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Hi all,
Last autumn, Tony Moss posted the recipe for anodising
aluminium on the Mailing List, and said that it should offer possibilities for
making multicoloured dials. I took up the challenge and, with Tony's help
and lots of failed attempts (Edison was right - invention is 1% inspiration and
99% perspiration!) I've got some useful results and a way forward.
A 36 kbyte thumbnail of a simple test piece (a basic
horizontal dial) is on my website at www.btinternet.com/~john.davis/anodial.jpg. A
higher resolution picture is at www.btinternet.com/~john.davis/anodial2.jpg.
I found that by following Tony's instructions, getting an
anodised film was easy. The difficult bit was to get a CAD drawing into
the film. The process that I came up with is (briefly) as
follows:
* anodise the aluminium dial plate or gnomon.
I used 12% sulphuric acid electrolyte with a current density of around 130 A/m2
for 40 minutes. It's important to keep the temperature down (to around
20C) which requires active cooling if the plate is relatively large compared to
the size of the electrolytic cell.
* rinse the plate and dye with the background colour
(optional).
* dry carefully - the porous layer is like blotting
paper and sensitive.
* laminate a dry film photoresist layer to the
porous layer. This is the key step. The aerosol can type of PCB
resist won't work as it gets sucked into the pores and won't develop out.
Lamination needs a roller temperature of 150 C. A domestic iron can be
made to work on small pieces, but is tricky.
* expose the film to UV through a mask. I make
my masks using an injet printer on OHP tranparency film, printing mirror image
so that it could be used ink side down. I could print lines down to
0.1mm. The dry film resist is negative working, so a light-field mask is
needed - much more convenient than the dark-field masks needed for the spray-on
positive PCB resist.
* develop the photoresist (it is hardend where
exposed to UV, so lines get narrower as the light diffuses around the edges of
the lines) and bake at 100 C to give good adhesion.
* the exposed porous anodisation can now be dyed a
second (darker) colour. It may be necessary to re-open the pores by a
brief re-anodisation or etch to get good dye take-up.
* alternatively, the exposed porous lines can be
etched out to bare aluminium. The film is only around 50 microns thick (2
thou to those that still think that way!) so it comes off quickly in NaOH.
This is another advantage of the dry film resist - it will tolerate the NaOH for
short periods, which other resists certainly won't. The base aluminium can
either be polished, or re-anodised in a second step.
* the photoresist is now stripped off, using a
proprietary stripper at 40C (or a long time at room temp).
* finally, the film is sealed in boiling water (with
a proprietary additive), locking in the dyes and closing off the pores so that
the film becomes silky smooth and corrosion resistant.
With relatively crude control, and no optimisation of the
exposure, I got lines about 0.15 mm wide from 0.2 mm as-drawn by CAD. I
think this is OK even for fairly precise dials, although I believe it would be
possible to do better.
Because the anodised film is so thin, its appearance is
dependant on the surface finish of the aluminium prior to anodisation.
Quite nice contrasts can be obtained between a brush finish and a mirror
polished one.
Masking of the porous film for the dyeing step can also be
achieved with dry rub-on lettering (Lettraset or Alfac in Europe). You
have to be careful not to rub too hard as this crushes the pores and they then
won't take up the dye. Cut-out letters and tape
strips would probably work too.
Interestingly, I used the same ink-jet masks and resist to
etch EoT plates in brass. Here, the undercutting of the Ferric Chloride
means the lines get wider than their printed values. I found that, etching
0.25 to 0.3 mm deep in a vertical bubble-etch tank at 40 C (about 35 minutes of
etching), the 0.2 mm as-drawn lines came out about 0.25 mm wide. This is
less undercutting than expected, because etchant trapped in the fine lines is
much more stagnant than that in larger features.
My experiments continue and I'm building a bigger
rig. I invite all you CAD "paper dial" experts to convert them into
something durable!
Best regards,
John
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- Anodised aluminium dials John Davis
- Re: Anodised aluminium dials Dave Bell
- RE: Anodised aluminium dials Ron Anthony
