Acid-Etching part 3

THE CONTACT TRANSPARENCY 'SANDWICH'

So, how to impart onto the brass sheets photo resist the specific imagery, the design, of the items we want cut from the metal? By directing UV light through a contact transparency, temporarily affixed to the surface of the sensitized sheet, that's how! The contact transparency forms a clear/opaque image that acts as a mask network controlling the location onto the sheets photosensitive film (the resist) where UV light will and will not fall.

This contact transparency (really, two of them needed) should be sized to fill the entire metal sheet - less a half-inch boarder around the sheets perimeter.

To produce simple acid-etched parts, possessing no relief to one face, requires the use of two identical transparencies.

... And yes, use of a single transparency will permit you to cut only one face of the brass sheet. However, unless the sheet is exceptionally thin, the long cut time results in a very sloppy looking brass part. Cut from both sides!

 (For you darkroom buffs: The use of the transparency here is akin to the task of making a photographic print from a 'contact negative' - though, in this example, our transparency is a positive of the drawing, not a negative).

Contact transparency, how do you get one made?

Well... for an old, out of touch fart, such as myself, you simply sit down at the drafting table and produce your art-work with paper and pen (remember when people actually used pens and drafting tools, boy's and girl's?).

It matters not a thing at this point if a negative or positive acting chemistry is used. The demands of those two different types is addressed later, as the art-work is reduced to a positive or negative transparency in the 'process camera'.

It's wise to draft out the outlines of the parts in black ink at a size significantly larger than the needed acid-etched parts. I typically size my drawing at a four-to-one ratio; the drawing is four times the actual size of the needed part. This is done so that when the work is optically reduced in the process camera (or Xerox machine), the line fidelity is tightened up and becomes denser. Remember: if the transparency will be of the 'positive' type, the white of the paper will copy as clear on the transparency and the black ink on the paper will copy as black on the transparency.

The drawing not only represents the outlines of the parts, but must also feature 'connecting nibs' between the items. The nibs will later become the connection points holding each acid-etched piece onto the fret ('fret' is the word most suitable for describing the complicated array of parts and nibs that form the interconnecting network of parts realized after the cut sheet is pulled out of the acid. To 'fret' is to eat away - look it up!). We've all heard he term, 'a fret of acid-etched parts is also included', in kit reviews. Without inclusion of the nibs, the acid-etched pieces will separate completely free of the dissolved brass and fall to the bottom of the vat and will be lost. No nibs, no acid-etched parts!

To produce an acid-cut fret of simple parts (no cut relief on one face) that's all you need to draft, just the one drawing. Sized to, upon reduction, to fit within half-inch boarders of the brass sheet that will be cut.

You then schlep the drawing to a printer in town who still has (and hopefully knows how to use) an in-house process camera.

Such outfits are getting hard to find, these days - so much of this work is passing into the hands of the computer geeks! Mention the words, 'process camera' to most of today's printing houses and you are likely to be greeted at the counter by some pizza stained fat guy with dirty classes, soft hands, an obvious lack of appreciation for the powers of soap, and spittle dribbling from his lips onto his pocket-protector. Ask him of the availability of a process camera there and you might be entertained with an answer like this:    "Process camera... ahh.... what? Is that... like... er... a 'real' camera or something? Just computers here, man. Hey, my Boss isn't here, wanna see my cool looking 3D Star Trek Models, man?... their killer!" (he informs me that in his off-hours he moderates an Internet chatroom, and would I like to join in some night?).

... And I'm off to another printer!

Assuming you find a printer who knows his business... instruct him to make two positive films, each shot to twenty-five percent of your art work (or whatever ratio you worked to). Further instruct your printer to shoot one film 'emulsion side down', and the second 'emulsion side up'.

The contact transparencies being registered and fixed in alignment. Care has to be taken to have these dense photographically produced positives (black will be saved, clear will be eaten away) produced so that the surface of the emulsion is in direct contact with the surface of the sensitized brass sheet.

With your two transparencies in hand identify the emulsion sides and place the two together with the emulsion sides facing each other. Hold them up to a light and shift them around until the outlines of the images overlap each other and register as one image. Tape one corner of the two to hold them in alignment.

Then, selecting a sheet of brass of the thickness you will be using to make the acid-etched parts, cut a strip and sandwich it between the two transparencies at another corner (clear of the image, within the half-inch boarder provided) and tape this shim in place. Check that the two transparencies are still in registartion. Affix another brass sheet shim at another edge of the sandwich. There, you can now secure a sensitized piece of brass between the two transparencies and be assured that the transparencies remain in perfect registration.

