Etching for Beginners
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Nigel Lawton: 3,507 Words Version 5
WARNING: ETCHANT CHEMICALS ARE POISONOUS AND SHOULD ONLY BE USED BY COMPETENT ADULTS WITH DUE CARE AND NOT USING IMPLIMENTS, VESSELS OR AREAS NORMALLY USED FOR FOOD PREPARATION
There have been many articles written on this subject covering the process of photo-etching, however I would like to describe how I have made use of a different technique which makes the whole process of home etching a much easier undertaking as it does NOT require any photographic steps. The set-up cost for this part of my method is therefore much lower and making my own etching tank from readily available materials has kept the overall costs very low.
The basis of etching is to selectively coat some thin brass or nickel-silver sheet with 'etch resist' then immerse it in a chemical that dissolves the exposed metal via a chemical reaction. The parts coated with 'resist' are left un-dissolved. It’s a great way of producing detailed parts from sheet material with the ability to take designs straight from plans or a computer and transfer them to metal. So the process has essentially two steps: -
1. Apply chemical-resistant 'resist' to sheet brass or nickel silver.
2. Immerse in a chemical 'etchant', removing when exposed metal has dissolved.
I will deal with these in two separate sections.
The Etch Resist
The method I am using to apply resist is known as the ‘toner transfer’ method. As many will know, ‘toner’ is a powdered medium used by standard photocopiers and laser printers. It is deposited on the paper or other substrate material by some magic electrostatic method and then fixed in place by ‘melting’ it with a heated roller. It is possible to print patterns of toner on a suitable substrate using a laser printer or toner-using photocopier and transfer these to brass or nickel silver to create a etch resist pattern. The toner is transferred by ‘ironing’ the printed substrate onto the brass with a domestic iron (like you use to iron your shirts) which re-melts the toner and makes it stick to the brass or nickel silver more than the film.
Initially I used a special coated film substrate which, when printed with a suitable resist pattern using a laser printer or standard photocopier can be ironed on to brass and peeled away to leave the printed areas behind along with the top layer of the film to form the areas not to be removed when etched. However I later discovered that some laser printer type and inkjet overhead projector acetates also works, the difference being that the special film’s coating comes away with the toner whereas using standard laser or inkjet acetate just the toner is transferred. The special film is easier to use, but expensive, £15 for 5 sheets at the time of writing, whereas you can buy 20 sheets of laser or inkjet acetate for as little as £6. The special film may also limit resolution as the backing tends to bridge small gaps – it’s intended for making tracks on a PCB rather than fine etched details on a model. No such limitations exist with standard laser or inkjet acetate and if you can print it, you can etch it! Using these materials I have made some very tiny nameplates with writing only 2mm high that can be read when etched (although you need a magnifying glass really). Depending on your subject material, pocket and adventurousness you may wish to try the special coated film first then graduate to standard acetates as I have done. Not all laser and inkjet acetates work. Some laser acetates hold onto the toner and you don’t get enough transfer to the metal. Some inkjet acetates may not tolerate the laser printer heater roller temperature, always test a piece with your domestic iron set just below the ‘three blobs’ mark first. You should look for acetates of both types that have some sort of ‘coating’ usually only on one side and print onto this coating. The coating seems to ‘let go’ of the toner quite well.
Before you get the iron out you need to develop your artwork, which is the shape of the parts you want to make. Most readers of this article will be familiar with the sort of etched brass and nickel silver 'frets' supplied by various 009 and other model manufacturers. Here are some of the important features of etching artwork:-
1. Don't leave large areas of metal exposed, it uses up your chemical etchant faster, just leave about 0.5mm-1mm around each of the parts.
2. Make sure you join your parts to the main part of the fret with thin ‘traces’ otherwise you'll lose them in the etch tank, these should be three times the material thickness wide at least.
