the balloon may be formed in a mold as illustrated in fig1 which includes a mold body 110 having an internal bore which defines the intended dimension of the finished balloon , indicated at 112 , and a pair of end members including a fixed end member 114 and a movable end member 116 . both end members include outwardly tapering portions 114a , 116a , respectively , which merge into smaller diameter end bores 114b , 116b , respectively . a water jacket 118 having inlet and outlet ports 123 , 124 surrounds the mold 110 . the mold parts are formed from a material , such as brass having good heat conductivity . the mold receives a tubular parison , indicated in phantom at 120 in fig1 . the parison 120 is gripped at its ends which extends outwardly of the mold , one of the ends being sealed and the other end being connected securely to a source of fluid ( such as a gas ) under pressure as by a fitting 122 . the clamp 121 and fitting 122 are mounted , by means not shown , to enable them to be drawn apart axially so as to impart an axial stretch to the parison 20 . the parison is formed from an orientable semicrystalline polymer such as polyethylene terephthalate ( pet ). in accordance with the invention , the parison is dimensioned with respect to the intended final configuration of the balloon . it is particularly important that the parison is relatively thin walled and that it is highly oriented being stretched radially close to the elastic limit of the material at the inner surface of the tube . the wall thickness is considered relative to the inside diameter of the parisons which have wall thickness - to - inside diameter ratios of less than 0 . 5 and , preferably between 0 . 45 and 0 . 09 or even lower . the use of such a thin walled parison enables the parison to be stretched radially to a greater and more uniform degree because there is less stress gradient through the wall from the surface of the inside diameter to the surface of the outside diameter . by utilizing thin wall starting parisons , there is less difference in the degree to which the inner and outer surfaces of the tubular parison are stretched . by maintaining a lowered stretch gradient across the wall of the parison , it is possible to produce a more uniformly and more highly oriented balloon having tensile strengths substantially higher than those of the prior art . orientation takes place at an elevated temperature , as controlled by a heat transfer fluid , such as hot water , circulated through the water jacket . the pet parison preferably is drawn axially and while being so drawn , is expanded radially within the mold . the orientation takes place at a temperature between the first and second order transition temperatures of the material , preferably about 80 ° c .- 99 ° c . and more preferably at about 90 ° c . the tube is drawn from a starting length l1 to a drawn length l2 which preferably is between 2 . 5 to 6 l1 . the tubular parison , which has an initial internal diameter id1 and an outer diameter od1 is expanded by gas emitted under pressure to the parison through fitting 22 to an internal diameter id 2 which preferably is 6 to 8 idi and an outer diameter od 2 which is about equal to or preferably greater than 4 od 1 . the expanded balloon then is subjected to a heat set step in which steam is circulated through the jacket 18 at a temperature above the stretching temperature and between 110 ° to 220 ° c . and preferably about 130 ° c . and is maintained for a fraction of a second or more , and preferably between about 5 to 30 seconds sufficiently to increase the degree of crystallinity in the balloon . the heat setting step is significant in assuring dimensional stability for the balloon , both during storage and also under inflation pressures . after the heat set step , the mold is cooled to a temperature less than the second order transition temperature of the material . the balloon thus formed may be removed from the mold by removing the end piece 16 and withdrawing the formed balloon from the mold . the degree of radial stretch in the resulting balloon is very close to the elastic limit of the material ( at the inner surface ) and , preferably is within 10 % of the maximum achievable radial expansion for the material under the process conditions used . the high degree of orientation strengthens the balloon to tensile strengths of more than 35 , 000 psi and as high as 90 , 000 psi or more . for example , balloons made in accordance with the invention have a wall thickness to balloon diameter ratio ( t / d ) of the order of 5 × 10 - 3 to 8 × 10 - 5 , with tensile strengths as high as 90 , 000 psi . table 1 below illustrates the t / d ratios for finished balloons of the present invention in comparison with t / d ratios for prior art balloons . data for the balloons identified a - e were taken from levy u . s . pat . no . 