Backflow prevention device and injection device provided with same

Abstract

The present invention provides a backflow prevention device that resists the intrusion of a piston ring into the gap between a backflow prevention ring and a cylinder. A backflow prevention device 4 of an injection device 3 has a backflow prevention ring 41 with an annular groove 42 facing the inner surface of a cylinder 32 of the injection device 3, and a piston ring 51 fitted in the annular groove 42. The annular groove 42 and the piston ring 51 each have a rear-section face 44, 55 in the injection direction of the injection device 3. The rear-section face 44 of the annular groove 42 and the rear-section face 55 of the piston ring 51 can contact each other only at respective radially inward sections 44B, 55B.

Description

Backflow prevention device and injection device equipped with the same

[0001] This application is based on and claims priority from Japanese Patent Application No. 2023-108728, filed on June 30, 2023, which is incorporated herein by reference in its entirety. The present invention relates to a backflow prevention device and an injection device equipped with the same.

[0002] The injection unit of an injection molding machine is equipped with a backflow prevention device that prevents the backflow of injected material during injection. Japanese Patent Application Laid-Open No. 7-314510 describes an injection unit having a screw housed in a cylinder, a presser foot attached to the screw, and a backflow prevention device provided between the screw head of the screw and the presser foot. When the screw moves forward during injection, the backflow prevention device is pressed against the presser foot, preventing the injected material from flowing behind the presser foot. The backflow prevention device includes a backflow prevention ring and a piston ring fitted into the backflow prevention ring.

[0003] Piston rings are installed 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 also damage the cylinder.

[0004] An object of the present invention is to provide a backflow prevention device that makes it difficult for piston rings to enter the gap between the backflow prevention ring and the cylinder.

[0005] 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, wherein 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 come into contact with each other only at their respective radially inner portions.

[0006] According to the present invention, it is possible to provide a backflow prevention device that makes it difficult for piston rings to enter the gap between the backflow prevention ring and the cylinder. The above and other objects, features, and advantages of the present application will become apparent from the following detailed description taken in conjunction with the accompanying drawings illustrating the present application.

[0007] FIG. 3 is a schematic front view of an injection molding machine according to a first embodiment. FIG. 4 is an enlarged view of a portion A in FIG. 1. FIG. 5 is an enlarged view of a portion B in FIG. 2. FIG. 6 is an enlarged view of a portion C in FIG. 2. FIG. 7 is a cross-sectional view taken along line 4-4 in FIG. 3A. FIG. 8 is an enlarged view of a portion corresponding to portion B in FIG. 2 in a second embodiment. FIG. 9 is an enlarged view of a portion corresponding to portion B in FIG. 2 in a third embodiment. FIG. 10 is an enlarged view of a portion corresponding to portion B in FIG. 2 in a fourth embodiment. FIG. 11 is an enlarged view of a portion corresponding to portion B in FIG. 2 in a fifth embodiment. FIG. 12 is an enlarged view of a portion corresponding to portion B in FIG. 2 in a sixth embodiment. FIG. 13 is an enlarged view of a portion corresponding to portion B in FIG. 2 in a seventh embodiment. FIG. 14 is an enlarged view of a portion corresponding to portion B in FIG. 2 in a comparative example. FIG. 15 is a diagram illustrating a problem in the comparative example.

[0008] Several embodiments of the present invention will be described below with reference to the drawings. The present invention is suitable for use in metal injection molding machines, particularly those that inject metals with low molten viscosity. However, the present invention can also be applied to injection molding machines that inject resin. In the following description, the X direction refers to the axial direction of the cylinder 32, the injection direction is referred to as the +X direction, and the direction opposite to the injection direction is referred to as the -X direction. Terms such as "front," "rear," "front portion," and "rear" are defined relative to the injection direction. Furthermore, the "radial direction" is defined relative to the central axis CL of the cylinder 32.

[0009] (First embodiment) <Overall configuration of injection molding machine> Figure 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.

[0010] <Mold Clamping Unit 2> The mold clamping unit 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. A hydraulic mold clamping cylinder may be provided instead of the link mechanism. A cavity M3 is formed between the fixed mold M1 and the movable mold M2 and is filled with molten metal.

[0011] <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 also 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.

[0012] The cylinder 32 is roughly divided into a supply section P1, a compression section P2, and a metering section P3 from rear to front. A hopper 36 is provided in the supply section P1, and pelletized metal material is supplied thereto. The metal material is heated by a heater 38 and sent to the compression section P2. 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 meters 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 that supplies molten metal to the cavity M3 is attached to the tip of the cylinder 32.

