JPH09143508A - Die setting device for powder molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent cracking of a green compact caused by difference of spring back quantities among respective punch elements at the time of releasing compressive load. SOLUTION: After compression molding is finished, the green compact W is held between an upper punch 4 and a lower punch 5 and is taken out of a die 2 while pressing an upper surface side of the green compact W by the upper punch 4 under pressure holding load smaller than molding load. At that time, a first punch 9 and a third punch 11 constituting the lower punch 5 are pushed up by prescribed quantities respectively by adjusting cylinders 33, 34. Thus, difference of spring back quantities between the first and third punches 9, 11 and the second punch 10 is absorbed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a sintered part such as a gear part such as a synchronous hub for an automobile, in which a metal powder is compressed to form a green compact having a predetermined shape. The present invention relates to a powder molding die set device.

[0002]

2. Description of the Related Art As a powder molding die set device of this type, as shown in FIG. 11, for example, a cylindrical die 2 supported by a top platen 1, a core 3, and an inner space of the die 2 are inserted. Is formed centering on an upper punch 4 and a lower punch 5 that are vertically opposed to each other, and the upper and lower punches 4 and 5 are arranged in order from the outer side as first, second and third punch elements, respectively. 3rd punch 6,7,
There is one that is divided into 8 and 9, 10 and 11, and each of the first to third punches 6, 7, 8 and 9, 10 and 11 is configured to be relatively movable in the axial direction. In FIG. 11, only the right half of the symmetrical cross section is shown.

As a molding procedure by this conventional die set device, as shown in FIG. 11, a filling cylinder provided between the stationary platen 12 and the standard platen 13 and between the stationary platen 12 and the bottom platen 14. With the (air cylinders) 15 and 16 being expanded, a predetermined powder P is filled in the space formed by the die 2, the core 3 and the lower punch 5.

In the compression molding process following powder filling, FIG.
When the upper punch 4 descends together with the upper ram 17, the first to third punches 6, 7 and 8 on the upper punch 4 side are in a so-called bottom-butting state with respect to the upper ram 17 and the punch holder 18 as shown in FIG. Therefore, the molding pressure of the upper ram 17 acts on all the powders through the first to third punches 6, 7, and 8, and the green compact W having a substantially H-shaped cross section and a ring shape as a whole is compression-molded. It Then, when the compressive load due to the compression overcomes the pressure in the extending direction of the filling cylinders 15 and 16, the second and third punches 10 and 11 on the lower punch 5 side together with the standard platen 13 and the bottom platen 14 are filled. While the cylinders 15 and 16 are contracted, the cylinders 15 and 16 are lowered by a predetermined amount, and at the same time, the die 2 is moved to the top platen 1
And, it slightly descends together with the lower holder 20 via the tie rod 19, and finally, when the upper ram 17 reaches the bottom dead center as shown in FIG. 12 and FIG. 15, the compression molding ends.

At about the same time as the end of the compression molding process, the pressure-holding process is started and the upper ram 17 begins to rise as shown in FIG.
The hydraulic pressure is introduced into the cylinder chamber 21 defined between the upper punch 4 and the upper punch 4, and the holding pressure acts on the entire upper punch 4.

The holding pressure referred to here is due to the frictional force at the time of withdrawing the green compact and the elastic deformation of each punch when the compressive load acting on the green compact W is completely removed at the same time as the completion of the compression molding. Since a defect such as cracking occurs in the green compact W due to the imbalance of the return amount (spring back amount), it is set to prevent such a defect, and usually 10% of the load during compression molding. It is set below.

Then, with the holding pressure applied with the holding pressure as described above, the extraction process shown in FIG.
The second punch 10 on the lower punch 5 side descends due to the contracting operation of one of the filling cylinders 15, and at the same time, the core 3 and the die 2 descend together with the top platen 1, the tie rods 19, and the lower holder 20, The green compact W after molding is extracted from the die 2.

At this time, in parallel with the above extracting operation,
Hydraulic pressure is introduced into the cylinder chamber 24 formed between the punch adapters 22 and 23 on the upper punch 4 side, and the third punch 8 is pushed down together with the punch adapter 23. As a result, the green compact W is extracted while being sandwiched between the third punch 8 on the upper punch 4 side and the first and third punches 9, 11 on the lower punch 5 side as shown in FIG. Become.

