Amazing what crap poses and passes as science these days. Holding
headphones near the implants of 100 people to see how many malfunction
is an experiment a dim adolescent could do if she could recruit enough
implanted fools to assist.
The safe limit specified by one ICD mfr for static (DC) B-field is 1
millitesla, and B-fields are easily measured.
I'm thinking E-field (volts per meter), not H field. The current is
microamperes. But it's a DC field so I'm probably being
over-conservative. The voltage is typically developed by a Villard
cascade or Cockcroft-Walton multiplier, both of which usually have
enough capacitance in the string to not have much ripple.
I'll try to answer this as brief as possible.
First just so you have some background I spent 14 years working in a
coating plant that did both liquid and powder coating. Did every job in
the place over the years from hanging parts on the lines to supervisor
including driving the trucks to P/U and deliver the parts.
First powder coating as a process is not hard to do. The trick to the
entire process is really simple. THE PART MUST BE CLEAN!!!!
After that it's a cakewalk. Well maybe a few minor items are a PIA but
money can fix those.
First your talking aluminum. I would rather coat my tongue than aluminum
First off - How long do you want the coating to last?
With aluminum this is the BIG problem. Aluminum Oxide forms VERY fast.
This is what causes the coating to fail. So you need to come up with a
cleaning method to remove it. In the plant we used a 5 step process,
First a simple degreasing bath, then through a 4 stage wash rig. First
stage rinsed off the panels with PURE water. Then into an acid wash and
rinse then a phosphate bath. Then through a quick rinse and into the
drying side of the oven. From there they could be handled WITH CLEAN
COTTON GLOVES. Touch the part or let it get dirty and you start over.
Next you hang the parts. The easy way is to drill a hole or holes and
use a steel wire to hang the part. Any tapped holes or unpainted areas
get masked off (Just about anything that is easy to remove can be used,
IF you remove it before curing the powder)
Now you coat the parts.
Here you need a powder gun, The cheap guns work BUT if you plan on doing
many of these items hit an industrial auction and get a REAL powder gun,
Graco, Sames, Norton all make good machines. Make sure you get the
COMPLETE unit, should be the control head, and a fluidized hopper you
put the powder in, plus the gun itself and the cables and color hose.
Powder to match the characteristics you want.
This can get interesting due to all of the different ones out there. ANY
color you like can be made, from Clear to Black, High Gloss to dead
flat, Textures, Hammertone, Mixed colors, Metallics, Candies, you name
it and it's out there.
Powder booth to contain the over spray and reclaim the unused CLEAN
powder. (easy to make one for the parts you list out of cardboard or
The racking for the parts, This can range from a single wire to a
complete conveyor system. For many folks the common bakers rack with
steel wheels works well.
The oven. This is the part that kicks most people in the guts. "any oven
can be used" Well yes/NO First you need an oven that holds the internal
temp at the cure point within a few degrees for 20-40 minutes. The
common home gas oven doesn't do that very well. An electric beats it
hands down. Then you need a way to vent any smell/fumes (yes curing
powder does STINK). For the parts you have the easy way out would be an
oven built out of tin with electric oven elements and fiberglass
insulation with a shell of tin. OR you could go with infrared heaters
and a simple insulated tunnel.
As for the process itself.
Hang and clean the part. OR clean it and handle it GENTLY while you hang
To coat it the part gets grounded to a ground strip through the hanging
wire. The powder goes out of the hopper and picks up a positive
electrostatic charge when it goes through the charging section of the
gun (usually at the very exit point). The powder goes through the air
and settles on the part due to the different charges.
This is the point where the pro gun beats all the home guns. CONTROL,
both in the powder stream, speed of discharge, amount of electrostatic
charge (the typical Sames gun we used put a 150KV charge on the powder,
if you got it close to a ground it would throw a 4" arc!!!) and powder
coat mil thickness. Rule of thumb is that the higher the charge the
better the powder sticks, BUT it also will apply the powder THICKER,
which isn't good. This is where the controls come in. On a good gun you
can adjust the entire process.
