Purging compounds can be a valuable aid in cleaning plastics equipment. They can help cut downtime and boost productivity. However, they are not a panacea, and in some cases processors rely on other ways of cleaning screws, barrels and hot runners.
Developed some 30 years ago, commercial purging compounds addressed the shortcomings of using cleaning materials such as regrind resin and even wood shavings. The compounds clean equipment of old resin and deposits during changeovers, shutdowns and startups.
Frank Van Haste, vice president of Novachem in Bridgeport, Conn., recommends a systematic, technical approach to purging to avoid the more drastic approaches of running large volumes of resin to flush a system, taking a system apart for mechanical cleaning, or using a purging compound only when problems pop up.
Analysis will help a processor see how much an unresolved purging issue affects productivity and costs.
``The processing industry is under a great deal of pressure from globalization,'' Van Haste said by telephone. ``Those processors active in the U.S. market have to be constantly vigilant to be competitive.''
Novachem supplies the major classes of purging compounds: chemical cleaners and products with a mechanical, scouring action. Depending on the problem, either type could be suitable to clean screws and barrels in injection molding, extrusion and blow molding machinery.
``Everyone needs purging compounds, but they need to be judicious as to what they choose,'' Van Haste said.
Some processors, he said, are not big fans of commercial purging compounds. But, he said, unsatisfactory experiences could have resulted from an inappropriate product selection or inadequate technical support.
RapidPurge, a U.S. pioneer in chemically active purging compounds, said flushing a machine with scrap resin doesn't lead to any real cleaning. At best, it displaces some plastic. But machinery needs a true chemical purge to loosen carbon deposits from metal surfaces, said the Stratford, Conn., firm. Maintenance purging is becoming more common to reduce the potential for carbon buildup, according to RapidPurge.
Asaclean-Sun Plastic Inc. of Parsippany, N.J., focuses on mechanical-type purging agents, which engineer John Pizzo said have a high affinity for deposits but are gentle on metal finishes. Such purging compounds are useful in multicavity hot-runner molds, still a fast-growing area of the injection molding market. For multicavity systems, a mechanical purger can be introduced and run with the mold closed, to give the benefit of machine pressure for cleaning. Many of Asaclean's products are moldable, Pizzo said.
``Processors need purging agents more than ever,'' Pizzo said. ``They can't afford downtime.'' Just-in-time manufacturing and an increasing number of changeovers for most processors means they can find cost and material savings with a purging program, he added.
Film production machinery supplier Brampton Engineering Inc. of Brampton, Ontario, generally does not recommend purging agents to its customers. ``Our system has few hang-ups and flushes well,'' said service manager Winston Lewis. But Brampton realizes that some film makers use them, so it's important to be aware of the purging compound's properties, he warned.
When running film lines in-house, Brampton typically uses a thermally stable, fractional-melt-index low density polyethylene to scour resin out of a system.
Shuman Plastics Inc. of Buffalo, N.Y., claims its mechanical type of compounds, sold through its Dyna-Purge business, are nonabrasive. One Dyna-Purge customer, Cy Plastics Works Inc. of Honeoye, N.Y., runs a dozen injection presses up to 420 tons and two blow molding machines processing engineering resins at temperatures up to 600° F.
The company is a short-run production specialist and purges many times a day for color or material changeovers. Its biggest recent challenge was changing from a black thermoplastic elastomer to clear high density PE in a blow molding machine. The previous practice was to pull the head for cleaning and to purge the barrel with about 65 pounds of polypropylene. The cost per purge was $321.
With the Dyna-Purge product, Cy Plastics eliminated streaking in 40 minutes vs. four hours for PP resin and no longer removes the head for cleaning - since about 35 pounds of the purging compound cleans the whole system, the company said. The average cost to purge fell to $174.
Custom injection molder Nypro Inc. uses purging compounds in many of its operations. But Nypro takes a more rigorous approach in its advanced technology center in Clinton, Mass., said the center's director, Doug Thorpe. The center is a clean room environment for molding clear acrylics and other resins for aesthetically demanding medical and other parts.
With the purging compounds, there was uncertainty as to when a screw and barrel were clean. Purging agents also tended to fume up and boost airborne particle counts in the clean room, Thorpe said.
For major changeovers, tech center operators remove the screw and barrel of an injection press and slap in a replacement - at a cost of about $10,000 a pop. The dirty screw and barrel are cleaned in a fluidized bed system. Pulling them takes less than 10 minutes and often can be done during a quick mold change.
Thorpe is bullish on the benefits, and the tech center has been doing it that way for more than 10 years. ``We took the guesswork out of screw cleaning,'' Thorpe said.
Purging agents supplier Neutrex Inc. of Houston finds the auto market hungry for its wares, largely due to JIT demands of carmakers and Tier 1 suppliers, according to Neutrex President and Chief Executive Officer Arthur Haag. An interior parts molder might do four quick color changes in a day running copolymer PP.
``They need to have a reliable purging compound,'' Haag said. He describes his firm's main products as hybrid chemical/mechanical purgers.
Husky Injection Molding Systems Ltd. of Bolton, Ontario, generally favors chemical purging agents for its injection presses and hot runners, partly because the agents do not require the high volumes that mechanical purging typically demands, says Jeff Carriero, global product manager for hot-runner components.
With mechanical purging agents, a processor has to be very aware of clearance specs for the compound so particles in the purge don't get hung up between screw flights and the barrel or in hot-runner nozzles, he said.