GB2067441A - Pressure die casting method and apparatus for production of rotor having radial vanes - Google Patents

Abstract

A split die for pressure die casting a vaned rotor which comprises a vane portion where radial vanes are formed on a central shaft and a base portion where the central shaft has no vanes thereon, is divided radially into a plurality of die segments that can be moved radially inwardly to define a die cavity and outwardly to open the die. Each die segment is divided in a plane perpendicular to the central axis of the die into first parts (54a) shaped correspondingly to the vane portion and second parts (54b) shaped correspondingly to the base portion of the rotor and held in slidable contact with each other. Immediately after solidification of molten metal in the die cavity in a region to become the vane portion of the rotor, initially only the first part (54a) of each die segment is moved radially outwardly (Figure 5A) to prevent this part of the die from significant wear. The second part (54b) of each die segment is moved radially outwardly (Figure 5B) after solidification of the molten metal in the remaining region to become the base portion of the rotor. <IMAGE>

Description

SPECIFICATION Pressure die casting method and apparatus for production of rotor having radial vanes This invention relates to a pressure die casting method for the production of a metal rotor of the type having radial vanes on the circumference of a central shaft over a limited length of the shaft and a pressure die casting apparatus for producing such a rotor.

There is a wide variety of vaned rotors for moving a fluid, and a large part of them are of the type having radial vanes integrally formed on the circumference of a revolving shaft. Usually the vanes of this type of rotor are intricate in shape and thin in sections and are required to be formed with very small tolerance in dimensions. Therefore, in the case of producing such rotors by casting of a metal, usually an alloy, it is usual to employ a precision casting process such as lost-wax process (investment casting) or plaster mold casting process. However, low production rate and high production costs are disadvantages common to conventional precision casting processes.

With a view to reducing the cost of production, it has been tried to produce rotors of the aforementioned type by ordinary pressure die casting process by using a split metal-die which is divided radially into a plurality of die segments which are shaped correspondingly to the rotor vanes and can be moved radially inwardly to define a die cavity and radially outwardly to allow removal of the casting from the opened die. In practice, however, rapid wear of the intricately and precisely shaped die segments presents a problem to this method. This problem becomes particuiarly serious when the rotor to be produced has its vanes only over a limited length of the central shaft and includes an axial end portion (which will herein be called the "base portion" of the rotor) where the shaft becomes relatively large in cross-section and has no vanes thereon.In each casting cycle for the production of such a rotor, the molten metal in the die cavity solidifies rapidly in a region to become the vane portion of the rotor having vanes of small thickness, but slowly in the remaining region to become the base portion of large cross section. The split die constituted of radially divided segments needs to be kept in the forcibly closed state until solidification of the whole molten metal in the die cavity, with the maintenance of a high pressure produced in the molten metal by the action of the injection cylinder. Due to heat transfer during the long contact with the molten or solidified metal, the precisely shaped vane part of the split die wears significantly to result in an unsatisfactorily short service life of the die.

Moreover, often cracking occurs in the vanes of the cast rotor because of internal strains produced by different thicknesses of the metal casting cooling and solidifying at different rates, combined with the influence of the above-mentioned high pressure which is maintained for such a long period of time and which is considered more than sufficient for the formation of the vane portion.

It is an object of the present invention to provide an improved pressure die casting method for the production of a rotor of the type comprising a vane portion having a plurality of vanes formed radially on the circumference of a central shaft and a base portion which is given by an axially extended endmost portion of the central shaft and has no vanes thereon, which method greatly mitigates the wearing of a vane part, the most important part, of a metal die for the casting process by the influence of the heat of molten metal and therefore can reduce the cost of production of the rotor.

It is another object of the invention to provide a pressure die casting method by which a rotor of the above type can be produced with remarkably lessened possibility of suffering cracking in the vanes.

It is still another object of the invention to provide an apparatus for performing a pressure die casting method according to the invention.

In a method according to the invention, the first step is to fit a split die of metal in a pressure die casting machine, which split die is divided radially of a central axis into a plurality of die segments that can be moved to a limited extent radially inwardly and outwardly with respect to the central axis. Moreover, each of these die segments is a combination of a first part shaped correspondingly to the vane portion of a rotor of the above stated type and a second part shaped correspondingly to the base portion, as defined above, of the same rotor. When the die is complete, the first and second parts of each die segment are brought into slidable contact with each other in a plane perpendicular to the central axis of the die so as to allow a relative displacement between the first and second parts when the die segment is moved radially of the central axis.

