Cleaning screws and barrels
Contamination in a continuous process like extrusion can take a profitable piece of business to un-profitable in no time at all. And once contamination begins, it is almost impossible to stop without aggressive intervention – namely either a screw pull and clean, or use of a purging compound. And, an unplanned downtime event like pulling a screw due to contamination, will wreak havoc on operations – labor, planning, logistics, unhappy customers, etc...
The best way to combat contamination is not to make it in the first place. Using a purge compound at regular intervals will keep contamination from beginning, and it typically only takes 1-3 barrels full of purging compound to prevent or correct contamination. Most operations would much rather have a planned purging related downtime, than an unplanned (or even planned for that matter) screw pull.
To purge out contamination, the first step is to select a purge agent grade that works in the same temperature as the production resin, to minimize downtime caused by waiting for temperatures to climb/drop. The production resin should be purged out, and the hopper should be wiped clean. Next, under slow screw speed, add 1-3 barrels full of purging compound to the hopper. Continue running slowly until purge compound is observed in the purge pile. If using a glassfilled grade, remove the screen pack and die (if polished or textured). Plug barrel vents if at all possible to increase pressure in the area of the vents. If the vents cannot be plugged, it may be helpful to add purge to the vents to increase cleaning locally.
If using a mechanical purge, increase screw speed to maximum levels to create the highest shear environment possible. This promotes scrubbing of the screw and barrel. Stop and start the screw several times to promote agitation. Repeat the process until the purge is clean purge compound, then chase immediately with production resin. Resume production when purge compound has been completely removed.
When using chemical purges, the process is slightly different. Increase front zone temperatures to the recommended level and purge out the production resin. Next, slowly introduce the purging agent until it is observed in the purge pile. At this point, stop the screw and let the material soak in the barrel – each supplier recommends a different soak time for their product, so speak to a technical representative for recommended soak times. After the purge agent has undergone its chemical reaction and expanded into hard-to-reach areas, increase the screw speed to maximum safe speed. This process should be repeated until the purge compound purge pile is free of contaminants.
Material and color changes can similarly be dramatically shortened using a purging compund. Many companies still use the old method of changing color or material – just push the next material behind it, and keep running until good product is produced. However, operators using a purging compound to eliminate the previous material or color will gain a competitive advantage due to greatly reduced changeover times and material reductions. The procedure is the same as is used for removing contamination. Most purging compounds are designed to remove stubborn carbon and color deposits in low pressure areas after vents, as well as dead spots within the screw/barrel that virgin resin simply cannot address. Just imagine a standard 4-6 hour color change reduced to 20 or 30 minutes using a purging compound! The reduced scrap and downtime will more than pay for the price of the purge. Just contact any purging compound supplier – their technical sales reps should be able to answer your technical questions as well as assist you with a cost anlysis.