Increasing Paint Application Transfer Efficiency on Robotic Waterborne Automotive Basecoat Application by James S. Hager, FANUC North America Inc., Auburn Hills, Mich. 97\53

aterborne paint application With the control of the paint parame- the same amount of linear travel at a is increasingly becoming ters (i.e., fan width, atomization, and constant relative speed is used to traverse the trend in automotive top- paint flow), can, therefore, deliver the vehicle. This is not always the case coatw painting. Waterborne paint offers a consistent and uniform film build. The with reciprocators or other paint applica- several advantages in quality and ap- and its control software ensure that tion equipment. pearance. The use of waterborne paint also minimizes the environmental im- pact. A robotic system has been devel- oped that allows automotive manufac- turers to apply waterborne paints with the assistance of electrostatic charging ‘see Fig. 1). Key to this system is an electrostatic spray gun capable of pro- viding the electrostatic isolation re- quired for waterborne paints. The gun, combined with the robot’s motion, fluid delivery, and control functions, provides an optimum paint application.

ADVANTAGES OF ROBOTIC PAINTING In order to appreciate the benefits of this new system, it’s important to un- derstand the advantages of robotic waterborne paint application and how to apply paint using the capabilities of a robot. The first step is to minimize paint usage by maximizing paint transfer ef- ficiency. Robots have the flexibility and dexterity to apply paint with the optimum orientation of the spray gun to the part. The distance between strokes, or the overlap, of one fan pattern to another is taught relative to the part. Once taught, a robot will maintain the same path relative to the part, even during line tracking operations. When line track- ing is used, the speed of the robot varies, depending on whether the robot is tracking in the same direction with, or against, the flow of the conveyor. The absolute speed will vary up and down with the conveyor speed, but the speed relative to the part will remain constant. This allows for the best ap- plication of paint. Figure 1. Robotic system for electrostatic application of waterborne basecoats.

METAL FINISHING OCTOBER 1995 0 Copyright Elsevier Science Inc. 47 SKETCH OF DEVICE MOUNTED NUC Robotics’ process engineers real- The following is a breakdown of the ON ROBOT ized that the reduction in the amount of system’s major components as illus- Robot Wrist paint being used per vehicle was an trated in Figure 2. important area to improve. It was also Measured Feedback obvious that, in order to reduce paint Components Mounted on the volume of usage, some type of electrostatic appli- Robot paint at cation would be required. The driving high - - .- - - .. Fill force for this development was the Reservoir with up to 1.5-L capacity for Port need to provide a cost-effective system paint for users. Displacement feedback device Paint /\/ The process engineers explored the Fluid regulator Regulator following electrostatic application so- High voltage cascade ... lutions, which currently exist for Filling valve q.:::..waterborne paint: Spray Gun Components Located Inside the External charging via a probe after at- Paint Booth Figure 2. Electrostatic waterborne delivery omization. system-sketch of device mounted on ro- Isolation of the entire circulation sys- Docking station with actuator for fill- bot. tem. ing valve Isolation with some sort of voltage Color drop box and color changer With robots, speed, gun distance Solvent and air from the part, and the approach vector block system. to the sprayed object remain constant They determined that the use of a The sequence of operation of the and the same amount of paint is used system is as follows: every time. This consistency of paint probe system with robots wouldn’t im- usage is a critical component to prove transfer efficiency enough to offset the packaging and cleanliness 1. The robot positions the filling valve achieving quality finishes while mini- to the docking station. mizing paint waste. concerns. The isolation of the entire circulation system was determined to 2. The docking station actuates the be too challenging to accomplish-and filling valve and cleans the reser- WHY WATERBORNE PAINT? it was a costly solution. voir. FANUC Robotics chose to develop 3. The color valve is selected and the Waterborne paint is an attractive al- reservoir is filled with the required ternative to high solids solvent paint an electrostatic solution based on the voltage block approach. This solution quantity for the next operation. because of its superior appearance. An- 4. The docking station closes the fill- other major advantage of using water- offers the unique opportunity to take full advantage of the control algo- ing valve and disconnects-voltage borne paint is the reduction in volatile block is established. organic compound (VOC) emissions. rithms available for robot control and fluid control, while improving on other 5. The robot paints the entire opera- VOC reductions are achieved because tion with voltage of 10-100 KV. deionized water is used as a large per- inefficiencies such as paint delivery length and the waste associated with 6. The robot returns to the docking centage of the solvent in the paint. In station. addition, deionized water can be used color change. FANUC Robotics’ volt- as the purge cleaning solvent. This re- age block system does not have the paint line charged at high potential and To reduce the amount of paint used, sults in the reduction of both sludge the reservoir is filled at a location as treatment chemical requirements and routed through the inside of, or around, the arm and wrist; therefore, pinholes close to the circulation and color disposal costs. change system as possible, with less Waterborne paint differs signifi- and paint hose life do not pose prob- lems. than 1 m of paint delivery pipe. The cantly from solvent paint in how it is device for filling+alled a dockingsta- applied electrostatically. Because the tion-is stationary and located inside majority of the solvent is water, water- THE ELECTROSTATIC the booth. borne paint is highly conductive. The WATER BORN E PA INT The robot executes a path to this conductivity of the paint presents DELIVERY SYSTEM docking station. The vessel to isolate unique challenges associated with the the paint is contained at the wrist of the association of the electrostatic charge The heart of this system is a reser- robot. This vessel is loaded with the from the normally grounded paint voir attached to the face plate of the correct and metered amount of paint to spraying equipment. robot wrist. The voltage block is support the next product to be painted. achieved because the paint and the res- The electrostatics are applied via a cas- WHY DEVELOP AN ervoir are insulated from the ground cade unit, which is similar to the units EL ECTR OSTATI C while painting. The device is only at- in production today. A feedback device WATERBORNE SOLUTION? tached to the circulating system while senses the displacement of fluid in the it is being filled, and never while it is reservoir. The flow rate required is Through their experience with auto- at high potential. The system includes controlled with the fluid regulator just motive paint finishing systems, FA- robot and control components. prior to the spray gun with control

