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Home In English In English Home made photo etching - part 1, part 2

Home made photo etching - part 1, part 2

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Quite often modelers face lack of aftermarket PE parts they need to detail their models. Simply either none of manufacturers decided to produce such PE set, or these are no longer available. Not a problem – PE elements can be practically made by oneself in domestic conditions!

For DIY photo etching the following materials are required: an aerosol photosensitive emulsion, a positive developer and an etching solution. All these can be acquired at electronic shops. The chemicals used as well as the technique of etching are almost identical to electronic circuits etching technique.

Photo etching – application of photosensitive layer and UV exposing.

A metal sheet we would use to make photo etched parts has got to be straight, of perfect surface and without any scratches and score marks or oxidized spots. In other case, photosensitive emulsion would not perfectly stick to the surface or might tend to chip off. It can also form a layer of uneven thickness. The metal sheet has got to be precut prior to applying photosensitive emulsion. Note the dimensions have to be 2cm wider than actual fret size. The excess material will accommodate holes designed for mounting the sheet in etching bath. It’s worth to check if we have a proper trey for etching bath and make sure that the metal sheet would fit inside.

Kitchen sponge with rough underside and dishwashing liquid are excellent for cleaning and degreasing of metal sheet. For surface imperfections which are difficult to remove I use CIF or other cleaning liquid that contains polishing granules. I rub the metal sheet constantly rinsing it with warm water. It must be done carefully not to bend the fragile sheet. A stiff plate slightly larger than the metal sheet is very useful for this operation. Having the metal sheet laid on the plate we minimize the risk of twisting or distorting it while rubbing. The sheet must be washed from both sides. After cleaning it should be generously rinsed with tap water to remove all remains of cleaning liquids. Properly cleaned and degreased sheet should have smooth, shiny and hydrophobic surface which means that drops of water would slide off it like from freshly waxed car body. Then the sheet must be wiped dry because water stains prevent proper adhesion of photosensitive emulsion. The sheet mustn’t be touched during rinsing nor drying.

In order to remove impurities and degrease the metal sheet I use kitchen sponge with rough underside and dishwashing liquid.

Alternatively an ultrasonic cleaner can be used to prepare metal sheet for etching.

Application of photosensitive emulsion.

The idea of photo etching is dissolving of metal in contact with etching bath chemicals and leaving intact material where no such contact occurs. Separating layer clung perfectly to etched surface prevents contact of etching bath with metal sheet. The photosensitive layer can be applied in several ways. The metal sheet can be laminated with photosensitive film which requires laminating machine. It could be also printed and dried in process of screen printing or it can be sprayed from an aerosol can. In this article I shall describe the use of spray emulsion as it is the only means available for domestic photo etching.

Right before application of photosensitive emulsion I blow off all dust particles.

Even layer of photosensitive emulsion has to be sprayed on both sides of clean and dry metal sheet. Prior to that operation all dust particles must be blown off. To prevent the sheet form sliding I recommend to secure it with Scotch tape on some flat surface like piece of cardboard. Removing of all impurities is very important because dust particles that stuck in emulsion would create lumps which prevent film from clinging to metal sheet and would cause poor photo etching. Such imperfections cannot be later removed. Hence it is mostly important in etching of regular meshes, anti slip surfaces of regular pattern as well as delicate and thin elements. Securing the sheets on horizontal surface would prevent runs. Having the photosensitive emulsion applied on one side we leave the sheet for hardening time (the temperature and time of drying are specified on the emulsion can). Usually it is few hours in room temperature or roughly a quarter in 70˚C. A kitchen oven with preset temperature can be used for quick drying by domestic means. Note the emulsion must not be dried longer than stated by manufacturer because over dried layer becomes crispy and easily chips off in case of accidental  scratch.

The aerosol emulsion is comfortable for domestic photo etching. I use Positive 20 photosensitive emulsion to cover the metal sheets.

Exposure and developing of photosensitive emulsion.

