Patent Publication Number: US-2021190706-A1

Title: Foreign substance inspection method and foreign substance inspection apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2019-232507 filed on Dec. 24, 2019, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a foreign substance inspection method and a foreign substance inspection apparatus each for inspecting the presence or absence of a foreign substance embedded in a tubular resin molded product and having a dimension larger than a predetermined threshold. 
     2. Description of Related Art 
     There has been conventionally known a fuel cell vehicle including a fuel cell configured to generate electric power by chemically reacting hydrogen taken in from a hydrogen tank provided in a vehicle with oxygen taken in from the atmosphere, and the fuel cell vehicle travels by use of the electric power generated by the fuel cell as a drive source. From the viewpoint of weight reduction, as the hydrogen tank provided in the fuel cell vehicle, it is general to use a high-pressure tank configured such that a sealed cylindrical liner constituted by a pipe and domes made of resin is provided as an inner shell, and a high-strength outer shell is formed by winding carbon fiber around the outer peripheral surface of the liner. 
     In the meantime, the liner constituting the inner shell of the hydrogen tank serves as a container in which high-pressure hydrogen gas is filled in an airtight manner, and therefore, the liner is required to have a gas barrier property to low-molecular gas. On this account, as a molding material for the pipe and the domes as component parts of the liner, it is major to use a nylon material having a high gas barrier property. Further, in consideration of productivity, it is general to mold the cylindrical pipe and the bottomed tubular domes by injection molding. 
     However, the pipe and the domes molded by injection molding by use of the nylon material have such problems that nylon is a material that easily deteriorates and degrades due to a heat history, and it is difficult to cause the material not to remain and stay at all in an injection path. In combination of these problems, a foreign substance (hereinafter also referred to as an “embedded foreign substance”) mainly made of a carbonized deteriorated substance and called a “black spot” may be included in the pipe and the domes. 
     In a case where a relatively large amount of embedded foreign substances or a relatively large embedded foreign substance is included in a product (the pipe and the domes), the mechanical strength of the liner decreases as a result. Accordingly, it is important to check the “amount” and “magnitude” of the embedded foreign substance included in the product. In this respect, the amount and magnitude of the embedded foreign substance present on a surface of the product are easily checkable by visual observation. Generally, in order to prevent weatherability deterioration due to ultraviolet rays, the nylon material as the molding material for the liner includes color pigment, in other words, the product is not transparent. Accordingly, it may be said that it is difficult to check the presence or absence of the embedded foreign substance present inside the product, and further, it is difficult to check the magnitude of the embedded foreign substance by visual observation. 
     In view of this, for example, Japanese Patent No. 6508435 (JP 6508435 B) describes the following inspection method for inspecting a resin molded product. That is, X-rays are emitted in a plurality of paths, and the X-rays that have passed through the resin molded product are detected at one or more positions. Then, a defect-suspected part (a gap or impurity) is detected from a detected X-ray image, and the height of the defect-suspected part is measured by a stereo matching method. After that, a logical product of an obtained height information image and an image obtained as the defect-suspected part is taken, so that the quality of the defect-suspected part is sorted based on the height position. 
     SUMMARY 
     However, the technique to detect an embedded foreign substance by use of X-rays like the technique of JP 6508435 B is unrealistic from the viewpoint of an increase in cost due to introduction of an X-radiation facility or the like, safety management of X-rays (radioactive rays), and so on. 
     On this account, currently, in a case where the presence of an embedded foreign substance inside products is slightly recognized by visual observation, the products are all handled as defective products without checking the magnitude of the embedded foreign substance. This decreases the yield of the liner and causes an increase in cost. 
     The present disclosure is accomplished in view of such a problem, and an object of the present disclosure is to provide a technology to calculate, with high accuracy, the dimension of a foreign substance embedded in a tubular resin molded product by a safe and simple technique. 
     In order to achieve the above object, a foreign substance inspection method according to the present disclosure is to calculate the dimension of an embedded foreign substance based on a projection image of the embedded foreign substance by light emitted from a light source, by use of such a property that nylon that does not include color pigment allows light to pass therethrough. 
     More specifically, the present disclosure is aimed at a foreign substance inspection method for inspecting the presence or absence of a foreign substance embedded in a tubular resin molded product and having a dimension larger than a predetermined threshold. 
     In the foreign substance inspection method, a resin molded product made of nylon that does not include color pigment is prepared as the resin molded product. 
     The foreign substance inspection method includes: a first measurement step of emitting light to a first peripheral surface of the resin molded product from a first light source and measuring a dimension of a first projection image, of an embedded foreign substance, that is projected on a second peripheral surface, the first light source being placed on either one of inside and outside of the resin molded product in the tubular radial direction; a second measurement step of emitting light to the second peripheral surface of the resin molded product from a second light source and measuring a dimension of a second projection image, of the embedded foreign substance, that is projected on the first peripheral surface, the second light source being placed on the other one of the inside and the outside of the resin molded product in the tubular radial direction; and a calculation step of calculating a dimension of the embedded foreign substance based on a proportional relationship between a dimension of each of the projection images projected by the light sources to a distance from a corresponding one of the light sources to the each of the projection images and a dimension of the embedded foreign substance to a distance from the corresponding one of the light sources to the embedded foreign substance. 
