The method for manufacturing a heat exchanger from a plastic material comprises feeding of at least two sheet elements (3, 4), and, by means of shaping means, shaping of said sheet elements to form heat exchanger plates (7). The shaping means are adapted to also undertake joining the sheet elements partly along edges (11) so that a fluid inlet (12) and outlet (13) are created, whereby the sheet elements are pressed together. The heat exchanger plates (7) are stacked and joined to a heat exchanger. The stack (34) of heat exchanger plates (7) is welded so that the fluid inlets and fluid outletsof the resective heat exchanger plates are combined to form a common fluid inlet and a common fluid outlet, and the sides of the heat exchanger plates are welded together, so that a fully functional heat exchanger is manufactured with plastic sheets as the only raw material.