YES you can cause all the same things with powder that you can do with
liquid, gobs stuck in the paint, dirt, bugs, runs, drips. BUT prior to
you actually curing that powder you can do one thing you cannot do with
ANY liquid paint, REMOVE THE PROBLEM. Simply grab a VERY filtered air
line and blow the powder off the part and recoat. (once you get the hang
of it you can even do spot touch-ups, blow off JUST the offending item
and even create paint designs with just the air gun.
Now you have a clean part, hanging on a wire, with a nice even coat of
powder on it. PERFECT. OOPS you sneezed....
OK you didn't sneeze this time... Believe it or not it takes a LOT of
force to knock that charged powder off that part if you use a good gun.
Now you transfer the part to the rack and GENTLY roll it into your oven.
OR transfer the part to the rack until you have enough coated to fill
the oven (you want room between parts if they move some but not a lot)
The oven will be PRE-HEATED to the cure temperature. Open the door, load
the oven and close the door.
WHY pre-Heat? Because you want those parts to come up to temp FAST so
the powder cures properly and bonds to the aluminum. Remember that
metals expand when heated, if the powder is already starting to gel as
the expansion occurs it will move with the part. If the powder isn't
starting to gel and the part expands you get micro-fissures in the paint
that let air/moisture in. NOT a good thing. Now for cooling you let them
cool together. The powder will again follow the metal. It will actually
stay in a semi-plastic state for quite a while when it is warm.
DO NOT TOUCH THE WARM PARTS!!!
Use the hanging wires to move them out of the oven to cool. Why not
leave them in the oven till it cools? Because powder CAN also be over
cured. Just like baking cookies, there is a point where the dough
softens, flows out and sets. Then they start to brown. If you leave them
in too long what happens? You get CHARCOAL COOKIES... Powder does almost
the same thing at a molecular level. Properly cured powder will move
with the substrate, I have taken .020 test panels, coated them, bent
them over. Flattened the bend down with a mallet and not had the coating
fail on properly cured powder! OVER cure that same powder and it will
flake off and fail.
You asked about spraying hot parts as well. YES you can do that and we
did that a lot on HEAVY cast parts or on parts where the customer needed
a specific thickness of coating for another purpose. For instance the
E-One company ( http://www.eone.com/sewer_systems/intro/index.htm ) had
us coating the pump housings and parts inside many of the sewage grinder
pumps they built. They wanted a VERY thick coat that would then get
machined flat and used as a gasketing surface. This kept all of the cast
iron housing covered and made those pumps last. To give them the proper
coat we would clean the parts and then pre-heat them up to cure
temperature. Then spray on the powder to build up the thickness. The
heated parts would instantly cause the powder to begin curing and then
we baked them a while longer till we reached a full cure.
This process also works on porous items like cast aluminum or iron
because it out-gasses the part and allows the powder to gel without
having gas bubbles in the coating. On 1/4" it isn't needed.
ANY more questions?
Very good write up. I would make one addition.
Use a good respirator when working with the powders. They are toxic
until they are cured. Cheap dust masks don't cut it.
Use a vacuum to clean up the area if not recycling powder overspray.
Same goes for your clothes.
Thats why a booth is used as a controlled area. Powder goes back into
the can or into the filter.
Also, if you make your own oven, check the cure temperature of the
powder you plan to use. Harbor Freight powders need a cure temp of 400
degrees F for 20 minutes after powder glosses over.
Powder would work OK but that first step of cleaning the part is the
problem part. We did a LOT of marine parts for a couple of the big
outfits as well as http://www.taylormadeproducts.com/ (hatches and
It can be done but not with the low end stuff.
No, this is a fitting that has a pin that fits into a socket on deck.
I suspect powder coating will get chipped, and in a salt water environment
aluminum would corrode away rather quickly.
