The subject of polymer filtration was broached in the first article of this series. Edie (1) stated, "It is ironic that filtration is one of the oldest processes known and still one of the least understood". One has to dig deeply in the literature to find only a handful of papers on the subject published during the last twenty years.
The first article of this series explained the need for filtration in some processes. This article will describe the types of filter media one may use. Future articles will expand on the subjects of pressure drop calculation and equipment design features.
As a consultant, I am often asked, "what type of screen pack do I need in my extruder?" I, in turn, must ask what specific particles must be removed from the plastic melt and why. You would be surprised at the number of converters who can not answer either of these questions.
The first article pointed to the melt pump as an example of a device which must be protected by adequate filtration to pre vent "scoring and possible seizure". Other processing equipment such as film dies, spinnerettes, and small profile dies are extremely vulnerable to plugging by contaminants in the melt stream.
Filtration media include one or more of the following elements:
1. Wire cloth screens.
2. Sintered powder.
3. Sintered fiber.
4. Sand packs.
Of these, wire cloth screens are the most popular. Some of the important variables involved in selecting the proper screen material are:
1. MESH - number of openings per lineal inch.
2. WEAVE - standard or Dutch as well as plain or twilled.
3. WIRE DIAMETER - normally in decimals of an inch with the first being the "warp" and the second the "schute".
Standard wire cloth is the most common screen material. Screens may be produced from a number of metals or alloys depending on the application.
The mesh value determines the opening width for a known wire size. Thus a 100 mesh screen with a .005° diameter wire will have a 25% open area and .005" wide opening.
If you wish to filter solid particles which have a minimum dimension of .007", a 100 mesh screw may be your best answer. If this filtration level is much too coarse, consider a 500 mesh, filter contaminants with a minimum width down to 30 microns.
Keep in mind that a filter cloth with a wire size of .0009" diameter is structurally quite weak. Such fine mesh cloth must be backed by one or more coarse screens to prevent wire failure in tension caused by the pressure drop of melt flow.
Carley and Smith (2) have reported a method of predicting the pressure drop of melt flow through one or more screens. In many applications, a fine mesh screen of the normal breaker plate diameter will plug through the accumulation of contaminants in a very short period of time. The saw tooth pressure rise profile coupled with the down time required for screen changes has led our industry to slide plate screen changers, continuous changers, and pot type filters which can be changed without interrupting the operation.
Certain process contaminants such as high molecular weight gels can only be removed by forcing the melt to flow through a complicated cross section with a small opening dimension and a sizable surface area. Porous media such as sintered metal or sand packs stand the best chance of gel removal. Morland and Williams (3) describe "tortuosity" in their paper as the effective fluid flow path length through the media divided by the thickness of the porous media.
It appears that the effective retention of contaminants is determined by the effective hydraulic radius of an average capillary opening through the porous media as well as the fluid tortuosity ratio.
Edie (1) is cited for a method of computing pressure drop through a porous media. In turn, cites references 4 through 10 his supporting literature.
The reader is encouraged to comment on this series of articles on melt filtration. Why not write to the author c/o Box 130, Intervale, NH 03845.
Literature Cited
1. Edie, 0. "Calculation of Filter Pressure Drop and Shear". Fiber Producers Conference 1-23, (1983)
2. Carley, J. and Smith, W., "Design and Operation of Screen Packs". Polymer Engineering and Science. 18, 408, (1975).
3. Moreland, C. and Williams, B., "Selecting Polymer Filtration Media". Fiber Producer. 32-65, (April 1980)
4. Savins, J.G. "Non-Newtonian Flow Through Porous Media." Industrial Engineering Chemistry., 61, 10:18-47, 1969.
5. Sadowski, T.J. and Bird, R.B. "Non-Newtonian Flow Through Porous Media. I. Theoretical." Transactions of the Society Rheology. 9, 2:243-250, 1965.
6. Christopher, R. H. and Middleman, S. "Power-Law Flow Through a Packed Tube." Industrial and Engineering Chemistry Fundamentals. 4, 4: 422-426, 1965
7. Gregory, D.R. and Griskey, R.G., "Flow of Molten Polymers Through Porous Media." AIChE Journal. 13, 1:122-125, 1967.
8. Marshal, R.J. and Metzner, A.B., "Flow of Viscoelastic Fluids Through Porous Media." industrial and Engineering Chemistry Fundamentals. 6, 3:393-400, 1967
9. Siskovich, N., Gregory, D.R., and Griskey, R.G., "Visco-elastic Behavior of Molten Polymers in Porous Media." AlChE Journal. 17, 2:281-285, 1971.
10. Kemblowshi, Z. and Dzuibinski, M., "Resistance to Flow of Molten Polymers through Granular Beds." Rheoligica Acta. 17: 176-187, 1978.
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