STEPPED DIE

Abstract

Provided is a stepped die which includes: an inner ring having a cylindrical shape, and an outer ring having a cylindrical shape which is fitted on an outer periphery of the inner ring by shrinkage fitting, in which a recessed portion for molding which has a stepped portion is formed on an inner side of the inner ring. A shrinkage fitting ratio of the outer ring to the inner ring is set to a value which falls within a range of from 0.12% to 0.25%.

Claims

1. A stepped die for powder molding of metal powder comprising: an inner ring made of a sintered hard alloy and having a cylindrical shape, and an outer ring having a cylindrical shape which is fitted on an outer periphery of the inner ring by shrinkage fitting, in which a recessed portion for molding which has a stepped portion is formed on an inner side of the inner ring, wherein a flange portion which is engaged with a die plate is formed on an outer periphery of the outer ring, wherein only the flange portion of the stepped die is supported by the die plate while a lower surface of the stepped die is not supported by other member, and wherein a shrinkage fitting ratio of the outer ring to the inner ring is set to a value which falls within a range of from 0.12% to 0.25%.

2. The stepped die according to claim 1, wherein a ratio between an outer diameter of the inner ring and a diameter of a maximum imaginary circle which is an imaginary circle having a center on a central axis of the inner ring and passes a corner portion of the stepped portion remotest from the center in a radially outward direction is set to 1.4 or more.

3. The stepped die according to claim 2, wherein the ratio is set to 2.0 or less.

4. The stepped die according to claim 1, wherein a wall thickness which is difference between an outer diameter of the inner ring and a diameter of a maximum imaginary circle which is an imaginary circle having a center on a central axis of the inner ring and passes a corner portion of the stepped portion remotest from the center in a radially outward direction is set to 5 mm or more.

5. The stepped die according to claim 1, wherein a material of the inner ring is a sintered hard alloy, and a material of the outer ring is hardened steel.

6. The stepped die according to claim 1, wherein the shrinkage fitting ratio of the outer ring to the inner ring is set to a value which falls within a range from 0.15% to 0.20%.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0015] [FIG. 1] FIG. 1 is a plan view of a stepped die according to one embodiment of the present invention.

[0016] [FIG. 2] FIG. 2 is a cross-sectional view of the stepped die shown in FIG. 1.

[0017] [FIG. 3] FIG. 3 is a perspective explanatory view of an inner ring of the stepped die shown in FIG. 1.

[0018] [FIG. 4] FIG. 4 is a graph showing a relationship between a strength ratio of a stepped corner R portion and a shrinkage fitting ratio.

[0019] [FIG. 5] FIG. 5 is a graph showing a relationship between a strength ratio of the stepped corner R portion and an inner ring ratio.

[0020] [FIG. 6] FIG. 6 is a graph showing a relationship between a compressive strength ratio and a wall thickness.

[0021] [FIG. 7] FIG. 7 is a graph showing a relationship between a compressive strength ratio and an inner ring ratio.

[0022] [FIG. 8] FIG. 8 is a perspective view showing one example of a powder molded product which has a stepped portion on an outer side thereof.

[0023] [FIG. 9] FIG. 9 is a plan view showing one example of a stepped die.

[0024] [FIG. 10] FIG. 10 is a cross-sectional view of the stepped die shown in FIG. 9.

[0025] [FIG. 11] FIG. 11 is a photograph showing a crack generated in a corner portion of a stepped portion.

DESCRIPTION OF EMBODIMENTS

[0026] A stepped die of the present invention includes an inner ring having a cylindrical shape, and an outer ring having a annular shape which is fitted on an outer periphery of the inner ring by shrinkage fitting, and a recessed portion for molding which has a stepped portion is formed on an inner side of the inner ring. A shrinkage fitting ratio of the outer ring to the inner ring is set to a value which falls within a range of from 0.12% to 0.25%.

[0027] It is preferable that a ratio between an outer diameter of the inner ring and a diameter of a maximum imaginary circle which is an imaginary circle having a center on a central axis of the inner ring and passes a corner portion of the stepped portion remotest from the center in a radially outward direction be set to 1.4 or more. In this case, by imparting a predetermined wall thickness to the inner ring, a resistance of the inner ring against a residual compressive stress applied to the inner ring due to shrinkage fitting of the outer ring can be increased.

[0028] Further, it is preferable that the ratio be set to 2.0 or less. In this case, by restricting a wall thickness of the inner ring to a predetermined amount or less, the large-sizing of the inner ring and eventually the large-sizing of the stepped die can be suppressed while maintaining a resistance of the inner ring against a residual compressive stress.

