One way to produce metal fibre-reinforced plastic-metal sandwich parts is to inject the polymer between the sheets during deep drawing. A new process has been designed in which parts with an interlayer of glas fibres including a thermoplastic matrix and two metallic sheets can be produced by combining deep drawing and thermoplastic-resin transfer moulding (T-RTM). With the possibility of forming sandwich sheets with a wide range of states of the interlayers' viscosity and due to the polymerisation of the monomers between the sheets, the process combines a lot of advantages but also faces challenges due to the viscous fluid interlayer being hindered from squeezing out. This contribution analyses the dependency of the interlayer on the forming behaviour of the sheets in this process. When forming circular cups with aluminium (AA5182) sheets and a viscous interlayer of different heights, the effect of bulging could be observed which affects a free formed shape during forming. The viscous interlayer does not remain at the same position during forming which leads to an inhomogeneous total blank thickness of the part. In experiments and FE simulations a shape with bulged zones could be observed. An analytical approach for the prediction of these bulge heights has been developed so that both bulge dome heights could be predicted analytically. This approach is verified by experiments and numerical simulations. The results serve to set the process window for the in-situ hybridisation method in which the starting point of injection, volume fraction of the thermoplastic matrix in the fibres, mechanical properties of the materials, and geometrical parameters can be adjusted so that this effect is reduced.