Reserve has shown many interesting effects are obtained from micro-layers and several papers are referenced here. However, flat dies produce films that are uniaxial in physical properties due to the molecular orientation in the machine direction. This causes vast property differences in the transverse and machine directions. Blown film equalizes these differences to a large extent. Why should anyone want to make films with so many micro-layers in either blown film or cast film? First of all many micro-layers begin to approach a blend without many of the incompatibility problems encountered with blended film and most resins are incompatible when melt blended. Deterioration in properties such as clarity, strength, tear resistance, permeability are just a few of the consequences of incompatible blends. Then there are new properties or other enhancements that are gained in films containing so many micro-layers with their inherently huge surface areas and equally numerous boundaries. These effects have been reported in the referenced literature but include such things as imposing control in making foamed film and molecular transfer or influences between micro-layers. Our films lab is exploring many of these reported effects now in blown film. Until now the modular disk die design had been limited to the smaller range of die sizes from 9” and under. Film delivery rates also had been limited to 300

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lbs/hr and under. While this die is ideal for lab and smaller scale production operations, a true full-scale production blown film die design has been eluded. About 2 years ago, a client who wishes to remain anonymous, asked the question about the possibility of scaling up the lab die after making several prototype micro-layer film runs at the BBS Laboratory. This client also shortly thereafter had purchased a used 6” die for their own laboratory and was now capable of making the prototype micro-layer films they desired at a rate of 110 lbs/hr. Initial positive product tests on the micro-layer films made from this line prompted the clients’ willingness to share in the development and the costs for building a scaled-up die through a lease-purchase agreement where the die performance information was to be shared. There were no guarantees of success or die performance because this had never been done before. Machine work was started on a 2x version using the existing die design wherever practical. Costs soon proved to be more in the order of 3x rather than 2x in this scale-up. However, sales pricing still was projected to match or be lower than similar classes of existing co-extrusion dies in the market. For example, an 11-layer die was the nearest comparable die capable of possibly making a simple version of a micro-layer film. The basic principles of the cells and their design variations will be shown. Assembly of 25 cells within the die module relative to each of the 3 extruders on the clients line is also shown. Once assembled in the module, the cells control the material formation into individual layers and define the micro-layer film structure. Rate, pressure and film quality measurements defined the die performance. Average internal pressure was determined and the pressure drop along the mandrel stem was projected from this and from a microphotograph obtained of layer thickness in an empirically derived inverse relationship to pressure. From the information developed, the source of backpressure at the extruder was brought into focus and a way to reduce this at even higher delivery rates suggested. Discussion: The individual cells, their geometry and position within the module determine the film structure and quality. Once cells are locked within the module, that module will always produce the same fixed structure even if it is removed, replaced with a differently assembled module and stored for later insertion. So, even though the initial cell assembly may seem complicated and tedious, the module may be operated continuously or intermittently for long periods without a total disassembly and cleaning. The modular disk die therefore may use several interchangeable modules to make different film structures. This feature enables the user to make many different and complicated film structures in a simple way without a lot of downtime for cleaning. Cell Design- Each Cell was made from Laser cut disks designed so that melt entering the cell followed a passageway that divided it in half three times so that it finally flowed equally through 8 circumferential equally spaced holes. The melt from these holes entered an 8-pointed star spreader that formed the melt into a layer. In another design, a floating spiral that tended to be more homogeneous in distributing the melt into a layer replaced the star spreader.

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Fig. 1 shows the 2 cell designs. The standard or star cell is the most compact and was chosen for the 25 micro-layer scale-up. The floating spiral cell can get quite elaborate even to a combination of 2 opposed spirals to create a floating spiral maze for maximum layer homogenization. Compact star cells were stacked one upon the other and varied in radial position to accept melt from a given melt stream filled from a specific extruder. Once the designated structure was geometrically achieved the whole stack was then bolted together to form the removable module (item 6). The inlet plate (1) was the source of melt flowing through the module (6) and it had provision for delivering up to 12 melts from 4 ports containing 3 melt holes each. The mandrel (2) and mandrel tip (5) were adjusted in relation to the exit plate (4) by cocking within a ball and socket melt-seal using a bolt secured adjusting plate (3). The die is depicted here blowing film downward and the disks are placed one upon the other in the same order.

