Inside the preheat zone the elastic bumpers are compressed and the distance between two clips has to be the exact clip spacing (clips in stack). To make sure that the clip spacing is always constant, the number of clips in the preheat zone is counted. To make sure, that the automatic counting is working proper, it should be checked a few times during the checkup by manually counting. To make sure that it works correct the distance between the two FDM-sensors, which are responsible for the counting, is always a multiple of the distance between two driven.