Coudray points out useful applications for carrier-based positives and tells how to put them together for accurate results in screenmaking.
Individual film elements are assembled in position, relative to the dummy under the carrier sheet. Follow the same procedure for assembly as you did when setting up the carrier sheet. That is, position the film element, tape the lead edge, use your wand to squeegee out the air, and tape the tail.
When taping the emulsion-up film images to the carrier sheet, follow a couple of simple precautions. First, make sure that no tape is within 1/8 in. of any image element. If it is, it will cause an air gap during exposure, re-sulting in halation or undercutting of the image detail. The second is to make sure that the tape is completely even and secured to the film. This is done by carefully adhering the tape to the film you are positioning, but making sure that you do not allow it to touch the carrier sheet.
With the tape secure to the film, lower the film to the carrier sheet. Then take the edge of your fingernail and run it across the tape along the edge of the film, using the edge you're affixing as a guide. This causes the tape to conform to the vertical thickness of the film and adhere precisely to the carrier sheet with no air gap between the tape and the carrier. Any air gap will result in the attached film moving due to vacuum drawdown in screen exposure.
When using tape to assemble images, it is important to use the right kind of tape. My choice is 3M Scotch 610. This clear tape is designed specifically for image assembly work. It will not yellow over time, has a high tack, and is very easy to handle. It is strong, so that if you need to pull it up, it will not tear or split. It is a little more expensive than a standard office tape, but it works so well that it is worth the difference.
When assembling multiple flats for a multicolor job, you will have several layers of carrier sheet and film assembly stacked on top of each other, often in excess of 0.02 in. total thickness. The opportunity for parallax error is very great with such a thick group of films. Parallax occurs when you look at an image at an angle. You tend to misregister layers because of the viewing angle.
To overcome this, always use a collimated loupe and look down at your work from directly overhead. The collimated loupe is made specifically for this purpose and is available from any good graphic-arts supplier.
In a short column like this, we cannot cover all of the possible problems that may occur during image assembly. The points presented here represent common procedures that are often overlooked, and when they are, they tend to result in misregistration on press that is difficult to track down. The key to good image assembly on carrier sheets is to be deliberate in your work habits, and meticulous in your attention to detail.
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