Backflow prevention device and injection device including the same

Abstract

To provide a backflow prevention device which prevents a piston ring from entering into a gap between a backflow prevention ring and a cylinder.SOLUTION: A backflow prevention device 4 of an injection device 3 has a backflow prevention ring 41 having an annular groove 42 facing an inner face of a cylinder 32 of the injection device 3, and a piston ring 51 fit into the annular groove 42. Each of the annular groove 42 and the piston ring 51 has rear side faces 44 and 55 in an injection direction of the injection device 3, and the rear side face 44 of the annular groove 42 and the rear side face 55 of the piston ring 51 can be brought into contact with each other by only their radial inside parts 44B and 55B.SELECTED DRAWING: Figure 3A

Description

[Technical field]

[0001] The present invention relates to a backflow prevention device and an injection device equipped with the same. [Background technology]

[0002] The injection device of an injection molding machine is equipped with a backflow prevention device that prevents the backflow of the material injected during injection. Patent Document 1 describes an injection device having a screw housed in a cylinder, a presser attached to the screw, and a backflow prevention device provided between the screw head of the screw and the presser. When the screw moves forward during injection, the backflow prevention device is pressed against the presser, preventing the injected material from flowing behind the presser. The backflow prevention device has a backflow prevention ring and a piston ring fitted into the backflow prevention ring. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 7-314510 Summary of the Invention [Problem to be solved by the invention]

[0004] The piston rings are provided to improve the sealing performance between the backflow prevention device and the cylinder. However, because the screw moves at high speed during injection, the piston rings may deform and enter the gap between the backflow prevention ring and the cylinder. In this case, not only will the piston rings wear out, but the piston rings that enter the gap between the backflow prevention ring and the cylinder may damage the cylinder.

[0005] An object of the present invention is to provide a backflow prevention device that prevents piston rings from entering the gap between the backflow prevention ring and the cylinder. [Means for solving the problem]

[0006] The backflow prevention device of the present invention includes a backflow prevention ring having an annular groove and a piston ring fitted in the annular groove. The rear side surface of the annular groove in the injection direction and the rear side surface of the piston ring in the injection direction can contact each other only at their respective radially inner portions. Effect of the Invention

[0007] According to the present invention, it is possible to provide a backflow prevention device in which the piston ring is less likely to enter the gap between the backflow prevention ring and the cylinder. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic front view of an injection molding machine according to a first embodiment. [Diagram 2] FIG. 2 is an enlarged view of part A in FIG. [Figure 3A] FIG. 3 is an enlarged view of part B in FIG. [Figure 3B] FIG. 3 is an enlarged view of part C in FIG. 2. [Figure 4] FIG. 4 is a cross-sectional view taken along line 4-4 in FIG. 3A. [Diagram 5] 3 is an enlarged view of a portion corresponding to part B in FIG. 2 in the second embodiment. [Figure 6] 10 is an enlarged view of a portion corresponding to part B in FIG. 2 in the third embodiment. [Figure 7] 10 is an enlarged view of a portion corresponding to part B in FIG. 2 in the fourth embodiment. [Figure 8] 10 is an enlarged view of a portion corresponding to part B in FIG. 2 in the fifth embodiment. [Figure 9] 13 is an enlarged view of a portion corresponding to part B in FIG. 2 in the sixth embodiment. [Figure 10] 13 is an enlarged view of a portion corresponding to part B in FIG. 2 in the seventh embodiment. [Figure 11A] 3 is an enlarged view of a portion corresponding to part B in FIG. 2 in a comparative example. [Figure 11B] FIG. 13 is a diagram showing a problem in the comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, several embodiments of the present invention will be described with reference to the drawings. The present invention can be suitably applied to a metal injection molding machine, particularly to a metal injection molding machine that injects a metal with low viscosity when molten. However, the present invention can also be applied to an injection molding machine that injects a resin. In the following description, the X direction refers to the axial direction of the cylinder 32, the injection direction is called the +X direction, and the direction opposite to the injection direction is called the -X direction. Terms such as "front", "rear", "front part", and "rear part" are defined based on the injection direction. In addition, the "radial direction" is defined based on the central axis CL of the cylinder 32.

