Screen Printing & Colour - a complete guide

Place the frame mesh side up, making sure the mesh is not touching anything, in a dark, light-proof place where warm air can circulate for 24 hours or until thoroughly dry. 

Place your screen mesh side down onto a piece of black, non-reflective card, and position your positive on top of it. Cover the positive with a piece of glass to hold it in place. The first step is to expose your final stencil. This is vital in order to achieve a strong positive and also, so that you do not over-expose, resulting in the emulsion curing to a hard, plastic finish which may be difficult to remove. Exposure times can vary considerably, depending on the type, wattage and age of your light source. With a Portable Photostencil Exposure Unit, begin your test strip at around 15 minutes. For larger units, times start from 10 seconds. The emulsion that has cured to a rubbery finish with clean, crisp lines has received the correct exposure time.

Once you have established the correct exposure time for your stencil, expose a new, coated screen for this length of time. Once the screen is exposed, it must be washed out immediately. Wet the screen well on both sides. Loosen the soft, unexposed emulsion by rinsing the areas with cold water. 

If the screen is properly exposed, the unexposed areas will be a different colour to the exposed emulsion, and will wash away revealing the clean mesh beneath. Once dry, it is ready to screen print with. Store the screen in a dark place when not in use. The exposed screen will still be sensitive to light, so in light conditions it will continue to react and the coat of emulsion will harden over time. If this occurs, the screen will need to be re-meshed or jet washed to remove the emulsion and reclaim the screen.

Reclaiming Your Screen Using De-Coating Agent 

If the stencil emulsion has cured to a rubbery finish, then De-Coating Agent can be applied to reclaim the screen. Brush the agent on to the screen making sure that the entire stencil is covered. Lay the screen into a tray containing De-Coating agent and allow to soak with the mesh fully submerged for 15 minutes. This process is important because, once the agent is applied, it will begin to thin the emulsion by reacting with, and dissolving, the surface layer. If the stencil is left exposed to the air at this stage of the process, the emulsion will begin to dry and solidify. Once this happens the stencil is difficult to remove and may need jet washing. 

With the screen still submerged in the de-coating agent, brush vigorously with a Screen Cleaning Brush until the emulsion is frothing off the screen. Do not use any water until this is evident.

Next, wash under the tap while continuing to brush. Eventually the emulsion will begin to ‘peel’ off like thin rubber. Keep scrubbing until the mesh is clear. 

You can also use solvent-based removers such as Stencil Strip Powder which is again diluted with water to create your solution. This can sometimes work better than using a water-based agent against water-based emulsions.

Reclaiming Your Screen Using a Pressure Washer 

When the stencil emulsion has cured to a hard, plastic finish, then a Pressure Washer must be used to reclaim the screen. Position the screen upright in a spray booth or other suitable location where the water jet will not damage the surroundings and can drain away. Wedge it in to a corner for added support, as the power of the pressure washer can move it around.

 
The water jet needs to pass through the screen from the opposite side to the stencil. The high-pressure water passes through the mesh and shatters the hard stencil, cleaning it away. The washer nozzle will need to be guided up and down the screen to remove the entire stencil from inside the mesh holes and may need more than one pass. By holding the screen up to the light, you will be able to determine if you have any blocked holes remaining. There may be a ‘shadow’ of the stencil left on the surface of the mesh and this can sometimes make it difficult to see whether the entire stencil has gone. To test this, carry out a screen print onto white paper. The print should be solid colour if it has all been removed

Stencil card is manufactured using refined paper furnish that is tub sized and wax coated, making it water-resistant and stronger than paper. This means that a single stencil can produce many prints, this 375micron manilla card can even run through printers and photocopiers. By placing the stencil card under the mesh prevents the squeegee from catching on the cut edges. 

1. Choose a bold graphic design, which will hold its rigid form when cut. You can either print out a design or draw your own. Your design should be formed with simple straight lines and curves to allow for easier cutting. 

2. Next measure the dimensions of your screen mesh and cut a piece of stencil card to the appropriate size for your mesh. Your card should cover some of the masking tape surrounding the frame, but ensure that your design does not protrude onto the masked area. 

3. Transfer your design over to stencil card. Use tracing paper to copy your design in pencil, and then rub on the reverse to transfer the pencil marks to the card. Make sure that the design doesn’t run too near to the outer edges of your card. 

