reference will now be made in detail to various embodiments of the present invention ( s ), examples of which are illustrated in the accompanying drawings and described below . while the invention ( s ) will be described in conjunction with exemplary embodiments , it will be understood that present description is not intended to limit the invention ( s ) to those exemplary embodiments . on the contrary , the invention ( s ) is / are intended to cover not only the exemplary embodiments , but also various alternatives , modifications , equivalents and other embodiments , which may be included within the spirit and scope of the invention as defined by the appended claims . referring first to fig2 to 3c , a crash pad according to an exemplary embodiment of the present invention is described . as shown in fig2 , the crash pad 100 according to various embodiments of the present invention comprises a main resin layer 110 made of polypropylene filled ( ppf ) resin , and upper and lower resin layers 120 , 130 made of thermoplastic olefin ( tpo ) resin that is more flexible than the ppf resin . the ppf resin is a combination of polypropylene resin and filler added thereto , for example talc - filled polypropylene resin . the crash pad 100 is integrally molded through a so - called over - molding process , in which the upper resin layer 120 may be an optional element . the main resin layer 110 may be divided into a door region 100 a which is defined by a hinge portion 111 and a tear line 112 , and a crash pad region 100 b positioned around the door region 100 a . in various embodiments , although the door region 100 a and the crash pad region 100 b are made of the same resin ( ppf ), both the regions may also be made of different resins . in various embodiments of the present invention , the hinge portion 111 is configured such that a recess is longitudinally formed along the lower surface of the main resin layer 110 . specifically , a transverse section of the hinge portion 111 has a gently curved arched shape with a predetermined curvature so that stress is not concentrated to bottom portion of the hinge portion 111 . the hinge portion 111 is provided on a lower surface with corrugations 111 a formed along the width of the hinge portion , in which the respective corrugations 111 a extend in a longitudinal direction of the hinge portion . the tear line 112 is formed on a lower surface of the main resin layer 110 to exhibit a neat appearance . of course , the invisible tear line may be formed on another area on the main resin layer 110 , and may be formed even after molding of the crash pad 100 . in other embodiments of the present invention , the hinge portion 111 has a predetermined thickness without corrugations . the thickness of lower resin layer 130 is configured sufficient enough to extend with elasticity as the main resin layer 110 is bent in various embodiments of the present invention , the upper resin layer 120 is provided on an upper surface of the main resin layer 110 to cover the door region 100 a . the upper resin layer 120 not only improves appearance and tactile sense of the crash pad 100 but also increases a rupture strength of the hinge portion 111 in conjunction with the lower resin layer 130 . meanwhile , a slit 113 having a width of 1 - 1 . 5 mm is formed along a boundary area between the door region 100 a and the crash pad region 100 b of the main resin layer 110 in order to prevent the upper resin 120 from propagating to the crash pad region 100 b from the door region 100 a during a molding operation of the upper resin layer 120 ( an injection molding of tpo resin ) and to ensure a neat boundary line between the door region 100 a and the crash pad region 100 b . the lower resin layer 130 is provided on a lower surface of the main resin layer 110 to cover the hinge portion 111 . the lower resin layer 130 includes a bending portion 131 having a corrugated shape corresponding to the corrugations 111 a of the hinge portion 111 . the bending portion 131 includes corrugations formed in a thickness direction such that the upper corrugated surface of the bending portion 131 is in close contact with the corrugations 111 a of the hinge portion 111 and the lower corrugated surface of the bending portion 131 is configured to correspond to the upper corrugated surface . as a result , the bending portion 131 exhibits excellent flexibility . referring to fig3 a to 3c , an effect of reinforcing rigidity of the hinge portion 111 of the crash pad 100 is described . as shown in fig3 a , in the case of crash pad 100 mounted on an automobile , a chute 220 of an air - bag module is fused to a back side of the door region 100 a , i . e ., a back side of a passenger air - bag door . the chute 220 is assembled with a housing 210 which accommodates a folded air - bag 200 therein . as shown in fig3 b and 3c , upon deployment of the passenger air - bag , the air - bag 200 breaks the tear line 112 with the aid of a gas pressure from an inflator . at the same time , the air - bag 200 pushes out the passenger air - bag door and expands towards the passenger . at this point , the bending portion 131 covering the hinge portion 111 is stretched thus increasing tension strength of the hinge portion 111 whereas the upper resin layer 120 is compressed with restoring force and thus a bending angle or a radius of curvature on a back side of the hinge portion 111 is increased thus mitigating the stress concentrated on the hinge portion 111 . in this regard , since the surface of the hinge portion 111 and the bending portion 131 of the lower resin layer 130 are provided with corrugations , respectively , distensibility of the area is excellent . furthermore , since a contact area between the hinge portion 111 and the lower resin layer 130 is increased due to the corrugations , separation of the hinge portion 111 from the main resin layer 110 is prevented even if the hinge portion 111 is broken at the time of deployment of the air - bag is efficiently avoided . in embodiments that the hinge portion 111 has a predetermined thickness without corrugations , the lower resin layer 130 may extend with elasticity as the main resin layer 110 is bent so that separation of the hinge portion 111 from the main resin layer 110 is prevented . referring to fig4 and 5 , an apparatus for manufacturing the above - described crash pad is now described . the apparatus comprises a mold and a resin supply unit . the mold comprises upper and lower mold 10 , 20 , and upper and lower cores 30 , 40 which are disposed in the upper and lower mold 10 , 20 , respectively . the resin supply unit comprises first and second resin supply portions 50 , 60 , and first to third supply lines 51 , 61 , 62 . the upper mold part 10 includes an upper recess 11 at a lower side , in which the upper recess 11 comprises a first cavity 11 a and a second cavity 11 b having a depth deeper than the first cavity 11 a . the lower mold part 20 includes a lower recess 21 at an upper side . the upper and lower mold 10 , 20 are matched with each other so that an enclosed space is defined by the upper and lower recesses 11 , 12 . furthermore , the lower mold part 20 includes a wedge 23 protruding from the upper surface thereof to provide a back side of the passenger air - bag door with the tear line 112 ( see fig2 ). the upper core 30 is disposed in the second cavity 11 b of the upper mold part 10 to be slidable up and down . the upper core 30 includes a dam 31 , which protrudes and is positioned along an edge of the lower surface of the upper core 30 facing the lower mold part 20 . the lower core 40 is disposed in the lower recess 21 of the lower mold 20 to be slidable up and down . the lower core 40 is provided at an upper surface facing the upper mold part 10 with a convex portion 41 having a predetermined length so as to form the hinge portion 111 ( see fig2 ) of the passenger air - bag door . the convex portion 41 includes corrugations which are arranged in a width direction . the first resin supply portion 50 communicates with the first supply line 51 connected to the first cavity 11 a of the upper recess 11 . the second resin supply portion 60 communicates with the second supply line 61 and the third supply line 62 , which are connected to the second cavity 11 b and the lower recess 21 , respectively . for exemplary purposes , the first resin supply portion 50 is intended to supply ppf resin , and the second resin supply portion 60 is intended to supply tpo resin . referring to the drawings , in particular , to fig5 to 9 , a process of manufacturing the crash pad using the apparatus is now described . while the upper core 30 and the lower core 40 are disposed at respective advanced positions thus defining a main resin layer - molding space 12 a ( see fig5 ), supply of ppf resin from the first resin supply portion 50 through the first supply line 51 results in the main resin layer 110 ( see fig6 ). at this point , the convex portion 41 of the lower core 40 forms the hinge portion 111 on the lower surface of the main resin layer 110 , and the wedge 23 of the lower mold part 20 forms the tear line 112 on the lower surface of the main resin layer 110 . during the formation of the main resin layer 110 , the dam 31 of the upper core 30 protrudes into the main resin layer - molding space 12 a . due to the presence of the dam 31 , the slit 113 is formed in the upper surface of the main resin layer 110 , in particular , in the area between the door region 100 a and the crash pad region 100 b . prior to solidification of the ppf resin supplied into the main resin layer - molding space 12 a , the upper and lower cores 30 , 40 are retracted to additionally define an upper resin layer - molding space 12 b and a lower resin layer - molding space 22 ( see fig7 ), and the upper resin layer - molding space 12 b and the lower resin layer - molding space 22 are simultaneously supplied with tpo resin from the second resin supply portion 60 through the second and third supply lines 61 , 62 ( see fig8 ), resulting in the creation of the upper resin layer 120 and the lower resin layer 130 . in various embodiments , although the simultaneous supply of the same kind of resin can contribute to improvement in efficiency of the process of manufacturing the crash pad , it is also to be understood that supplying different kinds of resins through the second and third supply lines 61 , 62 falls within the scope of the present invention . during the formation of the upper and lower resin layers 120 , 130 , the upper core 30 is positioned such that the dam 31 is partially disposed in the slit 113 of the main resin layer 110 . as shown in fig9 , when tpo resin is injected into the upper resin layer - molding space 12 b while the slit 113 is blocked by the dam 31 , the propagation of the tpo resin to the first cavity 11 a , i . e ., to the crash pad region is avoided . even if there is partial leak of the tpo resin , the leaking resin is held only in the slit 113 , and cannot infiltrate into a clearance between the upper mold part 10 and the main resin layer 110 . in various embodiments , the slit 113 has a width of about 1 - 2 mm , and preferably a width of about 1 - 1 . 5 mm . at this point , the tpo resin caught in the slit 113 is invisible from the outside . consequently , the resulting product exhibits a neat appearance on the boundary between the door region 100 a and the crash pad region 100 b . as described above , according to the present invention , a main resin layer of a crash pad is provided at upper and lower surfaces with resin layers which are more flexible than the main resin layer , and a hinge portion is provided with corrugations . consequently , even if the hinge portion is abruptly folded by the expansionary force of a passenger air - bag , it is possible to prevent breaking the hinge portion and separation of an air - bag door at the time of deployment of the air - bag . for convenience in explanation and accurate definition in the appended claims , the terms “ up ” or “ upper ”, “ down ”, and “ lower ” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures . the foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description . they are not intended to be exhaustive or to limit the invention to the precise forms disclosed , and obviously many modifications and variations are possible in light of the above teachings . the exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application , to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention , as well as various alternatives and modifications thereof it is intended that the scope of the invention be defined by the claims appended hereto and their equivalents .