Porous metal filters for polymer melt service are available in three basic configurations:

• disc shaped segments
• small diameter simple cylinders
• pleated high area elements

This article deals with segments which are the design of choice in critical applications requiring fine filtration, long on-stream life and premium extruded polymer product quality. Pall Corporation manufactures segments utilizing two different stainless steel media types:

• FS Series PMFTM porous metal fiber medium features a profiled pore construction. Coarse sintered fibers provide effective prefiltration, protecting the continuously varying downstream areas. The inner area contains pores of a consistent diameter, providing reliable absolute rated filtration. This unique structure maximizes the dirt holding capacity and maintains a high permeability. FS Series PMF medium is available in removal ratings from 2.5 to 20 microns absolute. See Table 1 for removal ratings.

Table 1: 'FS' Series PMF Metal Fiber Segments Competitive Comparison

Pall Grade	Efficiency		Nearest Competitive Grade	Efficiency
	98%	100%		98%	100%
FS025	1.2	2.5	1 µm	1	3
FS050	2.5	5.0	2 µm	2	6
FS075	4.0	7.5	3 µm	3	8
FS100	6.5	10.0	5 µm	5	12
FS150	8.5	15.0	10 µm	10	17
FS200	13.0	20.0	15 µm	14	20

• H Series PSS^® sintered metal powder medium is optimized specifically for polymer melt service. This medium has a higher void volume than powder metal media manufactured by conventional methods. As a result, it provides a higher dirt holding capacity and lower pressure drop than traditional media. PSS media are the strongest, longest lasting polymer filter media available. Operating experience has shown that media of this type offer good gel control characteristics in addition to fine particulate removal. H Series PSS medium is available in removal ratings from 8 to 25 microns absolute. See Table 2 for removal ratings.

Table 2: H Series PSS® Powder Metal Segments Competitive Comparison

Pall Grade	Efficiency		Nearest Competitive Grade	Efficiency
	98%	100%		98%	100%
H150	8	15	10 µm	9	20
H250	15	25	15 µm	17	30

To complement the high performance filter media that are now available, it is necessary to develop suitable drainage and support hardware designs. Their functions are to promote free drainage of polymer filtrate out of the filter medium and to provide a clear, unobstructed flow path to the segment hub and central core. In addition, the design must provide mechanical strength and stiffness without adversely affecting pressure drop, or hindering the economical packing of filter area into the multi-segment assembly.
A recently introduced, unique design approach is Pall's slotted support system (patents applied for). Used with either type of filter medium, the slotted support design objectives are:

• Minimize residence time to reduce polymer holdup and degradation
• Minimize pressure drop
• Improve flow distribution uniformity
• Maximize physical strength and durability

Display of FS Series PMF™ Segment Filters (2.5 to 20µm removal rating) and H Series PSS® Segment Filters (8 to 25µm removal rating). In progress, the assembly of a slotted support segment filter stack
Pall's Slotted Support* Component -- support member of SegmetTM and SegmaxTM Filters, enhances filter performance

The slotted support and its relevant design parameters are shown in Figure 1. A polymer flow model was developed to assist in optimizing the design to meet the above objectives. The complexity of this multivariate optimization is apparent from a consideration of the opposing effects each parameter has on filtration performance.

Figure 1: Slotted Support and Drainage Segment


Number of slots: Maximizing the number of slots minimizes the flow path that filtrate must travel to enter the nearest slot, hence, minimizing the residence time. Practical manufacturing criteria, however, limits the maximum number of slots that can be cut in the support plate.

Slot width: As slot width increases, the pressure drop decreases, but the residence time increases. The maximum slot width is limited by the extent of deformation of filter medium and drainage layer into the slot. The choice of drainage medium also influences the maximum slot width.

Plate thickness: Increasing the plate thickness increases the segment stiffness and reduces pressure drop. On the negative side, it increases the polymer residence time, and also, increases the overall segment thickness. This latter result may reduce the number of segments that can be packed into a fixed housing, hence, limiting total filter area.

Cutaway View of a Segmet Filter
Cutaway View of a Segmax Filter

The product of slot width and plate thickness is slot cross-sectioned area which, when multiplied by slot length, gives slot volume. Figure 2 illustrates the net effect of slot width and plate thickness on pressure drop and residence time.

Figure 2: Slotted Segment Optimization


Varying the pitch diameter and slot radius leads to a minimum residence time, as shown in Figure 3. At each pitch diameter, there is an optimum slot radius. The pitch diameter is defined as the locus of points from which slot radii are projected.

Figure 3: Slotted Segment Optimization


The optimization program takes into account all of these factors, many of which have opposing effects in predicting the net result of a specific set of parameters. By repeated iterations, it is possible to determine the optimum design parameters that will minimize polymer residence time without sacrificing pressure drop or physical strength. Furthermore, this permits the use of the profiled pore fiber metal medium, which is thicker than conventional media. The low pressure drop of the slotted support, coupled with the high dirt capacity of the FS Series PMF medium, combines the most desirable characteristics needed in a high performance polymer melt filter.