48 METAL FINISHING OCTOBER 1995 algorithms from a closed-loop fluid de- livery control system. Located within the spray gun sup- port manifold, this system provides high accuracy feedback for fluid con- trol. Metering of the paint flow into the reservoir ensures that only the amount of paint required for the operation and for filling the spray gun is delivered.

PAINT DELIVERY REQUIREMENTS One drawback of using robots for automotive painting is the location of the color changer and the length of the paint delivery lines. In some cases a paint line could be as long as 10 m. Due to the pressure drop in the line, the inside diameter is increased and the volume of paint filling it tends to be large. Just call 1-800-426-7273.You'll be in direct contact An advantage of the electrostatic with Goldschmidt Industrial Chemical Corp., waterborne delivery system, however, a leading supplier of inorganic tin compounds. is the location of the filling mechanism near the robot. The circulating system Whether you want your Stannous Sulfate in powder, or any moves the paint down to the filling station. With the isolated vessel on the standard or customized solution of your choosing - end of the robot, the robot positions we'll deliver quickly and cost-effectively. itself to the filling station, and the need for a long paint delivery line is elimi- Stannous Sulfate from Gold. What a great solution! nated. Only a small quantity of paint remains with the system on the piston, inside the regulator and the cavity of the gun itself. GOLDSCHMIDT INDUSTRIAL CHEMICAL CORP. P.O. Box 279,941 Robinson Highway McDonald, PA 15057-0279 1-800-426-7273 (412) 796-5584 FAX: (412) 922-6657 CONCLUSION Electrostatic capability for waterborne Circle 055 on reader information card paints delivered by robotic is an important new technology that is now The Phosphating of Metals available. By using this process, automo- by W. Rmrrh 41X pager $131.00 tive manufacturers canrealize the follow- This treatw cuvers theoretical principles of phosphating and its practical applications. It also includes such periph- ing significant advantages. eral aspects as rffluent treatment techoology, standard and specifications, plant construction, methods for cold working of treated surfaces and modern lacquering processes.

Capability to electrostatically charge Send Orders to: the paint (up to 100 KV), which is METAL FINISHING isolated in an application vessel as 660 Wiite Plains Road, Tarrytown, NY 10591 close to the gun as possible. For faster service, call (914) 333-2578 or FAXyour order to (914) 333-2570 Paint flow control and feedback is at All bwk ordim musr be Iwepaid. NY. NJ and MA residmts add appropriate sales tu. Pleare include $5.00 shipping and handling for ddiuery of rch brnk via UPS LO nildresses in the US.;JX.00 fur each bwk for Air Parcel Pmr shipment 10 the gun, which is the ultimate loca- Canada; and $20.00 brea~li hook brAir Parcel Post shipmenr ro all other countries. tion, and is in a small enough pack- age to be robot mounted. Paint delivery lines are significantly holes and other paint hose-related paint will be dispensed on the job shortened and do not travel with the problems. prior to starting of the cleaning op- robot, but are left in a stationary Less paint loss with color change due eration. position. to a short delivery channel between Fluid flow and usage control is path Paint lines, charged at high potential, the color drop and the filling valve. based. Adjustments to the path are do not travel with the robot or inside Less solvent usage due to a very short now considered in paint-fill calcula- the robot arm, which eliminates pin- delivery channel and most of the tions, minimizing paint waste. MF

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