A lot of printers utilize equipment for die cutting on machines that were converted from letterpress printing presses. The most popular of these are Heidelberg but there are others such as Millers. Recently we have seen more customers cutting on Vander Cook proofing presses. All of these presses are work horses and do a great job on die cutting registered work. One of the particularities of a cylinder is score heights. A cylinder press exerts different forces along the vertical and horizontal directions. Forces on scores that run parallel to the cylinder are less than scores that run perpendicular to the cylinder. We typically offset the score height by .005’’.
Score height is computed by taking the cut height (.918 virtually all letterpress equipment) and subtracting the caliper (in inches) of your paper. If you are using matrix you will need to subtract an additional .003’’ for plastic or fiberboard matrix and .009’’ for steel backed matrix
.918’’ cut height (type high)
-.010’’ paper caliper
-.003’’ plastic matrix
.905’’ crease Ht. horizontal .900’’ crease Ht. vertical
We actually made a pocket folder die for an Accu-cut table press last week. We had to offset the vertical scores by 0.015’’ because the vertical scores were die-cutting the stock. You may have to experiment with your die maker to get the perfect combination on your press.
Using matrix or some sort of female channel is imperative for achieving a true letterpress crease. Crushing the paper without a channel will give you a folding line but your paper will definitely tend to crack. This may be fine on white stocks but probably not acceptable on colored or printed papers.
The unequal forces may also be a problem when cutting or kiss cutting a grid pattern. Many times operators may damage the horizontal knives by increasing pressure because the vertical knives are not cutting. It may be possible with a very long make ready but we usually suggest running it on a platen style press like a Kluge or Bobst. Another solution may be to run the vertical knives in one pass and the horizontal knives in another pass.
Give us a call with any of your die cutting challenges or visit us at our website, graphicdies.com