comcom
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 Sulfate. 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.