Etching for
Beginners
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Lawton 009 home page http://www.nigellawton009.com/VeeTipper.html
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.
Etching
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).