A screen is coated with photo sensitive emulsion and allowed to dry. The stencil is made by placing a “film positive” (i.e. your design printed onto a transparency, or hand drawn onto draft film / polydraw using a permanent opaque marker) on the treated screen, which is then exposed to UV light. Where the light is blocked, which will be your design, the emulsion remains water-soluble and will wash away. The remaining emulsion will have been hardened by the light and act as the stencil (or resist) and the image is printed through the area left clear.
Instructions for using Photo Emulsion
(see also Ruth Brown’s notes on the process)
- Thoroughly degrease the mesh using Mesh Prep Gel.
- Dry the mesh completely and apply the photo emulsion using a coating trough. Coating should be carried out under low-level tungsten or yellow light. Exclude all sources of ultraviolet light and ensure that the coating and drying area is as dust free as possible.
- Apply one coat of Photo Emulsion (choose from Speedball Diazo Photo Emulsion or ready-mixed Kopimask Zoico Polymer 400C Photo Emulsion) to the underside of the mesh and turn the frame through 180Cº and apply two coats to the squeegee side of the mesh. Remove surplus emulsion from the screen by running the coating trough up each side of the screen with a dry blade edge. You should aim to achieve a thin, even coating.
- Dry the screen in the horizontal print position with the squeegee side uppermost, supported on blocks. Drying must be carried out in a U.V. light proof area, low level tungsten or yellow light is suitable. Dark conditions are better, but in all cases the area should be as dust free as possible. Gentle heat which must not exceed 35ºC is recommended.
- The air flow is more important than the velocity of air. When the screen is completely dry it may be dated and stored in a cool, dark, dry cabinet or used immediately.
Exposing the Emulsion on the Screen
- Place the film positive to the underside of the screen and tape it into position with clear cellulose tape. Place the screen into the “exposure unit” with your positive to the glass surface and apply U.V. light to the screen. Due to the various combinations of coating, mesh grade, mesh colour, output of lamp, distance etc it is not possible to give the optimum exposure times, therefore a trial step-wedge test should be made.
- After exposure, remove positive and wash, with a spray of cold water. Firstly spray the squeegee side of the mesh and immediately spray the underside of the mesh. The screen should be washed out in the vertical position starting on the squeegee side and turned to wash out the underside.
- When the stencil is fully developed it should immediately be blotted with either unprinted newspaper (Newsprint), or a well-wrung out chamois leather to remove excess water, and then fully dried preferably in the print position, again using heat not exceeding 35ºC. This final drying can be carried out under normal lighting.
N.B. The shelf-life of the Emulsion, once it is sensitised, is 2-4 weeks. It should be stored in a cool dark place. The shelf-life can be increased by storing the sensitised Emulsion in a refrigerator, but in this case, it must be removed from the refrigerator some hours in advance of using, to allow the Emulsion to warm to room temperature and become less viscous.
Frequently Asked Questions
Is it necessary to degrease the screen mesh?
The monofilament screen meshes we supply are very accurately woven, using a single, smooth, polyester fibre. This offers very little surface area for the emulsion to adhere to (this also applies to films and fillers, if you are using them). If there is any trace of grease, and this can be from your hands, then the stencil is likely to suffer premature breakdown. We would always recommend that you degrease your screen, prior to coating, as a regular practice.
Can you use washing up liquid as a degreasing agent instead of Mesh Prep Gel?
No! Washing up liquid contains softeners, which will leave traces on the mesh.
I have heard that you can use abrasive detergents to rough-up and degrease the surface of the mesh. Would you recommend this?
Do you have to have a dark room to carry out this procedure?
No, but do not carry it out in bright sun light. When the emulsion is in a liquid state it is not that sensitive. Therefore, coating a screen can be carried out under low-level light. As it dries it becomes more sensitive, so it should be put away in a cupboard or dark doom, to dry and be shielded from bright light and sources of heat, before processing further.
Can I assist the drying process of photo sensitive emulsion?
Yes, but please note, heat can have the same detrimental effect as a bright light (i.e. it can harden the emulsion so that it renders it unusable). A fan heater can be used on a low heat setting, blowing air across the screen. Do not blow directly onto it, as this can cause heat spots. Make sure that dust is not blow up onto the emulsion as this will cause pinholes in the stencil.
How many coats of emulsion should I apply?
What you should be aiming for, is a thin, even coating. One, might be quite sufficient, especially if you are applying it with a squeegee. If you have a coating trough, then you can apply as many coats as you like, but you must remove the excess from both sides of the screen, by running the dry blade, (i.e. uncharged with emulsion) up each side of the screen.
How do you coat a screen with a squeegee?
Lay the screen upside down. Using temporary tape (packing tape is good for this) put one or two strips across each end and another down each side, within the width of the squeegee. Pour a bead of emulsion across the tape at the top end of the screen and pull the emulsion down to the bottom end using your squeegee. Remove surplus emulsion from the tape and put it back into its tub and leave to dry. Strip away your tapes, and you will have a fine, clean coating.
How long is the exposure for photo sensitive emulsion?
This is a difficult question to answer. There are so many ponderables. It is very important that you have a very good film positive to work with when you calibrate your light source. Your design must be perfectly opaque to the light. The emulsion will harden where it is exposed to sufficient UV light, but will remain soft and wash away in water where it has been shielded.
Film positives produced from photocopiers and computer printers are not truly opaque and should be avoided for this procedure. Here are the points you have to consider:
- The light source you are using. Ideally this should be an appropriate Ultra Violet Reprographic Light. It is the emission of Ultra Violet, mainly in the invisible part of the spectrum, which hardens the emulsion.
- The area you are exposing with the photo sensitive emulsion.
- The thickness and colour of the mesh. A course mesh (eg; 43) will have a thicker stencil than a finer mesh (eg; 61), but a coloured mesh (eg; 77 Yellow) will require an extra 50% exposure time.
- The distance of your lamp is from your work.
- The age of your lamp. These lamps gradually wear out and although they might appear bright, they gradually loose some of the effective radiation. It will mean that the exposure will have to be gradually increased and eventually the bulb replaced
In the end, you will have to a make bold decision, chose a time and see what happens.
If the stencil is stubborn to wash out, but you can see the image, then you have overexposed it. If it washes out very quickly, with more besides and the working side of the emulsion is tacky, then it is underexposed. In the first case halve the time, in the latter, double the time and follow the same procedure if you are still getting the similar results.
Do I use hot or cold water for washing out the stencil?
Use cold water. Initially the unexposed areas are absorbing the water, swelling and then breaking away from the hardened areas. So to start with, the screen need only to be bathed in water and once your design has started to clear, it can be sprayed more rigorously. We would not recommend using a high pressure washer.
Why do I have to swab-off the excess water after washing out?
If water is left on the screen, then scumming can occur, leaving a translucent veil in your design, especially the most intricate parts, causing blockages.