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| An example of flexographic printing on a substrate |
There are several important considerations to make when designing art for flexographic printing.
Printing plates are mounted on cylinders, and there is always a small gap where the top and the bottom of the plate comes together. This is called a plate gap, and it's the result of mounting a flat item on a round item. This gap is typically between 1/8" and 1/16" wide, and in most cases it can be worked in to the design, making the gap barely noticeable.
However, when the design calls for a continuous solid color with random gaps, such as a reverse-out or a negative image, you will see a defined plate gap. One workaround for this is to use laser-engraved, or continuous sleeve. A continuous sleeve is a one-piece rubber or photopolymer tube that is threaded onto the printing cylinder, and it does not have any gaps or breaks in the design.
Unfortunately, continuous sleeves tend to be expensive and have long lead-times. The cost is often twice that of flat plates, and lead times can vary from 2-4 weeks. This time is often in addition to the lead-time on the film, since the order cannot be printed until the sleeve is in the plant.
There is another solution to the plate gap issue; on certain designs, a small strip can be printed to cover up the plate gap. It is called a butt-strip, and while it does not provide a seamless design like a continuous sleeve, it does cover up the gap rather well.
A butt strip can be used when there is an extra printing deck free, and
when the design allows for it.
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| A plate gap is typically 1/16" to 1/8", depending on the art work. | Depending on the art work, a plate gap may be hidden by a "butt strip". |
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| These bags were printed as a 2 item, side by side combo, and used 4 process colors and 4 spot colors: Cyan, Magenta, Yellow, Black, White spot, Blue spot, Red spot, Gold spot. |
When designing artwork for flexo printing, it is important to keep in mind the limitations of flexography, in order to get as good of a result as possible. Flexo printing typically uses a combination of process colors (CMYK, or Cyan, Magenta, Yellow, and Black) and spot colors. Because most presses have registration shifts of up to 1/16", CMYK will not work for small text. Even on larger print, edges can look blurry or dirty, as flexo printing uses dots to print process colors. Spot colors are needed where a large block of the artwork is a solid color, and most text will look the best if printed in separate spot colors.
In addition, almost every transparent film needs to be backed with a white color in order to make the colors stand out. Print on a transparent film will otherwise be hard to read, and chances are that the product will not look very good. If a window is not necessary, a white sealant web, such as white PE or CPP, can be used to save on ink cost.
In addition, screen tolerances for most flexo presses are 3-5%, and flexo plates cannot print a 0% screen. This is important to keep in mind when designing vignettes or gradients; there will be a hard line when the art fades into a 0% screen into another color.
Dot gain is also important to consider. Depending on the substrate,
the printed "dot" basically flows out (or "gains"), much like a water
color does on paper. Most presses have less than 10% dot gain, but it
can make very fine screens look dirty, and distort the color.
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| A good example of using common colors and plates. Only 6 plates were used to print 2 items with 5 colors each. |
As a rule of thumb, each color printed will need a separate plate, except for a flood coat that covers the whole web, which uses a standard in-house pad roller. When designing artwork for multiple SKUs, it is always a good idea to design common plates when possible to save cost. A different color can call out the flavor, rather than spelling it out.
Consider the Lant Chips bags on the left. Both items are 5 colors, White, Green, Brown Screen, Brown, Spot (Orange or Blue). The White, Green, Brown Screen, and Spot plates are all common, and the only changes between the two flavors are the Brown plate, and the color on the spot plate. This keeps the plate count at 6 for both items, with only 1 plate change and one color change between the two items.
A smart design can save both plate cost, but also minimize the change-over time between different SKUs in the same family of products, which ultimately leads to a lower film cost in the long run.