     In this configuration, the resin molded product made of nylon that does not include color pigment is used as an inspection target. Accordingly, when light is emitted to the inner peripheral surface of the resin molded product from the light source placed inside the resin molded product, for example, the light passes through the resin molded product, so that a projection image of the embedded foreign substance is projected on the outer peripheral surface of the resin molded product. Hereby, the dimension of the projection image of the embedded foreign substance can be measured. Similarly, when light is emitted to the outer peripheral surface of the resin molded product from the light source placed outside the resin molded product, a projection image of the embedded foreign substance is projected on the inner peripheral surface of the resin molded product, so that the dimension of the projection image of the embedded foreign substance can be measured. 
     Further, there is a proportional relationship between the dimension of the first projection image to the distance from the first light source to the first projection image and the dimension of the embedded foreign substance to the distance from the first light source to the embedded foreign substance, and there is also a similar proportional relationship between the dimension of the second projection image to the distance from the second light source to the second projection image and the dimension of the embedded foreign substance to the distance from the second light source to the embedded foreign substance. In other words, two equations (proportional relationships) are established for two unknown quantities, i.e., the “distance from the light source to the embedded foreign substance” and the “dimension of the embedded foreign substance.” Accordingly, the dimension of the embedded foreign substance can be calculated based on these proportional relationships. 
     Thus, with the present disclosure, it is possible to calculate the dimension of the embedded foreign substance with high accuracy by a safe and simple technique to emit light from the inner and outer light sources in the tubular radial direction to the resin molded product made of nylon that does not include color pigment. 
     Further, the calculation step may include: a first calculation step of calculating a distance from either one of the first and second light sources to the embedded foreign substance by use of an embedment depth of the embedded foreign substance, the embedment depth being calculated based on a proportional relationship between the dimension of the first projection image to a distance from the first light source to the first projection image and the dimension of the embedded foreign substance to a distance from the first light source to the embedded foreign substance, and a proportional relationship between the dimension of the second projection image to a distance from the second light source to the second projection image and the dimension of the embedded foreign substance to a distance from the second light source to the embedded foreign substance; and a second calculation step of calculating the dimension of the embedded foreign substance based on a proportional relationship between the dimension of a projection image projected by the one of the first and second light sources to the distance from the one of the first and second light sources to the projection image and the dimension of the embedded foreign substance to the distance from the one of the first and second light sources to the embedded foreign substance. 
     In this configuration, first, the embedment depth of the embedded foreign substance is calculated based on the two equations (proportional relationships), and then, by use of the embedment depth thus calculated, the “distance from the light source to the embedded foreign substance” as one of the unknown quantities is calculated. After that, the “distance from the light source to the embedded foreign substance” is substituted into either of the two equations, so that the “dimension of the embedded foreign substance” as the other one of the unknown quantities can be easily calculated. 
     In the meantime, the dimension of the projection image, of the embedded foreign substance, that is projected on the outer peripheral surface of the resin molded product by the light from the light source placed inside the resin molded product can be measured by an inspector by visual observation regardless of the size of the diameter of the tubular resin molded product. On the other hand, the dimension of the projection image, of the embedded foreign substance, that is projected on the inner peripheral surface of the resin molded product by the light from the light source placed outside the resin molded product might be difficult to be measured by the inspector by visual observation depending on the size of the diameter of the tubular resin molded product. On this account, in a case where the diameter of the resin molded product is relatively small, it is conceivable that an image of at least the projection image, of the embedded foreign substance, that is projected on the inner peripheral surface of the resin molded product is captured by a camera placeable inside the resin molded product, and the dimension of the projection image is measured based on the image thus captured. 
     However, in a case where the dimension of one of the projection images is directly measured by visual observation, and the dimension of the other one of the projection images is measured via a captured image, a slight measurement error might be caused between them. 
     In view of this, in the foreign substance inspection method, in the first and second measurement steps, images of the first and second projection images may be captured by respective cameras, and respective dimensions of the first and second projection images may be measured based on the images thus captured. 
     With this configuration, in a case where an image of the projection image, of the embedded foreign substance, that is projected on the inner peripheral surface of the resin molded product is captured by the camera, an image of the projection image, of the embedded foreign substance, that is projected on the outer peripheral surface of the resin molded product is also captured by the camera, and the dimension of the projection image is measured based on the image thus captured. Accordingly, respective dimensions of the projection images, of the embedded foreign substance, that are projected on the inner peripheral surface and the outer peripheral surface of the resin molded product, respectively, can be measured while the measurement error is restrained. 
     Further, the present disclosure is also aimed at a foreign substance inspection apparatus used for the foreign substance inspection method. 
