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Purging of Extruders

Modified on Saturday, 31 January 2015 02:39 AM by mpieler Categorized as Consultants Corner
Although purging an extruder with one material to remove another is certainly not new, it seems timely to present some reasons why purging is done, to provide some purging techniques, and to point out some limitations of purging.

One major reason for purging an extruder is to remove one material with another to save the expense of disassembling, cleaning, and assembling the extrusion system (i.e. screw, adaptor, die, etc.).

Another reason for purging an extruder is to remove a high melting resin such as nylon 66 with a lower melting polymer like nylon 6 so the extruder can be shut down at a lower temperature and therefore minimize thermal and oxidative degradation. In addition, restart of a full machine will be easier.

Purging may also be done to remove chemically or thermally unstable resins from the extruder (usually with polyethylene) so a cast acrylic or other purge compound can be used to clean the extruder.

Purging is usually achieved without any difficulty if the resins are in the same family (e.g., 2GT and 4GT polyester or HDPE and LDPE) and the purging material is of equal or higher melt viscosity than the resin being replaced. If the new material is of another color it should be darker than the previous one to prevent die and weld lines from appearing in the extrudate.

Purging a resin with a dissimilar material (e.g. nylon 66 purged with polyethylene) may be too expensive since defects like die lines, gels, discoloration, etc. may persist in the extrudate for hours or even days and the product quality may not be suitable for sale. Unless special purging techniques accomplish the resin changeover it will be more economical dismantle and clean the extruder.

One rather unusual purging technique which has proven successful in removing nylon from wire coating and film set ups is to purge with wet low melt index polyethylene (ca. one cup water per 50 pounds of PE). The water apparently reduces the adhesion of the nylon to steel and it hydrolyzes the nylon to a more fluid state making it easier to remove from the system. Care must be taken to stay away from the die since frothing and spitting of the melt will occur.

When the melt appears to be free of nylon particles dry low melt index PE is extruded until the melt is clear. Then a normal PE extrusion can be made.

Purge compounds for different applications are offered by companies like duPont (Alathon PE661 1), Rapid Purge Co. (Rapid Purge), Claude Bamberger Molding Compound Corp. (Bamberko Purge Compound) and others (1). These companies should be contacted for specific purging instructions.

Purging is often used to remove resin from the machine prior to dismantling and cleaning the system. The machine temperatures are maintained above the melting point of the material being removed while low melt index PE is being extruded. When the polyethylene melt appears homogenous either one of the following procedures may be followed. In the first case the extruder temperatures may be decreased to about 350° F (176° C) and the machine is purged with rigid PVC until the polyethylene is removed. The extruder is then run dry and the heaters turned off. The PVC is pulled away from the extruder components as they are removed. This procedure eliminates the necessity of burning out the die parts etc. as is usually the case with nylon, PE, and many other resins.

The second method is to purge the extruder with cast acrylic purge material following the PE purge. Molding or extrusion grade acrylic resin cannot be used as a purge material because their molecular weight and therefore melt viscosity is too low.

Purging of cast acrylic through a screen pack or die is not recommended because of potential high pressure development. Therefore these components are removed before extruding the purge compound.

The acrylic purge procedure consists of running the extruder screw at fairly high speeds and feeding several scoops of acrylic purge material to the hopper. The barrel temperatures must be at least 500° F (260° C) or more to prevent excessive motor load (high amperage) during purging. The compound is added until the PE is removed from the extruder and the acrylic purge extrudes as a powder. The extruder is then run dry and the screw is removed. Both screw and barrel are wiped free of any adhering acrylic particles before reassembling the extruder.

A word of caution! It is highly recommended to purge the extruder with PE before using the acrylic purge compound. Some resins like the acetate react chemically with the acrylic purge compound and form lachromators as well as cause excessive gassing.

- Robert M. Bonner

See also:
  • How effective are purge compounds
  • Melt block problems
  • Purging of extruders (1993)

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