[0013] <Backflow prevention device 4> Figure 2 shows a partial detailed view of the front portion of the cylinder 32 (an enlarged view of part A in Figure 1). For convenience, in Figure 2, the part above the center axis CL of the cylinder 32 shows the state of the backflow prevention device 4 during injection, and the part below shows the state of the backflow prevention device 4 during metering. Figure 3A is an enlarged view of part B in Figure 2, and Figure 3B is an enlarged view of part C in Figure 2, showing the positional relationship between the annular groove 42 and the piston ring 51. That is, Figure 3A shows the state during injection (the state of the backflow prevention ring 41 shown in the upper part of Figure 2), and Figure 3B shows the state during metering (the state of the backflow prevention ring 41 shown in the lower part of Figure 2). Figure 4 is a cross-sectional view taken along line 4-4 in Figure 3A.

[0014] As shown in Figure 2, the metering section P3 of the cylinder 32 of the injection device 3 is provided with a backflow prevention device 4 that prevents 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.

[0015] 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 one piston ring 51.

[0016] The piston ring 51 is an annular sealing component fitted into 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 at the end portions 63 overlap in the circumferential direction. Therefore, the steps at the end portions 63 shift circumferentially in response to the pressure applied to the inner circumferential surface 52 and 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.

[0017] As shown in Figure 2, a presser foot 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 foot 39 at the rear in the injection direction. The outer diameters of the screw head 35 and the presser foot 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 foot 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 foot 39.

[0018] As shown in part B of FIG. 2 and part A of FIG. 3, when molten metal is injected, the screw 33 moves forward in the +X direction, and the backflow prevention ring 41 moves backward in the -X direction relative to the screw 33. The backflow prevention ring 41 abuts against the presser foot 39, preventing the metered molten metal in front of the screw 33 from leaking backward during injection (when the screw 33 moves forward). As shown in part C of FIG. 2 and part A of FIG. 3, when metering molten metal, the screw 33 moves backward in the -X direction, and the backflow prevention ring 41 moves forward 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 foot 39 passes through a flow path 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 functions as a type of valve.

[0019] A through-hole 46 that penetrates the backflow prevention ring 41 radially is connected to the front annular groove 42 in the injection direction. High-pressure molten metal flows into the through-hole 46, applying 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 rear annular groove 42 is not connected to the through-hole 46. 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 applies pressure to the inner circumferential surface 52 of the rear piston ring 51. Therefore, both the front and rear piston rings 51 contribute to improving sealing performance.

[0020] Providing the through holes 46 improves sealing performance, but since a large pressure is applied to the piston rings 51, wear of the piston rings 51 is more likely to progress. Whether or not to provide the through holes 46 is determined appropriately taking into consideration sealing performance and suppression of wear of the piston rings 51. The through holes 46 may be provided in both annular grooves 42, or only in the rear annular groove 42, or none of the annular grooves 42 may have through holes 46. However, providing the through holes 46 only in the front annular groove 42 makes it easier to equalize the amount of wear of the front piston rings 51 and the rear piston rings 51.

[0021] The front annular groove 42 and the rear annular groove 42 have the same configuration except for whether or not the through-holes 46 are connected. 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.

[0022] <Backflow Prevention Device 104 of Comparative Example> Here, the backflow prevention device 104 of the comparative example will be described. FIG. 11A is an enlarged view of the area corresponding to portion B in FIG. 2 . The annular groove 142 and the piston ring 151 have rectangular cross sections. As described above, radially outward pressure is applied to the piston ring 151, so the piston ring 151 is pressed against the inner wall of the cylinder 32. During injection, molten metal flows in the −X direction through the gap between the backflow prevention ring 141 and the cylinder 32 and into the annular groove 142, so 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). However, because the speed of the screw 33 is higher during injection, the pressure pressing the piston ring 151 against the side surface of the annular groove 142 is greater during injection.

[0023] 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 does wear of the piston ring 151 progress, 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.

[0024] <Shapes of Annular Groove 42 and 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 Figure 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 from the rear side surface 44. The rear side surface 44 has a stepped portion 44C. The rear side surface 44 has a radially outer portion 44A and a radially inner portion 44B separated by the stepped 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.

[0025] The piston ring 51 has an inner circumferential surface 52, an outer circumferential surface 53, a front side surface 54, and a rear side surface 55. The inner circumferential surface 52 is located at the deepest portion of the annular groove 42 and separates the front side surface 54 from the rear side surface 55. The inner circumferential surface 52 faces the bottom surface 45 of the annular groove 42. The rear side surface 55 of the piston ring 51 has a stepped portion 55C. The rear side surface 55 of the piston ring 51 has a radially outer portion 55A and a radially inner portion 55B separated by the stepped portion 55C. The radially outer portion 55A, the radially inner portion 55B, and the stepped portion 55C face the radially outer portion 44A, the radially inner portion 44B, and the stepped 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.

[0026] The radially outer portion 44A of the rear side surface 44 of the annular groove 42 is located rearward in the injection direction from the radially inner portion 44B, 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) from 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.