     The foreign substance inspection apparatus includes an inner light source, an outer light source, a support base, a camera, a displaying unit, and a measuring unit. The inner light source is placed inside the resin molded product in the tubular radial direction such that the inner light source is movable in the tubular axial direction. The outer light source is placed outside the resin molded product in the tubular radial direction such that the outer light source is movable in the tubular axial direction. The support base is configured to support the resin molded product such that the resin molded product is rotatable in a circumferential direction. The camera is placed at least inside the resin molded product in the tubular radial direction such that the camera is movable in the tubular axial direction, the camera being configured to take an image of an inner peripheral surface of the resin molded product. The displaying unit is configured to display the image captured by the camera on a screen. The measuring unit is configured to measure a dimension of a projection image of the embedded foreign substance when an inspector specifies, on the screen, a border of the projection image, of the embedded foreign substance, that is displayed on the displaying unit. 
     In this configuration, two light sources are placed inside and outside the resin molded product in the tubular radial direction such that the light sources are movable in the tubular axial direction, and the resin molded product is supported by the support base such that the rein molded product is rotatable in the circumferential direction. Accordingly, it is possible to inspect the presence or absence of the embedded foreign substance over the whole length and the whole circumference of the tubular resin molded product. Besides, by such a simple operation that the inspector specifies, on the screen, a border of the projection image, of the embedded foreign substance, that is displayed on the displaying unit, the dimension of the projection image of the embedded foreign substance can be measured. 
     Thus, with the present disclosure, it is possible to calculate, with high accuracy, the dimension of the embedded foreign substance over the whole length and the whole circumference of the tubular resin molded product by a safe and simple technique. 
     Further, the foreign substance inspection apparatus may further include a camera placed outside the resin molded product in the tubular radial direction such that the camera is movable in the tubular axial direction, the camera being configured to capture an image of the outer peripheral surface of the resin molded product. 
     With this configuration, respective dimensions of the projection images, of the embedded foreign substance, that are projected on the inner peripheral surface and the outer peripheral surface of the resin molded product, respectively, can be measured by the same measurement technique, so that a measurement error can be restrained. 
     As described above, with the foreign substance inspection method and the foreign substance inspection apparatus according to the present disclosure, it is possible to calculate, with high accuracy, the dimension of the foreign substance embedded in the tubular resin molded product by a safe and simple technique. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein: 
         FIG. 1  is a perspective view schematically illustrating a liner according to Embodiment 1 of the present disclosure; 
         FIG. 2  is a perspective view schematically illustrating a dome; 
         FIG. 3  is a sectional arrow view taken along a line A-A′ in  FIG. 2  to schematically describe an inspection of an embedded foreign substance by visual observation; 
         FIG. 4  is a perspective view schematically illustrating a foreign substance inspection apparatus; 
         FIG. 5  is a view to schematically describe a foreign substance inspection method; 
         FIG. 6  is a view to schematically describe the foreign substance inspection method; 
         FIG. 7  is a view schematically illustrating a monitor and an image processor; and 
         FIG. 8  is a perspective view schematically illustrating a foreign substance inspection apparatus according to Embodiment 2 of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     With reference to the drawings, the following describes an embodiment to carry out the present disclosure. 
     Embodiment 1 
     Liner 
       FIG. 1  is a perspective view schematically illustrating a liner  1  according to the present embodiment. The liner  1  constitutes an inner shell of a hydrogen tank (not shown) provided in a fuel cell vehicle, for example. The liner  1  is formed in a cylindrical shape the opposite ends of which are closed, so that high-pressure hydrogen gas used for power generation of fuel cells can be stored in the liner  1  in an airtight manner. 
     From the viewpoint of weight reduction, the liner  1  is made of resin and includes a pipe  10  made of resin and two domes  20 ,  30  made of resin as illustrated in  FIG. 1 . The liner  1  is formed in an airtight cylindrical shape in such a manner that one pipe  10  having a cylindrical shape is joined (welded) to the two domes  20 ,  30  having a bottomed tubular shape in the axial direction such that the pipe  10  is sandwiched between the domes  20 ,  30 , and mouthpieces  2 ,  3  made of aluminum are assembled by press-fitting to the two domes  20 ,  30  placed at both ends. Then, carbon fiber (not shown) is wound around the outer periphery of the liner  1  formed as such, and hereby, a hydrogen tank having a high-strength outer shell is formed. 
       FIG. 2  is a perspective view schematically illustrating the dome  20 . The dome  20  includes a cylindrical portion  21 , and a semispherical dome portion  22  provided to close one end of the cylindrical portion  21  such that the mouthpiece  2  is assembled to the semispherical dome portion  22 . Thus, the dome  20  is formed in a bottomed tubular shape as described above. 
     Embedded Foreign Substance 
     In the meantime, as described above, the liner  1  serves as a container in which high-pressure hydrogen gas is filled in an airtight manner, and therefore, the liner  1  is required to have a gas barrier property to low-molecular gas. Examples of resin having such a rare characteristic called the gas barrier property include nylon and EVOH (ethylene-vinylalcohol copolymer). However, EVOH is hard and has poor extensibility, and its rigidity and weather resistance are also low. Accordingly, it is difficult to solely use EVOH, and EVOH is mainly used as a composite by extrusion molding and the like. In view of this, EVOH is not suitable for molding of the liner  1  that is required to have a high productivity. On this account, as a molding material for the pipe  10  and the domes  20 ,  30  as component parts of the liner  1 , it is major to use a nylon material having a high gas barrier property. Further, in a case where the liner  1  is manufactured, it is general to mold the cylindrical pipe  10  and the bottomed tubular domes  20 ,  30  by injection molding in consideration of the productivity. 