The brass shims at the corners of the transparency sandwich will assure that they don't wrinkle when compressed over the sensitized brass sheet within the negative holder.

Now, lets retrace our steps a bit and look at what you have to do to produce acid-etched pieces that have relief cut into one face, like these NX-01 'sensor' dish pieces with their oblong concentric rings on one face of each acid-etched piece:

Start with the drawing described above. Produce another identical drawing, a job for the Xerox machine, and change it by whiting out those areas of the images you want cut to one-half the thickness of the brass sheet. You then have one transparency shot from each drawing. Sandwich the two together as described above.



EXPOSING THE WORK

The particular photo resist emulsions I use (and most other photo resists available to both industry and hobbyist) are sensitive to light at the high end of the spectrum, Ultra Violet light.  A sizable component of the light from the noonday sun is UV. An exposure of only a few minutes per side is needed to adequately burn an image into the photo resist if you're using the sun as your light source.

However, if the sun is not available or convenient to you, a Sylvania 'Spot-Gro' bulb (rated at one-hundred-and-twenty Watts) will crank out enough UV to do the job. However, if you use the bulb you're compelled to position the light source about four feet over the contact negative holder and to expose each side of the transparency-brass sheet-transparency sandwich for a minimum of forty-five minutes.

I've used three hundred-Watt incandescent bulbs successfully as a source, but those bulbs put out so little UV as to require exposure times approaching two hours, each side! Not recommended.

Setting up for the exposures (two are needed, one for each side of the transparency-brass sheet-transparency sandwich) starts with securing the sandwich within a contact negative holder, clearly seen in the photo.

Five sets of transparencies with photo-resist clad brass sheet sandwiched between each. Compressed tightly under a glass lid of this contact exposure plate assures that the upward facing transparencies are pushed down tight onto the surface of the sensitized brass sheet, assuring no light 'leakage' under any dark portions of the transparency. Exposure is with an Ultra Violet light source.

The contact negative holder works to compress the sandwich elements together tightly. The base of the holder is slightly compressible rubber foam. Its top, which swings up on a hinge at one corner, permits installing, flipping, and removal of the work. The top is a clear piece of glass. The glass is there to permit the exposure source (UV bulb or sun) to reach the upward facing transparency.

After the initial exposure, the glass is unlatched, the sandwich is flipped, the glass mashed down onto the work again and the other side of the sandwich exposed to the UV source.

 
DEVELOPING THE EXPOSED PHOTO RESIST

After the exposure process (once again working under the 'yellow' safe light) the sensitized brass sheet is slipped out from between the transparencies and readied for 'development'.

The CG Electronics 'Positive Acting Developer Concentrate', part number 22-226, is cut with water as directed and poured into a shallow tray. This is a very base solution and is even more damaging to the eyes than hot acid - so eye protection, when mixing the CG developer, is a good idea. And have eyewash ready for quick use in the event of accident; have the goggles on and the eyewash ready before you pop the cap off the developer bottle!

After the photo-resist emulsion bonded to both faces of the brass is 'exposed', the contact transparencies are separated from the brass, and the brass sheets are taken to a developing bath. Slight agitation is provided by moving the tray slowly to assure the developer weakens and lifts off the exposed photo-resist off the brass.

The developer is maintained at room temperature. Grasping the exposed brass sheet at one corner with a hemostat the entire sheet is immersed in the developer. Gently, without touching the work against any portion of the tray, gently swish the sheet around to wash off the softened and lifted photo resist

Now, there is a neat feature to the CG Electronics photo resist, which becomes apparent at this point. As the developer does its work, the photo resist remaining on the brass sheet (those portions that will protect the sheet from acid attack) begins to take on a red color. This tinting of the remaining photo resist clearly defines the imagery transposed from the transparency onto the brass sheets photo resist emulsion - a means of visually verifying how the work is going.

As soon as you observe that there has been complete removal of the exposed photo resist from the sheet, the brass is pulled out of the developer, rinsed in a fresh water bath for a moment, then hung to dry.

(Not called out for in the instructions provided with the CG system, but a means for quickly checking the fidelity of the resist, I quickly dunk the work into room temperature acid and then rinse it off with fresh water. The shallow oxidation of the bare metal portions of the brass sheets surface reveals if I have a good transfer of the images to the sheets photo resist. If good I leave the work to dry out. If there is evidence of light 'bleed', poor adhesion of the resist, or other faults, the sheet is set aside for later scrubbing with lacquer and a repeat of the process).

Next THE ACID-ETCHING MACHINE AT WORK