hereinafter , the subject of the invention will be explained by example more in detail . the single figure illustrates a horizontally extending section through the left portion of a back - rest of a seat group structure . the back - rest of a seat group structure illustrated in the figure in a horizontal section includes an inner shell part 1 made of a glass - mat reinforced thermoplastic plastic material laminate ( gmt ). according to the illustrated embodiment an upholster 14 is connected to this inner shell part 1 , which can be for instance a foamed material . an outer covering 15 is arranged on this upholster 14 . depending from the design it can consist of a textile material , synthetic leather or true leather . this covering 15 extends along the back supporting area of the group structure and according to the illustration of the figure until over the lateral areas of the inner shell portion 1 . furthermore , an outer shell portion 2 is present . it includes also a covering 16 which may be for instance a textile product or also a part of a carpet , such as known among others for motor vehicles such as stations wagons . the covering 15 may , thereby ( see left end area of the figure ), extend over the inner shell part 1 up to the outer shell part 2 . the inner shell part 1 includes holding fixtures 17 with recesses 18 . the inner shell part 1 includes corresponding projections 19 by means of which it is set into the holding fixtures 17 of the outer shell part . the inner shell part 1 is mounted to the outer shell part 2 by means of a form - locked snap - lock connection , whereby the snap - lock connections extend along both sides of the respective backrest of the seat and along the upper part and / or also along its lower part . basically it is now to be mentioned that the structural members which now will be described , can be designed as continuous ledges which extend along the edge areas of the inner shell part 1 and the outer shell part 2 . preferred , however , they are in form of partial ledge group sections arranged at lateral distances from each other , i . e . that a plurality of ledge sections forming discrete snap - lock locations are arranged along an edge area . in order to form a respective snap - lock connection the inner shell part 1 includes an l - shaped portion 3 located at its outer edge area and extending in accordance with the extent of the snap - lock connection . this l - shaped portion 3 is formed by a first leg part 4 and a second , adjoining leg part 5 extending at least approximately perpendicularly thereto . the outer shell part 2 includes also a l - shaped portion 6 , which is formed by a third leg part 7 and a fourth leg part 8 projecting therefrom . the third leg part 7 projects hereby as such from the plate like main portion 12 of the outer shell part 2 . a first protrusion part 9 projects from the first leg part 4 of the l - shaped portion 3 at a location remote from the second leg part 5 . this protrusion part 9 extends at least approximately parallel to the second leg part 5 . furthermore , a second protrusion part 10 projects from the second leg part 5 , which in turn is located by a distance remote from the first leg part 4 . it can be seen that there two protrusion parts 9 and 10 are directed against each other and end at a distance from each other such that they define a l - shaped gap . the fourth leg part 8 and the directly adjacent third leg part 7 of the l - shaped section 6 of the outer shell part 2 are located in this l - shaped gap . a weakening area 11 which can be formed by a succession of slots analogue to the ledge sections extends at the area of this gap , which slots extend according to the snap - lock connection , e . g . along both sides of the back rest of the seat and along its upper edge . it can , thus , be seen , that the second leg part 5 can be elasticity bent relative to the first leg part 4 . a plate shaped main portion 12 of the outer shell part 2 which is structured somewhat thinner projects from the third leg part 7 of the l - shaped section 6 and it can be seen that also the third leg part 7 , i . e . the entire l - shaped position 6 can be resiliently bent relative to the plate shaped main portion 12 of the outer shell part 2 . the vertical distance between the second protrusion part 10 of the second leg part 5 and the surface portion of the inner shell part 1 facing same is selected at the area of the weakening area 11 such that this distance equals the thickness of the fourth leg part 8 of the l - shaped section 6 located therebetween . furthermore , the vertical distance between the first protrusion part 9 and the second protrusion part 10 , which protrusion parts 9 and 10 are directed against each other , is selected such that it is larger than the thickness of the portion of the third leg part 7 located in the area of this gap . it can been seen further that a curvilinear portion at the first leg part 4 is located between the surface portion of the first protrusion part 9 facing the fourth leg part 8 and the surface portion abutting the fourth leg part 8 at the region of the weakening area 11 . because the second leg part 5 and the third leg part 7 as well are resiliently bendable , it can be seen that the third leg part 7 can be snapped into mentioned gap such that the inner shell part 1 is connected to the outer shell part 2 . based on the above explained dimensions at the area of the gap and specifically of the fourth leg part 8 , it is obvious that this fourth leg part 8 is held in a clamped manner between the second protrusion part 10 and the surface area of the first leg part 4 , which rests on the leg part 8 . furthermore , the corresponding area of the second leg part 5 rests on the end surface 13 of the fourth leg part 8 . thus , a form locked snap - lock connection is formed between the inner shell part 1 and the outer shell part 2 . it can be further seen that in case of forces occurring e . g . due to an accident acting onto the area of the snap - lock connection can not cause a separation of the two shell parts 1 , 2 . any force , independent from the line of its action is directly transferred from one respective shell part onto the other shell part , wherewith a detaching of the snap - lock connection is impossible .
the present invention provides a double shell supporting device of a plastic material for a backrest of a seat - group structure with an inner shell part adapted to be a support of the back of a person sitting on the corresponding seat and an outer shell part which functions as a stiffening member .