This step is followed by the steps of forming a die cavity in the split die by moving the die segments radially inwardly to take a radially inner extreme position, then forcing a molten metal under pressure into the die cavity and allowing the molten metal to begin to solidify in the die cavity, initially moving the first parts of the die segments radially outwardly without moving the second parts immediately after solidification of the molten metal in the die cavity in a region to become the vane portion of the rotor to thereby separate the first parts of the die segments from the solidified metal, and thereafter moving the second parts of the die segments radially outwardly after solidification of the molten metal in the die cavity in a region to become the base portion of the rotor.

As will be understood from the above statement, the essential features of the present invention reside in that each of the radially divided die segments is divided into a first part which may be called a vane part and a second part or base part, that the first and second parts of each die segment in the assembled die are held in a radially slidably contacting state, and that immediately after solidification of the injected molten metal in the region to become the vane portion of the rotor, only the first parts of the die segments are quickly separated from the solidified metal.

Thus, the invention has succeeded in greatly reducing the length of the time period during which the vane-forming part of the split die is subjected to the high temperature and high pressure of molten metal in the die cavity and, therefore, has considerably mitigated the wearing tendency of the most important part of the split die. Because of the reduced rate of wearing, in many cases the service life of the vane-forming part of a split die used in the present invention becomes more than twice as long as that of an analogous split die of a conventional design.

Another important feature of the present invention is that it brings about a considerable decrease in the percentage of defective castings, particularly those having cracks in the vane portion.

A pressure die casting apparatus as another aspect of the present invention comprises a die assembly including a split die to form a die cavity corresponding in shape to a vaned rotor to be produced, means for moving the die assembly along a central axis of the die cavity and means for forcing a molten metal into the die cavity through an opening located axially at one end of the die cavity and maintaining the molten metal forced into the die cavity in a pressurized state. The split die is divided radially of the aforementioned central axis into a plurality of die segments that can be moved to a limited extent radially inwardly and outwardly so that the die cavity is formed when these die segments take a radially inner extreme position.Each of these die segments is divided by a plane perpendicular to the aforementioned central axis into a first part shaped corresponding to the vane portion of the rotor and a second part shaped corresponding to the base portion of the rotor, and the first and second parts of each die segment are in slidable contact with each other in the aforementioned plane. The die assembly further comprises means for moving the first and second parts of each die segment radially inwardly to allow the die segments to take the inner extreme position to form the die cavity and initially moving only the first part of each die segment radially outwardly from the inner extreme position and thereafter moving also the second part of each die segment radially outwardly from the inner extreme position.

As a preferable example of means for the differential movement of the two parts of each die segment, the first and second parts are coupled with each other by means of a key-slot formed on the contacting face of one of these two parts and a key-like ridge formed on the contacting face of the other part and shorter than the key-slot in length in the radial direction with respect to the central axis of the die, and only the second parts of each die segment are fixed to the piston rod of a hydraulic cylinder.

As another example, the first part of each die segment is fixedly connected to a coupling member with a connecting rod, and another connecting rod fixed to the second part is slidably coupled with the same coupling member. In this case the piston rod of a hydraulic cylinder is fixed to this coupling member.

In the accompanying drawings: Figure 1 is a perspective view of a rotor as an example of castings obtainable by the method and apparatus according to the invention; Figure 2 is a longitudinal sectional view of a principal part of a pressure die casting machine fitted with a die assembly including a split die according to the invention for casting of the rotor of Figure 1; Figure 3 is a plan view taken along the line 3-3 of Figure 2; Figure 4 is a partial enlargement of Figure 2 to illustrate the details of the split die; Figure 5(A) shows a partly opened state of the die in the die assembly of Figure 4; Figure 5(B) shows the same die in a further opened state; Figure 6(A) illustrates a variation of a slidable coupling mechanism suitable for the present invention; and Figure 6(B) illustrates partial-opening of the split die by the function of the same coupling mechanism.

Figure 1 shows a rotor for use in a turbocharger for an automotive engine as an example of a variety of rotors that can be produced by a pressure die casting method according to the invention. This rotor is a one-piece member having a central shaft 10 circular in cross section and thin and twisted vanes 12 extending generally radially from the circumference of the shaft 10 at circumferentially equal intervals. In Figure 1 the rotor is illustrated in the state as cast, and there is a large disc-like portion 14 at the lower end of the shaft 10.