[0010] (First embodiment) <Overall configuration of injection molding machine> 1 shows a schematic front view of an injection molding machine 1 according to a first embodiment. The injection molding machine 1 is a horizontal metal injection molding machine that injects molten metal such as a magnesium alloy or an aluminum alloy. The injection molding machine 1 is generally composed of a mold clamping device 2 that clamps a mold, and an injection device 3 that heats, melts, and injects the metal material to be injected.

[0011] <Mold clamping device 2> The mold clamping device 2 includes a fixed platen 21 fixed on a bed (not shown) to which a fixed mold M1 is attached, and a movable platen 22 slidable on the bed to which a movable mold M2 is attached. A mold clamping housing (not shown) slidable on the bed is provided on the opposite side of the fixed platen 21 with respect to the movable platen 22, and the fixed platen 21 and the mold clamping housing are connected by a plurality of tie bars 23. A link mechanism (not shown) for opening and closing the mold is provided between the movable platen 22 and the mold clamping housing. The link mechanism is driven by an electric ball screw. Instead of the link mechanism, a hydraulic mold clamping cylinder may be provided. A cavity M3 is formed between the fixed mold M1 and the movable mold M2 and is filled with molten metal.

[0012] <Injection device 3> The injection device 3 is provided on a base (not shown). The injection device 3 includes a hollow cylindrical cylinder 32 for heating and melting the metal material to be injected, a screw 33 housed in the cylinder 32, and a drive mechanism 34 for driving the screw 33. The screw 33 is coaxial with the cylinder 32. The screw 33 is driven to rotate by the drive mechanism 34 and is driven in the +X and -X directions. A heater 38 for heating and melting the metal material is provided on the outer periphery of the cylinder 32.

[0013] The cylinder 32 is roughly divided into a supply section P1, a compression section P2, and a metering section P3 from the rear to the front. A hopper 36 is provided in the supply section P1, and a pellet-shaped metal material is supplied to the supply section P1. The metal material is sent to the compression section P2 while being heated by a heater 38. The metal material is compressed, heated, and kneaded in the compression section P2 to become molten, and is then transported to the metering section P3. The metering section P3 measures the molten metal to be injected in one injection cycle (shot). The screw 33 is provided with a screw head 35 at its tip in the X direction. An injection nozzle 37 is attached to the tip of the cylinder 32 to supply molten metal to the cavity M3.

[0014] <Backflow prevention device 4> FIG. 2 shows a partial detailed view of the front part of the cylinder 32 (an enlarged view of part A in FIG. 1). For convenience, in FIG. 2, the upper side of the central axis CL of the cylinder 32 shows the state of the backflow prevention device 4 at the time of injection, and the lower side shows the state of the backflow prevention device 4 at the time of metering. FIG. 3A is an enlarged view of part B in FIG. 2, and FIG. 3B is an enlarged view of part C in FIG. 2, showing the positional relationship between the annular groove 42 and the piston ring 51. That is, FIG. 3A shows the state at the time of injection (the state of the backflow prevention ring 41 shown in the upper side of FIG. 2), and FIG. 3B shows the state at the time of metering (the state of the backflow prevention ring 41 shown in the lower side of FIG. 2). FIG. 4 is a cross-sectional view taken along line 4-4 in FIG. 3A.

[0015] As shown in Fig. 2, the metering section P3 of the cylinder 32 of the injection device 3 is provided with a backflow prevention device 4 for preventing the backflow of the injected metal material. The backflow prevention device 4 has a backflow prevention ring 41 made of a hollow cylindrical metal member, and the backflow prevention ring 41 has an annular groove 42 that faces the inner surface of the cylinder 32 of the injection device 3. The backflow prevention device 4 has a piston ring 51 fitted in the annular groove 42 of the backflow prevention ring 41. The backflow prevention ring 41, the annular groove 42, and the piston ring 51 are coaxial with the cylinder 32. The annular groove 42 runs around the outer periphery of the backflow prevention ring 41 and has the same cross section at each angular position around the central axis CL of the cylinder 32.

[0016] In this embodiment, the backflow prevention ring 41 has two annular grooves 42, and one piston ring 51 is fitted in each annular groove 42, but it is sufficient to have at least one annular groove 42 and at least one piston ring 51.

