the newsletter of the Chemical Coaters Association International.
Reprinted with the kind permission of
By Dave Wright [deceased]
- Mequon, Wisconsin
With sadness we note Dave's passing on Oct. 11, 2013. His longtime friend Anne Goyer offers a tribute to him in The Finishing Touch, Vol. 23 No. 4.
Excerpts from 'The Finishing Touch',
the newsletter of the Chemical Coaters Association International.
Reprinted with the kind permission of
by Dave Wright
Senior Technical Representative
Texo Corporation
Send your Questions to davewrit@execpc.com
Q. My company is in the process of making a bid for some plastics painting for one of the "Big 3". Can you describe a typical plastic cleaning operation? P.C., Farmington Hills, MI
A. Sure. A typical system would look something like this:
Stage #1 - Prewash. This removes water soluble salts, plastic burrs and dust.
Stage #2 - Wash. Cleaner removes soils and mold release. Can either dissolve or suspend soils, depending on nature of soil.
Stage #3 - Rinse. Removes residual film of cleaning solution and soil.
Stage #4 - Rinse. Removes remaining residual film of cleaning solution and soil.
Stage #5 - Recirculating D.I. Rinse. Removes all residual dissolved salts. A rinse additive may be added to this stage.
Stage #6 - Virgin D.I. Rinse. Removes all residual dissolved salts. A rinse additive may also be added to this stage.
Stage #7 - Dry. Removes residual water, usually by air blow off.
Q. Our company paints products that go to consumers finished as class "A" surfaces. Our problem is in surface dirt. How can we come up with a method to reduce dirt defects?
A. Although this really isn't pretreat related, maybe this will help. When dirt particles are encountered the problem usually is not cleaning from the washer. If it were, other problems such as adhesion and corrosion performance would be affected. What you need to do is to come up with a systematic method of reducing the defects, not taking the "shotgun" approach of cleaning everything. I would assess the problem, and build a library of dirt defects identifying their sources. Once you have done this, make up a pie chart containing the percentage of each type of defect. Doing this you can easily see where your efforts are best spent. Typically people are the biggest offenders! If you would like a detailed method of how to accomplish this, write to me a t CCAI and I will forward an outline of how to accomplish the reduction.
Q. We have been running a five stage iron phosphate system with liquid and powder coatings for a number of years. Our typical salt sprays go between 72 hours and 240 hours. How can we improve salt sprays to meet something like 500 hours? W.C. Milford, CT
A. First thing you need to do is assess why you want to go to 500 hours. If your system has been in use for a number of years, with no real problems due to defective coatings, why are you changing? Legitimate reasons could be customer complaints, warranty returns, customer specifications, competition, etc. Making an arbitrary decision to go to 500 hours might not. Once you have reviewed the desired specification, assess how capable the specific coating you wish to use is of meeting that spec. Do this by obtaining commercially available or lab pretreated panels ( at least 3) in the system you use (i.e. 3, 5, 7 stage, iron or zinc). Process them with the coating you wish to use. Have these panels run in salt spray "to failure" (determined either by A.S.T.M. standards or your desired specification). If you desire 500 hours and the pretreatment with that specific coating lasted 600 hours, you know you have a good chance of meeting the specification in your system. If the panels fail at 240 hours, try changing the coating, pretreatment or both. Using this method to "pre-screen" the system can save you many wasted hours and dollars!
Q. What is an "Iron phosphate" and why would we use it?
A. Iron phosphate is the most common type of metal pre-treatment for paint application. It is typically used where moderate corrosion resistance is required. Immersing steel in dilute phosphoric acid produces a very thin coating of water insoluble iron phosphate. It provides an excellent "base" for subsequent organic coatings, be they wet paint, powder or electrocoat. A number of different additives have been developed to accelerate or control the formation of the film. You will hear of "Moly" irons which use Molybdate, organic accelerators such as Hydroxyl Amine sulphate. Each of these has specific uses and limitations. Contact your favorite pretreatment vendor for specific applications. If you would like more information, CCAI will soon be publishing a manual on Pretreatment (available now!). This, the third in a series, will go into detail on the pros and cons of each system. Other manuals currently available include Wet Paint and Powder Coating. Contact CCAI for price and availability.
Back to finishing.com Home Page