4 , 490 , 421 for the correspondingly designated balloons in that patent . table 1______________________________________wall thickness ( t )/ balloon diameter ( d ) radio______________________________________prior art a b c d e______________________________________t . 028 mm . 038 mm . 028 mm . 038 mm . 045 mmd 3 . 7 mm 5 . 0 mm 3 . 7 mm 5 . 0 mm 6 . 0 mmt / d 7 . 57 × 10 . sup .- 3 7 . 6 × 10 . sup .- 3 7 . 57 × 10 . sup .- 3 7 . 6 × 10 . sup .- 3 7 . 5 × 10 . sup .- 3______________________________________present invention f g h i______________________________________t . 0136 mm . 0054 mm . 00806 mm . 0016 mmd 3 . 0 mm 4 . 0 mm 20 . 0 mm 20 . 0 mmt / d 4 . 5 × 10 . sup .- 3 1 . 8 × 10 . sup . - 3 4 . 03 × 10 . sup .- 4 8 . 05 × 10 . sup .- 5burst at 352 psi . sup . 220 psi . sup . 73 . 5 psi______________________________________ the balloons of the present invention display a remarkable increase in flexibility as compared to the prior pet balloons ( levy &# 39 ; 421 patent ). flexibility is a function of the cube of the wall thickness , assuming all other variables , such as balloon material , balloon diameter , etc . are held constant . table 2 illustrates the effect on flexibility of the thinner balloons of the present invention as compared with those of the prior art . table 2__________________________________________________________________________ improvement inprior art ( levy ) saab disclosure flexibility ( bars ) pressureburst ( psi ) strengthtensileradial ( mils ) thicknesswall ( t . sup . 3 ) stiffnessrelativeradial ( psi ) strengthtensile ( mils ) nessthick - wall ( t . sup . 3 ) nessstiff - relative ## str1 ## __________________________________________________________________________ 5 34 , 000 0 . 13 0 . 0022 90 , 000 0 . 048 0 . 00011 1 , 90010 34 , 000 0 . 25 0 . 016 75 , 000 0 . 114 0 . 0015 1 , 00015 34 , 000 0 . 38 0 . 054 65 , 000 0 . 20 0 . 0077 60020 34 , 000 0 . 50 0 . 127 52 , 000 0 . 33 0 . 15 6525 34 , 000 0 . 63 0 . 249 40 , 000 0 . 53 0 . 15 65__________________________________________________________________________ from table 2 it can be seen that for balloons having burst pressures up to about 25 bars , the relative stiffness of the balloons of the present invention is far less than that of the prior art pet balloons . the flexibility of the balloons of the present invention , considered as a function of the cube of the thickness ( t 2 ) of the balloons far exceeds those of the prior art . in the data presented in table 2 , the radial tensile strength was calculated at burst pressure . wall thicknesses were calculated using the well known pressure vessel equation ## equ1 ## where s c is the radial tensile strength , p is the burst pressure and d is the diameter of the balloon and t is the wall thickness . wall thicknesses similarly where calculated and the thicknesses were cubed to provide an indication of relative stiffness . fig2 shows the effect of the heat set step in the practice of the invention and compares the dimensional stability in storage as well as when inflated , of a balloon which has been heat set ( curve a ) to one which has not been heat set ( curve b ). both balloons were formed in a cylindrical mold having a diameter of 0 . 117 &# 34 ;, under identical conditions of temperature , time and pressure except that the heat set balloon was subjected to the heat set procedure as described above . the other balloon was cooled following the radial expansion , without any heat set . the balloons were removed from their molds and were permitted to remain in ambient conditions ( 20 ° c .) for 48 hours . the balloons then were tested by inflating them under increasing pressures and measuring the balloon diameter as the pressure was increased . as shown in fig2 the heat set balloon , when nominally inflated to remove wrinkles ( about 10 psi ) had a diameter of about 0 . 0116 &# 34 ;, reflecting a shrinkage of 0 . 001 &# 34 ;. by contrast , the non - heat sealed balloon showed a nominal diameter of 0 . 