PLUNGER TIP FOR DIE CASTING AND INJECTION DEVICE

Abstract

A plunger tip for die casting is configured to slide in a cylindrical sleeve to inject a molten metal in the cylindrical sleeve into a mold and includes a crushing portion and recesses. The crushing portion is provided on an outer peripheral surface on a front end side of the plunger tip and is capable of contacting a solidified layer of the molten metal generated on an inner peripheral surface of the cylindrical sleeve to crush the solidified layer. The recesses are provided on both sides of the crushing portion in a circumferential direction and recessed radially inward from the crushing portion. The crushing portion has a triangular shape and is a region between two of the recesses adjacent to each other in the circumferential direction and in which a front end side of the plunger tip is an apex.

Description

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent Application No. 2024-003162 filed on Jan. 12, 2024, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The disclosure relates to a plunger tip for die casting and an injection device, and more particularly to a plunger tip that slides in a sleeve into which a molten metal is poured, and an injection device including the plunger tip.

BACKGROUND

An injection device for die casting includes a sleeve that is coupled to a mold and into which molten aluminum is poured, and a plunger rod to which a plunger tip sliding in the sleeve is attached at a front end. In such an injection device, when the molten metal is supplied into the sleeve and the plunger rod is advanced toward the mold, the molten metal is pressurized by the plunger tip and injected into the cavity of the mold.

When the molten metal is poured into the sleeve, the molten metal is cooled and a solidified layer is generated on an inner peripheral surface of the sleeve. When the plunger tip advances in the sleeve in this state, a surface of a front end portion of the plunger tip comes into contact with the solidified layer generated in the sleeve, and the solidified layer is scraped. When a part of the scraped solidified layer, that is, a solidified piece is mixed into the molten metal and enters the cavity of the mold, the quality of the cast product is deteriorated.

In order to suppress mixing of the solidified piece into a product, Japanese Patent No. 5647848 describes a structure in which a plunger tip includes a cylindrical portion that slides in a sleeve, and a truncated cone-shaped protrusion whose outer diameter decreases from the cylindrical portion toward a front end side. In the plunger tip, a recess is formed at the tip of the protrusion. In the injection device including such a plunger tip, when the plunger tip advances toward the mold in the sleeve, the solidified layer generated in the sleeve is scraped by the cylindrical portion. The scraped solidified piece can be captured in a space between an outer peripheral surface of the protrusion of the plunger tip and an inner peripheral surface of the sleeve and in the recess of the protrusion.

SUMMARY

An aspect of the disclosure provides a plunger tip for die casting. The plunger tip being configured to slide in a cylindrical sleeve to inject a molten metal in the cylindrical sleeve into a mold. The plunger tip includes a crushing portion and recesses. The crushing portion is provided on an outer peripheral surface on a front end side of the plunger tip and is capable of contacting a solidified layer of the molten metal generated on an inner peripheral surface of the cylindrical sleeve to crush the solidified layer. The recesses are provided on both sides of the crushing portion in a circumferential direction and recessed radially inward from the crushing portion. The crushing portion has a triangular shape and is a region between two of the recesses adjacent to each other in the circumferential direction and in which a front end side of the plunger tip is an apex.

An aspect of the disclosure provides an injection device for die casting. The injection device includes a cylindrical sleeve and a plunger tip. The plunger tip is configured to slide in the cylindrical sleeve to inject a molten metal in the cylindrical sleeve into a mold. The plunger tip includes a crushing portion and recesses. The crushing portion is provided on an outer peripheral surface on a front end side and is capable of contacting a solidified layer of the molten metal generated on an inner peripheral surface of the cylindrical sleeve to crush the solidified layer. The recesses are provided on both sides of the crushing portion in a circumferential direction and recessed radially inward from the crushing portion. The crushing portion has a triangular shape and is a region between two of the recesses adjacent to each other in the circumferential direction and in which a front end side of the plunger tip is an apex.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and, together with the specification, serve to describe the principles of the disclosure.

