processing | Purging compounds
Above: These images from Asaclean show a PS screw before (left) and after purging
to talk about their purging practices, and many still prefer to make their own recipes. “Use of purging compounds in compounding is still a developing area, although we do have well-known resin compounding customers,” he says. “Some resin companies recom- mend their customers to use a fractional melt polyeth- ylene for cleaning (purging for 1-2 hours – lots of wasted resin), or even PVC for a screw change that will be inviting contamination and gas. It’s only understand- able if their customers get these resins from them.” Asaclean products are distributed by Velox in Europe
and the company recently launched online shops dedicated to Asaclean products. Available in English, French and German, the shops include a product finder tool, cost-savings calculator, and manuals and videos on how to purge. One of the oldest purging products around is Dow
Chemical’s Unipurge DFDB-0964 Natural 7. Dow only supplies the product directly for purging cable extrusion equipment. However, processors outside of the wire and cable industry that have tried Unipurge report that it works on compounding lines and also on film and profile extrusion lines and injection moulding equip- ment. While Dow developed the compound specifially for use with cable grade polyethylenes, it also appears
Right: This
glass reinforced PX2 grade by
Asaclean is said to provide a good option for
carbon removal from process- ing screws
to work well on lines processing materials running at higher temperatures. Recommended upper processing temperature limit is 250°C. “The use of Unipurge DFDB-0964 Natural 7 purge
compound just prior to equipment dismantling can reduce clean-up downtime significantly,” Dow says. The compound is said to strip easily from metal compo- nents. The company recommends that it is used periodically for preventative maintenance and also claims that it “dramatically” reduces the amount of scrap generated when changing from a lower melt index material to a higher melt index material, or from one colour to another. Dow recommends that in a routine program, on a 110-mm extruder purged once a month, around 45 kg of material is needed. Dow does not say exactly what Unipurge is, other
than a compound based on polyethylene, but informed opinion says it most likely contains a calcium carbonate filler and a slip agent (and possibly other additives). Augie Machado, Laboratory Manager at the US subsidi- ary of compounding machinery maker Leistritz, says it is not cheap (no commercial purging compound is), but it is cost-effective, especially on small extruders. While he believes the only way to totally get rid of
contamination is to take the extruder apart, he regularly trials different purging compounds and is a fan of the Unipurge product. He is also supportive of chipped acrylic, which he says does a very good scouring job. “But it doesn’t go through the die, so you have to take that off first.” When it comes to high temperature polymers like PEEK, PEI, PES and the like, he says ultra-high molecular weight polyethylene does a good job. Cost can certainly be an obstacle, however. Machado says it is easy to understand why compounders will often try to find home-made purging solutions. With prices for commercial products sometimes close to €10/kg, a purging time of around a minute on a machine with an output of 2,000kg/h, will cost well over €300 for a single purge. “We all know that in principle it’s the right thing to do, but then the real world gets in the way.”
58 COMPOUNDING WORLD | January 2016
www.compoundingworld.com
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