The above figure also shows a cross-section of the disks stacked one upon the other. The left side shows that 2 layers can be formed from 2 star cells in almost the same space as a single spiral maze cell. Fig. 1 depicts a 9” horizontal flex-lip lab die that used 6” diameter disks. A factor of 2 was used to scale up the 6” disks for the new production die. Each resulting 12” disk was laser cut from stainless steel sheet or plate depending on the thickness desired. Stainless steel plate was used for 0.25” thickness and SS sheet was used for 0.125” and below.

Fig. 1 Cell Design

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Fig. 2 shows the disk scale up. The original 4-disk cell was found to have internal leakage at higher extrusion rates. Bolting the distribution section together to its 2 cover disks stopped the internal leakage. However, the star section then also needed 2 thin cover disks (.063”) on both sides of each star layer-forming disk so as to keep the issuing melt from making contact with the 10-bolts and 2 strengthening pins of the covered distribution disk section. Once each star cell portion was secured within the 2 thin cover disks and itself bolted together this divided each cell into 2 melt contained and secured sections. When coupled together, a complete cell was assembled that behaved as 2 melt contained units within the module. Each sealed cell now had the potential of being removed from an opened module without a lot of melt contamination and cleaning problems. Each sealed cell measured 1 inch thick. Of course, while laser-cutting disks eliminated a lot of costly machining, there was slag that also had to be removed. So there was still substantial hand finishing work that had to be done before each cell could be assembled and bolted together as a unit. Because of the heat generated during laser cutting, some disks warp more than others. Grinding the surface of each disk would be a futile waste of metal. If the SS plate or sheet was free from scratches or other blemishes, the “as received” surfaces were actually flat enough to prevent melt leaks without further finishing and since the laser cut metal surfaces had been melted during the cutting process, they were microscopically smooth and required no further finishing either. Disk warping was therefore ignored in the scaled-up disks just as it had been on the smaller disks for the lab die. Bolting the 2 units of each cell tended to remove much of the disk warp anyway. This was clearly shown when the final 25 bolted cells were assembled within the module and it was compressed within a 60-ton press. Only .063” of module compression was observed. The assembled and bolted die was expected to exert this scale of compression on the module thus removing any last trace of remaining disk warp.

Fig. 2 Cell Scale-up

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The assembled module was then hoisted and secured within the die as a single unit only after 2-band heaters were fastened to it. Eight external 1.5”-12 tpi clamping studs were screwed in place and tightened with nuts to clamp the assembled module within the die. Of course, these external studs remained colder than the hotter module as the module and die was heated. As the heated module expanded with increasing temperature, the clamping force of the colder studs therefore increased. Now there is little doubt that after a year of leak free plant operation any disk warping had been removed even at the first die assembly. Further, as the module heat history increased especially now after exposure to 850F of a Beringer vacuum-cleaning oven, the initial warping caused by laser cutting has faded even further. Die Melt Inlets The die melt inlet plate is a precision machined die component that can deliver melt to the module from any number of extruders from 1 to 12. There are 4 inlet ports located every 90 degrees around this plate. Each melt port contains 3 - ¾ inch diameter melt inlet holes. Each inlet hole may admit either the same melt or 3 different melts by means of special adapters to each port. In this case, 3 special adapters were made to fit the 3 extruders. Each extruder was then connected to each of 3 ports of the die. As shown in Fig. 3, the melt delivered from each extruder was split into 3 melt streams by means of a split melt adapter at each of the 3 ports making a total of 9 melt streams now flowing through the .75” melt holes within the module. Parallel spreading of the melt delivery from one extruder in this manner is a new concept and departs from any other conventional co-extrusion blown film die melt delivery system. The advantage of parallel melt stream delivery is that the amount of melt delivered to the each of the cells was better equalized and it generally did not create a large pressure drop between cells sharing the same melt delivery stream. After all, one 65mm extruder fed 6 cells and the other fed 7 cells and the 50mm extruder even fed 12 cells. That amount of melt distribution amongst cells would most certainly create a large pressure drop problem from a single melt stream. The highest-pressure cell would receive more melt than the lowest-pressure cell. While the two or three cells sharing one melt stream were generally observed to be about equal in thickness implying an insignificant pressure drop, there were significant differences in thickness from the internal pressure drop along the long mandrel stem. This will be discussed later. Each extruder port therefore had a left (L), a middle (M) or center (C), and a right (R) melt delivery hole that extended through the module. In this case, all major component (75%) material cells from each of the 2-65 mm extruders were fed 2-cells from each of the 3 melt-streams/extruder except for one melt-stream that fed 3-cells. A total of 6 melt streams from both 65 mm extruders fed these 13 cells. The 65 mm-1 (A) extruder fed 7 cells and the 65 mm-2 (B) extruder fed 6 cells. The minor melt material (25%) 50 mm (C) extruder fed 4-cells /melt stream from a total of 3 melt streams to a total of 12 cells.