This portion 14 is a so-called biscuit that results from solidification of excess molten metal present in the end portion of the injection cylinder of a pressure die casting machine. Subsequently the shaft 10 is severed along a cutting line 15 to cut off the biscuit 14.

A lower endmost portion 16 of the central shaft 10 after separation of the biscuit 14 is a solid cylinder relatively large in diameter, and the cylindrical surface in this portion 16 is devoid of the vanes 12. In the present application this portion 16 is named the base portion of the rotor, while the remaining portion of the shaft 10 and all the vanes 12 are collectively called the vane portion 18 of the rotor.

Figure 2 shows a principal part of a pressure die casting machine which is fitted with a die assembly 40 to produce the rotor of Figure 1 by a method according to the invention. The die casting machine has a bed 20, vertical columns 22 and upper crossbeams 24. A toggle linkage 26 supported by the columns 22 and cross-beams 24 hold a ram 28 horizontally and, by the action of a hydraulic cylinder 30 installed on the crossbeams 24 to vertically reciprocate its piston rod 30a, allows the ram 28 to move downwards and upwards along guide rails 32 provided on the vertical columns 22.

A cylindrical tube called a shot-sleeve 34 is fitted vertically into a through-hole in the bed 20, and a hydraulic cylinder 36 for injection of molten metal into the die assembly 40 is installed in the interior of the bed 20 by a support frame 38 such that a plunger 42 fixed to the tip of the piston rod 36a of the cylinder 36 is slidably fitted into the sleeve 34 from the lower end of the sleeve.

Referring to both Figure 2 and Figure 3, the die assembly 40 has a framework comprising a disc-like upper back-up plate 46 fixed to the lower surface of the ram 28, a roughly cylindrical peripheral wall 48 vertically and coaxiallyfixed to the back-up plate 46 and a roughly annular lower back-up plate 50 fixed concentrically at the lower end of the peripheral wall 48.

As a part of a split die held in this framework, a disc-like plate 52 is fixedly and concentrically placed on the upper back-up plate 46, and there is a cylindrical through-hole at the center of these two plates 46 and 52. At the center of the ram 28, there is a through-hole which is axially in alignment with and somewhat larger in diameter than the hole in the two plates 46 and 52. A hydraulic cylinder 58 is mounted on the upper side of the ram 28 such that a knock-out pin 60 attached to the tip of the piston rod of this cylinder 58 passes through the hole in the ram 28 and slidably fits into the through-hole in the two plates 46 and 52. The lower end of this knock-out pin 60 also serves as a part of the split die. The axis P of the knock-out pin 60 coincides with the longitudinal axis of the shot-sleeve 34.

As the principal part of the split die, a plurality of identically shaped segments 54 are arranged radially with respect to the axis P and radially slidably held between the lower back-up plate 50 and the disc-like plate 52.

These die segments 54 are designed such that each vane 12 of the rotor is formed by solidification of molten metal in a gap between adjacent two die segments 54, so that the number of the die segments 54, agrees with the number of the vanes 12 (nine vanes in the illustrated example). For each of the nine die segments 54, a hydraulic cylinder 56 is mounted on the outer side of the peripheral wall 48 with its horizontally extending piston rod 56a fixed to the radially outer side of the die segment 54. Accordingly, all the die segments 54 can simultaneously be moved either radially inwardly or radially outwardly to a limited extent.

When the die segments 54 are forced to take a radially inner extreme position as shown in Figures 2 and 3 (also in Figure 4), a die cavity 62 corresponding in shape to the intended rotor is formed in the split die. In the description hereinafter, the radially inward movement of the die segments 54 will be expressed as advance and the radially outward movement as retreat.

As best can be seen in Figure 4, each of the die segments 54 is divided horizontally into a vane part or first part 54a shaped correspondingly to the vane portion 18 of the rotor, and a base part or second part 54b shaped correspondingly to the base portion 16 of the rotor. In the die assembly 40, the lower end face of the first part 54a is in slidable contact with the upper end face of the second part 54b of the same die segment 54, and the piston rod 56a is fixed to the first part 54a. In this example, the first and second parts 54a and 54b of each die segment 54 are coupled with each other by using a slot 66 formed on the lower end face of the first part 54a and a key-like ridge 68 formed on the upper end face of the second part 54b.The height of the ridge 68 just corresponding to the depth of the slot 68, but the length of the ridge 68 in the direction radially of the central axis P of the die assembly 40 being shorter than the length of the slot 66. Accordingly, in the coupled state with the ridge 68 inserted into the slot 66, there occurs a relative displacement between the first part 54a and second part 54b of each die segment 54 during either advance or retreat of the die segment 54. The extent of the displacement is determined by the difference in radial length between the slot 66 and the ridge 68. This manner of coupling of the first and second parts 54a and 54b can alternatively be accomplished by forming a slot on the upper surface of the second part 54b and a key-like ridge on the lower surface of the first part 54a.

