Electrocoat rack stripping is a substantial cost to coaters, so it is imperative to use practices that reduce high energy requirements and identify techniques to minimize these costs that contribute to the electrical contact defects.
Part coaters and job shops rely on multi-purpose racks to convey parts through their electrocoat systems, and while wet electrocoat material is highly conductive, the deposited film applied to the part and rack is very low in conductivity.
As film build is increased, the surface conductivity decreases until the deposition is complete. Afterward, the part and rack are cured, which essentially makes them non-conductive, with the exception of the small contact points where the parts are hung. Once the rack is coated, it increases the chances that no electrical contact will be made between newly hung parts and the rack, which could lead to defects, such as no-coats, low-coats and partial coats.
A no-coat—sometimes called a white elephant—will be observed if a rack hook is totally insulated from the part. A second type of defect, a low-coat or low film build part, is caused by poor or intermittent electrical contact. The third type of defect, a partial coat—where only a portion of the part is coated—is also caused by poor electrical contact. To rework these defective parts, no-coats can simply be reprocessed through the electrocoat system to attain an acceptable finish. However, low-coats and partial coats require that the coating is stripped from the part prior to reprocessing to attain acceptable film build and appearance.
Rack Maintenance Plan
A rack maintenance plan to periodically remove the paint from the racks may be needed to avoid these electrical contact defects. When a variety of part types are run, the electrical contact point is slightly different, and the coating will preferentially coat the former contact points because this is the only conductive point remaining on the rack. With each additional pass, the uncoated area will decrease, and in turn increase the chances of defects. A rack stripping process must be established in plants that observe this problem. Rack stripping can be accomplished by sandblasting; using a chemical removal process (which typically requires circulated chemicals with temperatures ranging from 160–200°F), or by a burn-off process, which requires temperatures ranging from 800–1100°F.
The following analysis will show how the use of a high voltage electrocoat can reduce rack stripping requirements or the defects associated with the lack thereof, which in turn will reduce costs from increased part handling and excessive energy consumption that these processes require in addition to the extra paint consumption from repetitive application to the rack.
In order to minimize energy consumption, BASF commissioned a study to evaluate the defect rates when processing a rack multiple times through the electrocoat system. To explore a worst-case scenario, this study focused on lightweight parts (<
To reduce rack maintenance, the grounding mechanism must be better understood. The rack coating is nearly uniform and complete after one cycle through the electrocoat system. The only uncoated or conductive point, after the first cycle, is the small contact area where the part and rack touched.
When the first no-coat is observed, the part is contacting the rack at an insulated point, so the previous point of electrical contact will also be insulated because it has now been exposed to the E-coat. Theoretically, at this point, the hook is totally insulated and for all practical purposes is a “dead” hook. If the hook is insulated, it should produce no-coats on all subsequent runs, unless there is a mechanism to break through this insulation.
The graph below compares the overall hook performance of each run with just those hooks that exhibited defects immediately prior to that run. In other words, how does the hook perform immediately after a defect? Since the electrical defects are observed no earlier than the second repetition, the performance after such a defect will be observed no earlier than the third run.
Performance degrades after defect
The graph on the previous page shows that the performance degrades slightly after a defect is observed; however, the hook is still “live” because the defect rates are substantially less than 100%, which means there remains a mechanism to transfer electricity and coat parts. The rack is insulated at a target film build of 0.8 mils (20 microns).
However, electrocoat tends to pull away from surface edges, resulting in lower film at the edges of the hooks, where parts typically contact the rack. The lower film thicknesses allow only partial insulation, and the voltage is able to bridge the thinner gap to ground the part.
Based on the post-defect hook performance, the next logical step is to determine whether a high voltage electrocoat with increased application voltage can better bridge this thinly insulated area. Increased coating voltage has shown substantial improvements to throwpower; so whether or not rack maintenance can be reduced depends on whether the electrical contact can be improved with higher voltage. This theory is based on a natural phenomenon as simple as lightning.
As the electrical potential, or voltage differential, builds between a cloud and the ground, it accumulates until the potential is high enough to bridge the insulating gap of air, which initiates the discharge and forms lightning. Similarly, in theory, the ease of bridging the insulating gap of the electrocoat on the rack should be facilitated by utilizing a higher voltage.
The graph to the left tests this theory, showing the performance difference in evaluating the occurrence of electrical contact defects between a low voltage technology and a high voltage technology. The low voltage electrocoat utilizes an application voltage of 150 V and the other uses double the voltage at 300 V.
High voltage cuts defects
As Figure 4 shows, the use of high voltage technology significantly reduces these defects, and virtually eliminates the need for rack stripping. The voltage increase reduced the average defect rate from 29.6% to 1.0% for a reduction of 97%. This represents a significant savings opportunity for the versatile job coater that coats a variety of parts. As for electricity costs, the energy consumption is a product of both the voltage and amperage. The high voltage technology draws fewer amps per square foot to compensate for its higher application voltage.
The above results, however, only represent a cost savings to the coater if target film builds are maintained. A doubling of voltage coupled with a doubling of film build will greatly increase paint consumption, substantially reducing the savings or eliminating them altogether. Therefore, it is important that the paint be formulated to achieve target film builds at this higher application voltage to receive the benefits. Furthermore, higher applied film builds on parts will lead to higher film builds on the racks, increasing the rack insulation. This, in turn, will increase the defect rates observed, nullifying the effects of increased voltage. As a result, it is important to use a paint formulated for high voltage application in order to achieve the full economic advantages. n
To contact BASF, call 248-948-2534, or visit BASF.com