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New process for coating metal parts economically prior to vulcanization.

With the global economy becoming so competitive, every dollar counts when manufacturing products here in the USA and Mexico. Companies that manufacture rubber parts are scrambling to reduce spending and cut out the excess, and yet many that are coating for rubber-metal bonding are still using uneconomical methods. Hanging small parts on racks, robotically spraying parts individually or using dip spin machines with primer and adhesive wastes gallons of coating material, which can dramatically reduce profitability. Plus, the secondary handling of these small parts after coating them causes an increase in labor costs.

These coating methods have been used in the rubber industry for years, but a new system has emerged that could cut extraneous labor and greatly reduce the amount of coating material used to coat metal parts for adhesion. The Rotamat, manufactured by the Walther Trowal Company of Grand Rapids, MI, uses PLC controlled tumble spraying technology, developed to eliminate these coating issues and help manufacturers regain their competitive edge.

The Rotamat works by tumbling small parts in an elliptical-shaped barrel beneath one or two spray nozzles. Heated, pre-filtered air is injected into the barrel to help the coating adhere to parts. Rotamat expert Frank Siegel of Walther Trowal said this air is also injected so that parts are ready to be packaged as soon as the process is complete.

The coating process is done by spraying a small amount of material while the parts are tumbling to be able to build up to a very specific coating thickness over the entire area of the part.

"The reason we use heated air is so that when we spray the coating material, the parts are dried immediately," Siegel said. "With normal room temperature air, you will end up with wet parts that could stick together." The air flow in the Rotamat is monitored by measuring the differential pressure in the spray zone of the drum by the PLC so that no coating material or volatile fumes can escape the machine. Due to the negative pressure generated by the machine, the operator will not be exposed to any hazardous fumes while the system is running.

Depending on the geometry and weight of the parts, they may tumble differently in the barrel. To accommodate this, the inclination of the barrel can be adjusted from 0 to 35 degrees to customize the coating angle and optimize the coating application for each batch of parts. Additionally, the barrel can rotate 125 degrees in either direction for easy part loading and unloading. The barrel can run anywhere from 2-30 rpm, and the processing temperature can range from ambient to 300[degrees]F, depending on the parts and finish being applied.

An exhaust fan with a post filter allows negative pressure to be maintained in the spray area where the parts are being coated. A Fanuc PLC controller monitors every aspect of the coating process and immediately notifies the operator if any deviations occur so no off-spec parts are produced. Siegel said because of these controls, the

operator's job is fairly simple.

"The operator's job is just to load the part, load the program and start it, that's all," Siegel said. "After that, the machine finishes everything. Just unload the part and that's it."

The use of an elliptical shaped barrel, rather than a conventional octagon, prevents parts from riding only along the outside of the barrel and ensures even paint coverage on all areas of the parts. When the process is complete, the barrel is tilted to unload the completely dry parts, which then can be placed directly into a shipping container or moved to an assembly area for further processing.

There are four different sizes of the Rotamat to accommodate manufacturers' needs. The R-55, R-70, R-90 and R-90C vary in power and drum size to suit the number of parts that need to be coated. Siegel said that the maximum volume for the R-90 is roughly 1.75 cubic feet, and for parts that are 1-1/2" inches in diameter by 3/8" thick, the Rotamat R-90 could hold between 4,500 to 5,500 parts per load.

The double spray nozzle is an extremely attractive feature when applying adhesive to metal parts for rubber-metal bonding. Conventionally, coating metal parts with a primer and an adhesive can be twice the effort; with either the dip spin method or rack drying, manufacturers have to apply one coat, wait for it to dry, and then apply another.

Using the Rotamat, the coating process is streamlined. Because of the two nozzles inside the barrel, manufacturers can load the pressure pot connected to the first nozzle with primer, and the pressure pot connected to the second nozzle with adhesive. Siegel said because the process dries the parts as they are sprayed, it is possible to apply the adhesive directly after the primer.

"A full batch with 4,500 parts would use 0.40 gallons of bonding agent primer," Siegel explains. "When we reach our weight of primer used, which is monitored by an electronic weight scale tied into the PLC, the machine changes automatically to spray gun number two and sprays the adhesive."

Using just one process can help save money over time. Whether it's supervising a two-step process or hanging parts on hooks, the dip spin and rack drying methods require constant handling of parts. Because the Rotamat is automated, this extra labor can be eliminated. Not only do conventional methods waste time and labor, but as much as 70% overspray can occur during the coating process. Siegel explained that because the Rotamat is spraying the coating on rotating parts, there is usually only 3-5% overspray. Additionally, the coating would be mixed with solvent to help smooth the application process.

"By diluting the coating material with solvent plus the minimal overspray achieved by the Rotamat, you would save more than 65% of the coating material used. That's a lot of money you save for coating your parts," Siegel said.

In addition to saving money, many customers using the Rotamat are seeing an improvement in coating quality as well.

"Customers are running into fewer problems because the coating is so much smoother," Siegel said.

When the dip spin coating method is used, parts may have built up clumps of coating during the process or may stick together. In turn, this can cause problems with the vulcanization of the rubber to the parts. Siegel said that because the Rotamat is constantly tumbling the parts, the coating is applied more evenly with both the primer and the adhesive. With a smoother surface, the rubber is more likely to have a stronger bond with the coated metal.

When rubber manufacturers are looking for a coating system that will lower labor costs, reduce wasted coating material and increase the bonding strength, the Rotamat small parts coating system from Walther Trowal is an attractive solution.
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Title Annotation:Process Machinery
Author:Raby, Andrea; Trowal, Walther
Publication:Rubber World
Date:Oct 1, 2013
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