3. You should make the parts slightly larger than you want to allow for 'undercut'. This should in theory be about 1/3 the material thickness (0.08mm for 0.01”/10 thou/0.25mm sheet) however for many items you can ignore this effect as it is very small.
Starting with the shape you want to produce you need to draw an artwork which is solid in the solid areas of the shape so for example the outline shape of my 009 brake stanchion is shown in figure 1
Figure 1 - Outline shape of item to be made by etching
Figure 2 - Solid shape of item combined with other parts on ‘fret’
This is translated into the following solid shape alongside the other parts needed as shown in figure 2.
The parts are joined together by thin areas of brass. Note that the main part of the stanchion is 0.7mm in width, scale 2", and its shown larger than life here for clarity. I used Microsoft Word ™ to generate this artwork on my PC; clearly you can use a wide range of PC or MAC programs or a photocopied hand drawing to produce the same result.
In theory you can make etchings with a single artwork etching from one side only, I tried this using electrical insulation tape to mask off the whole of the rear of the brass. This technique could be OK for making nameplates & works plates but has two big disadvantages if you're trying to make cut-out shapes (even for nameplates). Firstly the amount of 'undercut' is much worse and leads to ragged edges and secondly you can't create detail or fold lines by 'half etching'. To allow you to etch both sides you need to apply resist to both sides which requires two artworks of course, one being broadly the mirror image of the other. I have created pairs of artworks by first making a single artwork with all details on it then mirror imaging it and removing bits from each copy as necessary. I did all of this in Microsoft Word ™. I have added half-etch fold lines to the mirrored version shown in figure 3.
Figure 3 - Version of fret showing half etch fold lines.
When considering two-sided etching I was very concerned that I would not be able to achieved good enough alignment between the two sides of the brass leading to staggered and non-aligned etching from each side. However a useful hint I picked up on the internet provides a fairly easy and effective way to fix this potential problem. All you need to do is arrange your pairs of mirrored artworks along a 'fold line' with some cross-hair alignment targets equidistant from this same fold line. You can use the alignment tools available in most word processing and drawing packages to make sure everything is correctly aligned and centred. Figure 4 shows the final artwork to produce 7 sets of brake stanchion parts.
Figure 4 - 7- up fret with alignment crosses.
The way you use this artwork having printed it on the acetate is to fold it carefully so that the mirrored pairs of artworks are superimposed, very carefully align the cross hairs viewing through both thicknesses. If using the special coated film you will need to hold the two thicknesses of film against a cool light source as it is not clear but ‘translucent’, I use a low-power replacement light bulb, effectively a fluorescent tube. Once you have perfectly superimposed the two sets of cross-hairs staple the film in the folded position close to each set of cross-hairs but outside the area of the artwork.
Next you need to select and prepare some brass or nickel silver sheet. I have mainly used K&S Metals 0.01"/10 thou/0.25mm brass sheet as sold in most model shops. Nickel silver sheet is less commonly available but can be purchased from Eileen’s Emporium and others. You can also use other thicknesses of brass and nickel silver sheet from 0.005" to 0.015"/5 to 15 thou/0.13 to 0.38mm, possibly thicker. Brass is usual used for modelling body shells and more cosmetic parts, nickel-silver is only really used for chassis parts as it is more rigid and harder wearing. The only preparation needed is to cut to size to match your artwork (excluding alignment cross hairs!) with tin snips and clean thoroughly with ordinary dry steel wool (do not use Brillo pads or any other form of impregnated abrasive pad and do not use abrasive paper). The cleaning should be carried out immediately before applying the etch resist.
Next insert your prepared and suitably sized piece of brass sheet into the pocket formed by the folded film and staples with one edge firmly into the fold in the film. Set the iron to just below three ‘blobs’ and iron the acetate onto the brass on both sides being careful not to move the artwork around. Let it cool then pull the acetate substrate off (as instructed in the special film instructions) and you have a prepared brass or nickel silver sheet with resist pattern ready for etching. Etching should be carried out immediately after applying resist otherwise the surface of the brass will oxidize again and become resistant to etching. Figure 5 shows the various stages in this process. I usually cover any remaining areas of un-coated metal with an etch-resist marker to save my etchant (some laundry and general purpose markers work if you don’t have a purpose designed resist pen, these are also available from Maplin’s).
