Abstract: When two sheets of slack material, for example the two sides of a paper or plastics bag, are to be separated by means of a pointed tool, problems arise because the two sheets can easily bend over, both in the same direction. If moreover these sheets are formed from smooth material, they also adhere together by vacuum sealing or by electrostatical forces. The solution for these problems is to enclose the sheets into a positioning block, in which a hollow space is recessed or saved for receiving the volume of the pointed tool to be driven thereinbetween. This driving-in of the pointed tool can be combined with the issuance of pressurized air between the sheets. To this end the pointed tool is provided with grooves for guiding the pressurized air or with a separate blow-pipe slidable over the pointed tool.