Back to the Basics: Eletrocoating

Perhaps it’s a sign of the times. Perhaps it’s the state of the economy. For some reason, many of our questions seem to be focused on an effort to gain a better understanding of the principles behind an E-coat system. I’ve recently been engaged in an interesting e-mail dialogue with one reader in particular. Following is a recap of our discussion.

For crediting purposes, I refer you to one of my favorite resources: Electrocoating—A Guidebook for Finishers, published by The E-coat Association. To order a copy, go to electrocoat.org.

E-coat, a New Responsibility

Q. Lyle, our company makes stamped parts of various sizes for large trucks and farm equipment. I’m a mechanical engineer and have been working in the design group for 16 years. Until a couple of months ago I had never seen an E-coat system. We recently took over a plant that has an E-coat system that’s been shut down for almost a year. As a downsizing move, I’ve been assigned responsibility for the reincarnation of this system, which I think has been in the plant for about 14 years and is starting to act its age. Our entire team is made up of people who have been assigned to this operation from other stamping functions, so we all have a lot of learning to do. I’ve read several articles about E-coat applications and installations, but I seem to generate more questions than answers.

If I e-mail you my questions, although they might seem pretty basic, will you be able to answer them right away? 

A. I can certainly sympathize with your situation. We’re always looking for input in the way of questions from people in situations just like yours. Just as I have done with this question, I will get back to you as soon as my schedule permits with an initial answer via e-mail before re-writing the answer for publication. So, go ahead. Fire away!

Purpose of Ultrafiltration

Q. Thanks Lyle. I’ve had some past involvement with liquid immersion coating systems and I think I understand the basics of pretreatment. Now I am trying to understand how E-coat is different and how it works. For starters, could you tell me about the purpose of the ultrafiltration system? 

A. The ultrafiltration system is a key part of the E-coat process. It has two main purposes. First, it controls the level of soluble contaminants in the paint bath. These can come from dragout as parts come through the pretreatment baths. Second, when the paint is electrodeposited onto the part substrate, there are some particles not completely attached. This system provides the means to rinse those off.

If there was no way to purge the paint bath of soluble contaminants, they would accumulate and cause the bath to become unstable. The ultrafiltration system is what enables recovery and recycling of the undeposited paint particles and helps make E-coat such an efficient process. It also reduces hazardous waste disposal.

The Ultrafiltration Difference

Q. What makes the ultrafiltration system different from ordinary filter systems? And what is meant by the term “permeate?”

A. In conventional filtration, flow is perpendicular to filter media. That is, flow is passed directly through filter media and is not always pressure-driven. Ultrafiltration is pressure-driven, and the flow goes across or over the surface of the filter media. That way water and very small particles are forced through the filter media. The crossflow carries larger particles away and keeps the filter from becoming clogged for a considerable period of time. What comes out the other side is called permeate because this is what “permeates” through the filter. A conventional filter collects the larger particles more rapidly, which makes it clog faster and require frequent cleaning.

What is Flux?

Q. I keep seeing the term “flux.” What does it mean in the context of an E-coat system?

A. That question seems to come up a lot in training sessions. Here is how it is explained in the Electrocoating guide book: “The rate of permeate flow from a full-scale ultrafilter is usually indicated by a flow meter and is expressed in units of gallons per minute. The rate of permeate flow per unit membrane area is termed flux. Flux is used for sizing purposes. It can also be used to normalize and compare the permeate output from membranes of different areas and configurations.”

Flux Rate and Cleaning the System

Q. I also read where flux is used as an indicator for the need to clean the ultrafiltration system. What would be meant by that? 

A. That is correct. Even though the pores of the filter media do not clog up as fast in an ultrafilter system, they do get clogged up some as they are used. As this happens, the flux rate will drop. The ultrafilter system should be cleaned before the flux drops to 70% of the rate of a clean system. If the rate drops below that 70% level there is a possibility the cleaning function becomes much more difficult. A second cleaning could be required to regain a desirable flux rate.

Cleaning of the ultrafilter system should be part of your preventive maintenance program. During slow periods of the year, transfer your E-coat to a holding tank so you can clean up the paint tank and perform maintenance on it. That is also the ideal time to clean your ultrafilter system. As I recall, a couple of cleanings per year was effective.

Ultrafilter Cleaning Procedure

Q. One of our tech reps mentioned the cleaning requirement. We found part of an operating manual for the system but we have not found any procedure for ultrafilter cleaning. Do you have any procedure or guidelines?

A. Here’s a copy of the general procedure we developed for the Electrocoating guide book:

Most cleaning procedures involve the following steps:

1. Stop paint feed to ultrafilter system.
2. Displace paint from ultrafilter system to paint tank.
3. Rinse ultrafilter system with de-ionized water.
4. Prepare cleaning solution
5. Heat cleaning solution (if required).
6. Circulate cleaning solution.
7. After cleaning, send cleaning solution to drain.
8. Rinse ultrafilter system with de-ionized water
9. Store or go back on-line with paint.
10. Upon startup, send permeate to drain for a few minutes before sending to the rinse system.

This might not be all you need to know about the procedure, but it should give you a framework to start from.