referring now to fig1 there is shown a schematic view of a typical extruder apparatus 10 that can be used in carrying out the present invention . as is conventional in the extrusion process , there is a hopper 12 that contains a reservoir of the material 14 to be extruded . the material from the hopper 12 descends downwardly via a gravity feed into the extruder barrel 16 where the material is progressed forwardly by a spiral screw 18 through a breaker plate 20 . a gear pump 22 may be used to force the material 12 through the extrusion head 26 where the extruded form , such as a tubing , emerges from the die face 28 , that is , the end of the extrusion head 26 and which is in a plane that is generally at a right angle to the flow of the extruded material . depending upon the particular material being extruded , the material may be heated to flowing temperatures if the material is a thermoplastic material or , alternatively , if a thermosetting material , the material can be cooled as extruded . in any event , after the tubing has been extruded from the extrusion head 26 , there is a water bath 30 through which the extruded tubing is drawn by means of a puller 32 so that the extruded tubing is cooled to achieve a permanent set . finally , the extruded tubing passes by a cutter 33 that cuts the length of tubing desired by the user . as will be later explained , there is also present a pressure system to establish and maintain a pressure in the interior of the extruded tubing so that the tubing does not collapse during the progress of the tubing as it passes through the water bath 30 until the tubing is sufficiently rigid to maintain itself . there also may be a laser 34 or other measuring device to measure the outside dimensions of the tubing . turning now to fig2 there is shown a cross - sectional view of a typical prior art extrusion head 36 . as can be seen , the extrusion head 36 includes a crosshead body 38 having a helicoid 40 which helps to direct the flow of the material . a mandrel 42 fits into the helicoid 40 and a die 44 and the mandrel 42 extend outwardly from the crosshead body 38 to terminate at a die face 46 which is a face that is formed in a plane that is generally at a right angle to the flow of an extrudable material from the die face 46 . the material to be extruded enters into the crosshead body 38 by means of a flow inlet 48 and passes in the space 50 between the mandrel 42 and the die 44 , such that the outer surface of the extruded material is established by the inner surface of the die 44 and the inner surface of the extruded material is established by the exterior surface of the mandrel 42 . accordingly , the relative location of the die 44 and the mandrel 42 , governs the inner and outer surfaces of the extruded material . if the material is being extruded into , for example , a hollow , cylindrical tube , the outer diameter of the tubing is established by the inner surface of the die 44 and the inner diameter of the tubing is determined by the outer diameter of the mandrel 42 . obviously , by having the mandrel 42 or the die 44 of differing configurations , the outer peripheral configuration of the tubing can be square , rectangular , oval , hexagonal or other desired peripheral configuration . in addition , the wall thickness of the extruded tubing is determined by the space between the die 44 and mandrel 42 at the die face 46 . turning now to fig3 - 5 , there are shown cross - sectional views of an extrusion head 52 constructed in accordance with the present invention . taking first fig3 it can be seen that the mandrel 54 is a specially designed mandrel and comprises a core 56 that can be mounted so as to be in a fixed position relative to the crosshead body 38 . other of the components shown in fig3 - 5 are the same as described with respect to fig2 and have been given the same identification numbers as they carry out the same functions in the same manner . therefore , the components and features include the crosshead body 38 , helicord 40 , die 44 , die face 46 , flow inlet 48 and a space 50 . as will be seen , the space 50 does have a changing function in accordance with the present invention as does the specially constructed mandrel 54 . as such , in fig3 the mandrel 54 comprises a core 56 that can be non - movably mounted within the crosshead body 38 and has a distal end 57 that extends to the die face 46 of the extrusion head 52 and therefore establishes the internal diameter of a tubing extruded from the die face 46 . since the core 56 is fixed with respect to the crosshead body 54 , it will be seen that the core 56 establishes the smallest internal dimensions for the hollow tubing extruded therefrom , and , of course , if the tubing is circular , the smallest dimension will be the smallest inner diameter . as will also be seen , however , in fig3 there are a pair of sleeves , namely , an inner sleeve 58 and an outer sleeve 60 that telescope over the core 56 and can slide thereover . the inner sleeve 58 has a smaller outer dimension than the outer sleeve 60 in order to provide the telescoping relationship and , again , the sleeves 58 , 60 may have any of a wide variety of outer configurations and shapes and sizes depending upon the particular tubing being extruded . in addition , as will now be appreciated , while only two sleeves 58 , 60 are shown in fig3 - 5 , there can be a lesser number or