Figure 5 - Preparation of special film and standard acetate masters and toner transfer to brass sheet.
WARNING: ETCHANT CHEMICALS ARE POISONOUS AND SHOULD ONLY BE USED BY COMPETENT ADULTS WITH DUE CARE AND NOT USING IMPLIMENTS, VESSELS OR AREAS NORMALLY USED FOR FOOD PREPARATION
The next thing you need to do is immerse the selectively resist-coated brass in a suitable chemical etchant. There is more than one option here however I recommend the use of Ferric Chloride which is sold for this purpose by electronics shops for use in making PCBs (printed circuit boards). It is supplied as either crystals or as a pre-dissolved solution; I use crystals because they are cheaper. These should be mixed according to the instructions on the pack and to a volume suited to your etching tank (see later in the article). If buying a ready mixed solution check whether it needs more water to be added before use. I recommend the use of distilled or de-ionised water as the chemicals added to tap water will react with the etchant partly ‘using it up’. De-ionised water can be obtained at car accessory shops.
At this point you may be tempted to think that all you have to do is pour some etchant into a shallow bath and put your brass in it. I tried this and the results were discouraging to say the least. The problem seems to be that the chemicals do not get stirred enough (even if you stand there stirring it) to ensure that 'reacted' etchant is carried away from the brass allowing new 'active' etchant to react with the exposed brass. Also metallic iron is generated as a product of the reaction and this can deposit on the item being etched preventing fresh etchant getting to the surface. The result is that you can leave the metal in there overnight without it etching properly at which point the resist starts to fall off anyway!
I almost gave up at this point but having done some internet
research (try typing 'etch tank' into a search engine) I found that one popular
way of achieving home PCB etching is to use a 'bubble tank'. This is a tank sized to accommodate your item
to be etched having an arrangement to allow air bubbles to be blown through the
etchant in a similar manner to that practiced by most 4-6 year old children
with Coke and a straw in McDonalds. The
bubbles should pass over the surfaces to be etched thus providing continuous
agitation. Of course you can buy such an
etch tank for perhaps £100-£200 (much less in the
Figure 6 - Home made etch tank with bubble bar, aquarium pump and spotlight heater.
The essential element we now need to add to the tank is a 'bubble bar’; you can buy such a thing at an aquarium shop where you will find it described as an 'air curtain'. Don't be tempted to try an 'air stone' or similar diffuser, these are attacked by the etchant, will not work and will eventually fall apart, use the type of air curtain which is made from flexible foam rubber as this will give the most even bubble distribution. The type I use is an ‘Interpret Aqua Air Aquarium Flexible Air Wall’ and cost a few pounds. I had to remove its plastic fittings and bendy metal insert (because it would be affected by the etchant) and only needed half of its length. I closed one end up with superglue and found that the fish tank air pipe fitted well inside the other end. It’s worth noting that all items used to make the etch tank or used in the etch tank must be plastic, glass or glazed ceramic. Most other things will react with the etchant. Initially I made my own bubble bar by drilling holes in a length of plastruct pipe about 4mm diameter. I do not recommend this method as it’s surprisingly hard to get a consistent hole size so I found it difficult to get bubbles to come out evenly along the length of the bar. I did use some plastic tubing with a bend at the end (made using a heat gun) to allow me to position the flexible air curtain tube at the bottom of the tank more easily.
Install your bubble bar into the bottom of the tank, mine was a reasonable side jam fit and I do not recommend permanently gluing this in place as you really need to remove it after each session and clean it to prevent clogging with precipitated iron. I keep mine in water (clear plastic or glass container) and run it in water to make sure its working OK before putting it in the etch tank each time. If it seems clogged roll it between your thumb and finger in the affected area whilst its running underwater to clear it.
