Image Assembly on a Carrier Base
Coudray points out useful applications for carrier-based positives and tells how to put them together for accurate results in screenmaking.
Even with all of the digital prepress software and equipment at your disposal, you may find it necessary to assemble multiple image elements on a carrier or strip base for imaging onto the screen. Often, this assembly of elements can become very complex, presenting the opportunity for any one of a number of different types of errors.
This month, in a departure from the normal digital emphasis of this column, I'll revisit some of the time-tested techniques that can aid you in creating better-assembled composites on carrier bases, which are typically referred to as <I>flats</I>. The techniques you'll learn about are designed to aid in progressive, accurate registration of design elements and their accurate translation to the stencil.
The need for a carrier base
You can use a carrier or strip base for any of several very good reasons. The most obvious is that the image you need to print is larger than the film you are outputting. In order to save on expensive imagesetter film, small elements are assembled on a carrier sheet. Any number of elements can be handled this way, but as you shall soon see, some limitations apply.
One situation when a carrier base could be useful is when you are using a four-color-process image as part of a poster, and the actual separations are considerably smaller than the overall poster dimensions. To save on costs, you could or-der smaller separations and assemble them on carrier sheets in the appropriate positions. The remaining design elements of the poster would then be assembled around the separations.
A variation on this theme would be if you needed to change information in a selected portion of the image based on where the graphics are going to be used. This changing information is called variable data, and it might be used to customize prints for different store locations or perhaps different pricing in different markets.
Another reason for using a carrier base is if you rely on a pin-registration system that uses master carrier sheets. Such systems are based on the premise that smaller elements will be taped to the carrier sheets for precise transfer and registration to screens. This is usually a two-step process, with the carrier sheet being positioned on pins, followed by the screen being positioned onto its own set of registration locator points (either pins or three-point set stops) relative to the film. The carrier sheet, complete with the assembled images and registered accurately to the other carrier sheets in the job, is then transferred to the screen for exposure.
Using carrier sheets
The use of carrier sheets begins with some type of common registration system for the master carrier sheets. Typically, printers use the lay stops of the press or the gripper edge of the press sheet as the registration guides. You will need both a head stop and side guide so that all of the carrier sheets will have at least two common edges to align with.
From this fixed set of coordinates, a master press sheet (sometimes called a dummy) is laid out on conventional paper. The layout should include an allowance for a sheet gripper (if you are using a press with a take-off unit), color bars, and top, bottom, and side trim marks. The press sheet should also be bisected for head-to-tail and side-to-side centerlines.
The amount of time you spend on carrier-sheet layout is determined by how precise the final product needs to be. If it requires a high degree of precision, you need to check each flat for parallel and squareness. Don't assume the layout table is accurate--verify the position of design elements yourself. The few minutes it takes to double check this can prevent a huge amount of grief later.
If you have access to a large-format inkjet printer, you could create the entire dummy electronically and print it to paper. This print then becomes the composited press sheet, with all elements in position. If you are careful in the scaling, the final random elements can be very quickly assembled using the printed sheet as your guide. It is much faster than manually laying out the sheet.
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.
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.