Abstract:
A net-shaped protective material for wire harness sheathing which has flame retardancy and is free from mesh deformation. Vertical strands ( 2 ) and horizontal strands ( 3 ) which are constituted of a flame-retardant resin wire having an elliptic sectional shape have been pressed and hot melt bonded at the intersections ( 4 ) thereof to form a net shape. The vertical strands and horizontal strands are superposed at the intersections without interlacing, and one or both of the vertical strands and horizontal strands are melted in a state where 40% to 50% of the sectional area has been melted and embedded in the other strands. The protective metal has a cut end which has undergone fraying prevention processing.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a net-shaped protective material for a wire harness to be wired on a vehicle. The net-shaped protective material consists of a sheet or a tube sheathing the peripheral surface of a group of electric wires of the wire harness. 
         [0003]    2. Description of the Related Art 
         [0004]    To bind and protect a group of a plurality of electric wires of a wire harness to be wired on a vehicle and prevent the wire harness from interfering with external interference materials, an adhesive tape is wound around the group of the electric wires or the group of the electric wires are inserted through a tube such as a corrugate tube, a round tube, and the like. 
         [0005]    As a sheathing material consisting of the tape and the tube, a vinyl chloride is generally used for the tape, and polypropylene or the like is used for the corrugate tube. The tape and the tube are made of resin. 
         [0006]    A net tube consisting of resin fibers as proposed in Japanese Utility Model Application Laid-Open No. 4-21119 is used. 
         [0007]    Because a small amount of resin is used for the net tube, the net tube has advantages that it is possible to make the net tube lightweight, produce the cost low, and make the net tube excellent in its stretchability and flexibility. Therefore it is easy to perform an operation of inserting the net tube through the wire harness and bend the net tube along a wiring path of the wire harness. 
         [0008]    As compared with a case in which the wire harness is inserted into a closed space inside the corrugate tube or a cylindrical tube, the net tube has an inferior protection function because the group of the electric wires is exposed to the outside through vacancies of a net. But the net tube can be used in a wiring region in which there is no fear of interference of the net tube with external interference materials. 
         [0009]    As apparent from the foregoing description, the net tube has advantages. But in the conventional net tube, the net is formed by braiding resin fibers such as polyester, polyethylene or polypropylene. Thus the conventional net tube is not excellent in its flame retardance and thus unsuitable as a sheathing material for the wire harness to be wired inside an engine room. 
         [0010]    In addition, in forming the conventional net tube by braiding warps and wefts, the vertical position of the warp and that of the weft at an intersection point of the warp and weft are reversed at an adjacent intersection point thereof. Thus the binding force between the warp and the weft is low at the intersection points. Although the warp and the weft are fused to each other at the intersection point thereof, as shown in  FIG. 10 , the peripheral surface of a warp  100  and that of a weft  101  are fused to each other in line contact. Thus when a pull strength and a tear strength are applied to the intersection point, the warp  100  and the weft  101  peel from each other at the intersection point thereof, i.e., the shape and size of meshes of the net are prevented from being changed. Thus the conventional net tube has a problem that it has a low shape retention force. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention has been made in view of the above-described problem. It is an object of the present invention to provide a net-shaped protective material, consisting of a sheet or a tube for a wire harness to be wired inside an engine room, which can be used as a sheathing material for the wire harness, in which fray is not generated at an intersection point at a cut end, and has a high binding force so that peeling does not peel at the intersection point. 