FIG. 1 is a cross-sectional view illustrating a casting apparatus including an injection device according to an embodiment of the disclosure;

FIG. 2 is a cross-sectional view illustrating a state in which a mold is released in the casting apparatus;

FIG. 3 is a perspective view of a plunger tip according to the embodiment of the disclosure;

FIG. 4 is a partial cross-sectional side view of the plunger tip;

FIG. 5 is an enlarged cross-sectional view of a main part of the plunger tip;

FIG. 6 is a view describing a sliding state of the plunger tip in a sleeve;

FIG. 7 is a photograph of a biscuit of a cast product;

FIG. 8 is a perspective view illustrating an embodiment of the plunger tip;

FIG. 9 is a perspective view illustrating an embodiment of the plunger tip;

FIG. 10 is a perspective view illustrating an embodiment of the plunger tip;

FIG. 11 is a perspective view illustrating an embodiment of the plunger tip; and

FIG. 12 is a perspective view illustrating an embodiment of the plunger tip.

DETAILED DESCRIPTION

In the injection device described in Japanese Patent No. 5647848, it is possible to capture the solidified piece and suppress the solidified piece from entering the mold. However, the solidified layer generated in the sleeve cannot be sufficiently crushed, and there is a concern that the solidified layer remains in the sleeve. The solidified layer remaining in the sleeve may lead to a subsequent decrease in the quality of the cast product, and the solidified layer is fused with the metal sleeve to be in a welded state, which leads to a decrease in the life of the sleeve.

It is desirable to provide a plunger tip for die casting and an injection device capable of forcibly crushing a solidified layer generated in a sleeve and suppressing a solidified piece from entering a mold.

In the following, some embodiments of the disclosure are described in detail with reference to the accompanying drawings. Note that the following description is directed to illustrative examples of the disclosure and not to be construed as limiting to the disclosure. Factors including, without limitation, numerical values, shapes, materials, components, positions of the components, and how the components are coupled to each other are illustrative only and not to be construed as limiting to the disclosure. Further, elements in the following example embodiments which are not recited in a most-generic independent claim of the disclosure are optional and may be provided on an as-needed basis. The drawings are schematic and are not intended to be drawn to scale. Throughout the present specification and the drawings, elements having substantially the same function and configuration are denoted with the same numerals to avoid any redundant description.

FIGS. 1 and 2 are cross-sectional views illustrating a die casting apparatus 1 including an injection device 10 for die casting according to an embodiment of the disclosure. The die casting apparatus 1 includes the injection device 10 and a mold 40. The injection device 10 includes a sleeve 12, a ladle 14, a plunger rod 16, an injection cylinder 18, and a plunger tip 20. The plunger tip 20 for die casting is attached to a front end portion of the plunger rod 16 and slides in the sleeve 12.

The mold 40 includes a fixed mold 44 fixed to a fixed platen 42 and a movable mold 46 that can approach and separate from the fixed mold 44. As illustrated in FIG. 1, the mold 40 is configured so that a cavity 41 as a casting space is formed inside the mold 40 in a mold-closed state in which the movable mold 46 is brought close to the fixed mold 44.

The injection device 10 is a device for injecting a molten metal 11 into the cavity 41 of the mold 40. In the present embodiment, an aluminum alloy molten metal is used as an example of the molten metal 11. In the following description, the molten metal 11 is also simply referred to as “molten metal 11”.

The sleeve 12 is formed in a cylindrical shape, and a first end 12a, which is one end, is fixed to the fixed mold 44 and extends in a horizontal direction. A spout 13 opening upward is formed on a peripheral surface of the sleeve 12 on a second end 12b side. In the injection device 10, a predetermined amount of the molten metal 11 is poured into the sleeve 12 through the spout 13 from the ladle 14 as a pouring means. The molten metal 11 in the sleeve 12 is pressurized by the plunger tip 20 and injected into the mold 40.

The plunger rod 16 is formed in a rod shape extending linearly. A rear end portion of the plunger rod 16 is coupled to a front end portion of a piston rod 17 of the injection cylinder 18. The injection cylinder 18 is a hydraulic cylinder that operates by hydraulic pressure, and moves the piston rod 17 forward and backward by controlling the amount of hydraulic oil and the hydraulic pressure of the hydraulic oil by a hydraulic control circuit (not illustrated). The plunger rod 16 receives power from the injection cylinder 18 and is configured to be able to move forward and backward in the sleeve 12. A cylindrical plunger tip 20 sliding in the sleeve 12 is attached to the tip of the plunger rod 16.