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Knowing that the melts from each of the three extruders was split into 3 melt streams making a total of 9 flowing through the module helps to understand the nomenclature used to identify each cell within the assembled module. Fig. 4 shows the entire die with the module in place. The 8-1.5”-12 external clamping studs were omitted for clarity.

Fig. 4 Overall Die Assembly

65 mm-1 (A)

65 mm-2 (B)

50 mm (C)

Fig. 3 Split Melt Adapters

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Die Assembly- Figure 4 shows 2 options that were considered to spread the melt equitably amongst the cells. Case 2 was chosen as the best because the melt would be spread more evenly between the higher output cells and even the low output cells. The 65 mm-1 extruder was designated as A, the 65 mm-2 extruder as B and the 50 mm extruder as C in this drawing. The extruder letter is then followed by a left (L), middle (M) and right (R) melt stream hole along with a cell number on each particular melt stream. Please note that each of the 9-melt streams eventually had to be terminated at some point within the module and so the final 9 cells capping each melt stream were designed to stop further melt flow at that point. These were all designated with a T in Fig. 4. All other cells within the module were designed so that these cells picked off a portion from the melt stream but also permitted the remainder of it to continue flowing further to also feed a subsequent cell or cells. In this manner each of the 9-melt streams was further split into 25 individual cells that ended in a 25-layer melt array with an alternating micro-layer structure. Further notation should be made about the overall die assembly schematic drawing of Fig. 4. A superimposed outline of the ball and socket mandrel alignment devise was outlined to show the method used to make mandrel adjustments. This was a push-pull mandrel cocking adjusting arrangement. The pull bolts insured a firm melt seal while the push bolts aligned the ball portion by sliding it within the socket. This caused the mandrel to cock so that the exit annulus was aligned using feeler gauges and then locked in position. The die annulus was designed with a flexible inner lip plate that was secured and aligned by attaching it to the melt flare portion. Once the die lip was correctly positioned by screwing both along the threaded track on the mandrel, the position was then locked in place with the back up plate. Individual set screws within the thick backup plate could then make further lip adjustments by bending the thin inner lip plate, if needed, to insure perfect die lip alignment. As it turned out, the individual setscrews were kept in a neutral position meaning that the flex-lip did not need to be flexed for good alignment during this assembly. First assembly took place at BBS and the stainless steel disks of each cell were chemically etched to positively identify each one of them. All other metal parts exposed to the melt were nickel plated after polishing. Picture 1 shows the actual first die assembly. There were many more assemblies to follow once the die was delivered to the client’s plant. We all wanted things to work right the first time. Once extruders were repositioned and the

