Analyzing a Waterborne Paint Line

Considering that more than 1,000 parts of ranging shapes and sizes are regularly hung from its hooks, the finishing line at Jervis B. Webb Co. (Mount Vernon, OH) was performing reasonably well.

But, for the line operators, close inspection revealed several lingering finishing issues that needed attention. And, for an international leader in the manufacture of conveyor systems, "performing reasonably well" left plenty of room for improvement.

So, when plant manager Chris Hook and paint line foreman Brad Coon were offered a finishing line analysis that would address some of their concerns with the line, they decided to take advantage of the opportunity.

"We don't paint widgets," acknowledged Mr. Hook. "We paint conveyor system pieces ranging from 20 ft long to about 2 × 2 inches, and they all have nooks, crannies, pockets and channels that can be difficult to paint properly. Plus we're set up to spray both automatically and manually, so our paint line has to be diverse and adaptable. But, we had a few issues with our paint line. We had been given some off-the-cuff recommendations, but no one really came in, addressed each of those issues and gave us a systematic analysis as Sherwin-Williams did. And, frankly, at the time they did it, we weren't even using any of their products. This happened before we had an actual business relationship with them."

Chemical Streaking

Painted parts leaving the facility's paint line occasionally showed chemical streaking through the topcoat. The line analysis helped determine that the streaking was the result of cleaning agents in use in the two-stage washer. For various reasons, the rinse stage of the washer had not been in use, so the line analysis, along with help from a representative from the pretreatment company, helped prompt a switch to a rinse-free pretreatment solution that could be employed without leaving a trace. "We were ending up with iron phosphate tracing. So, we made the decision to switch from an acidic wash solution to a neutral solution," said Mr. Hook. "We drain and clean the tank annually anyway, so we made the switch then. That solved the problem."

Safe for Waterbornes

A switch to waterborne coatings at Webb some 4 years prior made good environmental sense as well as safety sense for the plant's employees. But, where solvent-borne coatings could be applied to parts still hot from the dry-off oven, parts entering the waterborne paint booth at more than 100F, even 140-150F sometimes, left some concerns regarding the use of waterborne paints.

The solution to the problem was simple. Four fans were placed at the point where the conveyor line leaves the dry-off oven. As a result, the metal temperature of parts entering the spray booth is now 70-90F, a temperature that eliminates many waterborne application and adhesion concerns.

Line Slow Down

Dry-off ovens had been set at temperatures up to 260F. At line speeds up to 20-22 fpm, such high temperatures were necessary to achieve dry-off before entering the spray booth, even with the addition of fans to the line. But, Mr. Hook and Mr. Coon were concerned about the energy required to keep the oven heated to such levels.

In what turned out to be a related concern, some parts were leaving the line without a complete cure. Mr. Hook and Mr. Coon dedicated their automated electrostatic paint equipment to coating a 20-ft conveyor track piece. But, according to Mr. Hook, it became standard procedure to remove the track pieces from the end of the line and then spread them out on the floor for overnight curing despite their recent pass through the cure oven. The labor necessary to perform these duties practically negated any advantages brought on by automated application. "What we wanted was pretty simple," said Mr. Coon. "We wanted to be able to stack these parts once they got off the end of the line, but we obviously needed them to come off the line in a tack-free condition."

Sherwin-Williams proposed a solution that addressed this concern and more. The company suggested a drastic reduction in line speed, from 20-22 fpm down to 10-12 fpm, with an inversely proportionate increase in line density that would allow the plant to maintain production levels. Mr. Hook and Mr. Coon bought the idea and settled for a slight compromise in line speed of 14 fpm.

The resulting benefits were numerous. Parts now spend more time in the washer, leaving the unit cleaner than in the past. And, since parts now spend nearly twice as long in the dry-off oven, complete dry-off is achieved at a considerably lower, energy-saving temperature. The dry-off oven is now set at 225F, down from 260F.

In the spray booth, painters are more comfortable with the line speed and have maintained, and in some cases improved, production levels. At the higher line speed, painters occasionally stopped the line in order to keep up with the movement of parts on the conveyor. These shutdowns, however brief, not only slowed production, but led to potential problems in other areas of the line. A part spending extended time in the dry-off oven, for example, heated to a temperature inconsistent with the others on the line. At the slower line speed, however, such shutdowns are rare.

Farther down the line, the effects of the slower line speed are also significant. With the cure oven set at an efficient 175F, a better cure is achieved. Metal temperature reaches or exceeds 140F for more than 4 min compared to about 2.5 min at the faster line speed. The slower line speed also lengthens the cool down cycle. With four additional cool down fans positioned at the exit end of the cure oven, parts are ready for handling as soon as they reach the finishing line. The days of spreading 20-ft pieces of track on the floor for overnight cure are almost forgotten memories.

"We've achieved better appearance and adhesion," stated Mr. Hook. "Our customers have noticed. And, the best part is that all of this was achieved with no trade-off. We didn't sacrifice productivity or anything to make these improvements."

 

If you're using a solvent-borne paint and would like to switch to a waterborne paint. Check out Coatings Alternative Guide, also knows as CAGE.