How to Guide - Tips and tricks for indirect films
Why is hardening necessary?
Gelatine stencils use ferric salt sensitisers that need to be chemically hardened to complete the tanning action started during exposure.
To prevent the image regressing and subsequently disappearing, the film must be placed in Autotype Activator Powder (or hydrogen peroxide solution) within 10-15 minutes of exposure.
The correct method
The success of the hardening process depends on the solution strength, temperature, the technique and length of immersion.
In the absence of Autotype Activator Powder, which comes with detailed instructions, a 1.2% (4 volume) solution of hydrogen peroxide should be made up. The exposed films should be immersed in the peroxide for 1 minute after which the film is washed out with water of 40-45°C (104-113°F). Temperatures of the bath above 19-21°C (66-70°F) tend to accelerate the solution’s decomposition.
Caution: Concentrations of 6% H2O2 can cause mild stinging. Stronger solutions can burn the skin. Always handle carefully. If in doubt about the strength of local supplies of hydrogen peroxide use Autotype Activator Powder.
Importance of solution strength and solution life
If the peroxide solution is weak either due to an error in its make up or old age, the stencil film can be ruined after exposure. Insufficient solution strength will increase swelling of the gelatine resist and in severe cases, the gelatine will ‘wrinkle’ and wash away from the polyester support.
Excess solution strength will produce thinner, harder stencils which reduce adhesion to the mesh. The recommended strengths have been calculated to give optimum hardening with reasonable solution life and should be adhered to.
As an approximate guide, 1 litre (1.76 pints) of fresh solution hardens 1 square metre (10.7 square feet) of film. A fresh solution should be made up once this quantity of film has been processed and ideally, thrown away after 2 days. Solutions exposed to UV light decompose more quickly.
For even hardening, always immerse the film quickly and evenly. Failure to do so will produce variations in stencil thickness - particularly noticeable on halftone work.
NEVER wet the stencil with water before immersion in the hardening solution as this will cause premature swelling.
Baths should be slightly larger than the maximum stencil size and ideally made of plastic (acrylic) or stainless steel. Rust adversely effects the peroxide solution so enamelled steel dishes must not be chipped and galvanized dishes should not be used at all.
Immerse the film emulsion side up in the hardening solution to a depth of at least 1 cm (½"). Rock the dish gently to remove air bubbles from the film’s surface, or, place the film, emulsion side up, into a dry dish and pour the solution evenly over the film.
If it can be ensured that coverage will be adequate and quick enough, peroxide can be sprayed onto the film.
Mounting/adhering of indirect films
After washout the indirect photostencil is mounted and adhered to the underneath of the printing screen. In mounting the stencil, the entire emulsion surface must be pressed into contact with the individual mesh filaments to ensure adhesion of the finest detail.
After the stencil is mounted, it is adhered by blotting with newsprint placed in contact with the screen from the squeegee side. This draws the moisture through the mesh and pushes the emulsion into contact with the mesh filaments. It is essential that the mesh is correctly pre-treated and degreased before adhering the stencil. Failure to do this will result in poor stencil adhesion.
Recommended Mounting/Adhering Techniques
This technique is considered the best since it ensures good contact between the stencil emulsion and the screen mesh. It is particularly recommended if using warped, poorly tensioned or hand-stretched screens. The build up should be larger than the stencil, though within the printing frame dimension.
If processing different sized screens a number of build ups will be needed. The build up can be of plate glass or melamine (Formica) topped chipboard or similar timber.
Unless the mounting surface has been matted by shot blasting it is necessary to lay a sheet of film (old stencil film base support is ideal), or a sheet of paper between the build up and the processed stencil. This prevents the stencil sticking in the build up when the frame is lifted off.
- Select a suitably sized build up and place on a mounting table, with a sheet of paper on top.
- Place wet stencil, emulsion side upwards on build up.
- Position screen in register over stencil and place in contact.
- Adhere stencil by blotting off surplus moisture with 5 - 6 sheets of newsprint applied firmly with a roller.
Mounting Table Only
Build ups are not always convenient when processing a number of different sized screens, and they can make stencil pre-registering difficult. A flat surface is acceptable providing the mesh is in contact with the mounting table. This would exclude warped frames or some self-tensioning types. (If there is too great an off-contact distance the stencil will strip from the screen as the roller is passed over it.)
The advantage of this technique is that the mounting table can also act as a pre-register device. All that is required is a flat melamine or glass-topped table capable of accepting the largest frame being processed.
- Lay plastic sheet on table and position wet stencil on it, emulsion side up.
- Position stencil over register marks.
- Lay the screen in register on the stencil.
- Blot off as above.
Use a wet screen to dilute any soft emulsion in the open areas. Wetting removes dust particles from the mesh surface and ensures good capillary attraction between mesh and stencil.
Adhering: the suitability of a particular adhering technique is mainly determined by the evenness of pressure that is applied to the newsprint and therefore to the stencil. The palm or the back of the hand is not suitable since it will produce pressure spots. Ideally a roller should be used as this will guarantee an evenness of pressure over the stencil. A decorating roller is quite satisfactory.
Paper staining; the blotting up paper can show some staining. This varies with the amount of Soft Top and the degree to which the film has been chilled. Less staining occurs with coarser meshes or if the stencil has been over-exposed.
Use a smooth hard surface: avoid soft surfaces, such as plastic foam, as they will depress unevenly during blotting and cause the film to delaminate. Uneven surfaces, especially those with localised high spots, cause additional pressure in these areas when adhering the stencil. This forces the mesh deeper into the soft emulsion, and even punctures the stencil, resulting in early breakdown.
When working with screen printing machines using fixed pre-register stops or lays, the stencil must be pre-registered on the screen. Even with simple hand tables, fixed lays reduce setting-up time. Stencil pre-register boards greatly simplify stencil positioning, ensure greater accuracy and avoid accidental damage to stencils through faulty positioning.
‘Scumming’ is caused by a thin layer of emulsion which blocks the open areas of mesh. It is generally transparent but can be seen in reflected light as it has a shiny appearance. ‘Scumming’ will block the mesh and render the stencil useless and so must be avoided at all costs. Invariable ‘scumming’ is cause by improper stencil processing.
How to prevent scumming
Only careful handling between the washout and adhering stages will avoid scumming:
Ensure that the stencil is thoroughly washed out. Continue washing for at least 30 seconds after the open areas appear clear.
Ensure that the entire stencil area is thoroughly washed out. Large areas of the stencil, particularly those at the edges, can be easily overlooked and if insufficiently washed will contain an excessive amount of soft gelatine.
Chill the stencil to the recommended temperature and for the recommended time.
Rinse the stencil immediately before transfer to the mesh. Do NOT leave it lying on the washout unit for any period of time without rinsing as the soft gelatine will run into the open areas.
Rinse the mesh before mounting. As well as ensuring that the mesh is free from dust and dirt, this ensures plenty of water on the screen to dilute any emulsion that may have reached the open areas of the stencil.
Use the recommended mounting technique. Try and ensure that all the excess water is removed from the open areas as the ‘semi-soluble’ soft gelatine can ‘leach’ into the water in the open areas and if drying is slow, ‘scum’ will be left after the water has evaporated.
Avoid moving the stencil on the screen. Some emulsion is normally left on the mesh and can find its way into the open areas once the stencil is repositioned.