Notes on Airbrush Needles 
by Zsolt
Needles influence
airbrushing for two main reasons. The first is related to the needle shape,
which defines how focused is the spray pattern (together with the nozzle of
course). Therefore, if a needle loses the tip or is hooked, it must be replaced
or rebuild. The second important aspect is the needle surface, in particular,
it should not let the paint stick to it. A simple method to reduce tip dry is
polishing the needle surface, because a smooth surface has less bumps for the
paint to stick to. Using reducers, retarders and whatever one can think of is
just a possible paint-based solution to the same problem. The needle surface
adhesion propriety is a very important point to address in an airbrush.
However, thin lines can be traced using any airbrush with the right technique,
while clogging an airbrush every five seconds makes the painting process really
painful...
Rebuilding the needle tip:
The following image shows a
brand new needle:
The needle length is 130mm,
the diameter is 1.20mm and the tip cone height is 13.7mm. The needle material
was measured and it is made of austenitic stainless steel.
The images below are
microscope images of the cone region and the tip, respectively:
Finally, this is the tip of
a brand new Iwata CM-B needle:
Sometimes, a needle may
hook badly and eventually break, so it must be replaced or the tip rebuild. The
following sequence shows simply how I rebuild the tip.
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The hooked
tip is cut off first, then using a 220 grit sand paper, or a grinding wheel,
the raw cone is created by rotating the needle with a drill. |
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Using an
800 grit sand paper, the shape is refined using as reference a brand new
needle. |
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A
1200 refines the shape even more. |
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The
polishing phase starts with a 2500 grit sand paper smoothing the surface. The
tip of the needle can not be smoothed by rotating it because the relative
speed of the tip is near to zero. |
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Finally,
a 1um or 0.25um grain polishing compound can create a very smooth surface. I
used diamond powder (DP-Spray) on velvet in this case. |
The complete procedure
takes about 20 minutes. Using the 1200 grit sand paper, the needle should be
checked in the airbrush to make sure that color leakage doesn't occur. Using
the 2500 grit paper and the polishing compound the shape is not changed
anymore, just the surface is smoothed to reduce clogging. Smoothing the needle
at home, I’ve found some toothpastes with micro-crystals in them working
fine. If the crystals are fine, using a velvet with light pressure, the needle
can be smoothed reasonably well.
The following macro image shows the result of the final needle check. Diluted
black paint is sprayed on paper creating hair thin lines (with no shielding of
course). The image, therefore, shows a hair as reference and a ruler with marks
at 1mm spacing:
The following images are
microscope images of the cone region and the tip, respectively:
The tip is the most
difficult part to smooth without loosing it, but the most important area is the
cone region around the nozzle rim to reduce clogging.
Microscope image of an
untouched needle body (diameter = 1.19mm):
Image of the needle after a
coarse smoothing using diamond powder of 1.0 micron on velvet:
Image of the needle after a
further polishing using diamond powder of 0.25 micron on velvet:
Image of the needle after a
further polishing with colloidal diamond on velvet:
The diamond suspension
smoothes the borders of the small irregularities of the steel.
Polishing better the needle
with 1.00 micron diamond powder and further smoothing it using the 0.25micron powder
first and eventually the diamond suspension, a mirror smooth surface can be
obtained.
Polishing
the needle tip:
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Step 0: Brand new needle tip. Physical image size: H =
1.3mm, W = 1.7mm Visible tip length = 2.1mm |
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Step 1: After polishing with 2500
grit sand paper. |
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Step 2: After polishing with 4000
grit sand paper. |
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Step 3: After polishing with 1um
size diamond powder sprayed on velvet. |
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Step 4: After polishing with
0.25um size diamond powder sprayed on velvet. |
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Step 5: Final polishing with
colloidal diamond on velvet. |
Experimenting
with new needles:
Thanks to many friends and
colleagues, 10 prototype needles
were created (starting with brand new CM-B needles) to experiment with
different technologies and techniques (this activity was part of airbrush
improvements done in 2007: link to details). All
these needles were sent to Eddy as a gift in 2007:
1) Sursulf: courtesy
of Sergio TROGLIO (Bodycote,
The Sursulf process is a
salt bath nitriding process utilising sulphur activated non-polluting salts.
Operating temperature of Sursulf is 540-
Due to high temperature,
the needle bends and it must be straitened (this is not so easy as it seems).
The resulting surface reduces clogging, just because of the smoothness. Thanks
to this hardening process, the smooth surface lasts longer. The process itself
reduces the surface smoothness, so a final polishing step must be applied.
2, 3, 4) atomic layer deposition of ZnO, TiO2 Rutile, TiO2 Anatase: courtesy of Elza BONTEMPI (Structural
Chemistry Laboratory - University of Brescia,
This method is particularly
suitable for making uniform conformal layers through atomic layer deposition
(ALD), also known as atomic layer epitaxy. An ALD process deposits thin layers
of solid materials by using two or more different vapour-phase reactants.
First, a dose of vapour from one precursor is brought to the surface of a
substrate onto which a film is to be deposited. Then any excess unreacted
vapour of that reactant is pumped away. Next, a vapour dose of the second
reactant is brought to the surface and allowed to react, and the excess is
pumped away. This cycle of steps normally deposits a monolayer or less of
material. These cycles can be repeated to build up thicker films.
As far as I could check,
ZnO and TiO2 don’t reduce clogging.
