The invention relates to a device and a method for welding two polymer parts via fusion using a heater mat knitted from a resistive wire coated with an insulating varnish, the heater mat being given a final shape. The heater mat is resistive wire having a diameter ranging from 0.2 mm to 0.3 mm, and has approximately parallelepiped meshes, the dimensions of which range from 1.5×3 mm2 to 2.5×4.5 mm2.