[0009]

When the upper punch 4 and the lower punch 5 are each divided into three punch elements as described above, the first to third punches 6, which are the punch elements, are used.
Since the projected areas and lengths of 7, 8 and 9, 10, 11 and each adapter are different from each other, a difference occurs in the elastic deformation amount of each punch element or adapter due to compression molding.
Therefore, if the compression load is removed after the completion of compression molding, a difference occurs in the springback amount of each of the punch elements and the adapter described above, and an unreasonable force is applied to the green compact W at the time of extracting the green compact W after molding. Defects such as cracks occur.

Here, the holding pressure applied to the green compact W in the pressure holding step shown in FIG. 13 is set in order to prevent the above defects when the green compact is extracted. However, since the above holding pressure can be set only on the upper punch 4 side and cannot be adjusted independently for each punch element, the intended purpose cannot always be sufficiently achieved.

As a result, even with the above holding pressure, the first to third punch elements which are the punch elements on the lower punch 5 side in particular.
Due to the imbalance of the spring back amount of the punches 9, 10, 11, a gap is created between the green compact W and each punch element, and as shown in FIG. 16, for example, the green compact W and the first punch 5 on the lower punch 5 side. When a gap Q is generated between the first punch 9 and the first punch 9, a crack C ₁ occurs at the corner portion on the tip side of the second punch 10 due to the frictional force when the die 2 descends, while the third punch 11 has a crack. Even when the springback amount is larger than the springback amount of the first punch 9, a crack C ₂ is generated in the same portion as shown in FIG.

The occurrence of molding defects that cannot be prevented even by setting the holding pressure is dealt with by adjusting the molding density of the green compact W and changing the cross-sectional area of each punch element and adapter. However, the former requires adjustment of the molding density by the operator, and the latter requires a lot of time and man-hours to change the cross-sectional area of each punch element, etc. There is still a problem in terms of ease of adjustment and reproducibility.

The present invention has been made in view of the above problems, and makes it possible to easily and reproducibly adjust the difference in the amount of spring back of each punch element or the like, and to prevent cracks or the like. It is an object of the present invention to provide a structure capable of reliably preventing the occurrence of molding defects.

[0014]

According to the present invention, a die and a core arranged in the central portion of the die are inserted into the die concentrically with the core from below and the first, A lower punch, which is divided into second and third punch elements, each of which is capable of relative movement in the axial direction of the core, and an upper punch which is inserted into the die so as to face the lower punch from above. And compressing the powder contained in the space formed by the die, the core and the lower punch with the lower punch and the upper punch to form an annular green compact having a substantially H-shaped cross section, After compression molding, while pressing the upper surface side of the green compact with the upper punch under a holding load smaller than the molding load, hold the green compact with the upper punch and the lower punch and extract it from the die. Die set equipment for powder molding Depending on the difference in the springback amount between any one of the second punch elements or the first and third punch elements that constitute the lower punch and the other punch element. It is characterized in that a position correction mechanism for displacing any one of the punch elements in the longitudinal direction in parallel with the extracting operation of the green compact is provided.

The above-mentioned position correcting mechanism is provided, for example, individually on the first and third punch elements and the support body supporting the punch elements and between them, and extends at the same time when the powder compact extracting operation is started. It is formed by an operating fluid pressure cylinder or an elastic body provided separately between the first and third punch elements and the support body supporting the punch elements.

Further, the above-mentioned position correction mechanism includes a stopper block which comes into contact with the second punch at the end of the compression molding process and restrains the second punch at a predetermined height position until the start of the withdrawal process and stops it. It may be configured by an actuator that lowers the second punch together with the stopper block by a predetermined amount after the stopper block is stopped.

As described above, after the compression molding, the upper punch and the lower punch hold the green compact while pressing the upper side of the green compact with the upper punch under a holding load smaller than the molding load. The first punching element, the third punching element, or the second punching element, which is the punching element of the lower punch, is lifted by a predetermined amount in parallel with the withdrawal operation of the green compact, so that the springs between the punching elements are pulled out. By correcting the difference in the back amount, it becomes possible to prevent the occurrence of defects such as cracking of the green compact.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing a first embodiment which is a typical embodiment of the present invention, and the portions common to the conventional example shown in FIGS. I am doing it.