FIG. 3 is a perspective view of the plunger tip 20, and FIG. 4 is a partial cross-sectional side view of the plunger tip 20. In the following description, a front end side of the plunger tip 20 is the first end 12a side of the sleeve 12, that is, the mold 40 side in a state of being assembled to the injection device 10 illustrated in FIG. 1, and a rear end side of the plunger tip 20 is the second end 12b side of the sleeve 12, that is, the side coupled to the plunger rod 16. The plunger tip 20 is formed to be slidable in the sleeve 12, and includes a crushing portion 30 and a recess 32 on the outer peripheral surface on a front end side. In the present embodiment, the plunger tip 20 includes a cylindrical main body 22 and a protrusion 24 provided on the front end side of the main body 22, and the crushing portion 30 and the recess 32 are provided at a front end portion of the main body 22.

The main body 22 has a hollow cylindrical shape, and has an outer diameter substantially equal to an inner diameter of the sleeve 12 so as to slide in the sleeve 12. In the present exemplary embodiment, a missing part 23 where a part of the outer peripheral surface is missing is formed at the rear end portion of the main body 22. The missing part 23 is a part used for coupling to another member, and the plunger tip 20 may have a shape without the missing part 23.

The crushing portion 30 is a portion capable of contacting a solidified layer formed by the molten metal 11 generated on an inner peripheral surface of the sleeve 12 and crushing the solidified layer when the plunger tip 20 slides in the sleeve 12. The recess 32 is provided on both sides of the crushing portion 30 in a circumferential direction of the plunger tip 20, and is a portion recessed radially inward from the crushing portion 30. The recess 32 is formed so that a recess width, that is, a dimension in the circumferential direction increases toward the front end side of the plunger tip 20, and is formed in a trapezoidal shape in which the front end side of the plunger tip 20 is a lower base and the rear end side is an upper base in the present embodiment.

The crushing portion 30 is a region between the two recesses 32 adjacent in the circumferential direction, and is formed in a triangular shape in which the front end side of the plunger tip 20 is an apex 30a. In the example illustrated in FIGS. 3 and 4, the apex 30a of the crushing portion 30 is formed in a pointed shape forming an acute angle. In the plunger tip 20 of the present embodiment, the crushing portion 30 and the recess 32 are alternately arranged in the circumferential direction of the plunger tip 20 and formed on the entire circumference of the plunger tip 20.

In the plunger tip 20, an outer diameter of the plunger tip 20 gradually decreases toward the front end side of the plunger tip 20 in a region where the crushing portion 30 is formed, that is, in the region which is the front end portion of the main body 22 and where the crushing portion 30 exists. In other words, a surface of the triangular shape of the crushing portion 30 is inclined radially inward toward the front end side of the plunger tip 20. The inclination angle of the surface of the crushing portion 30 can be, for example, up to 0.5° or less, and preferably around 0.2°. In the main body 22, in the region of the front end portion of the plunger tip 20 in which the recess 32 is formed, the area to be in contact with the molten metal 11 at a high temperature increases, and thus the rate of thermal expansion also increases. As in the present embodiment, the influence of thermal expansion can be absorbed by gradually decreasing the outer diameter of the plunger tip 20 toward the front end side in the region where the crushing portion 30 exists.

As illustrated in FIG. 5, a bottom surface 32a of the recess 32 is inclined inward in the radial direction of the plunger tip 20 toward the front end side of the plunger tip 20. The plunger tip 20 of the present embodiment is molded as a single piece using a mold, and it is possible to smoothly release the mold after molding by providing a draft angle on the bottom surface 32a of the recess 32. The inclination angle θ of the bottom surface 32a can be, for example, in a range of 2° to 10°.

The protrusion 24 of the plunger tip 20 is a portion protruding from the front end surface of the main body 22, and is formed in a truncated cone shape whose outer diameter gradually decreases toward the front end side of the plunger tip 20. The central axis of the protrusion 24 is coaxial with the central axis of the main body 22. At a rear end of the protrusion 24 having the largest outer diameter, the outer diameter of the protrusion is set to be smaller than the outer diameter of the main body 22, and as illustrated in FIG. 4, the outer peripheral surface of the rear end portion of the protrusion 24 is located radially inside the recess 32. An axial length of the protrusion 24 is preferably equal to or less than 1/10 of an axial length of the main body 22, and in the present embodiment, as an example, the main body 22 is set to 165 mm and the protrusion 24 is set to 15 mm.

