A Technique developed by Graeme Ockleshaw, life member of the Society of Model & Experimental Engineers, Perth, Western Australia. (an incredibly talented model engineer).
Foreword by Howard Jones..... There is nothing really mystical about anodising. The anodising process develops a spongy translucent oxide over the aluminium. This oxide layer is permeable initially and will absorb liquid pigments into its structure. Immersion in boiling liquid will seal the outer surface of the oxide, trapping any pigment within its structure permanently.
Graeme demonstrated the technique at a model engineers meeting on the table top, completing an anodised sample in a little over half an hour. The following text is a handout he made available at the meeting; where I have needed to add a supplemental note it occurs in brackets...().
EQUIPMENT
1. An acid proof tank, to be used at room temperatures, large enough to
give about 50mm clearance all round the work piece. Notch two opposite
top edges to locate a work support bar. (big square tupperware container)
2. A cathode cut from pure aluminium sheet to cover one side of the tank then pass across the bottom and up the other side (on the inside). Bend the sheet out over the top edge of the tank to hold in position and leave a tag for electrical connection. The thickness is not important. (Graeme used a bitumen coated aluminium sheet sold for damp courses in brick work called "Ormonoid Alcor damp proofing". The bitumen washes off easily with kerosene to leave a soft aluminium sheet which is easy to form)
3. Enough dilute Sulphuric Acid to three quarter fill the tank. An acid concentration of 10 percent is satisfactory but can vary widely. Pour 100ml of concentrated Sulphuric Acid [1.84 SG] into 900ml of deionized (distilled) water WITH CARE. Alternatively pour 500ml of Battery Acid [1.23 SG] into 500ml of deionized water WITH CARE. Add Sodium Sulphate [Glaubers Salts] at the rate of 30g per litre of dilute acid.
4. A 12 to 15 volt DC power supply capable of 15 amps per sq. foot of workpiece surface area. [if there is no control on the voltage then there will be a heavy current inrush as the power is first turned on and as the oxide coating starts to form. Graeme used a bridge rectifier with a 4000 micro Farad smoothing condenser].
JOB PREPARATION TIPS
1. Material with a matt finish anodises better than one with a mirror finish.
i.e. dont polish the surfaces to be anodised.
2. Because of its porosity you do not get a satisfactory anodised finish on cast aluminium which has been machined. An as-cast aluminium surface will not have a lusterous finish when anodised. (looks poor)
3. Don't bother trying to anodise riveted or bolted assemblies or parts which are loosely screwed together. (the acid continues to weep from the joins no matter what you do and ruins the finished appearance. anodise components separated).
4. ALL work supports and fixings in the acid bath must be aluminium. preferably pure aluminium.
DEGREASING
(The anodised finish is prevented from forming by any traces of grease)
"Tricleanium" at 70 deg Celcius will mildly etch the surface. Leave the
workpiece in it until water does not run into globules when raised above
the surface. (Tricleanium is an alkali cleaner available in Australia).
The job may take on a blue hue but this will disappear as soon as current
starts to flow.
Wash under the tap and hang in the acid bath while wet. (wear rubber gloves
for this, you must not touch the degreased surface).
ANODISING
With the power turned on myriad small bubbles will rise off the cathode
and workpiece. Agitate the job every 5 min. to make sure the bubbles dont
hold on (to a fixed position on the surface of the work).
If the work is to be dyed black leave it in the anodising tank for 70 mins.
For other colours 20 to 30 mins.
After this time the job will have taken on a slightly milky appearance.
Other work can be added to or removed from the tank at any time as the
voltage remains constant.
If dying is not required take the job out of the acid, without touching it,
and wash it VERY well under the tap.
Hang it in boiling water for 15 mins. to close up the spongy oxide.
This makes it more durable.
Sealing Salts are sold which can be added to the boiling water to assist the
sealing process and make any dye more UV resistant.
The concentration of Aluminium Sulphate in the bath will slowly build up.
Replace the acid solution from time to time.
A damaged anodised finish can be removed by VERY brief immersion in a strong
caustic soda solution.
Once having sealed up the oxide film it is not possible to increase the
film thickness by reimmersing it in the acid bath because of the high
electrical resistance of the oxide layer.
DYEING
1. Dyes for cotton fabric are satisfactory but don't add mordants or anything
to the dye, e.g. salt, vinegar etc.
2. Vegetable dyes are reputedly better than chemical ones. (Black dyes, like tulips, are harder to achieve. A commercial Black! dye retails for $aus170/kg. ouch! Experiments with a black vegetable base hair dye produced a golden yellow colour).
3. Most colours can be used at 20gms of dye per 600ml of tap water. Black needs to be much stronger at 60gms per 600ml or thereabouts.
4. When a sufficient thickness of oxide is built up take the work out of the acid bath and WITHOUT TOUCHING IT wash really well under the tap. Place into the cold dye bath without allowing it to dry out.
5. Raise temperature of dye bath to boiling point and allow it to boil for ten minutes.
6. Remove workpiece and hang it in clean boiling water for 15 minutes. Remove, cool, then wash it under the tap.
7. The job may be coated with lanoline (wool grease) to make it more durable. (a trick used commercially).
MASKING
Bare areas of aluminium may be masked with 3 coats of nitro cellulose laquer
(model aircraft dope) or one coat of
"Freundorfer" "Freko Positivlack".
Remove both with lacquer thinners after completion of dyeing.
SUMMARY
The work is cleaned of any grease and must remain free of all grease to
prevent blemished surface finishes.
A spongy oxide layer is built up in an acid sulphate bath.
The workpiece is washed then placed in the cold dye for a period to
absorb the dye.
The workpiece is then boiled to harden and seal the oxide layer.
The workpiece is then washed and dried and may be coated with lanoline
to protect the finish.
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End of Graeme Ockleshaw's original work on a simplified Anodising technique.
Please acknowledge Graeme's creation of these notes in any publication based
on these notes.
Graeme Ockleshaw, Society of Model & Experimental Engineers,
Perth, Western Australia. 3rd of April 1995.
You may email thanks to graeme via howard@perth.dialix.oz.au (I'll pass em on)
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Howard Jones, (howard@perth.dialix.oz.au)
66 Towton Way, Langford 6147, Western Australia
Toy Aeroplane Flyer VH22143. Tyre Kicker & Current Pilot!
Corby Starlet Plans #279....RAAF Association Flying Club Member
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