Acid-Etching part 2

THE ACID-ETCHING PROCESS

Within the field of scale model building, acid-etching is principally employed to fabricate metal detail parts specifically intended to supplement the contents of a pre-existing commercially available model kit; these 'after-market' add-on packages used to 'super detail' a specific product.

Until a few years ago, acid-etched parts were only available from third parties. Manufacturers who produce replacement/additional parts for kit-assemblers who wish to enhance a kit of other manufacture.

As I mentioned at the start, we have seen the traditional injection formed (styrene) kit manufactures providing acid-etched parts. Many of today's high-dollar airplane and armor styrene kits routinely feature highly detailed acid-etched and white metal parts.

In essence, acid-etching is the process of chemically reducing a metal sheets surface(s) in a controlled manner - some form of mask or 'resist' film is bonded to those areas of the metal that must resist the chemical attack. Of course, this resist film is so applied as to represent the shape of the part we wish to produce, be the finished acid-etched piece a ship railing, the silhouette of a man, or the most complicated lattice type radar antenna reflector.

The acid proof mask can be applied directly to the metal sheet with pen, iron-on transfer, press-on transfer, decal, or done photographically - exposing a photo-reactive type resist through an image captured on a transparency, the imagery representing the metal shapes to be cut from the sheet. And that is the process I'm going to describe for you, the use of photosensitive resists as applied to the process of acid-etching metal sheet.

Now, unlike the other model kit part fabrication techniques I described above, acid-etched parts often require further manipulation to derive the desired shape: That 'ship railing' has to be snipped off the fret and bent to follow the outline of a model ships hull. The small 'man' silhouette has to be built up with super glue or putty to thicken the figures waist, legs, arms, neck, head and so-forth. That dish shaped 'radar antenna' has to be annealed and then pounded to shape upon a purpose built hardwood mandrel. Acid-etched parts often demand much more attention from the kit-assembler than other type model kit parts.

And care has to be taken to layout the drawing (from which the transparency will be shot) so the resulting acid-etched pieces (their form dictated by the transparency) can be worked into three-dimensional structures, if that's their end-use. Much as a sheet-metal craftsman will lay-out the most complicated of shapes onto a two-dimensional sheet of metal, the originator of the drawing has to do much the same.

The example here is a series of acid-etched parts I was asked to produce by Thomas Sasser of Thomas Models. Specifically, the small ovoid shaped 'sensor' dish to accompany his NX-01 kit representing the ship seen in the new Star Trek series, Enterprise.

Below is a reasonably complete description of the acid-etching method I employ. But, for the impatient out there, here's a nutshell version of what I do: A sheet of brass is coated with a photosensitive 'resist' film. A contact transparency is applied over each face and the work then exposure under a UV light source. The 'sensitized' sheet is soaked in a development bath. The sheet is then subjected to an acid attack were those portions not protected by the resist are either completely eaten away or are eaten to approximately one-half the thickness of the sheet. Bingo! A fret of acid-etched parts ready for use.

Now, the process in detail:

THE ACID ETCHING MACHINE

Typically, those casually employing the acid-etching technique will settle on simply pouring Ferric Chloride acid, at room temperature, into a shallow tray to immerse and cut the metal sheet. Doing it this way is relatively simple, safe, cheap, and easy to set up. But, doing it this way produces very long cut times, liberates significant amounts of acid vapor into the shop, and produces less than perfect cuts, as there is significant 'undercutting' of the work owing to the long immersion time in the acid.

Soaking the processed sheet of metal in a tray of room temperature acid, with only the occasional shaking of the tray as agitation, does not produce the best possible work. Yet, that is how I performed my acid cutting for years. However, I eventually smartened up and designed and built a proper heated acid agitation vat, my 'acid-etching machine'. The quality of my acid-etching work has since improved significantly.

My 'air-agitation/hot acid' acid-etching machine. A clear acrylic vat was welded together with cement. An 'air agitation manifold' was constructed from PVC water pipe and then installed within the vat. A twenty-five Watt fish-tank heater, installed near the bottom, of the vat raised the temperature of the acid to about one hundred degrees. Air introduced to the manifold releases bubbles, which work the hot acid around to wash oxides off the work. Sorry guys... no 'kits' of this type tool are available... back to the chat rooms and boards with you. Run along now.

My 'machine' is a clear acrylic tank (vat) with a PVC pipe air agitation manifold within it to discharge low-pressure air bubbles from the bottom of the vat. The machine employs a one hundred-Watt fish-tank heater to bring the acid to and maintain it at a working temperature of one hundred-and-twenty degrees.

The fidelity/crispness of the acid cut is directly related to the speed of the cut. Quick cutting evidences less undercutting and 'raggedness' of piece outline. The less working time in the acid, the better! Two things affect the speed of the cut:

The temperature of the acid has to be high. The hotter the acid that comes into contact with the metal being attacked, the quicker those surfaces are reduced to an oxide.

And some form of agitation has to be employed to 'scrub off' the accumulated metal oxides in order to get fresh acid to the working area. The air bubbles in my machine froth the acid, working it around to wash oxides off the work.

The air introduced for agitation has to be provided a means of escape well clear of the operator (me!) and the shop. Should this acid rich air get into the shop it would quickly rust my iron bearing tools. Those same vapors, Ellie and I can assure you, will do one hell of a number on the lungs and eyes. The vent line from the machine runs out the door and well into the yard!