         [0012]    To solve the above-described problem, the present invention provides a net-shaped protective material for a wire harness in which intersection points of vertical strands and horizontal strands both consisting of identical sectionally elliptic flame-retardant resin fibers are thermally fused to each other under pressure to allow the net-shaped protective material to be net-shaped. A configuration of each of vacancies surrounded with the vertical strands and the horizontal strands is any one of a square, a rectangle, a rhomb, a hexagon or a circular shape consisting of a true circle, an ellipse, and an oblong. A size of each of the vacancies is adjusted according to a demanded degree of stretchability and strength; and the vertical strands and the horizontal strands are vertically overlapped each other at the intersection points without braiding the vertical strands and the horizontal strands. One of the vertical strands and the horizontal strands is melted at 40 to 50% of a sectional area thereof and fused to the other of the vertical strands and the horizontal strands with one of the vertical strands and the horizontal strands being embedded in the other of the vertical strands and the horizontal strands or both of the vertical strands and the horizontal strands are melted at 40 to 50% of a sectional area thereof and fused to each other with the vertical strands and the horizontal strands being embedded in each other. Thus, a cut end of the vertical strands and that of the horizontal strands are prevented from being frayed. 
         [0013]    As described above, resin fibers composing the net-shaped protective material of the present invention consisting of a sheet or a tube are sectionally elliptic and flame-retardant. Thus a wire harness to be wired inside an engine room of a vehicle can be sheathed with the net-shaped protective material. Further at the intersection point of the vertical strand and the horizontal strand, the peripheral surfaces thereof are fused to each other not in line contact, but the vertical strand and the horizontal strand are sectionally elliptic and in addition fused to each other with one of the vertical strand and the horizontal strand being embedded in the other of the vertical strand and the horizontal strand or both of the vertical strand and the horizontal strand are fused to each other with the vertical strand and the horizontal strand being embedded in each other. Therefore the vertical strand and the horizontal strand are firmly fixed to each other. Thus even though a pull strength and a tear strength are applied to the net-shaped protective material, disconnection between the vertical strand and the horizontal strand does not occur at the intersection point thereof and the shape and size of meshes of the net are prevented from being changed. Therefore the net-shaped protective material has a high shape retention force and is thus highly reliable. 
         [0014]    The flame-retardant resin fibers are composed of not less than one kind of a halogen-free resin component selected from among mixtures each consisting of not less than two kinds of polypropylene, polyester, polyethylene terephthalate, polyamide, polyphenylene ether, polybutylene terephthalate, and polyphenylene sulfide and a flame retardant, added to the halogen-free resin component, which is selected from among a bromine-based flame retardant, phosphates, metal hydroxides, and melamine. 
         [0015]    As the resin component, the polypropylene is most favorably used because it can be processed easily and is inexpensive. 
         [0016]    In detail, in the net-shaped protective material, 0.5 to 100 parts by mass of the flame retardant is added to 100 parts by mass of the resin component. If necessary, 0 to 50 parts by mass of a filler consisting of magnesium oxide or/and calcium carbonate is added to 100 parts by mass of the resin component. 
         [0017]    The vertical strand and the horizontal strand of the present invention are braided such that the vertical position of the vertical strand and that of the horizontal strand at an intersection point of the warp and weft are not reversed at an adjacent intersection point thereof, but are molded into the net-shaped sheet by a sizing machine which is described later. For example, the vertical strand and the horizontal strand are overlapped on each other with the vertical strand disposed at the upper side and the horizontal strand disposed at the lower side and in surface contact at the intersection point thereof. Further the vertical strand and the horizontal strand are fused to each other with both strands being embedded into each other, as described above. 
         [0018]    The net-shaped protective material of the present invention consisting of the vertical strands and the horizontal strands is formed as a tube or a sheet. 
         [0019]    The sheet is so bent that it has an undeformable cylindrical configuration to overlap both ends thereof in a width direction thereof on each other. 