Next, a die casting method using the plunger tip 20 for die casting and the injection device 10 will be described. First, in the die casting apparatus 1, the mold 40 moves the movable mold 46 close to the fixed mold 44 to bring the fixed mold 44 and the movable mold 46 into a mold-clamping state in which the molds are aligned. The injection device 10 reverses the piston rod 17 and the plunger tip 20 so that the plunger tip 20 is at a standby position retracted from the spout 13. In this state, a lubricant (not illustrated) is applied to the inner peripheral surface of the sleeve 12 or the plunger tip 20. The lubricant is for lubricating the movement of the plunger tip 20 with respect to the sleeve 12, and for example, lubricating oil can be used. The lubricant can be applied by, for example, spraying from the spout 13, or directly dropping onto the plunger tip 20. Next, as illustrated in FIG. 1, the molten metal 11 is poured into the sleeve 12 from the spout 13 using the ladle 14. Thereafter, the plunger rod 16 and the plunger tip 20 are moved forward toward the mold 40 by the driving force of the injection cylinder 18.

FIG. 6 is a view describing a sliding state of the plunger tip 20 in the sleeve 12 in the injection device 10, and illustrates a state in which the plunger tip 20 moves forward in the sleeve 12. The molten metal 11 injected into the sleeve 12 is cooled by the sleeve 12 having a temperature lower than this temperature, and a solidified layer 54 in which the molten metal 11 is solidified, a so-called “chill layer” is generated on the inner peripheral surface of the sleeve 12. When the plunger tip 20 advances in the sleeve 12 in which the solidified layer 54 is generated on the inner wall, the crushing portion 30 of the plunger tip 20 comes into contact with the solidified layer 54, and the solidified layer 54 is crushed. Since the crushing portion 30 is formed in a triangular shape in which the front end side is the apex 30a, the solidified layer 54 can be forcibly crushed by the sharp apex 30a. In the present embodiment, since the apex 30a of the crushing portion 30 has a pointed tip shape, the crushing effect is further enhanced.

When the solidified layer 54 is crushed by the crushing portion 30, a part of the solidified layer 54 is peeled off as a solidified piece 56. The peeled solidified piece 56, a so-called “rupture chill layer”, enters the inside of the recess 32 provided on both sides of the crushing portion 30 and is collected. As described above, in the plunger tip 20 of the present embodiment, the solidified layer 54 generated in the sleeve 12 can be forcibly crushed, and the peeled solidified piece 56 can be collected into the recess 32 to suppress the solidified piece 56 from entering the cavity 41 of the mold 40.

Furthermore, in the injection device 10 of the present embodiment, the excessive lubricant adhering to the inside of the sleeve 12 can also be collected in the recess 32. When the lubricant comes into contact with the molten metal 11 and is exposed to a high temperature, a gas is generated by combustion of the oil component, which may affect the quality of the cast product. In the plunger tip 20 of the present embodiment, it is possible to suppress generation of gas by collecting the lubricant into the recess 32 and suppressing contact with the molten metal 11.

When the plunger tip 20 comes into contact with the molten metal 11, the plunger tip 20 is thermally expanded by the high-temperature molten metal 11. In the main body 22 of the plunger tip 20, in the region where the recess 32 and the crushing portion 30 are formed, the contact area with the molten metal 11 increases, and thus the ratio of thermal expansion increases. In the present embodiment, the outer diameter of the main body 22 of the plunger tip 20 is gradually reduced toward the front end side at the front end portion of the plunger tip 20 in which the recess 32 and the crushing portion 30 are formed, so that the outer diameter of the main body 22 can be made uniform in the axial direction when thermally expanded.

When the plunger tip 20 moves forward toward the mold 40 by the driving force of the injection cylinder 18, the molten metal 11 in the sleeve 12 is pressurized by the plunger tip 20, and the molten metal 11 is injected into the cavity 41 of the mold 40. Thereafter, as illustrated in FIG. 2, the movable mold 46 is separated from the fixed mold 44 to open the mold, and a cast product 50 along the shape of the cavity 41 is taken out from the mold 40.

