Effective gap control and adjustment in rotary die-cutting.
In label die-cutting the gap between the magnetic and anvil cylinder, also referred to as clearance, is of fundamental importance. The gap can vary in the course of time due to various influences and during the production process itself, which has a negative effect on the die-cutting result. This article provides background knowledge on the gap and how it can be controlled and adjusted in practice by adjustable anvil systems. Read the full article in our eDossier “Small gap – big effect.“
Definition of the gap
In rotary die-cutting the clearance is one of the most important variables influencing the cutting result. It is defined as the distance between the magnetic cylinder (magnetic zone) and the anvil cylinder given normal pretension, or to express it as a formula:
Gap = (dia. of bearer rings – dia. of magnetic zone) / 2
The pretension is the force with which the magnetic cylinder presses from above against the anvil cylinder. It must be at least as high as the resistance which the lateral lines generate during die-cutting, to prevent vibration of the die. The optimum pretension depends, among other things, on the material, the number of lateral lines in the die, and the condition of the tool. The contact pressure on both sides of the machine should be continuously monitored and regulated by pressure cells, since for example the effects of heat will raise the pretension and thus influence the gap. In order to get more information, visit our shop to download the whole article.
The standard gap for label processing in Europe is usually 0.480 mm (0.0189”), while American cylinder manufacturers normally supply a clearance of 0.019” (0.483 mm). This might be considered a negligible difference. However, this difference of just three micrometres actually can cause problems in die-cutting, because manufacturers precisely match the height of the flexible die to the material depending on the gap. If the gap is considerably less than specified in an order, the cutting is too deep, while with a gap which is (too) wide it will accordingly not be strong enough.
Standard paper material on a glassine liner is still relatively robust if the gap alters, as the upper material can be cut even at low compression while the backing is compressible and therefore less “sensitive”. On the other hand, filmic materials such as PE or MDO behave considerably more critically, in particular in combination with thin PET backings. Here literally every micrometre counts in die-cutting, so that perfect condition of the cutting unit and a precisely defined cylinder gap running in parallel are all the more important for successful die-cutting.
Variables effecting the gap
The gap can vary considerably over the course of time, so that a satisfactory cutting result may no longer be achievable. Although weak cutting can be compensated to a certain degree by adjusting the pretension (i.e. increasing the cutting pressure), this procedure is detrimental to the life expectancy of the die, the bearer rings and other components. Learn more about “Small gap – big effect” – you can easily download it in our shop.