1060 , indicating a very substantial diametral shrinkage of 0 . 011 &# 34 ;. as the pressure was increased , the heat set balloon showed relatively steady and slow yielding , reaching 5 % radial expansion at about 170 psi . in contrast , the non - heat set balloon displayed very substantially yielding and radial expansion in the range of 50 to 100 psi , reaching 5 % radial expansion at about 85 psi . the superior properties of balloons made in accordance with the invention may be appreciated further from the &# 34 ; f5 &# 34 ; characteristics of the balloon . in oriented polymers the tensile strength at 5 % elongation is referred to as &# 34 ; f5 &# 34 ;. fig2 illustrates , on curve a , the point on which a balloon made in accordance with the invention has achieved 5 % radial expansion . in curve a the f5 point occurs at about 170 psi inflation pressure . the balloon reflected in curve a is the balloon identified as balloon g in table 1 ( and in example 2 below ), having a wall thickness of 0 . 0054 mm and a diameter of 3 . 0 mm . from the pressure vessel equation , the radial tensile strength at 5 % radial expansion is calculated to be 47 , 600 psi . that is contrasted with the prior art ( levy &# 39 ; 421 ) balloons in which the maximum tensile strength achievable to burst is of the order of 34 , 000 psi . thus , the present invention provides for f5 radial tensile strengths greater than 30 , 000 psi . the following examples illustrate several balloons which may be made in the range of balloons achievable in accordance with the present invention . a tubular parison was extruded from high molecular weight pet homopolyester resin having an initial intrinsic viscosity of about 111111 . 04 . the parison had an inner diameter of 0 . 789 mm , a wall thickness of 0 . 180 for a wall thickness - to - id ratio of 0 . 42 . the parison was stretched axially 3 ×, was stretched 7 × id and 3 . 8 × od to form a 3 . 0 mm balloon having a wall thickness of 0 . 0136 mm . the balloon was heat set . the balloon had a burst pressure of 24 bars ( 23 . 7 atm ) and a calculated radial tensile strength of about 38 , 900 psi . this balloon corresponds to the balloon designated &# 34 ; f &# 34 ; in table 1 . a parison was formed in the manner described above in example 1 to have an inner diameter of 0 . 429 mm and an outer diameter of 0 . 638 mm , having a parison wall thickness / id ratio of 0 . 25 . the parison was stretched axially 3 . 3 ×, was stretched 7 × id and 4 . 7 × od to produce a heat set 3 . 0 mm balloon having a wall thickness of 0 . 0054 mm measured burst strength of 15 bars ( 14 . 8 atm ) and a calculated radial tensile strength of about 61 , 250 psi . this balloon corresponds to balloon g in table 1 . a parison was formed in the manner described about in example 1 to have an id of 2 . 86 mm and an od of 3 . 39 mm , with a wall thickness / id ratio of 0 . 09 . the parison was stretched at 90 ° c . in an axial direction 3 . 75 × and radially 7 × id and 5 . 9 × od to produce a heat set 20 mm balloon having a wall thickness of 0 . 00806 , a measured burst strength of 5 bars ( 4 . 9 atm ) and calculated radial tensile strength of 91 , 200 psi . this balloon corresponds to balloon h in table 1 . it should be understood that the foregoing description of the invention is intended merely to be illustrative and that other embodiments and modifications may be apparent to those skilled in the art without departing from the spirit . for example , the preferred aromatic linear polyester described for making balloons in accordance with the invention is polyethylene terephthalate derived from an aromatic dicarboxylic acid or its derivatives as a main acid component and an aliphatic glycol as a main glycol component . this polyester is a melt extrudable orientable semicrystalline polymer that can be fabricated into a variety of formed structures . typical examples of other aromatic dicarboxylic acid polymers that meet these criteria utilize materials such as terepthalic acid , isothalic acid , naphthalene dicarboxylic acid , together with aliphatic polyethylene glycols having two to ten carbon atoms . among these are ethylene glycol , trimethylene glycol , tetramethylene glycol , pentamethylene glycol , hexamethylene glycol , didecamethylene glycol and cyclohexane dimethanol . fig3 illustrates a balloon dilatation catheter of the type with which the present invention is concerned . the catheter 10 has a proximal end ( to the left in fig3 ) and a distal end ( to the right in fig3 ). an elongate flexible shaft 12 typically is provided with appropriate lumens , for example , a guidewire lumen ( not shown ) that extends the length of the shaft and an inflation lumen ( not shown ) that extends from the proximal end of the shaft to the distal region of the shaft and communicates with the interior of a dilatation balloon 14 that is mounted to the distal region of the shaft . reference is made to the aforementioned leary u . s . pat . no . 