Using the above described apparatus, one cycle of a pressure die casting method according to the invention is carried out in the following way.

Firstly, the hydraulic cylinders 56 are actuated to advance the die segments 54 and hold them in the radially inner extreme position. The second part 54b of each die segment 54 may not begin to advance simultaneously with the first part 54a, but after a short while the continued advance of the first part 54a causes the second part 54b to advance together with the first part 54a. Next, the cylinder 58 is actuated to keep the knock-out pin 60 in a predetermined position as shown in Figure 4 to thereby complete the formation of the die cavity 62.

Next, a suitable quantity of molten metal 64 is supplied into the interior of the shot-sleeve 34, and immediately thereafter the ram 28 is lowered by the action of the hydraulic cylinder 30 until the lower surface of the assembly of the advanced split die segments 54 comes into close contact with the upper end face of the shot-sleeve 34. The cylinder 30 is kept actuated to keep the die in forcibly closed state with the application of a sufficient pressure. Without unnecessary delay, the hydraulic cylinder 36 is actuated to move the plunger 42 upwards to thereby force the molten metal 64 into the die cavity 62.The quantity of the molten metal 64 supplied into the sleeve 34 and the extent of the upward movement of the plunger 42 are determined such that when the die cavity 62 is completely filled with the injected molten metal the upper end face of the plunger 42 is slightly below the upper end of the sleeve 34. The cylinder 36 is kept actuated even after completion of the upward movement of the plunger 42, so that the molten metal in the die cavity 62 is continuously subjected to the injection pressure until its solidification. This is necessary to afford the casting with a very tight structure.

The molten metal in the die cavity 62 solidifies in a short period of time in the upper region defined by the first parts 54a of the die segments 54, i.e. a region relatively small in cross-sectional dimensions and to become the vane portion 18 of the rotor. When the metal in this upper region solidifies sufficiently, the nine hydraulic cylinders 56 are all switched to the reverse to retract the piston rods 56a to thereby retreat the first parts 54a of all the die segments 54, as illustrated in Figure 5(A).Because of the existence of a gap between the radially inner end of each slot 66 and the radially inner end of the engaged ridge 68, the retreat of the first parts 54a of the die segments 54 does not instantly cause retreat of the second parts 54b. Therefore, the first parts 54a can be separated from the solidified metal while the second parts 54b are still in the extreme advanced position.

After solidification of the molten metal in the lower region of the die cavity 62 to become the base portion 16 of the rotor, the second parts 54b of the die segments 54 are retracted by further retraction of the piston rods 56a as illustrated in Figure 5(B). When the difference in radial length between the slot 66 and the ridge 68 can be made sufficiently large, appropriately delayed retreat of the second parts 54b of the die segments 54 can be achieved by continuously retreating the first parts 54a at an adequate rate. Alternatively, the delayed retreat of the second parts 54b can be achieved by temporarily stopping the retracting operation of the cylinders 56 by means of, for example, electromagnetic control valves provided in the hydraulic pressure line.

Just before the retreat of the second parts 54b of the die segments 54, the ram 28 is moved upwards by the action of the cylinder 30 and the plunger 42 is moved downwards by the action of the cylinder 36. After completion of the retreat of the die segments 54 the knock-out pin 60 is thrust downwards to remove the casting from the opened die. The rotor cast by this method is finished by cutting the biscuit 14 off as mentioned hereinbefore and removing flash if necessary.

As can be understood from the foregoing description of the casting operation, the two-stage retreat of the die segments 54 according to the invention does not prolong the total time of one casting cycle. For example, when the diameter of the base portion 16 of the rotor of Figure 1 is 24 mm and the thickness of the vanes 12 is 2.5 mm at the root and 0.7 mm at the tip, the first parts 54a of the die segments 54 can be retreated after the lapse of 2 to 5 seconds from the injection of molten metal (using a nickel-base heat-resistant alloy) and the second parts 54b can be retreated after the lapse of 10 to 15 seconds from the injection.When each die segment 54 for casting of the same rotor is a one-piece member not divided into the first and second parts 54a and 54b, the retreat of such die segments as a whole becomes possible after the lapse of 10 to 15 seconds from the injection of the molten metal.