[0017] The piston ring 51 is an annular sealing part fitted in the annular groove 42 of the backflow prevention ring 41, and separates the front and rear of the backflow prevention ring 41. The piston ring 51 is made of metal. As shown in FIG. 4, the piston ring 51 has stepped end portions 63, and is fitted into the annular groove 42 so that the steps of the end portions 63 overlap in the circumferential direction. Therefore, the steps of the end portions 63 shift in the circumferential direction depending on the pressure applied to the inner circumferential surface 52 and the outer circumferential surface 53 of the piston ring 51, thereby changing the diameter of the piston ring 51. The piston ring 51 has the same cross section at each angular position around the central axis CL of the cylinder 32, except for the vicinity of the end portions 63.

[0018] As shown in Fig. 2, a presser plate 39 made of an annular plate is inserted into the screw head 35. A backflow prevention ring 41 is inserted into the screw 33 between the screw head 35 at the front in the injection direction and the presser plate 39 at the rear in the injection direction. The outer diameters of the screw head 35 and the presser plate 39 are larger than the inner diameter of the backflow prevention ring 41, and the length of the backflow prevention ring 41 in the X direction is smaller than the distance between the screw head 35 and the presser plate 39 in the X direction. Therefore, the backflow prevention ring 41 is movable in the X direction while being restricted by the screw head 35 and the presser plate 39.

[0019] As shown in part B of FIG. 2 and part A of FIG. 3, when the molten metal is injected, the screw 33 advances in the +X direction, and the backflow prevention ring 41 retreats in the -X direction relative to the screw 33. The backflow prevention ring 41 abuts against the presser 39, and prevents the molten metal measured in front of the screw 33 from leaking backward when injected (when the screw 33 advances). As shown in part C of FIG. 2 and part B of FIG. 3, when the molten metal is metered, the screw 33 retreats in the -X direction, and the backflow prevention ring 41 advances in the +X direction relative to the screw 33. The backflow prevention ring 41 abuts against the screw head 35, but the molten metal behind the presser 39 passes through the flow passage 35A inside the screw head 35 and is supplied to the space in front of the screw head 35. In this way, the backflow prevention ring 41 is a kind of valve in function.

[0020] A through hole 46 that penetrates the backflow prevention ring 41 in the radial direction is connected to the front annular groove 42 in the injection direction. High-pressure molten metal flows into the through hole 46 and applies radially outward pressure to the piston ring 51. This pressure presses the piston ring 51 against the inner wall of the cylinder 32, improving the sealing performance of the piston ring 51. The through hole 46 is not connected to the rear annular groove 42. However, some of the molten metal passes through the gap between the front piston ring 51 and the cylinder 32, the gap between the backflow prevention ring 41 and the cylinder 32, and the rear annular groove 42 and exerts pressure on the inner circumferential surface 52 of the rear piston ring 51. Therefore, both the front and rear piston rings 51 contribute to improving the sealing performance.

[0021] Providing the through holes 46 improves sealing performance, but since a large pressure is applied to the piston ring 51, wear of the piston ring 51 is likely to progress. Whether or not to provide the through holes 46 is appropriately determined in consideration of sealing performance and suppression of wear of the piston ring 51. The through holes 46 may be provided in both annular grooves 42, the through holes 46 may be provided only in the rear annular groove 42, or none of the annular grooves 42 may have the through holes 46. However, by providing the through holes 46 only in the front annular groove 42, it becomes easier to equalize the amount of wear of the front piston ring 51 and the rear piston ring 51.

[0022] The front annular groove 42 and the rear annular groove 42 have the same configuration except for whether the through holes 46 are connected or not. The two piston rings 51 have the same configuration. Therefore, the following description will mainly focus on the front annular groove 42 and the piston ring 51 fitted in the front annular groove 42.

[0023] <Comparative Example Backflow Prevention Device 104> Here, the backflow prevention device 104 of the comparative example will be described. Fig. 11A is an enlarged view of a portion corresponding to part B in Fig. 2. The annular groove 142 and the piston ring 151 have a rectangular cross section. As described above, the piston ring 151 is subjected to radially outward pressure, so that the piston ring 151 is pressed against the inner wall of the cylinder 32. Also, during injection, the molten metal flows through the gap between the backflow prevention ring 141 and the cylinder 32 in the -X direction and flows into the annular groove 142, so that the piston ring 151 is pressed against the rear side surface 144 of the annular groove 142. During metering, the piston ring 151 is pressed against the front side surface 143 of the annular groove 142 (not shown), but since the speed of the screw 33 is higher during injection, the pressure with which the piston ring 151 is pressed against the side surface of the annular groove 142 is greater during injection.

