JP2013202638A - Hollow piston for disk brake and molding die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hollow piston for a disk brake capable of encouraging non-cutting processes by improving plastic liquidity during forging in an almost cup shape and capable of achieving a lost cost by reducing cutting processes.SOLUTION: A hollow piston for a disk brake, which is formed with a backward extrusion method, has a cup shape with a cylindrical section and a bottom section formed on one end of the cylindrical section. A portion connecting an outer peripheral surface of the cylindrical section and a bottom surface of the bottom section includes is comprised of a curvature section come into contact with the bottom surface, a tapered surface coming into contact with the curvature section and crossing with an outer peripheral surface at a fixed angle, a plurality of projections arranged on a circumference of the bottom surface, and a recessed surface at a central part of the bottom surface. The bottom surface, curvature section, and tapered surface have continued plastic liquidity.

Description

  The present invention relates to a hollow piston for a disc brake and a molding die used for braking a vehicle.

Conventionally, a hollow piston for a disc brake has been mainly formed by forging.
After cutting the bar material, it is upset to prepare a substantially cylindrical blank, and after annealing and lubrication, it is forged into a substantially cup shape by an extrusion method called backward extrusion. After that, in the first cutting process, the substantially cup-shaped opening side end face and the outer peripheral groove for assembling the piston boot are processed, and in the second cutting process, the R-shape and taper are formed at the corners connecting the substantially cup-shaped outer peripheral surface and the bottom surface. The surface is processed and finally the outer peripheral surface is ground to complete.

  As described above, a manufacturing method in which a rough product is formed by forging and then removed by cutting or grinding to obtain a finished product is often applied to mass production of automobile parts because of a high material yield and high productivity.

  However, in recent years, there are many cases where the manufacturing cost is reduced by further reducing the machining allowance or eliminating the cutting, and the molded product is required to have a shape close to the finished product. Among them, a method not using forging has been studied.

  Patent Document 1 proposes a method for producing a hollow piston at low cost by spinning a circular plate material to form a hollow piston.

  This proposal may be simple in the work process, but the conventional forging and forming equipment cannot be used, and a new spinning processing equipment is required.

  Therefore, it is difficult to switch the manufacturing method because capital investment is required, and conventional forging molding remains the mainstream.

  The conventional process for producing a hollow piston for disc brakes using forging is to cut the bar material, prepare an approximately cylindrical blank by upsetting, and after annealing and lubrication, an extrusion method called backward extrusion is used. Forging into a generally cup shape, and then processing the substantially cup-shaped opening side end face and the outer peripheral groove for assembling the piston boot in the first cutting process, and connecting the substantially cup-shaped outer peripheral surface and bottom surface in the second cutting process. R shape and taper surface are processed at the corner, and finally the outer peripheral surface is ground and completed.

  If it is going to omit the cutting process in this manufacturing process, when forging into a substantially cup shape by backward extrusion, an R shape and a tapered surface are simultaneously formed at the corner connecting the outer peripheral surface and the bottom surface of the substantially cup shape. Thus, it can be easily considered that the second cutting step is omitted.

  For example, although the backward extrusion shown in FIG. 3 of Patent Document 2 is the most basic backward extrusion, when an R shape is simultaneously formed at the corner portion connecting the outer peripheral surface and the bottom surface of the substantially cup shape, 3 is provided with an R shape (mold surface 2b) on the die (mold 2).

JP 2002-70902 A JP 2000-233255 A JP 2004-358589 A

  When this R shape is formed on the die as in the above prior art, depending on the size of R, concentrated stress is applied to the R shape portion during molding, and cracks are likely to occur in the R shape portion of the die. As a result, mass production was difficult.

  In this case, it is a general countermeasure to form a dividing surface in the vicinity of the R shape (in the case of FIG. 2 of Patent Document 3, in the cross-sectional direction near the boundary between 2a and 2b). The molding lubricant is easy to bite, and the material or molding lubricant that has entered the minute amount into the split surface accumulates inside the split surface during repeated molding, and the split surface is opened and the die is easily damaged. The method is also difficult to mass produce.

  Therefore, as a method of reducing the concentrated stress applied to the R-shaped portion of the die, there is a method of projecting the position of the upper surface of the counter punch (3a in the case of FIG. 2 of Patent Document 3) from the bottom surface of the die. Although this method improves the mold life, the material that enters the groove formed by the R shape of the die, the bottom surface, and the counter punch locally becomes a dead metal (the material does not flow), and the R shape portion However, there was a concern about the occurrence of macre, so finishing by the cutting process was necessary.