I've never had the chance to machine stainless - or any steel for that matter.
I've whittled aluminum with a mill though, and could make what I needed.
So the idea of making parts from stainless is kind of over the top for me.
I honestly don't know how hard it is to do, or how long it would take.
But if I have the choice - all other things being equal...
Like Forest Gump always said, "One less thing to worry about".
Stainless isn't too bad to machine, depending on thge alloy. However, I
woudln't be surprised if the best stainless for marine use is harder to
machine. The problem with SS is that machining it work hardens it. You
want to be more agressive with it than aluminum, which might tax your
machine tools. Carbide helps a lot because it doesn't care so much that
the work is hardened.
You got it.
"Aren't cats Libertarian? They just want to be left alone.
I think our dog is a Democrat, as he is always looking for a handout"
Unknown Usnet Poster
Heh, heh, I'm pretty sure my dog is a liberal - he has no balls.
Powder coating without spray gun, charges, or oven:
I watched a CalTrans crew today repaint street markings in front of an
intersection. They laid down metal stencils, then used a weedburner type
torch to heat the asphalt inside the letter cutouts. Then white powder
was dusted over the stencil.
Is there an exam?
Do I get college credit? :)
Why does the oxide of aluminum cause problems for powder coating? It's
electrical conductivity is much lower than aluminum (mebbe even an
insulator?), but that shouldn't affect the static electricity effect, unless
alum oxide in fact affects that as well.
Speaking of alum oxide/powder coating, I guess that's why god invented
Which is another option.
That was cool, machining the powder coat itself! Super-neat! Some creative
engineers, on that one!
The insulating properties are not the problem. It is that the oxide
coating acts as a contaminant and prevents a good bond between the
powder and material. Then the paint fails.
Same problem with chrome plating on aluminum.
Yup, and it's a better one for some applications.
They picked up that trick from one of our other customers. Small outfit
called GE Turbine Systems. We coated ALL of the shaft seals used on the
various turbines they made for about 10 years. In that case we used a
special powder that provided insulation properties for the shaft.
We also did some parts for Raymond Corp. (fork lift parts) Those were
interesting. The powder was a special high temp cured nylon blend. Cure
temp was 650 degrees. Those parts had to be preheated, coated then
cured. ALL of this had to be done HOT. These were done in a batch oven.
You would hang the parts, let them get HOT, then open the oven door,
grab a couple parts, close the door, spray them, open the door, then
hang them back up. Repeat until you finished the batch.
These parts and items like the seals and some medical items we did were
done by two people. We also were the ones who taught all the other
sprayers the ropes and did all the gun maintainance as well.
AHH the good old days.....
I can see Al2O3 being a problem for chrome plating, which depends on where
materials fall in the chemical "emf series", but it is less clear why such a
stable compound, useful in its own right, could not be powder coated.
Surely Al2O3 can be *painted*, right? Epoxied? Glued?
Seems to me it should be powder coat-able, which is, in essence, a kind of
Not argering, just thinking out loud.
From what you describe, it seems like there are various chemical
configurations for powder coats beyond simple pigments, for different
applications/properties -- none for oxided alum, or even anodized alum?
The oxide forms a layer on the aluminum which acts almost like you
coated the part with some dust prior to coating.
The way the powder bonds is interesting.
There are literally thousands of different chemical blends for powder
For instance lets say you desire a flat black coating 1 mil thick.
What UV stability do you need?
Will the part be inside or outside?
What final surface finish do you wish, smooth, pebbled, sand textured?
Does the coating need to be temperature resistant?, How many degrees?
What temperature will the substrate withstand and for how long?
What wear characteristics do you need?
Electrical Characteristics? Gloss retention?
These are just a few things that you decide prior to making the blends.
All of them have to be determined PRIOR to producing the powder. They
cannot be changed later, Unlike a liquid paint which you can add a pearl
or a flatting agent or a gloss additive or whatever.
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