PW Technology News - October 1993
Fabricator boosts quality with speedy seam test
Seaming problems are nipped in the bud within 5 minutes, not 40 hours after they're made.
Environmental Protection Inc. (EPI), a Mancelona, Mich.-based fabricator of poIyvinyl chloride (PVC) geomembrane liners, has developed a process that permits it to test the integrity of a welded seam within 5 minutes.
Dubbed the Wolschon Test after its developer, the 5-minute process directly correlates with the ASTM's and NSF's (National Sanitation Foundation) standards for testing weld-seam integrity, which requires that sampIes be brought from the production floor into the lab to "acclimate" for 40 hours-allowing the seam to cure before peel-strength tests are done. The end result of the accelerated test is that EPI can correct flawed seams about 10 minutes after they’re made, instead of nearly three days later.
"The procedure has allowed us to make any corrections in the seam before the problem gets away from us and a lot of flawed product is made," said Fred Rohe, [former] president of EPI. "And since the operator is getting such fast feedback, he is learning how to make the seam right in the first place more often".
The numbers bear this out. Before implementing the system in December 1991, EPI seams didn’t meet the minimum NSF peel-strength standard of 10 lb/inch wide 25 times per 1,000 samples taken. The rate is down to 5 per 1,000 samples now.
There’s no black magic involved in the test itself: samples are pulled from the production process and after 5 minutes tested for peel strength in a tensiometer. The tricky part was correlating the results with ASTM/NSF 40-hour standard.
EPI achieved this by comparing peel strength results from the Wolschon Tests with results from the ASTM/NSF procedure and looking for patterns. They soon became apparent; as the strength of the Wolschon Test sample average increased, so did the 40-hour test, and vice-versa. After 200 samples were taken, EPI established that Wolschon Test samples had to have a minimum peel strength of 4.25 lb/inch width in order to meet the 10 lb/inch wide standard after 40 hours of acclimation.
"What this test basically does is allow us to predict from a 5-minute test what the results will be after 40 hours," Rohe elaborated. "It has also isolated the critical factors of what makes a good seam and what doesn't.
"We found that what makes a difference is the time the chemical is allowed to react with the surface of the PVC before pressure is applied. Too little time and the surface won't be dissolved: too much time and the solvent will evaporate and the material will harden."
PVC geomembrane liners are generally seamed twice: in the plant and at the point of installation. EPI buys 76-inch wide calendered PVC in rolls and seams them until a panel as large as 30,000 sq ft is formed. That web is then shipped to the field, where it is seamed again. EPI's Wolschon Test currently is applied to seams in the plant only, though the company is working on extending it to field welds as well. -Jim Callari
Jim Callari, (1993), "Fabricator Boosts Quality With Speedy Seam Test", Plastics World, October 1993, Page 18
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