It Starts at the Finish
How the final surface finish governs coating prep
Say a custom powder coater has coated a product for years. Then one day the customer calls to complain about the coating flaking off. What happened?
Industry consultant and Products Finishing columnist Rodger Talbert, Grand Rapids, Mich., has seen many cases like this, and sometimes the problem isn’t with the powder coat line itself, but instead can be traced back to surface preparation.
Talbert recently worked with a company that had complained about paint coming off of a piece of metal furniture. The problem was traced back to a finishing step that the fabricator had eliminated to cut costs. Talbert stressed that it wasn’t the fabricator’s fault that a finishing step was eliminated. After all, finishing is (as its name implies) one of the last fabrication processes and, as such, sets the pace for every preceding manufacturing step. In this case, though, the fabricator inadvertently eliminated a necessary process--one that provided a solid anchor for coating.
Final Finish First
The desired finish can entail a specified roughness average (Ra), often attained after running test panels using various abrasive grades. High-solid-content coatings like powder generally adhere well to deep scratches, while low-solid coatings like some wet paints may require a finer finish. A color’s opacity and gloss also factor into the equation.
“Many times test panels are required, prepared with varying abrasive products, to determine the best adhesion and paint coverage,” says Erik Vanstrum, technical service manager for 3M Abrasive Systems Division, St. Paul, Minn. Guidelines from the abrasive manufacturer give a range of surface finishes attainable with a certain product. But these guidelines may only be a starting point, because “different paints react differently to certain surfaces,” he says.
With the required surface finish determined, a fabricator then can go about developing an efficient grinding and finishing process. So where does a fabricator start?
“Start at the finish.”
So says Mike Marshall, technical sales and marketing manager at CS Unitec, Norwalk, Conn. “Regardless of what you’re trying to accomplish, every scratch you put in that metal has to be removed to get to your final, required surface finish.”
Say a workpiece requires a 220-grit finish. In a perfect world, the metal would arrive at the finishing station without any imperfections, which would mean the worker could simply use a 220-grit abrasive media. But this isn’t a perfect world, of course. Most workpiece surfaces have imperfections such as scratches and welds, and these areas need to be removed first with coarser abrasives.
“We may need to use an 80-grit grinding disc to get a weldment down,” Marshall says, “but we will certainly not touch the whole piece with the 80-grit.”
A coarse abrasive effectively removes imperfections quickly, but it also puts in large scratches that must be taken out by a subsequent, finer-grit media. If a surface area doesn’t have an imperfection requiring removal, why use any grit but the one that will put on the desired scratch pattern for coating?
First, finishers can identify areas on the workpiece without imperfections (which admittedly are sometimes rare) that may need only a final polish with the final-step abrasive. Next, they can identify imperfections (welds and other scratches from material handling) and determine how many steps (different abrasive grit sizes and tools) can remove and blend them with the rest of the surface. Ultimately, the scratch depth, width, and direction should be the same over the entire surface.
“In an effort to get the job done quickly, some people use an extremely coarse product to grind the weld down,” says Paul Krupa, senior product manager at Norton Abrasives, Watervliet, N.Y. “Then they come back with a fine grit product to cut out steps in the process.”
The number of steps matter in a finishing procedure. Say the operator starts with an 80-grit, and then moves to a 100-grit, and then to a 150-grit for the final finish. He could save a step by starting with the 80-grit to remove the imperfection, then move right to the 150-grit. Still, he will spend more time with the 150-grit to remove the large scratches left behind by the 80-grit material.
If the jump between grit sizes is too drastic, the final step will take so long that the worker may wear out the abrasive completely and have to change discs midprocess. Worse yet, the finisher may send on the workpiece to powder coating prematurely, before completely removing those deep scratches.
Thinking of the process on a microscopic scale helps explain why using certain grit sizes matter. Say a finisher jumps from a 60- to 220-grit. Large scratches from a 60-grit abrasive have high peaks and deep valleys. This means that tiny particles on a 220-grit abrasive must grind down those peaks and valleys before those grit particles can put in their much smaller scratches. The more drastic the jump in grit size, the longer it will take to grind those peaks and valleys left by the previous abrasive media.
No matter how carefully a finisher works to grind, blend, and polish a surface, problems arise if a workpiece isn’t cleaned properly. The parts should be cleaned before using any abrasive, because doing so will increase abrasive life.
As Simon Wainwright, brand and product manager at CS Unitec, explained, “The first thing in the finisher’s toolbox will probably be a cleaner and degreaser. The surface will have all sorts of oils and handprints. That will be taken off first, and it’s the No. 1 most neglected duty for paint prep.”
Pressure and Speed
The RPMs of the abrasive media should be high enough so that the abrasive grains move quickly enough to cut material. Think of abrasive media as a cutting tool on a machining center. Move too slowly, and the teeth (or abrasive grains) don’t remove much material.
