The method involves preparing a lower element (2) made of polymer material resistant to high temperature and adding carbon powder to the element so that the element is opaque to visible light radiation and to absorb a laser radiation (1). An upper element (4) made of polymer material is prepared. The elements (2, 4) are welded using the laser radiation that crosses the element (4). The element (4) is made opaque to the light radiation and transparent to the laser radiation by adding organic or mineral pigment to the polymer material, without modifying properties of the material. An independent claim is also included for an assembly of thermoplastic elements resistant to high temperature, comprising upper and lower elements.