greater number of such sleeves in the spirit of the present invention . in any event , the inner sleeve 58 slides along the outer surface of the core 56 and , correspondingly , the outer sleeve 60 slides along the outer surface of the inner sleeve 58 in a telescoping arrangement . as shown in fig3 both of the sleeves 58 , 60 have their distal ends 62 , 64 respectively , displaced away from the die face 46 so that , in the positions shown in fig3 the sleeves 58 , 60 do not have any influence upon the inner dimensions of the extruded tubing . in fig4 however , it can be seen that the inner sleeve 58 has been moved from the retracted or displaced position of fig3 to an extended position where the distal end 62 of the inner sleeve 58 is located at the die face 46 and therefore does determine the inner dimensions of the extruded tubing . as can be understood , by moving the inner sleeve 58 from its retracted position of fig3 to the extended position of fig4 the inner dimension of the extruded tubing , that is , the inner diameter if a circular tubing is being produced , is increased . accordingly , since the outer dimensions of the extruded tubing has not changed , the wall thickness of the extruded tubing has consequently been reduced . the reduction of the wall thickness can be abrupt such as when the inner sleeve 58 is moved rapidly from its retracted position to the extended position , or the change in the wall thickness can be gradual where the inner sleeve 58 is moved at a slower speed from the retracted to the extended positions . thus , the slope of the changes in the wall thickness can be controlled by the user in accordance with the desired end product tubing profile by controlling the speed of movement of the inner sleeve 58 between its retracted and extended positions and the reverse is equally true , when the inner sleeve 58 is moved back from its extended position to its retracted position of fig3 so that the wall thickness can again be increased at whatever slope is desired by the user in creating the end product . taking now fig5 there is shown a cross - sectional view of the extrusion head 52 shown with the outer sleeve 60 in its extended position such that the distal end 64 of the outer sleeve 60 is in the die face 46 and therefore the inner dimensions of the extruded tubing are established by the outer dimensions of the outer sleeve 60 , the outer dimensions of the extruded tubing still being the same as the inner dimensions of the die 44 . therefore , the wall thickness of the extruded tubing has again been thinned over the fig2 positions of the sleeves 58 , 60 at the control of the user and , again , the control over the rate of the advancement or retraction of the outer sleeve 60 can be used to shape the inner profile of the extruded tubing . in the movement of the inner and outer sleeves 58 , 60 , it should be noted that the movement of the sleeves 58 , 60 should be carried out in a sequential manner , that is , the inner sleeve 58 is advanced to the extended position first and then as it is desired to further narrow the wall thickness of the extruded tubing , the outer sleeve 60 can then be advanced and the same is true of the withdrawal of the sleeves 58 , 60 back to their retracted positions , i , e , the outer sleeve 60 is retracted first and then the inner sleeve 58 is retracted so that the sleeves 58 , 60 are moved in a sequential order determined by the outer dimensions of the particular sleeve with the smaller of the sleeves being first moved from the retracted to the extended position and the larger sleeve being moved first from the extended position to the retracted position . the sequence of motion is carried out even if there are more than two sleeves and would be applicable to any number of multiple sleeves used with the present invention . turning now to fig6 a , 6b and 6 c , there are shown , schematic views illustrating the use of multiple or a plurality of sleeves in accordance with the present invention . in fig6 a the core 56 is shown and is the innermost component of the mandrel 54 of fig3 - 5 . as also can be seen there is a opening 66 that passes through the core 56 and which will be later explained for its use to produce a pressure within an extruded tubing to prevent the tubing from collapsing . in fig6 b , the inner sleeve 58 is shown positioned in telescoping arrangement over the core 56 and the distal end 62 of the inner sleeve 58 is located at the die face 46 where the outer dimensions of the inner sleeve 58 determine the inner dimensions of the extruded tubing ( fig5 ). next , in fig6 c , there is shown the core 56 , the inner sleeve 58 and the outer sleeve 60 of the extrusion head 52 , all telescoped together with the inner and outer sleeves 58 , 60 in the extended positions with the distal ends 62 and 64 , respectively , located at the die face 46 ( fig5 ) so that the inner dimensions of the extruded tubing is determined by the outer dimensions of the outer sleeve 60 . turning next to fig7 there is shown a schematic view of a set up for the present invention and basically illustrates the components of the mandrel 54 . the core 56 is held in a fixed position within the extrusion head and that mounting is schematically shown by the clamp 68 . there is also a gas fitting 70 at the proximal end 72 of the core 56 and , as explained , the gas fitting 70 is adapted to be affixed to a regulated