  Also, in forging, by increasing the thickness of the substantially cup-shaped bottom, the material flow around the R-shape can be reduced as a whole, so that the material does not bite into the split surface and mass production is possible. Although it is considered possible, in this case, the product mass is increased, so that the material yield is deteriorated and the cost effect cannot be obtained. In addition, there is a concern that the vehicle weight increases and the running performance decreases.

  The present invention has been made in view of the above circumstances, and by making plastic flow good when forging into a substantially cup shape, it is possible to eliminate cutting, reduce the cutting process, and reduce the cost. The present invention provides a hollow piston for a disc brake that achieves the above.

  A hollow piston for a disc brake formed by backward extrusion of the present invention, which has a cup shape having a cylindrical portion and a bottom portion formed at one end of the cylindrical portion, and an outer peripheral surface and a bottom portion of the cylindrical portion The portion connecting the bottom surfaces of the bottom surface, the curvature portion in contact with the bottom surface, a tapered surface in contact with the curvature portion and intersecting with the outer peripheral surface at a certain angle, a plurality of protrusions arranged on the circumference on the bottom surface, The bottom surface, the curved portion, and the tapered surface have a continuous plastic flow.

  Further, it is a forming die for forming a hollow piston for a disc brake by backward extrusion, and the hollow piston has a cup shape having a cylindrical portion and a bottom portion formed at one end of the cylindrical portion, and the cylindrical portion The portion connecting the outer peripheral surface and the bottom surface of the bottom portion is formed of a curvature portion that is in contact with the bottom surface, and a tapered surface that is in contact with the curvature portion and intersects the outer peripheral surface at a certain angle. And an upper die separated from the upper die, and a lower die having a second cavity for forming a tapered surface, a curvature shape and a bottom surface, and the first cavity diameter on the upper die dividing surface of the upper die is lower die It is set to be larger than the second cavity diameter in the lower mold dividing surface, and a step is formed in the upper mold and lower mold dividing portions.

  The present invention provides a hollow piston for a disc brake that achieves low cutting cost by reducing plasticity by making plastic flow good when forging into a substantially cup shape. be able to.

It is a figure showing an example of a section of a disc brake of the present invention. It is a figure which shows the cross-sectional example of the hollow piston shape | molded with the shaping | molding die of the hollow piston for disc brakes which concerns on this invention. It is a figure which shows the manufacturing process of the hollow piston of this invention shown in FIG. It is a figure which shows the cross section of the metal mold | die structure of the shaping | molding die of the hollow piston for disc brakes of this invention. It is a figure which shows the cross section which expanded the division | segmentation part of the shaping | molding die of the hollow piston for disc brakes of this invention.

Hereinafter, the present invention will be described based on examples.
FIG. 1 is a cross-sectional view when a hollow piston is used for a disc brake.
In FIG. 1, the caliper 1 is formed with a cylinder 2 on one leg 1 </ b> A, and the other leg 1 </ b> B extends to the opposite side of the disk D via a bridge 1 </ b> C straddling the disk D. The hollow piston 3 is slidably inserted into the cylinder 2, and the hollow piston 3 is formed in a substantially cup shape, and forms a hydraulic pressure chamber 4 between the bottom portion 13 of the cylinder 2. The liquid introduction hole 5 is drilled in the bottom of the leg 1A of the caliper 1, and the liquid introduction hole 5 introduces the hydraulic pressure from the master cylinder (not shown) into the hydraulic pressure chamber 4, and this hydraulic pressure causes the hollow piston 3 is slid toward the disk D.

  The friction pads 6 and 7 are located between the hollow piston 3 and the leg 1B of the caliper 1 and are arranged on both sides of the disk D. The friction pads 6 and 7 slide on the hollow piston 3 by hydraulic pressure. When pressed, both sides of the disk D are pressed, and a braking force is applied to the disk D. That is, when the friction pad 6 is pressed against one side of the disk D by the hollow piston 3, the entire caliper 1 is moved in the opposite direction to the hollow piston 3 by the reaction force, and the other leg portion 1 B pushes the friction pad 7. The disk D is pressed against the other side.

  The annular boot mounting groove 8 is formed on the inner peripheral surface of the opening side of the cylinder 2, the annular boot mounting groove 9 is formed on the outer periphery of the hollow piston 3, and the piston boot 10 is disposed between the cylinder 2 and the hollow piston 3. The piston boot 10 is formed on the bellows by a flexible resin material, and both end sides thereof are assembled in the boot mounting grooves 8 and 9. The piston boot 10 prevents external earth and sand, rainwater and the like from entering between the cylinder 2 and the hollow piston 3.