[0024] 11B shows the state of the piston ring 151 after it has been deformed. The piston ring 151 is made of a relatively soft metal. Therefore, when the piston ring 151 is pressed radially outward and in the -X direction, it may deform and part of it may enter the gap G in the -X direction between the backflow prevention ring 141 and the cylinder 32. When this situation occurs, not only will the piston ring 151 wear further, but the part of the piston ring 151 that has entered the gap G between the backflow prevention ring 141 and the cylinder 32 may damage the cylinder 32.

[0025] <Shapes of the annular groove 42 and the piston ring 51> The shapes of the annular groove 42 and the piston ring 51 of this embodiment will be described in more detail, mainly with reference to FIG. 3A. The annular groove 42 has a front side surface 43, a rear side surface 44, and a bottom surface 45. The bottom surface 45 is the deepest part of the annular groove 42 and separates the front side surface 43 and the rear side surface 44. The rear side surface 44 has a step portion 44C. The rear side surface 44 has a radial outer portion 44A and a radial inner portion 44B separated by the step portion 44C. The front side surface 43 is a plane perpendicular to the central axis CL of the cylinder 32. A through hole 46 is connected to the bottom surface 45.

[0026] The piston ring 51 has an inner peripheral surface 52, an outer peripheral surface 53, a front side surface 54, and a rear side surface 55. The inner peripheral surface 52 is located at the deepest part of the annular groove 42 and separates the front side surface 54 and the rear side surface 55. The inner peripheral surface 52 faces the bottom surface 45 of the annular groove 42. The rear side surface 55 of the piston ring 51 has a step portion 55C. The rear side surface 55 of the piston ring 51 has a radial outer portion 55A and a radial inner portion 55B separated by the step portion 55C. The radial outer portion 55A, the radial inner portion 55B, and the step portion 55C face the radial outer portion 44A, the radial inner portion 44B, and the step portion 44C of the rear side surface 44 of the annular groove 42, respectively. The front side surface 54 is a plane perpendicular to the central axis CL of the cylinder 32, and faces the front side surface 43 of the annular groove 42.

[0027] The radially outer portion 44A of the rear side surface 44 of the annular groove 42 is located rearward of the radially inner portion 44B in the injection direction, and the radially outer portion 55A of the rear side surface 55 of the piston ring 51 protrudes rearward in the injection direction (-X direction) relative to the radially inner portion 55B. The inner diameter of the piston ring 51 is larger at the rear portion in the injection direction than at the front portion in the injection direction. A distance D1 in the X direction between the radially outer portion 44A and the radially inner portion 44B of the rear side surface 44 of the annular groove 42 is larger than a distance D2 in the X direction between the radially outer portion 55A and the radially inner portion 55B of the rear side surface 55 of the piston ring 51.

[0028] For this reason, when the piston ring 51 moves relative to the annular groove 42 in the −X direction during injection, the radial inner portion 55B of the rear side surface 55 of the piston ring 51 can come into contact with the radial inner portion 44B of the rear side surface 44 of the annular groove 42, while the radial outer portion 55A of the rear side surface 55 of the piston ring 51 cannot come into contact with the radial outer portion 44A of the rear side surface 44 of the annular groove 42. In other words, when the radial inner portion 55B of the rear side surface 55 of the piston ring 51 comes into contact with the radial inner portion 44B of the rear side surface 44 of the annular groove 42, a gap S equal to the distance D1-D2 is formed between the radial outer portion 55A of the rear side surface 55 of the piston ring 51 and the radial outer portion 44A of the rear side surface 44 of the annular groove 42.

[0029] In this way, the rear side surface 44 of the annular groove 42 and the rear side surface 55 of the piston ring 51 can come into contact with each other only at their respective radially inner portions 44B, 55B. In this embodiment, the shapes of the annular groove 42 and the piston ring 51 are changed from those of the comparative example, so that it is less likely that a part of the piston ring 51 will enter the gap G on the -X side between the backflow prevention ring 41 and the cylinder 32.