[0027] For this reason, when the piston ring 51 moves relative to the annular groove 42 in the −X direction during injection, the radially inner portion 55B of the rear side surface 55 of the piston ring 51 can come into contact with the radially inner portion 44B of the rear side surface 44 of the annular groove 42, whereas the radially outer portion 55A of the rear side surface 55 of the piston ring 51 cannot come into contact with the radially outer portion 44A of the rear side surface 44 of the annular groove 42. In other words, when the radially inner portion 55B of the rear side surface 55 of the piston ring 51 comes into contact with the radially 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 radially outer portion 55A of the rear side surface 55 of the piston ring 51 and the radially outer portion 44A of the rear side surface 44 of the annular groove 42.

[0028] 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, by changing the shapes of the annular groove 42 and the piston ring 51 from those of the comparative example, it is made more difficult for part of the piston ring 51 to enter the gap G on the −X side between the backflow prevention ring 41 and the cylinder 32.

[0029] As described above, radially outward pressure is applied to the piston ring 51, but this embodiment makes it 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 circumferential surface 52 of the piston ring 51. If the pressure applied to the inner circumferential surface 52 of the piston ring 51 is P1, the pressure with which the piston ring 51 presses against the cylinder 32 is P2, the width of the inner circumferential surface 52 of the piston ring 51 is W1, and the width of the outer circumferential surface 53 of the piston ring 51 is W2, then P2 = (W1 / W2) × P1. Since P2 < P1, wear of the piston ring 51 can be suppressed. Moreover, because the pressure P2 can be adjusted by W1 / W2, it is possible to balance sealing performance and suppression of wear of the piston ring 51.

[0030] Other embodiments of the present invention will be described below. In the following embodiments, the configurations and effects that are not described are the same as those of the first embodiment.

[0031] Second Embodiment Figure 5 is an enlarged view of a portion corresponding to part B in Figure 2, showing a 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 forward 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 separated 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 separated by the step portion 54C.

[0032] The radially outer portion 43A of the front side surface 43 of the annular groove 42 is located forward of the radially inner portion 43B in the injection direction, and the radially outer portion 54A of the front side surface 54 of the piston ring 51 protrudes in the injection direction (X direction) relative to the radially inner portion 54B. A distance D3 in the X direction between the radially outer portion 43A and the radially 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 radially outer portion 54A and the radially inner portion 54B of the front side surface 54 of the piston ring 51. Therefore, as shown by the dashed line in Figure 5, when the piston ring 51 moves relative to the annular groove 42 in the X direction during metering, the radially inner portion 54B of the front side surface 54 of the piston ring 51 can come into contact with the radially inner portion 43B of the front side surface 43 of the annular groove 42, while the radially outer portion 54A of the front side surface 54 of the piston ring 51 cannot come into contact with the radially outer portion 43A of the front side surface 43 of the annular groove 42.

[0033] 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, because the speed of the screw 33 is slow during metering, there is little chance 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, this embodiment can further reduce this possibility.

[0034] (Third Embodiment) Figure 6 is an enlarged view of a portion corresponding to portion B in Figure 2, showing a 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 circumferential surface 52 form an obtuse angle. Therefore, during injection, the radially inner portion 55B of the rear side surface 55 of the piston ring 51 evenly contacts the radially inner portion 44B of the rear side surface 44 of the annular groove 42. 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 acting on the piston ring 51 in the −X direction is converted into pressure P4 acting radially outward, thereby improving the sealing performance.

[0035] (Fourth Embodiment) Figure 7 is an enlarged view of a portion corresponding to portion B in Figure 2, showing a fourth embodiment. The rear side surface 44 of the annular groove 42 has a radially outer portion 44A, a radially inner portion 44B, and a stepped portion 44C, and the radially outer portion 44A and the radially inner portion 44B are separated by the stepped portion 44C. The inner diameter of the piston ring 51 is larger at its rear portion in the injection direction than at its 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 radially inner portion 55B of the rear side surface 55 of the piston ring 51 can come into contact with the radially inner portion 44B of the rear side surface 44 of the annular groove 42, while the radially outer portion 55A of the rear side surface 55 of the piston ring 51 cannot come into contact with the radially outer portion 44A of the rear side surface 44 of the annular groove 42.

[0036] 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 of the outer peripheral surface 53 in the injection direction is located forward of the rearmost end portion of the housing portion 56 in the injection direction (a radially outer portion 55A of the 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 further away from the gap G than in the first embodiment, and therefore the piston ring 51 is less likely to enter the gap G.

[0037] The surface connecting the rear end 53A of the outer peripheral surface 53 and the outermost radial portion 58 of the rear end of the housing portion 56 has a cylindrical surface 60 whose center 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 peripheral 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.