  An annular seal mounting groove 11 is formed on the inner peripheral surface of the cylinder 2, and a seal member 12 is assembled in the mounting groove. The seal member 12 is slidable with the outer peripheral surface of the hollow piston 3 to prevent the liquid in the hydraulic chamber 4 from leaking to the outside.

Next, the hollow piston 3 obtained by the molding method of the present invention will be described with reference to FIG.
2 is a cross-sectional view of the hollow piston 3, in which P is an arrow view and Q is an enlarged view.
The hollow piston 3 includes a cylindrical portion 31 and a bottom portion 32 formed at one end thereof, and has a substantially cup shape.

  Since the outer peripheral surface 33 of the cylindrical part 31 slides with the cylinder 2 and the seal member 12 in FIG. 1, surface accuracy is required.

  A boot mounting groove 9 is formed in an annular shape on the outer periphery on the opening side, and a pressing surface 34 for pressing the friction pads 6 and 7 in FIG.

  In addition to reducing the weight of the inner circumferential space 35 of the cylindrical portion 31, the heat transmitted from the friction pad is difficult to be transmitted to the hydraulic pressure chamber 4 in FIG. 1 during braking and serves to prevent boiling and deterioration of the liquid. Yes.

  In the assembly with the caliper 1 of FIG. 1, the inner peripheral surface 36 is automatically fitted into the caliper 1 by fitting a jig as disclosed in Japanese Patent Laid-Open No. 3-24331. Coaxial accuracy is required.

  A protrusion 38 is formed on the bottom surface 37 of the bottom portion 32 at the position shown in the P arrow view of FIG. This is provided in order to always secure the hydraulic chamber 4 so that the bottom surface 37 of the hollow piston 3 does not stick to the bottom 13 of the cylinder of FIG. A recess 42 is formed at the center of the bottom surface 37. A portion connecting the outer peripheral surface 33 and the bottom surface 37 will be described with reference to FIG. 2Q enlarged view.

  A portion connecting the outer peripheral surface 33 and the bottom surface 37 is formed by an R portion 39 which is a curved portion in contact with the bottom surface 37 and a tapered surface 40 in contact with the R portion 39 and intersecting the outer peripheral surface 33 at an angle θ. In the present invention, the bottom surface 37, the curved portion R portion 39 and the tapered surface 40 have a continuous plastic flow. The curved portion R portion 39 and the taper surface 40 are scratched to the piston boot 10, the cylinder 2, and the seal member 12 when the hollow piston 3 is automatically assembled as described in Japanese Patent Laid-Open No. 3-24331. It is a shape necessary to incorporate so as not to attach.

  An object of the present invention is to form the R portion 39 and the tapered surface 40 by forging, to achieve no cutting, to reduce the cutting process, and to reduce the manufacturing cost of the hollow piston 3.

Next, the manufacturing process of the hollow piston 3 shown in FIG. 2 will be described with reference to FIG.
A cylindrical blank 51 is prepared by the first blank process 50. This cylindrical blank 51 is formed by upsetting a bar material that has been press-cut or saw-cut into a predetermined mass, and is formed slightly smaller than the cavity diameter of the die for backward extrusion in the next step. Moreover, it annealed and lubricated in preparation for back extrusion of the next process.

  The next process is a process of forming a substantially cup-shaped piston material 53 by backward extrusion in the forging process 52. Although the mold structure will be described later, the backward extrusion is called backward extrusion because the material is extruded toward the rear in the pressing direction of the forming punch.

  In FIG. 2, the parts that become the completed shape of the hollow piston 3 by the backward extrusion are the inner peripheral surface 36, the inner peripheral bottom surface 41, the bottom surface 37, the protrusion 38, the concave portion 42, the R portion 39, and the tapered surface 40.

  The next process is a cutting process 54 using a lathe or a single-purpose board, chucking the outer peripheral surface 33 with the axial direction as the reference surface 55 at the bottom surface 37, and processing from the boot mounting groove 9 on the opening side to the pressing surface 34 (conventional technology). In this case, after this cutting process, the outer peripheral surface 33 must be re-chucked in the axial direction with reference to the pressing surface 34 to process the R portion 39 and the tapered surface 40 in FIG.

  Thereafter, the outer peripheral surface 33 is ground and finished in a grinding step 56 with a centerless polishing machine, and the hollow piston 3 is completed.