Having the metal sheet prepared as described above, it should be placed between two films with two sided picture of elements. Crosses on both films are markers used for aligning of the films. Note the films must be sandwiched with emulsion covered sides facing inwards. Then after aligning markers films can be fixed together with double sided sticky tape. To distinguish which side of film is covered with emulsion a glance against the light is required. Emulsion can be recognized by matt finish it gives to black areas. It is crucial that emulsion covered sides of film (note the film emulsion is different than photosensitive emulsion described above) face the metal sheet because in other case the picture on top of the film would be separated from the metal sheet by the wall of film. It causes the risk of refraction of light on the edge of the drawing which as an effect gives irregular and uneven edge of the final PE element. A UV light source is required for exposing. It can be UV glow tube in a simple fixture as well as a polygraphic copy-frame – a device present in most every print house. Vacu-membrane pressure system is the main advantage of this professional tool. It provides a perfect fit of the film to the metal sheet – which is one of the factors that affect the sharpness and accuracy of the etched bits. However it can be substituted with PC flatbed scanner with its glow tube replaced with an UV one. In this case or if an external UV light source is used – the film must be properly pressed to the sheet. Placing the films previously fixed with the metal sheet between two pieces of glass or plexiglass can also do the job.

I put the metal sheet with dry emulsion between two films fixed together and use polygraphic copy-frame for UV exposing.

The polygraphic copy-frame provides the best fit of the film to metal sheet. Good results can be achieved with use of flatbed PC scanner with its lamp swapped to UV glow tube. (photo: Marcin Witkowski)

Time is the most important parameter in exposing. In practice the time of exposing is defined by experience. A quick and simple method is use of photographic step wedge. It works by sticking onto the metal sheet a row of square little pieces of isolating tape. Next the sheet is exposed in 30 sec. cycles. By removing one square after each cycle we obtain exposed squares with exposing time of 30, 60, 90 and 120… seconds. Correct exposing can be easily recognized because during developing the emulsion will be completely washed off only from properly exposed square. The exposure time defined this way will be the same for further exposing unless we change the exposing conditions – sort of UV light source and its distance from exposed piece, sort of material and thickness of the pressure plates that hold films and metal sheet.

Developing (removal of unneeded layer of photosensitive emulsion) must be with aid of developer corresponding to the photosensitive emulsion – seek information on compatibility from retail sellers. A simple tray of the size of developed plate can be used. Time of developing, concentration, and the temperature of bath are given on the leaflet attached to developer. Once observed that emulsion is flushed off the exposed places, the developing should be stopped, the plate rinsed to remove remains of emulsion and dried.

Exposed emulsion is being flushed in developer. After developing the metal sheet is rinsed with cold water.

The ready plate should be carefully examined in search of any possible errors which might have occurred in previous stages of preparation, emulsion application, exposing or developing. This analyze will help decide whether to continue with etching or should it be formerly fixed. I shall describe the correction of errors and repairing of emulsion layer in the following article.

Errors may often appear on the developed plate. However mostly they can be corrected.

Other techniques of etching preparation.

It happens that modeler needs parts which do not require extra accuracy. This is when alternative method of  application of the layer which separates the metal sheet from developing bath can be used.

The first method is based on use of adhesive film trimmed with cutting plotter – similar to painting masks. This method is applicable in one sided etching only. Note the back side of the etched plate must be completely covered with adhesive film. The main advantage of this technique is short preparation time – it’s enough to trim and stick the masks – with no need to careful cleaning, application of photosensitive layer nor exposing with UV light. Another advantage is no necessity of making bridges which hold the element in fret – the function is fulfilled with adhesive film on the back side of the plate which holds the etched parts.

Another method is based on special laser printing paper. This paper is designed for single-handed etching of electronic circuits, and can be purchased in electronics shops. The toner of this paper is pressed into metal sheet with aid of iron and this way forms a layer that separates the metal sheet from developing bath.

Both of these methods are excellent for modelers who make their first steps in photo etching.