         [0020]    The tube is formed by thermally fusing both edges of one sheet in the width direction thereof to each other. 
         [0021]    Regarding the strength of the net-shaped protective material of the present invention, when the major axis of each of the sectionally elliptic vertical strand and horizontal strand is 0.35 mm to 0.5 mm and a thickness of a net-shaped sheet is 0.4 mm to 0.8 mm, a tensile strength of the net-shaped sheet in a vertical direction and a horizontal direction, an elongation thereof, and a tear strength thereof are set to not less than 15.7N, not less than 15.7N, and not less than 150% respectively. The tensile strength of the net-shaped protective material in its vertical and horizontal directions is a value measured by using JIS No. 3 Dumbbell. 
         [0022]    The configuration of each of the vacancies surrounded with the vertical strands and the horizontal strands is set to any one of a square, a rectangle, a rhomb, a hexagon or a circular shape consisting of a true circle, an ellipse, and an oblong. 
         [0023]    The configuration of the vacancy is adjusted according to a demanded degree of stretchability and strength of the net-shaped protective material. 
         [0024]    That is, regarding the size of the vacancies, when a high stretchability is demanded, the size of the mesh of the net is set large to form large vacancies. When a high strength and wear resistance are demanded, the size of the mesh of the net is set small to form small vacancies. Regarding the configuration of the vacancy, to allow the net to be stretchy, the vacancy is set to a rhombic configuration, whereas to allow the net to be unlikely to stretch, the vacancy is set to a square or a rectangle. 
         [0025]    As vertical strands and horizontal strands, it is possible to form large-diameter strands and small-diameter strands respectively, arrange the large-diameter strands and the small-diameter strands by sandwiching a plurality of the small-diameter strands between the large-diameter strands, and dispose net-shaped portions formed with the small-diameter strands in vacancies surrounded with the large-diameter strands. 
         [0026]    The net-shaped protective material may have a selvage, in which vacancies are not formed, disposed at both ends in the longitudinal direction thereof to allow an adhesive tape to be easily wound around a group of electric wires of a wire harness and around both ends of the net-shaped protective material in its longitudinal direction. 
         [0027]    Further, in the net-shaped protective material of the present invention, the vertical strands and the horizontal strands are molded into a net-shaped sheet by using a sizing machine; the net-shaped sheet is heated and pressurized when the molding is carried out; and intersection points of the vertical strands and the horizontal strands are fused to each other under a pressurized state; and the net-shaped sheet is cut to a required length; and the cut net-shaped sheet is so bent that the net-shaped sheet has an undeformable cylindrical configuration to overlap both edges of the cut net-shaped sheet in a width direction thereof on each other. 
         [0028]    As described above, the sectionally elliptic vertical strands and horizontal strands are molded into the net-shaped sheet by passing them through dies of the sizing machine. Because the vertical strands and the horizontal strands are heated and pressurized in a molding operation, the vertical strands and the horizontal strands make a surface contact at the intersection point thereof and are fused to each other with one of the vertical strand and the horizontal strand being penetrated into the other of the vertical strand and the horizontal strand or both of the vertical strands and the horizontal strands are fused to each other with the vertical strands and the horizontal strands being embedded in each other. Thereby it is possible to enhance a connection force at the intersection point. Thus even though a pull strength and a tear strength are applied to the net-shaped sheet, the vertical strand and the horizontal strand is prevented from peeling from each other at the intersection point. 
         [0029]    As described above, the net-shaped sheet so bent that the net-shaped sheet has the undeformable cylindrical configuration can be easily wound around the peripheral surface of a group of electric wires of a wire harness. In a state in which the net-shaped sheet wound around the peripheral surface of the group of the electric wires, the adhesive tape is wound around both ends of the net-shaped sheet in its longitudinal direction to fix the adhesive tape and the group of the electric wires to each other. 
