Back to basics - entry level screen printing using lith / line film

Discussion in 'Enlarging' started by robgunby, Jun 16, 2012.

  1. robgunby

    robgunby Member

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    Hi there

    Having used darkrooms for a short while and got to grips with the standard bw processes and some colour work, I would like to try screen printing a poster or two using the traditional technique of making a halftone image using lith or line film. However, the books I have read on the subject have led to confusion, either because they were published several years ago and the equipment mentioned is no longer available, or because they assume a level of knowledge that I do not possess.

    I understand that I should create a halftone image by contact printing a bw neg onto lith or line film (I have both but have never used either) and developing in high contrast developer, and use this to make each of the single colour stencils for making the screens.

    I also understand that the screens are made using some sort of photographic emulsion, but the information I have read on this and each subsequent step has been scant at best.

    Could anyone point me in the direction of a book or website that will spell the process out nice and simply for a dummy like myself please?

    Also, if anyone has any tips for using lith / line film, for this purpose or any other, I'd be interested to hear them. I intend on making some bas-relief style images too, but there must be other uses for this steep film?

    /edit I should point out that I am not looking to become heavily involved with this, nor to outlay a lot of money. Any stages that can be done on the cheap, DIY, or even done in a rough and ready fashion will suffice. This is to satisfy idle curiosity, not to produce stellar images for sale.

    Thanks in advance!
     
    Last edited by a moderator: Jun 16, 2012
  2. Steve Smith

    Steve Smith Member

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    Most screen printing emulsion is similar to gum bichromate. It's a PVA type of adhesive sensitised with potassium dichromate. It is applied either by hand from a coating trough or by an automatic machine. It is also possible to buy screen emulsion in sheet form which is applied to the mesh before exposure. Autotype Cappilex is one which we use.

    http://www.macdermidautotype.com/products

    The image you use needs to be either clear film base or solid black. It is applied to the screen emulsion and exposed to ultra violet light. The UV light hardens the emulsion and areas not exposed are washed out. What was black on the photographic image gets printed.

    When looking at the artwork the right way round, the emulsion should be on the top. This is then placed emulsion to emulsion on the underside of the frame to make a contact exposure. It is normal to coat the underside but at work, we have a machine which coats both sides at once.

    A halftone screen will turn a full tone image into a series of varying size dots just as is used for newspaper pictures. You might get suitable results by just exposing line film without a halftone screen. It has a high enough contrast that the output will be clear or black and the amount of exposure will determine the changeover point.


    Steve.
     
  3. robgunby

    robgunby Member

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    Steve, this is most succinct and helpful, thank you very much. I am hoping that a local gallery who used to offer screen printing tutorials will start up again later this summer as planned, which would give me a very simple introduction to the screen printing process. In the meantime I will practice creating a suitable image with the line and lith film.

    Using a halftone screen sounds intriguing though. I have seen 35mm contact texture screens on firstcall's website for a pretty penny, but that's for a range of other texture screens that I wouldn't use, and only permits the use of 35mm images, which is no good. Any idea where I can get one these days?

    Oh and one other thing - you mention screen emulsion requires UV light - will daylight suffice given enough time?

    Thanks again
     
  4. Steve Smith

    Steve Smith Member

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    I'm not sure where you could get a half tone screen today (although I'm sure someone will still be making them). It is usually done digitally now.

    Sunlight will work. Even the light from fluorescent lights will probably expose screen emulsion. Our screen room has yellow filters over the lights and orange over the windows to prevent accidental exposure.


    Steve.
     
  5. glbeas

    glbeas Member

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    The halftone lines per inch measurement needs to be a bit coarse, usually about 1/3 the mesh count of the silkscreen. I've done decent halftones at 75 lpi with a 300 mesh screen. Oherwise the screen mesh will moire with the halftone screen and produce poor halftones.
     
  6. richard ide

    richard ide Member

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    To prevent moire the angle of the lines on the halftone image need to be at the right angle to the screen mesh. With four colour printing, the angle is different for each colour. There will be one colour which gives a moire pattern and this is generally the yellow printer. The moire is not noticeable because the yellow is a lot weaker than the other colours. You can visualize this by rotating your halftone positive on the screen so the pattern disappears and use that angle when exposing your screen emulsion.
     
  7. robgunby

    robgunby Member

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    Thanks again folks. I'm not sure I understand how I control the coarseness of my lines in the halftone image - using lith and line film is very new to me, but presumably a lot will become clear once I go through the process.

    Richard, neat little trick there thanks. I will be printing black and white halftones as simple colour screen prints in maybe 4 or so different colour inks.