Picture 1 First Assembly

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melt adapters attached to each extruder, all was set for installation of the assembled die and the initial start up. Unfortunately, I had to take a leave of absence for medical reasons. Since everything was now ready to attach, the cautious and dedicated R&D people proceeded to finish this task very well and I was grateful that my absence caused no delay to the startup. First melt presented no issues such as melt leaks or flow variations. The first bubble was blown and thickness measurements were on the desired target. Again there were no issues and the client was very anxious to start making product variations for his own tests. I had no problem waiting for the actual die performance tests because everything appeared to be functioning as desired. Picture 2 shows the client’s film being blown from the 25 micro-layer die at 640 lbs/hr. This was taken close to startup to show that there were no melt leakage or other issues at this rate and that the film quality was more than good enough to make ample product variations to test. Picture 3 shows a cross-section of the 25 micro-layer film being produced from the 18” die shown in picture 2. There is a gradual layer thickness increase from bottom to top. The bottom layer is the inner layer of the bubble and the upper layer is the outer layer of the bubble. While this could be a potential “issue” for some products, all concerned saw no reason why it should greatly interfere with the final film properties in this case. Later in the discussion, the reason for this layer variance in thickness will be addressed. The line was kept very busy during the first 6 months of operation and we did not want to interfere with this operation. We understand that at one point it turned into a 158-day part time around the clock and a steady 12-hour shift operation. Finally after all the variations and tests had been completed, the dust settled a bit and we had the opportunity to run tests on the die itself.

Picture 3 25 Micro-layer 0.9 mil Film

Picture 2 First Melt

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Die Performance tests- After about 9 months of continuous and semi-continuous film manufacture; we had the opportunity to finally test the die performance and to hopefully increase our understanding of things that were taking place within the die. Starting at the normal production rate of 650 lbs/hr, the die rates were gradually increased to 704, 770 and 800. Acceptable film was made and the thickness variation recorded. But at 840 lbs/hr, the bubble was unstable because of insufficient air-cooling. At this rate we were approaching a screw speed of over 100 rpm usually considered an upper limit of desirable screw speed for any sustained operation in most plants. The proprietary LLDPE resin used was a fractional .75MI resin. At a general extrusion and die temperature profile of about 400 F, the following pressure-rate data was collected at each extruder:

Die Performance at Higher Rates

106 Screw RPM

464 F Melt

101.8 Screw RPM

462 F Melt

93.7 Screw RPM

457 F Melt

85.2 Screw RPM

453 F Melt

65 mm (2)

EXTRUDER

3805 psi3755 psi3610 psi3460 psiDie Pressure

99.9 Screw RPM

406 F Melt

96.1 Screw RPM

406 F Melt

88.1 Screw RPM

404 F Melt

80 Screw RPM

405 F Melt

50 mm

EXTRUDER

800#/hr770#/hr704#/hr650 #/hrRate:

305 fpm290 fpm265 fpm240 fpmLine Speed

5030 psi4945 psi4755 psi4540 psiDie Pressure

106.2 Screw RPM

446 F Melt

101.9 Screw RPM

443 F Melt

93.6 Screw RPM

439 F Melt

85.3 Screw RPM

436 F Melt

65 mm (1)

