A method for mixing a first and at least a second polymeric reactive component by a self-cleaning high-pressure mixing device comprising a pressure chamber configured with a forwardly converging fore portion in fluid communication with a mixing chamber through a settable co-injection orifice; comprising the steps of: feeding the first and at least the second reactive components (A, B) into the pressure chamber maintaining the reactive components (A, B) inside the pressure chamber at a same pressure and in an unmixed state; flowing the reactive components (A, B) at said pressure and in the unmixed state, into the pressure chamber and into the forwardly converging fore portion to increase the flow velocity; co-injecting the unmixed components (A, B) into the mixing chamber, causing a pressure drop by the settable co-injection orifice; and ejecting the remaining unmixed reactive components (A, B) from the pressure chamber and from the forwardly converging fore portion, and ejecting a remaining mixture from the mixing chamber, by first and respectively second cleaning members (20, 28) having a fore end, the cleaning members (20, 28) being reciprocable between a retracted and an advanced condition in the pressure chamber, respectively in the mixing chamber (12, 15) of the mixing device; characterised by the steps of: configuring the co-injection orifice in the form of an elongated restriction longitudinally extending on a side wall of the mixing chamber, transversely oriented and intersecting the forwardly converging fore portion of the pressure chamber; adjusting the pressure of the unmixed reactive components (A, B) in the pressure chamber, setting an open section of the elongated restriction of the co-injection orifice by adjusting, in respect to the same co-injection orifice, the position of the fore end of the second cleaning member at the retracted condition thereof; and co-injecting the unmixed reactive components (A, B) into the mixing chamber, causing a pressure drop and mixing the reactive components (A, B), while they are flowing through the set open section of the co-injection orifice and into the mixing chamber of the mixing device. feeding the first and at least the second reactive components (A, B) into the pressure chamber maintaining the reactive components (A, B) inside the pressure chamber at a same pressure and in an unmixed state; flowing the reactive components (A, B) at said pressure and in the unmixed state, into the pressure chamber and into the forwardly converging fore portion to increase the flow velocity; co-injecting the unmixed components (A, B) into the mixing chamber, causing a pressure drop by the settable co-injection orifice; and ejecting the remaining unmixed reactive components (A, B) from the pressure chamber and from the forwardly converging fore portion, and ejecting a remaining mixture from the mixing chamber, by first and respectively second cleaning members (20, 28) having a fore end, the cleaning members (20, 28) being reciprocable between a retracted and an advanced condition in the pressure chamber, respectively in the mixing chamber (12, 15) of the mixing device; characterised by the steps of: configuring the co-injection orifice in the form of an elongated restriction longitudinally extending on a side wall of the mixing chamber, transversely oriented and intersecting the forwardly converging fore portion of the pressure chamber; adjusting the pressure of the unmixed reactive components (A, B) in the pressure chamber, setting an open section of the elongated restriction of the co-injection orifice by adjusting, in respect to the same co-injection orifice, the position of the fore end of the second cleaning member at the retracted condition thereof; and co-injecting the unmixed reactive components (A, B) into the mixing chamber, causing a pressure drop and mixing the reactive components (A, B), while they are flowing through the set open section of the co-injection orifice and into the mixing chamber of the mixing device. configuring the co-injection orifice in the form of an elongated restriction longitudinally extending on a side wall of the mixing chamber, transversely oriented and intersecting the forwardly converging fore portion of the pressure chamber; adjusting the pressure of the unmixed reactive components (A, B) in the pressure chamber, setting an open section of the elongated restriction of the co-injection orifice by adjusting, in respect to the same co-injection orifice, the position of the fore end of the second cleaning member at the retracted condition thereof; and co-injecting the unmixed reactive components (A, B) into the mixing chamber, causing a pressure drop and mixing the reactive components (A, B), while they are flowing through the set open section of the co-injection orifice and into the mixing chamber of the mixing device.