as seen from fig1 to 3 , guiding tube 11 for medical instruments according to one embodiment of the present invention comprises a tubular body opened at opposite ends in the shape of a hollow tube for guiding a rod - like medical instrument such as a catheter and converged at one end in contact with an inner needle , not shown , and enlarged at the other cylindrical base . the tubular body is formed , as shown in fig2 with brittle portions 12a and 12b so made at two parts of the circle of the body , i . e ., at two positions opposite to each other in the radial direction along the length of the tubular body or along the entire length of the body except the opposite end that the splitting strengths are smaller than those of the other circular portion . brittle portions 12a and 12b of the tubular body are formed with extrusion molding machine or extruder 13 as shown , for example , in fig4 and 5 . molding machine 13 comprises a pair of baffles 15a and 15b so positioned oppositely to each other on the inner peripheral surface of inner die 16 and projected oppositely from the inner peripheral surface of die 16 in the radial direction in annular passageway 18 in the vicinity of a nozzle of annular die 14 that the molded parts of the tubular body passing baffles 15a and 15b naturally become brittle portions 12a and 12b . baffles 15a and 15b may be determined at the positions and in the heights properly in relation to the type and the mixture of synthetic resin to be used . when distance l between the end positions of baffles 15a and 15b and the end of die 14 is 0 to 5 mm and distance h between the upper ends of baffles 15a , 15b and the inner wall of die 17 is set to h : h = 0 to 1 / 2 : 1 , preferably 0 to 1 / 5 : 1 to the interval h of resin passageway 18 , brittle portions 12a , 12b having desired splitting strength are provided . when the tubular body of the guiding tube for the medical instrument is molded by the extruder provided with baffles 15a and 15b near the nozzle of die 14 under the above - mentioned conditions , softened resin guided into die 14 by a screw , not shown , is fed in die 14 temporarily substantially split state by baffles 15a , 15b , is again bonded to each other in the nozzle or in the vicinity of the nozzle of die 14 to be molded in a tubular shape . the tubular body thus molded is formed substantially smoothly at a glance in a continuous circular shape , but is formed with welding lines , i . e ., brittle portions 12a , 12b in the portions passing baffles 15a , 15b . since brittle portions 12a , 12b remarkably decrease in bonding strength between synthetic resins of other circular portion , portions 12a , 12b are readily broken by applying an external stress thereto , and are , for example , readily split as shown in fig3 . the splitting strength of brittle portions 12a , 12b may be regulated by adjusting the positions , the heights and the widths ( lengths or thicknesses ) of baffles 15a , 15b or arbitrarily adjusting the mixture of two or more resins . thermoplastic resins such as polypropylene , fluorinated plastics , chlorinated polyethylene , blend polymer of polyolefin resin and ethylene / vinyl acetate copolymer are used properly selectively as the material of the tubular body of the guiding tube for the medical instruments . the case that the extruder in which baffles 15a , 15b are projected on the inner die has been used has been described as shown in fig5 and 6 . however , the present invention is not limited to the particular embodiment . various other changes and modifications may be made within the spirit and scope of the present invention . in the embodiment described above , other arbitrary molding means may be employed . for example , as shown in fig6 and 7 , the same baffles 15a , 15b as those which have been described with reference to fig4 and 5 may be positioned oppositely to the annular die 14 ( in which symbols designate all the same as those in fig4 and 5 ). further , though not shown , similar baffles may be projected oppositely to annular and inner dies . in any case , the positions ( l ), the heights ( h ) and the sizes of the baffles may be regulated and selected similarly to the case of fig4 and 5 . a method of using the guiding tube for the medical instrument of the present invention will be described . guiding tube 11 of the present invention is , for example , mounted to syringe 2 as shown in fig8 with respect to guiding tube 1 , and is then positioned together with the inner needle of syringe within a blood vessel , as shown in fig9 . under this condition , inner needle 3 is withdrawn from the blood vessel , with guiding tube 11 kept attached to blood vessel 4 . then , a catheter is inserted into guiding tube 11 so that catheter is positioned within the blood vessel . then , in withdrawing guiding tube 11 after use from the catheter , an external stress is applied to guiding tube 11 as shown in fig3 to cut the base ends of brittle portions 12a , 12b , and tube 11 is split through the base ends of brittle portions 12a , 12b . then , guiding tube 11 can be readily withdrawn from catheter 5 . in the embodiments described above , the catheter has been guided as the guiding tube for the medical instruments . however , the present invention is not limited to the catheter , but may be applied to the case that all types of rod - shaped medical instruments are positioned within a living body . the above - mentioned brittle portions to be split are not limited to the two linear bodies as in the embodiments described above , but may be provided with one or three or more linear bodies of the brittle portions . the guiding tube for the medical instrument according to the present invention as described above is not necessarily provided with the linear bodies formed of a synthetic resin foreign to the material forming the tubular body of the guiding tube as the conventional art but is molded of a sole plastic composition . thus , the linear body can avoid to be unintentionally cracked during the after - treatment , such as cutting or edge - processing of the guiding tube , the guiding tube can be manufactured readily , leading to a low manufacturing cost of the guiding tube . a pair of baffles having 22 mm long and 0 . 5 wide were provided , as shown in fig7 in contact with the inner wall of the inner die at the annular die of an extruder so that the tip thereof was disposed at the position of 0 . 5 mm before the nozzle of the die ( i . e ., &# 34 ; l &# 34 ;= 0 . 5 mm in fig7 ). then , a guiding tube fitting a 16g inner needle was prepared by polypropylene . even when the tip of the guiding tube was processed in contact with the 16g inner needle , the guiding tube was found to be free of problems such as cracking of the tip portion . also , the pin portion of the guiding tube was free from roughening , cracking , etc . when the guiding tube was stuck in a blood vessel of a dog together with the inner needle . further , the linear body was readily peeled from the tubular body in removing the guiding tube . a catheter guiding tube was prepared by extrusion molding similarly to the example 1 except that the similar baffles to those in the example 1 were disposed at a position of 6 mm before the nozzle of the die . however , in this case , the guiding tube was not substantially molded with brittle portions and could not be split . similar baffles to those of the example 1 were projected on the inner wall of the annular die having 5 mm of inner diameter . however , in this case , the heights of the baffles were separated 0 . 6 mm from the peripheral surface of the inner die having 4 mm of outer diameter , and the ends of the baffles were positioned 0 . 5 mm before the nozzle of the die . when a catheter guiding tube was molded similarly to the example by this extruder , a brittle portion was not substantially molded and the guiding tube could not be split .
a method for molding a tube for guiding a rod - like medical instrument into a living body , which includes a tubular body made of a synthetic resin and having a longitudinal hollow portion to permit the passage of the rod - like medical instrument . the whole of the tubular body is formed to have a chemical uniformity , but at least one brittle portion such as a weld line is longitudinally formed over the entire length of the tubular body .