[0029] It is preferable that a wall thickness which is the difference between an outer diameter of the inner ring and the diameter of the maximum imaginary circle which is the imaginary circle having the center on the central axis of the inner ring and passes the corner portion of the stepped portion remotest from the center in a radially outward direction be set to 5 mm or more. In this case, by imparting a predetermined wall thickness to the inner ring, a resistance of the inner ring against a residual compressive stress applied to the inner ring due to shrinkage fitting of the outer ring can be increased.

[0030] A material of the inner ring may be a sintered hard alloy, and a material of the outer ring may be hardened steel. In this case, compressive strength and fatigue strength required for the inner ring can be ensured.

[0031] Hereinafter, a stepped die according to an embodiment of the present invention is described in detail with reference to attached drawings. FIG. 1 is a plan view of a stepped die 1 according to one embodiment of the present invention, and FIG. 2 is a cross-sectional view of the stepped die 1 shown in FIG. 1.

[0032] The stepped die 1 according to the present embodiment is a die used in manufacturing a green compact formed by compressing powder for metallurgy. The stepped die 1 includes an inner ring 2, and an outer ring 3 which is fitted on an outer periphery of the inner ring 2 by shrinkage fitting. A recessed portion 4 for molding is formed on an inner side of the inner ring 2.

[0033] The inner ring 2 has a cylindrical shape, and can be manufactured using a sintered hard alloy such as a WC-Co alloy or a WC-TiC-Co alloy, for example. The outer ring 3 also has a cylindrical shape, and can be manufactured using general hardened steel. A flange portion 6 which is engaged with a die plate 5 is formed on an outer periphery of the outer ring 3 over the whole circumference.

[0034] The recessed portion 4 has a rectangular shape as viewed in a plan view on an upper surface side (upper side in FIG. 2) of the inner ring 2, and has a circular shape as viewed in a plan view on a lower surface side (lower side in FIG. 2) of the inner ring 2. A stepped portion 7 is formed on a boundary portion between an upper recessed portion having a rectangular shape as viewed in a plan view and a lower recessed portion having a circular shape as viewed in a plan view. The stepped portion 7 is a portion corresponding to a step of a molded product (see FIG. 8) which is formed by molding using the stepped die 1.

[0035] In this embodiment, an outer diameter of the inner ring 2 and an inner diameter of the outer ring 3 are set such that a shrinkage fitting ratio or a shrinkage fitting amount expressed by the following formula (1) (hereinafter, represented as shrinkage fitting ratio) takes a value which falls within a range of from 0.12% to 0.25%.

Shrinkage fitting ratio (%)={1(inner diameter of outer ring/outer diameter of inner ring)}100 (1)

[0036] When a shrinkage fitting ratio (%) is less than 0.12%, there is a possibility that a residual compressive stress is insufficient so that a crack occurs at the time of molding. On the other hand, when a shrinkage fitting ratio (%) is more than 0.25%, there is a possibility that a crack occurs at the time of shrinkage fitting. From a viewpoint of surely preventing the occurrence of a crack and also suppressing the large-sizing of the inner ring, it is preferable that a shrinkage fitting ratio (%) be set to a value which falls within a range of from 0.15% to 0.20%.

[0037] In this embodiment, a ratio between an outer diameter d1 of the inner ring 2 and a diameter d2 of an imaginary circle P which is an imaginary circle having a center on a central axis O of the inner ring 2 and passes a corner portion 7a of the stepped portion 7 remotest from the center O in a radially outward direction (hereinafter, the imaginary circle is also referred to as maximum imaginary circle) is set to 1.4 or more. Hereinafter, this ratio is also referred to as inner ring ratio. When the inner ring ratio is less than 1.4, there is a possibility that a crack occurs in a thin wall thickness portion of the inner ring 2 due to a residual compressive stress generated in the inner ring 2 brought about by fitting the outer ring 3 on the outer periphery of the inner ring 2 by shrinkage fitting. On the other hand, when the inner ring ratio is set to 1.4 or more, there is no possibility that the above-mentioned drawback occurs. However, when the inner ring ratio is excessively large, the inner ring 2 and eventually the stepped die 1 becomes large-sized and hence, it is preferable that the inner ring ratio be set to 2.0 or less.

