An Automated Line for Painting and Coating / 1988

Technology Reflecting Real-World Experience

An Automated Line for Painting and Coating / 1988

Automating the Line with Welding Robots

Japan emerged as a key worldwide auto exporter in 1980. As export volumes increased, Japanese automobile manufacturers were asked to improve the paint quality of their cars in order to provide sufficient corrosion resistance in certain regions of the U.S. and Europe, where vehicles were subject to damage from the application of salt to icy winter roadways. The manufacturers were also asked to improve the exterior quality of their cars in the hope of answering an emerging consumer preference for expensive-looking cars.

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The sealing process used a robot with a larger operating range that was ideal for wide application areas.

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The undercoating robot was capable of applying a coating over the bottom surface of the floor without using masking tape. The use of a dual-head spraygun allowed the robot to handle different coating specifications for multiple models, helping integrate the process.

Most automobiles then employed a triple-coat, triple-bake coating system - the so-called standard process - except for certain luxury models. In this system the body was first cleaned in a pretreatment process before passing through the electro-deposition coating process to receive the rust-prevention treatment. The rust-proofed body was then forwarded to the surface-preparation processes: first to the sealing process, where sealant was applied along the seams between steel sheets for waterproofing and dust-proofing; and then to the undercoating application process, where a protective coating was applied to the underside of the floorpan in order to inhibit rust and excessive vibration. An intermediate coating was then applied over the undercoat to enhance its performance. The final application of an overcoat completed the process.

Honda's coating lines had certain drawbacks, however. Although the methods used by Sayama Plant and Suzuka Factory differed somewhat, the coating lines at both facilities were basically mixed lines comprising automated machines and human operators. As such, the line facilities were substantially large and lengthy. There were also many processes that depended upon operator skills. Each time the number of operators increased, so did the amount of dust that could enter the system and contaminate the coating material. Likewise, production costs were rising as a result. Therefore, in order to achieve a high degree of exterior quality, Honda had to address some challenging questions.

Another concern was the nature of coating/painting lines, in which large quantities of solvents were employed. From the standpoint of work safety, it was preferable that manual operations be minimized or eliminated from the process.

The sealing and undercoat application processes in particular required numerous operators, who had to work in unnatural positions such as crouching forward and leaning back. Also, the operators had to clean their workplace frequently because the area was easily soiled by spilled materials. This was a burden that only added to the fatigue the operators experienced. Moreover, these processes were prone to inconsistencies of quality. In light of such detrimental conditions, sealing and undercoating applications soon topped the list of processes requiring automation.

Honda Engineering (EG) began its effort to automate the coating/painting line in mid-1984. As an initial step, EG directed its attention to surface-preparation processes such as sealant and undercoating applications.

In October of the following year, EG's special development team working on coating projects installed a robot for sealant application on the second line at Suzuka Factory. This prototype, a modified version of the welding robot, was part of a control experiment designed to examine the feasibility of robots for use in mass production. However, the process of sealant application was more complicated than the team had expected, so the experiment served only to highlight numerous areas needing further improvement.

Before work could begin in the sealing operation, the team had to improve on the accuracy of positioning for the body as it was sent down the line. Even a slight offset in its position, which the operators would probably have ignored, could have a significant affect on the robot's performance in applying sealant. Another drawback of the modified welding robot was that it was unable to move smoothly in accordance with the flow of the line. Not only did this prevent the robot from applying a smooth coating of sealant over the welded surface of the car body, it led to inconsistencies in coating thickness. Accordingly, the operators had to correct problems manually in order to stabilize the fluctuating quality of robot operation.

EG's project team also learned that in the coating operation the robot had to process much more information than they had expected. Therefore, in order to process large amounts of data, there was a need to increase the robot's intelligence.

EG introduced a robot for undercoating application to Sayama Plant's second line in 1986. However, the robot was something less than perfect. The body forwarded to the undercoat application process had approximately ten holes in the back of the floor that were used as positioning references during welding or to assemble portions of the drivetrain. During undercoating these holes had to be covered with masking tape, which then had to be removed once the application was completed. Although the undercoating application was being performed by the robot, the attachment and removal of masking tape before and after the application still needed to be done by hand. The matter of automating this step posed an important question for Honda's engineers.

During 1986 and into 1987, the development team created a number of new technologies, including a high-density, high-speed, high-accuracy robot offering a larger range of operation, making it ideal for general areas requiring sealant application. It also provided a sealant supply system and a spraygun capable of controlling the spreading of sealant as it was ejected. The team even worked to enhance the accuracy with which the car body was positioned on the line.

The team also developed a series of new technologies in the area of undercoating application, including a small, orthogonal robot suitable for applying undercoating over the floor's bottom surface; a constant-quantity undercoating supply system for improved accuracy of the application pattern; and a dual-head spraygun capable of coating two sections differently.

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