As illustrated in FIGS. 2 and 7, the solidified pieces 56 and the lubricant collected in the recess 32 of the plunger tip 20 adhere to a biscuit 51 of the cast product 50 formed in the vicinity of the injection port of the mold 40 at the time of casting, and are discharged to the outside of the sleeve 12. The biscuit 51 is metal in which the molten metal 11 remaining in the space between the sleeve 12 and the cavity 41 is solidified, and is unnecessary to be cast to be separated from the cast product 50. In the plunger tip 20 and the injection device 10 of the present embodiment, the solidified layer 54 is forcibly crushed, and the collected solidified piece 56 can be discharged to the outside of the sleeve 12 for each injection, so that it is possible to suppress the solidified layer 54 and the solidified piece 56 from remaining in the sleeve 12. Thus, the quality of the product can be improved, and it is possible to prevent the solidified layer 54 from being fused with the sleeve 12 to be in a welded state, and to prevent the life of the sleeve 12 from being reduced.

Next, another embodiment of the plunger tip 20 will be described with reference to FIGS. 8, 9, 10, 11, and 12. The plunger tip 20 illustrated in FIG. 8 includes a main body 22 and a protrusion 24. The main body 22 includes a crushing portion 30 and a recess 32 provided at the front end portion, and a rear crushing portion 36 and a rear recess 38 provided at a rear end portion. Note that, in the plunger tip 20 illustrated in FIG. 8, the structure other than the rear crushing portion 36 and the rear recess 38 is similar to that of the plunger tip 20 illustrated in FIG. 3, and thus details are omitted here.

The rear crushing portion 36 is a portion that can come into contact with the solidified layer 54 formed of the molten metal 11 generated on the inner peripheral surface of the sleeve 12 and crush the solidified layer 54 when the plunger tip 20 slides in the sleeve 12. The rear recess 38 is provided on both sides of the rear crushing portion 36 in the circumferential direction of the plunger tip 20, and is a portion recessed radially inward from the rear crushing portion 36. The rear recess 38 is formed so that a recess width, that is, a dimension in the circumferential direction increases toward the rear end side of the plunger tip 20, and is formed in a trapezoidal shape in which the rear end side of the plunger tip 20 is a lower base and the front end side is an upper base in the illustrated example.

The rear crushing portion 36 is a region between two rear recesses 38 adjacent in the circumferential direction, and is formed in a triangular shape in which the rear end side of the plunger tip 20 is an apex 36a. In the example illustrated in FIG. 8, the apex 36a of the rear crushing portion 36 is formed in a pointed shape forming an acute angle, and the rear crushing portion 36 and the rear recess 38 are alternately arranged in the circumferential direction of the plunger tip 20A and formed on the entire circumference of the plunger tip 20. In the plunger tip 20 illustrated in FIG. 8, the rear crushing portion 36 and the rear recess 38 have the same shapes as the crushing portion 30 and the recess 32, respectively, and are formed so that the direction of the apex 36a of the rear crushing portion 36 is opposite to the direction of the apex 30a of the crushing portion 30 in the axial direction of the plunger tip 20.

In the injection device 10 using the plunger tip 20 of the present embodiment, when the plunger tip 20 moves backward so as to be separated from the mold 40 in the sleeve 12 in which the solidified layer 54 is generated on the inner wall, the rear crushing portion 36 can come into contact with the solidified layer 54 and crush the solidified layer 54. Since the apex 36a of the rear crushing portion 36 is formed in a pointed shape, the solidified layer 54 can be forcibly crushed by the sharp apex 36a. The solidified piece 56 crushed and peeled off from the solidified layer 54 by the rear crushing portion 36 can be collected inside the rear recess 38 provided on both sides of the rear crushing portion 36.

In another embodiment of the plunger tip 20 illustrated in FIG. 9, at the front end portion of the plunger tip 20, the crushing portion 30 and the recesses 32 on both sides thereof, and the rear crushing portion 36 and the rear recesses 38 on both sides thereof are formed at intervals in the entire circumference of the plunger tip 20. As described above, in the plunger tip 20, it is not necessary that one crushing portion 30 and the pair of recesses 32 formed on both sides thereof are formed on the entire circumference, and it is sufficient if at least one set is formed. In this case, for example, by advancing the inside of the sleeve 12 while rotating the plunger tip 20, it is possible to crush the solidified layer 54 generated in a wide range in the circumferential direction by one crushing portion 30.