[0030] As described above, a radially outward pressure is applied to the piston ring 51, but in this embodiment, it is easy to reduce or adjust this pressure. The pressure with which the piston ring 51 presses against the inner surface of the cylinder 32 is generated by the pressure applied to the inner peripheral surface 52 of the piston ring 51. Let the pressure applied to the inner peripheral surface 52 of the piston ring 51 be P1, the pressure with which the piston ring 51 presses against the cylinder 32 be P2, the width of the inner peripheral surface 52 of the piston ring 51 be W1, and the width of the outer peripheral surface 53 of the piston ring 51 be W2. Then P2 = (W1 / W2) × P1. Since P2 < P1, wear of the piston ring 51 can be suppressed. Moreover, since the pressure P2 can be adjusted by W1 / W2, it is possible to balance the sealing performance and the suppression of wear of the piston ring 51.

[0031] Hereinafter, other embodiments of the present invention will be described. In the embodiments shown below, the configurations and effects for which the description is omitted are the same as those of the first embodiment.

[0032] (Second Embodiment) FIG. 5 is an enlarged view of a portion corresponding to part B of FIG. 2, showing the second embodiment. In this embodiment, a step portion 43C of the annular groove 42 and a step portion 54C of the piston ring 51 are also formed on the front side in the injection direction. The front side surface 43 of the annular groove 42 has a radially outer portion 43A and a radially inner portion 43B partitioned by the step portion 43C. The front side surface 54 of the piston ring 51 has a radially inner portion 54A and a radially inner portion 54B partitioned by the step portion 54C.

[0033] The radial outer portion 43A of the front side surface 43 of the annular groove 42 is located forward of the radial inner portion 43B in the injection direction, and the radial outer portion 54A of the front side surface 54 of the piston ring 51 protrudes in the injection direction (X direction) relative to the radial inner portion 54B. A distance D3 in the X direction between the radial outer portion 43A and the radial inner portion 43B of the front side surface 43 of the annular groove 42 is greater than a distance D4 in the X direction between the radial outer portion 54A and the radial inner portion 54B of the front side surface 54 of the piston ring 51. For this reason, as shown by the dashed line in FIG. 5 , when the piston ring 51 moves relative to the annular groove 42 in the X direction during metering, the radial inner portion 54B of the front side surface 54 of the piston ring 51 can come into contact with the radial inner portion 43B of the front side surface 43 of the annular groove 42, whereas the radial outer portion 54A of the front side surface 54 of the piston ring 51 cannot come into contact with the radial outer portion 43A of the front side surface 43 of the annular groove 42.

[0034] Therefore, the front side surface 43 of the annular groove 42 and the front side surface 54 of the piston ring 51 can come into contact with each other only at their respective radially inner portions 43B, 54B. As described above, since the speed of the screw 33 is slow during metering, it is unlikely that the piston ring 51 will enter the gap G1 on the +X side between the backflow prevention ring 41 and the cylinder 32. However, according to this embodiment, this possibility can be further reduced.

[0035] (Third embodiment) FIG. 6 is an enlarged view of a portion corresponding to the portion B in FIG. 2, showing the third embodiment. The radially inner portion 44B of the rear side surface 44 of the annular groove 42 and the radially inner portion 55B of the rear side surface 55 of the piston ring 51 are inclined at the same angle with respect to the central axis CL of the cylinder 32. The radially inner portion 44B of the annular groove 42 and the bottom surface 45 form an obtuse angle, and the radially inner portion 55B of the piston ring 51 and the inner peripheral surface 52 form an obtuse angle. Therefore, the radially inner portion 55B of the rear side surface 55 of the piston ring 51 contacts the radially inner portion 44B of the rear side surface 44 of the annular groove 42 evenly during injection. The radially outer portion 44A of the rear side surface 44 of the annular groove 42 and the radially outer portion 55A of the rear side surface 55 of the piston ring 51 are perpendicular to the central axis CL of the cylinder 32. In this embodiment, the pressure P3 applied to the piston ring 51 in the -X direction is converted into a pressure P4 directed radially outward, thereby improving the sealing performance.

