How to design prepress artwork for flexo printing using continuous engraving ‘in the round’ sleeves

Thin sleeve flexo sleeve - image courtesy of Stork Prints

Mercian Labels are early adopters of technology in the label printing industry, and one area where we have been trialing new products recently is in the area of continuous sleeve technology for flexography, the mainstay printing technology for global label printing.  Sometimes these projects require us to tackle some interesting problems, and yesterday we encountered and solved one interesting problem that is bound to be encoutered by others in due course (and googleing it didnt find a solution!).

Here is our solution to the problem of how to design artwork to fit a continuous sleeve printing, a flexo printing technique that produces a continuous image that does not have a join, unlike conventional flexo printing where the plate is a sheet that obvious has a join as it is wrapped around the printing cylinder.  If you want to know more about flexography, then please read about it here.  The great advantage of continuous images for clients and brand owners is that you get a great quality images with no breaks  – a very useful benefit when producing continuous packaging.

The problem:
If you want to design a continuous sleeve then the images has to repeat exactly in the repeat of the print cylinder diameter. This is fairly easy to do when the images are just repeating around the cylinder in the direction of the cylinder, but when they go in a diagonal like many of our premier Label Lock security label text, then these are much much harder to design to match up precisely.

For those in the label printing, prepress or reprographic industry looking to solve this problem, here is our solution:


Solution:

Decide the following:

  1. For each individual image to be repeated around the sleeve the approximate width and height you wish it to be including gaps
  2. the angle of the text across the plate
  3. the repeat size of the printing cylinder
  4. the width of the printing cylinder

For an example calculation
1 – about 40mm by 6.7mm image size for a 30mm image with a 10mm horizontal gap, and a 3mm image with a 3.7mm vertical gap
2 – 45 degrees
3 – 304.8mm for a 12 inch repeat

Steps:

1 Calculate the repeat length into which you have to fit the images

To do this use Pythagoras’ theorem http://en.wikipedia.org/wiki/Pythagorean_theorem

a^2 + b^2 = c^2!,

where for a 45 degree angle a=b (but if the angle is not 45 degrees see later as this step wont work)
so that a or b are the same
a=b=square root of [(304.8*304.8)/2]
so that a or b (the same) are 215.5261469mm

OR IF THE ANGLE IS NOT 45 DEGREES

If you want to do an angle that is not 45 degrees, then the Pythagoras’ theorem method wont work as a is not equal to b, but  I would expect the following to work (but not tried…)
Repeat length  = COS angle* repeat length of the cylinder
eg
Repeat length  = COS 45* 304.8 = 215.526 etc as above

2) work out how many EXACT repeats you can get into the repeat length

  • label width…..     215.5261469 / 40mm = 5.388 repeats, not exact therefore use judgment to decide on either 5 or 6 repeats per repeat length.  I would pick 5 here, so that the EXACT repeat of the image width is not 40mm, but is 215.5261469/5 = 43.10522938mm
  • label height…..     215.5261469 / 6.7mm =32.16808163 repeats, not exact therefore use judgment to decide on either 32 or 33 repeats per repeat length.  I would pick 32 here, so that the EXACT repeat of the image width is not 6.7mm, but is 215.5261469/32 = 6.735192091mm

3) layout the artwork in straight lines to be about double the size of the actual plate you need with image size and spacing as calculated in step 2

4) Rotate the grouped image to the angle you wish, usually 45 degrees

5) add a horizontal box of 304.8 x 250mm, the size of the print cylinder

6) Crop to the rectangle, and add further images at a different angle ALONG the web fitting the images into a 304.8m repeat as usual using the same method as in step 2 but for 304.8mm repeat

7) dispro  (if requested by the plate maker) and you are finished


Copyright statement: The text and method described here is copyright of Mercian Labels Ltd and may not be copied or reproduced in any way without our explicit permission.  If you wish to reference this work then please link to it – https://www.mercianlabels.com/blog/2009/08/06/how-to-design-prespress-artwork-for-flexo-printing-using-continuous-engraving-in-the-round-sleeves/