         [0030]    In forming the net-shaped tube, both overlapped ends of the net-shaped sheet are thermally fused to each other. 
         [0031]    As described above, in the net-shaped protective material, of the present invention for the wire harness, consisting of the sheet or the tube, the vertical strand and the horizontal strand consisting of the sectionally elliptic and flame-retardant resin fibers are fused to each other at the intersection point thereof. Thus a wire harness to be wired inside an engine room of a vehicle can be sheathed with the net-shaped protective material. Further at the intersection point of the vertical strand and the horizontal strand, the peripheral surfaces thereof are fused to each other not in line contact, but the sectionally elliptic vertical strands and horizontal strands make a surface contact and in addition fused to each other with one of the vertical strand and the horizontal strand being embedded in the other of the vertical strand and the horizontal or both of the vertical strands and the horizontal strands are fused to each other with the vertical strands and the horizontal strands being embedded in each other. Therefore the vertical strand and the horizontal strand are firmly fixed to each other. Thus even though a pull strength and a tear strength are applied to the net-shaped protective material, the vertical strand and the horizontal strand are prevented from peeling from each other at the intersection point, and the shape and size of meshes of the net are prevented from being changed. Therefore the net-shaped protective material has a high shape retention force. 
         [0032]    Further because the net-shaped protective material is net-shaped, it is possible to reduce the weight of the net-shaped protective material by not more than half the weight of a conventional round tube made of resin. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  shows a net-shaped sheet of the present invention, in which  1 (A) is a perspective view, and  1 (B) is a partly enlarged view. 
           [0034]      FIG. 2(A)  is a perspective view showing a flame-retardant resin fiber to be formed as a vertical strand and a horizontal strand, FIG.  2 (B) is a perspective view showing a state in which the vertical strands and the horizontal strands are overlapped each other, and  2 (C) is an enlarged view of an intersection point of the vertical strand and the horizontal strand. 
           [0035]      FIGS. 3(A) , (B), and (C) show a state in which a wire harness is sheathed with the net-shaped sheet. 
           [0036]      FIGS. 4(A) and 4(B)  show a method for producing the net-shaped sheet. 
           [0037]      FIGS. 5(A) ,  5 (B) show the process of shaping a net-shaped sheet into a circular-arc sheet. 
           [0038]      FIG. 6  is an explanatory view for comparing the size of a vacancy of a net and the function of the vacancy thereof with each other. 
           [0039]      FIGS. 7(A) through 7(H)  show the configurations of meshes of a net tube. 
           [0040]      FIGS. 8(A) and 8(B)  show a test method. 
           [0041]      FIGS. 9(A) ,  9 (B), and  9 (C) show an embodiment of the net-shaped tube. 
           [0042]      FIG. 10  shows a problem of a conventional net tube. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    An embodiment of the present invention is described below with reference to drawings. 
         [0044]    A net-shaped protective material of the embodiment shown in  FIG. 1  is a sheet. The sheet  1  is net-shaped by fusing sectionally elliptic vertical strands  2  and horizontal strands  3  to each other at intersection points  4  thereof to form rhombic vacancies  5  surrounded with the vertical strands  2  and the horizontal strands  3 . Because the vacancies  5  are rhombic, the vertical strands  2  are not parallel with a longitudinal direction (axis direction) L of the sheet  1 , and the horizontal strands  3  are not orthogonal to the axial direction L, but both the vertical strands  2  and the horizontal strands  3  are in a tilt direction to the axial direction L. Strands are distinguishably named the vertical strand  2  and the horizontal strand  3 . 
         [0045]    Both the vertical strands  2  and the horizontal strands  3  consist of identical flame-retardant resin fibers  10  and are elliptic in section, as described above. 
         [0046]    In detail, the flame-retardant resin fiber  10  is composed of not less than one kind of a halogen-free resin component selected from among mixtures each consisting of not less than two kinds of polypropylene, polyester, polyethylene terephthalate, polyamide, polyphenylene ether, polybutylene terephthalate, and polyphenylene sulfide and a flame retardant, added to the halogen-free resin component, which is selected from among a bromine-based flame retardant, phosphates, metal hydroxides, and melamine. 