In this embodiment, the stationary stationary platen 12 is supported by the tie rod 31 so that the punch adapter 32 positively moves the first punch 9, which is a part of the lower punch 5, up and down. An adjusting cylinder (hydraulic cylinder) 33 serving as a position correcting mechanism is additionally provided, and an adjusting cylinder (position adjusting mechanism serving as a position correcting mechanism for positively moving the third punch 11 which is also a part of the lower punch 5 up and down on the bottom platen 14 ( Hydraulic cylinder)
This is different from the conventional structure in that 34 is attached separately from the filling cylinders 15 and 16.

More specifically, the punch adapter 32, which was conventionally integrally connected to the stationary stationary platen 12, is divided into an upper punch adapter 35 and a lower punch adapter 36, and the upper punch adapter 35 is provided with the punch adapter 35. While supporting the first punch 9, the lower punch adapter 36 is provided with an adjusting cylinder 33, and the piston rod 37 of the adjusting cylinder 33 is connected to the upper punch adapter 35.

A punch holder 38 is interposed between the third punch 11 and the bottom platen 14 supporting the third punch 11, and the lower portion of the punch holder 38 is accommodated in the space formed in the bottom platen 14, Punch holder 3
Forming an adjusting cylinder 34 having 8 as a piston,
The adjusting cylinder 34 is made to coexist with the filling cylinder 16 provided between the stationary platen 12 and the bottom platen 14.

The second punch 10 forming the lower punch 5 together with the first and third punches 9 and 11 is fixedly supported by the standard platen 13 as in the conventional case, and the standard platen 13 is a filling cylinder. It is supported by the stationary platen 12 via 15.

Further, the adjusting cylinders 33, 3 described above
4 and each of the filling cylinders 15 and 16 are provided in plurality to maintain the upper punch adapter 35, the standard platen 13 and the bottom platen 14 in a horizontal state.

The stationary platen 12 is provided with a stopper device 39 for temporarily stopping and positioning the standard platen 13 at the end position of the compression molding at the end of the compression molding before shifting to the pressure holding process. .

The stopper device 39 is provided with a wedge block 40 provided on the inclined surface 12a of the stationary platen 12, a stopper block 41 slidably provided on the wedge block 40, and vertically movable with respect to the standard platen 13. Drive cam 4 provided
2, and a pusher pin 43 fixedly supported by the top platen 1 so as to be positioned on the same axis as the drive cam 42. The stopper block 41 is provided with a cam follower roller 44 capable of sliding contact with the inclined cam surface 42a of the drive cam 42, while the wedge block 40 is provided with the adjusting screw 4
5 is attached, and the adjusting screw 45 is screwed into the female screw portion on the side of the stationary platen 12. Accordingly, when the wedge block 40 is moved back and forth by rotating the adjust screw 45, the stopper block 41 on the wedge block 40 is correspondingly moved.
The height of can be adjusted arbitrarily.

Although the standard platen 13 is lowered as the compression molding progresses, the standard platen 1 is contacted with the stopper block 41.
3 temporarily stops at that position, at which point compression molding is complete. When the top platen 1 comes down in the process of shifting to the pressure holding process and the extraction process following the completion of this compression molding, the pusher pin 43 causes the drive cam 42 to move.
Since the slanted cam surface 42a of the drive cam 42 and the cam follower roller 44 are in sliding contact with each other, the stopper block 41 is retracted, and the stopper block 41 is disengaged from the standard platen 13. As a result, the standard platen 13 can be lowered again, and is lowered together with the top platen 1.

The stopper device 39 is shown in FIG.
It is also provided in the conventional devices shown in FIGS.

Therefore, according to the die set apparatus of the present embodiment, at the time of powder filling, as shown in FIG. 1, the filling cylinders 15 and 16 are in the extended state and push up the standard platen 13 and the bottom platen 14 ( (The same state as the conventional powder filling shown in FIG. 11), and the adjusting cylinders 33 and 34 attached to the punch adapter 36 and the bottom platen 14 are both in a contracted state. In this state, the space formed by the die 2, the core 3 and the lower punch 5 is filled with a predetermined metal powder P.
Then, the upper punch 4 descends together with the upper ram, and
The compression molding proceeds in exactly the same manner as in the above case, and finally the compression molding ends when the upper ram reaches the bottom dead center. Note that FIG. 3 shows an operation diagram of the die 2, the upper punch (upper ram) 4, and the first and third punches 9 and 11 on the lower punch 5 side.

