The pressure build-up zone is of fundamental importance for the general conception of processes on co-rotating twin screw extruders. Depending on its pressure build-up capacity, it influences the temperature curve as well as the shear- and residence-time distribution. It is the task of the pressure build-up zone to bring up the necessary pressure to overrun the die. At the moment, a downstreamed single-screw extruder or a melt pump are used for the pressure build-up in many processes. The co-rotating twin-screw extruder could however assume this task in many areas. This would reduce the machine technology, which would cause the costs to be reduced immensely. The condition for this however, is a geometrically optimal layout of the pressure build-up zone. Apart from the flight angle, the screw clearances, which refer to the clearance between the inner diameter of the first screw and the outer diameter of the second screw and the clearance between the outer diameter of the screw and the barrel housing, are particularly relevant. In the following the influence of these clearances on the pressure build-up will be examined on the basis of 3D flow simulations. In order to do this, the clearances, the flight angle and the pressure difference that is to be built up for machines with nominal diameters of 25mm and 75mm are full factorial varied in three steps. On the basis of the results it is shown that the clearance has a tremendous influence on the pressure build-up in small machines. However, in machines with a large nominal diameter, the influence is considerably less. In conclusion, general recommendations for action for the geometrical layout of a pressure build-up zone will be formulated.