As the industry introduces new presses, inks and substrates, it is important to remember that it is often simple best practices that determine the success of a print run. Methods for consistent impression are some of the most important best practices a printer can establish and the “kiss” impression is one of the most essential.
What is a kiss impression? Simply put, on an ideal printrun, the printing sleeve or plate should just “kiss” the substrate. Technically, a kiss impression is the minimum impression needed to transfer all text, solids and halftones to the substrate. The advantages to a kiss impression are numerous. The kiss allows pressmen to hold a fine dot while increasing graphic fidelity. It allows for printing at faster speeds, with less wear, and will protect the most fragile dot: the highlight.
But a kiss impression is an ideal. In most “real world” environments, it is difficult to achieve. Lots of things in a print environment can ruin the perfect impression, so every printer has to look at the printing process from the center of the press outward and ask questions like these:
- Is the substrate a perfect thickness on the roll and on every different roll?
- Are the mandrels perfect in total indicated runout (TIR) without tapering?
- Does the bridge mandrel have low spots?
- Is there any “play” or movement in the mandrels from one unit to the next?
- Do you trust that the plates are mounted to the cushion tape without waves?
- Is the printing sleeve perfect in TIR and without tapering?
- Have you ensured proper and consistent ink acetate levels and viscosity?
- Have you ensured consistent surface tension that is within the substrate’s specifications?
- Is the corona treatment of the substrate working correctly?
“IT IS MUCH EASIER TO HAVE A CUSTOMER
APPROVE A JOB THAT’S OVER IMPRESSED
WITH TOO MUCH GAIN THAN IT IS TO HAVE
A CUSTOMER APPROVE A JOB THAT IS
MISSING GRAPHICS BECAUSE IT WAS
Print managers have to be risk managers. Most pressmen and print managers will err on the side of over impressing a printrun to compensate for those real world problems. It is much easier to have a customer approve a job that’s over impressed with too much gain, than it is to have a customer approve a job that is missing graphics because it was under impressed. And the difference between a perfect impression and an under impression may be as small as 0.001-in.
In addition, substrates vary in thickness. A 40-μm. variation in a substrate may produce a “wave” pattern of impression variances in a printrun. And every printer knows that even if a substrate has a rated thickness tolerance specification, it can still receive substrates that are out of tolerance. Many substrates are also prone to stretching during the print process. Printers regularly adopt a “make it work” mentality, because they do not have time to deal with a manufacturer on an out of spec substrate. It is easier to squeeze it and get the job done with some over impression than it might be to change out a stock or miss a press date.
Finally, plate and sleeve defects may necessitate over impressing. Plates, by their nature, have more variables than sleeves:
- Mounting tape
- Cushion adaptors
- Mounting errors
These can all change the pressure needed across the web. Heavy impressing may be necessary to mitigate low spots, press bounce and/or maximize the usable print area across the web when using plates.
“…WHILE MANY MODERN PRESSES
TAKE CARE OF THESE PRACTICES WITH
AUTOMATION, THERE IS NO SUBSTITUTE
FOR AN EXPERIENCED PRESSMAN
OPERATING WITH A COMPANY’S UNIQUE
SET OF BEST PRACTICES FOR ITS SPECIFIC
MITIGATING CONSEQUENCES OF OVER IMPRESSION
As we’ve shown, there are a lot of reasons printers feel the need to slightly over impress a job. But there are strategies for mitigating the effect and need for over impression. Gray Component Replacement (GCR) in the file prep stage can help immensely to minimize the negative effects of over impression. GCR is the removal of most of the multichannel whites and grays from a 4-color (or more than 4-color) process. Those grays are then seamlessly added back as a full range black, maintaining the shape and depth of the original. Neutrals that were once three colors are now mostly black. With less ink behind grays there is less variation in color throughout, as pressure is increased. GCR also produces better highlights because a majority of colors are created by only one color (in the case of a spot) or two colors (CMYK or expanded gamut) for hue, and then black is added for shading.
Engraved elastomer sleeves mitigate the effects of over impression and allow for wider tolerance in impression pressures. Elastomer compounds can be specifically tailored to printers’ ink types, which allow for optimized ink transference. A key component to direct ablated elastomer sleeves is known as “below surface imaging.” By taking the highlight dots below surface (1 percent to 5 percent) we can alleviate dot gain and problems with damaging highlight dots, especially at higher pressures. Laser engraved sharp shoulders on the edge of text and borders help to minimize fat text and fill in on reverse text.
Engraved sleeves can also offer increased uniformity across the web, especially when compared to flat plates. This can mitigate the need to over impress and can greatly increase the life of a sleeve.
Another reason to use engraved elastomer sleeves is their durability. Elastomer is inherently more durable than polymer. You can expect two to four times the run length without degradation in highlight dots compared to polymer. What does this mean? Elastomer decreases the need to over impress to achieve the kiss impression at the later stages of a typical print run. There is no breaking down of cushion layers like a cushion adapter with polymer ITR or sticky back with flat plates.
“AS THE INDUSTRY INTRODUCES NEW PRESSES, INKS AND
SUBSTRATES, IT IS IMPORTANT TO REMEMBER THAT IT IS
OFTEN SIMPLE BEST PRACTICES THAT DETERMINE THE
SUCCESS OF A PRINTRUN.”
We’ve developed a set of best practices for establishing kiss impression when running fingerprints, discussed in Adam’s article. It is important to note that while many modern presses take care of these practices with automation, there is no substitute for an experienced pressman operating with a company’s unique set of best practices for its specific press. This way, the press operator has the ability to manually make adjustments for best impression, if conditions change during the course of a printrun.