The early retreat of the first parts 54a of the die segments 54 means early separation of them from a very hot metal and also early release from a high pressure. Therefore, this method is greatly effective for reducing the rate of wear of the first parts 54a which are intricately shaped with high dimensional accuracy. In practical applications, this effect is appreciated as a prolonged service life of the die. As an additional and also important effect of the invention, the vane portion 16 of the rotor can be cast-formed with minimized internal strains, and this effect is reflected in a remarkable decrease in the percentage of defective castings having cracks in the vane portion 18. The following experimental results demonstrate these effects of the method of the invention.

Casting of the rotor of Figure 1 having the aforementioned dimensions was repeatedly performed by using the apparatus of Figures 2 to 4 and a nickel-base heat-resistant alloy as the material. In addition to the die segments 54 divided into the first and second parts 54a, 54h, also use was made of similarly shaped conventional (one-piece) die segments for the sake of comparison. The service life of each die in terms of the number of possible casting cycles, i.e. the number of castings that can be produced from each die, and the percentage of cracked castings were examined with respect to three kinds of die materials for both types of die segments. The results are presented in the following table.

Die material Service life of die Cracked castings (number of casting cycles) (% in 200 sampies) Conventional Invention Conventional Invention Hot tool steel 120 260 2.5 0.5 Ni-base super heat-resistant 270 600 3.0 0.5 alloy W-base super heat-resistant 1500 3000 3.5 1.0 alloy Figures 6(A) and 6(B) show another example of a variety of methods to realize the early retreat of the first part 54a of each die segment 54.

The first part 54a of each die segment 54 is placed on the second part 54b so as to make slidable contact, but in this case neither a slot nor a ridge is formed on the lower end face of the first part 54a and the upper end face of the second part 54b. The piston rod 56a of each hydraulic cylinder 56 does not extend to the die segment 54 but is fixed to a coupling member 70 to which is fixed a connecting rod 72 to extend parallel to the piston rod 56a toward the first part 54a of the die segment 54 by a pin 74 or the like. At the other end, the connecting rod 72 is fixed to the radially outer side of the first part 54a, so that the coupling member 70 is kept at a fixed distance radially outwardly from the first part 54a.The coupling member 70 has a cylindrical hole 71 elongate in the direction parallel to the piston rod 56a, and an elongate slit 73 is formed in the side wall of the coupling member 70 parallel to the hole 71 to provide lateral access to the cylindrical hole 71. A connecting rod 76 is fixed at one end to the radially outer side of the second part 54b to extend parallel to the aforementioned connecting rod 72. In the other end portion, this connecting rod 76 slidably fits into the cylindrical hole 71 in the coupling member 70 and is slidably coupled with the coupling member by a stopper-pin 78 fixed to the end portion of the connecting rod 76 to transversely and slidably engage the slit 73.

The length of the slit 73 and the distance between the pin 78 and the second part 54b of the die segment 54 are determined such that, when the coupling member 70 is thrust radially inwardly by the piston rod 56a to advance the first part 54a, the pin 78 takes the radially outer extreme position within the slits 73 as shown in Figure 6(A) before completion of the advance of the first part 54a. Therefore, the second part 54b also is advanced to the radially inner extreme position. When the piston rod 56a is retracted, the first part 54a begins to retreat in compliance with the movement of the piston rod 56a and coupling member 70. However, the second part 54b remains in the inner extreme position until the coupling member 70 reaches such a position that the pin 78 takes the radially inner extreme position within the slit 73, as shown in Figure 6(B).

Accordingly, the casting operation described with reference to Figures 2 to 5(B) can identically be performed also when the first and second parts 54a, 54b of each die segment 54 are coupled in the manner illustrated in Figures 6(A) and 6(B). Of course it is possible to effect the early retreat of only the first part 54a of each die segment 54 according to the invention by a still different method. For example, use may be made of a toggle linkage or a hinge mechanism. Also it is possible to provide a hydraulic cylinder for each first part 54a and second part 54b of each die segment 54 so as to individually move the separate parts by operating the two cylinders individually. However, the two devices illustrated herein are advantageous in simplicity of construction.

