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Ask Joe Powder
"Ask Joe Powder" is a question and answer column authored by Kevin Biller of
the Powder Coating Research Group. Mr. Biller has over 30 years experience
formulating and manufacturing powder coatings. He welcomes your questions
regarding powder coating technology. Please write to: firstname.lastname@example.org
The Powder Coating Research Group
15 W. Cherry Street, 3rd Floor
Columbus, OH 43215
Is there any method, equation or software program to calculate powder
density? Thanking you in advance,
I am aware of two methods used to determine powder density. Both are covered
in detail in ASTM D5965 - 02(2007) Standard Test Methods for Specific
Gravity of Coating Powders.
One uses the volume displacement of the powder into a fluid (kerosene or
hexane) with a known density. The weight of the powder is known so the
relationship between weight and volume can then be calculated.
Powder Specific Gravity = [Weight of Powder (grams)] /[ Final Volume –
Original Volume (milliliters)]
This method involves introducing the fluid into a graduated cylinder. The
volume and weight of the fluid is recorded. Next a given weight of powder is
mixed into the fluid and the displaced volume is determined. It is essential
that you eliminate all air pockets in the mixture to obtain a reasonably
accurate measurement. Please be aware that this method doesn’t easily
account for the surface porosity common with most powder coatings and
typically results is a lower than true specific gravity. Nonetheless it can
be used as a decent tool to compare powders.
A much more accurate method is based on the Ideal Gas Law and utilizes a gas
pyncometer instrument which measures volume of a known weight of powder by
gas displacement. These are relatively expensive instruments and are
available from a number of commercial instrument suppliers. Each instrument
is slightly different; some measure volume, others can measure volume and
density. You would have to consult the specific procedure provided by the
instrument manufacturer to successfully measure specific gravity of powders.
I recommend you use the simpler fluid method but always run a control sample
of know specific gravity along with your samples to be evaluated.
I hope that this helps.
Powder Coating Magnesium Alloys
I have a cleaner/phosphate that is supposed to treat Magnesium but how
should it be handled as far as dry-off and cure temps?
I tried a couple of parts this morning, with a low gloss clear coat and they
came out looking like desert storm Camouflage. This was cured for 12 minutes
Thanks for the question. Magnesium alloys are a tricky substrate to powder
coat unless you know how to do it. Most magnesium fabricated products are
cast resulting in a certain degree of porosity on its surface. Cleaning the
substrate is a great idea however the cleaners/ pretreatment can remain
harbored in the pores. Indeed, even without cleaning air resides in the
pores. As the powder melts and flows the cleaners and air escape from the
pores. Most powders are curing at this point and can't recover or reseal the
holes caused by the volatiles. The result is pinholes, low gloss and
unsightly surface disruptions.
My advice is to continue cleaning as you are but run the parts through a
relatively high temperature dry-off before you apply the powder coating.
It's preferable to coat the parts very soon after the dry-off, even while
they are still warm so they don't re-absorb ambient moisture. As for dry-off
temperature - 200°C for 10 minutes is a good place to start.
You should also be aware that many powder suppliers offer product lines that
are better suited for porous substrates such as magnesium. It may be best to
use one of these with a well- controlled dry-off process.