At about the same time as the end of the compression molding process, the pressure-holding process is started as shown in FIG. 13, and as shown in FIG.
The holding pressure acts on each of the adjusting cylinders 33, 33 at almost the same time as shown in FIG.
34 expands under a preset pressure, and the first punch 9 and the third punch 11 on the lower punch 5 side are respectively pushed up by a predetermined amount. As a result, the first punch 9 and the second punch 1 which are the punch elements of the lower punch 5 are provided.
The difference in the amount of springback between the 0th and 3rd punches 11 is corrected, and the tip surfaces of the 1st to 3rd punches 9, 10, 11 are surely brought into close contact with the powder compact W, and the springs described above are used as in the conventional case. It is possible to prevent a gap from being formed between the green compact W and the powder compact W due to the imbalance of the back amount, and as a result, FIG.
It becomes possible to prevent the occurrence of cracks C ₁ and C ₂ as shown in Nos. 6 and 17 in advance.

Then, thereafter, similarly to FIG. 14, the process moves to the extraction process, and the green compact W is extracted from the die 2.

Here, each of the adjusting cylinders 33,
It is sufficient for the extension stroke of 34 to be about 5 mm at the maximum, and in practice, the first and third punches 9 and 11 are 0.1 to
If it can be moved about 1.0 mm, the above-mentioned molding crack C
It is possible to reliably prevent ₁ and C ₂ .

FIG. 4 shows the adjusting cylinders 33,
34 of the stroke adjusting mechanism, the upper limit position of the upper punch adapter 35 connected to the piston 37 of the adjusting cylinder 33 on the first punch 9 side is the head of the stopper bolt 46 provided on the lower punch adapter 36. It is regulated by abutting against the portion, and a spacer 47 for stroke adjustment is interposed between the punch adapter 35 and the stopper bolt 46 on the upper side thereof.
Therefore, by exchanging the spacer 47 with one having a different thickness, it is possible to finely adjust the movement amount of the first punch 9 for correcting the stroke of the adjusting cylinder 33 and the spring back amount.

Similarly, the upper limit position of the punch adapter 38 connecting the piston 48 of the adjusting cylinder 34 on the side of the third punch 11 and the third punch 11 is the punch stopper 49 provided on the side of the bottom platen 14. The contact is regulated. A spacer 50 is interposed between the punch adapter 38 and the punch stopper 49 in the same manner as described above. By replacing the spacer 50 with a spacer having a different thickness, the stroke of the adjusting cylinder 34 and, by extension, the springback amount can be corrected. It is possible to adjust the movement amount of the third punch 11 for.

FIG. 5 shows the second embodiment of the present invention.
Cylindrical urethane springs 51, 52 as elastic bodies instead of the adjusting cylinders 33, 34 in the above embodiment.
Is used.

More specifically, the urethane spring 51 is provided between the upper punch adapter 35 on the first punch 9 side and the lower punch adapter 36 which are the punch elements of the lower punch 5.
Is compressed, and the upper limit position of the upper punch adapter 35 relative to the lower punch adapter 36 is regulated by the stopper 53 that also serves as a guide rod, while the spring force of the urethane spring 51 is adjusted to the upper punch adapter 35. It can be prepared by the adjusting screw 54 screwed into the.

The same applies to the third punch 11 side, that is, the bottom platen 14 and the punch adapter 55.
The urethane rubber 52 is interposed between the punch adapter 55 and the bottom platen 14, and the upper limit position of the punch adapter 55 with respect to the bottom platen 14 is restricted by the stopper 56 that also serves as a guide rod. It can be prepared by the adjusting screw 57 that is screwed into the.

According to this embodiment, in the compression molding process similar to that shown in FIG. 12, the upper punch holder 35 is bottom-butted to the lower punch holder 36 on the first punch 9 side. Similarly, on the side of the third punch 11, the punch adapter 55 is in a bottom-butting state with respect to the bottom platen 14, but the urethane springs 51, 52 are arranged in the same pressure-holding process and extracting process as those shown in FIG.
However, as in the first embodiment, the first and third punches 9 and 11 are pushed up by a predetermined amount.