EXTRUDER

2680 psi2615 psi2490 psi2350 psiDie Pressure

Die pressure increased at each extruder as the total output was increased from 650 lbs/hr to 800 lbs/hr. The 50 mm extruder C increased 330 psi; the 65 mm-1 extruder A increased 490 psi and the 65 mm-2 extruder B increased 345 psi. The A, B, C extruder designation used in Fig 4 is applied to the appropriate plant extruder used in these tests so that each layer of the film structure can be traced back to the appropriate extruder designation used to make it. Extruder A 65 mm-1 operated at a higher pressure and pressure difference than extruder B 65 mm-2 because it ran at a lower melt temperature. Extruder A 65 mm-1 had a lower pressure drop (1300 psi) across the screen changer but extruder B 65 mm-2 had a higher-pressure drop across its screen changer (3000 psi). The more restrictive screen changer on Extruder B 65 mm-2 is believed to be responsible for the 20 degree rise in melt temperature from this extruder because of the extra shear heat input to the passing melt. At the final rate of 800 lbs/hr all extruder screws were turning at 100 rpm or above. Referring back to Fig. 4, Extruder B 65 mm-2 fed the first 3 cells and the last 3 cells within the module. This corresponded to a total of 6 major layers, the 3 inside and the 3 outside layers of the blown bubble. Obviously, both film surfaces could be treated separately from the other internal layers of the film. Different surface sealants, slip & anti-block packages, clarified resin etc. could be issued to B 65 mm-2 extruder in different proportions from the other layers within the film. More expensive materials and blends could therefore be controlled to help reduce raw material costs.

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Extruder A 65 mm-1 fed all of the remaining 7 internal major layers of the bubble. These layers could consist of film scrap reclaim, high performance resins or anything else that might affect the functionality of the surface layers. Finally, extruder C 50 mm fed all remaining 12 alternating micro-layers with a minor amount of a special performance resin that affected the entire film properties. As can be seen, each extruder had a specialized function to maximize film performance and minimize material cost. The following thickness distribution curves show the film thickness variation around the entire circumference of the bubble using only the above .75 MI LLDPE in all extruders driven at screw speeds required to deliver the appropriate 25/75 ratio. This removed the effects of different resin viscosity and flow differences from distorting the true picture of film quality. Of course, matching resin rheology of different resins for maximum film quality is another subject that goes beyond the scope of this discussion and is best discussed on a case-by-case basis.

The above thickness scans taken from film made at 704 lbs/hr and at 800 lbs/hr. are easily compared and show that a high quality film was made and there was no deterioration due to the rate increase. These scans were similar to those monitored throughout the entire 9 months of film production even with the special

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co-extruded materials used. Comparisons of films made from the modular disk die to other films made in the same plant were considered by the R&D personnel to be generally of better quality. The die gap that was set at the startup did not require any re-adjustments throughout the entire production period even as we increased rate during these performance tests. The machine operator was delighted with the die thickness uniformity and consistency of performance that was generally considered more stable than other dies operating in this plant. It is very true that fewer manufacturing headaches make people happier. Internal Die Pressure - Total pressure measurements taken at the die inlets and at the outlets of each extruder reflected the pressure caused by the most restricted areas within the die. The pressure difference between the two reflected the pressure drop across each screen changer. The most restricted areas were thought to be at the melt inlets of each cell operating along the inlet melt paths from each extruder but we could not be certain of this until we knew that the internal pressure along the mandrel stem was low and not controlling the internal pressure of each cell. Doubling or tripling the thickness of each distribution disk within each cell would appear to reduce inlet cell backpressure. On the other hand, increasing the internal cell pressure above the pressure along the mandrel stem by decreasing the cell exit annulus would seemingly lower the layer thickness differential observed in the film. The key was to find the average pressure along the mandrel stem. The following tests conducted below show how the mandrel pressure was obtained using the C 50 mm extruder as a pressure-measuring gauge. Average mandrel pressure could be determined by using one extruder as a pressure sensing devise. If one extruder was put through a series of rate reduction steps while proportionately increasing rate on the other 2 to maintain a constant 650 lb/hr total output, the pressure of that extruder would drop at each reduction step. When the chosen extruder finally reached 0 output, that should be the average pressure at the mandrel stem. The C 50 mm extruder that formed alternating micro-layers along the entire mandrel length was chosen to sample average pressure along the mandrel stem. The screw speed of the C 50 mm extruder was gradually lowered in several steps while the pressure on both sides of the screen changer was recorded. The rates of the 2 calibrated A & B 65 mm extruders were adjusted upward at each interval to compensate for the reduction in output from the 50 mm extruder and to maintain a steady output of 650 lbs/hr. Graph 1 with its tabulated C 50 mm extruder data shows the pressure reduction progression on both sides of the screen changer as its screw speed was reduced. These converged to a common pressure value of 1200 psi when the C 50 mm extruder was momentarily finally shut completely off. We took this value to be the average mandrel pressure. Clearly this was below the pressure developed at each extruder and so the restricted cell inlets seemed to be the only other possible cause of observed die pressure.

