processing | Purging compounds Fractional melt high density polyethylene is also a
quite good solution, says Machado, even for products such as polyamides and polyesters. “You are right at the edge of its processing window though,” he says. Some processors will add 15-20% of a mineral filler to provide extra scrub, but glass fibres are not recommended, as they are too abrasive. The quest to formulate in-house seems to inspire the imagination on the part of formulators. Machado’s colleague, Leistritz US Engineering Manager Bert Elliott says: “Compounders have also been known to use homemade purges such as walnut shell blast media mixed in with some LDPE, diatomaceous earth mixed in with some LDPE, a slurry of powdered kitchen cleanser/water, even cow corn. Compounders are always looking for the most inexpensive solutions, of course, so lots of times they’ll keep off-spec resin or
re-ground start-up waste around for purging.” Both he and Machado agree, though, that sooner or
later you are going to have to take the machine apart to get it properly clean. “I have seen machines that have not been stripped for a while, and we find all colours of the rainbow inside when we pull the front off,” says Machado.
Click on the links for more information: ❙
http://ultrapurge.com ❙
www.dynapurge.com ❙
www.chemtrend.com ❙
www.rapidpurge.com ❙
http://asaclean.com ❙
www.velox.de ❙
www.dow.com ❙
www.leistritz.com
Hot tips for effective extruder purging
Read on for a few tips from purging compound developers on how to effec- tively purge compounding equipment and avoid contamination build up (with thanks to purging compound suppliers Shuman Plastics and Sun Plastech). l Seal extruders with a heat-stable purging compound during extended shutdowns. Many purge compounds will shrink when they cool, pulling contami- nation off the surfaces of the screw and barrel so that it can be flushed out during start-up. l If a screw pull cannot be avoided, use of a glass-filled grade of purging compound to clean before pulling the screw is often the safest, most efficient way to get the job done (but remember removal of the screen pack, etc., will be required). l Knowing your barrel capacity prior to purging will allow you to control the amount of purge being used. l Rotate the screw slowly to fill the flights then, once the purge starts to extrude, increase screw speed to maximize purging compound performance. Mechanical purges are most effective at the highest safe screw speeds, where agitation and turbulence are maximized. l If experiencing difficult colour change or contamination, try repeatedly increas- ing and then decreasing the screw speed.
60 COMPOUNDING WORLD | January 2016
Short bursts of agitation at higher screw speed help loosen the carbon or pigment while subsequent lowering of the screw speed allows the purge to expand and push the loosened contamination out. This procedure is often referred to as the “disco purge.” l Manually plug vents during purging to increase local pressure. If plugging the vents is not possible (or allowed), feed the purge into the vents at the same time it is being fed into the hopper. l Chemical purges may perform best for complex die heads, where pressures are often low, and for mixing areas as expansion during the soak time can allow the purge to access dead spots not reach- able by a mechanical purge. l Raising the temperature of the die by 15-30°C (while remaining within the service temperature range for both resin and the purging compound) may help remove material deposits more quickly. l Make sure that the specific purge compound is rated to use with the smallest clearance in your die. l To maximize pressure in the head during purging, leave screen packs or breaker plates in place during cleaning when possible. And here is some step-by-step advice from compounding machinery maker
Entek covering effective purging of the extruder in order to pull the screws for cleaning them and the inside of the extruder barrel. 1) Set barrel temperatures to 250°C. 2) Fill one bucket with fractional melt HDPE or PP, and another with a wax. 3) Begin the purge by feeding the polymer alone until it is coming out the end of the extruder. 4) Introduce a scoop of wax and follow up with a scoop of polymer. 5) Feed another scoop of wax and follow up with a partial scoop of polymer in order to push the wax out the end of the extruder. Determining the correct amount of polymer to follow the second scoop of wax is important as too much polymer makes it difficult to pull the screws out, while too little does not push the molten wax out. 6) Run the screws at full speed to push as much residual polymer and wax out of the extruder . 7) The purge has been successful when: l some wax pours out the end of the machine like water; l the extruder torque is extremely low (0% to 2%) at nominal RPM (300 to 500); l you can turn the screws at the motor / gearbox coupling by hand. 8) The extruder screws are now ready to be removed.
www.compoundingworld.com
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