4. Using a sharp scalpel or craft knife, carefully cut out your design. The areas that are cut out will allow ink to pass through the mesh onto your chosen substrate. 

5. Lay the cut-out stencil on the mesh on the reverse of the screen and smooth all the edges down until they are flush to the mesh. To keep the stencil in position, stick the outer edges down using brown polypropylene tape. The screen is now ready for printing.

6. Position the screen with the stencil side down onto your substrate. Lay a strip of ink at one end of the screen. Pull a squeegee across the surface of the mesh to push the ink through using a smooth, gliding motion. The stencil can be used for multiple prints. Once you have finished screen printing, remove the tape and stencil. Clean the mesh with screen wash to remove any ink. Greaseproof Paper can also be used with this method, instead of stencil card.

Fabrics and Preparation

Many fabrics are difficult to identify, particularly synthetics and mixtures such as Terylene/Wool, Polyester/Cotton. If fibres are not named, check for any washing instructions or labels as this may be a guide. 

Fibres that are difficult to Home Dye or Print are most commonly: Acrylics (Courtelle, Dralon, Orlon, Neospun), pleated Tricel, anything with a special finish (drip dry cotton, flameproof nighties and pyjamas, mothproof and waterproof garments), cashmere, glass fibre fabric, mohair, felt, plastic sponges, anything which has a bonded interlining, foam or plastic backing. Suede and leather have special paint on dyes of their own. 

It is essential that all fabrics are washed before dying to remove all dressings, impurities, dirt or stains.

Specialist Crafts water-based inks will work best on paper and board and perform brilliantly when combined with stencils. They are easy to clean up and won’t block your mesh. Unfortunately, these inks will not fix on to fabric.

The inks can be diluted with up to 50% water, and to make these more opaque, add Opaque White to tone down colour and increase opacity. 

Acrylic paints can be mixed with screen printing medium to produce a good quality screen printing ink.

Your design does not wash clear from the screen after exposing:

 The screen may have been OVEREXPOSED. Wash the screen out with stencil strip or decorating agent and conduct a test strip. If necessary, reduce the amount of time each strip is exposed for (try 2 minutes per strip, reducing to 20 seconds per strip if required). The design may also have not have been opaque enough, allowing light to reach the areas of the screen where your design should have been. Re-colour your design, colour on the reverse of the acetate or print multiple copies on a printer and layer the sheets together. 

When washing your design out, some of the detailed areas will not wash out: 

The screen may have been OVEREXPOSED. Very detailed designs require a much more accurate judgment of the exposure time required. Once you have completed a test strip and worked out that the correct exposure is, for example, between 12 and 16 minutes, it may be necessary to carry out another test strip starting at 12 minutes and working along every 10 or 20 seconds up to 16 minutes to find the precise exposure time which works with your design. 
Detailed areas are susceptible to LIGHT SEEPAGE were, during long exposures, light creeps under the edges of the design due to the thickness of the acetate raising the design marginally above the surface of the emulsion. Draw your design on both sides of the acetate and ensure the acetate sheet is held flat to the surface of the screen with a glass or acrylic sheet to help to prevent this problem. 

The emulsion will not wash out of the screen using the stencil strip or de-coating agent: 

The screen may have been OVEREXPOSED, either through incorrect exposure timing or due to being stored in a non-light safe environment. The screen can be cleaned using a pressure washer as detailed on page 9. 
The screen may have been partially cleaned with stencil strip or de-coating agent and then left to dry before the emulsion was fully removed. Once the emulsion has dried at this stage, it can only be cleaned using a pressure washer. 

Storing the emulsion after mixing: 

After mixing, the emulsion should be stored in a cool, dark place. The shelf life after mixing is 4 weeks. We advise you write the mixing date on the label to ensure the product is used within this time. The manufacturer does not state that storing the product in a refrigerator will increase the shelf life, so we are unable to guarantee that this method will be effective. 

You have washed your screen but the design is still visible: 

It is normal to see some shadowing of a design on your screens where the ink has discoloured the mesh. Hold the screen up to a light source to check whether the mesh is clear. If there is some emulsion or ink blocking the holes in the mesh, clean these areas with a pressure washer as detailed on page 9.