“If you push down too hard, you slow the tool down, the revolutions per minute decrease, the surface feet per minute of the abrasive media is reduced, and the whole process slows down,” Marshall says. “The idea is, let the tool do the work. You educate the operator to use feel and finesse as opposed to brute force.”
Krupa added that experienced operators may increase pressure slightly as the abrasive begins to wear. “But you have to let the tool do its work,” he says, “and apply more pressure only once the product slows its cutting as it begins to wear. If you use too much pressure in the beginning, you can shorten the life of the product severely.”
Discs and other media have speed limits. Products are rated for maximum operating speed (MOS) by manufacturers and, as such, shouldn’t be placed on a tool that can exceed those limits. A 20,000-RPM tool should never hold an abrasive media rated for only 19,000 RPM. “You are in danger of having the product fly apart if you operate above MOS,” Vanstrum says.
“You need to make sure you have the right speed for optimal use,” Vanstrum added. “Running at the maximum operating speed isn’t necessarily the best for performance, depending on the product and the application.”
For instance, products like nonwoven media may be rated to run a little slower to get the best performance. Heat buildup from excessively high finishing speeds can cause the media to break down faster.
The Right Tool
The finisher (ideally) has a wide assortment of power tools at his disposal, like a variable-speed drum sander for large surfaces and an angle finishing tool for corners and crevices. Lacking the right tool can slow overall throughput, not only because it may take longer to finish a part, but also because the part may not leave the finishing area with a uniform surface. The result: Coating doesn’t adhere and rework ensues.
The shape and type of the abrasive media also make a difference. Consider a flap disc, which has a series of abrasive flaps overlapping each other. As Krupa explained, a flap disc’s high number of cutting points gives it aggressive cutting action. But all those cutting points--that is, the abrasive grits--are on flexible flaps. Because they aren’t held rigidly in place, they don’t remove as much material with a single pass.
How a finisher approaches a specific workpiece depends on the tool used. Unlike a right-angle grinder, a random orbital sander usually sits flush on the surface, without any, or perhaps only a slight, angle, depending on the workpiece’s shape. Along with rotational motion, these sanders move the disc linearly back and forth--a motion called the orbit. “A bigger orbit on the tool changes the aggressiveness of the product,” Vanstrum says. The larger the orbit, the more aggressive the sander can be. So even if the sander has a fine-grit media, it still may leave an excessively deep scratch pattern if it has a large orbit.
Another abrasive in the finisher’s toolbox, nonwoven fiber discs designed for blending and finishing can have a cushioning effect, allowing them to conform to different shapes in the workpiece. “The nonwoven abrasive material is a much more conformable type of product,” Krupa says.
And then there are power brushes. “In typical metalworking application, a power brush will clean and prepare a surface and promote adhesion because it works with the same mechanism that blasting works through,” says Brad Hopkins, senior product manager at Weiler Corp., Cresco, Pa. “The impact of the sharp wires strips away surface contamination and provides an anchor pattern for paint or powder coating.”
He says that the key sign workers aren’t using power brushes effectively is wire breakage--specifically, the breakage of long pieces of wire from the brush after only a short period of use. This is usually caused by excessive pressure. Operators may push too hard because they may be using a power tool without enough torque and speed to do the job, or the brush itself may not be aggressive enough for the application. “Using excessive pressure is the single most common misapplication of wire brushes,” Hopkins says.
The speed at which the brush tips strike the workpiece is critical, and that speed is a function of both the diameter of the abrasive media as well as the spindle speed of the tool. The wire brush finishes metal like power blasting. The impact of the wire tips does the work. “So when you have insufficient operating speed, you aren’t getting enough wire tips hitting the surface with enough velocity to accomplish the work.”
A brush is effective only when it’s on a power tool that can rotate at sufficient speeds. “The combination of a small-diameter product running at a very low operating speed creates the tendency to use more pressure to try to get the work done,” Hopkins says.
Coating Prep: In-house or Outsource?
If workpiece surfaces require coating, fabricators can prepare them in-house or send them out for blasting.
“A lot of people out there struggle with that decision,” CS Unitec’s Wainwright says, “because sending parts out for blasting can be expensive. And it takes time. But [blasting] also gets the job done thoroughly.”
It may make economic sense to send some work out for blasting. For some jobs, blasting may be the only way to get the job done right. Regardless, tools are available to finish many jobs in-house. And as sources explained, many challenges can be overcome with proper training and—perhaps most important—good communication between the coating and fabricating operations. After all, any process change in one can affect the other. n
For more information from the companies mentioned, call 3M Abrasive Systems Division at 866-279-1235, or visit 3m.com; CS Unitec, call 800-700-5919 or visit csunitec.com; Norton Abrasives, call 508-795-5000 or visit nortonabrasives.com; Talbert Consulting call 616-356-6190 or visit talbertconsulting.com; Weiler Corp., call 570-595-7495, or visit weilercorp.com.
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This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.