[0038] Further, based on a viewpoint substantially equal to the viewpoint taken with respect to the inner ring ratio described previously, in this embodiment, a wall thickness which is a value obtained by dividing a difference between the outer diameter d1 of the inner ring 2 and a diameter d2 of the previously-mentioned maximum imaginary circle by 2 is set to 5 mm or more. When the wall thickness is less than 5 mm, there is a possibility that a crack occurs in a thin wall thickness portion of the inner ring 2 due to a residual compressive stress generated in the inner ring 2 brought about by fitting the outer ring 3 on the outer periphery of the inner ring 2 by shrinkage fitting. On the other hand, when the wall thickness is equal to or more than 5 mm, there is no possibility that the above-mentioned drawback occurs. However, when the wall thickness is excessively large, the inner ring 2 and eventually the stepped die 1 becomes large-sized and hence, it is preferable that the wall thickness be set to 40 mm or less.

Test Example 1

[0039] Green compacts were prepared by pressure molding such that metal powder was filled in the recessed portion for molding and was press-molded at a molding pressure of 10 t/cm.sup.2 while variously changing, as described in Table 1, a diameter of the inner ring, an inner ring ratio, a wall thickness (a value obtained by dividing a difference between an outer diameter of the inner ring and an diameter of the maximum imaginary circle described previously by 2), and a shrinkage fitting ratio (see the formula (1)) in a stepped die having the configuration and shape shown in FIG. 1 and FIG. 2.

[0040] A height h of the stepped die (see FIG. 2) was set to 40 mm. A length w1 of a long side of the rectangular portion of the recessed portion for molding was set to 21 mm, a length w2 of a short side of the rectangular portion was set to 16 mm, and a diameter d3 of a circular columnar portion of the recessed portion was set to 10 mm. Further, a material of the inner ring was a WC-Co based sintered hard alloy, and a material of the outer ring was hot die steel.

Table 1

[0041]

TABLE-US-00001 TABLE 1 Equivalent stress aeq of stepped corner R portion [MPa] Inner ring diameter 70 60 50 45 40 35 32 Inner ring ratio 2.8 2.4 2.0 1.8 1.6 1.4 1.3 Wall thickness [mm] 23 18 13 10 8 5 4 Shrinkage 0.00 765 776 790 799 807 817 823 fitting 0.10 681 679 664 654 644 628 616 ratio [%] 0.12 672 664 652 641 629 610 599 0.15 666 663 651 639 629 608 591 0.20 659 658 653 642 629 607 594 0.25 659 658 651 643 632 612 601 0.35 664 661 654 646 640 633 627 0.50 693 693 695 696 697 697 697 * Molding pressure: 10 t/cm.sup.2

[0042] Table 1 shows an equivalent stress aeq of the stepped corner R portion when the diameter, the inner ring ratio, the wall thickness, and the shrinkage fitting ratio of each inner ring were variously changed. As shown in FIG. 3, stepped corner R portion means a short-side edge portion 7h of the stepped portion 7 having a rectangular shape as viewed in a plan view, and side-surface corner portion in Tables 3 and 4 described later means a boundary portion between two neighboring surfaces out of inner surfaces of the inner ring which face the recessed portion 4 having a rectangular shape as viewed in a plan view, and is the same portion as the corner portion 7a described previously.

[0043] The equivalent stress aeq is a value calculated by the following formula (2).

aeq=a/(1m/.sub.B) (2)

[0044] In the formula (2), a is an amplitude of stress generated at the time of molding metal powder by pressure molding, and m indicates an average stress. .sub.B is a tensile strength which is a value unique to a material. In the present test example 1, a WC-Co sintered hard alloy was used as a material of the inner ring so that the value of GB is 1600 MPa.

[0045] Table 2 shows a strength ratio (fatigue strength/aeq) calculated based on the equivalent stress aeq shown in Table 1 and a fatigue strength which is a value unique to a material. In the present test example 1, a WC-Co sintered hard alloy was used as a material of the inner ring so that the fatigue strength was 700 MPa.

