Methods of forming a photopolymerizable flexographic printing element for making flexographic printing plates which is developable by use of washout media comprising hydrocarbons and which comprises - arranged on top of each other in the stated order - at least • a dimensionally stable PET supporting sheet having a thickness of 50 μm to 300 μm, whose shrinkage in the longitudinal direction (MD shrinkage) and in the transversal direction (TD shrinkage) as determined by annealing the sheet at 100°C for 15 minutes is in each case less than 0.02%, • a photopolymerizable, relief-forming layer which is soluble or dispersible in organic solvents and has a layer thickness of 300 μm to 7000 μm, comprising at least an elastomeric binder, an ethylenically unsaturated monomer and a photoinitiator or photoinitiator system, and • a coversheet, wherein the method comprises at least the steps of (1.1) providing a covering element comprising at least one coversheet, (1.2) melting and mixing the components of the photopolymerizable, relief-forming layer in a mixing assembly at a temperature of 120°C to 170°C, (1.3) exporting the melt through a sheet die into the nip of a calender while importing into said nip a temporary supporting sheet via one roll of the calender and the covering element via the other roll of the calender, (1.4) calendering the composite of temporary supporting sheet, photopolymerizable layer and covering element, (1.5) cooling the composite of temporary supporting sheet, photopolymerizable layer and covering element down to a temperature < 60°C, (1.6) peeling the temporary supporting sheet off the photopolymerizable layer, (1.7) bonding the composite, cooled down to temperatures of less than 60°C, with the side of the photopolymerizable layer to a PET supporting sheet having a thickness of 50 μm to 300 μm and whose shrinkage in the longitudinal direction (MD shrinkage) and in the transversal direction (TD shrinkage) as determined by annealing the sheet at 100°C for 15 minutes is in each case less than 0.02%. • a dimensionally stable PET supporting sheet having a thickness of 50 μm to 300 μm, whose shrinkage in the longitudinal direction (MD shrinkage) and in the transversal direction (TD shrinkage) as determined by annealing the sheet at 100°C for 15 minutes is in each case less than 0.02%, • a photopolymerizable, relief-forming layer which is soluble or dispersible in organic solvents and has a layer thickness of 300 μm to 7000 μm, comprising at least an elastomeric binder, an ethylenically unsaturated monomer and a photoinitiator or photoinitiator system, and • a coversheet, (1.1) providing a covering element comprising at least one coversheet, (1.2) melting and mixing the components of the photopolymerizable, relief-forming layer in a mixing assembly at a temperature of 120°C to 170°C, (1.3) exporting the melt through a sheet die into the nip of a calender while importing into said nip a temporary supporting sheet via one roll of the calender and the covering element via the other roll of the calender, (1.4) calendering the composite of temporary supporting sheet, photopolymerizable layer and covering element, (1.5) cooling the composite of temporary supporting sheet, photopolymerizable layer and covering element down to a temperature < 60°C, (1.6) peeling the temporary supporting sheet off the photopolymerizable layer, (1.7) bonding the composite, cooled down to temperatures of less than 60°C, with the side of the photopolymerizable layer to a PET supporting sheet having a thickness of 50 μm to 300 μm and whose shrinkage in the longitudinal direction (MD shrinkage) and in the transversal direction (TD shrinkage) as determined by annealing the sheet at 100°C for 15 minutes is in each case less than 0.02%.