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Cell Geometry - The internal cell melt exit gaps that produced the layers were wide open at .125” and represented about (.125”x 3.14x4” diameter)= 1.57 sq inches of cell exit area. Meanwhile, the rectangular entrance area to each cell was only ¾”x .25”= .188 sq inches. Melt leaving here was split in half into 2 rectangular passageways at 2x(.5”x.25”)= .25 sq inches total cross sectional area. Each one of these was again split in half to 4 passageways widening the total melt cross section area still further to 4x(.375”x.25”)= .375 sq inches. Once again these were split to now 8 passageways widening the total cross sectional area to 8x(.25”x.25”)= .5 sq inches total cross sectional area. Lower and lower pressures would naturally be expected at each area increase. The melt issued from here through 8 – ½” melt holes that were again open in similar fashion to 8x(.5”diax.125” thick)= .5 sq inches total melt cross sectional area at the apex of each star spreader inlet point. Clearly the most restricted point of each cell was the initial inlet and this was the cause of the backpressure at each extruder. However as said before, the cell inlet could easily be doubled to .375 sq inches by the insertion of another .25” thick distribution disk within each cell needing reduced backpressure. This will be reserved for future work involving increased output with internal bubble cooling (IBC). 50 mm pressure drop @ 650 lbs/hr Screw rpm Screw psi Die psi 80 3170 2340 50 2600 1980 20 1885 1520 0 1210 1190 Knowing that the average pressure along the length of the mandrel was 1200 psi at 650 lbs/hr, the next step was to empirically obtain an estimate of the pressure at different points along the mandrel. If we assumed that the layer thickness was inversely proportional to the pressure at any given point, then measurement of layer thickness should correlate to the pressure at that point. A thicker layer would represent a lower pressure than a thinner layer at the same output. The internal pressure drop along the mandrel stem certainly appeared to be the controlling factor in governing the layer thickness distribution as shown in Picture 3. Thicker layers appeared to be located closer to the die exit where the pressure was expected to be lower and thinner layers were located closer to the other end where the pressure was expected to be higher. Certainly the layer thickness

Graph 1 Mandrel Pressure

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could be made more uniform by reducing the rather large 0.125” die gap of each star cell so that backpressure could be controlled by cell exit gap rather than the variable mandrel pressure. Another way to control layer thickness along the mandrel would be to control the input to these cells rather than the backpressure of the cell. This was experimentally demonstrated in a joint paper with the Naval Research laboratory and is cited as a reference. While the increasing layer thickness in the film was not considered to be an immediate issue affecting the film performance, sooner or later it could become an issue. So it is nice to know that there are at least 2 solutions available for future use. Judicious delivery from separate extruders is perhaps a third option. Mandrel Pressure Drop Estimates - Graph 2 Data shown below contains 2 elements: the actual cross section of film from which the thickness measurements were derived and then the position of these thicknesses according to their relative position along the mandrel stem. Graph 2 Data Estimated Mandrel pressure drop using layer thickness

Since the many thin alternating micro-layers from the C 50 mm extruder were due mainly its intended low rate, these were averaged into the adjacent thicker layers or essentially ignored. Each of the A & B 65 mm extruders delivered melt to either 6 in one case or 7 individual cells or layers in the other case. So there was a discrepancy in output between some of these layers. The total averaged number of layer thickness measured was therefore 13 not 25.

