Ecoat Clinic: High Throwpower Impacts Ecoat Performance
Q. Why is high throwpower so important to the overall applied cost for an electrocoat system? What specific elements of the system are affected by it? How can I measure the throwpower effect?
A. The conventional definition of throwpower is the capability of an electrocoat material to deposit paint in recessed, enclosed or hard-to-reach or areas that otherwise would end up with low film builds or remain totally uncoated. A more scientific definition would be that throwpower is the capability of an electrocoat material to deposit uniform and even film thicknesses at varying electrode or anode distances.
For some, throwpower is one of the most critical formula and application characteristics of the electrocoat technology, and one of the main reasons why electrocoat is chosen over many other competing paint application technologies.
Using electrocoat materials with high throwpower capabilities can have a positive effect on higher production capabilities, higher efficiencies, enhanced corrosion performance and overall lower operating costs. They can provide even film builds between the hard-to-reach areas versus the easy-to-reach areas, equalizing inside versus outside film thicknesses. In other words, they eliminate or minimize the film thickness gradient, which is unwanted and excessive paint film. This excessive and unwanted paint film is typically high film build on outside surfaces or outside racks located closest to the anodes.
There are several methods to measure “standard” or conventional throwpower properties of an electrocoat material. Standard tests typically determine how far into a pipe or an open-ended box of panels the electrocoat is capable of depositing film. The tests determine the differential film thickness gradient between the inside and outside of the pipe or between panels within the test box. Either a two-panel throw box or a four-panel throw box test apparatus are used, depending on the industry and the application.
The standard throwpower tests are performed under laboratory standard application conditions of cycle time, bath temperature, percent solids, bath conductivity and voltage, in an effort to simulate and correlate film thickness deposition across multiple parts or racks. The throwpower results are typically reported as minimum film thickness obtained, the film distribution gradients as well as the percentage of film thickness achieved between inside and outside surfaces.
During a cathodic electrocoat cycle, the electrocoat paint solids initially deposit into the part areas that are the closest to the counter electrodes, being the anodes, in this case. As the resistance to the applied DC current increases—because of the resistance offered by the freshly deposited wet electrocoat film—paint solids are increasingly forced more and more into areas with predominantly bare metal surfaces. The coating deposition will continue on the parts further and further away from the anodes until complete coverage of the part is accomplished.
The electrical insulation value of the deposited electrocoat wet film is the key to throwpower performance and directly responsible for the film build characteristics of the electrocoat material. For an electrocoat material, this electrical insulation property is determined as the wet film resistance or inversely measured as the wet film conductivity.
The wet film conductivity is defined by the electrocoat main active ingredients in the wet film stage, which are primarily resin, pigments, solvents and water. Modifying the composition or physicochemical properties between the main active ingredients or its proportions will have an affect on electrocoat throwpower capability.
The lower the wet film conductivity of the electrocoat material, the higher the throwpower capability. The low wet film conductivity allows the electrocoat material to search for uncoated bare metal areas with higher conductivity than the wet electrocoat film.
Although low wet film conductivity electrocoats produce deeper coating reach and provide more uniform film thickness distribution between inside and outside areas, one physical limitation is their inability to build heavy film builds. Electrocoats with low wet film conductivities are self-limiting, low-film electrocoats only capable of maximum film builds between 20 to 24 microns. This limitation is important to consider when evaluating new high, superior or extreme throwpower performance electrocoats.
On the other hand, the higher the wet film conductivity, the less an electrocoat material will throw. The high wet film conductivity allows the electrocoat material to continue to build on wet deposited electrocoat preferently, rendering low coating reach and poor film distributions because of high films on the outside surfaces and little film on the inside areas. Because the low resistance of the wet film to build thickness, low throwpower electrocoats can build significantly higher film builds than their counterparts.
The throwpower effect of an electrocoat formula is not the same for all cathodic electrocoat applications. The electrocoating technology serves many different industries and OEM specifications, and encounters a large variatey of part geometries and application systems. The percent improvement provided by an electrocoat formula is very specific to each application and system. Not all claims out there are completely true, and all data from other systems should be used at face value.
To quantify the throwpower effect of your electrocoat system, start by monitoring the film thickness of parts during several normal production periods. Choose at least 10 racks or fixtures and determine the square footage of parts for each rack as accurately as you can. Don’t include the square footage of the racks, just the parts.
It is recommended to do the study several times to ensure that the calculations and measured effects are accurate and repetitive, as the type of parts, racks and part density all play a significant role on the results. The throwpower effect should be more evident using process racks with high square footage loads due to the size, geometry or rack density of the parts.
Once you know the square foot load, then calculate the theoretical material efficiency using the minimum film build requirement as the target for the calculation. Make sure that the electrocoat weight shrinkage during cure and a 95 percent overall transfer efficiency are all included in the theoretical calculations. Please note that using the minimum film thickness target in the calculations will allow us to obtain calculated material efficiencies without the influence of throwpower.
Measure the film thicknesses across all parts in the chosen 10 racks. Depending on the size of the parts, use at least five different locations within the area of each part. Film builds may vary as much as 17 microns when comparing outside and inside racked parts, depending on the actual parts and racks configuration.
Using the actual film thickness readings obtained, perform the same material efficiency calculations. Using the film thickness gradient in the calculation allows us to obtain calculated material efficiencies with the influence of throwpower. Performing both material efficiency calculations allows you to compare the theoretical material efficiency to the actual material efficiency and determine the actual effect of throwpower on the quality (minimum film thicknesses) and overall cost of your system.
The film thickness gradient across the parts and racks is unwanted excess paint film deposited per square foot over the minimum. Multiplying this number by the square footage production load of the system is an easy way to calculate the cost impact of the ecoat material’s throwpower on the system.
For a specific electrocoat system, altering any application parameters—like bath temperature, percent solids, bath conductivity, voltage or cycle time—can have a positive influence in the throwpower properties of a specific electrocoat formulation. Although they can provide varying degrees of throwpower levels, often times, the improvements can be marginal, as those variables do not govern the final throwpower characteristics of the electrocoat system.
In other cases, it is necessary to evaluate newer electrocoat materials and have the opportunity to select an electrocoat paint formula that improves the overall efficiency and lowers the total applied cost of your specific finishing process.
Originally published in the December 2016 issue.
E-coat can produce uniform finishes with excellent coverage and outstanding corrosion resistance.
This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 13, 2012.
Selective brush plating is much more than just a touch-up repair process. Hundreds of applications are using the selective brush plating process to provide surface enhancement coatings to aircraft OEM applications.