TABLE-US-00002 TABLE 2 Strength ratio of stepped corner R portion (fatigue strength of material aeq) Inner ring diameter 70 60 50 45 40 35 32 Inner ring ratio 2.8 2.4 2.0 1.8 1.6 1.4 1.3 Wall thickness [mm] 23 18 13 10 8 5 4 Shrinkage 0.00 0.92 0.90 0.89 0.88 0.87 0.86 0.85 fitting 0.10 1.03 1.03 1.05 1.07 1.09 1.12 1.14 ratio [%] 0.12 1.04 1.05 1.07 1.09 1.11 1.15 1.17 0.15 1.05 1.06 1.07 1.10 1.11 1.15 1.18 0.20 1.06 1.06 1.07 1.09 1.11 1.15 1.18 0.25 1.06 1.06 1.07 1.09 1.11 1.14 1.16 0.35 1.05 1.06 1.07 1.08 1.09 1.11 1.12 0.50 1.01 1.01 1.01 1.01 1.00 1.00 1.00 *Molding pressure: 10 t/cm.sup.2

[0046] FIG. 4 shows the result shown in Table 2 in a graphical form for respective inner ring ratios, and FIG. 5 shows the result in Table 2 in a graphical form for respective shrinkage fitting ratios. In FIG. 4, a strength ratio of the stepped corner R portion is taken on an axis of ordinates, and a shrinkage fitting ratio (%) is taken on an axis of abscissas. Further, in FIG. 5, a strength ratio of the stepped corner R portion is taken on an axis of ordinates, and an inner ring ratio is taken on an axis of abscissas.

[0047] From FIG. 4, it is understood that a strength ratio of the stepped corner R portion is substantially fixed in a stable manner when a shrinkage fitting ratio (%) falls within a range of from 0.12 to 0.25. Also from FIG. 5, it is understood that a strength ratio of the stepped corner R portion takes a substantially fixed value when the inner ring ratio exceeds 2.0.

[0048] In the test example 1, it was confirmed (visually recognized) that a crack was generated in the sample (strength ratio: 1.06) where a shrinkage fitting ratio was set to 0.35% and an inner ring ratio was set to 2.4. On the other hand, a crack was not confirmed in the sample (strength ratio: 1.11) where a shrinkage fitting ratio was set to 0.15%, and an inner ring ratio was set to 1.6.

Test Example 2

[0049] A compressive stress which was generated in a side surface corner portion of the stepped portion of the inner ring (the portion indicated by 7a in FIG. 3 as descried previously) was obtained while variously changing, as described in Table 3, a diameter of the inner ring, an inner ring ratio, a wall thickness (a value obtained by dividing a difference between an outer diameter of the inner ring and a diameter of the maximum imaginary circle described previously by 2), and a shrinkage fitting ratio (see the formula (1)) in a stepped die having the configuration and shape shown in FIG. 1 and FIG. 2.

[0050] A height h of the stepped die (see FIG. 2) was set to 40 mm. A length w1 of a long side of the rectangular portion of the recessed portion for molding was set to 21 mm, a length w2 of a short side of the rectangular portion was set to 16 mm, and a diameter d3 of a circular columnar portion of the recessed portion was set to 10 mm. A material of the inner ring was a WC-Co based sintered hard alloy, and a material of the outer ring was hot die steel.

TABLE-US-00003 TABLE 3 Compressive stress of side-surface corner portion [MPa] Inner ring diameter 70 60 50 45 40 37 35 32 30 Inner ring ratio 2.8 2.4 2.0 1.8 1.6 1.5 1.4 1.3 1.2 Wall thickness [mm] 23 18 13 10 8 6 5 4 3 Shrinkage fitting 0.00 0 0 0 0 0 0 0 0 0 ratio [%] 0.15 501 541 571 601 642 676 744 918 1346 0.20 639 723 751 793 868 901 992 1224 1794 0.35 1142 1179 1330 1387 1501 1576 1736 2141 3140 0.50 1612 1681 1832 1982 2145 2252 2480 3059 4486

[0051] Table 4 shows a compressive strength ratio (compressive strength/generated compressive stress) calculated based on generated compressive stress shown in Table 3 and a compressive strength which is a unique value that a material has. In the present test example 2, a WC-Co sintered hard alloy was used as a material of the inner ring so that the compressive strength was 4000 MPa.

TABLE-US-00004 TABLE 4 Compressive strength ratio (compressive strength generated compressive stress) Inner ring diameter 70 60 50 45 40 37 35 32 30 Inner ring ratio 2.8 2.4 2.0 1.8 1.6 1.5 1.4 1.3 1.2 Wall thickness [mm] 23 18 13 10 8 6 5 4 3 Shrinkage fitting 0.00 ratio [%] 0.15 8.0 7.4 7.0 6.7 6.2 5.9 5.4 4.4 3.0 0.20 6.3 5.5 5.3 5.0 4.6 4.4 4.0 3.3 2.2 0.35 3.5 3.4 3.0 2.9 2.7 2.5 2.3 1.9 1.3 0.50 2.5 2.4 2.2 2.0 1.9 1.8 1.6 1.3 0.9

[0052] FIG. 6 shows the result in Table 4 in a graphical form with respect to the case where a shrinkage fitting ratio (%) was set to 0.15%. In FIG. 6, a compressive strength ratio is taken on an axis of ordinates, and a wall thickness (mm) is taken on an axis of abscissas. FIG. 7 also shows the result in Table 4 in a graphical form with respect to the same case. In FIG. 7, a compressive strength ratio is taken on an axis of ordinates, and an inner ring ratio is taken on an axis of abscissas.