         [0047]    0.5 to 100 parts by mass of the flame retardant is added to 100 parts by mass of the resin component. If necessary 0 to 50 parts by mass of a filler consisting of magnesium oxide or/and calcium carbonate is added to 100 parts by mass of the resin component. 
         [0048]    In this embodiment, the polypropylene (PP) is used as the resin component, and the bromine-based flame retardant consisting of tetrabromobisphenol is used as the flame retardant. 
         [0049]    The vertical strands  2  and the horizontal strands  3  are not braided, but fused to each other at intersection points  4  by layering the vertical strands  2  on the horizontal strands  3  with the vertical strands  2  being disposed at an upper side and the horizontal strands  3  being disposed at an upper side. 
         [0050]    As shown in  FIG. 2(C) , the sectionally elliptic vertical strands  2  disposed at the upper side and the sectionally elliptic horizontal strands  3  disposed at the lower side make surface contact at the intersection points  4  and are fused to each other with a portion  3   m , of the lower-side horizontal strand  3 , which occupies 40 to 50% of the sectional area thereof being penetrated into the upper-side vertical strands  2 . 
         [0051]    The major axis of each of the vertical strand  2  and the horizontal strand  3  is set to 0.35 mm to 0.5 mm. The thickness of the net-shaped sheet  1  composed of the vertical strands  2  and the horizontal strands  3  is set to 0.4 mm to 0.8 mm. 
         [0052]    In the net-shaped sheet  1 , the length of the vertical strand  2  and that of the horizontal strand  3  surrounding the rhombic vacancy  5  are set to 1.4 mm×1.4 mm in this embodiment. 
         [0053]    The tensile strength, tear strength, and tensile elongation of the sheet  1  in the vertical direction (axial direction) and in the horizontal direction are set to not less than 15.7N, not less than 15.7N, and 180 to 230% respectively. 
         [0054]    As shown in  FIG. 3 , by using a method described later, the net-shaped sheet  1  is so bent in advance that the net-shaped sheet  1  has an undeformable cylindrical configuration to overlap both ends thereof in the width direction thereof orthogonal to the longitudinal direction thereof, namely, the axial direction L thereof on each other. 
         [0055]    At the process of mounting the sheet  1  on a wire harness  20 , the sheet  1  is disposed along the longitudinal direction of a group W of electric wires of the wire harness  20 . In this state, the bent sheet  1  having the undeformable cylindrical configuration is wound around the peripheral surface of the group W of the electric wires. In this state, both ends of the net-shaped sheet  1  in the width direction thereof are overlapped each other. Thus the net-shaped sheet  1  is capable of completely covering the entire peripheral surface of the group W of the electric wires. Thereafter an adhesive tape T is wound around both ends of the sheet  1  in its longitudinal direction and the group W of the electric wires drawn out of the sheet  1  to fix the adhesive tape T and the sheet  1  as well as the electric wires to each other. 
         [0056]    The method for producing the net-shaped sheet  1  is described below. 
         [0057]    The sheet  1  is formed by using a sizing machine shown in  FIGS. 4(A) and 4(B) . In detail, after the resin component, the flame retardant, a stabilizer, and the like are supplied to a hopper  25 , they are stirred to mix them with one another. A mixture obtained by stirring and mixing the components is transported to a die set  27  with the mixture being kneaded by a screw conveyor  26 . The die set  27  molds the kneaded material into the net-shaped sheet consisting of the flame-retardant resin fiber  10 . 
         [0058]    The die set  27  is constructed of an outer die  27   a  and an inner die  27   b . The outer die  27   a  and the inner die  27   b  are rotated in opposite directions by a motor  29 . An intersection portion (intersection point) where the vertical strand  2  and the horizontal strand  3  overlap on each other is formed at a portion  27   c  where a hole of the outer die  27   a  and a hole of the inner die  27   b  overlap on each other. When the hole of the outer die  27   a  and that of the inner die  27   b  move apart, a grating shape (rhombic shape in this embodiment) is formed. 