 

 

 

Part 2 - 04.09.2011

Quality control of the etching preparation processes

It is worth to check the surface quality prior to etching. Some errors might appear on the sheet surface during application of the photosensitive layer, UV exposing or during developing. Particular errors can be corrected still before etching. After developing it can be observed that some areas were not completely covered by the photosensitive emulsion or some dust particles or little hair stuck to the emulsion before it set. Such errors can be easily corrected with an aid of a permanent marker. It is very easy to scratch off the emulsion while operating the sheet e.g. during drying, UV exposing or developing. This type of damage can be also repaired with the use of a permanent marker. The places where these types of damage occur must be taken into account. It’s pointless to correct the errors, which are on the fret, outside the elements. It might be tricky to correct if the damage happens to be on an edge of an element, because it is very difficult to mark a perfect line which would also form the edge of the part. Fine elements like thin lines, small openings or meshes pose the same problem. The only way to solve it is to accept that errors do happen and copy the problematic elements in the design preparation phase. In this way the risk of encountering such errors in similar elements is minimized.

After developing of the photosensitive emulsion there may be some spots where the emulsion did not fully cover or some dust particles or little hair which stuck to the emulsion before it set. Many errors can be easily corrected prior to etching with the help of a permanent marker.

The etching bath preparation

The process of etching can be conducted with the use of many chemical agents. After many trials I decided to use sodium persulphate – it is easily available in electronics shops. Moreover, due to its clarity it is possible to watch the process of etching, and it is relatively safe in use. The amount of water needed to dissolve the sodium persulphate and the optimal temperature are given on the leaflet attached to the agent. No special equipment is needed for etching – only a simple receptacle which can contain a suspended sheet prepared for etching. It is important that the sheet should be suspended in such a way that prevents turning over and additionally leaving enough space around it to allow stirring of the etching bath. If you plan to etch many frets or look for comfort, you should make a special receptacle designed for etching. Securing the etching bath receptacle inside another bowl filled with water allows for an easy temperature control. The etching bath is stirred by air bubbles coming out of the perforated hose connected to the airbrush compressor.

Only a simple receptacle that can contain the suspended sheet is needed for etching process. For frequent etching – making a special receptacle is recommended.

Etching

Two things are most important during the etching process. These are the temperature and the heterogeneity of the etching solution. I have tested the proces where the etching bath receptacle was secured inside another bowl filled with water to keep the temperature constant. I find this simple method worthy to recommend. The temperature of water in the outer bowl can be adjusted by a flow of preset tap water or by installing an aquarium heater with a thermostat. The etching rate decreases with the increasing mass of etched copper or brass. Also the temperature fall can decrease the rate of etching. Therefore, it is very important to maintain a constant temperature and densification in the whole volume of the etching bath. Otherwise the elements in different areas of a fret would be etched at different rate which will cause poor quality of etching. Under-etched elements would not completely separate from the fret and the ones that were etched for a too long time would fall out of the fret, change their dimensions or even completely dissolve. These are the reasons for continuous stirring of the etching bath. For the same reasons, from time to time the etched piece should be rotated by 180 degrees. If a simple receptacle is being used, the etching solution should be stirred by hand but if the aerification equipment is installed the process of stirring goes on automatically. At the temperature of 40 degrees Celsius the recommended densification of the solution and with the use of the fresh solution etching of 0.1 mm sheet takes about 15 – 20 minutes. During this time the etching solution should be stirred about ten times and the fret must be turned over about four times. Along with the etched material mass increase, the etching capability of the solution decreases, its color turns more azure, and the etching time increases. The etching solution can be stored and used even for few weeks until it completely loses the etching capability.

A good way of ensuring a constant temperature is securing the etching bath receptacle inside another bowl filled with water.

It is very important to recognize the moment when the plate should be recovered from the etching solution and washed. The sodium persulphate is clear and allows to watch the etching progress. The picture below shows the moment when first perforations appear. From this moment on the fret should be carefully watched in order to stop the etching as soon as the areas which should be etched throughout will have even edges. The etching time should not be exceeded due to risk of etching through the fret bridges and loosing the elements in the etching solution. After removing the fret should be rinsed with tap water and dried out. The remains of photosensitive emulsion can be easily removed with a nitro solvent.

First perforations appear – from now on the process should be carefully watched in order to stop etching at the right moment.

A nitro solvent is excellent for removing the remains of the photosensitive emulsion.

Text and photos: Wojtek Fajga,
Translation: Piotr Mazurek
Original article was published in polish language in AeroPlan 1, 2, 3, 4/1010