doubling problem

Discussion in 'Web-Presses' started by printing.hari, Aug 3, 2009.

  1. printing.hari

    printing.hari Member

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    Location:
    india
    hi,
    i'm facing the doubling problem often in black unit. i'm working in a blanket to blanket, single width single circum web press and the black unit is the top one. i'm in need of all your valuable reply
     
  2. blazini36

    blazini36 New Member

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    try checking iron-to-iron (bearerless press) or bearer pressures
     
  3. nosefo

    nosefo Member

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    You say doubling. Do you mean like a shadow/ghost/overprint?
     
  4. printing.hari

    printing.hari Member

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    it is just like a shadow of the image, but we can c the full image of that doubling letter.
     

  5. nosefo

    nosefo Member

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    Location:
    Norway,Stavanger
    One of my first thoughts is that this doubling (shadow) is cosed by a combination of the layout on the printing plate and the distance of one of your form rollers (probably the last roller before print).

    I do have reason to believe that you experience this doubling problem (shadow) when you print positive or negative letters or other elements in a distance that is approximately one revolution of your form roller. And only when you have either a flat raster area or a picture that match with this distance.

    What happens is this:
    With negative letters/elements:
    After your rollers have done their job with delivering the ink to the printing plate your form rollers have "left over ink" (not consumed) that matches with the negative fields in the plate. And then when your form rollers again meet the ink transfer rollers your form roller just get rid of some of the ink witch means that your roller then have a ticker ink film in some of the areas. When this thicker film of ink meets the flat raster area or the picture your dots get overinked, and you have a darker shadow (ink gain effect).

    With positive letters/elements:
    The opposite of the negative effect. You have used more ink to print these letters/elements and when your form rollers meets the ink transfer roller they are not capable to deliver enough ink to "flattened" out the ink film on the form roller before the next revolution. You will then have a thinner ink film in some areas on the form roller and you will experience underinking. a lighter shadow in the rasered area or in the picture.

    Basic solution: Reduce your water to and absolutely minimum and make sure that you are printing with the correct density for that color.

    Solution1: You can try to increase the pressure between your last form roller and the ink transfer roller (to get more squeeze effect here). If you also decrease the pressure between your last form roller against the plate to a minimum I do think you have reduced some of the problem. This setup is recommended anyway, because your main inking to the plate is done by your first form rollers and the last form roller act like a smoothening roller.

    Solution 2:
    Change the type of ink used. I'm thinking especially of the "tack". More "tack" gives you better ink transfer. But the negative is more drag between the blanket and the paper (risk for splitting the paper). And if you are printing wet in wet you can pick up some of the previus printed color from the web/sheet.

    Solution 3: (The heavy one)
    If this does not have any effect you can start thinking of adding/mounting one or two new small rollers riding one top of the last form roller. The job for those rollers will be as an extra contact against the form roller and then distribute the ink more evenly before contact with the ink transfer roller.
     
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