titanium powder and base alloy powder of 60 wt . % aluminum and 40 wt . % vanadium , both having the average particle size of 20 μm and the maximum particle size of 40 μm , were dry - mixed together at a weight ratio of 9 : 1 in a ball mill to obtain raw powder of ti - 6 wt . % al - 4 wt . % v alloy . 200 parts in weight ( all parts are defined in weight throughout the specification unless otherwise specified ) of this raw powder was added to 100 parts of a binder consisting of 1 wt . % solution of ammonium alginate , followed by mixing together in the ball mill for 24 hours . thus , a slurry compound having viscosity of 900 cps was prepared . referring to fig1 and 2 , this slurry compound 1 was poured into a cylindrical mold 2 made of special grade gypsum , through a gate opening 3 . after the mold cavity had been filled with the slurry compound 1 , the gate opening 3 was closed with a plug made of special grade gypsum . the closed mold 2 was fixedly supported within a cylindrical holder 5 , as can be seen in fig4 . the holder 5 was rotated about its axis at 100 rpm for 10 minutes by means of a motor 6 ( fig3 ). during such rotation , the holder 5 was subjected twice to a round - trip swinging movement over 180 ° rotation , at 10 rpm , by means of a servo motor 7 ( fig4 ). more particularly , first swinging movement was applied immediately after beginning of rotation and second application was made 5 minutes later . by combined application of such rotating and swinging movement , the raw alloy powder in the mold cavity was centrifugally forced to move toward the inner wall of the mold cavity . a substantial amount of the binder in the slurry compound 1 was absorbed into porosity of the mold 2 . thus , within the cavity of the standing mold 2 , there was formed a body 8 having a fully closed hollow interior extending over a femoral bone head portion 8a and a stem portion 8b . the sectional mold 2 ( fig2 ) was divided to remove the formed body 8 , which was then subjected to debinding treatment in an algonic atmosphere at a temperature condition wherein a temperature was elevated to 700 ° c . at a speed of 50 ° c . per minute and 700 ° c . temperature was kept for 3 hours . the formed body 8 was then subjected to sintering in a vacuum ( 1 × 10 - 5 torr ) at 1300 ° c . for 3 hours . a hip prosthesis thus produced had a fully closed hollow interior and relative density of 95 % at the outer shell portion . 40 wt . % of smaller size titanium powder having the average particle size of 20 μm and the particle range of 3 - 45 μm , 30 wt . % of medium size titanium powder having the average of 140 μm and the range of 100 - 180 μm and larger size titanium powder having the average of 650 μm and the range of 600 - 700 μm were dry - mixed together in a ball mill for 10 hours to obtain pure titanium raw powder . 400 parts of this raw powder was added to 100 parts of a binder consisting of 1 wt . % solution of ammonium alginate , followed by mixing together in the ball mill for 24 hours to prepare a slurry compound having viscosity of 1500 cps . the following operation was performed with the mold 2 , the holder 5 and the rotating / swinging device which were used in example 1 . the slurry compound 1 was poured into the mold 2 through the gate opening 3 , and after the mold cavity had been filled with the slurry compound 1 , the gate opening 3 was tightly closed . the holder 5 containing the closed mold 2 was rotated about its axis at a low speed of 4 rpm for 30 minutes by the motor 6 . during such rotation , the holder 5 was subjected five times to a round - trip swinging movement over 180 ° rotation , at 5 rpm , by the servo motor 7 . more particularly , first swinging movement was applied immediately after beginning of rotation and the following applications were made with 6 minutes intervals . in this example , the raw powder was prepared by mixing three groups of titanium powder having different particle ranges . a larger and therefore heavier particle was given a larger gravity . accordingly , a formed body 9 in the cavity of the mold 2 after application of such rotating and swinging movement showed a distribution that larger particles were concentrated toward the surface of the cavity and smaller particles were likely to move toward the center , as shown in fig6 . such distribution was shown throughout the body 9 , including the femoral bone head portion 9a and the stem portion 9b . the formed body 9 was removed from the mold 2 and then subjected to debinding and sintering in the same manner as in example 1 , thereby making a hip prosthesis 10 made of pure titanium . the hip prosthesis 10 thus produced showed a density gradient with a rough surface and a dense center . more particularly , as shown in fig7 finer particles densely gathered near the central portion which constitutes a high density core 11 and larger particles were sintered generally in vicinity to the surface which became a low density , rough surface 12 having a plurality of recesses or grooves of approximately 500 μm diameter and 500 - 1000 μm depth . an intermediate layer 14 formed between the extreme layers 11 and 12 may have micro - voids 13 . the hip prosthesis 10 produced in example 2 , suspended in a bath heated to a temperature of 400 ° c ., was introduced into a spray booth so that only the femoral bone head portion was coated with a resin coating on the following conditions : immediately after coating , it was subjected to a pressure of 0 . 