Using Photo-etch Techniques to Build a 1/76 Medium Mark III

The finished Medium Mark III

When I decided to build a Medium Mark III I considered the various techniques available to me. Building from sheet polystyrene is a well established technique that I have often used in the past, but which was a fairly daunting prospect given the amount of rivet detail required for this vehicle. Also, I fancied trying some new technique that I had not used before. I had etched electronic circuit boards before and decided that this was a possible way forward. It took a lot of experimentation to develop suitable methods that gave me the results I wanted. Even then, there are unsolved problems, but I thought that my experiences are worth passing on.

The process outlined

In the simplest case, to prepare photo-etched parts, you expose a photoresist coated brass sheet to UV light through a mask. When the exposed photoresist is developed by immersion in a developing solution an etch resistant pattern formed by the masked areas is left on the surface of the brass. The brass sheet is then etched by immersion in a suitable etching fluid, such as ferric chloride solution and the unprotected brass surface is eaten away, leaving the pattern that you want. It sounds simple, but as ever, there are pitfalls for the unwary.

Equipment and materials

The table lists the essential items required. Some of these materials are hazardous and it is very important that appropriate safety precautions are observed. If you are not experienced working with such materials, read up all you can about safe practices and try to get some training with someone who is experienced. Always read Material Safety Data Sheets and observe the precautions given. If in doubt, don’t!

Table 1.              Equipment and Materials Needed

The developer and etching solutions should be made up as in the supplier’s instructions. I used half strength ferric chloride solution (0.5 g/cm³ rather than 1 g/cm³). Dissolving ferric chloride in water is exothermic – it gets very hot. It is best to use deionised or distilled water whenever water is required. Always use rubber gloves and eye protection (goggles or safety glasses) when handling chemicals. Ferric chloride is corrosive, toxic and stains heavily. Always read and observe the precautions in the Material Safety Data Sheets. Always clear up spillages immediately. Always label containers clearly. Keep out of reach of children and away from food and drink.

Methods

Preparation of the brass sheet

Correct preparation is vital. Most problems are caused by poor preparation.

The sheet must be perfectly clean. It is best to wear rubber gloves to avoid getting fingerprints on the brass surface. I used wire wool to clean the surface and then finished off by degreasing with methylated spirit, soap and water and then a final water rinse. Other solvents may be used if safety precautions are observed. The plate can then be wiped dry with a paper towel and finally blow dried with the hair dryer. The sheet can curl in preparation. Avoid this by preparing both sides. Beware particles of dust, fluff, etc. adhering to the plate.

Photoresist coating

This is the most difficult part. It is essential to achieve a smooth, uniform, flawless coat. This also can appear to be nigh on impossible. Practice may not make perfect, but it certainly assists.

In a well ventilated, poorly lit area lay out a wide expanse of newspaper. Place the brass sheet to be sprayed flat on this surface. Start spraying well off of the brass surface, about 6 – 9 inches from the paper surface and at an oblique angle level with the top of the plate edge. Smoothly traverse the plate, ending well off the plate on the newspaper and then, spraying continuously, reverse the direction of traverse a little lower down the plate. Continue until the plate surface is covered. Dry the plate for a few seconds with the hair dryer so that the plate is not wet. Rotate the plate by ninety degrees and repeat the process. Continue until the plate has been rotated and coated through a full 360º. You may wish to repeat, depending upon your technique and accumulated experience. If you can inspect the plate under high magnification this is helpful. A mottled or pitted finish is not suitable. The smoother the better. This method is at variance with that suggested by RS, but their advice is aimed at preparing one-off circuit boards and the finish is not so critical. I used up a whole can just experimenting. Dry the finally coated plate with the hair dryer for at least 30 minutes.

Unexposed photoresist coatings should be protected from intense light, especially UV rich light, such as sunlight.

Preparation of the mask

I used a graphics program on my computer to prepare the patterns for masking the photoresist film. Your favourite program will do fine. For developing the net of awkward shaped items, such as the conic sections of the turret and the cylindrical MG turrets, a little mathematics helps. For convenience, I prepared masks for only fairly small sheets of brass at any one time and divided the tasks up into suitable units, such as wheels, box bogies, turrets and so on. I often printed sufficient patterns to make myself some spare items in case of problems.

The masks were printed onto overhead projector transparency material (OHP) using an inkjet printer. You need to ensure that the black areas are as opaque as possible. This is a function of the printer settings and requires experimentation. The densest ink may not be achieved with the OHP recommended settings. I found my Epson Stylus 860 appeared to use denser black ink than the Canon i850 I replaced it with, although alignment was adequate enough to enable me to overprint OHP again with the Canon so as to achieve a denser black. I have yet to try a laser printer.