5) TiCN: courtesy
of Delia MONDINI (Protim Lafer,
TiCN is a ceramic coating
through Physical Vapour Deposition (PVD). TiCN can be a range of colours
(blue-grey through to pink) depending on the Ti:C ratio. The TiCN coating is
harder (3000Hv) and has a lower coefficient of friction (0.3) than TiN. However
it is not a direct replacement for TiN in all applications and is only
recommended for cutting, punching, and wear applications where moderate
temperatures will be generated. Real toughness, hardness and wear resistance:
these properties make TiCN the best coating for interrupted cutting
applications, provided that the temperature on the cutting edge doesn't exceed
temperature about
As far as I could check,
TiCN doesn’t reduce clogging.
6) CrCN: courtesy
of Delia MONDINI (Protim Lafer,
CrCN is a ceramic coating
obtained by Chemical Vapour Deposition (CVD). It has a surface hardness of
2200Hv and a friction factor of 0.3.
As far as I could check,
CrCN doesn’t reduce clogging.
7) ZrN: courtesy
of Sergio TROGLIO (Bodycote,
ZrN is a ceramic coating
obtained by Chemical Vapour Deposition (CVD). It is hard and refractory. It has
been used recently as an alternative to titanium nitride for coating drill
bits. Both coatings are supposed to keep the bit sharper and cooler during cutting.
It is also used in refractories, cermets and laboratory crucibles.
As far as I could check,
ZrN doesn’t reduce clogging.
8) Diamond Like Carbon: courtesy of Delia MONDINI (Protim
Lafer,
DLC Lafer is a CVD DLC
process leading to a surface hardness of 2000-4000Hv and a friction factor of
0.05. It's the most innovative coating that all the researchers are developing.
Extremely low deposition temperature, the possibility given by CVD technology
to deposit even on internal surfaces and the lowest friction factor among the
existing coatings are the reasons for such interest in DLC films. Lafer also
has given credit to this coating potential and has developed, after many years
of research, its own version of this film, working mainly on process gases and
power supplies with the aim of reaching a wide range of applications for this
film, that is normally employed in strict and specific uses.
DLC reduces clogging. The
clean and uniform deposition on the tip region is difficult.
9) NiPTFE: courtesy
of Mariangela BRISOTTO (Metal Work,
NIPLATE 500PTFE is a High
Phosphorous (10-13%) Electroless Nickel plating with an amorphous structure
containing about 25-35% by volume of sub-micron particles of PTFE also known as
Teflon (Dupont trademark). It is the most performant Nickel-PTFE coating in the
market. The very high content of PTFE particles gives the lower coefficient of
friction compared to other similar platings. NIPLATE 500PTFE is preferred to
other electroless Nickel platings in the case of the need of a very low
coefficient of friction even without lubrication. There are no metals with the
same hardness and with a so low coefficient of friction. The sliding
performances are so improved to allow a re-planning of machines and moving parts.
It finds applications, moreover, thanks to the anti-stick property of the PTFE,
as anti-fouling coating on heat exchangers and parts exposed to scale
formation; NIPLATE 500PTFE assures good conductivity and
anti-fouling-anti-stick ability that cannot be found in any other surface
coating.
NiPTFE reduces clogging.
The clean and uniform deposition on the tip region is difficult. The plating
thickness can not be neglected. The overall coating is softer compared to
steel, so it can be damaged by the nozzle during the painting process: in this
case clogging becomes an issue even more than using bare steel.
10) Kolsterizing process (K33): courtesy of Vittorio BORDIGA (Bodycote,
The Kolsterising process
(formerly known as the Hardcor process) is marketed by Bodycote. Kolsterising
improves the wear resistance of stainless steel part surfaces, without
degrading their corrosion resistance. There are no additions of chemical
elements to the steel during the process. Kolsterising does not apply a coating
on the surface but is a low temperature surface carbon diffusion treatment.
Although large quantities of carbon are diffused into the surface visible
chromium carbides are not formed. The resulting surface treated layers can have
hardnesses in the range of 1000 to 1200 VPN (approx 72 HRC). The thickness of
the hardened layer is dependant on the process conditions used, but includes 22
or 33 micron effective case depths (K22 and K33, respectively). Complex shapes
can be effectively hardened by this process.
K33 reduces clogging,
because of the final smoothness. Thanks to this hardening process, the smooth
surface lasts longer. The process itself reduces the surface smoothness, so a
final polishing step must be applied.
Acknowledgments:
I would like to thank very
much my friends at the Department of Mechanical and
Industrial Engineering, namely in alphabetic order, Michela FACCOLI, Marcello GELFI and Roberto
ROBERTI, for ideas and precious technical information.
A warm thank you to Alberto
FERRERO (Anest Iwata EU) and to “Marissa Art Productions” for the
donation of all the material I needed to experiment with.
- Elza BONTEMPI (Structural Chemistry Laboratory, Brescia University, Italy)
- Mariangiola BRISOTTO (Structural Chemistry Laboratory, Brescia University, Italy), Metal Work, Concesio
- Aldo BORDIGA (Micron s.r.l., Soncino, Italy)
- Delia MONDINI (ProtimLafer S.r.l., Bedizzole, Italy)
- Sergio TROGLIO and Vittorio BORDIGA (Bodycote, Rodengo Saiano, Italy)
- Vanni ZACCHE' (ATP, Modena, Italy)
Link to the main page of
Color Experiments.