         [0059]    When the net-shaped sheet is extruded from the die set  27 , the vertical strand  2  and the horizontal strand  3  are heated and pressurized to fuse them to each other. Because the vertical strand  2  and the horizontal strand  3  are heated and pressurized, both are thermally fused to each other with the vertical strand  2  and the horizontal strand  3  being penetrated into each other at the intersection point  4  of the vertical strand  2  and the horizontal strand  3 , as shown in  FIG. 2  (C). 
         [0060]    Thereafter a net-shaped sheet  40  is transported to a cooling bath  31  and thermally stretched by a draw roll  32 . Thereafter the net-shaped sheet  40  is transported to a stretching bath  33  and wound in a coil  41 . 
         [0061]    The net-shaped sheet  40  is so bent that the net-shaped sheet  40  has an undeformable cylindrical configuration to overlap both ends thereof in the width direction thereof. 
         [0062]      FIG. 5(A)  shows a method of producing a sheet  1 -A having a predetermined length from the net-shaped sheet  40 .  FIG. 5(B)  shows a method of producing a continuous sheet  1 -B from the net-shaped sheet  40 . 
         [0063]    In the method shown in  FIG. 5(A) , the uncoiled sheet  40  is cut to a predetermined length by a sheet-cutting machine (not shown) to obtain a cut sheet  42 . Thereafter the cut sheet  42  is passed through a heated roll  43  to obtain a cut sheet  42 C so bent that an obtained cut sheet  40  has an undeformable circular arc configuration. 
         [0064]    In the method shown in  FIG. 5(B) , a conic cylindrical molding machine  45  is used, and pull rolls  46 A and  46 B are disposed upstream and downstream from the molding machine  45  respectively. The sheet  40  is passed through the molding machine  45  from the roll  46 A to shape the sheet  40  into an undeformable circular arc configuration inside the molding machine  45  so that both ends of sheet  40  in its width direction overlap on each other at a small-diameter portion of the molding machine  45  disposed at the rear portion thereof. The sheet  40  is drawn out by a roll  46 B in this state. In this manner, the continuous sheet  1 -B is produced. The sheet  1 -B is cut to a required length by a cutting machine  47  in dependence on a use condition. 
         [0065]    As shown in  FIG. 2(C) , in the net-shaped sheet  1  having the above-described construction, at the intersection point  4  of the vertical strand  2  and the horizontal strand  3 , the vertical strand  2  and the horizontal strand  3  are deformed sectionally elliptically and thermally fused to each other with the vertical strand  2  and the horizontal strand  3  being penetrated into each other. Therefore unlike a case in which the peripheral surface of the vertical strand and that of the horizontal strand are welded to each other, the vertical strand  2  and the horizontal strand  3  are unlikely to peel from each other. Therefore the net-shaped sheet  1  has the above-described degree of tensile strength and tear strength and the shape and size of meshes of the net are prevented from being changed and is thus reliable. 
         [0066]    Because the vertical strand  2  and the horizontal strand  3  are formed from the flame-retardant resin fiber  10 , the net-shaped sheet  1  has flame retardance and thus can be used as a sheathing material for the wire harness to be wired inside the engine room. Further because the vertical strand  2  and the horizontal strand  3  are firmly fixed to each other, each of the vertical strand  2  and the horizontal strand  3  do not generate fray at a cut end thereof. Furthermore because the sheet  1  is net-shaped, the weight thereof can be reduced to half the weight of a round tube. Particularly by using the sheet  1  as a sheathing material for a large number of wire harnesses to be wired on a car, the sheet  1  is capable of contributing to a decrease of the weight of the car and thus decreasing fuel consumption. 
         [0067]    In the net-shaped sheet  1  having the above-described construction, by adjusting the position of the intersection point  4  of the vertical strand  2  and the horizontal strand  3 , it is possible to arbitrarily alter the size of the vacancy  5  and the configuration thereof. 
         [0068]      FIG. 6  shows a coarse net N-1 having large vacancies  5  and a fine net N-2 having small vacancies  5 . Regarding the relationship between the size of the vacancy  5  and the stretchability, wear resistance, and strength of the net-shaped sheet  1 , as shown in  FIG. 6 , the coarse net N-1 has a high stretchability, but has a low wear resistance and strength. The fine net N-2 has a low stretchability, but has a high wear resistance and strength. 
