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Image Assembly on a Carrier Base

(January 2001) posted on Fri Oct 19, 2001

Coudray points out useful applications for carrier-based positives and tells how to put them together for accurate results in screenmaking.


By Mark A. Coudray

In the next prepress step, the press-sheet dummy is taped to the layout table in relation to how the press sheet will move through the press. The carrier sheet is positioned against the pins or lay stops and taped to the layout table in preparation for image assembly. This is the first point where problems can appear.

 

Misregistration can occur for two primary reasons. The first is that the carrier sheet is not completely resting against the lay stops. However, also be careful to avoid buckling the sheet by jamming it against the stops; it does not take much to misregister a sheet. The second registration problem is called <I>radial error</I>. This is a wedge-shaped distortion that occurs over a distance. The farther out from the stops, the greater the radial error. This type of error can result from something as simple as air trapped under the carrier sheet. It can also occur when surface static on the layout table grabs the sheet unevenly. If static pools on the table surface, the sheet will bind and twist as it is being positioned.

 

To counteract this condition, follow these recommendations: Begin by carefully squeezing out the air layer under the carrier sheet. This is best done using a wand that you can make yourself. It consists of a 1 x 1-in. piece of square hardwood about 24 in. long. You can often get a scrap for free from the local lumberyard. Oak or maple work very well. Wrap the wand with several layers of cheesecloth and tape each end of the stick to secure the cheesecloth to the wand.

 

When you sweep the wand over the carrier sheet, start at the head (top) center of the sheet. With the sheet secured against the lay stops (it's okay to tape it on the leading edge), sweep straight back toward the tail (bottom) of the sheet. Then, lightly squeegee the air out, side-to-side, working from the center of the sheet out.

 

When you are done removing the air, tape the tail of the sheet to the layout table. A better solution would be to use a tail pin. This is a register tab that is placed over a taped-down male register pin and then taped to the carrier sheet. This will secure the tail of the carrier sheet, which will now be ready to accept random films for assembly.


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