Meeting the Challenges of Contract Electrocoating

Carolina Coating Systems, Greenwood, SC, is a contract coating business specializing in electrocoating. It was founded in 1993 on the strength of a recommendation by the local economic development agency, which had become aware of the need for electrocoating services in the area. The business began operation with one customer and a used hoist system, painting after-market trucking accessories. Seven years later, more than 20 customers are served within a 200-mile radius of Greenwood, with an emphasis on components for the automotive industry.

The original hoist system was lost in a fire just 3 months after startup. Current equipment includes a square transfer system purchased in 1994 and a second square transfer system that became operational in April, 2000. Increasing workloads from existing customers and an influx of new customers taxed capacity to the point where three shifts each work day were required to meet demands. The new system allows a reduction to two daily shifts while maintaining extra capacity for expansion.

Selling the business

Although the initial opportunity was presented by a governmental agency with a customer ready to supply work at startup, the success of Carolina Coating depended on filling capacity in order to grow the business. Being the only “local” company able to provide an electrocoating service attracted a great enough volume of work to survive during an admittedly difficult period of on-the-job training. Specifications for automotive components to include electrocoat as a final coating or a primer were also instrumental in attracting customers to the company, as we stood ready to meet requirements set by automotive manufacturers. The square transfer system purchased in late 1994 had a versatile work envelope that matched the relatively modest size of our customers’ products. Even better, the system had been built on spec by the supplier and was quickly shipped and installed to begin production.

Developing sales for most businesses means selling the process, as well as the company. We have been fortunate that product specifications mandate electrocoating for automotive components manufactured by several relatively small companies in our area that cannot justify installing a system on their own. None of our present clientele knew coming in that they had decided on electrocoating—or electrocoating had been specified by their customers.

Beyond that, word of mouth recommendations have been our most effective means of expanding our customer base. Carolina Coating has seen success as a niche industry in an area where the nearest direct competitor is some distance away. Dependability has become our most important selling point.

Operation

As a startup company, Carolina Coating underwent a steep learning curve involving electrocoating. Although general information was received from the paint supplier, many of the intricacies of troubleshooting coating defects were solved early on through trial and error. A combination of tank testing and systematic adjustment of process variables has been critical to maximizing throughput and minimizing rework.

Carolina Coating currently applies epoxy paint on a variety of parts. The vast majority of those parts are machined or stamped automotive components. Our square transfer systems are equipped with an eight-stage zinc phosphating pretreatment. The electrocoating stage is followed by three post-rinses and a drip zone prior to curing.

Cleaning concerns

Although the products processed by Carolina Coating vary in size and shape, most are similar in configuration. Few enclosures are painted, meaning that dragout, draining and trapped air concerns are minimal. The gauge of metal used is fairly consistent.

The biggest problem in obtaining uniform cleaning has been seen with deep draws and motor mounts. These parts have tight areas where normal agitated solution in a regular immersion system cannot readily reach. To obtain better cleaning, these parts are prewashed in a separate batch system with spray impingement. The spray loosens and removes impurities in holes and bends that are less likely to be completely cleaned in the regular pretreatment system.

On our square transfer systems, counterflowing allows fresh water to enter at the cleaning rinse and flow back into the second cleaning stage. Similarly, the second cleaning tank counterflows into the first cleaning tank. This helps to return dragout to the dirtiest tank and lower contamination in the rinse stage. Solution from the cleaning tanks is then run through bag filters to remove particulates.

Oils used in the manufacturing process or for protecting the bare metal prior to coating are the major causes of cleaning tank contamination. When removed from the parts, oil will become suspended in the cleaning solution and, over time, foul the bath and cause excess carryout to subsequent tanks. Although a skimming device can eliminate significant amounts of oil, the primary cleaning tank solution must be closely monitored to ensure that high oil content and weakened cleaning solution do not lead to poor cleaning and an unacceptable reject rate.

Case study

Among the products electrocoated by Carolina Coating are covers for automotive EGR valves. Initial attempts to process these products resulted in a 50% reject rate due to major coating defects. The problem was traced to a paraffin-based drawing compound used by the client. This is an extremely difficult coating to remove due to its thick consistency and aggressive bonding properties.