     However, the pipe  10  and the domes  20 ,  30  molded by injection molding by use of the nylon material have such problems that nylon is a material that easily deteriorates and degrades due to a heat history, and it is difficult to cause the material not to remain and stay at all in an injection path. In combination of these problems, a foreign substance  40  (see  FIG. 3 ) (hereinafter also referred to as an “embedded foreign substance  40 ”) mainly made of a carbonized deteriorated substance and called a “black spot” may be included in the pipe  10  or the domes  20 ,  30 . More specifically, a gap is set between a distal end of a barrel (not shown) constituting an injection path and a distal end of a screw (not shown) configured to extrude molten resin, so as to prevent a collision or bite between the barrel and the screw even in a state where the screw is advanced most. Nylon remaining and staying in such a gap is turned into a carbonized deteriorated substance due to a heat history, and the carbonized deteriorated substance peeled off is emitted into a cavity of a metal mold together with the molten resin (nylon). Hereby, the carbonized deteriorated substance is turned into the “black point.” In a case where a relatively large amount of the embedded foreign substances  40  or the embedded foreign substance  40  that is relatively large is included in a product (the pipe  10  and the domes  20 ,  30 ), the mechanical strength of the liner  1  decreases as a result. In view of this, it is important to check the “amount” and “magnitude” of the embedded foreign substance  40  included in the product. 
       FIG. 3  is a sectional arrow view taken along a line A-A′ in  FIG. 2  to schematically describe an inspection of the embedded foreign substance  40  by visual observation. As illustrated in  FIG. 3 , the amount and magnitude of the embedded foreign substance  40  present on a surface (an outer peripheral surface  21   b ) of the dome  20  are easily checkable by visual observation. Generally, in order to prevent weatherability deterioration due to ultraviolet rays, the nylon material as the molding material for the liner  1  includes color pigment, in other words, the dome  20  is not transparent. Accordingly, as illustrated in  FIG. 3 , it may be said that it is difficult to check the presence or absence of the embedded foreign substance  40  present inside the dome  20 , and further, it is difficult to check the magnitude of the embedded foreign substance  40  by visual observation. 
     Here, it is also conceivable that the embedded foreign substance  40  is detected by use of X-rays. However, from the viewpoint of an increase in cost due to introduction of an X-radiation facility or the like, safety management of X-rays (radioactive rays), and so on, the inspection by use of X-rays is unrealistic. On this account, in a case where the presence of the embedded foreign substance  40  inside the pipe  10  and the domes  20 ,  30  is slightly recognized by visual observation, they are all handled as defective products without checking the magnitude of the embedded foreign substance  40 . This decreases the yield of the liner  1  and causes an increase in cost. 
     In view of this, in the present embodiment, by use of such a property that nylon that does not include color pigment allows light to pass therethrough, a dimension W 0  of the embedded foreign substance  40  is calculated based on a projection image of the embedded foreign substance  40  by light emitted from a light source. The following describes details of a foreign substance inspection apparatus  50  (see  FIG. 4 ) and a foreign substance inspection method each of which can calculate the dimension of the foreign substance  40  embedded in a tubular resin molded product (e.g., the dome  20 ) with high accuracy. 
     Foreign Substance Inspection Apparatus 
       FIG. 4  is a perspective view schematically illustrating the foreign substance inspection apparatus  50 . As illustrated in  FIG. 4 , the foreign substance inspection apparatus  50  includes a base plate  51 , a first slide block  61 , a second slide block  62 , an inner light source  71 , an outer light source  73 , an inner camera  81 , a monitor  85 , and an image processor  87  (see  FIG. 7 ). 
     The base plate  51  is a rectangular disc body, and slide rails  53 ,  55  extending in the longitudinal direction are formed in the opposite end portions of the base plate  51  in the longitudinal right angle direction. Further, four guide rollers  57  that can support the dome  20  are provided in a central part of the base plate  51 . Hereby, for example, when an inspector applies a force, in the circumferential direction, to the dome  20  placed on the guide rollers  57 , the dome  20  rotates smoothly. Therefore, in terms of correspondence relations with claims, the base plate  51  provided with the four guide rollers  57  corresponds to a “support base configured to support the resin molded product such that the resin molded product is rotatable in a circumferential direction” in the present disclosure. 
