coating materials and application methods

POWDER COATING WITH ROBOTS

AND DENSE PHASE TECHNOLOGY—A

CRITICAL BLEND OF ADVANCED

TECHNOLOGIES

BY JOHN BINDER, MARKETING MANAGER, NORDSON POWDER COATING

SYSTEMS, NORDSON CORP., AMHERST, OHIO

The efficiencies of robotics designed specifically for painting—coupled withadvances in powder coating application and delivery technology through theuse of dense phase technology guns, pumps and controls—provides greater productivity,higher repeatability and savings in daily powder coating operations.There was a time when powder coaters would adapt large welding robots topowder coating applications. The result was costly over-sized machines that didnot lend themselves well to powder booths or precise powder paint applications.Often the window or opening for the robot would need to be sized to accommodatethe robot’s painting motion. This caused higher than desired openbooth area and up-sized collectors to allow for the necessary booth airflows tocontain powder.Since then, robot technologies have progressed at a rapid pace. Today, there arerobots designed specifically for powder painting that are smaller, more cost effectiveand more precise with advanced part identification and controls that communicatebetween the powder application equipment and the robot. As a result,powder coaters achieve higher productivity, greater repeatability – and savings!

Robotics and powder coating—for greater process control and efficiency.

Because of their virtually unlimited range of motion, robots are more flexiblethan fixed axis machines–be they single, dual or even triple axis machines. Sincerobots can maintain a gun-to-part distance that is more precise and flexible thanfixed axis machine, they are able to contour a part as well as reach into hard-toreachareas. This allows for more uniform coverage of the powder coating onall coated surfaces. Additionally, a robot makes the same motion every time foreach and every part, translating into less variability of the coating than if coatedwith a manual powder gun.Some might ask, then–what if the part is not hung correctly or the same wayevery time? Fortunately, there are robots available with sophisticated part identificationthat can sense if a part to be coated is misaligned.In fact, robot controls when combined with powder system controls cannotonly sense part orientation but can also sense part type and shape. This makesit possible for both the robot to know what motions to make and the powdergun to know what application parameters to use. In other words, part identificationinformation can be sent to the powder gun controller such that the correctpresets for powder flow and electrostatic control match the part to be powdercoated. This same sophisticated part identification technology allows the robotto then powder coat the same parts with precise robot arm speed, repeatablemotion and consistent gun-to-part distance parameters that are both optimizedand harmonized with the powder coating equipment application settings. Thistranslates into consistent and repeatable part quality that eliminates powderwaste, minimizes rejects and rework, and reduces cycle time when compared tomanual powder coating operations. The greater repeatability that is achievedthrough robotics—when coupled with dense phase powder coating delivery andapplication equipment—delivers even greater levels of productivity and moresavings than would otherwise be achieved with conventional methods of movingguns (fixed axis or manual operator) and venturi pump powder delivery andapplication equipment.

Dense phase technology and robotics—for greater application control and

efficiency.

So, what is dense phase technology? It is essentially dense phase powder pumps,guns and controls that are superior to venturi powder coating equipment andall other dense phase powder equipment on the market. Previously, automaticpowder guns with conventional venturi pumps could not paint fast enough, oreffectively enough, to take full advantage of robotic technology. However, theadvent of dense phase powder pumps and spray guns has created a perfect matchfor the speed, accuracy and repeatability of today’s painting robots.Dense phase technology was born of the need for 20-second color changes,hundreds of colors on a powder coater’s color palette, and same day shipments–therefore, the need for many color changes not only within a single 8-hour shift,but within a single hour. Dense phase technology was initially developed forNorth American office furniture manufacturers that were spraying hundreds ofdifferent colors and needed to ship complete office sets (desk, chair, file, drawers,etc.) the same day they received an order. They eventually determined that it wasless costly to throw powder away than to have any amount of significant downtimefor color changes. These same customers were also running line speeds asfast as their conventional powder equipment would allow them—in order to coatas much product as possible. This was initially achieved by manually switchingair and powder lines between multiple hoppers with different colors, equippedwith venturi pumps and manual powder spray guns. However, venturi pumpsdid not lend themselves well to powder coating at high line speeds because somuch air was required to propel the powder—propelling much of it past the partwhen the air flows were turned up to achieve greater powder flow in order toaccommodate higher line speeds.It was here that a “dense phase” delivery pump was developed (Figure 1). Thispump was designed to deliver more powder at lower velocities because it wasmore of a metering pump with two chambers. The first chamber with two valves

