process feature | Melt filtration


Typical uses in speciality resin and film


l More than 95% of the PET film industry uses high-area filter systems with disc-type elements made with sintered fibre type media


l Over 95% of the polycarbonate resin and film industry uses a combination of sintered fibre and powder medium in high-area melt filter systems with candles or disc-type elements


l Almost every optical film or resin uses sintered fibre or powder medium with a high-area melt filter system


l High-end PP film manufacturers tend to use sintered fibre media in candle and disc format, while low-end manufacturers use wire mesh medium in candle and disc format. This is primarily because there is no known solvent that can dissolve PP


l Virgin polyolefin resin producers use screenchangers with wire mesh medium for melt filtration. This requires downstream producers of high-quality film to use finer melt filtration


plished with limited interruption to the flow. There is also a range of continuously indexing rotary-style screen changers with back-flush capability, which enables a plugged screen to be back-flushed with clean polymer for multiple re-use. In spite of the large variety in style and sizes, there


are two basic challenges inherent to all screenchang- ers: they offer very limited filter area, and; they can use only wire mesh as the filter medium. Screenchangers offer anywhere from 0.1ft2


to 20ft2 of


filter area with 60 microns and coarser wire mesh medium. Typically a screenchanger is sized for a flux rate of 4000lbs/hr/ft2


. Shear rates this high limit the ability of


the filter to capture anything. It also results in melt shearing and can impact film or resin colour index value. For high-viscosity melts, a limited area screen changer will offer a high pressure drop, which requires the use of a large melt pump.


Table 2: Screenchangers vs. High-area melt filter systems Screenchanger


Limited to wire mesh as filter medium Only removes hard particles of certain size and above;


no gel removal ability Very high flux rate, typically 4,000lbs/hr/ft2 Residence time less than 2.5 minutes


Very high pressure drop due to limited filter area On-stream life in hours/days Rotary equipment


Uses screens that require disposal Cannot handle product, grade, or micron rating changes 42


High-area melt filters High-area melt filter systems come in a variety of styles and sizes and can be bulkier in comparison to small screenchangers. They are available in simplex-type (single housing with pleated candles or discs) or duplex type (dual housings connected with valves for continuous operation). When using a simplex unit, the process must be interrupted to change dirty housings and candles. Continuous equipment enables shifting the polymer


flow from the dirty housing to a clean housing by deploying flow diverter valves at the inlet and outlet. A high-area melt filter system (duplex or simplex)


can use pleated cartridges or disc-type filter elements that can use either of the above mentioned media choices. They offer a very large filter area in comparison to screen changers (1-1500ft2 for a flux rate of 30-75lbs/hr/ft2


). Typically they are sized .


The biggest benefit of such systems is that they can be


customised to fit end-user requirements of size, performance, on-stream life, and residence time. The two systems are compared and contrasted in Table 2 below. In the end, selecting a melt filter can be compared to buying an insurance policy for your car: if you can count on the distance you will drive every day, the road and weather conditions, and the driving habits of other drivers, then buying a low-coverage policy might suffice. But if your car will be driven by other members in your family, on a wide variety of roads and weather condi- tions, it is better to buy a policy that offers across-the- board coverage.


At the end of the day, if your product passes through


a melt filter, careful consideration should be made in making the best choice for your application. A supplier with the right knowledge and expertise can be crucial in helping you make the best choice. l Raj Shah is global market leader for polymers at Pall Corporation. ❙ www.pall.com


High-Area Melt Filter Systems Can use any type of metal media Primarily used for gel filtration


Very low flux rate, typically 50lbs/hr/ft2 Residence time less than 2.5 minutes


Very low pressure drop due to high filter area On-stream life in days/months Static equipment


Uses filter elements that can be cleaned and re-used for many cycles Can easily handle any changes, including process upsets and feedstock changes


FILM & SHEET EXTRUSION | December 2012/January 2013 www.filmandsheet.com


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