PRODUCTION | PURGING COMPOUNDS
Right: Dyna-Purge’s compounds can very effectively remove black specks from processing machines
some cases lead to carbon deposits, often a result of excessive oxidation,” he says. “As an example, many black or blue pigments
are carbon-based. They tend to stain and molecu- lar bonding can take place. Without regular purging, these pigments can build up on metal process surfaces, eventually breaking loose leading to product contamination,” Grzegorek explains. “Allowing oxygen to enter the process system is
frequently the cause of carbon formation. When- ever you have extended down time, it is best to purge the system entirely, leaving the purge compound to insulate the screw and barrel,” he says. “A non-chemical acting, heat stable purge compound allows the extruder to fully seal the entire process from feed throat to die, minimising oxygen risk to the system. This reduces the risk of polymer oxidation which leads to the formation of carbon deposits.”
Home or custom The next decision that needs to be taken, accord- ing to Grzegorek, is whether to use a homemade purge, or one of the pre-formulated, engineered grades of commercial purging compounds (CPCs). “As a provider of such compounds, we see the value of using a commercially designed purge over a homemade solution,” he says. “In the vast majority of trials, CPCs outperform homemade purge or a simple resin used for purging. The overall cost of using a commercial compound versus other methods is by far more cost-effective. CPCs require less volume to be effective and work faster as well.” Ultimately, the best approach is to carry out a cost comparison. Grzegorek says this can be done using calculation models hosted on CPC suppliers’ websites. These will capture downtime and scrap, as well as raw material costs, labour and resin use, allowing a decision to be made on which method is the best value option. “It is best to do your research, and work closely with your provider of choice. They will work with you to recommend the best CPC for your specific process, resin and application. Follow that up with proper trials and document your results. Finally, there will be opportunities for the provider to suggest fine tuning and optimising your purging process to enable you to achieve optimised results. Heat settings, RPMs, pressures and torque can all be considered for this,” he says. A CPC supplier will also likely be able to suggest
process tips that may enhance purging perfor- mance, such as varying the screw RPM to allow the purge to loosen and grab a hold of the residue
70 COMPOUNDING WORLD | September 2019
build-up from the screw and barrel. This will help it to be flushed out. The final step is to post-purge to remove the purging compound and ensure the system is clean and ready for the next production run. “Professional commercial purge suppliers can provide the resources and support to assist with all the steps necessary to improve your production performance,” Grzegorek says.
Tackling small runs Technology advances in both resins and additives have influenced how compounders run their materials, as well as run size, according to Joseph T Serell, President of RapidPurge. “Many compound- ers are being asked to accommodate smaller runs so that their customers – injection moulders and extruders – can address the needs of their custom- ers. This advancement and specialisation has meant that many compounders are faced with more frequent changeovers,” he says. The result is a more urgent need to quickly remove resin and additives from the previous run in order to begin the next one. He says the need for a high quality purging compound becomes even more a priority as many of the latest additives and resins can be more difficult to remove than their predecessors. Serell explains that purging a compounding
machine, in particular a twin-screw extruder, can be much more difficult than purging an injection moulding machine. “In response to our many compounding customers, we have just introduced a product that has three-pronged approach to cleaning these machines. Firstly, a proprietary mineral blend is used that is tough enough to remove most resin or contamination, including black specs and brown streaks, while not being as aggressive on the metals of the machine as glass fibre is. A customised expanding agent is also
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PHOTO: DYNA-PURGE
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