Maro Publications

Melt Filtration



Maro Topics


Patents with Abstracts

Polymeric Melts




“Melt filters are used in compounding to remove foreign particles of contamination or burnt polymer or additives & fillers/reinforcements. Melt filtration is critical for strand pelletizing, as imperfections in the process could cause a strand to break, for conversion processes such as thin film or fibers, or for any product requiring high quality that is free from contaminants. Pelletizing systems are used to produce pellets for feeding into another device. Cooling may be either in air, on cooling belts or water. Typically, pellets do not have tight dimensional tolerances as compared to final products. Strand pelletizing is the most widely utilized and versatile technology. An extruder forces the melt through a strand die which is then pulled through a cooling medium by feed rolls and cut by a bed knife/rotor assembly. Strand pellets are typically in the 3-4 mm range, but can be as short as 1 mm and as long as 25 mm based upon the comparative speeds of the feed rolls and rotor in combination with the number of rotor teeth. The strands are typically cooled in a water bath, or sometimes on a waterslide. Prior to entering the pelletizer, an air stripper device removes moisture from the surface of the strands. The end product is a cylinder die face pelletizing where the extrudate is cut at the die face while still molten and then cooled via air or liquid. The advantage of this system, if workable, is that strand breakage during the cooling phase is eliminated. If the process is amenable, die face pelletizing is often preferred for high volume and reclaim applications.

 There are essentially two types of melt filtration:

• Discontinuous melt filtration

• Continuous melt filtration

Discontinuous melt filtration: Discontinuous melt filters are used where screen changes are rarely needed, and it is not a problem to stop the production when changing screens. Small compounders use these manual or hydraulic slide-plate or slide-bolt screen changers because they cost less and are simple to operate. The major limitation of manual screen changers is that they require to be changed often to prevent contamination. This demands processing equipment to be closed for the time it takes to change the screen changers. Hydraulic screen changers are more popular as they do not require shutting down the line. They briefly interrupt the flow as the hydraulic system quickly inserts a clean screen.

Continuous melt filtration: Allows for filtration of most all polymers without interrupting production during a screen change. The continuous screen changer consists of two screen bolts with each containing a filter cavity. When a screen change is required, one of the bolts is moved out of the housing while the other remains in the operating position. The dirty screen pack is removed and replaced with a new one. The bolt is then moved back into the housing to its venting stages before resuming operation. These steps are then repeated for the other bolt. This concept allows for continuous filtration with no interruption or loss of production. This type of melt filtration system is more expensive compared to discontinuous melt filtration system. When screens need to be changed more frequently, compounders are likely to find that continuous systems more economical over their service life. A major benefit of continuous melt filtration systems is reduction or elimination of large pressure changes in the extruder. In discontinuous screen changers, the pressure builds as dirt begins to collect in the screen, spikes during the screen change, and drops when a clean screen is inserted. Strand pelletizers require very consistent flow to avoid strand breakage. Die-face or underwater pelletizers are less sensitive, although large pressure changes increase the chance for die freeze-off and misshapen pellets. Continuous melt filtration system types include single- or dual-piston, continuous belt or ribbon, and multi-segment rotary disc. Large-area or candle filters are used in polymer production to provide very fine filtration that can even remove polymeric gels, but these are rarely used in compounding.”

(Plastemart, Melt Filtration, 6/27/2012


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(RDC 6/5/2012)


Roger D. Corneliussen

Maro Polymer Links
Tel: 610 363 9920
Fax: 610 363 9921


Copyright 2012 by Roger D. Corneliussen.
No part of this transmission is to be duplicated in any manner or forwarded by electronic mail without the express written permission of Roger D. Corneliussen

* Date of latest addition; date of first entry is 6/27/2012.