         [0069]    As apparent from the foregoing description, in dependence on a use condition of the wire harness, the net tube  1  is allowed to have a demanded degree of elongation and strength by adjusting the size of the vacancy  5 . 
         [0070]    The vacancy  5  can be shaped as shown in  FIGS. 7(A) through 7(H) . 
         [0071]    The net tube  1  shown in  FIG. 7(A)  has rectangular meshes so that the net tube  1  is unlikely to stretch. 
         [0072]    The net tube  1  shown in  FIG. 7  (B) has rhombic meshes so that the net tube  1  is stretchy. 
         [0073]    The net tube  1  shown in  FIG. 7  (C) has hexagonal meshes so that the net tube  1  has a strength higher than that of the net tube  1  of (B) and has a stretchability a little lower than that of the net tube  1  of (B). 
         [0074]    The net tube  1  shown in  FIG. 7  (D) has circular meshes so that the net tube  1  has a strength higher than that of the net tube  1  of (C) and does not have stretchability. 
         [0075]    The net tube  1  shown in  FIG. 7  (E) has narrow rhombic meshes obtained by stretching the vertical strands  2  and the horizontal strands  3 . Although the net tube  1  has a low strength, it has an excellent stretchability. 
         [0076]    In the net tube  1  shown in  FIG. 7  (F), as the vertical strands and the horizontal strands, large-diameter strands  2   a  and  3   a  and small-diameter strands  2   b  and  3   b  are formed respectively. The large-diameter strands  2   a  and  3   a  and the small-diameter strands  2   b  and  3   b  are arranged by sandwiching a plurality of the small-diameter strands  2   b  and  3   b  between the large-diameter strands  2   a  and  3   a . Net-shaped portions formed with the small-diameter strands  2   b  and  3   b  are disposed in vacancies surrounded with the large-diameter strands  2   a  and  3   a . The net tube  1  has a strength higher than that of the net tube  1  of (D) and stretchability to some extent. 
         [0077]    The net tube  1  shown in  FIG. 7  (G) has rectangular meshes. The net tube  1  has a selvage, having a solid portion NS, which is formed at both ends of the net tube  1  in its longitudinal direction to allow an adhesive tape to be easily wound around a group of electric wires and the front end of the net tube  1 . The solid portion is formed by heating resin fibers to melt them and immersing them in a cooling bath to solidify them. 
         [0078]    The net tube  1  shown in  FIG. 7  (H) has rhombic meshes, and the net tube  1  has a selvage. 
         [0079]    As apparent from the above description, the configuration of the vacancy  5  can be easily adjusted according to a demand for a stretchy net tube or a net tube unlikely to stretch. 
         [0080]    Comparison between physical properties of the net-shaped sheets of the examples and those of net-shaped sheets of the comparison examples is described below. 
       Examples 
       [0081]    In examples 1, 2, and 3, four parts by mass of a bromine-based flame retardant was added to 100 parts by mass of polypropylene to form vertical strands and horizontal strands. At the intersection point of the vertical strand and the horizontal strand, as described in the embodiment, the vertical strand and the horizontal strand were thermally fused to each other with 40 to 50% of the sectional area of the horizontal strand being penetrated into the vertical strand to obtain a net-shaped sheet having rhombic meshes. 
         [0082]    The lengths of sides of vacancies of the examples 1, 2, and 3 were different from each other, as shown in table 1. 
       Comparison Examples 
       [0083]    In the net tube of the comparison example 1, the sectionally circular vertical strand and horizontal strand were made of the polypropylene, and the surface of the vertical strand and that of the horizontal strand at an intersection point thereof were fused to each other in line contact. The net-shaped sheet had rhombic meshes similarly to the net-shaped sheets of the examples. 
         [0084]    In a tube of the comparison example 2, the vertical strand and the horizontal strand were made of polyester and woven densely. 
         [0085]    A tape of the comparison example 3 was a vinyl chloride tape generally used to be wound around a wire harness. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Target value 
                 Comparison example 
                 Example 1 
                 Example 2 
                 Example 3 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Thickness 
                 0.65 
                 0.4 
                 0.6~0.8 
                 0.7~0.8 
                 0.8~0.9 
               
