Early in my career, I was very fortunate to work for a small company that sold metal pretreatment and cutting fluids. The company had a lot of seasoned veterans: chemical engineers and chemists who had over 25 years of experience and were willing to impart a lot of great knowledge. They had a deep understanding of chemicals and chemical processes, but they could also break down the processes and make it easy to learn.  

As I was digging through old files, I found a paper written by Tom Land. Tom passed several years ago, but he taught me so much about operations and how to help customers. When you think about rinsing in your pretreatment system, it seems like the simplest process. As simple as it seems, it is critically important to produce a high-quality product, and I think Tom wrote about it in an impactful way. 

Proper Rinsing

Proper rinsing between process stages is essential to successful pretreatment. Poor rinsing will leave residues on the part surface that will interfere with the remaining process. If the alkaline cleaner is not completely rinsed off, it will carry over into the phosphate state and cause excessive chemical usage and poor performance of the phosphate due to the neutralization of the phosphate bath. If the final rinse is inadequate, salt will be left on the part surface and interfere with paint adhesion and consequent failure of corrosion resistance. 

The rinse following the cleaner should have aggressive pressure to provide complete removal of all residual alkalinity and soils. The overflow rate of the rinse stage should be set to maintain a level of alkalinity (recommend by the chemical supplier) to prevent flash rusting, reduce precipitation and settling of soil and cleaner and assure complete rinsing.  

 Surface damage due to heavily contaminated final rinse. 

Overflow rates and frequent dump cycles are also very important factors in maintaining a clean rinse. Normal overflow rates range from 3 to 10 gallons per minute and are set according to the line speed, part configuration, racking and production volume which all vary. Dump cycles are also set by the same parameters, but in addition are often controlled by total alkalinity titration and/or Total Dissolved Solids meters set by the chemical manufacturer on the process recommendations. If the overflow rates and dump cycles are ignored, accumulation of contaminates will occur in the rinse water.  

Finally, zone length of the drain stages is probably the most important insurance of a properly rinsed part. As mentioned above, line speed, part configuration or profile, racking and production volume are variables. A washer design must include careful attention to what is being processed and consider these factors. It is a well-known fact that rinsing is as important as cleaning or any other stage in the process. There are three methods for determining the zone length of drain stages for spray washers: 

1. Line Speed (ft per minute)+Part Profile (ft)= distance from last riser to apex

                  2     

Example: 19 ft/min + 8 ft = 17.5 ft drain zone 

                       2

2. (Profile (ft) + 1 ft) * 5/3 = distance from last riser to apex  

Example: (8 ft. + 1ft) * 5/3 = 15 ft drain zone 

3. The minimum distance from the last riser to the apex should supply at least 30 seconds drain time at the maximum line speed on a variable speed line.  

Example: 9 ft total drain stage@19.5 ft/min. Only 6 ft to apex (based on normal apex at 60% of total length)  

     6 ft.    * 60 sec = 18.5 seconds total drain time  

    19.5 ft/min 

Scott Crosley is a 28 year veteran in the paint and coatings industry. After a long career at Sherwin Williams, Scott currently serves as the Vice President – Business Development & Sales at ACT Test Panels. 

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