[0053] From FIG. 6, it is understood that, with a wall thickness in the vicinity of 5 mm set as a boundary value, the way that a compressive strength ratio changes largely differed between the case where the wall thickness is smaller than the boundary value and the case where the wall thickness is larger than the boundary value. To be more specific, a relationship between a compressive strength ratio and a wall thickness in three test examples where the wall thickness is set equal to or less than 5 mm can be expressed by y=0.94x+0.65 (R.sup.2=0.96), and a relationship between a compressive strength ratio and a wall thickness in seven test examples where the wall thickness is set equal to or more than 5 mm can be expressed by y=0.13x+5.08 (R.sup.2=0.94). It is understood that an inclination of a regression line largely changes above and below 5 mm which functions as a maximum value.

[0054] From FIG. 7, it is understood that, with an inner ring ratio in the vicinity of 1.4 set as a boundary value, the way that a compressive strength ratio changes largely differed between the case where the inner ring ratio is smaller than the boundary value and the case where the inner ring ratio is larger than the boundary value. To be more specific, a relationship between a compressive strength ratio and an inner ring ratio in three test examples where the inner ring ratio is set equal to or less than 1.4 can be expressed by y=12.02x+11.39 (R.sup.2=0.99), and a relationship between a compressive strength ratio and an inner ring ratio in seven test examples where the inner ring ratio is set equal to or more than 1.4 can be expressed by y=1.65x+3.44 (R.sup.2=0.94). It is understood that an inclination of a regression line largely changes above and below 1.4 which functions as a maximum value.

[0055] From the result of the test example 1 and the result of the test example 2, it is understood that it is preferable to set a shrinkage fitting ratio (%) to a value which falls within a range of from 0.12 to 0.25% since a substantially fixed strength ratio of the stepped corner R portion can be obtained. It is also understood that it is preferable to set an inner ring ratio to 1.4 or more. It is also understood that it is preferable to set a wall thickness to 5 mm or more. On the other hand, it is understood that it is preferable to set an upper limit value of an inner ring ratio to 2.0 or less.

Other Modifications

[0056] It should be construed that the embodiments are disclosed merely in an exemplifying purpose and are not limitative in any aspects. The scope of the present invention should not be determined by the meanings disclosed in the embodiments, and the present invention intends to embrace all modifications which are described in Claims and fall within the meaning and the scope equivalent to the meaning and the scope of Claims.

[0057] For example, in the above-mentioned embodiment, the recessed portion for molding has a rectangular shape as viewed in a plan view. However, the shape and the size of the recessed portion can be suitably selected corresponding to a molded product and, for example, the recessed portion may have a circular shape or a polygonal shape as viewed in a plan view.

REFERENCE SIGNS LIST

[0058] 1: STEPPED DIE

[0059] 2: INNER RING

[0060] 3: OUTER RING

[0061] 4: RECESSED PORTION

[0062] 5: DIE PLATE

[0063] 6: FLANGE PORTION

[0064] 7: STEPPED PORTION

[0065] 7A: CORNER PORTION

[0066] 7B: STEPPED CORNER R PORTION

[0067] 21: STEPPED DIE

[0068] 22: INNER RING

[0069] 23: OUTER RING

[0070] 24: RECESSED PORTION

[0071] 25: STEPPED PORTION

[0072] 26: DIE PLATE

[0073] 27: FLANGE PORTION

[0074] 28: LOWER PUNCH

[0075] 30: STEP

[0076] 31: PART

[0077] O: CENTRAL AXIS

[0078] C: CRACK

[0079] P: IMAGINARY CIRCLE

[0080] S: LOWER SPACE

[0081] d1: OUTER DIAMETER OF INNER RING

[0082] d2: DIAMETER OF MAXIMUM IMAGINARY CIRCLE

[0083] d3: DIAMETER OF RECESSED PORTION

[0084] w1: LONG SIDE OF RECESSED PORTION

[0085] w2: SHORT SIDE OF RECESSED PORTION

[0086] h: HEIGHT OF STEPPED DIE