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 (14, 24), which faces the components (T, A) to be welded in operation, and an opposite entry side (16, 26), 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 (14, 24), which faces the components (T, A) to be welded in operation, and an opposite entry side (16, 26), 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.