Cleaning Process for Draw Bridge


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Q. What cleaning process do you think would work best for our operation of painting various industrial brakes used on overhead bridge cranes and draw bridges? I think a manual iron phosphate hot pressure wash with pressure rinse in a booth would be best. Our profile:

A. Machined steel.
B. Cast iron or ductile iron castings that are machined. All machining is done at a supplier, although we do some grinding.
C. Masking. We plug holes with silicone and apply high-temperature tape after cleaning parts where required. 

A. Small, the size of a quarter to light sheet metal junction boxes. This is less than 5 percent of our overall parts volume.
B. Mid-sized, about ¼- to ¾-inch thick parts (i.e., 2 × 16 inches or 8-13 inches in diameter). This is about 75 percent of our overall parts volume.
C. Large and extra-large, greater than ¾-inch to up to 3-inch thick plate parts or castings. These are similar to the mid-sized parts but just much bigger, weighing as much as 500 lbs. About 5-10 percent of our overall volume falls in this category.

Current process:
A. Manually clean parts with isopropyl alcohol using rags and bristle brushes.
B. Hang parts onto rolling carts with paint hooks. Several smaller parts are hung onto the top rails and larger parts on the lower rails. We segregate smaller parts from bigger parts to help prevent over-cooking or under-cooking the other parts on the same cart, but we can’t batch since our mix of parts sizes goes with the brake assembly order.
C. Apply powder. We use a grey epoxy on nearly 99 percent of all volume. 
D. Bake 2-3 carts per batch at 350°F for various times, typically from 40 to 60 minutes. We pre-heat the extra-large parts for 30 minutes to get them warmer. Some parts then cure for as long as 90 minutes. 
E. Test. We do cross-hatch adhesion checking 1-2 times per day on a cast and a plate part. We try to stay to a minimum 4B level, but no customer specifications require this.

Cleaning options considered:
A. A local metal fab house that does electrical enclosures for us has a conveyor line and a batch line with a pressure washer using iron phosphate and a hot rinse in a walk-in booth with a floor drain (concrete floor, stainless walls, exhaust venting above). It then oven-dries the parts, and then powders and bakes them again.
B. A few distributors previously had dunk systems, front and top loaders and recycling systems. The process of masking so much seems to be more of a challenge of parts handling and plug after dry-off, and requires double or triple handling of the parts in the process. 
C. We also discussed the potential of spritzing the parts with a rust inhibitor to avoid flash rusting using a crude, weed-killer, pressure container.
I am trying to justify a new system, and any input would be greatly appreciated. —B.S.

A. As you have laid out, you have a large size range of parts that require cleaning and coating. Unfortunately, the cleaning systems need to be sized to clean the largest parts you have, and you have to decide how to approach the process since costs can quickly escalate with the size of the system.

One aspect for consideration is the need to keep all parts together in a single batch, as you describe in your current process. Based on your description, you will likely need to eliminate the idea of immersion cleaning all parts, since the low volume of large parts will escalate the cost of the project unnecessarily. This will likely lead you to the spray pretreatment booth as the most economical alternative for your mid-size and larger parts. However, the spray booth is less than optimal for cleaning and pretreating the smaller parts you describe. In this case, a small immersion parts washer could be your best alternative to minimize parts handling and damage. In this case, you could also utilize a no-rinse cleaner that could provide moderate interim rust protection. 

The iron phosphate would also be very compatible with a spray wand type of pretreatment system. This will provide good interim rust protection and a base for the powder paint you are applying. Overall, I think you are definitely headed in the right direction with your project and would tend to agree that a single-stage, iron phosphate spray booth would be the best way to handle the variety of shapes, sizes and volume of parts you have to deal with on a daily basis. 

The only other area of consideration would be to examine potential alternatives for the handling the small parts in your process. However, since this makes up less than five percent of the total volume, it may be difficult to justify. You would need to compare the material handling costs.


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