Fill the tank to the required depth with water, don't use etchant straight away. Its easier to see what's happening with water as Ferric Chloride etchant is hardly transparent. Connect the air pump to the bubble bar and wind up the pressure until you get some bubbles then adjust for best results. Ideally you are looking for an even distribution of bubbles throughout the tank or at least consistent bubble density across a wide enough section to fit your job. Make whatever adjustments seem necessary then when you're happy that you'll be able to get the bubbles to cover your brass or nickel silver work piece you're ready to fill the tank with etchant instead of water and have a go at etching! One final and simple suggestion with regard to the tank is that the bubbles create a fine mist of etchant when they ‘pop’ at the surface, to catch most of this stand the tank in a shallow plastic tray, I use one of the type in which Chinese takeaways are commonly supplied.
At this point I would like to mention an 'optional extra' which you can fit to your bubble tank. This is a heater to warm the etchant, the fastest etching is supposed to be obtained at temperatures of 25-30C which is warmer than you will get using a tank at ambient temperature in most rooms (probably garages). Some commercially available bubble tanks employ an aquarium heater - these are glass and suitable for the purpose. I did not have one of these to hand so I have experimented with low voltage 'mat' style heaters. Such mat heaters are commercially available for around £10-£15 and typically operated on 12V DC delivering a few watts (say in the range 3-10W). They are usually self-adhesive and you should fit such a heater to one side (nominally the back) of your bubble tank at the bottom. Run the heater so that the etchant is around 30C for faster etching. Another method which is simple but effective is to shine a 100W spotlight on the tank from a few inches away. In fact I find that this is likely to heat up the etchant too much if anything! Figure 6 shows my home made etch tank, bubble generator and spotlight heater with the tank filled with water.
For those who are laughing at my 'Heath Robinson' approach I make no apologies, I believe the British are second to none as improvisers and am a fan of 'Robot Wars', 'Scrap heap challenge', 'Salvage Squad' and similar TV programs which illustrate this point.
Figure 7 – Workpiece holder design and photo
One final piece of equipment you need to improvise is something with which to dangle your brass or nickel silver into the etchant. This like everything else needs to be plastic. I made a simple clamp using some acrylic off cuts and two nylon nuts and bolts. The springiness of the acrylic provides the clamping action.
Clearly the clamp should be sized to fit your tank, workpiece and etchant fill level and I have made use of clothes pegs (which are mainly plastic) to support the clamps on the edge of the tank and provide some adjustment. The peg’s metal spring will fall apart after a while but I consider these disposable items. Figure 7 shows the design of the clamp and my (rather grubby) example.
Now for the payoff, time to etch! Having made all the necessary preparations you simply switch on the bubbles (and heater or spotlight if used - wait until temperature has reached around 30C) and immerse your resist coated brass or nickel silver. Make sure the bubbles pass on both sides and across the full area of the workpiece as uniformly as possible. Check the workpiece by removing it from the tank every 5 minutes until the first holes appear then every 2 minutes until all the exposed metal has been dissolved. Don't leave it too long! This results in the parts you want to keep being dissolved too! Remove the etched brass or nickel silver once complete and wash in water to remove all the etchant. I also give a rinse with some mild household bleach to stop any residual etching action. Don't forget to switch off the bubbles (and heater) and cover the tank to reduce evaporation if you are not using the tank again immediately.
Finally you can remove the etch resist with a solvent, I use etching primer thinners or Finnegan’s thinners (sold for use with ‘Hammerite’ paints), something stronger than white spirit is needed. You could possibly leave the resist in place or sand it off depending on what you are making.
Figures 8 shows my homemade etch tank, aquarium air pump and work piece holder in action.
Figure 9 shows my completed brake stanchion etch on the fret and assembled (the shaft is a length of 0.45mm wire).