Exposing the mask

I used an RS light box which consists of a pair of UV fluorescent tubes mounted beneath a glass plate in a lidded box. The mask is placed on the glass plate (be careful with orientating right and left handed patterns) and the brass sheet is placed on top of the mask. When the lid is closed, the plate and mask are pressed tightly together by a foam pad inside the lid. The UV lights are turned on and exposure proceeds. Typically this takes 4 – 6 minutes, but again experimentation will be required to get this right for the thickness of resist and opacity of mask that you achieve. It would be quite possible to make a light box or to use another UV source. Ordinary filament lamps do produce some UV, but exposure times would be very long. Sunlight is UV rich, but, in the UK at least, is very unpredictable. Also, the sun keeps moving!

Developing the exposed plate

The exposed plate is immersed in the developer solution and continually kept in motion. The exposed areas will wash away in a matter of a minute or two, leaving a well defined pattern corresponding to the mask. The plate can then be washed in water and dried. It is better to slightly underexpose, rather than to suffer exposure through not completely opaque areas of the mask. Gentle rubbing with a tissue will often clean up a slightly underexposed pattern. An ink eraser can also be used for cleaning up. Inadvertent exposure through inadequately opaque masking will lead to pitting of the masked areas of the etched plate.

Pre-etch preparation

Unmasked borders of the brass sheet and also the reverse of the sheet should be covered with PVC insulation tape. This masks these areas from etching. It also performs another function: if a brass sheet is etched from both sides, small parts may fall out. If PVC tape is used for a one sided etch, this cannot happen as the adhesive prevents small parts falling away from the sheet.

I have tried double sided etching with some success, although light bleed around the plate during exposure can be a problem. Alignment is of course critical and a transparent jig has to be prepared to assist in correct alignment. The hull underside of the Medium Mark III was etched on both sides in order to allow score marks to assist in bending the brass to shape.

Etching

I use a large flat dish (a casserole dish lid). The plate to be etched is immersed flat in the etching fluid on one side of the dish with the side to be etched uppermost. On the other side of the dish is the follower of a magnetic stirrer. It is important to ensure that the fluid is continually mixed by stirring. Commercial etching baths suspend the plate above the etching fluid and continually recycle the fluid by spraying the plate and allowing the fluid to run back into the bulk liquid below, but I find continuous stirring to be adequate.

It is wise to remove the brass plate from the etching medium at intervals of a few minutes, rinse it and closely examine it in order to estimate the rate of etch, to spot any potential problems and hopefully to correct them. Plastic tweezers are ideal for handling the brass plate in the etching bath.

Where a partial etch is required, for example if raised rivet detail is being etched, the plate can be washed and dried once the desired depth of etch has been achieved, and the areas masked from further etching with an etch resist pen. Once the resist is dry, the plate is returned to the bath for further etching. Similarly, an etch resist pen can be used for retouching the mask pattern, as required. Etch resist pens do not permit fine detail to be retouched, however, and are prone to flooding if care is not taken. I have used nail varnish and a size 00 brush.

I find a stereo zoom microscope invaluable for examining brass plates, for retouching work and indeed for many aspects of general modelling. A good single lens magnifier will in most cases be perfectly adequate, however.

 

 

The etched brass sheet should be thoroughly washed to remove all traces of ferric chloride and the spray etch resist may be removed with methylated spirit. The etch resist from RS resist pens is not soluble in methylated spirit and it is probably best to consult the RS data sheet for methods of removal. It is soluble in 1,1,1 trichloroethane, formerly available as correcting fluid diluent, but as this is potentially toxic and now difficult to obtain, I cannot recommend this.

It is usually best to always use freshly prepared ferric chloride solution. There are, however, uses for nearly exhausted etching solution. Brass is an alloy of copper and zinc. Zinc is more reactive than copper and old nearly exhausted etching solution will tend to remove zinc in preference to copper, hence leaving a rusty reddish finish to the etched surface. This can be useful in simulating rust on exposed brass surfaces.

Problems and solutions

Even following the methods given here, it is extremely difficult to get consistent good quality results. When the techniques work well, the resolution of the etching can make the dot pattern from the inkjet printer used to print the masks visible and you can’t expect to get better than that. It is, however, very difficult to get a consistent technique for cleaning and coating the brass and lack of opacity of printing can also be problematic. I understand that there are photoresist films available (e.g. Riston from Du Pont) that can be laminated onto brass using an office pouch laminator. I have yet to locate a source of this material for the small quantities required for the hobbyist. Do let me know if you know of a source as this would solve at least some of the problems. I suspect that a good laser printer may overcome the ink opacity problem.

Some people appear to have had problems with undercutting of detail during etching. I have not found this to be the case. I think this may be related to the stirring technique used and also to the maximum thickness of the brass sheet. In my case this was 10 thou.

Where photoresist has been inadequately removed due to, for example, underexposure, when etched a rough surface may result. If this is spotted early, patches of unwanted resist may be removed with a scalpel blade, a mounted needle or a hard pencil or ink eraser.

Other Pages in this Group of Articles

Part I: Medium Mark III

Part III: Construction in Photoetched Brass

Finished Model

The Rest of the Site

 

(pre-GS 'A' number designations)

 

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