would pull the powder in with vacuum and push the powder out with pressure.The second chamber would operate similarly but its timing would be oppositethat of the first chamber, such that when thesecond chamber applied pressue,for example, to push powder out—the first chamber would apply vacuum topull powder in.Alternating cycles resulted in smooth dense powder flow with no pulsing andvery little air to propel the powder. This was a quantum leap for powder deliveryand application equipment—providing higher transfer efficiency with lessoverspray—allowing powder coaters to increase line speeds and, in some cases,even eliminate manual gun stations.Not visible to the naked eye is the ability of dense phase technology to chargepowder better due to the lower powder velocity and the increased relaxation timeof the corona charge. In other words, more powder at a lower velocity meansthat more powder has a greater time to attract a charge as it passes through thecorona field in front of the powder gun. When compared head-to-head with amanual spray gun and a venturi pump, the dense phase technology gun andpump will apply more powder, with fewer strokes, in a shorter amount of time.The ability of dense phase powderapplication to reduce gun-topartdistance stands out as beingmore transferable to robot paintingthan all other benefits. Due to thelower velocity and relatively shorter“throw” of the powder as comparedto conventional venturi pump technology,the gun-to-part distance fordense phase powder coating gunsbecomes shorter.Please note: this does not meanthat the effective fan pattern widthis reduced as there is a wide rangeof nozzle configurations for densephase guns that allow them tomatch, and in some case, exceedthe maximum fan pattern widthof powder guns using conventionalventuri pumps. Similarly, the same features and benefits of dense phase deliverypumps that make them useful with robots also makes them perfect for use with automatic guns.

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Figure 1. In this schematic, a dense phase technology

pump illustrates alternating cycles of the two chambers,

each containing two valves

Dense phase technology and robotics—a perfect marriage of advanced technologies.

As more and more powder coaters adopted dense phase technology for usewith manual guns and fast 20 second color changes, it was a natural evolutionto adapt this same technology to automatic guns. This was done not only foruse in spray-to-waste systems, butalso for use in reclaim systems thatrecovered the over-spray and reusedit. It was determined during thisevolution that the decreased gunto-part distance that applied to themanual guns, not only applied tothe automatic gun as well, but alsolent itself very well to the combinationof an dense phase gun ona robot.Because of the almost infinitemovement and longer reach of arobot (as compared to a manualoperator), the dense phase guncould track very close to the partat a high speed, resulting in exceptionalpowder uniformity, betterstandard deviation of film build,reduced over-spray, and reducedcycle time. Coupled with AFC(automatic feedback control) currentlimiting technology, thisallowed the robot to not only contourthe part quickly, but to reachback into cavities and coat them

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Figure 2. The gun-to-part distance of a dense phase

gun is much closer to the part, making it the perfect

powder coating tool for use with robotics.

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Figure 3. When combined with color-on-demand technology, dense phase application equipment can

provide color changes in 20 seconds or less.

uniformly without bounce-back due to the low velocity of powder and withoutdefects caused by too high of a current.When a powder gun is at its optimum distance from the part–the effectivecharging voltage of the powder is optimized and current draw is low. When thegun-to-part distance decreases, effective voltage also decreases and the currentincreases. This causes a defect on the powder coated surface of the part commonlyknown as back ionization. In the case of cavities, this can result in whatis known as the Faraday Cage Effect, resulting in poor powder coverage of thesurface of the cavity. Dense phase with its low powder velocity and the resultingreduced bounce back—coupled with AFC current control—makes it the perfectmatch for use with robots for all powder coating applications. This is especiallytrue for those that require frequent color changes.Because dense phase pump is purge-able in both directions, the powder deliveryline can be cleaned quickly and effectively. When coupled with “color-on-demand”technology, dense phase technology provides automatic color changes in less than20 seconds. When combined with automatic gun presets that control applicationparameters—such as powder flow and electrostatics (KV andAFC) for each andevery part as well as each and every powder—powder coaters now have the ultimatetool for productivity and flexibility in their powder coating operations.Office furniture manufacturers in Japan have been using dense phase technology,robotics and color-on-demand technology for several years now to powdercoat effectively and change colors quickly using multiple dense phase gunson a single robot—and sometimes using multiple robots on the same booth.Additionally, dense phase technology is not limited to organic powders. It canalso be used to pump and spray PE (porcelain enamel) powders. An appliancemanufacturer in North America is currently using dense phase guns on robotsto apply porcelain enamel powder to oven cavities. In the end, these manu-facturers are a testimony to thesuperior process control of roboticsand the unparalleled applicationcontrol of dense phase technologywhen married together.

SUMMARY

The benefits of dense phase powdercoating equipment, when coupledwith robotics, deliver advancedapplication and process technology.As a result, it provides increased productivity,greater repeatability–andsavings!Increased

Productivity

• Faster cycle times

• Higher line speeds

• Increased up-time andreduced downtime

• Higher first pass yieldsGreater

Repeatability

• Reduced process variations with robotics

• Reduced powder application variations with presets (part recipes)

• Elimination of human interaction variables

Savings

• Higher first pass transfer efficiency—more powder on the part and lessover-spray

• Reduced compressed air usage with dense phase technology as compared toconventional venturi pumps

• Decrease in required number of manual gun operators

• Less rework and fewer rejects