Commercial Purging Compounds: Now More Than Ever

(September, 2008; Reprinted from Canadian Plastics Magazine)

Economizing on resin costs is the new reality for plastics processors. Investing in more efficient commercial purging aids can help.
Few, if any, plastics processors need to be told that resin prices are skyrocketing, with no sign of coming down. But a survival strategy many are adopting – making more “just in time” product, with shorter runs and more frequent colour changes – has its own potential problem: a great deal of valuable virgin resin is wasted if products are contaminated by streaks or black specks. Luckily, a ready, cost-effective solution is at hand. Using commercial purging agents can clean equipment with the least amount of resin and expensive downtime.

Purging 101
There are two types of commercial purging agent: mechanical and chemical. Mechanical or physical purges, typically used in injection molding, use abrasive or high-viscosity plastics to scour other materials from screws, barrels and dies. “A mechanical purge is a flush, designed to break down and displace plastic,” said Barbara Giaquinto, technical manager at RapidPurge, a maker of chemical purging compounds. “There’s nothing fancy about mechanical purging grades, including ours.”

Chemical purges, by contrast, use ammonia or other substances to break down the polymer residue chemically in the machine, reducing the molecular weight and viscosity so it can be flushed out easily. “Chemical purges require residency time inside the machine to soften and loosen any hard carbon deposits, giving a greater degree of cleaning than mechanical purges,” said Giaquinto. The process can become tricky with blown film and coextrusion lines, she added, with the chemical agents requiring strict procedures for measuring, blending and “soaking.”

There’s a rough economic balance between the two processes. Chemical agents in pellet form can cost up to 70 per cent more than mechanical purging agents, but are also used in much smaller quantities.

In-House is Out
Notwithstanding the availability of mechanical and chemical purging products, the tried and true purging method for many processors has long involved using in-house material: virgin resin (typically styrene), as well as scrap or regrind, heated and run through a machine.

For a number of reasons, commercial purging compound suppliers say, this is no longer the economical way to operate.

“Frankly, using in-house material is the best way for a company to lose money,” said John Pizzo, technical service and development engineer with mechanical purging compound maker ASACLEAN -Sun Plastech Inc. “In-house material is simply not designed for use as a purging agent. It’s ineffective and requires constant purging, which results in a lot of downtime.”

There are signs that times are changing, however. “There’s a transition underway in the industry, going from purely resin purging to purging with a commercial purging compound, as companies realize they have to reduce their costs to remain competitive,” said Arthur Haag, president of commercial purging compound manufacturer PURGEX-Neutrex Inc. “Companies are doing short run applications with custom colour changes, and they’re realizing in-house purging doesn’t help them reduce their scrap rate.”

Using commercial purging grades can save time, as well, “A processor can conceivably cut the purge time on a machine from two hours with an in-house material, to 45 minutes with a commercial grade,” said Michael Muth, sales manager at chemical purging compound maker Slide Products Inc. “They’re also using much less purging compound than they would in-house resin, and the aggregate savings are very high.”

And with resin prices on their steady rise, it’s no wonder commercial purging compound suppliers are experiencing a boom. “Our business is up dramatically in the past two years, and to a certain extent the market is coming to us,” said Tim Cutler, business manager at Dyna-Purge, a maker of mechanical purging compounds.

Nor is business likely to fall off anytime soon. “Resin price increases appear to be sticking, whereas in the past they didn’t,” said Arthur Haag. “With commodity resins now hitting the US$1.00 per pound range, very few molders want to waste their material doing purges, and this makes commercial purging agents almost mandatory.”

Learning Curves
While product demand may have increased, commercial compound makers know they must still satisfy the industry’s need for a ready-to-use purging product.

The good news is they’re doing it. First, suppliers say, today’s commercial purging compounds can do much more than previous generations. “At one time, they were used only to remove expensive high-temperature engineering resins,” said Tim Cutler. “Now, they can be used even with commodity polyethylene, polystyrene, polypropylene and polyvinyl chloride.”

The catch, however, is that the commercial grades must be used properly. Fortunately, for molders carrying out mechanical purges, switching from in-house material to a commercial grade is relatively simple. “Processors may have to tweak a few parameters in using commercial grades for a mechanical purge, but these are very easily accomplished,” said John Pizzo. “There’s still no mixing, and no downtime or soak time involved.”

There are still certain steps to follow, though. “Increasing the back pressure, increasing the screw speed, and keeping the screw forward for injection molding are very important steps in making a mechanical purge work, and processors that don’t do these things won’t get the optimum cleaning power,” Pizzo continued. “And while it may sound obvious, it’s also important to use the correct grade for a particular job.”

As for chemical purges, it’s no longer the case that, being more complex, they always take longer to perform. “We have a process with our chemical purge that we call a through purge,” said Michael Muth. “A processor doesn’t have to change a machine’s back pressure, temperature or settings, and as a result the process doesn’t require much longer to complete than a mechanical purge.”

Assuming a processor follows simple rules of operation, they’ll find today’s commercial compounds are easier to use than ever. “In the past, customers had problems by using too much of the active ingredient, thinking it would be more effective,” said Arthur Haag. “We’re trying to make our compounds more efficient, so processors can optimize the amount of commercial purging compound required to maximize the economic benefit.”

Depending on experience levels, suppliers note that a shop’s employees may still encounter a learning curve when using either a mechanical or commercial purging compound for the first time, and stress the importance of performing trial runs. “A processing team should start with a series of trials, with the proper information in front of them, and then measure the results,” said Tim Cutler.

Commercial purging compound suppliers caution that no purging agent removes all contamination from equipment; serious contamination still requires a machine shutdown, disassembly and cleaning with a wire brush. The best use of a purging agent, they say, is as a preventative maintenance tool, used to remove heat-sensitive resins at shutdown or at the start of a changeover to avoid degradation in the early stages.

Investing in commercial grade purging compounds might appear to be a low priority, some suppliers say, but the benefits will quickly become apparent. “From a molder’s point of view, purging can seem like a non-value added function,” said Michael Muth. “But it’s becoming a necessity in this economic climate, and if we can find a way to make it less of a burden, then we’ve done our jobs.”

– Mark Stephen, managing editor