     The first slide block  61  is attached to the slide rail  53  on the far side in  FIG. 4  so as to be slidable in the longitudinal direction. The first slide block  61  is provided with an attachment arm  63  having a generally lateral U-shape. The attachment arm  63  includes: a first arm portion  63   a  extending to a first side in the longitudinal direction of the base plate  51 ; a second arm portion  63   b  extending to be inclined upward toward the central side of the base plate  51  from a distal end portion of the first arm portion  63   a;  and a third arm portion  63   c  extending from a distal end portion of the second arm portion  63   b  toward a second side in the longitudinal direction of the base plate  51 . The attachment arm  63  is configured such that, when the inspector slides the first slide block  61  on the slide rail  53 , for example, in a state where the dome  20  is placed on the guide rollers  57 , the cylindrical portion  21  of the dome  20  enters between the first arm portion  63   a  and the third arm portion  63   c  so that the third arm portion  63   c  moves in the tubular axial direction inside the dome  20 . 
     The inner light source  71  is a commercial white LED lamp, and as illustrated in  FIG. 4 , the inner light source  71  is attached to a distal end portion of the third arm portion  63   c.  Therefore, for example, when the inspector slides the first slide block  61  on the slide rail  53 , the inner light source  71  attached to the distal end portion of the third arm portion  63   c  moves in the tubular axial direction inside the dome  20 , and light is emitted from the inner light source  71  to the inner peripheral surface  21   a  of the dome  20 . Note that a distance L 1  from the inner light source  71  to the inner peripheral surface  21   a  of the dome  20  is set to a sufficiently short distance so that a first projection image  41  (see  FIG. 5 ), of the embedded foreign substance  40 , that is projected on the outer peripheral surface  21   b  of the dome  20  by the light from the inner light source  71  does not become blurred. 
     As illustrated in  FIG. 4 , the inner camera  81  is attached to the distal end portion of the third arm portion  63   c  together with the inner light source  71 . Accordingly, for example, when the inspector slides the first slide block  61  on the slide rail  53 , the inner camera  81  attached to the distal end portion of the third arm portion  63   c  moves in the tubular axial direction inside the dome  20  and captures an image of the inner peripheral surface  21   a  of the dome  20 . The inner camera  81  is electrically connected to the monitor (displaying unit)  85  via a cable  82 , and hereby, the image of the inner peripheral surface  21   a  of the dome  20 , the image being captured by the inner camera  81 , is displayed on a screen  85   a  of the monitor  85  with an appropriate reduced scale. Note that the image processor  87  will be described later. 
     The second slide block  62  is attached to the slide rail  55  on the near side in  FIG. 4  so as to be slidable in the longitudinal direction. The second slide block  62  is provided with a generally L-shaped attachment arm  64 . 
     Similarly to the inner light source  71 , the outer light source  73  is a commercial white LED lamp, and as illustrated in  FIG. 4 , the outer light source  73  is attached to the attachment arm  64 . Accordingly, for example, when the inspector slides the second slide block  62  on the slide rail  55 , the outer light source  73  attached to the attachment arm  64  moves in the tubular axial direction outside the dome  20 , and light is emitted from the outer light source  73  to the outer peripheral surface  21   b  of the dome  20 . Note that a distance L 2  from the outer light source  73  to the outer peripheral surface  21   b  of the dome  20  is set to a sufficiently short distance so that a second projection image  42  (see  FIG. 6 ), of the embedded foreign substance  40 , that is projected on the inner peripheral surface  21   a  of the dome  20  by the light from the outer light source  73  does not become blurred. 
     In the present embodiment, the distance L 1  is set to a sufficiently short distance. Accordingly, a relatively small white LED lamp is employed as the inner light source  71  so that the inner light source  71  does not interfere with the arcuate inner peripheral surface  21   a  of the dome  20 . In the meantime, as illustrated in  FIG. 4 , a relatively large white LED lamp is employed as the outer light source  73  that does not have such a restriction. However, the outer light source  73  is not limited to this, and a white LED lamp having the same size as the inner light source  71  may be employed as the outer light source  73 . Note that the distance L 1  from the inner light source  71  to the inner peripheral surface  21   a  of the dome  20  and the distance L 2  from the outer light source  73  to the outer peripheral surface  21   b  of the dome  20  are set to the same value L. In the following description, the distance from the inner light source  71  to the inner peripheral surface  21   a  of the dome  20  and the distance from the outer light source  73  to the outer peripheral surface  21   b  of the dome  20  are both indicated by L. 
     Foreign Substance Inspection Method 
     Next will be described a foreign substance inspection method for inspecting the presence or absence of the embedded foreign substance  40  having a dimension larger than a predetermined threshold, the foreign substance inspection method being performed by use of the foreign substance inspection apparatus  50 . 
     First, as an inspection target, the dome  20  made of nylon that does not include color pigment is prepared, and the dome  20  is placed on the guide rollers  57 . As described above, generally, the nylon material as the molding material for the liner  1  includes color pigment in order to prevent weatherability deterioration due to ultraviolet rays. However, the present embodiment focuses on a point that carbon fiber is wound around the outer periphery of the liner  1  of the hydrogen tank (ultraviolet rays are prevented by carbon fiber), and the nylon material that does not include color pigment is employed as the molding material for the liner  1 . Therefore, the liner  1  of the present embodiment is constituted by the pipe  10  and the domes  20 ,  30  that are semi-translucent such that light is passed therethrough. As such, the liner  1  that employs, as the molding material, the nylon material that does not include color pigment has such an advantage that the low-temperature tensile strength is relatively high because stress concentration due to variations in distribution of the color pigment is hard to occur. 