5 mp for 2 - 3 minutes until the resin coating manifested a smooth surface . thus , a porous surface 12 ( fig7 ) of the femoral bone head portion of the hip prosthesis 10 was coated with and smoothened by a peek resin layer 15 having thickness of about 0 . 2 mm . a molten peek resin has viscosity of 3000 - 4000 cps at an optimum coating temperature range . even with such a relatively high viscosity of the coating material , the resin layer 15 showed a good adhesion to the rough surface of the femoral bone head portion of the hip prosthesis 10 . this was because the hip prosthesis 10 produced in example 2 had relatively large surface porosity of the order of 500 μm , which allowed the coating material to reach the bottom of the surface recesses or grooves . on the other hand , when the femoral bone head portion should have surface porosity smaller than 0 . 1 mm , the molten peek resin could not flow into the recesses or grooves , degrading adhesion to the surface . increase of the molten temperature of the coating material will lower viscosity but may result in decrease of physical properties . another coating method which is considered better applicable to the surface having porosity of 0 . 1 mm or less includes the step of immersing the femoral bone head portion in a solution of polyethlene resin which is diluted with methylene chloride at a ratio of 2 : 8 to lower its viscosity to approximately 50 cps , in a vacuum , for 5 minutes so that the diluted resin solution may effectively be filled within the surface recesses or grooves . then , it is treated in a compressable volatile oven of 200 ° c . temperature for 10 minutes to volatilize methylene chloride , followed by being applied a pressure of 0 . 5 mp for 5 minutes . by these treatment , a voidless resin coating layer 16 of polyethlene may be formed with a thickness of about 0 . 05 mm on the surface of the femoral bone head portion . an additional polyethlene layer 17 may overlay the layer 16 by electrostatic coating method so that a composite resin layer has a sufficient thickness . 35 vol . % of alumina ceramics powder having the average particle size of 60 μm and the particle range of 45 - 75 μm and 65 vol . % of sus316l stainless steel powder having the average of 3 . 5 μm and the range of 0 . 5 - 5 μm were dry - mixed together in a ball mill for 10 hours to obtain composite raw powder . 350 parts of this raw powder was added to 100 parts of a binder consisting of 1 wt . % solution of ammonium alginate , followed by mixing together in the ball mill for 35 hours to prepare a slurry compound having viscosity of 1900 cps . the following operation was performed with the mold 2 , the holder 5 and the rotating / swinging device which were used in example 1 . the slurry compound 1 was poured into the mold 2 through the gate opening 3 , and after the mold cavity had been filled with the slurry compound 1 , the gate opening 3 was tightly closed . the holder 5 containing the closed mold 2 was rotated about its axis at a low speed of 4 rpm for 40 minutes by the motor 6 . during such rotation , the holder 5 was subjected the following swinging movement by the servo motor 7 . ( 1 ) immediately after beginning of rotation , the holder 5 was rotated over 90 ° in a counterclockwise direction so that it stands upright with the femoral bone head portion being directed downwardly , as clearly seen in fig1 . ( 2 ) each one round - trip swinging movement over 60 ° c . angle was imparted to the holder 5 five times , namely first swinging movement was applied immediately after the position of fig1 had been obtained and the following applications were given 2 minutes , 4 minutes , 6 minutes and 8 minutes later . ( 3 ) after the lapse of 10 minutes , the holder 5 was rotated over 90 ° in a clockwise direction to return the original position of fig4 . ( 4 ) each one round - trip swinging movement over 180 ° angle was applied to the holder five times , namely first one being applied immediately after obtaining the original position of fig4 ( i . e ., after the lapse of 10 minutes ) and the following applications being made with 6 minutes intervals ( i . e ., after the lapse of 16 , 22 , 28 and 34 minutes respectively ). this example employed the composite raw powder made of larger particles of alumina ceramics and smaller particles of stainless steel and the slurry compound prepared was relatively viscous , which