and controlled source of gas to establish and maintain a controlled gas pressure at the distal end 74 of the core 56 . thus , there is also a gas pressure controller 76 that is used to control the pressure of the gas at the distal end 74 of the core 56 . that gas pressure controller 76 can operate off various inputs , one of which being the location of the inner and outer cylinders 58 , 60 as will be explained . turning finally to fig8 there is shown a schematic view of the inner and outer sleeves 58 , 60 and also showing the motive means or mechanism that can be used to move each of the sleeves 58 , 60 between their extended and retracted positions . thus , taking the outer sleeve 60 first , there can be a clamp 78 that affixes the outer sleeve 60 to a bracket 80 and that bracket 80 moved by a first motive means 82 . as such , the first motive means 82 can be a linear motor 84 that is connected to the bracket 80 and moved by a lead screw 86 . the first motive means 82 can be any means of providing motion to the bracket 80 and , of course , the outer sleeve 60 , and can include a pneumatic system , a hydraulic system or some other type of motive means or system that is effective to slide the outer sleeve 60 between the retracted and extended positions . in a similar manner , there is a second motive means 88 that moves the inner sleeve 58 in the same manner as the first motive means 82 and , again , may comprise a linear motor 90 that acts through a lead screw 92 to move the inner sleeve 58 by moving bracket 94 affixed to the inner sleeve 58 by a clamp 96 . therefore , as can be seen , the first motive means 82 and the second motive means 88 serve to move , individually , the inner and outer sleeves 58 , 60 and each can be operated independently of the other and can move at different speeds depending upon the desired internal profile of an extruded tubing . as noted , there is also a control system to establish and maintain a predetermined pressure within the tubing being extruded and that pressure is controlled by the gas pressure controller 78 . the purpose of the gas pressure is to prevent the newly extruded tubing from collapsing while it is being cooled and eventually set and thereby strengthened . in the preferred embodiment , the gas pressure is established and then changed in accordance with the wall thickness of the extruded tubing , thus , as the mandrel dimensions get larger , the wall thickness becomes smaller and additional pressure is needed to maintain the wall tubing and prevent its collapse . the reverse is , of course , also true for the wall thickness as it gets larger , that is , less gas pressure is required within the larger wall thickness extruded tubing . therefore , there may be a feedback system that changes the set gas pressure established by the gas pressure controller 78 based upon the outer dimensions of the mandrel 54 ( fig3 - 5 ). accordingly , one such means of providing the necessary feedback can be by means of position sensors 98 , 100 that sense the position of the inner and outer sleeves 58 , 60 respectively so that the gas pressure controller 76 can receive the information via communication lines 104 and adjust the gas pressure accordingly . as an example , when the inner sleeve 58 is moved to its extended position , the position sensor 98 can sense that the inner sleeve 58 has reached its extended position and therefore recognizes that the wall thickness of the extruded tubing has been narrowed . that information is provided to the gas pressure controller via the communication line 104 so that the pressure is then raised . obviously , the pressure can be lowered if the wall thickness is again thickened or increased by the reverse process . in the event some mechanism other than sleeves is used to change the dimensions of the mandrel 54 ( fig3 - 5 ), other means can be used to change the gas pressure within the extruded tubing based upon the outer dimensions of the mandrel and / or the wall thickness of the extruded tubing . those skilled in the art will readily recognize numerous adaptations and modifications which can be made to the extruder and method of extruding tubing of the present invention which will result in an improvement , yet all of which will fall within the scope and spirit of the present as defined in the following claims . accordingly , the invention is to be limited only by the following claims and their equivalents .
an extruder apparatus and method for producing an extruded tubing having varying inner dimensions without changing the outer dimensions . the tubing is forced from the die face of an extrusion head having an extruder die and an internal mandrel determining the outer and inner dimensions , respectively , of the extruded tubing . the outer dimensions of the mandrel varied by the user to change the inner dimensions of the extruded tubing . a pressure is maintained in the extruded tubing to prevent its collapse and that pressure is changed based upon the change of the external dimensions of the mandrel and the wall thickness of the tubing . in one embodiment , the mandrel is comprised of a core and a plurality of telescoping sleeves . each sleeve is movable between a retracted position where the sleeve is displaced away from the die face and an extended position where sleeve is at the die face . there may be multiple mandrels in the case of multi - lumen catheters .