  Next, a die 61 for forming the hollow piston 3 for disc brake according to the present invention will be described with reference to FIGS.

  FIG. 4 is a sectional view of the mold structure in which the rear extrusion die of the present invention is installed, and FIG. 5 is a sectional view of the die 61 alone in which the division portion 69 of the rear extrusion die of the present invention is enlarged. The piston material 53 is omitted so that it can be easily understood.

The mold structure in FIG. 4 will be described.
3 is a state immediately after backward extrusion, the blank 51 in FIG. 3 is put into the first cavity 62 of the die 61, the punch 111 presses the blank 51 in the pressing direction 112, and the first cavity 62 and the punch 111 are formed. A state in which the material flows in the direction opposite to the pressing direction 112 of the punch 111, that is, in the backward direction from the gap formed by the surface 113 is shown.

  More specifically, the die 61 is fixed to the pressure receiving table 91. The die 61 includes a first cavity 62 in FIG. 5 that molds the outer peripheral surface 33 of the hollow piston 3 in FIG. 2, a second cavity bottom surface 64 in FIG. 5 that molds the bottom surface 37 of the hollow piston 3 in FIG. The concave portion 65 for forming the protrusion 38 of FIG. 2, the R surface 66 of FIG. 5 for forming the R portion 39 of FIG. 2, and the tapered surface 67 of FIG. 5 for forming the tapered surface 40 of FIG. Two cavities 63 are provided.

  In FIG. 4, a counter punch 101 is placed in a slightly protruding manner at the inner peripheral center of the concave portion 65 of the second cavity bottom surface 64 to mold the concave portion 42 of the hollow piston 3 in FIG. 2. The bottom inner surface 68 of the die 61 and the counter punch outer periphery 102 are fitted and slidable. After the molding is completed, the counter punch 101 moves in the discharge direction 103 in order to take out the piston material 53 from the die 61. The piston material 53 is discharged.

  In backward extrusion, a counter punch must be installed in the discharge mechanism of the molded product. The reason why the counter punch 101 is slightly protruded at the center of the inner periphery of the recess 65 of the second cavity bottom surface 64 as in the present invention will be described.

  If the diameter of the counter punch 101 is enlarged and the recess 65 is formed on the counter punch forming surface 104, a groove is formed between the R surface 66 and the counter punch 101. When material flows into this groove, the material becomes dead metal (the material does not flow), and macre-in occurs at the boundary between the counter punch forming surface and the R surface and the material flowing across the groove. It is.

  As in the present invention, if the counter punch 101 is installed so as to protrude slightly at the center of the inner periphery of the concave portion 65 of the second cavity bottom surface 64, a space that becomes a groove will not be formed, so that the above-described macre-in does not occur. . Even if it occurs, the creaking does not reach the R surface 66.

  Since the amount of protrusion of the counter punch 101 must be taken into account by elastic deformation of the mold, it varies depending on the mold material. In the embodiment, the depth of the recess 42 of the hollow piston 3 in FIG. With .15, molding without macre was possible. As the depth of the concave portion is increased, the molding end surface 114 of the punch 111 and the molding surface 104 of the counter punch 101 are brought closer to each other, so that the molding load is extremely increased and the mold life is deteriorated. .

The backward extrusion die of the present invention will be described with reference to FIGS.
The die 61 includes an upper mold 70 having a first cavity 62 that molds the outer peripheral surface 33 of the hollow piston 3, a cavity bottom surface 64 that molds the bottom surface 37 of the hollow piston 3, and a recess 65 that molds the protrusion 38 on the bottom surface 37. A lower die 71 having a second cavity 63 integrally formed with an R surface 66 for forming the R portion 39 connecting the bottom surface 37 and the outer peripheral surface 33 and a tapered surface 67 for forming the tapered surface 40 in contact with the R portion 39; Are produced individually.

  In order to reinforce the upper mold 72 that forms the first cavity 62, the upper mold 70 is shrink-fitted in a double manner by the first reinforcing member 74 and the second reinforcing member 75.

  The lower mold 71 is shrink-fitted in a single layer by a third reinforcing member 76 in order to reinforce the lower mold 73 that forms the second cavity.

The above-described reinforcement by shrink fitting can be performed by taper press-fitting, for example.
The lower inner periphery 77 of the second reinforcing member 75 of the upper mold 70 is set so that the lower mold 71 is fitted into the lower mold outer periphery 78 by press fitting, and the lower mold dividing surface 79 and the upper mold dividing surface 80 are brought into contact with each other. Temporarily fixed. The press-fitting may be shrink fitting with a small tightening margin.

