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Prepress & Screen Making



I recently attended the Print 05 exhibition in Chicago and gave a presentation about the state of the screen-printing industry. As I walked the show and contemplated the exhibitors’ offerings, it became obvious to me that digital technology, which has influenced the way some screen printers do business, has also had an effect on traditional sheet-fed offset printers.

In that market, which includes equipment from manufacturers such as Heidelberg, Kormori, MAN Roland, and others, printers have realized numerous benefits from computer-to-plate systems. Bringing digital prepress in house has helped them stay competitive as other printing technologies make inroads. In fact, it’s abundantly clear that many segments of the graphics-printing industry have made wholesale changes in the prepress workflow in order to strip unproductive waste and cost from their processes.

Computer-to screen (CTS) technology is one solution that screen printers use to maximize efficiency in prepress. The current state of CTS equipment was the focus of my column in the July 2005 issue of Screen Printing (page 22). In that column, I concluded that inkjet-based CTS, whether the consumable is ink or wax, is viable for operations that use as few as 25 screens per day. This month, I would like to go into some of the economies you can realize by implementing CTS technology.

How CTS benefits prepress

Increases in digital-imaging speeds and reductions in investment costs should be enough reason for traditional screen printers to develop serious cost-reduction plans and implement productivity enhancements in order to minimize the inevitable confrontation with digital printing. The biggest bang for the buck comes at the front end of the process. Anything you can do to reduce overall steps, handling, and process time in prepress will be magnified and multiplied down stream. In addition, you have the opportunity to insert other productivity-enhancing capabilities into the prepress segment.

A traditional, film-based workflow can have 14 or more steps from the creation of the artwork until the screen is ready to print. Table 1 offers a typical comparison of a CTS workflow to a traditional workflow. In this comparison, there are 17 steps in the traditional workflow and seven steps in the CTS workflow. All steps are not equal, and each step introduces variables into the final outcome. Each step must further have its own quality-control point. In the film workflow, there are huge problems with exposure and processing of film.

Table 1: Comparing steps in traditional and cts-based workflows
Traditional Workflow CTS workflow
Create digital artwork Create digital artwork
RIP file RIP file
Image film Image screen
Process film Expose screen
Retouch film Remove screen from CTS system
Pin register film Develop screen
Transport film to screenroom Print
Mount film to screen
Load vacuum frame
Pull vacuum
Expose screen
Remove screen from vacuum frame
Remove film
Archive film
Develop screen

Consider the flaws of three of the most common methods of generating film. Inkjet-printed film can suffer from density and laydown issues. Thermal printing can lead to problems with density, bloom, and streaking issues. Conventional photo-based film is known for producing challenges with density and developing. All can hinder registration and create fit problems if the films are capstan or roll driven.

Mounting film to the screen poses another set of variables. Is the film in the exact position (accurate register) if pin registration is even in use? If the film is not taped perfectly flat to the screen, then buckling, twist, and distortion become problems. Any time you handle or move film, you risk damaging it by crimping, scratching, or transferring dirt and debris from tape or exposure glass.

Improper application of vacuum when creating stencils creates one more layer of quality problems. Besides adding 30 seconds or more to the exposure of each screen, the problems range from poor registration to destructive moiré. Image undercutting is common if the vacuum is incomplete or localized. A weak vacuum combined with low film density is a recipe for highlight-halftone disaster. If you’re even able to capture the dot, you will likely generate moiré in the tone.

CTS works without a vacuum. In fact, the systems create perfect contact between the image and the emulsion surface


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