To determine the improvements in segment stiffness, a series of simple load versus deflection measurements were conducted. The results, summarized in Figure 4, show that the segment with the slotted support is about three times stiffer than a conventionally supported segment relying on a coarse mesh. This comparison is based on a slotted support optimized by the procedure described previously. In operating service, it is less prone to twisting and buckling, which results from considerable shear forces generated by high viscosity polymer flow. Further enhanced by suitable spacers between segments, insures that the full filter area is available at all times. It also insures that the sealing load is borne by the segment hubs and never by the outer perimeter of the disc. This can occur in less rigid designs

Figure 4: Segment Deflection


and have a greater tendency to warp. Finally, the overall ruggedness and durability that the slotted support imparts to the segments, reduces the possibility of damage during assembly, disassembly, and cleaning.

A brief review of some technical data helps illustrate the benefits of the slotted support. Consider the clean pressure drop and maximum residence time data presented in Table 3 for a few different grades -- both with and without slotted supports. This data was generated by the polymer flow model used in the support design optimization program. Many interesting comparisons can be made:

• The slotted support reduces the pressure drop by 18% for the finest grade of fiber metal (2.5 micron) and by 48% in the case of 20 micron fiber metal.

• The maximum polymer residence time (the time it takes polymer entering at the outermost edge of the segment to flow through the filter medium, drainage layer, support member and hub into the core) is reduced by a factor of nearly three for a 30 micron fiber metal segment.

• A 2.5 micron fiber metal segment with the slotted support has a lower pressure drop than a conventionally supported 5 micron segment. The program printout for a sample run is shown in Table 4.

Note: The reduction in pressure drop results from savings in both drainage and hub pressure drops. In this particular application, the slotted support reduces the maximum residence time by a factor of 3.

Table 3: Representative Technical Data 12 Inch Diameter Pall Segments

Media Type	Grade µm	Support	Clean Delta P (1) PSI/LB/HR/FT2	Maximum Res. Time (Min) (2)
PMF	2.5	Slotted Mesh	16.8 20.5	3.7 9.6
	5.0	Slotted Mesh	11.7 15.4	3.8 10.0
	7.5	Slotted Mesh	9.0 12.6	4.0 10.6
	10.0	Slotted Mesh	7.8 11.4	4.1 11.1
	15.0	Slotted Mesh	5.2 8.6	4.8 13.9
	20.0	Slotted Mesh	5.0 8.3	4.9 14.7
	30.0	Slotted Mesh	3.8 6.9	7.1 22.5

(1) and (2) Based on: Viscosity = 1,000 Poise, Density = 1 G/cm³, Flow = 10 LB/HR/FT²


Table 4: Sample Program Run

Inputs:	12” with PMF FS200 Media
	Polymer Viscosity: 1,500 Poise Polymer Density: 1.34 G/cm3 Flow Density: 15 LB/HR/FT2

Clean Pressure Drop	Slotted Support	Mesh Support
Media & Drainage	62 PSI	94 PSI
Hub	20 PSI	46 PSI
Total	82 PSI	140 PSI
Maximum Residence Time	4.6 MIN	13.1 MIN

The accuracy of these predictions has been confirmed through actual operating experience where possible. Many other observations can be made from this data, greatly assisting in the sizing and selection of polymer melt filters.

The flow simulation model has been used to calculate the flow gradient across the surface of the segment. Figures 5 and 6 show a comparison of flow gradients for 12 inch diameter segments, with the slotted support and with a conventional mesh support. Clearly, the slotted support has significantly enhanced flow uniformity. There are two chief benefits:

• Life on-stream is further enhanced by utilizing the entire surface area at a far more uniform flow density. In contrast, a mesh support tends to overload the areas nearest the hub and underutilize the outermost periphery, reducing the real dirt capacity and; hence, on-stream life.

• Most polymers tend to degrade, to some extent, at the high temperature conditions of melt extrusion. Since the slotted support flattens the flow gradient, it greatly reduces the potential effect on molecular weight distribution that a wide flow distribution may have on the polymer. Since most of the residence time is downstream of the filter medium; and, assuming that this is the final filter, any degradation products formed in the center of the segment remain in the final polymer product. By minimizing the residence time, the slotted support minimized this risk.

Figure 5: Radial Flow Gradient


Figure 6: Radial Flow Gradient

Filter Segments are Used for the Following Critical Applications:

• Polyester Film for Audio and Video Tape
• Polyester Film for Reprographic and Photographic Products
• Optical Grade Polycarbonate for Compact Discs
• High Performance Polyolefins for Multilayer, Coextruded Packaging Films
  

The Proprietary Filter Medium Provides:

• Absolute Removal Ratings
• High Dirt Holding Capacity
• Reliable, Consistent Performance

 

The Unique Slotted Support Provides:

• Minimum Residence Time to Minimize the Risk of Polymer Degradation
• Minimum Pressure Drop
• Uniform Flow Distribution
• Strength, Stiffness, Durability