A first waveguide sub-segment of a negative waveguide for plastic welding, especially for laser transmission welding, comprising: a. a first wall having a first inner face by means of which laser light can be reflected, b. a second wall having a second inner face by means of which laser light can be reflected, wherein c. the first and the second inner face are arranged opposite to each other and a first depth (D 1 ) of the first waveguide sub-segment is defined by a distance between the first and the second inner face , and d. the first and the second wall comprise an exit side , which faces the components (T, A) to be welded in operation, and an opposite entry side , wherein e. a first height (H 1 ) of the first wall is defined by a distance between the exit and the entry side of the first wall and a second height (H 2 ) of the second wall is defined by a distance between the exit and the entry side of the second wall and f. a first width (W 1 ) of the first wall is defined by an extension of the first wall perpendicular to the first height (H 1 ) as well as to the first depth (D 1 ) and a second width (W 2 ) of the second wall is defined by an extension of the second wall perpendicular to the second height (H 2 ) as well as to the first depth (D 1 ), characterized in that g. the entry side of the first wall extends at least in a first portion in a first angle α 1 with respect to the first height (H 1 ), for which applies: 0° < α 1 < 90°, preferably 5° < α 1 < 85° and especially preferred 10° < α 1 < 80°, and h. the entry side of the second wall extends at least in a second portion in a second angle α 2 with respect to the second height (H 2 ), for which applies: 0° < α 2 < 90°, preferably 5° < α 2 < 85° and especially preferred 10° < α 2 < 80°, wherein i. during usage of the first waveguide sub-segment , laser light from another waveguide sub-segment enters the first waveguide sub-segment between the first and the second portion . a. a first wall having a first inner face by means of which laser light can be reflected, b. a second wall having a second inner face by means of which laser light can be reflected, wherein c. the first and the second inner face are arranged opposite to each other and a first depth (D 1 ) of the first waveguide sub-segment is defined by a distance between the first and the second inner face , and d. the first and the second wall comprise an exit side , which faces the components (T, A) to be welded in operation, and an opposite entry side , wherein e. a first height (H 1 ) of the first wall is defined by a distance between the exit and the entry side of the first wall and a second height (H 2 ) of the second wall is defined by a distance between the exit and the entry side of the second wall and f. a first width (W 1 ) of the first wall is defined by an extension of the first wall perpendicular to the first height (H 1 ) as well as to the first depth (D 1 ) and a second width (W 2 ) of the second wall is defined by an extension of the second wall perpendicular to the second height (H 2 ) as well as to the first depth (D 1 ), characterized in that g. the entry side of the first wall extends at least in a first portion in a first angle α 1 with respect to the first height (H 1 ), for which applies: 0° < α 1 < 90°, preferably 5° < α 1 < 85° and especially preferred 10° < α 1 < 80°, and h. the entry side of the second wall extends at least in a second portion in a second angle α 2 with respect to the second height (H 2 ), for which applies: 0° < α 2 < 90°, preferably 5° < α 2 < 85° and especially preferred 10° < α 2 < 80°, wherein i. during usage of the first waveguide sub-segment , laser light from another waveguide sub-segment enters the first waveguide sub-segment between the first and the second portion . <EOS>