[0038] Fifth Embodiment Figure 8 is an enlarged view of a portion corresponding to portion B in Figure 2, showing a fifth embodiment. In this embodiment, the radial outermost portion 58 of the rear end of the housing portion 56 is located inside the annular groove 42, and is otherwise the same as the fourth embodiment. This embodiment achieves the effects of the fourth embodiment. Furthermore, in this embodiment, the piston ring 51 is further away from the gap G than in the first and fourth embodiments, making it even more difficult for the piston ring 51 to enter the gap G.

[0039] 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 located 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.

[0040] Sixth Embodiment Figure 9 is an enlarged view of a portion corresponding to part B in Figure 2, showing a sixth embodiment. A surface 62 connecting the rear end 53A of the outer peripheral surface 53 and the radially outermost portion 58 of the rear end of the accommodating portion 56 is generally inclined with respect to the central axis CL of the cylinder 32. The remaining features are the same as those of the fourth embodiment. The radially outermost portion 58 of the rear end of the accommodating portion 56 is located at approximately the same radial position as the opening 59 of the annular groove 42. This embodiment achieves substantially the same effects as the fourth embodiment, but because the shape of the surface 62 connecting the rear end 53A of the outer peripheral surface 53 and the radially outermost portion 58 of the rear end of the accommodating portion 56 is simple, the piston ring 51 can be easily machined.

[0041] Seventh Embodiment FIG. 10 is an enlarged view of a portion corresponding to portion B in FIG. 2 , showing a seventh embodiment. In this embodiment, the radially outermost portion 58 of the rear end of the housing portion 56 is located inside the annular groove 42, and is otherwise the same as the sixth embodiment. This embodiment achieves the same effects as the fifth embodiment. That is, because the outermost portion 58 is located inside the annular groove 42, it takes a longer time for the outermost portion 58 to contact the cylinder 32 even if the piston ring 51 wears and thins. In this embodiment, the life of the piston ring 51 can be extended, and the frequency of replacement of the piston ring 51 can be reduced. Furthermore, in this embodiment, the piston ring 51 is located farther from the gap G than in the first embodiment, making it even less likely for the piston ring 51 to intrude into the gap G.

[0042] Although several preferred embodiments of the present invention have been shown and described in detail, it will be understood that various changes and modifications can be made therein without departing from the spirit or scope of the appended claims.

[0043] 3 Injection device 4 Backflow prevention device 32 Cylinder 33 Screw 41 Backflow prevention ring 42 Annular groove 44 Rear side surface of annular groove 44A Radially outer portion of rear side surface of annular groove 44B Radially inner portion of rear side surface of annular groove 46 Through hole 51 Piston ring 53 Outer peripheral surface 53A Rear end portion of outer peripheral surface 55 Rear side surface of piston ring 55A Radially outer portion of rear side surface of piston ring 55B Radially inner portion of rear side surface of piston ring 56 Storage portion

Claims

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

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 backflow prevention device as described in claim 2, wherein the radially outer portion of the rear side surface of the at least one annular groove is located rearward in the injection direction from the radially inner portion of the rear side surface of the at least one annular groove, and the radially outer portion of the rear side surface of the at least one piston ring protrudes rearward in the injection direction from the radially inner portion of the rear side surface of the at least one piston ring.

4. The backflow prevention device according to 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 the central axis of the cylinder.

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.

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

7. A backflow prevention device as described in any one of claims 1 to 6, wherein the at least one piston ring has a receiving portion received 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 receiving portion.

8. A backflow prevention device for an injection device as described in claim 7, wherein a surface connecting the rear end of the outer peripheral surface and the rear end of the accommodating portion has: a cylindrical surface whose central axis is the central axis of the cylinder; and an inclined surface that connects the front end of the cylindrical surface in the injection direction and the rear end of the outer peripheral surface, and is inclined with respect to the central axis of the cylinder.

9. A backflow prevention device for an injection device as set forth in claim 7, wherein a surface connecting said rear end of said outer circumferential surface and said rear end of said storage portion is generally inclined with respect to a central axis of said cylinder.

10. A backflow prevention device for an injection device as described in claim 8 or 9, wherein the radially outermost portion of the rear end of the accommodating portion is located at approximately the same position in the radial direction as the opening of the annular groove.

11. A backflow prevention device for an injection apparatus as described in claim 8 or 9, wherein the radially outermost portion of said rear end portion of said accommodating portion is inside said annular groove.

12. A backflow prevention device as claimed in any one of claims 1 to 11, wherein the at least one annular groove and the at least one piston ring each have a front side surface in the injection direction, and the front side surface of the at least one annular groove and the front side surface of the at least one piston ring are capable of contacting each other only at their respective radially inner portions.

13. A backflow prevention device according to any one of claims 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.

14. An injection device comprising: a backflow prevention device according to any one of claims 1 to 13; the cylinder; a screw housed in the cylinder; and an injection nozzle attached to the tip of the cylinder.

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