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Graph 2 shown below is a plot of the layer thickness vs relative layer position. Superimposed on this graph is an averaged straight line following the plotted points. If the average measured pressure of 1200 psi at the mandrel were represented by a relative thickness value of 2 at the mandrel midpoint, then the pressure at the thicker value of 3.25 and the thinner value of .75 could be calculated by inverse proportionality. Pressure at the mandrel stem outlet end would = 1200 psi x 2 / 3.25 = 738.5 psi. Pressure at the mandrel stem inlet end would = 1200 psi x 2 / .80 = 3000 psi

. The higher-pressure value of 3000-psi is certainly not higher than the die pressures measured on any 65 mm extruder operating at the 640-lb/hr rate and is therefore approximately consistent with these actual extrusion values. However, the 50 mm extruder C had a lower pressure of 2350 psi (see table on page 9). So the estimated 3000-psi value may be a bit too high. Graph 2 layer positions 6,7,8 were fed less melt from extruder A 65 mm –1 because these were all fed from the middle (M) melt stream of this extruder. These positions are depicted as the encircled Low Output Cells that should and actually have suppressed layer thickness values because of this division.

Low output cells

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The above evidence was obtained from Fig. 4 on page 6, the A middle melt stream (M) fed the 3 cells at positions 11-12, 13-14, 15-16 corresponding to positions 6, 7, 8 on graph 2. This meant that these three layers should be thinner because of less melt delivery per layer. All other major layers were fed from melt streams containing 2 cells so they delivered more melt to each layer and those layers should be and are thicker. If this is factored into Graph 2, it explains why positions 6, 7, 8 are thinner. Perhaps the output corrected line should really be depicted as the two thinner lines joining at position 7 where the average pressure of 1200 psi might correspond to 1.5 rather than 2. In that case, the pressures would be estimated lower as follows:

Pressure at the mandrel stem outlet end would = 1200 psi x 1.5 / 3.5 = 514.3 psi. Pressure at the mandrel stem inlet end would = 1200 psi x 1.5 / 1.1 = 1636 psi. The higher pressure of 1636 psi in this case may be actually closer to the real value. In any event, the pressure ranges both at the top and bottom of the mandrel stem give aim points to consider when addressing the problem of changing individual cell geometry to better equalize the melt flow. The main point to make here is that the actual average pressure at the mandrel tip is projected to be surprisingly low. Actual pressure measurements taken along the mandrel tip will better define this and our understanding of the die. Die Exit Geometry - While more direct measurements are planned for determining pressure drop along the flex-lip mandrel tip, the ever-expanding area of the horizontal melt path points to an even more significant pressure drop as the melt progresses to the die exit and 15 psi. As the melt flares horizontally outward along a path with a height of approximately .25” to the exit of the .090” 18” diameter flex-lip annulus, the increasing geometric volume of melt needed to fill this expanding void should be reflected in a melt reservoir at low pressure. The exit area after all is 5.09 sq in. (18x3.14x.090) compared to the melt cross sectional area along the mandrel of 3.14 sq in. (4x3.14x.25). This large low-pressure melt reservoir contained along the flex-lip mandrel tip probably is responsible for the exceptionally good thickness control of the films made throughout the year of filmmaking. This melt reservoir also provided a buffer zone so that there was no need to make one thickness correction adjustment during the operating period. The main operator of the film line even made his own observation after operating the line for about a year and commented that he thought it was unusual that the melt should exit the die evenly around the circumference during each startup. This was an acute often neglected observation most likely another manifestation of the buffering action of the large melt reservoir just before its exit. Plans are to provide a series actual small pressure tap holes along the flex-lip mandrel tip exit route or better still provide a thicker test plate with several pressure gages along its radius to make actual pressure measurements within the blown bubble along this buffer zone of melt. The good results obtained from this die certainly warrant a more definitive further understanding of it.

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Summary and Conclusions:

1. The Modular Disk Die was built double the scale of the existing laboratory design and installed in a client’s plant where it produced 25-microlayer films at production rates even up to 800 lbs/hr for about a year without an issue.

2. Film quality including thickness variation was equal to or better than the other existing dies in the plant where the die was installed.

