What is the difference between a Direct Imaging press and an offset press?

What is the difference between a Direct Imaging press and an offset press?

A Direct Imaging (DI) press takes CtP (Computer to Plate) one step further. Instead of imaging plates on a separate platesetter, a DI press images the plates directly on press. The laser imaging heads are actually part of the press, and when the operator hits “print,” the plates are automatically mounted and the file begins imaging. Because the plates are mounted before they are imaged, the registration is about as good as you can get it from the very first press sheet. Obviously there is also a labor and time savings in mounting the plates themselves. In addition, DI plates use CIP3 data to determine the amount of ink needed for each ink key setting on the press. CIP3 is an industry standard for passing data directly from the job file to the press. The computer determines exactly how much ink will be needed for each portion of the plate according to how many dots are actually imaged in that area. The press’s ink keys are automatically set. The savings in make ready time can be substantial. While some conventional presses can also use CIP3 data, all DI presses make use of this capability.

Recently two important things have happened in the DI press market. Presstek, the chief supplier of imaging heads for most DI presses, has come up with a way to make higher resolution imaging heads, which makes new DI presses capable of 300 lpi (lines per inch) and stochastic screening. This means, in terms of quality, DI presses can be competitive with very high end offset presses. Additionally, the purchase price of these presses has come down significantly.

Stephen Beals is a digital pre-press manager and has been writing for major print publications for many years. He is the author of A Practical Primer for Painless Print Production. He can be reached at stephenbeals@mac.com.
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