A major supplier of automotive underbody components recently expanded its operations by opening two new plants in the United States. The new factories were individually planned and purchased their E-coat systems based on available space and customer requirements. For one plant, this resulted in a monorail system, while the other facility opted for a square transfer unit.

The monorail electrocoating system is similar in design and operation to other electrocoating systems used by the company. The intent was to reproduce, as closely as possible, the racking, pretreatment chemical use, oven dwell time, etc. used in processing parts at other facilities.

The system handles 200 sq ft of product per minute with a 2-ft wide by 5-ft high work envelope on 3-inch I-beam conveyor. The pretreatment regimen comprises eleven stages:

1. Spray clean, 160F;
2. Immersion clean, 160F;
3. Spray rinse;
4. Immersion rinse;
5. Immersion conditioning rinse;
6. Immersion zinc phosphate, 135F;
7. Spray rinse;
8. Immersion rinse;
9. Spray non-chrome seal;
10. Immersion RO water rinse; and
11. Spray RO water rinse
.

Immersion stages on monorail pretreatment systems require much larger process tanks, bigger pumps to circulate higher volumes of fluids, increased chemical charging costs and higher utility costs for heated solutions. However, they also allow more complete interaction between the parts and process solutions than spray stages, especially on parts with complex shapes. This is important in stages where a chemical reaction or deposition of material is required, such as the conditioning rinse, zinc phosphate stage and non-chrome seal. Back-to-back combinations of spray and immersion stages also result in more thorough cleaning and rinsing and minimize carryout. The extra investment in six immersion stages ensures that products will receive the ultimate in pretreatment.

Photo I: Monorail electrocoat systems provide high capacity electrocoating along a reliable chain conveyor.

Tank solutions are counterflowed where possible to return active chemicals back to the process baths and continually cleanse rinses, prolonging the operational life of both. For instance, the stage five tank (conditioning rinse) is counterflowed to stage four and stage three cleaner rinses. Solution from stage three provides makeup water to the first two pretreatment stages as required to replace carryout and evaporation, with any excess piped to drain.

Likewise, the final spray rinse is made up with reverse osmosis (RO) water, counterflowed to the previous stage (immersion RO rinse) and then piped back to stage eight and, in turn, seven - the rinses following the zinc phosphating stage. Stage seven then overflows to drain. This arrangement keeps the final two pretreatment rinses extremely clean and minimizes water use.

Support equipment for the monorail system includes a 30 gpm RO system and 30 gpm wastewater treatment system, both provided by TTX Environmental, a sister company to Therma-Tron-X, which provided the electrocoat systems.

The monorail itself takes up approximately 7,000 sq ft of floor space, although the cure oven and walkways are raised, allowing storage underneath. The entire system, including wastewater treatment equipment, RO system, other support equipment and burn-off oven, occupies a total of 9,500 sq ft.

At the second plant personnel first evaluated the electrocoating processes at their other facilities, including the new monorail system, and compared those processes to the square transfer electrocoating system. They determined that the SST would best meet the company's needs at the second facility because of limited space availability, modular design for faster installation, ability to process high density racks, part handling versatility and lower cost compared to an equivalent capacity monorail system.

The SST system processes 100 sq ft of product per minute with a 8-ft 6-inch wide by 5-ft high by 2-ft long work envelope. The pretreatment regimen comprises ten stages:

1. Immersion clean, 160F;
2. Spray clean, 160F;
3. Immersion rinse;
4. Immersion rinse;
5. Immersion conditioning rinse;
6. Immersion zinc phosphate, 135F;
7. Immersion rinse;
8. Immersion non-chrome seal;
9. Immersion RO water rinse; and
10. Spray RO water rinse.

SlideRail Square transfer systems cut floor space requirements for immersion pretreatment and electrocoating operations, especially when working with larger parts.

The vertical dipping capabilities of the SST allow for more economical use of immersion stages compared to a monorail system, due to smaller tanks and lower heating costs and circulation requirements. The usefulness of back-to-back spray and immersion cleaning tanks is retained, as well as immersion and spray final RO.

As with the monorail system, the SST system greatly reduces water use by counterflowing solutions where possible. RO water constantly makes up the tenth stage spray rinse. Excess capacity is pumped into the ninth (immersion) stage, from which overflow is piped to the seventh stage (immersion zinc phosphate rinse) and then to drain. The conditioning rinse (stage five) counterflows into the final cleaner rinse (stage four), which overflows to the first cleaner rinse (stage three). Once again, stage three provides makeup to the two cleaner stages as needed to replace carryout and evaporation.

The SST design places the cure oven directly over the process tanks. The system itself occupies approximately 2,300 sq ft, with a total of 4,400 sq ft when including support equipment such as the 20 gpm RO water system, 20 gpm wastewater treatment equipment and burn-off oven. A monorail able to process a similar maximum part length of 8 ft 6 inches would have longer and more expensive immersion tanks, as well as large turning radii where parts round corners on the conveyor, requiring more floor space.

Vertical immersion into the SST tanks also offers the advantage of dead entry in the paint tank to eliminate striping on the parts and allows the use of automatic voltage control (AVC) to prevent over- and undercoating. The PLC automatically adjusts applied voltage to the amperage draw measured from the parts in the tank. Smaller loads are coated at a lower peak voltage, while overloaded racks still receive the proper film thickness through an automatic increase in dwell time, as calculated by the PLC.

This automotive supplier recognizes that monorail and square transfer systems can meet its production requirements and quality standards. Determining factors for each application include space, throughput requirements and the size, weight and configuration of the parts.

Only the zinc phosphate stage is isolated from the counterflow water conservation scheme employed by this electrocoating system. Zinc must be removed from the waste stream prior to combination with other system waste flows.