  A circumferential groove 81 is formed on the opening side of the lower inner periphery 77 of the upper mold 70, while a notch 82 is formed at the position of the lower mold 71 facing the circumferential groove 81, A ring-shaped coupling member 83 having substantially the same shape is embedded in the notch 82, and the end surface 84 of the coupling member 83 is pressed by a press using a jig (not shown), and the material of the coupling member 83 is changed. The lower die 71 is fixed to the upper die 70 by plastic flow in the circumferential groove 81. At this time, since the compressive stress transmitted when the coupling member 83 is pressed remains as the residual stress 85 in the split portion 69, the split portion 69 can be prevented from opening when the piston material 53 is formed.

The dividing part 69 of the die 61 will be described in detail.
In the die 61, the first cavity diameter D1 in the upper mold dividing surface 80 is set larger than the second cavity diameter D2 in the lower mold dividing surface 79, and a step δ is formed in the divided portion. It is said.

The purpose of the step δ will be described.
3 is inserted into the first cavity 62 of the die 61, the punch 111 presses the blank 51, and backward extrusion starts. At the beginning of the backward extrusion, the distance between the second cavity 63 and the punch forming end surface 114 is long, so that the molding load is dispersed in the first cavity 62 and the second cavity 63. Therefore, since the R surface 66 and the tapered surface 67 of the second cavity 63 are square portions, the filling of the material does not proceed and the material flow is small.

  However, as the pressing of the punch 111 progresses and the bottom thickness t decreases, most of the molding load transmitted by the pressing of the punch 111 is received by the second cavity 62.

  At the same time, filling of the material into the R surface 66 and the tapered surface 67 of the second cavity 62 proceeds, and a material flow 86 indicated by an arrow is generated.

  At this time, the lower mold 71 is crushed in the axial direction, and at the same time, the second cavity diameter D2 is enlarged. If the enlarged second cavity diameter D2 is smaller than the first cavity diameter D1 and the step δ is slightly secured, the material flow is not blocked.

  If the second cavity diameter D2 is larger than the first cavity diameter D1 and the step δ becomes negative, the material flowing on the surface of the tapered surface 67 is dammed to the upper mold dividing surface 80 of the first cavity 62. As a result, it enters into the internal gap 87 and accumulates as molding is repeated, and the dividing portion 69 opens, making molding impossible. Further, galling occurs on the outer peripheral surface 33 of the molded piston material 53.

  Therefore, in order to keep the material flow 86 normal, it is necessary to set the step δ in advance.

  If the step δ is set too large, the step mark remains on the outer peripheral surface 33 of the piston material 53 and the normal material flow cannot be maintained. However, in the embodiment, a normal material flow 86 is obtained when δ is about 0.06 mm. However, since it varies depending on various conditions, it is desirable to obtain it experimentally.

  Next, regarding the method of forming the step δ, when manufacturing by shrink fitting is considered, the upper die and the lower die are reinforced by shrink fitting with the same reinforcing ring, and then the cavity inner diameter is ground or Since it is finished by electric discharge machining, the position of the machining tool or electrode is shifted from the divided surface or becomes a sagging shape, and it is difficult to process and finish a minute step on the divided surface.

  However, as described above, the upper mold 70 having the first cavity 62 and the lower mold 71 having the second cavity 63 are individually manufactured to manage the dimensions, and press-fit and plastic coupling by the coupling member 83 are used. By using the die 61 as a result, a step δ can be formed in the dividing portion 69.

  Next, a method for discharging the molding lubricant 121 entering the internal gap 87 from the dividing portion 69 from the die 61 will be described.

  The molding lubricant 121 may be a bond process applied to the blank 51. A powdered solid lubricant is applied to the surface of the blank 51 after the bond treatment.

  The solid lubricant becomes high pressure and becomes liquid during molding and enters the internal gap 87 from the divided portion 69, but then returns to a solid and accumulates in the divided gap 87, causing the divided portion 69 to open.

  In the embodiment, a plurality of holes 88 (first holes) that are radially opened from the inner gap 87 of the die 61 in the outer circumferential direction and the third reinforcing member 76 of the lower die 71 are arranged on the circumference in the axial direction. A plurality of opened holes 89 (second holes) are provided, and high-pressure air 122 that is high-pressure air is passed through the holes 89 that are the second holes, so that the internal gap 87 is separated from the dividing portion 69. The molding lubricant 121 entering the rim is discharged together with the high-pressure air 122 to the outside.