Of course, the choice of materials from which to build the acid-etching machine had to be given careful attention. I certainly did not want any failure occurring to a vat containing over one gallon of hot corrosive, skin eating acid as a consequence of material failure. As you can imagine, such an incident would constitute a 'bad day' for anyone... except, maybe, Vincent Price.

So, I took care to construct my machine from acrylic, glass, silicon, PVC, cyanoacrylate, and urethane rubber. Substances little affected by hot Ferric Chloride acid.

The acrylic from which the vat, base, and lid were fabricated was secured from a local plastics supply house. I got the PVC tube and silicon hose from the local Lowe's hardware store. Everything else was available in-shop.

The 'heater' used is manufactured by Aquaculture and I've seen these things sold at K-mart and Wal-Mart as well. Go to the 'pet supply' section.

Examine the photo detailing the lid of my machine. See the black rubber seal running about its perimeter? That gasket makes a gas-tight seal between the lid and the acid filled vat. Isolation of the air space above the acid is necessary as the acid vapor rich bubbles that pop at the surface have to be contained. The white PVC pipe atop the lid makes up to a hose that vents off the acid rich air to the outside.

Note the strip of white plastic projecting down off the lid and into the vat. The brass sheet is secured to this foundation with masking tape. As the lid is placed down on the vat, the brass sheet becomes totally immersed in the hot acid.


THE PHOTO RESIST

Photo resist, in this application, is a photosensitive liquid that when applied to a surface (the two faces of the brass sheet) dries to a hard film. In this condition the photo resist can be made to undergo a chemical change when exposed to a specific frequency of light.

In the old days these photo resists, as used by we 'hobbyists', were either brushed onto the work by hand or loaded into a spray-brush and applied to the metal that way. Another problem way back when was that this stuff was packaged and marketed for 'the big guy's' (this stuff finds is major use in the printing industry, for creation of imagery on the face of offset printing plates), thirty years ago it was easier to secure a fifty-five gallon drum of Kodak Photo Resist (KPR) than an ounce of the stuff! I still have half a gallon of old crystallized KRP taking up space in the shed somewhere).

However, in recent times, photo resist, owing mostly to the use of the acid-etching process by those wishing to cut their own circuit boards (and the reason that most of the chemicals needed are found at electronic supply outlets) is today packaged in more reasonably sized quantities. I've even found a source of a very reasonable priced, spray-can packaged photo resist. Very convenient!

Two types of photo-resist chemistries are available to us: 'Positive' and 'negative'. The positive type, the one I favor, produces a photosensitive film that will fail when exposed to UV light. By 'fail' I mean that exposed portions of the photo resist will change in such a way that when subjected to a special developer it will dissolve and lift off the metal sheet, exposing the metal to direct contact with the environment. With a positive type photo resist, those portions of the photo resist not exposed to an UV source (and that's what the contact transparency, described later, does) will remain unaffected by the developer and remain tightly bonded to the sheet, protecting its surface(s) from the environment.

The positive photo resist I have been using recently is produced by CG Electronics. They call it, 'Etch Resist Sensitizer', and is packaged as an aerosol. Ask for part number, 22-074.

Obviously, a 'negative' type photo resist reacts oppositely - exposed areas of a negative acting photo resist are 'hardened' against the developer, and those portions of a negative type photo resist that remain unexposed will readily dissolve when developed.

The transparency needed to impose the desired image onto the positive type photo resist (bonded to the faces of a metal sheet) must be a 'positive'. That is to say, that the dark lines on the transparency will cause the photosensitive film bonded to the sheet to 'save' the metal underneath. Those clear portions of the positive transparency which permit UV light to strike the sheets photosensitive film, will be removed during development, causing those portions of the sheet to be subject to later acid attack.

Application of the photo resist to the sheet starts with selection of the appropriate thickness. I prefer to use the K&S four-inch by ten-inch sheet brass. For the NX-01 sensor dish job I selected their five thousands of an inch thick sheet (part number #250).

To assure adhesion of the photo resist to the brass sheet, the brass must be scrubbed to remove any oils and oxides which would otherwise inhibit the adhesion and distribution of the liquid photo resist. Both faces were scrubbed with a lacquer soaked wad of steel wool and wiped clean.

Now, since the photo resist is photo sensitive to a portion of the light spectrum most of us work under (florescent and tungsten), care has to be taken to work under a light closer to the infra-red end of the spectrum, light that will not 'activate' the photo resist film. A yellow 'bug light' is the ideal safe-light for this type work. During photo resist application and subsequent handling of the coated sheet, all work is done under this safe-light, the bulbs can be gotten at any hardware store.

After the selected brass sheet (for this job I used three-thousands of an inch thick brass sheet) was scrubbed with a lacquer soaked wad of steel wool and wiped off, I spray coated both surfaces (under a yellow 'bug light', safe-light) with a positive type photo-resist emulsion. The work was then placed in a box to dry out thoroughly. The emulsion dries to a rock hard clear film.
 

The two faces of the cleaned brass sheet were then spray coated with the positive type photo-resist. The work was then placed in a light tight box to permit the emulsion to dry out thoroughly and harden.

On to the THE CONTACT TRANSPARENCY 'SANDWICH'