Therefore, also in the case of this embodiment, the same operational effects as those of the first embodiment can be obtained.

FIGS. 6, 7 and 8 are views showing a third embodiment of the present invention. Instead of the adjusting cylinders 33, 34 and urethane springs 51, 52 in the first and second embodiments, existing stoppers are used. The device 39 also has a function as a position correcting mechanism equivalent to the adjusting cylinders 33 and 34 and the urethane springs 51 and 52.

More specifically, as shown in FIGS. 6 and 7, the wedge block 40 of the stopper device 39 is connected to the piston rod 62 of the hydraulic servo cylinder 61, and the wedge block 4 is expanded and contracted by the expansion / contraction operation of the hydraulic servo cylinder 61.
The configuration is such that 0 can be positively moved back and forth.

According to the structure of this embodiment, the wedge block 40 is at the advance limit position P ₁ shown in FIGS. 6 and 7 in the powder filling process similar to the state shown in FIG. In the compression molding process shown in FIG. 12, the second punch 10, which is the punch element of the lower punch 5, gradually descends together with the standard platen 13 as described above, but the standard platen 13 becomes the stopper block 41 on the wedge block 40. When it comes into contact, it stops, and the compression molding process ends therewith. In FIG. 10, the die 2, the upper punch 4 and the second punch 10 are shown.
The operating diagram of is shown.

Simultaneously with the end of the compression molding process, the pressure holding process similar to the state shown in FIG. 13 is started. However, the hydraulic servo cylinder 61 contracts at the same time as the start of the pressure holding process. As shown in FIG. 8, the wedge block 40 is moved backward by a predetermined amount and is positioned at the backward limit position P ₂ . At this time, the wedge block 40 is attached to the stationary platen 12 and the stopper block 41.
However, since the holding pressure on the side of the standard platen 13 acts on the stopper block 41, the relative positional relationship between the stopper block 41 and the standard platen 13 remains unchanged. It remains.

As a result, the second punch 10 is lowered by a predetermined amount relative to the first and third punches 9 and 11, whereby the first and third punches 9 and 11 and the second punch 10 are moved.
The difference in the amount of springback between
It becomes possible to prevent the occurrence of cracks C ₁ and C ₂ as shown in Nos. 6 and 17 in advance.

After that, when the extraction stroke similar to the state shown in FIG. 14 is entered, the drive cam 4 is moved as in FIG.
The slide contact between the cam follower roller 2 and the cam follower roller 44 causes the stopper block 41 to retreat with respect to the wedge block 40 to be disengaged from the standard platen 13, and the standard platen 13 can be lowered again.

Here, the movement amount of the wedge block 40 can be easily adjusted by changing the command amount for the hydraulic servo cylinder 61, and the position where the standard platen 13 comes into contact with the stopper block 40 and stops. That is, the end position of the compression molding process can be easily adjusted in the same manner as described above, and there is an advantage that the wall thickness and the molding density of the green compact W can be controlled more precisely.

FIG. 9 shows a modification of the third embodiment shown in FIGS. 6 and 7, and the hydraulic servo cylinder 61 shown in FIGS.
Is replaced by a servo motor 71, and the wedge block 40 is inserted through the gears 72, 73 and the ball screw 74.
Is moved forward and backward, and this embodiment also operates in exactly the same way as in the third embodiment.

[0047]

As described above, according to the present invention, any one of the punch element and the other punch element corresponding to the second punch element or the first and third punch elements constituting the lower punch are respectively provided. By providing a position correction mechanism for displacing any one of the punch elements in the longitudinal direction in parallel with the operation of extracting the powder compact according to the difference in the springback amount between the punch elements, Since the difference is corrected and the difference in the springback amount can be adjusted easily and with good reproducibility, the occurrence of defects such as cracking of the green compact can be prevented and contributes greatly to the improvement of molding quality. In addition to being able to do so, there is an effect that it can be dealt with by only a small modification of existing equipment.

