The ins and outs of extrusion purging

Photo Credit: Chem-Trend L.P.

When plastics manufacturers think about purging, a lot of them probably also immediately think injection molding. And no wonder: Commercial purge compounds (CPCs) were originally designed and marketed to the injection molding industry, and it remains the dominant process for CPC usage. But purging makes as much sense for extrusion, which is a continuous process. Meaning that, once contamination begins, it’s almost impossible to stop it without the kind of aggressive intervention that a purging compound provides.

But that’s the irony – precisely because it’s a continuous process, CPCs have, historically, been used less frequently in extrusion than injection molding, which has long been considered more natural and easier for purging because it’s cyclical.

That thinking isn’t just old school, it’s from the school they tore down to build the old school. Extrusion systems can have areas with little or no polymer flow where material can build up after colour and material changes, leading to frustrating production challenges, including higher scrap rates, more downtime, and lower quality of finished products. Add that to the challenges that even the best-run extrusion shops are facing today – such as long runs and high outputs, heavy reliance on just-in-time processing, and increased pressure for cost savings on materials – and the case for performing a proper extrusion purge has never been stronger.

The good news is, most CPC suppliers offer a full product line of purging grades that address different resins as well as purging issues; and more and more have formulations that have been developed specifically for extrusion, including more aggressive glass or mineral-filled grades.

THOUGHTS ON RESIN AND REGRIND

CPCs can be used for a wide array of extrusion processes – pipe, profile, tubing, sheet, film, compounding, and extrusion blow molding – and there are some things to consider in order to choose the best purging compound for your system, beginning with the old strategies of purging with either production resin or regrind. In some situations, some CPC supplier say, either could work. “Injection purges may work in extrusion depending on the problem and the machines, considering that both have barrels and screws in common,” said Hector Sanchez, national sales manager with Asahi Kasei Plastics North America. Perhaps the best-case scenario for using resin or regrind is for pipe extruders running dedicated lines, without colour or material changes. “Since these machines have bigger orifices and bigger and less intricate dies than other types of extruders, non-CPCs can usually do the job,” said Jeff Lewis, sales and technical manager with Slide Products Inc. “But with resin prices going up, regrind is becoming the more popular choice – a lot of plants these days generate a lot of regrind, whether they like it or not, and if they don’t have a source to put the regrind back into, it makes sense to purge with it. Meanwhile, the only rationale for using production resin now is if a job is cancelled and the resin is just sitting in a box picking up moisture because the extrusion shop has no other use for it.”

For extrusion processes with frequent changeovers and more complicated dies, meanwhile, using production resin or regrind might not end well for the simple reason that, at the end of the day, they aren’t designed to clean machines. “Purging with production resin just could just create another layer over existing resins, colour deposits or carbonized material within the barrel and on the screw, and these layers build up to become additional sources of contamination, making purging even more difficult,” said Graziano Pestarino, business development director for thermoplastics with Chem-Trend L.P. “And an injection molding-grade CPC might not be very effective on a complex extrusion die because it’s designed to work with high pressures and lots of agitation, and these factors don’t occur in extrusion. Some extrusion shops used to be okay with using resin for purges even though it took longer, because the cost of resin was cheap compared to CPCs – but resin isn’t so cheap today, and supply-chain issues make commodity and engineering resins much harder to come by, so it doesn’t even make sense for some shops on that level anymore; when you consider the price-per-purge, not the price-per-pound, CPCs pay for themselves very quickly.”

NARROWING IT DOWN

Finding the right CPC for the job begins with factoring in the characteristics of a specific extruder, including the type of extrusion process and the size and processing temperatures of the machine. “Machines have different characteristics, accessories, and restrictions that require different purges,” said Hector Sanchez. “Some purges deal better in low-pressure environments and difficult zones to reach; others have viscosities and additives that scrub better, and the machine forces and flow patterns require purges to be easier to displace.”

Since using the wrong chemicals or tools can damage screws and lead to production issues and since purging procedures for the extrusion process vary depending on the grade you’re using, it’s best to reach out to your purging compound supplier directly if you’re unsure on how to most efficiently clean your extruder. “Your CPC provider should provide standard procedures for using their products,” said Bob Grzegorek, global technical services manager with Shuman Plastics Inc., which supplies the Dyna-Purge line. “We recommend running initial trials to optimize the procedure for your equipment, and we provide technical support on-site to assure the most efficient procedure.”

Another part of this decision is choosing the type of CPC to use, of which there are three categories: mechanical, which cleans by scouring with a particulate such as glass or mineral; chemical, which requires a soak time to clean by using chemical reaction; or hybrid, which cleans using the components of both mechanical and chemical purge types. And the truth is, different CPC suppliers offer, and recommend, different categories. “Typically, mechanical purges work very well for extrusion, and we also have a fast-acting chemical/mechanical hybrid for polypropylene that requires no soak time,” Grzegorek said. The only problem, some experts say, is that mechanical CPCs may contain particles that will lodge or block areas of the process such as extruder screens. A chemical-type purge, or even one that foams or expands, works extremely well in low pressure and low flow environments, which are often hard to reach and clean, CPC suppliers say, and may prove useful in cleaning the dead areas that are common in dies – the areas that, if neglected, can result in the most serious contamination. “Our rule of thumb is, if the part defect is bad enough that you can see it, a chemical purge is the better choice,” Jeff Lewis said.