4 , 545 , 390 for further description of the type of catheter , the leary patent being incorporated by reference herein . by way of example , the catheter shaft 12 may be of the order of 150 cm long and of the order of 0 . 50 &# 34 ; diameter . the balloon may vary in size from about 1 . 5 mm to 4 . 5 mm diameter , for coronary use . the balloon may be considered as having a constant diameter cylindrical midportion 14m which expands to the nominal diameter , a pair of end cones 14c at the ends of the midsection 14m and a pair of neck sections 14n that extend outwardly from the narrow ends of the cones 14c . the balloon 14 is attached to the catheter shaft 12 by adhesively bonding the necks 14n to the catheter shaft 12 . fig4 shows a conventional balloon formed in one - piece , shown with its wall thicknesses exaggerated for ease of illustration . such a balloon may be made according to the procedure described in u . s . pat . no . 4 , 490 , 421 and u . s . application ser . no . 001 , 759 , filed jan . 9 , 1987 , now abandoned , the disclosures of which are hereby incorporated by reference . the balloon is formed in a biaxially stretching process that includes blow molding in a mold of the type illustrated in fig6 . as described in further detail in the levy patent , which is incorporated herein by reference , a tubular parison 15 of uniform inner and outer diameters and wall thickness is extended through the mold 17 . the tubular parison is stretched axially and is blow molded radially within the mold 17 . the portion of the tube 15 that forms the cylindrical midportion 14m is subjected to a greater degree of radial stretching than the neck portions 14n . consequently , the midportion 14m will have less wall thickness than the neck portion 14n . the cones 14c are radially stretched to progressively varying degree as the diameter of the cones change . thus , as illustrated in fig4 the midportion 14m will have the thinnest wall , the neck 14n will have the thickest wall and the cones 14c will have a progressively increasing wall thickness in a direction extending from the ends of the midportion 14m to the necks 14n . the cones 14c and necks 14n thus are thicker than required . the increased thickness in the cones adversely affects the ability of the balloon to contract to a low profile . the greater thickness of the cones and the necks detracts from the trackability of the catheter . in accordance with the present invention , the balloon is formed from a plurality of relatively thin layers rather than a single unitary relatively thick layer . the configuration of a balloon made in accordance with the invention is shown in fig5 in highly diagrammatic enlarged and exaggerated form . the illustrated balloon , indicated generally at 16 , is formed from three layers including an outer layer 18 , an intermediate layer 20 and an inner layer 22 . the layers 18 , 20 , 22 are each in continuous intimate contact with each other and need not be adhesively attached . the balloon 16 is formed in a procedure illustrated diagrammatically in fig7 a - 7e . the mold 17 , described in further detail in the levy patent , receives a tubular parison 15 of the polymer from which the balloon is to be formed . the parison is relatively thin walled . the parison is stretched and expanded biaxially by combined axial stretching and blow molding as described in the levy patent , to form a balloon having cones and neck extensions . in accordance with the invention , a first such balloon 18 is formed and is heat set at an elevated temperature as described in said u . s . application ser . no . 