One of the most important characteristics of E-coat is throwpower—the ability to coat interior or recessed surfaces to provide corrosion protection in those difficult-to-coat areas. Many auto suppliers, for example, require a minimum interior thickness or use a test method that defines a target distance that the E-coat must "throw."
Seeking to put its products to the ultimate test, E-coat manufacturer BASF (Southfield, MI) used an experiment similar to Toyota's Nagoya box, coined the "4-panel Box Method," which it says clearly illustrates that its E-coat products, CathoGuard 525 & 525HE provide industry leading throwpower performance.
Several test methods are used in the automotive industry to measure the throwpower of an E-coat technology or product. But BASF says that after thorough testing, it has concluded that the 4-Panel Box method not only demonstrates which technologies provide optimal interior coverage and protection, but correlates to the jobcoater market as well.
The 4-Panel Box method consists of four evenly spaced panels with holes drilled in three of the four panels. The four panels are enclosed on all sides and bottom with electrical tape. The holes not only allow the paint to enter the box, but provide the only means for the electrical fields necessary to deposit the E-coat to penetrate the box. The box is coated with an E-coat bath in the laboratory under defined conditions, after which the film thickness on the front and back of each panel is measured after curing. "Superior" throwpower E-coats are able to coat all panels within the box, while minimizing film thickness on the exterior panels and minimizing film thickness variation across the box.
Both the CathoGuard 525 and 525 HE products achieved a film thickness on the exterior panels lower than or equivalent to the competitive products, and both were able to coat all panels within the box, says the company. In fact, at an exterior thickness of 0.7–0.8 mils, both products were able to achieve a minimum thickness of 0.45–0.50 mils on all panels. According to BASF, the competitive products were unable to coat multiple panels and showed a rapid film thickness decline from exterior to interior panels.
So how does this method correlate to the jobcoater market? The 4-Panel Box represents multiple racks of parts coated simultaneously at the customer facility. The ability of a superior throwpower electrocoat to minimize the film thickness variation across the box as mentioned above correlates to the ability to minimize the film thickness variation across a rack or multiple racks of parts.
Robert Doty, BASF's E-coat tech service manager, says CathoGuard's throwpower is unique to the industry. "It's formulated to search for the next bit of bare metal rather than continue to build on already painted areas."
For example, if a minimum film thickness is required on all parts, an E-coat with great throwpower is able to achieve the minimum target on the parts in the middle of the racks while minimizing the film thickness on the parts at the outside of the racks.
In contrast, for a standard throwpower E-coat to achieve the minimum target on the parts in the middle of the racks, the film thickness on the parts at the outside of the racks may significantly exceed the target—thus wasting both paint and money.
In addition, a standard throwpower E-coat that's unable to coat the interior panels of the box may also lead to increased "partial coats" or "no coats" in the jobcoater market (which can lead to scrap and rework) unless the excess paint is applied. This is why there tends to be a trade-off between excess paint consumption and the number of partial coats.
Using a superior throwpower electrocoat, says Doty, reduces paint consumption, the number of partial or uncoated parts and the reprocessing associated with them while increasing rack density and throughput.
Doty provides an example of a company that switched to CathoGuard 525 in 2008. This jobcoater runs an indexing system and loads racks very densely to maximize throughput. Prior to converting to CathoGuard 525, in order to achieve a minimum film thickness of 0.6 mils on all parts, the jobcoater was applying more than 1.2 mils of electrocoat to the parts at the outside of the rack in order to meet the minimum requirements on the parts on the interior of the rack. After converting to CathoGuard 525, the customer was able to reduce the film thickness on the parts at the outside of the rack to 0.8 mils, thereby minimizing the film thickness variation resulting in a 30% reduction in overall paint costs and a 48% reduction in scrapped parts.