Anode switching works by adjusting the current density at any given point on a part. This is the major effect that results from turning an anode’s current flow on or off. The following chart shows the relationship between current density and efficiency (See Figure 1).

If the overall current density is raised, overall efficiency is also raised. This suggests that higher current density leads to a higher deposition rate at a better efficiency. In terms of cost savings, the higher the current density, the less cost to apply paint per piece per mil. The ideal point to coat is at the highest current density during the entire coating cycle as long as the maximum voltage across the film is observed to avoid rupture. This curve breaks down at a point of rupture when the voltage necessary to increase current density violates the resistance of the applied film. The highest tolerable current density varies from the beginning of the cycle to the end of the cycle and varies depending on the part painted.

Tank Design. Tank specifications are important considerations when implementing an anode switching system. The tank’s specifications can impact the efficiency of deposition. Any design that would “choke” the ability to transmit current to a part will also degrade switching performance. Limitations such as depth of paint and part submersion must be dealt with early in the design process. Switching can help improve performance when a part does not “fit” the tank, but it will not overcome all of the tank’s design limitations. Conversely, the more efficient the tank’s design, the more the switching effect will be enhanced.

This approach has been used in both monorail and square transfer systems, although the data presented in this paper is related to the latter.

Type of Paint. Paint formulas vary within as well as across manufacturers, leading to much variation. The question concerning which paint is a better choice can only be addressed by the user and the part being painted. Switching will work regardless of paint formula and adjustments to those formulas. Any adjustments made to paint formulas that affect throwing power will also affect switching results; therefore, switching can both enhance and degrade throwing power by simply switching anodes on or off at the correct times during paint cycles.

Figure 1: Relationship between current density and efficiency

Rectifier Requirements. Since switching can be done without direct control of the rectifier, it is possible to add a switching system to an existing system without implementing any control of the rectifier. However, without control of the rectifier, overall reduction in savings will not be as pronounced. It is therefore recommended that a rectifier control package be implemented along with the switching system.

The most effective rectifier control system seems to be a simple current limiting system that offers the ability to limit the rectifiers amperage to the maximum current that any one switch (or anode) can handle. This allows any anode to be on and pushed to its maximum current density during the initial ramp cycle. Holding all “working” anodes to their maximum current density allows the paint cycle to reach maximum efficiency.

This method of control also protects the wiring system from overload by dynamically adjusting the rectifier based on load changes from part to part and cycle to cycle. The most effective adjustment seems to be at one second intervals. Of course, the shorter the cycle, the shorter the recommended adjustment period.

Figure 2: Reduction in paint usage through the use of anode switching

It is also important to size the rectification system based on the maximum amperage required to coat the largest part. Switching works great to squeeze more amperage from the system but is still limited to the maximum amperage available.

Anode Requirements. Anode design impacts the effectiveness of switching. When too few anodes are used, a large zone is created leading to the inability to effectively control current density in small areas. However, if the desire is to reduce the paint film build over a large surface, then fewer anodes will work. Large anodes are more costly to switch since larger currents cost more to control. Too many small anodes can also cost more since large amounts of anode switches and wiring can add to infrastructure and cost. Both of these situations do not seem to be the norm, and most electrocoaters can immediately benefit from switch with existing anode designs. Individually wiring anodes is usually the only anode related change required to implement switching.

A Real World Example
The following discussion is based on a real installation of a switching system with rectifier control. All data has been “adjusted” to protect the client since this system is considered a “highly competitive advantage.” No data will be misrepresented in its relationship to the topic.

Figure 3: Reduction in paint thickness through the use of anode switching

Paint Use. The customer specifications require 1.0 mil minimum anywhere on the part. They have been seeing up to 1.45 mil in high current density areas and 1.0 mil in low. This means alot of waste just to coat the low current density areas to 1.0 mil.

Reduced Paint Usage with Switching. Switching off anodes with too low of a currentflow eliminates anodes that are operating in the low efficiency range. This keeps only anodes that are actually painting the part active. Once the high current density area is coated, the anode in that area is turned off. This allows the rectifier to increase in voltage and also increase the average current density in the unpainted area of the part. This increases the efficiency in which the low current density areas are coated. The net result here is an overall reduction in the high current density areas of the part to 1.11 mil while still maintaining a 1.0 mil overall thickness (see Figure 2).

Paint Use Value of Improvements. Overall value in this case is huge. This operation is now seeing a reduction of 0.34 mil per piece painted (See Figure 3).

Improved Consistency with Switching. The switching times are controlled dynamically. This means part-to-part variations are reduced. Also, individual part recipes control all aspects of the rectifier and anode amperages. Desired variations are enhanced but consistent. Undesired variations are reduced and consistent.

Figure 4: Reduction in cycle times as a function of part size

Productivity Increase with Switching. Original cycle times have been reduced by 25% to 50% (See Figure 4). This reduction is especially realized on large surface area loads. The reduction of time is the direct effect of running at higher current density and therefore always running the part at a higher efficiency. The target of 1.0 mil is reached sooner since the low current density areas are turned off and not acting as a “parasitic” current drain.

Additional Savings with Switching. This installation can now realize these savings:

• Produce up to twice as many parts per year;
• Up to 25%less paint per part;
• Less energy used by the rectifier;
• Less time and less amp/hours per piece means less chiller cost per piece; and
• More pieces per year means less overhead cost per piece.

Value of Switching to Productivity. There is an overwhelming case where switching has dramatically increased productivity and cut cost per piece painted. The increased efficiency has reduced cycle time by up to 50%, increasing throughput up to 100%.