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Emerald Group Publishing Limited
Copyright © 2006, Emerald Group Publishing Limited
Bell-bell processes save time and money for metallic paint application
Bell-bell processes save time and money for metallic paint application
Keywords: Robots, Painting
A significant trend over the past 5 years has been an increase in the use of rotary atomizers, or bells, for metallic painting applications. This approach is commonly called a “bell-bell” process to distinguish it from the previous standard process that utilized a spray gun for the second coat of basecoat application to obtain the desired color characteristics (Plate 3). The bell-bell process is being used by OEMs as well as auto component and general industrial suppliers, and is generally implemented to take advantage of the cost, quality and environmental benefits of using higher transfer efficiency application equipment. Recent advances in both material and application equipment technologies have made this processing approach a very practical and beneficial application for many companies.
Plate 3 Bells have a significantly higher transfer efficiency rate than conventional spray guns
Why implement a bell-bell process?
A primary motivation for adapting this process is cost savings. Bells have a significantly higher transfer efficiency rate than conventional spray guns. Typical transfer efficiency improvements expected for bells compared to non-electrostatic and electrostatic spray guns are 40-50 and 25-35 percent, respectively. In addition to material cost savings, manufacturers will also see savings due to reduced hazardous emissions as well as reduced spray booth cleaning frequency. Dirt related defects can be reduced because higher transfer-efficient bells generate less overspray than spray guns and consequently less paint buildup on equipment. It is not uncommon for customers who have replaced spray guns with bells to see a two to three times reduction in the cleaning frequency of their spray booths and application equipment.
Quality improvements can also be expected with a bell-bell process. This is due to the improved ability of a bell to atomize the paint compared to a spray gun. The bell-bell process can improve overall paint film uniformity, especially if robots, with their accuracy and path repeatability, are used as the automation equipment transporting the bells. This improved uniformity will reduce defects such as paint sags and other film thickness related problems. In addition, manufacturers have utilized this improved uniformity to either reduce the target film build requirement (for material savings benefits) or to increase the target film build (for quality improvements). Both of these are enabled because the uniform application allows the end- user to operate more robustly on either the low- or high-end of the film build requirements.
Considerations when applying a bell-bell application to your facility
With the proper application, planning and validation, a bell-bell application is generally an excellent choice for many painting applications. However, there are some things to consider in determining if this process is right for your application.
Is there another painting process that the bell-bell system needs to match? Some plants may have a different paint line that is using alternative application equipment such as spray guns to paint the same parts. Also, a different process may be used to paint other parts, such as fascias, and the parts must match the vehicle being painted with the new bell- bell process. If these conditions exist, additional testing and flexibility in color matching standards is generally required.
Because a bell atomizes paint differently than a spray gun, the color and appearance will generally be different with the two applications if the identical paint is used. Adjustments to the paint formulation or solvents used are generally required when converting to a bell-bell process. If the bell-bell process is being retrofitted into a facility that also has a gun-gun or bell-gun process with the same paint, the ability to adjust the paint is more limited. A similar condition exists if a bell-bell process is being adapted to an OEM facility that also receives parts such as fascias from an outside supplier. A small color shift to one side of the paint color standard as a result of the bell-bell process may be more significant if a part supplier is operating on the other side of the standard.
The number of colors currently being sprayed in the facility is also a factor to consider with a bell-bell process. If the existing paint needs to be adjusted with the implementation of the new process, this can present a problem if a large color palette exists. For most automotive suppliers, the typical color palette is 18- 24 colors, which is very manageable in converting to a new process. On the other hand, an industrial truck supplier with a palette of 300 colors would have a much greater challenge in implementing the new process.
Cost is a third item to consider when implementing a bell-bell process. In most cases, the return on investment for these applications is very fast due to the significant savings realized. However, in addition to the added cost of the bell application equipment, the spray validation testing costs, as well as the cost of training the manufacturing staff should be considered when doing a business case for a specific application.
Keys to success
The three-legged stool
Probably the most important key for success in implementing a bell-bell process is strong cooperation between the end-user, the paint supplier and the application/automation equipment supplier. The end-user will be critical to determine the color and quality requirements and trade-offs for the process both up front and during the spray validation process. The paint supplier plays a key role in making adjustments to the paint during the spray testing for the new process. The application equipment supplier will provide the flow rate estimates, recommend process parameter adjustments, and will generally be responsible for programming the automation equipment on site. It is critical that all three parties work closely through this process to find the best solution. Also, since these people will be working together through the launch of the system, the training, relationships and teamwork established during the testing phase will pay large dividends during the critical launch periods.
Plan early, test early
In many cases, new application and automation systems are installed just a few months after purchase. This provides a limited timeline for performing the desired spray testing and paint adjustment process. Spray testing to find the optimal process is accomplished by performing panel spray testing at either the paint supplier's or application equipment supplier's facility. This precious lab time generally needs to be reserved far in advance.
For spray testing, three to five colors which represent the entire color palette are chosen. Typically silver (or the brightest color) is chosen to make sure the brightness and travel requirements are optimized. Another bright metallic (such as gold) is included for brightness and hue optimization. In addition, two to three other colors from the medium and dark metallic paints in the palette are selected. If the paint palette contains a high film build and low solid content color, it should be selected to determine the application parameters and resulting appearance. The testing of all of these colors is focused on finding the optimal quality at the required color standards. It is common to have slightly different process settings for the paints within the different color families.
Testing these colors takes approximately 2 weeks. If adjustments to the material are required based on the data from the first round of testing, a second round of tests may be performed on some of the colors. Finally, once optimal process settings are determined, confirmation panels on all colors may be sprayed.
Training at all levels
Training of engineering, maintenance and system operators is crucial for continued success of a newly implemented bell-bell system. In many cases, manufacturers may already have bells in some parts of their paint shop (such as a clearcoat zone) which will be beneficial from an operation and maintenance perspective. However, the bell equipment developed for the bell-bell process may be different from the previous equipment and almost certainly will have different guidelines for how to optimize or adjust the process. For instance, understanding how adjustments to bell speed, shaping air, electrostatic settings, and film build ratios impact color and quality needs to be understood by those responsible for the new system in production.
If the facility does not currently contain bells, the maintenance, operation, and troubleshooting methods are significantly different than those of spray guns. Therefore, up-front planning of training with hands-on experience through the launch is key to a smooth transition of the new process.
The proliferation of the bell-bell process in the painting industry has been one of the most relevant recent advances in the industry in terms of its impact on finish paint quality, environmental benefits and cost reductions. With proper planning, testing, and cooperation from all parties involved, a bell-bell process may be just what you need to gain that next level of performance in your paint shop.
For further information, please contact: Martin Rola, Director of Engineering, FANUC Robotics America, Inc.