Selecting the perfect plate and sleeve for any given flexible packaging job isn’t rocket science, but the wrong decision can take years off your life. OK, maybe that is a bit of an exaggeration, but at the very least it will surely ruin your day.
So, it is always worth giving some thought to the needs of the job that should dictate which plate and sleeve will work best, and then selecting the products that have the most appropriate features to suit those needs.
Before diving into the nuts and bolts of what those needs and features are, let’s agree that for the sake of this discussion we are focusing on typical, run-of-the-mill wide web, flexible packaging process printing on film with solvent-based inks.
While it is tempting to base plate selection solely on cost, this can be a recipe for disaster. At the very root of deciding what plate to use for a job, the answers to these three questions are of most importance:
- Is the job especially long and/or do the plates need to be reused over and over?
- Are the graphics able to be staggered to maintain constant contact of the plate to the anilox, and the plate to the substrate?
- Are there really important highlights, such as critical fades to zero?
What plate features are to be decided upon? In the not-too-distant past, this discussion would have started with a debate about whether the plates should be round top or flat top, and whether or not surface texturization was necessary to enhance ink transfer (via surface screening or through a special method employed by the photopolymer plate manufacturer). These days, there really is not much need for discussion.
In this application, the plates should be both flat top and texturized. The advantage of a textured plate in flexible packaging is well documented, and the structure of flat top dots makes both theoretical and practical sense for ensuring plate-to-plate consistency and stable color on press. It is arguably this exact application that has been helped the most by flat top technology, and the very many articles written on the subject support this statement.
So with that out of the way, the following three plate features are essential enough to merit a discussion and decision.
Do the Plates Need Exceptional Ink Resistance?
If the job is extremely long, or if there is a need to reuse the plate over and over, a plate with superior resistance to solvent-based ink should be used. Some plates are better than others in regard to minimizing the swelling effect of solvent from the ink, and your plate manufacturer or supplier of choice can advise on its offerings. Plate swell can lead to both softening of the plate and an increase in plate thickness. Both of these effects can lead to unwanted dot gain. Soft plates will naturally gain more on press and a swollen plate is more likely to wear and slowly break down dots, making them larger. Unwanted dot gain means unwanted color shift.
Ideal Plate Hardness
Traditionally, hard plates were better for printing highlights, but solids suffered. Softer plates were better for printing solids, but highlights suffered. These days, we can more or less have our cake and eat it, too, with the use of surface screening and inherently textured plates, making plate hardness less critical. Therefore, the rule of thumb is to generally print with a hard plate to get the best highlights, while letting the texture do the work in the solids.
If the graphics of a job have poor stagger, with big gaps in graphics coverage in the print direction (either by chance or by design), a hard plate is always going to be preferred—especially in conjunction with a soft cushion tape or compressible sleeve. Without continual contact between the plate and anilox and plate and substrate, bounce will be created at the desired press speed. A hard plate will resist compression, while the soft cushion absorbs the blow of the bounce. A medium or soft plate in this scenario will be more susceptible to compressing and showing off the bounce in the form of banding and skips. Not pretty.
However, if the solids are absolutely a perfection must-have, bounce isn’t an issue and highlights are not a concern, a medium hardness plate—in combination with the right texture—can do a little better job than a hard plate when it comes to printing nice solids. This is especially true in printing pinhole-free, high-opacity whites.
How to Handle Highlights
Using specialty highlight screening to space apart the dots is a great way to reduce highlight gain and fade off vignettes to avoid harsh lines in subtle detail areas. The key downside to this is that when spaced apart, these dots no longer enjoy the close support of their fellow dots that they would find packed together on an AM grid. Therefore, they are more susceptible to damage than AM dots—both from general cleaning and from friction on press.
Using a hard plate can up the odds of survival, but expectations for longevity with such highlights should be minimized. For really long-running jobs, plates that will see frequent reuse, or when aggressive inks will be used, FM highlights should be used selectively and with great caution—if at all. Otherwise, AM-only dots will make the practicality of getting the job run much easier.
Long story short: For printing wide web flexible packaging with solvent-based inks, here is a simple guideline:
- Use a flat top plate with a texture—either via surface screening or inherently texturized
- Generally speaking, a hard plate with conservative use of FM highlights should be used
- If the job is really long or the plates will frequently be reused, speak with your plate supplier to be sure you are using the most ink-resistant hard plate available, and consider using only AM dots in the highlights
- If the job has especially important solids, and there is no concern with bounce or printing highlights, consider using a plate with medium hardness for maximum impact, but realize that the tradeoff could be longevity and reuse