3. The ball and socket die gap adjusting system prevented melt leakage while permitting the accurate adjustment and locking of the external die gap into a permanent setting that lasted during the entire production period of 25 micro-layer films manufacture without the need for making any other adjustments.

4. Each micro-layer was formed from a cell consisting of laser cut stainless steel disks that needed no further machining except for some hand-work to remove slag along with some drilling and tapping for securing bolts on the distribution section.

5. Laser cut disks warped to varying degrees because of the high cutting temperature. Bolting the distribution portion disks of each cell helped to flatten these disks and prevent internal cell leakage at the steady production rates of 640 lbs/hr.

6. Adding cover disks to the star disk layer forming portion of each cell kept melt from interacting with the 10 bolts that unitized the distribution portion and made the outside surface of each 2 section cell free of the melt contained within.

7. The average mandrel pressure at about 400F was experimentally determined to be 1200 psi at 650 lbs/hr using a 0.75 Melt Index LLDPE.

8. The estimated pressure drop along the mandrel stem at 650 lbs/hr was empirically derived from the average stem pressure and a graphical plot depicting the 13 averaged micro-layer thickness measurements within the film. Straight-line inverse relationships between pressure and averaged layer thickness were superimposed on the measured layer thickness distribution plot and empirically yielded a low-pressure of 739-514 psi at the top exit of the mandrel and a high-pressure of 3200-1636 psi at the bottom inlet to the mandrel.

9. The low pressure values at the mandrel exit coupled with the ever expanding volume of the horizontal melt path along the flex-lip mandrel tip point to a surprisingly low pressure melt reservoir of melt approaching the exit gap.

10. The buffer action of the melt reservoir along the exit path of the flex-lip mandrel tip was most likely responsible for the excellent quality of the film produced and for keeping the die gap stabilized and without the need for correction for the entire year that was observed and reported.

References:

1. Ethylene-Octene Based Foam-Film Structures via Micro-layer Co-extrusion – Renate, Hiltner, Baer, Barger, Dooley ANTEC 2006.

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2. Structure-Property Relationships in Coextruded Foam/Film Micro-layers – Renate, Hiltner, Baer, Bland ANTEC 2004

3. Comparison of Irreversible Deformation & Yielding in Micro-layers of PC with PMMA & Poly(styrene-co-acrylonitrile) Kerns, Hsieh, Hiltner, Baer J. of Applied Science Vol.77, 1545-1557 (2000)

4. The Modular Disk Co-extrusion Die – Schirmer Polyolefins 2000 5. New Compositions of Matter from The Modular Disk Co-extrusion Die-

Schirmer, Love, Schelling, Loschialpo ANTEC 2000 6. Breathable Polymer Films Produced by the Micro-layer Co-extrusion

Process Mueller,Topolkaraev, Soerens, Hiltner, Baer J. Applied Science Vol. 78, 816-828 (2000)

7. Microlayer Coextrusion Technology Baer, Jarus, Hiltner ANTEC 1999 8. Modular Disk Coextrusion: Production Rate Tests with the 9” flex-Lip

Die Schirmer Future-Pak 1999 9. Oxygen Barrier Enhancement of PET Through Physical Modification

Sekelik, Nazarenko, Stepanov, Hiltner, Baer ANTEC 1998 10. Novel Structures by Layer Multiplier Co-extrusion - Nazarenko, Snyder,

Ebeling, Schuman, Hiltner, Baer ANTEC 1996 Acknowledgements: Credit is given by BBS Corporation to the Research and Development, Management and Plant people who contributed so much to this effort both with their time, untiring work, suggestions and financial backing to make this project actually happen. Without them we would not have moved this development so far and so fast. Thank you very much for your help even if I am not at liberty to publish your names. Credit also is given to Clyde B. Taylor who was part of the BBS team responsible for the precision machining of the critical parts of the die and to Dennis Dunagin who was also a BBS team member responsible for overseeing and finishing of all the Laser-cut disks that were assembled into the micro-layer cells that were the heart of the module that defined the film structures made from the die.