       FIGS. 5 and 6  are views to schematically describe the foreign substance inspection method. The foreign substance inspection method includes a marking step, a first measurement step illustrated in  FIG. 5 , a second measurement step illustrated in  FIG. 6 , a calculation step, and a quality determination step. 
     First, in the marking step, a presence position of the embedded foreign substance  40  is marked on the dome  20 . More specifically, while light is emitted to the inner peripheral surface  21   a  of the dome  20  from the inner light source  71  placed inside the dome  20  in the tubular radial direction, the first slide block  61  is roughly slid on the slide rail  53 , and the dome  20  is roughly rotated on the guide rollers  57 . As a result, in a case where the foreign substance  40  is embedded at a position irradiated with the light, the first projection image  41  of the embedded foreign substance  40  is projected on the outer peripheral surface  21   b  of the dome  20 , as illustrated in  FIG. 5 . Then, a tape or the like is attached in the vicinity of the first projection image  41  on the outer peripheral surface  21   b  of the dome  20 , and thus, the presence position of the embedded foreign substance  40  is marked on the outer peripheral surface  21   b  of the dome  20 . By performing such an operation over the whole length and the whole circumference of the dome  20  through the sliding of the first slide block  61  and the rotation of the dome  20 , presence positions of embedded foreign substances  40  in the dome  20  are investigated. 
     Subsequently, in the first measurement step, light is emitted to the inner peripheral surface  21   a  of the dome  20  from the inner light source  71  placed inside the dome  20  in the tubular radial direction, so that a dimension W 1  of the first projection image  41 , of the embedded foreign substance  40 , that is projected on the outer peripheral surface  21   b  is measured. More specifically, light is emitted to the inner peripheral surface  21   a  of the dome  20  from the inner light source  71  with the tape attached in the marking step as a mark, and as illustrated in  FIG. 5 , the first projection image  41  of the embedded foreign substance  40  is projected on the outer peripheral surface  21   b  of the dome  20 . As such, since the first projection image  41  projected on the outer peripheral surface  21   b  of the dome  20  is checkable by the inspector by visual observation, the dimension W 1  of the first projection image  41  is measured by use of a gauge or the like. 
     Subsequently, in the second measurement step, light is emitted to the outer peripheral surface  21   b  of the dome  20  from the outer light source  73  placed outside the dome  20  in the tubular radial direction, and a dimension W 2  of the second projection image  42 , of the embedded foreign substance  40 , that is projected on the inner peripheral surface  21   a  is measured. More specifically, light is emitted to the outer peripheral surface  21   b  of the dome  20  from the outer light source  73  with the tape attached in the marking step as a mark, and as illustrated in  FIG. 6 , the second projection image  42  of the embedded foreign substance  40  is projected on the inner peripheral surface  21   a  of the dome  20 . As such, since it is difficult for the inspector to check, by visual observation, the second projection image  42  projected on the inner peripheral surface  21   a  of the dome  20 , an image of the second projection image  42  is captured by the inner camera  81 , and the captured image of the second projection image  42  is displayed on the screen  85   a  of the monitor  85 . 
       FIG. 7  is a view schematically illustrating the monitor  85  and the image processor  87 . As illustrated in  FIG. 7 , the image of the second projection image  42  is displayed on the screen  85   a  of the monitor  85 . However, a border  42   a  of the second projection image  42  may be blurred as indicated by dot hatching in  FIG. 7 , and therefore, determination by the inspector is required. More specifically, the inspector specifies the border  42   a  of the second projection image  42  displayed on the screen  85   a  by use of a pointer (not shown) or the like on the screen, so that the image processor  87  measures the dimension of the second projection image  42 . The image processor  87  is a device that can measure the dimension of the embedded foreign substance  40  based on image data or the like, and the image processor  87  can be achieved, for example, such that an image processing program, a determination program, and so on are incorporated into a general-purpose personal computer. Note that, in terms of correspondence relations with claims, the image processor  87  corresponds to a “measuring unit configured to measure a dimension of a projection image of the embedded foreign substance when an inspector specifies, on the screen, a border of the projection image, of the embedded foreign substance, that is displayed on the displaying unit” in the present disclosure. 
     By performing such an operation on all the embedded foreign substances  40  marked in the marking step, the dimensions W 1 , W 2  of the first and second projection images  41 ,  42  of the embedded foreign substances  40  can be obtained. Note that, in the present embodiment, the dimensions of the projection images are measured in order of the first measurement step and the second measurement step. However, the first measurement step and the second measurement step may be performed in reverse order to the above. 
