Some Thoughts on Purging

A reader asked for some information on purging LDPE in his twin screw extrusion process. By and large, the screws within a twin screw extruder are “self-wiping” due to the close contact between elements and the high shear rates caused by this close contact. That being said, there are regions within the screw design that are low-shear regions which do not receive the same wiping action as the high-shear regions. These low-shear regions are thus susceptible to build-up of carbon, color concentrates, or any other additive used in the process.

Various “purge compounds”, such as DuPont’s 6611, are commercially offered that purportedly abrade and scour polymer flow surfaces, thus obviating the need to disassemble and clean the machinery. If this were an effective option, it would definitely be a preferable alternative to the alternative mechanical cleaning method. My experience is that polymeric/other purging techniques are of some limited value, and in cases where there is minimal build-up and obstruction, it is an effective alternative, especially when purging through the die.

One exception to this rule is if you have the option of removing the screen changer/head components and do not have to purge through the downstream equipment. In this situation, rigid acrylic pellets are a very effective purge medium and can save hours of downtime to the processor. These materials are easily found (just Google “acrylic purge compounds”).

Because of fluid dynamics principles, such as “the velocity at the wall is zero”, it is very difficult to remove materials that are firmly adhered to metal surfaces with purge compounds alone. One way to improve your chances of success in purging the extruder/die is to use what has been called “The Disco Purge Technique”, first discussed by Al Soutar of DuPont some years ago. With this technique, the flow rate (screw speed) is drastically varied from high to low to medium to low to high, etc in an effort to alter the flow patterns (shear stress) at the walls so as to dislodge build-up in those areas. This, in combination with high-viscosity, abrasive-filled purge compounds like 6611, have proven to be successful and can help eliminate a complete machine disassembly in many operations.

Again, if the build-up is not severe, purge compounds can be effective. If the build-up is sever and “caked on” so to speak, mechanical intervention is likely needed. When I say “mechanical intervention”, I mean either the use of brass shimming materials to clean the die lip area or full mechanical disassembly of the die and components to clean them manually.

Belief it or not, this subject is probably half the reason why I make consulting visits to clients. The more preferable way to reduce or eliminate the need to mechanically disassemble the die and other components is to modify start-up and shut-down procedures so as to minimize thermal exposure of the resin. Excessive thermal history will degrade polymeric materials, quite often causing carbon build-up, which can result in die lines gauge bands and product imperfections in the finished product. All this being said, it is an unavoidable reality that the die and extrusion components muse occasionally be disassembled and mechanically cleaned to return the equipment to a “like-new” state.

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