facilitated precipitation of larger alumina ceramics particles . the smaller stainless steel particles had a tendency of floating in the binder , rather than of precipitating . after remaining the mold 2 at a rest for a long period of time , the compound 1 filled in the mold 2 should show a composition gradient . once the larger particles of alumina ceramics were adhered to the porous inner wall of the mold cavity to form a ceramic layer , this layer was hardly separated from the inner wall of the cavity even when applying the rotating / swinging movement to the mold . a body 18 formed in the mold cavity after application of the above - programmed rotating and swinging movement showed a composition gradient with a femoral bone head portion 18a rich in alumina ceramics particles 19 and a stem portion 18b rich in stainless steel particles 20 , as shown in fig1 . the formed body 18 was removed from the mold 2 and then subjected to debinding treatment in an atmosphere of nitrogen gas at a temperature condition wherein a temperature was elevated to 500 ° c . at a speed of 30 ° c . per minute and 500 ° c . temperature was kept for 3 hours . the formed body 18 was then subjected to sintering in a vacuum ( 1 × 10 - 2 torr ) at 1350 ° c . for 2 hours . a hip prosthesis thus produced had a fully closed hollow interior and relative density of 95 % at the outer shell portion . a bundle of four titanium wires 21 of 1 . 0 mm diameter was prepared ( fig1 ). 0 . 5 mm diameter wires 22 made of titanium were crossedly wound around the bundle with 20 mesh intervals to prepare a skeleton 23 ( fig1 ) formed into a hip prosthesis to be finally produced . the skeleton 23 was press - molded in a mold 25 with a pressure of 2 ton / cm 2 at a room temperature , followed by being subjected to sizing treatment . then , the skeleton 23 was set in the mold 2 of fig1 . titanium powder having the particle range of 3 - 100 μm was mixed with 1 % solution of ammonium alginate to prepare a slurry compound 1 having viscosity of 1800 cps . this compound 1 was poured via the gate opening 3 into the mold cavity and the gate opening 3 was then closed , the holder 5 containing the closed mold 2 was subjected to rotation and swinging movement on the same conditions as in example 4 , thereby forming such a body 26 as shown in fig1 . more specifically , as a result of the rotating and swinging movement applied to the mold 2 , a major portion of the titanium particles 24 in the compound 1 was concentrated at the femoral bone head portion 26a . gaps between the winding titanium wires 22 were completely filled with a plenty of the titanium particles 24 . further , the titanium particles 24 were densely gathered to provide a smooth surface of the femoral bone head portion 26a . on the contrary , the stem portion 26b was poor in the titanium particles 24 which could be seen only at intersections of the wires 22 . the titanium wires 22 were exposed to the outside and provide a rough surface of the stem portion 26b . the formed body 26 was removed from the mold 2 and then subjected to debinding and sintering in the same manner as in example 1 , thereby making a hip prosthesis 10 made of pure titanium . the hip prosthesis 10 thus produced comprised a stem portion covered with a mesh of the titanium wires 22 and having surface porosity of 0 . 1 - 0 . 3 mm and a femoral bone head portion showing a smooth surface made of sintered fine particles of titanium . the rough surface of the stem portion would facilitate development of surrounding bone tissue . the smooth surface of the femoral bone head portion would provide smooth rotation or movement relative to a socket implanted in the human body . although the invention has been described in conjunction with specific embodiments thereof , it is to be understood that many variations and modifications may be made without departing from spirit and scope of the invention as defined in the appended claims .
a slurry compound prepared by a sintering powdery material and a binder is press - molded to obtain a contour for the final product applicable to a bone - implant such as hip prosthesis . the molded body is given a programmed movement of rotation and / or swinging to impart a centrifugal force to the sintering particles which direct toward the inner wall of the mold cavity . the final product obtained after sintering has a hollow interior having no communication to the outside . imparting conditions of rotating / swinging movement may be changed or programmed in order to achieve a desired structure or constitution of the final product . by way of example , larger particles concentrate near the inner wall of the mold to provide a rough , porous surface of the body , whereas it has a dense core consisting mainly of sintered fine particles . a hip prosthesis having a ceramic - rich femoral head and a metal - rich stem may also be produced by so programming the movement imparting conditions .