     In the calculation step, the dimension W 0  of the embedded foreign substance  40  is calculated based on a proportional relationship between the dimension W 1  of the first projection image  41  (or the dimension W 2  of the second projection image  42 ) to the distance from the inner light source  71  (or the outer light source  73 ) to the first projection image  41  (or the second projection image  42 ) and the dimension W 0  of the embedded foreign substance  40  to the distance from the inner light source  71  (or the outer light source  73 ) to the embedded foreign substance  40 . The calculation step includes a first calculation step and a second calculation step. 
     In the first calculation step, an embedment depth d of the embedded foreign substance  40  is calculated based on the proportional relationship between the dimension W 1  of the first projection image  41  to the distance from the inner light source  71  to the first projection image  41  (the outer peripheral surface  21   b ) and the dimension W 0  of the embedded foreign substance  40  to the distance from the inner light source  71  to the embedded foreign substance  40 , and the proportional relationship between the dimension W 2  of the second projection image  42  to the distance from the outer light source  73  to the second projection image  42  (the inner peripheral surface  21   a ) and the dimension W 0  of the embedded foreign substance  40  to the distance from the outer light source  73  to the embedded foreign substance  40 . Then, by use of the embedment depth d thus calculated, a distance L 0  from the outer light source  73  to the embedded foreign substance  40  is calculated. 
     More specifically, when the thickness of the cylindrical portion  21  of the dome  20  is taken as t, and the embedment depth of the embedded foreign substance  40  from the outer peripheral surface  21   b  is taken as d, a proportional relationship as expressed by Equation (1) is established between the dimension W 1  of the first projection image  41  and the dimension W 0  of the embedded foreign substance  40 , as seen from  FIG. 5 . 
         W 0: W 1= L+t−d:L+t    Equation (1)
 
     When Equation (1) is rearranged, Equation (2) is established. 
         W 0=( L+t−d )/( L+t )× W 1   Equation (2)
 
     Further, as seen from  FIG. 6 , a proportional relationship as expressed by Equation (3) is established between the dimension W 2  of the second projection image  42  and the dimension W 0  of the embedded foreign substance  40 . 
         W 0: W 2= L+d:L+t    Equation (3)
 
     When Equation (3) is rearranged, Equation (4) is established. 
         W 0=( L+d )/( L+t )× W 2   Equation (4)
 
     Here, the distance L, the thickness t, the dimension W 1 , and the dimension W 2  are known, and only two values, i.e., the dimension W 0  of the embedded foreign substance  40  and the embedment depth d of the embedded foreign substance  40  are unknown. Accordingly, the dimension W 0  and the embedment depth d of the embedded foreign substance  40  can be calculated from two equations, i.e., Equation (2) and Equation (4). 
     First, the dimension W 0  of the embedded foreign substance  40  is eliminated from Equation (2) and Equation (4), and an obtained equation is rearranged in terms of the embedment depth d of the embedded foreign substance  40 . Hereby, Equation (5) is obtained. 
         d =[ W 1× t +( W 1− W 2)× L ]/( W 1+ W 2)   Equation (5)
 
     Hereby, the distance L 0  (=L+d) from the outer light source  73  to the embedded foreign substance  40  is calculated. 
     In the subsequent second calculation step, the dimension W 0  of the embedded foreign substance  40  is calculated based on the proportional relationship between the dimension W 2  of the second projection image  42  to the distance from the outer light source  73  to the second projection image  42  and the dimension W 0  of the embedded foreign substance  40  to the distance L 0 . 
     More specifically, by substituting the distance L 0  into Equation (4), the dimension W 0  of the embedded foreign substance  40  is calculated by Equation (6). 
         W 0= L 0/( L+t )× W 2   Equation (6)
 
     Subsequently, in the quality determination step, it is determined whether or not the dimension W 0  of the embedded foreign substance  40 , calculated in the calculation step, is a predetermined threshold or less. More specifically, when the dimension W 0  of the embedded foreign substance  40  exceeds the threshold (e.g., 0.6 mm), the dome  20  in which the embedded foreign substance  40  is embedded is handled as a defective product. In the meantime, when the dimension W 0  of the embedded foreign substance  40  is the threshold or less, the dome  20  in which the embedded foreign substance  40  is embedded is handled as a non-defective product. Even in a case where the dimension W 0  of the embedded foreign substance  40  is the threshold or less, when the number of the embedded foreign substances  40  exceeds a predetermined number (e.g., 5 per 100 mm 2 ), the dome  20  including the embedded foreign substances  40  the number of which exceeds the predetermined number is handled as a defective product. 
     Thus, with the present embodiment, the dimension W 0  of the embedded foreign substance  40  can be calculated with high accuracy by a safe and simple technique to emit light from the inner and outer light sources  71 ,  73  in the tubular radial direction to the dome  20  made of nylon that does not include color pigment. 
     Embodiment 2 
     The present embodiment is different from Embodiment 1 in that an image of the first projection image  41  projected on the outer peripheral surface  21   b  of the dome  20  by the inner light source  71  is also captured by a camera. The following mainly describes points different from Embodiment 1. 