The above position correcting mechanism is provided separately, for example, between the first and third punching elements, the supporter supporting the punching elements, and the support between them, and extends at the same time when the operation of extracting the green compact is started. It may be composed of a fluid pressure cylinder that operates, or may be composed of an elastic body such as a urethane spring provided individually between the first and third punch elements and a support body supporting the punch elements. Yes, again
At the end of the compression molding process, the position correction mechanism comes into contact with the second punch and stops the second punch at a predetermined height position until the extraction process starts. The second punch may be composed of an actuator that lowers the stopper block by a predetermined amount. However, particularly when the position correction mechanism is composed of a fluid pressure cylinder, the pressure of the cylinder and the operation timing can be arbitrarily adjusted. When the position correction mechanism is made of an elastic body such as urethane spring, there is an advantage that the structure can be simplified.

[Brief description of the drawings]

FIG. 1 is a view showing a first embodiment of a die set device of the present invention and is a cross sectional explanatory view in a powder filling process.

FIG. 2 is an explanatory cross-sectional view when the pressure-holding stroke is changed from the state of FIG.

3 is an operation diagram of the die set device shown in FIG.

FIG. 4 is an enlarged view of a main part of FIG.

FIG. 5 is a view showing a second embodiment of the die set device of the present invention, and a sectional explanatory view in the powder filling process.

FIG. 6 is a view showing a third embodiment of the die set device of the present invention and is a cross-sectional explanatory view in the powder filling process.

FIG. 7 is an enlarged view of a main part of FIG. 6;

8 is an operation explanatory view of FIG. 7. FIG.

9 is a cross-sectional explanatory view showing a modified example of FIG.

10 is an operation explanatory view of the die set device shown in FIG.

FIG. 11 is an explanatory cross-sectional view of a powder filling process of a conventional die set device.

FIG. 12 is an explanatory cross-sectional view of a conventional die set device in a compression molding process.

FIG. 13 is an explanatory cross-sectional view of a conventional die set device in a pressure holding process.

FIG. 14 is an explanatory cross-sectional view of a conventional die set device in an extraction process.

FIG. 15 is an enlarged view of a main part of FIG.

FIG. 16 is an enlarged view of a main part showing a crack occurrence state of the green compact.

FIG. 17 is an enlarged view of a main part showing a crack occurrence state of the green compact.

[Explanation of symbols]

 2 ... Die 3 ... Core 4 ... Lower punch 5 ... Upper punch 9 ... First punch 10 ... Second punch 11 ... Third punch 33, 34 ... Adjust cylinder (position correction mechanism) 40 ... Wedge block 41 ... Stopper block 51, 52 ... Urethane spring (position correction mechanism) 61 ... Hydraulic servo cylinder 71 ... Servo motor W ... Powder compact

Claims (4)

[Claims] 1. A die and a core arranged in the center of the die, and a core which is inserted into the die concentrically with the core from the lower side thereof, and which are sequentially arranged from the outside in the order of first, second and third punch elements. A lower punch that is divided into two parts, each of which is capable of relative movement in the axial direction of the core, and an upper punch that is inserted into the die from above to face the lower punch. The powder housed in the space formed by the lower punch and the lower punch is compressed by the lower punch and the upper punch to form an annular green compact having a substantially H-shaped cross section. With a die set device for powder molding, the upper side of the green compact is pressed by the upper punch under a small holding pressure and the green compact is held between the upper and lower punches and pulled out from the die. Yes, the lower pan Any one of the second punch element or the first and third punch elements that compose the punch punch element corresponding to the difference in the springback amount between the corresponding punch element and the other punch element. A powder molding die set device comprising a position correction mechanism for displacing in the longitudinal direction in parallel with the operation of extracting the powder compact. 2. The position correcting mechanism is separately provided between the first and third punch elements, a support body supporting the punch elements, and a space between them, and an extension operation is performed at the same time when the powder compact extracting operation is started. The die set device for powder molding according to claim 1, which is a fluid pressure cylinder. 3. The position correction mechanism is an elastic body provided individually between the first and third punch elements and a support body supporting the punch elements.
The powder molding die set device described. 4. The stopper block, wherein the position correction mechanism contacts the second punch at the end of the compression molding process and restrains and stops the second punch at a predetermined height position until the start of the extraction process, and the stopper block. Second after the block stops
2. The powder molding die set device according to claim 1, further comprising an actuator for lowering the punch together with the stopper block by a predetermined amount.