[0036] (Fourth embodiment) 7 is an enlarged view of a portion corresponding to part B in FIG. 2, showing the fourth embodiment. The rear side surface 44 of the annular groove 42 has a radial outer portion 44A, a radial inner portion 44B, and a step portion 44C, and the radial outer portion 44A and the radial inner portion 44B are partitioned by the step portion 44C. The inner diameter of the piston ring 51 is larger at the rear portion in the injection direction than at the front portion in the injection direction. When the piston ring 51 moves relative to the annular groove 42 in the -X direction during injection, the radial inner portion 55B of the rear side surface 55 of the piston ring 51 can come into contact with the radial inner portion 44B of the rear side surface 44 of the annular groove 42, while the radial outer portion 55A of the rear side surface 55 of the piston ring 51 cannot come into contact with the radial outer portion 44A of the rear side surface 44 of the annular groove 42.

[0037] The piston ring 51 has a housing portion 56 housed in the annular groove 42 and a protruding portion 57 located outside the annular groove 42. An outer peripheral surface 53 of the piston ring 51 is located radially outside the annular groove 42. A rear end portion 53A in the injection direction of the outer peripheral surface 53 is located forward of a rearmost end portion in the injection direction of the housing portion 56 (a radially outer portion 55A of a rear side surface 55), and the outer diameter of the piston ring 51 is smaller at the rear portion in the injection direction than at the front portion in the injection direction. In this embodiment, the piston ring 51 is farther away from the gap G than in the first embodiment, and therefore the piston ring 51 is less likely to enter the gap G.

[0038] The surface connecting the rear end 53A of the outer circumferential surface 53 and the outermost portion 58 in the radial direction of the rear end of the housing portion 56 has a cylindrical surface 60 whose central axis is the central axis CL of the cylinder 32, and an inclined surface 61 that connects the cylindrical surface 60 and the rear end 53A of the outer circumferential surface 53 and is inclined with respect to the central axis CL of the cylinder 32. This makes it easy to move the piston ring 51 away from the gap G, and makes it difficult for the piston ring 51 to enter the gap G. The diameter of the cylindrical surface 60 is constant in the X direction, but may increase in the +X direction.

[0039] Fifth embodiment 8 is an enlarged view of a portion corresponding to part B in FIG. 2, showing the fifth embodiment. In this embodiment, the outermost part 58 in the radial direction of the rear end part of the housing part 56 is inside the annular groove 42, and the other points are the same as those of the fourth embodiment. This embodiment has the effects of the fourth embodiment. In addition, in this embodiment, the piston ring 51 is further away from the gap G than in the first and fourth embodiments, so that the piston ring 51 is even less likely to enter the gap G.

[0040] As the piston ring 51 wears with use, the thickness of the protruding portion 57 gradually decreases, and eventually the cylindrical surface 60 may come into contact with the cylinder 32. However, because the cylindrical surface 60 is inside the annular groove 42, it takes a longer time for the cylindrical surface 60 to come into contact with the cylinder 32 even if the piston ring 51 wears and becomes thinner. In this embodiment, the life of the piston ring 51 can be extended, and the frequency of replacing the piston ring 51 can be reduced.

[0041] Sixth embodiment 9 is an enlarged view of a portion corresponding to part B in FIG. 2, showing the sixth embodiment. A surface 62 connecting the rear end 53A of the outer peripheral surface 53 and the outermost portion 58 in the radial direction of the rear end of the accommodating portion 56 is generally inclined with respect to the central axis CL of the cylinder 32. The other points are the same as those of the fourth embodiment. The outermost portion 58 in the radial direction of the rear end of the accommodating portion 56 is located at approximately the same position in the radial direction as the opening 59 of the annular groove 42. This embodiment has almost the same effects as the fourth embodiment, but since the shape of the surface 62 connecting the rear end 53A of the outer peripheral surface 53 and the outermost portion 58 in the radial direction of the rear end of the accommodating portion 56 is simple, the piston ring 51 can be easily machined.

