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Advances in Robotic Finishing

POSTED 07/23/2008  | By: Mary Kay Morel, Staff Writer/Editor

Manufacturers looking to improve painting operations for parts of all sizes and shapes are turning to advanced robotic technology to increase finishing quality, consistency and throughput, while dramatically lowering operating costs and decreasing wasted material and hazardous environments for human workers.  Easy to install and program, robots are fast, reliable and can apply the exact same high-quality finish time after time without tiring.  Robotic painting provides an estimated 25-30% paint savings over a manual process, providing a very quick Return on Investment (ROI).  Other advantages of robotic painting include less maintenance and cleanup, lower filter/water wash chemical cost and reduced volatile organic compound (VOC) emissions.  Paint robots improve safety by reducing the exposure of human workers to paint fumes and other environmental risks, as well as by reducing repetitive motion injuries.

Robots can be used for primer, base coat, finish coat, clear coat and spray dispensing,  using water-based, solvent-based, powder, glaze and glue/adhesive materials.  Today’s  flexible, high-performance paint robots can efficiently coat intricate parts with recesses, curved and contoured surfaces, and even picture-frame-like shapes.   A robot can be programmed to use a complex spray path to apply coating materials to different areas of the part to various film thicknesses -- without runs or sags.  

Painting-Specific Robots

Example of a painting robot with a three-roll wristIn addition to traditional paint robots that use external hoses and cabling to feed the paint gun, robot manufacturers now offer application-specific robots designed to optimize finishing operations.  These hollow arm models feature integrated cabling and paint hoses through the upper arm to make programming easier and also improve access into tight spaces that otherwise could not be painted robotically. 

Various types of robot wrists are available – each designed to facilitate painting of specific types of parts.  Three-roll wrist – provides three axes of motion – (RBT axes -- roll/bend/twist), all in a compact wrist assembly.  A three-roll wrist is well-suited for painting complex contours, such as car body interiors and the insides of box-shaped objects and other enclosures. 

Lemma wrist – also provides three axes of motion, but has slightly less flexibility to maneuver in tight spots.  A Lemma wrist is well suited for high-speed painting or coating of less complex part shapes in horizontal and vertical planes, such as a frame or the outside of a cabinet.   

Hollow wrist – has the same type of movement as a three-roll wrist, but includes a large opening through the robot base, arm and wrist for hoses, cables and direct connection of various spray application devices to the robot wrist.   With a hollow wrist, interference between the hoses and parts/fixtures is avoided, ensuring optimum cycle time and robot reach/access.  Programming is also simplified without hose interference worries.  However, hollow wrists cost approximately 10-15 percent more than non-hollow models.

Some robot manufacturers offer a variety of fully integrated spray gun options for these different wrist types, including traditional air spray guns, electrostatic guns and high-speed bell applicators, as well as powder coating applicators.

Common Options for Paint Robots

Painting robots are often equipped with closed-loop fluid control, such as flushable gear pumps or Air Operated Pressure Regulators (AOPRs) with flow meters.  Closed-loop fluid control guarantees the amount of fluid dispensed, which affects paint quality and film thickness. With a manual application, the worker adjusts the path used to paint a part to accommodate changes in viscosity, etc.  However, a robot is blind, so closed-loop fluid control is critical to monitor the actual paint fluid delivery output.

Most robot manufacturers offer a built-in (external axis) servo motor that can be used to drive the flushable gear pumps.  This type of servo motor option is controlled by the robot controller, which provides better control of fluid delivery, as well as the capability to control paint operations through the robot teach pendant. 

Color change valves (a type of air pilot valves) are often used when painting multiple colors.  Mounted on the upper arm of the robot, these valves allow automatic color change in as little as 15 to 30 seconds. 

Solenoid valves also can be mounted on the upper arms of some of the larger, floor-mounted painting robots to provide faster response time for color change or fluid delivery control.  Solenoid valves convert electrical signals from the PLC or robot  to air pilot (pneumatic) signals. 

Some applications use disposable paint robot covers to protect the manipulator from material overspray and allow easier clean up.  Some paint robots now include a Teflon coating inside and outside the hollow upper arm casting and wrist to reduce hose wear and maintenance.

