process feature | Melt fi ltration


Table 1: Comparison of common fi lter media Wire Mesh Medium


Surface medium (fi ltration takes place on the surface only)


Fixed pore size (classifying medium, sharp cut off)


Removes hard particles only (soft particles are pushed through)


Short on-stream life (fi xed count of pores, surface fi ltration only)


Lowest pressure drop, highest porosity


Pleatable Lowest cost Crude form of fi ltration


Sintered Random Fibre Metal Medium Depth fi lter medium (fi ltration takes


place both on the surface and inside the medium depth)


Tapered pore size with tortuous path (broad removal capability)


Removes both hard and soft particles; tapered pores catch sheared gels or smaller particles across the depth


Longest on-stream life (broad pore size distribution, depth fi ltration)


Low pressure drop, 75% porosity on average, excellent permeability


Pleatable Medium cost Excellent fi ltration characteristics


or both directions? Will it be processed at high stretch ratio and high speed?


 Is there a gel specifi cation on the fi nal fi lm or end product?


 Is the end-product used in an industry trending tighter gel specifi cations or trending to thinner gauge?


Filter media


The fi rst decision to make when choosing a melt fi lter is the type of fi lter medium, as it is at the heart of any melt fi lter. Of all the components in a fi lter system, the fi lter medium has the most infl uence on performance. Since almost all melts are at temperatures beyond the range of polymeric materials, metal is the main material used in melt fi ltration. There are many choices when it comes to metal medium, and understanding these choices is critical to comparing and selecting a media type that best suits the application. The three most basic choices in fi lter media are wire mesh, sintered random fi bre metal, and sintered powder metal. They are outlined and


THESE IMAGES, AT 200 TIMES MAGNIFICATION, SHOW (FROM LEFT): FIBRE MEDIUM,


SINTERED METAL AND WIRE CLOTH


Sintered Powder Metal Medium Depth fi lter medium (fi ltration takes


place both on the surface and inside the medium depth)


Tapered pore size with tortuous path (broad removal capability)


Removes both hard and soft particles; tapered pores catch sheared gels or smaller particles across the depth


Medium on-stream life (broad pore size


distribution, depth fi ltration, but fewer pores than fi bre metal media)


Highest pressure drop, 40% porosity on average


Cannot be pleated Highest cost


Excellent fi ltration characteristics


contrasted in Table 1 above. If the end-user knows the nature, size, and count of the impurities, this matrix can be used to determine the type of fi lter media required. However, the nature and count of the particles to be fi ltered is usually unknown, especially when the feedstock source is unstable or includes recycled material. In such cases, it is better to use a fi lter media such as sintered random fi bre medium, which can remove a broad range of particles.


Filter systems


The two most common technologies are screenchang- ers and high-area melt fi lter systems. Screenchangers are typically compact-style rotary equipment with multiple moving parts. They house a cavity that includes a small wire mesh screen, which the polymer fl ow passes through. When plugged, the dirty screen can be replaced with a new one. In some basic models, screen replacement requires process interruption; in more sophisticated models, the replacement can be accom-


Photo 1: This illustrates a view of a 40 micron abolute


rated random fi bre media at 200 magnifi cation


40


Photo 2: This illustrates a side view of a 40 micron


absolute rated sintered metal at 200 magnifi cation


FILM & SHEET EXTRUSION | December 2012/January 2013


Photo 3: This illustrates a 165 x 800 wire cloth, rated at 40 micron absolute. The magnifi cation is 200x


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