001 , 759 and is then removed from the mold . the first balloon then is trimmed at each end between the ends of its cones as suggested at 24 in fig7 a , thus leaving a balloon midportion 18m and a pair of partial cones 18c . the first balloon , thus trimmed , then is replaced in the mold . a second elongate tubular parison 20p then is inserted into the mold as suggested in fig7 b and it is biaxially stretched and expanded . when the second parison 20p expands , it contacts fully and intimately the inner surface of the outer layer 18 contained within the mold . the second balloon thus formed also is heat set . after the intermediate balloon 20 has been formed , the combined outer and intermediate balloons 18 , 20 are removed from the mold . the ends of the intermediate balloon layer 20 then are trimmed as suggested at 26 in fig7 c so that the intermediate cones 20c extend slightly outwardly of the shorter outer cones 18c . the two layer , partially formed balloon then may be reinserted into the mold and the process repeated with a third parison 22p of polymeric material as suggested in fig7 d . when the third layer 22 has been formed , the assembly of layers is again removed from the mold . the ends of the inner layer 22 then may be trimmed as suggested at 28 in fig7 e to leave the necks 22n and an exposed portion of the cones 22c . the balloon thus formed may be attached to the catheter shaft 12 by adhesively bonding the necks 22n to the catheter shaft . it will be appreciated from the foregoing that the neck of the multiluminal balloon which is formed from an initial thin parison , although not expanded as much as the balloon midportion still is substantially thinner than the corresponding neck in a balloon formed in one - piece in a single layer . the regions of the cones similarly define a series of stepped thicknesses in which the thickness of the cone decreases in a direction extending away from the balloon midportion . thus , although the cone segment in each of the three layers will tend to have increased thickness in an outward direction , the stepped down configuration of the cones , considered together , results overall in a cone thickness that is relatively small . for example , even in a thin wall high strength balloon made in accordance with the procedure described in u . s . application ser . no . 001 , 759 , filed jan . 9 , 1987 , now abandoned , the wall thickness in the cone ranges from about 0 . 0003 &# 34 ; at its juncture with the cylindrical midportion to approximately 0 . 001 &# 34 ; at its juncture with the neck . the neck portion may be of the order of 0 . 001 &# 34 ;. by way of example , balloons made in accordance with the present invention may be formed from a tubular parison of polyethylene terephthalate having an inner diameter of the order of 0 . 0168 &# 34 ; inner diameter and a wall thickness of the order of 0 . 0022 &# 34 ;. the parison is biaxially stretched about 3 × in an axial direction and radially about 7 × inner diameter stretch and about 5 . 5 × outer diameter stretch . the resulting balloon will have a wall thickness in the cylindrical midportion region of the order of 0 . 0001 &# 34 ;, a cone thickness gradually increases from 0 . 0001 &# 34 ; to about 0 . 0004 &# 34 ; where the cone joins the neck and a neck portion having a wall thickness of the order of 0 . 0004 &# 34 ;. the aggregate wall thickness in the cylindrical midportion of the multiple layers is of the order of 0 . 0003 &# 34 ; which is comparable to currently commercially available balloons . from the foregoing , it will be appreciated that the invention provides a new balloon dilatation catheter comprising an elongate , flexible shaft having a dilatation balloon mounted at the distal end . the balloon is formed from polymeric material and having a cylindrical midportion , an outwardly tapering conical portion at each end of the midportion and a cylindrical neck portion at the ends of the conical portions . the balloon is formed in at least two layers including an inner and outer layer , one of which is shortened and terminates in the region of the cones of the other , the necks of the balloon being adhesively attached to the shaft . from the foregoing , it will be appreciated that the invention provides a new construction for a dilatation balloon , a new method for its manufacture and a new resulting catheter that will display improved characteristics as to trackability and reduced profile . it should be understood , however , that the foregoing description of the invention is intended merely to be illustrative thereof and that other embodiments , modifications and equivalents may be apparent to those skilled in the art without departing from its spirit .