Because the pretreatment capabilities of the square transfer system alone were insufficient to remove the compound, the EGR valves were first prewashed in the batch cleaning system. The reject rate was found to improve somewhat but was still unacceptable. A similar slight improvement was found by performing a second prewash. It then became apparent that aqueous cleaning alone would not satisfactorily remove the paraffin compound.

In cases such as this, it is sometimes possible to work with the customer to find a comparable metal lubricant with similar performance but with a more cleaning friendly chemistry. Unfortunately, our customer was unable or unwilling to find such a product.

As a result, an even more aggressive strategy was implemented in which parts fouled by the paraffin compound were hand cleaned with an all metal safe cleaner prior to entry into the electrocoating system. Using this method, the reject rate on the EGR valves dropped to 4-5%, which, although certainly not ideal, was acceptable.

Coating concerns

Although tap holes and other tight places significantly affect immersion cleaning, the attractive forces present in the electrocoating tank overcome the relatively low flow rates through such areas. Unless the paint bath has been contaminated in some way or the racks overloaded, poor pretreatment has been found to be the cause of most, if not all, paint defects in our operation.

Prior to installation of the second square transfer system, we had been using a two-part electrocoat consisting of resin and pigment components that were added to the tank separately. We have now gone to a single-component premixed by the supplier. This eliminates worries about mix ratios, which can easily become skewed during the course of daily operations. A bulk tank has been installed to store the new paint, which will be automatically metered into the electrocoating tanks on both systems. Buying paint in bulk has brought a significant reduction in the cost per unit.

Curing concerns

In order for complete curing to take place, it is essential that the product substrate reach the optimum curing temperature and is held there for the minimum recommended amount of time. Problems may arise if parts with varying material thicknesses are processed together, as heavier metal gauges take longer to heat through than sheet metal.

Most of the parts processed by Carolina Coating are made of similar substrates. This means that no special action is usually needed for good curing. To deal with the heaviest gauge parts, the process cycle timing on the square transfer system is slowed by 5-7% to allow for extra cure time. The paint curing window allows us to extend oven time without overbaking the paint on lighter substrates. This means that heavier and lighter parts may be run together without segregation, although throughput is increased by the 5-7% factor when only lighter gauge parts are run through the system.

Racking concerns

Obviously, the main interest of any contract coating company is to maximize throughput. Running an electrocoating system at highest efficiency means producing as much finished product as possible in the shortest time frame with the fewest defects possible—in other words, getting the most out of the machine.

Correctly racking product is one of the most important aspects of achieving high throughput. Proper racking maximizes the square footage of product in any one load, eliminates rework due to poor grounding, prevents loss of parts in the tanks and ensures that tank solutions have easy access to every area of the parts during the cleaning and coating processes.

Premanufactured racking systems do exist but are targeted almost exclusively at spray coating operations. The immersion nature of the electrocoating process allows for three-dimensional racking with no loss of coating efficiency. It is essential that the contract electrocoater become an expert at designing and building racking systems customized to those products handled on a regular basis.

Adjustable racks are a mixed blessing, although they can be useful for short run projects and miscellaneous items. It is always better to load only one type of product than several different products on any one rack because mixing partsmeans processing less square footage and realizing less efficient loading and unloading. Special racks dedicated to each type of high-volume product can best process that part with maximum proficiency.

The three elements of superior part racking are close spacing, good air release/solution drain and stability in process. Parts that have bends in them, for instance, should be racked so that the bends all face in the same direction. “Nesting” parts in this way increases the square footage in any one load. Creative use of the three-dimensional work envelope can squeeze extra profits out of any electrocoating tank.

Parts that are manufactured with internal voids must be positioned so that air is able to escape quickly and easily at the beginning of the immersion process. This will prevent air pockets from interfering with free access of tank solutions to all surface areas. When parts are removed from the solution, racking should encourage draining to minimize dragout, as well as puddling in bends and corners that can result in poor curing. Air release is more important than draining, but air knives can be added to the drain area after the final electrocoat rinse tank to blow off excess solution before parts enter the oven.