[0042] Seventh embodiment FIG. 10 is an enlarged view of a portion corresponding to part B in FIG. 2, showing the seventh embodiment. In this embodiment, the outermost part 58 in the radial direction of the rear end of the housing part 56 is inside the annular groove 42, and the other points are the same as those of the sixth embodiment. This embodiment has the same effect as the fifth embodiment. That is, since the outermost part 58 is inside the annular groove 42, it takes a longer time for the outermost part 58 to contact the cylinder 32 even if the piston ring 51 wears and is reduced in thickness. In this embodiment, the life of the piston ring 51 can be extended, and the frequency of replacing the piston ring 51 can be reduced. In addition, in this embodiment, the piston ring 51 is farther away from the gap G than in the first embodiment, so that the piston ring 51 is less likely to enter the gap G.

[0043] (Additional Note) This specification includes the following disclosure. [Configuration 1] a backflow prevention ring having at least one annular groove facing an inner surface of the cylinder of the injection device; At least one piston ring is fitted in the at least one annular groove, the at least one annular groove and the at least one piston ring each have a rear side in an injection direction of the injection device; A backflow prevention device for an injection apparatus, wherein the rear side surface of the at least one annular groove and the rear side surface of the at least one piston ring are capable of contacting each other only at their respective radially inner portions. [Configuration 2] 2. The backflow prevention device of the injection apparatus as described in configuration 1, wherein a gap is formed between a radial outer portion of the rear side surface of the at least one piston ring and a radial outer portion of the rear side surface of the at least one annular groove when the radial inner portion of the rear side surface of the at least one piston ring contacts the radial inner portion of the rear side surface of the at least one annular groove. [Configuration 3] the radially outer portion of the rear side surface of the at least one annular groove is rearward in the injection direction relative to the radially inner portion of the rear side surface of the at least one annular groove; 3. The backflow prevention device according to claim 2, wherein the radially outer portion of the rear side surface of the at least one piston ring protrudes rearward in the injection direction relative to the radially inner portion of the rear side surface of the at least one piston ring. [Configuration 4] 4. The backflow prevention device of claim 3, wherein the radially inner portion of the rear side surface of the at least one annular groove and the radially inner portion of the rear side surface of the at least one piston ring are planes perpendicular to a central axis of the cylinder. [Configuration 5] 5. The backflow prevention device of claim 4, wherein a distance between the radially outer portion and the radially inner portion of the rear side of the at least one annular groove is greater than a distance between the radially outer portion and the radially inner portion of the rear side of the at least one piston ring. [Configuration 6] 4. The backflow prevention device of claim 3, wherein the radially inner portion of the rear side surface of the at least one annular groove and the radially inner portion of the rear side surface of the at least one piston ring are inclined with respect to a central axis of the cylinder. [Configuration 7] The backflow prevention device according to any one of configurations 1 to 6, wherein the at least one piston ring has a housing portion housed in the at least one annular groove and an outer peripheral surface located radially outward from the at least one annular groove, and a rear end portion of the outer peripheral surface in the injection direction is forward in the injection direction of a rear end portion of the housing portion in the injection direction. [Configuration 8] A surface connecting the rear end of the outer circumferential surface and the rear end of the storage portion is A cylindrical surface having a central axis along the central axis of the cylinder; an inclined surface that connects a front end of the cylindrical surface in the injection direction and a rear end of the outer circumferential surface and is inclined with respect to a central axis of the cylinder; 8. The backflow prevention device of the injection device according to claim 7, [Configuration 9] 8. The backflow prevention device for the injection device according to configuration 7, wherein a surface connecting the rear end of the outer circumferential surface and the rear end of the storage portion is generally inclined with respect to a central axis of the cylinder. [Configuration 10] 10. The backflow prevention device of the injection device according to configuration 8 or 9, wherein a radially outermost portion of the rear end portion of the accommodating portion is located at approximately the same position in the radial direction as an opening of the annular groove. [Configuration 11] 10. The backflow prevention device of the injection device according to configuration 8 or 9, wherein a radially outermost portion of the rear end portion of the accommodating portion is inside the annular groove. [Configuration 12] the at least one annular groove and the at least one piston ring each have a front side in the injection direction; 12. The backflow prevention device of any one of claims 1 to 11, wherein the front side of the at least one annular groove and the front side of the at least one piston ring are capable of contacting each other only at their respective radially inner portions. [Configuration 13] 13. The backflow prevention device according to any one of configurations 1 to 12, wherein the inner diameter of the piston ring is larger at a rear portion in the injection direction than at a front portion in the injection direction. [Configuration 14] A backflow prevention device according to any one of configurations 1 to 13, The cylinder; A screw housed in the cylinder; and an injection nozzle attached to the tip of the cylinder. [Explanation of symbols]