     The dimension of the first projection image  41  projected on the outer peripheral surface  21   b  of the dome  20  by light from the inner light source  71  can be measured by the inspector by visual observation regardless of the size of the diameter of the liner  1 . On the other hand, it might be difficult for the inspector to measure, by visual observation, the dimension of the second projection image  42  projected on the inner peripheral surface  21   a  of the dome  20  by light from the outer light source  73  depending on the size of the diameter of the liner  1 . On this account, in Embodiment 1, an image of the second projection image  42  projected on the inner peripheral surface  21   a  of the dome  20  is captured by the inner camera  81 , so that the dimension W 2  of the second projection image  42  is measured based on the image thus captured. 
     However, in a case where the dimension of a first projection image is directly measured by visual inspection, and the dimension of a second projection image is measured via a captured image, a slight measurement error might be caused between them. 
     In view of this, in the present embodiment, images of the first and second projection images  41 ,  42  are both captured by respective cameras, and the dimensions W 1 , W 2  of the first and second projection images  41 ,  42  are measured based on the images thus captured. 
       FIG. 8  is a perspective view schematically illustrating a foreign substance inspection apparatus  50 ′ according to the present embodiment. As illustrated in  FIG. 8 , the second slide block  62  of the foreign substance inspection apparatus  50 ′ is provided with a bar-shaped attachment arm  65  extending upward, in addition to the generally L-shaped attachment arm  64 . An outer camera  83  is attached to a distal end portion of the attachment arm  65 . Accordingly, for example, when the inspector slides the second slide block  62  on the slide rail  55 , the outer camera  83  attached to the distal end portion of the attachment arm  65  moves outside the dome  20  in the tubular axial direction and captures an image of the outer peripheral surface  21   b  of the dome  20 . The outer camera  83  is electrically connected to the monitor  85  via a cable  84 , and hereby, the image, of the outer peripheral surface  21   b  of the dome  20 , that is captured by the outer camera  83  is displayed on the screen  85   a  of the monitor  85  with an appropriate reduced scale. 
     Thus, in the first measurement step, light is emitted to the inner peripheral surface  21   a  of the dome  20  from the inner light source  71  with the tape attached in the marking step as a mark, and the first projection image  41  of the embedded foreign substance  40  is projected on the outer peripheral surface  21   b  of the dome  20 . Hereby, an image of the first projection image  41  projected on the outer peripheral surface  21   b  of the dome  20  is captured by the outer camera  83 , and the captured image of the first projection image  41  is displayed on the screen  85   a  of the monitor  85 . When the inspector specifies a border of the first projection image  41  displayed on the screen  85   a  by use of a pointer or the like on the screen, the dimension W 1  of the first projection image  41  is measured by the image processor  87 . 
     In the present embodiment, the dimensions W 1 , W 2  of the first and second projection images  41 ,  42 , of the embedded foreign substance  40 , that are projected on the inner peripheral surface  21   a  and the outer peripheral surface  21   b,  respectively, are measured by the same measurement technique, so that a measurement error can be restrained. 
     Other Embodiments 
     The present disclosure is not limited to the above embodiments and can be carried out in other various forms without departing from the spirit or main feature of the present disclosure. 
     In the above embodiments, the dome  20  is an inspection target. However, the present disclosure is not limited to this. The pipe  10  and the dome  30  may be inspection targets. 
     Further, in the above embodiments, the present disclosure is applied to a member (the dome  20 ) constituting the liner  1 . However, the present disclosure may be applied to a member other than members constituting the liner  1 , provided that the member is a tubular member made of resin which does not include color pigment and which has a property that allows light to pass through the resin. 
     Further, in the above embodiments, the dome  20  is rotated manually, and the inner and outer light sources  71 ,  73  and so on are moved manually. However, the present disclosure is not limited to this. For example, the guide roller  57  and the first and second slide blocks  61 ,  62  may be configured to be electrically driven, and the rotation of the dome  20  and the movement of the inner and outer light sources  71 ,  73 , and the like may be controlled by a computer. 
     Further, in the above embodiments, the distance L 1  from the inner light source  71  to the inner peripheral surface  21   a  of the dome  20  and the distance L 2  from the outer light source  73  to the outer peripheral surface  21   b  of the dome  20  are set to the same distance L. However, the present disclosure is not limited to this. The distance L 1  and the distance L 2  may be set to different distances. 
     Further, in the above embodiments, at least the inner camera  81  is used. However, in a case where the diameter of the liner  1  is sufficiently large, the present disclosure is not limited to this, and the dimension W 2  of the second projection image  42  may be also measured by the inspector by visual observation. 
     Thus, the above embodiments are just examples in every respect and must not be interpreted restrictively. Further, modifications and alterations belonging to an equivalent range of Claims are all included in the present disclosure. 
     With the present disclosure, it is possible to calculate, with high accuracy, the dimension of a foreign substance embedded in a tubular resin molded product by a safe and simple technique. Accordingly, the present disclosure is extremely useful when the present disclosure is applied to a foreign substance inspection method and a foreign substance inspection apparatus each for inspecting the presence or absence of a foreign substance embedded in a tubular resin molded product and having a dimension larger than a predetermined threshold.