John, Great stuff. I have been outsourcing all of my anodized worked. Especially since they can do a brass - like color, but your success encourages me to bring my one of a kind work home.
Are you saying that you use a positive mask?, i.e., Black lines on a clear field. If so that's great. I usually have to have a photo negative made from my positive black on white drawing. If I can eliminate that step it might be worth the effort to do the chemical parts. Also, are the results resistant to fading in the sun? ++ron ------Original Message------ From: "John Davis" <[EMAIL PROTECTED]> To: Sundial mailing list <sundial@rrz.uni-koeln.de> Sent: April 5, 2000 8:28:26 AM GMT Subject: Anodised aluminium dials 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 ---------------------------------------------------------------------------- Dr J R Davis Flowton, UK 52.08N, 1.043E email: [EMAIL PROTECTED]