SCREWS AND BARRELS, AND BEYOND

How often to purge an extruder depends on a variety of factors. “Any extrusion process involving frequent material or colour changes will require more frequent purging,” said Bruce Gervason, senior technical sales manager with Purgex Purging Compounds. Traditionally this meant profile extrusion and compounding, where changeovers were more frequent and the resins, colours, and additives were more varied. But some CPC suppliers are now reporting a rise in usage among film sheet and extrusion blow molders. “Both of these processes have added lots of colours lately to become more cosmetic, and additives to become more durable; and especially with sheet, extrusion shops are adding flame retardants, which makes the plastic almost like taffy, and very difficult to get out of the die without an effective purge,” said Jeff Lewis.

For most extrusion processes, cleaning the machine is accomplished by flushing the system with clean resin or a purge material until the system is no longer contaminated. The purge time could be as short as a couple of volumetric exchanges or as long as several hours, depending upon extruder screw/barrel wear, the die design, the presence of flow inhibitors in the system such as static mixers, and the type of contaminant. Purging the screw and barrel is typically easy and straightforward, CPC suppliers say, and the purging instructions are almost always the same for any particular CPC. The compound should be fed slowly in order to ensure that all the flights are full, and the screw speed is then increased to the maximum safe rpm once the purge starts to come out of the machine.

During the purge, periodically stop the screw and allow the purge to settle into any “dead” areas, and then, after a few minutes, start rotating the screw slowly, and then go back up to high screw speed. This stop-and-go purging – sometimes called a “disco purge” – adds turbulence to negative flow areas and is the first and easiest method of extrusion cleaning. “The disco purge can be a very effective mechanical technique, and works especially well for stubborn carbon buildup and colours that are more difficult to remove,” said Bob Grzegorek. After continuing this process until most of the compound is purged from the barrel, the pile should be inspected. “The extruder is clean and the process complete when the purging compound coming from the machine is free of contaminants,” Graziano Pestarino said.

A problem with disco purges, some CPC suppliers say, is that shops don’t always do them properly. “The operator who’s been trained to do the disco purge isn’t always the operator on that machine when it’s being done, and anytime you’re changing heats and speeds and pressures, you can run into problems,” said Jeff Lewis. And chemical CPCs now are so efficient, Lewis continued, there’s little reason to do the disco purge in lieu of a chemical purge anymore.

Some areas of an extruder can be especially challenging to purge. “Adapter tubes – which is a setup that requires resin to travel for a long distance – transition areas, melt pumps, and dies, are all areas that need to be purged very carefully to prevent resin buildup and degradation,” said Bruce Gervason. The die is usually very big and always intricate, and therefore typically the most difficult area of the extrusion line to clean, period. “A chemical purge, which foams and expands, is much better than a mechanical purge at cleaning the dead areas that are common in dies,” Graziano Pestarino said. The key is to move the purge to cover the whole die, Hector Sanchez added. “This means creating enough pressure to reach the die edges, and sometimes temperature and gauge adjustments can increase the purge effectivity here,” he said. Specifically, some experts say, raising the die temperatures by up to 50° F may help soften any deposits within the die, but this should not be attempted when running heat-sensitive resins.

Vented barrels are another part of the extruder that can be difficult to clean because the vent is an area of low pressure and lower agitation. “Sometimes a standard purging procedure will work, and other times you can feed the purge through an open vent to improve cleaning of the vent area,” said Bob Grzegorek. “In cases where the purge is flowing out of the vent, you might need to allow the purge to harden in the vent before proceeding, or you may need to close off the vent.”

OUNCE OF PREVENTION

Many extrusion shops use purging compounds only when absolutely necessary, but establishing a regular and preventative-maintenance purging program will prevent contamination build-up and other problems in the first place – and it typically only takes between one to two barrel capacities of purging compound to prevent contamination, CPC suppliers say. “Even if you’re running the same material and colour 24/7, we recommend a preventative maintenance purge to assure no build-up in low pressure areas,” Grzegorek said. “These are typically done every two weeks to at least monthly.”

Also, extruders should be purged before temporary or extended machine shutdowns, especially over weekends or holidays. “I recommend purging any type of machine that has a barrel and screw before extended shutdown, including extruders,” said Jeff Lewis. “If you don’t, you risk allowing the last thing you’ve run to degrade in the barrel. I recommend purging the extruder and then adding a bit more compound and shutting down full, which keeps out the oxygen that would allow degradation to occur. Even for shops that mainly purge with regrind and only use a CPC once in a while, a shutdown – along with screw pulls – is the best time to use it.”

And last but definitely not least, follow the instructions unique to each CPC. “Each CPC manufacturer has developed its own purging procedures that work best with their particular purging compounds, and it’s imperative that these procedures be followed for best results,” said Bruce Gervason. “This is probably the most important factor in successful purging.”

Given the many headwinds of today’s competitive plastics industry, using CPCs for thermoplastic processing should no longer be up for debate. And that definitely includes extrusion, where processors can benefit substantially from the material and time savings in downtime and scrap that CPCs offer.