A 3D printing pen for extruding and curing a radiation-curable pasty polymer composition has a housing that contains a disposable cartridge filled with the composition. The cartridge also contains a piston that is slidable in the cartridge due to engagement of the nut secured in the piston with a lead screw that is located in the cartridge and driven from a motor so that displacement of the piston inside the cartridge expels the composition from the pen through an orifice at the end of the cartridge opposite the motor. The front end of the housing also contains LEDs that emit light onto the thread of composition on exit of the composition from the device. The mode of operation of the 3D printing pen and viscosity of the composition are selected so that after curing the extruded thread maintains its shape without supports.