Commercial purging compounds have been aiding color changes and removing carbon in injection molding machines since your grandfather's time. But they're not just for I/M anymore. Here's why no extrusion shop should be without them.
February 1, 2013 by Mark Stephen, editor
We’ve all heard the old myth that an apple falling onto his noggin inspired Sir Isaac Newton to start thinking about gravity. A more recent myth holds that commercial purging compounds (CPCs) are for injection molding machines only. While the first tale may or may not be true, there’s no doubt that the second is a bunch of bull. While it’s true that CPCs are considered most often for injection molding, they also can provide the same benefits to extrusion processors.
Blame the technical ins and outs of the injection molding and extrusion processes for any misunderstandings. “Historically, CPCs have been used less frequently in extrusion than injection molding because of equipment design and the fact that extrusion is a continuous process whereas injection molding is a cyclic process,” said John Pizzo, technical service and development engineer for Sun Plastech Inc. Truth is, though, when it comes to handling color and material changes and removing carbon and color deposits, CPCs can be invaluable tools for almost any extrusion shop.
First, a quick backgrounder. Purging compounds operate on either chemical or mechanical principles. In a nutshell, chemical purging compounds use reactions during the purge that have an effect on resin residues at the molecular level; mechanical purging compounds exploit differences in melt viscosity or other flow characteristics to displace resin residues from the equipment being purged.
Each method has its strong points. “Chemical purging compounds are very effective in dealing with challenging purges; as a group, however, they’re likely to involve more process complexity than mechanical purging compounds, and are likely to be somewhat more expensive on a ‘per pound’ basis,” said Frank Van Haste, general manager for NovaChem. “Mechanical purging compounds can provide excellent service and value in more routine purge situations, and they offer the added benefit of being simple to use.” These are rules of thumb that most injection molders probably know as well as their own thumbs, but they also apply to extrusion purges.
Another rule that applies to the extrusion world is that, as with injection molding, a CPC can be used for a wide array of processes: pipe, profile, tubing, sheet, film, compounding, and extrusion blow molding. “CPCs are more commonly used in profile extrusion and compounding because changeovers are more frequent and the resins, colors, and additives are more varied,” said John Pizzo. 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 many have formulations that are tailored specifically for extrusion, including more aggressive glass or mineral-filled grades.
It’s largely because of this last point, in fact, that CPCs are becoming increasingly popular among extruders. “CPCs are being used much more frequently today than 10 years ago, primarily because extrusion shops are making things that require mineral-filled grades or flame retardants to be added,” said Jeff Lewis, sales manager for Slide Products Inc. “These materials really change the mechanics of what’s going on inside the mold, causing considerable build-up on the screw.”
Why not just run a similar resin through the machine when it comes time to purge? Several reasons, actually. “Color and material changes are faster using a CPC than with the traditional method of running hundreds or thousands of pounds of virgin material through the extruder to purge the previous material,” said John Pizzo. “Most CPCs are designed to remove stubborn carbon and color deposits while ordinary processing resins cannot. CPCs can also prevent the need for screw-pulls, and can drastically reduce the time and effort required to overhaul the machine if the pull is required.”
BARRELS OF INFORMATION
So now that we know why, let’s talk about how. Purging the screw and barrel of an extruder is typically straightforward and easy, the experts say, though it tends to become more difficult when more mixing/kneading elements are used. In most cases, one to three full barrels of a CPC should be sufficient to clean an extruder. If that seems a touch vague, CPC suppliers should be able to provide estimates on how much purge to use based on the screw diameter and how severe the purging application.
To maximize performance of the purging compound, it should be fed slowly in order to ensure that all the flights are full; then, increase the screw speed to the maximum safe rpm once the purge starts to come out of the machine. Keep in mind that a mechanical purge benefits from the greater turbulence and agitation caused by higher screw rpm. “During the purge, periodically stop the screw and allow the purge to settle into any dead areas,” said John Pizzo. “After a few minutes, start rotating the screw slowly, and then go back up to high screw speed. This stop-and-go purging provides a fluctuating environment of pressure and agitation that allows a purging compound to work its best.” This is done until the purge is visibly free from any contamination.
Sounds simple, but there are areas where it can get tricky. Vented barrels are often difficult to clean because the vent is an area of low pressure and lower agitation. “If the barrel has a vent, we recommend plugging the vent,” Pizzo said. “If no plug is available, or if your company has a policy against doing so, we recommend that the CPC be fed into the vent simultaneously with its feed into the main hopper.”
THE DIE IS CAST
While the purging instructions for cleaning the screw and barrel are almost always the same for any particular CPC regardless of the type of extrusion process, the instructions will likely vary when it comes to cleaning the die. The die is, in fact, typically the most difficult area of the extrusion line to clean. “While a mechanical-type CPC is often a great choice to clean the screw and barrel, a chemical-type purge, or even one that foams or expands, may prove useful in cleaning the dead areas that are common in dies,” Pizzo said. “Raising the die temperatures by up to 50°F, or 10°C, may help soften any deposits within the die, but don’t try it if running heat-sensitive resins.” In order to build up pressure, in many cases it helps to leave in place screen packs and breaker plates (if used). “But ensure that the screen packs have openings sufficient to let the purging compound and the contamination it will remove pass through,” Pizzo added.
And a word of caution about melt pumps/gear pumps. Some CPCs have no problems passing through these high-precision devices, but it’s important to triple-check with your CPC supplier about the particular purge compound before you run it. Also, a glass-filled purge shouldn’t be run through a melt pump.
As we’ve seen, mechanical and chemical purges each have strengths and weaknesses that will impact the end result. “Mechanical purging compounds are normally efficient at cleaning the screw, but less efficient at cleaning the head,” said Graziano Pestarino, president of UltraPurge representative Moulds Plus International USA. “Chemical purges tend to be better at expanding inside the accumulator and reaching the dead spots where material builds up.” One the other hand, longer residence times are a possible strike against chemical purges. “The chemical reaction is paramount when doing a chemical purge, and that reaction usually takes time,” said Tim Cutler, vice president of the Dyna-Purge division of Shuman Plastics Inc. “Processors don’t always give the material enough time to soak, however, because it adds to machine downtime.” &nbs
But no matter where they are on the learning curve, it’s clear that extrusion processors are seeing more benefits from the material and time savings that CPCs can offer. And that’s no myth.
Dyna-Purge Division of Shuman Plastics Inc. (Depew, N.Y.);
NovaChem (Bridgeport, Conn.);
Slide Products Inc. (Wheeling, Ill.);
Acetronic Industrial Controls
(Mississauga, Ont.); www.acetronic.com;
Sun Plastech Inc. (Parsippany, N.J.);
UltraPurge/Moulds Plus International USA
(Santa Anna, Calif.); www.ultrapurge.com;