Continuous melt filtration offers better quality and longer production outputs

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.

Developments in continuous systems
Cofit�s design for its AP AutoPurge automatic, self-cleaning, dual-channel, continuous screen changer, uses valves to divert flow to one filter while the other is being cleaned. In the new design, material flows in a straight line, avoiding dead spots and the orifice is enlarged to enable handling high contaminant levels. Cylindrical screens with three to four times the surface area of flat screens are able to catch more contamination. Such systems offer advantages for recycled materials as well as for large volume of long run production cycle. The particularities of this screen changer are that the filter is made out of a micro-perforated plate, easily available on the market and comparatively cheap and the patented system of cleaning the plate that makes the screen changer self cleaning and continuous. From time to time, the screen changer itself cleans the filtering plate from all the hold impurities thanks to a rotating scraper. This happens with the maximum simplicity without any operator's intervention and, above all, it guarantees a high final quality of the plastic material.
The AP.R screen changer has two possible working modalities:
Automatic, Pressure Controlled Mode: The most commonly used system is that which allows a gradual build up of the impurities on the micro-perforated filter plate, thus producing a constant increase in pressure: when a predetermined threshold is reached the cleaning of the filter by the scraper begins automatically.
Continuous Mode: The self cleaning system can also work in a continuous mode, in which case the rotation of the scraper carriers is non stop, cleaning the plate continuously. This type of use is advised when there is a very high concentration of the impurities in the molten material, or when the material or the system of working requires a constant operating pressure.
Advantages
• Pollution free processing : The cleaning, by scraping, of the filtering plate makes possible an accurate processing of materials, even those very polluted with metallic residues, wood, paper, textile fibres, stones and other impurities. The screen changer allows the recovery of even highly polluted materials and can be used in combination with other Cofit series screen changers to achieve a high purity material.
• Self Cleaning: An internal device in the filter periodically cleans, with maximum simplicity, the filtering plate of all impurities. The possibility of cleaning the plate, avoids the need for its frequent substitution, reducing the necessity of intervention by an operator and above all guarantees a high final quality
• Automatic: All cleaning operations take place automatically, thanks to an internal PLC, hence it allows a reduction in the costs of management and labour, it intervenes only when necessary, avoiding waste of time and materials.

Gneiss introduced next-generation versions of its SFX Magnus and RSF genius continuous, rotary disc filtration systems. The improved models offer significantly reduced energy consumption, and the new heating design promotes an even melt temperature.
RSF Genius: Is a fully-automatic and process-constant filtration that meets the highest product quality demands all the time. The filter elements can be re-used up to 400 times due to the integrated self-cleaning system. Fully-automatic operation practically excludes operating errors or other disturbances caused by otherwise necessary manual interventions.
SFX: Is a fully-automatic and process-constant filtration that meets the highest product quality demands all the time. Screen changes are carried out without influencing the production process. Fully-automatic operation practically excludes operating errors or other disturbances caused by otherwise necessary manual interventions.

Kreyenborg has several types of screen changers. The range includes
Discontinuous Screen Changers: The Main application fields include:
• quality filtration of virgin polymers
• protection of downstream equipment (pump protection)
• Masterbatch production and compounding (small batches)
Continuous Screen changers: The Main application fields include:
• Direct extrusion (films, fibres/straps, profiles, sheets, tubes, etc.)
• Compounding (underwater-, strand-, watering pelletizing)
• Raw material production (polymerization)
• Recycling (genuine recycled material and production waste/in-house recycling)
Continuous Backflush Screen Changers: The Main application fields include:
• All kinds of recycling (pelletizing and direct extrusion)
• Processes with high demands concerning the process constancy
• For increasing the screen life times in polymerisation and direct extrusion
Pressure and volume flow constant Backflush Screen Changers: The Main application fields include:
• Very sensitive processes with high portion of recycled material (fibres/straps/non-wovens, films, especially PET recycling) for ensuring a high process and volume flow constancy.
Kreyenborg has introduced a melt reservoir system combining a back flush, piston-type screen changer with a melt pump in order to provide constant pressure and flow volume. This system, as well as Kreyenborg�s V-type screen changer is intended for direct extrusion processes. In some applications the use of a KREYENBORG melt reservoir offers an alternative to the V-type screen changer. In combination with a continuous backflush screen changer and a melt pump it constitutes a control oriented unit and provides a pressure and volume flow constant melt filtration. In this combination the melt reservoir lives up to its name. During the production - shortly before a backflushing becomes necessary - it constantly takes a small quantity of the melt flow and stores it. This process is affected by a fine control, so that there will be no pressure fluctuations in the following process. In that moment in which the backflushing is affected at the screen changer and the flushed material is removed from the process, the melt reservoir will release the stored melt for compensation. Thus a continuously constant volume flow behind the screen changer and hence in the following process is ensured. The use of a melt reservoir instead of a V-type screen changer depends on the exact process data. In principle, existing backflush screen changers may be upgraded by a melt reservoir for pressure and volume flow constant filtration. It is suited for almost all polymers, especially PET. They find application in: very sensitive processes (productions of packing straps), compounding, fibre production, direct extrusion, and processing of an especially high portion of recycled material with high pollution degree (PET bottle flakes).

Extruders and pelletizers running recycled materials can greatly benefit from the use of the continuous backflush screen changer (CSC/BF) from PSI-Polymer Systems Inc. The CSC/BF provides maximum melt filtration capabilities, separating the impurities out of the material and allowing processors to continue running with no downtime. Because of the unique design of the CSC/BF, it provides as much as 25x the life span of the screen pack filters compared to standard styles of screen changers. This allows processors to confidently process recycled materials without fear of clogging the system. The CSC/BF is the ideal product as processors don�t need to shut down their extruders or pelletizers due to clogged screen packs. With the continuous backflush technology, the material being processed is divided into two melt streams each with two filtration cavities. When upstream pressure rises due to a high contaminant level, PSI�s Extrudepak PLC control automatically actuates each piston to its backflush position, allowing pressure to push contaminants off the dirty screen pack. The contaminants are purged out of ports in the housing, and the purged/cleaned screens are returned to their original position until the backflush cycle takes place again. The CSC/BF features guaranteed leak-proof operation to 10,000 psi and operate with process temperatures to 700�F. Because there are no seals or contact parts, PSI provides a five-year warranty against melt leaks. They are available in sizes ranging from a screen diameter of 4.58" (116 mm) to 9.07" (230 mm).