Finally, parts must be held securely during cleaning, coating and curing processes that can last for nearly 2 hours. At Carolina Coating, products undergo 12 immersion processes in baths of varying temperatures with strong circulation currents and are subjected to high heat in the oven where expansion of the metal substrate can cause separation from the racking system. Even heavy gauge parts with relatively low surface areas can be affected to a surprising degree by tank currents. The penalty for poorly secured parts is lost product in the tanks which interfere with circulation, block drain access and, in many cases, intrude into the work envelope and knock additional parts loose.

New electrocoating system

Continuing growth filled the capacity of our existing equipment and necessitated a three-shift working schedule. This growth came from increased workload from existing customers as well as an influx of new customers.

Carolina Coating is completing its first month of operation with a second square transfer electrocoating system. Although this system has a work envelope identical in size to the previous square transfer system, it has doubled the rated processing capacity in terms of maximum square footage per load. We have not been able to realize the full increase, however, due to size, shape and configuration of most of the products we process. Even with optimal racking strategies, we expect the overall average throughput of the new system to be somewhat less than 100% of the rated maximum.

The second square transfer system does give us the ability to continue production if, for some reason, one system goes offline. In fact, the extra capacity of the new system should allow us to meet current production requirements in two 8-hr shifts, rather than 3. Even with further workload increases, plans are to shut down at least one square transfer system every workday for one shift in order to perform routine maintenance.

Changing the formulation of the paint used in our systems has been discussed previously. Two other major changes have been made regarding the new system, which have also been made to serve the original square transfer system: the use of reverse osmosis (RO) conditioned water for electrocoat tank makeup and final pretreatment rinse; and changing to a zinc phosphate conversion coating from iron phosphate.

DI water vs. RO water

When Carolina Coating purchased its first square transfer system in late 1994, dual-bed deionization (DI) was the accepted method of obtaining high-quality water for electrocoating bath makeup and final pretreatment rinse. Since that time, reverse osmosis (RO) has been developed to provide water of near similar quality with none of the chemical and waste treatment costs associated with the resin bed regeneration required by DI systems.

A RO system capable of supplying both square transfer systems has been installed with the new electrocoating equipment. The DI system has been retained as an emergency backup. Use of RO water is expected to save thousands of dollars per year in water conditioning costs.

Iron phosphate vs. zinc phosphate

The first square transfer system purchased by Carolina Coating used iron phosphate as a conversion coating. Iron phosphate is a nontoxic substance that readily remains in solution and generates minimal sludge. Spent solution may also be pumped directly to drain if phosphates are within discharge limits.

Zinc phosphate serves the same function, except with much better performance. The coating it produces on the metal substrate is tougher against corrosion and builds more heavily, offering an even more receptive surface for the paint. However, zinc phosphate is not easily held in solution and produces a sludge that must be removed from the tank to prevent fouling of the piping equipment. Additionally, zinc sludge is classified as a hazardous waste that must be disposed of in a regulated manner.

Because product specifications from automobile manufacturers demand high corrosion resistance on painted parts, zinc phosphate is the only viable choice for a conversion coating. Just as the type of paint used in our systems is specified by our customers, so too is their demand for the ability to apply a zinc phosphate conversion coating.

The recent purchase of our second square transfer system allowed us to not only offer zinc phosphate in the new system but in the older system as well. The waste treatment system installed with the new square transfer unit is sized and configured to process waste from both systems and handle the associated sludge. This extra corrosion protection gives the products we electrocoat a Class A automotive finish—and is a service for which we can charge a premium price to offset disposal costs of the zinc wastes.

Electrocoating is a high-quality coating that provides long lasting protection for a variety of products. One advantage to running an electrocoating business is that many small contract manufacturers are constrained by their clients in the type of coating required on their parts. In the case of automotive tier suppliers, this almost always includes electrocoat. The success of Carolina Coating is due not just to dependable service but also somewhat to our location and readily available customer base.