Solving the Ecoat Mysteries of the Deep

 

 

 

The human eye cannot witness the application of electrocoat; it occurs below the surface in the deep abyss of resin, pigment and water. A part slips beneath the surface into the depth of the paint, electricity is applied then the part emerges completely coated. It would almost seem like magic to the cause observer.

The hidden nature of applying electrocoat makes it is hard to troubleshoot problems, determine the health of the system and optimize system performance.

The application of electrocoat is dependent upon numerous factors including voltage, current, time, temperature and chemistry. Traditionally, electrocoat tank monitoring devices measure the total output of voltage and current at the rectifier/anode, and the average temperature of the electrocoat bath. It is hard to determine precisely what is happening inside the electrocoat tank with data gathered from these devices. Gaining insight into what actually happens in an electrocoat tank requires an instrument that measures and records DC and AC voltage, current and temperature on specific points of a part during the electrocoat process.

There is a diagnostic testing device, the Submersible Data Acquisition Unit, that can measure and record these variables. The Submersible Data Acquisition Unit—nicknamed “the Sub”—consists of a data recorder encased in a leak-proof container with probes and grounding wires. The Sub is placed on the part prior to the electrocoat tank and the probes are placed on the part. The device is removed after the electrocoat tank and data from the run is downloaded to a compute (see Illustration 1).

 

Illustration 1: Typical Voltage and Current Curves from the Sub

 

 

Data Uses

Data gathered by the Sub has many uses, but to date, it primarily determines the condition of an anolyte cell as well as the condition of the rectifiers and electrical connections. When used on a regular basis, data from the Sub can be used to predict when a component of the electrical system might fail. Data from the current probes can be used to troubleshoot appearance issues, such as pinholing. In addition, data has also been used to determine if anode placement can be changed to improve electrocoat film builds.

The voltage curves created from data gathered by the Sub are particularly useful in diagnosing equipment problems. The curves created from the data can illustrate problems with anolyte cells, rectifiers, and electrical connections. Anolyte cells can be plotted on the voltage curves to determine if they are working (see Chart 1), where the line graph is the voltage curve and where the bars are the anolyte cells plotted against the curve. The voltage increases when the probe is in front of an anolyte cell, and if the voltage decreases when the probe is in front of an anolyte cell, it indicates that the cell is not working. 

 

Chart 1

 

 

Chart 2

 

DC Voltage is supplied to the anolyte cells from a DC rectifier. The rectifier takes AC voltage and converts it to DC voltage, and when the rectifier is not working properly, AC voltage can pass through, which can cause problems in the electrocoating process. The problems usually associated with AC leakage include poor appearance, pin holing and gassing. The Sub measures the amount of AC voltage that reaches the vehicle. Chart 3 demonstrates a tank that had one of the two rectifiers leaking AC voltage and, in Chart 4, the rectifier was repaired. 

 

Chart 3

 

 

Chart 4

 

 

Electrical Connections 

Electrical connections—especially the connection between the bus bar and the carrier—are often overlooked and can have an impact on the coating process. The example in Chart 5 shows a plant that assumed to be operating fine, but instead experienced problems with the connection from busbar to carrier. The Sub was initially run at the plant to determine if the anolyte cells could be optimized to improve film build variability. The first run at the plant produced Chart 5.

The large dips in voltage are from a loss of connection. In addition, the insulators on the racks were flawed, causing the racks to coat at the end of the tank when they should not have been. The plant was able to reduce the operating voltage 50 VDC after the repairs. The film build variability improved and usage reduced (see the plant's corrections in Chart 6).

 

Chart 5

 

 

Chart 6

 

The placement of anodes can affect film build and its variability, including minimum film build. In addition, the power-on mode of the anode system will affect film builds. Data from the Sub—in conjunction with film build data—is analyzed to determine if the anode placement or power-on mode can be changed to improve electrocoat film builds. Film build data is reviewed to determine where deficiencies might exist. Sub data is analyzed to determine the root cause of the deficiencies. Once the root cause is identified, actions are formulated to eliminate it. This technique has been used at several plants with great success.

For example, Assembly Plant X’s electrocoat tank was designed for cold entry, which is when the power to the anodes is turned on after the vehicle or part is completely submerged. The power stays on until the next vehicle starts to enter, causing a gap in the applied voltage to the vehicle (see Chart 7). The interior film build was affected by this gap. The voltages in V1 and V2 operated at a higher set point to achieve interior film build. The higher voltages caused an excessive amount of film build on the exterior of the vehicle, especially the vertical surfaces. This problem was resolved by switching the power-on mode to hot entry, indicating that the power to the anodes is on when the vehicle enters the tank.

 

Chart 7

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Chart 8

 

 

Chart 9

 

Film Build Changes

Switching the tank to hot entry enabled the plant to reduce exterior film build and increase the interior film build.

Pinholing is a defect generally caused by voltage and current blowing through the film, leaving holes or gaps in the film, and usually occurs in the front half of an electrocoat tank, where the film is thinner. Pinholing can be seen on the current curves of the Sub. Typically, the current will increase rapidly at the beginning of the coating process, quickly decline to a lower level, and then taper off through the rest of the coating time (see Chart 10).

 

Chart 10

 

A plant was experiencing pinholing on the top of the right door only; the left was fine. The normal containment action was performed—reduce voltage in the front of the tank— but this did not help, so the temperature was increased and the front-end voltage was further reduced. This action proved unhelpful; the problem appeared to get worse. The Sub was run with the current probes in the area of the defect. The pinholing occurred at the back end of the tank, the problem being the overhead anode on the right side and drop down. The anode was raised and the problem disappeared (see Chart 11).

 

 

Chart 11

 

These are just a few of the cases from the Sub’s files, but they illustrate the potential of the device. Further refinements and uses are still being explored to enhance the Sub’s effectiveness.  

Joseph Subda is an electrocoating expert with Axalta Coating Systems. Reach him at joseph.j.subda@axaltacs.com, or visit axaltacs.com. The Sub is available from JP Tech, jptech.com 
or 262-642-7671.

 

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Originally published in the October 2016 issue.