In another embodiment of the plunger tip 20 illustrated in FIG. 10, the width of the recess 32 is increased so that there is no region where the crushing portion 30 and the recess 32 are not formed in the circumferential direction. Similarly, on the rear end side of the plunger tip 20, a rear recess 38 having a large recess width is provided together with the rear crushing portion 36. As described above, by increasing the recess widths of the recess 32 and the rear recess 38, the collected amount of the solidified piece 56 can be increased.

In another embodiment of the plunger tip 20 illustrated in FIGS. 11 and 12, the crushing portion 30 and the rear crushing portion 36 are formed in a triangular shape, and R is attached to the apexes 30a and 36a thereof. In this manner, the apexes 30a and 36a may have a rounded shape. In addition, as in the example of the plunger tip 20 illustrated in FIG. 12, the crushing portion 30 and the rear crushing portion 36 may have an asymmetric triangular shape that is not line-symmetric.

Note that the disclosure is not limited to the above-described embodiments and modifications, and various modifications can be made without departing from the gist of the disclosure.

Claims

1. A plunger tip for die casting, the plunger tip being configured to slide in a cylindrical sleeve to inject a molten metal in the cylindrical sleeve into a mold, the plunger tip comprising: a crushing portion that is provided on an outer peripheral surface on a front end side of the plunger tip and is capable of contacting a solidified layer of the molten metal generated on an inner peripheral surface of the cylindrical sleeve to crush the solidified layer; andrecesses provided on both sides of the crushing portion in a circumferential direction and recessed radially inward from the crushing portion, whereinthe crushing portion has a triangular shape and is a region between two of the recesses adjacent to each other in the circumferential direction and in which a front end side of the plunger tip is an apex.

2. The plunger tip according to claim 1, wherein the plunger tip comprises crushing portions comprising the crushing portion,the crushing portions and the recesses are provided on an entire circumference of the plunger tip such that the crushing portions and the recesses are alternately arranged in the circumferential direction.

3. The plunger tip according to claim 1, wherein an outer diameter of the plunger tip gradually decreases toward a front end side of the plunger tip in a region where the crushing portions is provided.

4. The plunger tip according to claim 2, wherein an outer diameter of the plunger tip gradually decreases toward a front end side of the plunger tip in a region where the crushing portions is provided.

5. The plunger tip according to claim 1, wherein a bottom surface of each of the recesses is inclined inward in a radial direction of the plunger tip toward a front end side of the plunger tip.

6. The plunger tip according to claim 2, wherein a bottom surface of each of the recesses is inclined inward in a radial direction of the plunger tip toward a front end side of the plunger tip.

7. The plunger tip according to claim 1, further comprising: a rear crushing portion provided on an outer peripheral surface of a rear end portion of the plunger tip and capable of contacting the solidified layer generated on an inner peripheral surface of the cylindrical sleeve to crush the solidified layer; andrear recesses provided on both sides of the rear crushing portion in the circumferential direction and recessed radially inward from the rear crushing portion, whereinthe rear crushing portion has a triangular shape in which a rear end side of the plunger tip is an apex in a region between two of the rear recesses adjacent in the circumferential direction.

8. The plunger tip according to claim 2, further comprising: a rear crushing portion provided on an outer peripheral surface of a rear end portion of the plunger tip and capable of contacting the solidified layer generated on an inner peripheral surface of the cylindrical sleeve to crush the solidified layer; andrear recesses provided on both sides of the rear crushing portion in the circumferential direction and recessed radially inward from the rear crushing portion, whereinthe rear crushing portion has a triangular shape in which a rear end side of the plunger tip is an apex in a region between two of the rear recesses adjacent in the circumferential direction.

9. An injection device for die casting, the injection device comprising: a cylindrical sleeve; anda plunger tip configured to slide in the cylindrical sleeve to inject a molten metal in the cylindrical sleeve into a mold, whereinthe plunger tip comprises:a crushing portion that is provided on an outer peripheral surface on a front end side and is capable of contacting a solidified layer of the molten metal generated on an inner peripheral surface of the cylindrical sleeve to crush the solidified layer; andrecesses provided on both sides of the crushing portion in a circumferential direction and recessed radially inward from the crushing portion, andthe crushing portion has a triangular shape and is a region between two of the recesses adjacent to each other in the circumferential direction and in which a front end side of the plunger tip is an apex.