The notes provided on this page relate to hand printing ctp (computer-to-plate) aluminum lithographic plates via a direct lithography press. Ctp lithography is a process whereby an image is imaged via laser directly onto a photosensitive aluminum plate from a digital file. The notes presented here assume the reader is familiar with the process of stone lithography.

The notes have been organised into three sections:

• file preparation
• the plate-maker
• printing

File preparation

The design of the image is critical in achieving good printing results. We have found that large flat blocks of 75%-95% of a single colour (spot) tone are difficult to manage and have a tendency to 'fill in'. We believe there are a number of contributing factors such as water dispersion, ink coverage, and roller pressure but think speed to be the most significant factor as hand printing ctp plate can not match the machine speeds of a commercial press. Design with this in mind. For example build up an 80% tone from two 40% plates, or other multiple combinations.

This ‘fill in’ posed less of a threat in full colour photographs due to the undulating nature of the half tone structure, and the way in which full tonal colour is created from four process colour plates.

To prevent pixelation, artwork should be scanned or constructed within the computer as 600dpi for line art images and 300dpi for grayscale or colour images. This is for images at 100%. If for example, a 20x20cm image is required to print at 40x40cm, scan the artwork at 600dpi and then adjust the image size rescaling it to 40x40cm at 300dpi.

Measure your equipment and design accordingly. Create your printable image size smaller than the roller width. This prevents roller edge marks appearing through the image, which is especially evident in continuous tone images.

As with stone lithography leave an adequate border around the image and an even longer border in the rolling-up orientation.

Ctp plates come in many sizes with two different gauges. We have found that thinner gauge plates tend to curl & dent easier and so we prefer the thicker gauge of which the smallest plate size is 730x600mm. Below is a table of plate sizes from one Melbourne plate maker. Call your plate maker beforehand to check plate sizes and thickness.

Sending work to the plate maker

Plate makers can be found in the yellow pages under "pre-press production services" or "printers-lithographic". Ring them up and talk about your requirements, the cost involved, the best format and method to deliver your work and the time required to produce the plates.

When supplying your artwork make sure you specify:

• reverse reading. This means that the art is imaged in reverse on the plate and printed onto the paper in the intended orientation.
• 85 lpi. This is the density of the halftone pattern.
• no grip or printers marks. For commercial work the grip is an imaged strip along the long edge which is fixed to the printing drum. The printer’s marks can be: crop marks, plate name information, test strip. These items can get in the way especially if you are leaving a border around your work. Again talk to your plate maker about these things.
• centre image on plate
• required plate size. We recommend the thicker gauge for its durability.

Currently the pdf format provides the best manner to package your artwork from your layout or image application. But be warned the pdf format can still be problematic. When you supply the plate maker with files also provide a print out of the given job and any separations if you are doing multi-colour work. Check your printout to see if they are printing correctly!

For hand printing ctp plates, we have experimented with various lpi rulings and found that a line screen ruling of 85lpi is suitable for most jobs.

Printing

In addition to lithography printing equipment we have found that the following items improve printing:

• bedbase. A super flat surface on which to place the printing plates ensures even contact with ink roller and paper, and reduces damage to the plate. 18mm mdf board is inexpensive but is easily damaged if exposed to water!
• dampening roller. A dampening roller is a small diameter roller wrapped in absorbant material. The roller is used to dampen the plate by rolling out the water rather than the dragging technique used with a sponge. The roller helps maintain fine ink areas that would be dragged away by a sponge. Keep the dampening roller wet with your water solution via a hand spray bottle.
• isopropyl alcohol. Mixing about 8% isopropyl in the water solution helps reduce the water tension. This is useful because water tension can create a beading effect by pulling small amounts of water from non-image areas leaving dry areas, which attract ink.

Once set up on the lithography press, the plate only requires a washout before inking. Wash out thoroughly with a sponge and then use the dampening roller to create an even dispersion of water over the plate. Ctp plates require minimum ink. Do not have the ink slab overloaded with ink as this can cause plate fill. A few drops (or about 1% of volume) of gum arabic added to the water solution help keep the plate clean. Clean up plates with a mixture of gum arabic and mineral turpentine (10 parts to 1) using a soft cotton cloth. Finish by buffing down with pure gum arabic. Store the plate with paper cover to help avoid scratches.

top