Pad Printing Articles

Selecting the Correct Pad Printing Cliche

The following list describes the different types of cliches that are available currently:

Polymer Cliches:

(Important to note that their are many different qualities)
  1. Single exposure Polymer Cliches ( Plate has same depth of etch usually 30microns)
  2. Double exposure Polymer Cliches ( Depth can be varied, see later explanation)

Steel Cliches:

  1. Thin Steel Cliches ( Flexible approx .020" thick )
  2. Thick Steel Cliches (From .25" to approx. .5")

My personal experience with Pad Printers covers the old style doktor blade units and the new style Sealed ink cup units with ceramic ring doktoring system and I mention this as each has certain specific characteristics when it comes to the choice of clichg.

When choosing a cliche the following points should be considered:

  1. Size of the run.
  2. Amount of coverage of the largest/ srnalIest ink area.
  3. The thickness or opacity of ink required.

The first consideration is usually cost (this is not my personal opinion) as it relates to the size of the Pad printing job for which the clichk is being chosen. If the open inkwell doktor blade system is being used it's far quicker to change from job to job when polymers are used; therefore short jobs Iend themselves to polymer due to both cost and set up speed.

Any print job that exceeds 10,000 cycles will usually consume at least one or more polymer cliches and if you cannot make your own cliches in house, you are probably better off with a thin steel or thick steel clichg to prevent interruption of the print job. You could also order more than one polymer clichk to keep as a back up but please consider the potential rejects as well as down time which will rapidly consume the savings you made by ordering the cheaper polymer cliches. The use of Polymer cliches also requires chosing a more suitable (usually more flexible) doktor blade to prevent tearing up the fragile polymer surface of the cliche'.

This flexible doktor blade will also tend to more easily dip into image areas that lie parallel with the blade or larger areas of ink causing images with faded areas. This is commonly referred to as scooping. In many cases even multiple hits will not eliminate the problem and if rotating the image does not solve the problem you may have to resort to a firmer doktor blade. Using a firmer blade usually necessitates the use of the steel plate with half tones in the large areas to prevent scooping and keep the ink layer more even.

The fundamentals of half tone:

The etched area, instead of being simply a large open space, has small peaks which are created by exposing the cliche with a half tone screen (lines per inch and percentage can be as tow as 70 line 70 % or as high as 300 line 90%). The etched area takes on the appearance of a crater with tiny even ty spaced bumps or peaks.

Pad Printing Half-Tone Peacks

These bumps or peaks serve 4 purposes:

  1. Help support the doktor blade and eliminate the scooping that rnay otherwise occur.
  2. Help breakdown the surFace tension in the ink and allows for a more even layer of ink for pick up by the pad.
  3. Prevent ink from moving around theirnage during the doctoring process.
  4. Prevent excessive ink travel due to pad pressure during pick up.

The old style open inkwell machines lend themselves to steel cliches and in many cases are used exclusively with steel cliches as setting up these units on polymers requires some finesse and experience. The use of thick steel cliches is never necessary with the ceramic ring cup system and in fact can be problematic if the clIch6 is not perfectly flat.

The newer style ink cup machines (at least those with high quality ceramic rings) lend themselves well to the use of polymer cliches in the case of print runs of less than 100,000 cycles. In cases where runs exceed the 100,000 mark the use of the thin steel ciiche is recommended as these cliches are relatively inexpensive and will last easify in excess of 1,000,000 cycles.

In my experience scooping problems when using ceramic ring cups on polymers can occur but rarely. By using extreme care during the etching technique, and using the double exposure high quaIity polymers, these problems can usuaIly be overcome. These scooping issues are not as pronounced when using the ceramic ring cups as with the doktor blade system. This is due to the fact that the ring itself wiIl not flex into the etched areas. Any scooping that does occur is caused by the flexing of the cliche. It is therefore self evident that a very thin flexible cliche will create more scooping than a less flexible heavier polymer or thin steel cliche. In cases where the solid ink areas exceed 1" in diameter the use of a well made half toned thin steel will always work very well.

The double exposure polymers always have the half toned etched areas and in many cases I have seen the elimination of pin holes by using this type of cliche. The double exposure cliche also enabfes the pad printer to create an etch depth that best suites the appIication, eg. Printing on wood or leather would always require a deeper etched ciiche than when printing onto less porous materials such as most plastics or glass.

The single exposure polymer has redly only one advantage over the double exposure cliche and that is if you have extremely fine detaiI images which tend to be lost due to the second exposure, where the screen masks the extremely fine details. The single exposure cliche is usually more expensive and is recommended only when the artwork to be etched consists of mostly very fine to fine images. If you were to use the single exposure clichk with larger images, chances are scooping would occur.

Choosing the correct cliche for your print job becomes less of an issue if you are equipped with your own in house polymer clichri making equipment as you can always very quickly replace a worn or damaged cliche.

This article is by no means a very extensive explanation of any of the concepts raised and I hope that if you have any comments suggestions or questions that you contact the writer at 1-800-272-7764.

Written by: Julian Joffe (1/O6/98)