The high-pressure air 122 is also useful for preventing the temperature of the die 61 from rising.
As described above, the disc piston hollow piston 3 formed by the backward extrusion die 61 of the present invention has the R portion 39 and the tapered surface 40 at the corner portion connecting the substantially cup-shaped outer peripheral surface 33 and the bottom surface 37. Since molding can be performed simultaneously with the backward extrusion, no cutting is possible, and the cutting process can be reduced and the cost can be reduced.

  Further, the disc brake hollow piston 3 formed by the backward extrusion die 61 of the present invention has no material clogging or the like in the R portion 39, and the material flow is continuous. Since plastic flow is good, quality can be stabilized.

  Furthermore, the disc brake hollow piston 3 formed by the backward extrusion die 61 of the present invention has good plastic flow, so that there is no biting of material in the divided portion 69, and there is no gap between the divided portion 69 and the internal gap. Since the molding lubricant 121 that has entered 87 can be discharged, there is no accumulation of the molding lubricant 121 in the internal gap 87, so that the dividing portion 69 does not open and the mold life is good.

DESCRIPTION OF SYMBOLS 1 Caliper 2 Cylinder 3 Hollow piston 4 Hydraulic chamber 5 Liquid introduction hole 6, 7 Friction pad 9 Boot attachment groove 10 Piston boot 11 Seal attachment groove 12 Seal member 13, 32 Bottom part 31 Cylindrical part 33 Outer surface 34 Press surface 35 Inner circumference Space 36 Inner peripheral surface 37 Bottom surface 38 Projection 39 R portion 40 Tapered surface 41 Inner peripheral bottom surface 42 Recessed portion 51 Blank 53 Piston material 61 Die 70 Upper die 71 Lower die 83 Coupling member 91 Pressure receiving base 101 Counter punch 102 Counter punch outer periphery 103 Discharging direction 104 Counter punch molding surface 111 Punch 112 Pressing direction 113 Molding surface 114 Punch molding end surface

Claims (6)

A hollow piston for a disc brake formed by backward extrusion,
It has a cup shape having a tube portion and a bottom portion formed at one end of the tube portion,
The portion connecting the outer peripheral surface of the cylindrical portion and the bottom surface of the bottom portion is,
A curvature portion in contact with the bottom surface;
A tapered surface in contact with the curvature portion and intersecting the outer peripheral surface at a constant angle;
A plurality of protrusions disposed on the circumference on the bottom surface;
A concave surface at the bottom center, and
The bottom surface, the curved portion, and the tapered surface are hollow pistons having a continuous plastic flow. The hollow piston according to claim 1,
A hollow piston in which the bottom surface, the curvature portion, and the tapered surface are not cut. A molding die for molding a hollow piston for a disc brake by backward extrusion,
The hollow piston has a cup shape having a cylindrical portion and a bottom portion formed at one end of the cylindrical portion, and a portion connecting the outer peripheral surface of the cylindrical portion and the bottom surface of the bottom portion is in contact with the bottom surface. A curved portion, and a tapered surface that is in contact with the curved portion and intersects the outer peripheral surface at a certain angle;
An upper mold having a first cavity for molding the outer peripheral surface, and a lower mold separated from the upper mold and having a second cavity for molding the tapered surface, the curvature shape and the bottom surface,
The first cavity diameter in the upper mold dividing surface of the upper mold is set larger than the second cavity diameter in the lower mold dividing surface of the lower mold,
A forming die in which a step is formed in a divided portion of the upper mold and the lower mold. A forming die according to claim 3, wherein
A plurality of protrusions are formed on the bottom surface of the hollow piston,
The plurality of protrusions are formed by a plurality of recesses provided on the bottom surface of the second cavity of the second cavity,
The second cavity is a molding die in which the plurality of concave portions, a taper molding surface, a curvature molding surface, and the bottom surface are integrally formed. A forming die according to claim 3 or claim 4,
A first hole that is radially opened from the division gap between the upper mold and the lower mold in an outer peripheral direction, and a second hole that is opened in the axial direction in the reinforcing member of the lower mold,
A molding die in which molding lubricant that has entered through the dividing gap during molding can be discharged to the outside through high-pressure air through the second hole. A method for forming a hollow piston for a disc brake using the forming die according to claim 4,
The method of forming a hollow piston, wherein the recess of the hollow piston is formed by a counter punch that is slidably installed at the inner peripheral center of the recess provided on the bottom surface of the second cavity.