[0044] 3 Injection device 4 Backflow prevention device 32 cylinders 33 Screw 41 Backflow prevention ring 42 Annular groove 44 Rear side of annular groove 44A Radially outer portion of rear side of annular groove 44B: radially inner portion of rear side of annular groove 46 Through hole 51 Piston ring 53 Outer surface 53A Rear end of outer periphery 55 Rear side of piston ring 55A Radial outer part of rear side of piston ring 55B Radial inner part of rear side of piston ring 56 Storage unit

Claims

1. a backflow prevention ring having at least one annular groove facing an inner surface of the cylinder of the injection device; At least one piston ring fitted in the at least one annular groove, the at least one annular groove and the at least one piston ring each have a rear side in an injection direction of the injection device; A backflow prevention device for an injection apparatus, wherein the rear side surface of the at least one annular groove and the rear side surface of the at least one piston ring are capable of contacting each other only at their respective radially inner portions.

2. 2. The backflow prevention device of claim 1, wherein when the radially inner portion of the rear side surface of the at least one piston ring contacts the radially inner portion of the rear side surface of the at least one annular groove, a gap is formed between the radially outer portion of the rear side surface of the at least one piston ring and the radially outer portion of the rear side surface of the at least one annular groove.

3. the radially outer portion of the rear side surface of the at least one annular groove is rearward in the injection direction relative to the radially inner portion of the rear side surface of the at least one annular groove; 3. The backflow prevention device according to claim 2, wherein the radially outer portion of the rear side surface of the at least one piston ring protrudes rearward in the injection direction relative to the radially inner portion of the rear side surface of the at least one piston ring.

4. 4. The backflow prevention device of claim 3, wherein the radially inner portion of the rear side surface of the at least one annular groove and the radially inner portion of the rear side surface of the at least one piston ring are planes perpendicular to a central axis of the cylinder.

5. 5. The backflow prevention device of claim 4, wherein a distance between the radially outer and radially inner portions of the rear side of the at least one annular groove is greater than a distance between the radially outer and radially inner portions of the rear side of the at least one piston ring.

6. 4. The backflow prevention device of claim 3, wherein the radially inner portion of the rear side surface of the at least one annular groove and the radially inner portion of the rear side surface of the at least one piston ring are inclined with respect to a central axis of the cylinder.

7. 2. The backflow prevention device according to claim 1, wherein the at least one piston ring has a housing portion housed in the at least one annular groove and an outer peripheral surface located radially outward from the at least one annular groove, and a rear end portion in the injection direction of the outer peripheral surface is located forward in the injection direction of a rear end portion in the injection direction of the housing portion.

8. A surface connecting the rear end of the outer circumferential surface and the rear end of the storage portion is A cylindrical surface having a central axis along the central axis of the cylinder; an inclined surface that connects a front end of the cylindrical surface in the injection direction and a rear end of the outer circumferential surface and is inclined with respect to a central axis of the cylinder; The backflow prevention device of claim 7, further comprising:

9. 8. The backflow prevention device for an injection apparatus according to claim 7, wherein a surface connecting the rear end of the outer circumferential surface and the rear end of the housing portion is generally inclined with respect to a central axis of the cylinder.

10. 10. The backflow prevention device of the injection apparatus according to claim 8, wherein a radially outermost portion of the rear end portion of the housing portion is located at substantially the same position in the radial direction as an opening of the annular groove.

11. 10. The backflow prevention device for an injection apparatus according to claim 8, wherein a radially outermost portion of the rear end portion of the accommodation portion is located inside the annular groove.

12. the at least one annular groove and the at least one piston ring each have a front side in the injection direction; 2. The backflow prevention device of claim 1, wherein the front side of the at least one annular groove and the front side of the at least one piston ring are capable of contacting each other only at their respective radially inner portions.

13. 2. The backflow prevention device according to claim 1, wherein an inner diameter of the piston ring is larger at a rear portion in the injection direction than at a front portion in the injection direction.

14. A backflow prevention device according to claim 1 ; The cylinder; A screw housed in the cylinder; and an injection nozzle attached to the tip of the cylinder.

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