How to Measure Ecoat Throwpower in the Field
Axalta’s Dan Rose says the regular monitoring, recording and charting of throwpower can be a very valuable tool for ensuring quality.
Q: What is throwpower and can you measure it in the field?
A: Throwpower is the term used for the electrocoat (ecoat) film build achieved in recessed areas of a part. The interior area of a rack with multiple parts can be considered a recessed area. Throwpower tests try to simulate an interior part condition and act as an indicator of the throw effectiveness.
Interior sections of vehicles, parts and or racks of parts can be more challenging to apply ecoat. These areas can have multiple complex layers of metal and limited openings for the ecoat and electrical current to flow.
There are various methods of measuring throwpower in the field:
Part Cutups: The part or vehicle is physically cut up and the films are measured. This is the most accurate way of determining throwpower and is common for small parts manufacturing. However, this is cost-prohibitive in the automotive and other industries, so alternative methods are used.
Throw Tube: This consists of a PVC pipe that is sealed at one end. A metal strip is secured inside the tube and grounded to the vehicle. This is mostly used in the automotive industry and is secured to the interior floor pan, but it can be used in industrial applications and attached to the parts rack. The type and number of parts on a rack need to be consistent from test to test when using the throw tube. This will ensure accuracy of the results. The throw tube is insulated and the current can only reach the throw strip through the open end. This method attempts to replicate a recessed part such as a rocker panel.
Throw Strips: Thin metal strips are placed inside of a part and grounded. This has limited use as it requires access through a drain hole or areas with open ends.
Throw Box Channel: Two metal panels are sandwiched together, forming an open channel. The panel is suspended horizontally in a vehicle with metal wire and grounded.
Film Build Gauge: The interior sections are inaccessible, in most cases, with traditional film build instruments. Some companies do offer long, thin probes that can access these areas.
Lab Testing: In some situations, it is not feasible to perform on-site throw tests. The two-panel and four-panel throw box tests can be performed in the lab or, ideally, in addition to field testing. The disadvantage of strictly performing lab testing is it will only show ecoat bath chemistry issues. It does not highlight potential process issues, while on-site testing can show both.
The methods to measure throwpower vary per test and per company, but the basic premise remains the same:
- The metal panel or strip is baked after ecoating.
- The film build is measured on the panel or strip in 1-3 cm increments. The films should be higher on the front of the strip closest to the opening and gradually decrease.
- Record the distance from the start of the throw strip until bare substrate is exposed. This is referred to as “Total Throwpower.”
- Record the distance from the beginning of the throw strip until a desired film build is achieved. The desired film build is usually the interior target film build specification of 0.2-0.5 mils.
Q: Is there a general specification for field throwpower?
A: No, it is relative to the specific process. Bath chemistry, part surface area, rack density, coating time and more will vary significantly from plant to plant and impact the throwpower results.
Throw power tests should be used as a gauge to verify that the interior film build specifications are being met.
The recessed film builds first need to be determined. This is done by ecoating a part, dissecting it and measuring the interior film build. The metal substrate film build needs to be subtracted from the total ecoat and substrate film to determine the true ecoat-only value.
The throwpower test is performed at the same time and the values are recorded to be used as the baseline. Plant-specific specifications can then be implemented.
The throwpower validation process should be performed yearly, at a minimum, or any time there is a significant process change. This would include any changes to the vehicle or part, new model year, line speed increase and more.
Benefits of Testing Throw Power On Site
Q: What are the benefits of testing throwpower on site?
A: Visual quality inspections are not possible in the recessed areas and film build cannot be easily measured. Throw tests should be performed regularly to ensure that the interior areas are meeting quality specifications. A change in throwpower can indicate issues that otherwise may have been missed and potentially prevent warranty claims, returns and liability issues.
A significant shift in throwpower indicates a process change and needs to be investigated. These can include:
- Poor part grounding or connection.
- Rectifier issues.
- Bath heat exchanger issues.
- The rack density is too high.
- High concentration of bacteria.
- An unusual large shift in ecoat bath chemistry.
- Issue with the throwpower testing method. Common issues are air entrapment or a poor ground.
Gradual changes in the throw results can indicate:
- An imbalance of pigment, resin, acid, solvent or other material additions.
- Degradation of the anode surface area and ecoat buildup on the anolyte cell membrane.
- The start of a bacterial infection.
- Poor grounding from ecoat buildup on the carrier/rack over time.
- Anolyte cell leak associated with a decline in pH.
The regular monitoring, recording and charting of throwpower can be a very valuable tool for ensuring quality and indicating potential issues.
Dan Rose is a senior product specialist at Axalta Coating Systems.
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