There are more cliché options than most shops realize, and the wrong one shows up as faded patches, pin holes, or a plate that dies halfway through the run. Here is how I would decide.
The clichés available to you
Polymer — and it is important to note that there are many different qualities.
- Single exposure. The plate has the same depth of etch throughout, usually about 30 microns.
- Double exposure. The depth can be varied, which I will come back to.
Steel.
- Thin steel — flexible, approximately 0.020″ thick.
- Thick steel — from 0.25″ to approximately 0.5″.
My experience with pad printers covers both the old style doctor blade units and the newer sealed ink cup units with a ceramic ring doctoring system, and I mention that because each has specific characteristics when it comes to the choice of cliché.
What should actually drive the choice
Three things:
- The size of the run
- The amount of coverage in the largest and smallest ink areas
- The thickness or opacity of ink required
In practice the first consideration is usually cost — and I want to be clear that this is not my personal opinion of how it should be, only an observation of how the decision usually gets made. If you are running an open inkwell doctor blade system, it is far quicker to change from job to job with polymers, so short jobs lend themselves to polymer on both cost and setup speed.
Where polymer stops making sense
Any print job that exceeds 10,000 cycles will usually consume at least one polymer cliché, and if you cannot make your own in house you are probably better off with thin or thick steel to avoid interrupting the run.
You could order spare polymer clichés as backup instead. But consider the rejects as well as the downtime, because between them they will rapidly consume whatever you saved by buying the cheaper plate.
Polymer also forces a choice further down the line: it requires a more flexible doctor blade to avoid tearing up the fragile polymer surface. That flexible blade will then dip more easily into image areas lying parallel with it, and into larger areas of ink, producing images with faded patches. This is what people mean by scooping.
In many cases even multiple hits will not eliminate it. If rotating the image does not solve it either, you may have to go to a firmer blade — and a firmer blade usually means moving to a steel plate with half-tones in the large areas to prevent scooping and keep the ink layer even.
The fundamentals of half-tone
Instead of being one large open space, the etched area is given small peaks, created by exposing the cliché through a half-tone screen. The ruling and percentage can be anywhere from 70 line at 70% up to 300 line at 90%. The etched area ends up looking like a crater with tiny, evenly spaced bumps across its floor.
Those peaks do four things:
- Support the doctor blade and eliminate the scooping that would otherwise occur.
- Break down the surface tension in the ink, giving a more even layer for the pad to pick up.
- Stop ink moving around the image during doctoring.
- Stop excessive ink travel under pad pressure during pick-up.
The machine changes the answer
The old open inkwell machines lend themselves to steel clichés, and in many cases are used exclusively with steel, because setting those units up on polymer takes some finesse and experience.
Thick steel is never necessary with a ceramic ring cup system, and can in fact be problematic if the cliché is not perfectly flat.
The newer ink cup machines — at least those with high quality ceramic rings — take polymer clichés well for runs under 100,000 cycles. Above that, thin steel is the recommendation: it is relatively inexpensive and will comfortably exceed a million cycles.
In my experience scooping on polymer with ceramic ring cups does happen, but rarely. With careful etching technique and good double-exposure polymer it can usually be overcome. The problem is far less pronounced than with a doctor blade system, because the ring itself will not flex into the etched areas — any scooping that does occur comes from the cliché flexing. It follows that a very thin, flexible cliché will scoop more than a heavier polymer or a thin steel one. Where solid ink areas exceed an inch in diameter, a well-made half-toned thin steel plate will always work very well.
Double exposure or single exposure
Double exposure polymers always have half-toned etched areas, and I have seen them eliminate pin holes in many cases. They also let you create an etch depth suited to the application — printing onto wood or leather always wants a deeper etch than printing onto less porous materials such as most plastics or glass.
The single exposure polymer has really only one advantage: where you have extremely fine detail that tends to be lost in the second exposure, because the screen masks it. It is usually more expensive, and I would recommend it only where the artwork is mostly fine to very fine images. Use it with larger images and the chances are you will get scooping.
If you make your own
Choosing the correct cliché becomes much less of an issue if you are equipped with your own in-house polymer cliché making equipment, because you can always replace a worn or damaged plate quickly. If that is the direction you are going, our polymer plate development procedure covers both exposures, developing, and the exposure times for each plate material.
This article is by no means an exhaustive explanation of the concepts raised. If you have comments, suggestions or questions, call us on 800-272-7764 — or send us the part and the artwork and we will tell you which cliché the job wants.
First published as an EPS technical article; Julian Joffe was President of Engineered Printing Solutions at the time of writing. The original PDF is still available.