Claims (12)

1. A pressure die casting method for the production of a rotor comprising a vane portion having a plurality of vanes formed radially on the circumference of a central shaft and a base portion which is given by an axially extended endmost part of the central shaft and has no vanes thereon, the method comprising the steps of:: fitting a split die made of a metal in a pressure die casting machine, said split die being divided radially of a central axis into a plurality of die segments that can be moved to a limited extent radially inwardly and outwardly with respect to said central axis, each of said die segments being a combination of a first part shaped correspondingly to said vane portion of the rotor and a second part shaped correspondingly to said base portion of the rotor, said first and second parts being brought into slidable contact with each other in a plane perpendicular to said central axis so as to allow a relative displacement between said first and second parts when said die segments are moved radially of said central axis; forming a die cavity corresponding in shape to the rotor in said split die by moving said die segments radially inwardly to take a radially inner extreme position;; forcing a molten metal under pressure into said die cavity and allowing the molten metal to begin to solidify in said die cavity; initially moving only said first parts of said die segments radially outwardly immediately after solidification of the molten metal in said die cavity in a region to become said vane portion of the rotor to thereby separate said first parts from the solidified metal while said second parts remain in said radially inner extreme position; and moving said second parts of said die segments radially outwardly after solidification of the molten metal in said die cavity in a region to become said base portion of the rotor.

2. A method according to Claim 1, wherein the step of moving said first parts radially outwardly is continued at least until commencement of the step of moving said second parts radially outwardly.

3. A method according to Claim 1, wherein the step of moving said first parts radially outwardly is suspended for a determined period of time before the commencement of the step of moving said second parts radially outwardly.

4. A method according to any one of Claims 1 to 3, wherein the molten metal under pressure is forced into said die cavity along said central axis through an opening located axially at one end of said die cavity.

5. A method according to Claim 1, wherein the number of said plurality of die segments agrees with the number of the vanes of the rotor.

6. A pressure die casting apparatus for the production of a rotor comprising a vane portion having a plurality of vanes formed radially on the circumference of a central shaft and a base portion which is given by an axially extended endmost part of the central shaft and has no vanes thereon, the apparatus comprising:: a die assembly comprising a split die made of a metal to form a die cavity corresponding in shape to the rotor to be produced, said split die being divided radially of a central axis common to said split die and said die cavity into a plurality of die segments that can be moved to a limited extent radially inwardly and outwardly so that said die cavity is formed when said die segments take a radially inner extreme position, each of said die segments being divided by a plane perpendicular to said central axis into a first part shaped corresponding to said vane portion of the rotor and a second part shaped corresponding to said base portion of the rotor such that said first and second parts of each die segment are in slidable contact with each other in said plane, and first means for moving said first and second parts of each of said die segments radially inwardly to allow said die segments to take said inner extreme position and initially moving only said first parts of said die segments radially outwardly from said inner extreme position and thereafter moving also said second parts of said die segments radially outwardly from said inner extreme position; second means for moving said die assembly along said central axis; and third means for forcing a molten metal into said die cavity through an opening located at one end of said die cavity and maintaining the molten metal forced into said die cavity in a pressurized state until solidification of the molten metal.

7. An apparatus according to Claim 6, wherein said first means comprise, for each of said die segments, a slot formed on one of the slidably contacting surfaces of said first and second parts of each die segment, a ridge which is formed on the other of said slidably contacting surfaces to engage said slot and is shorter than said slot in length in the radial direction with respect to said central axis and a reciprocable piston rod fixed at one end thereof to said first part and extending radially of said central axis.

8. An apparatus according to Claim 6, wherein said first means comprise, for each of said die segments, a coupling member arranged radially outwardly of each die segment, a first connecting rod fixed at one end thereof to said first part of each die segment and at the other end to said coupling member, a second connecting rod fixed at one end thereof to said second part so as to extend radially of said central axis and being slidably coupled with said coupling member and a reciprocable piston rod fixed at one end thereof to said coupling member and extending radially of said central axis.

9. An apparatus according to Claim 8, wherein said coupling member is formed with an elongate hole which slidably receives an end portion of said second connecting rod and an elongate slit which is parallel to said elongate hole and provides lateral access to said elongate hole, said first means further comprising a stopper pin fixed to said end portion of said second connecting rod to transversely and slidably engage said slit.

10. An apparatus according to Claim 6, wherein the number of said plurality of die segments agrees with the number of the vanes of the rotor to be produced.

11. An apparatus according to Claim 6, substantially as herein described with reference to Figures 2 to 5(B), or Figures 2,3,6(A) and 6(B) of the accompanying drawings.

12. A pressure die casting method according to Claim 1, performed by using an apparatus according to Claim 11.