processing feature | Top tips for twin-screws actions[2] . Chemical purges work by attacking the plastic
residue like a solvent. Many of them also incorporate a foaming action, and are intended to be run to fill the machine, and then the extruder drive is shut off for around five minutes to allow the chemical action to work. The drive is then restarted to run the purge out of the front end. Mechanical abrasive purges work by scouring and scrubbing material off the metal surfaces, using friction and shear. Most processors do a lot of trial and error experimentation to find the right purge material and technique. Since there is such a wide variety of plastics and additives being processed, there is no “magic” purge material is going to work best for all materials. Cost is also a factor. Commercial purge products are
Figure 5: It is very important to select a suitable anti-seize for spline shafts
Figure 6: Screw set being
pulled from
extruder after purging
small gap tolerance, plus low viscosity polymers tend to creep down between adjacent elements and get into this gap. With time and heat, this polymer forms a high-strength adhesive just like an epoxy. If screws are left together in an extruder without being taken apart periodically, then they can be extremely difficult to remove from the shafts. This is why the choice of anti-seize is so important. Many types of anti-seize are acceptable for a short period of time, but when used over long periods at high temperatures, they can degrade into a fine powder. This makes them a poor choice for spline shafts, where you need the anti-seize to perform when you’re ready to disassemble the screws, which can be months or years after it was applied. After testing a very wide range of anti-seize brands on
the market over a period of many years, Leistritz has found one which is ideal for this application: Gleitmo 820, which is manufactured by Fuchs, a leading global supplier of lubricants (www.fuchs.com). The product is a white grease containing high solids, with very slippery qualities. It is rated for 1,150˚C. What makes it perfect for spline shafts is that it does not degrade into a powder. When the screw elements are removed after months of use, the Gleitmo 820 is still a slippery grease[1]
. 1 3 Purging techniques
Purging is a technique widely used as an easy way to clean out materials from the extruder and die to avoid a time-consuming strip-down cleaning. There are many types of materials used for purging, with both commer- cial purge products and “home engineered” purges being used extensively (Figure 6). Purge materials generally fall into two categories:
chemical purges, and mechanical abrasive purges. Many materials combine both chemical and abrasive
30 COMPOUNDING WORLD | June 2012
generally more expensive per lb/kg than “home engineered” purges. However, if you can use less material with the commercial purge, or get the job done quicker, then the total cost may not be more. Probably the most important habit to get into is to
purge often. The object is to prevent old materials from being “baked on” to the metal surfaces. Once this happens, purging will generally not be able to remove
the material, and a manual strip-down cleaning will be the only remedy. One mistake many processors make is to run the
purge only at a fixed screw RPM. It’s much more effective to vary the screw RPM, alternating from low to medium to high speeds, running for perhaps 30 seconds at each level. This induces different shear rates against the metal surfaces, helping to dislodge old material from the walls.
1 4
Home-brewed extruder/die cleanout methods
Processors have run all sorts of materials through extruders, in the quest for a cheap and effective purge material. For instance, in the Midwest USA many operators use cow corn, because it is readily available and does a fairly good job of scouring the machine out.