               
                 (mm) 
               
               
                 Open size of strand 
                 1.1 × 1.1 mm 
                 1.7 × 1.7 
                 1.4 × 1.4 
                 1.2 × 1.2 
                 1.0 × 0.9 
               
               
                 (mm) 
               
               
                 Flame retardance 
                 No less than 23.5 
                 Non-flame-retardant (17) 
                 Flame-retardant (22) 
                 Flame-retardant (24.5) 
                 Flame-retardant (24.5) 
               
               
                 (OI value) 
               
               
                 Material 
                 Flame-retardant PP 
                 PP 
                 Flame-retardant PP 
                 Flame-retardant PP 
                 Flame-retardant PP 
               
               
                 Tensile strength 
                 No less than 15.7 
                 2.8 
                  7.9~10.1 
                 12~18 
                 21 (0.8t) 
               
               
                 (N) Horizontal 
               
               
                 JIS No. 3 Dumbbell 
               
               
                 measured value 
               
               
                 Tensile strength 
                 No less than 15.7 
                 2.4 
                 4~7 
                 3~9 
                 14 (0.8t) 
               
               
                 (N) vertical 
               
               
                 JIS No. 3 Dumbbell 
               
               
                 measured value 
               
               
                 Tensile elongation 
                 No less than 150 
                 210    
                 180~230 
                 140~160 
                 150~213 
               
               
                 (%) 
               
               
                 Wear test of tape 
                 No less than 1000 
                 500~1000 
                  670~3080 
                 2250~4420 
                 3200~6100 
               
               
                 (mm) 
               
               
                   
               
             
          
         
       
     
         [0086]    The wear volumes of tapes shown in table 1 were measured by using a test apparatus shown in  FIG. 8 . 
         [0087]    In the test apparatus, an auxiliary weight  31  was mounted on a supporting bar  30 , and a bracket  32  is disposed below the auxiliary weight  31 . The bracket  32  was coupled to the front end of a cantilevered shaft arm  33 . A test tape  40  was fixed to a horizontal supporting bar  34 . In this state, a wear tape  36  consisting of 150 A sand paper was moved in a direction shown with arrows with the wear tape  36  being held by a supporting pin  35  and sliding the test tape  40 . The wear volume of the test tape  40  was measured at a portion thereof where the wear tape  36  slid. 
         [0088]    The tear strength shown in table 1 was measured as follows: As shown in  FIG. 8(B) , a slit  41   a  having a length of 25 mm was formed from the center of one side of a test tape  41  having vertical and horizontal lengths of 50 mm. The test tape  41  was pulled in left-ward and right-ward directions by setting the slit  41   a  as the boundary. 
         [0089]    As shown in table 1, the test net-shaped sheets of the examples 1, 2, and 3 were excellent in the flame retardance (OI value) thereof. The OI values of the test net-shaped sheets of the examples 2 and 3 were more than the target value of 23.5. 
         [0090]    The test net-shaped sheets of the examples 1, 2, and 3 had a tensile strength 3 to 10 times higher than the test net tube of the comparison example 1 in which intersection points of the vertical strands and the horizontal strands were fused to each other in line contact and tear strengths higher than the test net tube of the comparison example 1. In the wear test, the test net-shaped sheets of the examples 1, 2, and 3 had wear volumes not less than the target value of 1000, which proved that they were excellent in the wear resistance thereof. 
         [0091]    In the above-described embodiment, the net-shaped sheet  1  is shaped into a circular arc configuration such that both ends in its width direction overlap each other. Besides, as shown in  FIG. 9(A) , overlapped both ends of the net-shaped sheet  1  may be thermally fused to each other to form a tube  50 . 
         [0092]    As shown in  FIG. 9(B) , after a wire harness  20  is inserted through the contracted net-shaped tube  50 , the net-shaped tube  50  is stretched as shown in  FIG. 9(C)  to fix a group of electric wires to an adhesive tape T.