Purging – The Competitive Edge

The primary reason for using a commercial purging compound is to increase productivity, which is ultimately attained through increased uptime. The challenge is to convince plastic processors who have been doing lengthy color or resin changes for years with whatever resin is on hand that there is a better, more productive alternative.

“One of the most exciting frontiers in the plastic processors’ fight for competitive advantage is purging to improve operating efficiencies,” says Tim Cutler, Dyna-Purge business manager with Shuman Plastics Inc. “There is a growing acceptance of using commercial purging compounds over regrind resins or homemade concoctions.”

Michael Muth, vice-president of Slide Products Inc., says most processors are familiar with the benefits of purging compounds, and may even have some in their facility somewhere, but don’t necessarily use it regularly. “Purging is one of those things where you have to look at the big picture. You need to look at purging compounds as a way to minimize your losses due to downtime.”

“When business is slow, it’s easier to get away with not using purging compound,” he adds.

Slide offers both a mechanical, liquid purge called PDQ, and a chemical purging compound called NPT (NuPurge Technology).

“Some companies have great difficulty seeing the economic value of purging compounds,” says Glen Billinger of Plastics Machinery Inc. PMI sells a chemical purge containing what Billinger describes as a unique formulation. “There are tremendous savings to be had, but people generally have scrap plastic around that can be used instead.”

Tom Barrow of Blackbrook Chemical, manufacturer of HD Purge, argues that resins and regrind were never intended to clean the barrel or equipment. “While it may clean out some of the old resin, it will not clean the carbon deposits and hard-to-get areas in the barrel and manifolds. Although regrind looks like a bargain, it will end up costing processors more in terms of downtime and labor.”

HD Purge is a fast-acting chemical purge, manufactured by Blackbrook Chemical in Canada. Barrow notes that it is not temperature or resin specific.

The cost of purging is not strictly a function of the purging material cost per unit, says Cutler. A disciplined cost-to-purge analysis includes cost of machine downtime (including mixing and soaking time, if any), cost of carrier resin, post purge costs, such as the cost of resin to clean out the purging compound after use, plus time lost for post purge operations.

Purging suits a preventative mentality

Although users of purging compounds run the gamut of industries and size, John Pizzo of Asaclean-Sun Plastech Inc., says the majority of his customers tend to be mid-size companies. Pizzo is a technical service and development engineer with Sun Plastech. He believes larger companies tend to have dedicated lines or longer runs, and therefore don’t need purging compounds as frequently. He says the type of company that chooses purging compounds is generally a quality-conscious firm, or one that experiences a lot of changeovers.

Pizzo says Asaclean has also been educating its customers on the benefits of using purging compounds during shutdown. Because it is thermally stable, a mechanical purge such as Asaclean can be left in the barrel during shutdown, say, over a weekend. “It’s not what most customers think of off the bat. We introduce the idea to them.”

Asaclean is a mechanical purge, with both abrasive and non-abrasive grades. Pizzo explains that the compound can withstand a wide temperature range, and is generally more viscous than resin, which contributes to its scrubbing abilities.

Some customers do use purging compounds in a preventative manner as well. Barbara Giaquinto of RapidPurge notes that many users in the extrusion field will do a thorough purge once per quarter to get rid of carbon buildup.

The bigger the machine, the more troublesome the purge, she adds. Giaquinto describes one case in which a blown film processor using a spiral die in a co-extrusion operation used to spend half a day using regrind when changing from black to clear resins. Using RapidPurge, the customer is producing without die lines in about 2 hours.

Chemical purges, such as RapidPurge, attack the resin left inside the equipment and break down its molecular structure, as well as attacking carbon deposits.

“They are generally more expensive, but you’re getting a more thorough cleaning,” says Giaquinto.

“Even the cheapest purging product will work better than polyethylene.”

Are there deeper issues?

While purging compounds offer a quick fix, Frank Van Haste, general manager of purging compound manufacturer Novachem, suggests that sometimes fixing the cause of the problem, rather than the symptoms, is warranted. In his white paper entitled “Toward a Systematic Approach to Evaluation and Resolution of Purging Issues in Thermoplastic Processing,” he speculates that purging issues (downtime due to color or material changes, and contamination in process equipment) had been accepted as a fact of life for two reasons:

because every facility faced the same problems, there was no differential effect on competitiveness;

in the hierarchy of problems to be solved by expected benefit, these issues were not at or near the top of the list.

However, Van Haste warns that “as some leading practitioners begin to deal with purging issues in an effective way they will gain a competitive advantage.”

Van Haste counsels that ad hoc approaches to deal with purging issues neglect the underlying causes. “An informed and systematic technical approach — the antithesis of the heretofore-typical episodic approach — is required to truly minimize the negative productivity impact of purging issues.”

When assessing your current situation, Van Haste suggests that the productivity impact of the purging issue be stated, in dollar amounts if possible. In addition to the obvious costs, your assessment should include lost profit due to saleable product not made.

Van Haste says that solutions to purging issues may include equipment modifications, re-specification of materials, process changes or introducing tools such as commercial purging compounds. (Visit www. novachem.net/docs.htm for a copy of the paper.)