Painting Robots Require Special Controllers

Robots are ideal for liquid painting operations where fumes might be hazardous to human workers.   Generally, robots used for liquid painting operations are required to have a Factory Mutual (FM) Class 1, Division 1 intrinsically safe (explosion-proof)  rating.   To reduce the risk of explosion, air purge is used to positively pressurize the robot and keep flammable vapors away from the electrical motors.  Powder coatings are usually not flammable like liquid coatings; however, in some cases the intrinsically safe painting robots and controllers might still be required due to the explosion risk for power-coating materials. 

Motoman SK6 robot is used to pre-heat plastic parts prior to painting operation.Some advanced controllers for painting robots feature programming pendants with built-in menus specific to painting with functions such as Gun On/Gun Off, Color change, and paint condition files that control fluid, fan air, atomizing air, electrostatic high-voltage, bell speed, and shaping air.  (Shaping air is used to control the size of the pattern created by the painting bell as it spins.  More shaping air provides a narrower paint pattern, less shaping air provides a wider paint pattern.

Powder Coating

Due to the powder application process, it is not unusual for the robot speed to be significantly slower for powder than liquid coatings.  High-volume robot powder applicators should be considered to reduce the cycle time or number of robots required.

Robotic Plasma/Flame Treating of Plastic Parts

When robots are used to paint plastic parts, sometimes another robot is used to pre-treat the  parts, which changes the molecular structure of the plastic, allowing paint to better adhere to the parts. In some cases, flame treating is also used in lieu of traditional water-based (aqueous) part cleaning systems. 

Vision Systems

Vision systems are becoming more affordable and are being used more often in painting applications or closely related tasks.  For example, one automotive supplier uses a laser in conjunction with high-end cameras to inspect automotive body panels prior to painting operations to detect dings or other flaws.  To determine defects, the vision system matches the laser scan and camera images of the auto body panel with a computer model and confirms that the part is within tolerance.  Another manufacturer that runs more than 70 different part families uses bar code readers and vision systems to read part tags and automatically tell their painting robots which spray pattern program to run.  If no bar code read is detected, the robot uses a “generic” program that will cover most parts.  Results are dramatic – material savings alone can easily pay for a robot system in just a few short months, depending on product volumes and system complexity.  The manufacturer in the later example above achieved payback in only seven months.

Advanced PC-Based Software

When planning your robotic painting system, PC-based simulation can be used to select the robot model and wrist type, and optimize cell layout by determining the best placement of the robot(s) in relation to conveyors and spray booth walls to eliminate potential interference.  Users can then simulate part and conveyor movements, as well as the actual spray process to determine the best coating pattern that delivers the required finish with the least waste of materials and fastest throughput time.

Simulation programs allow manufacturers to develop robot programs off-line on a PC and download them directly to the robot.  This reduces or eliminates down time required for point-to-point robot programming.

New Approaches in Automotive Exterior Paint Lines

New robot designs are starting to change paint lines in automotive exterior paint applications.   Manipulator arms are narrower, and feature hollow wrists, providing better part access.  Previously, paint robots were nearly always floor-mounted, which created a limited working area due to interference between the robot, applicator, and car body coming down the line.  Newer design paint robots can be shelf- or overhead-mounted, which offers significant advantages by expanding the effective work envelope and decreasing potential interference.  One automotive company has been able to reduce the width of their paint booth by 25 percent (from approximately 6,000 mm to 4,500 mm).  Overhead robot layouts reduce floorspace requirements, resulting in additional cost savings.  Overhead-mounting configurations also provide less contamination from paint overspray onto the robots and base risers, which decreases maintenance requirements. 

Growth Markets

Automotive companies and suppliers were early users of paint robots, and they continue to take advantage of today’s more flexible robots to minimize floorspace on paint lines.  Use of robotic painting has expanded into Tier 1 automotive suppliers, as well as to general industry.  Aircraft manufacturers are also turning to paint robots to provide the kind of high-quality finishes needed on large components.  Paint robots are easy to cost-justify for virtually any application.