Screw head structure and injection molding machine

Abstract

The invention relates to a screw head structure and an injection molding machine. Provided are a screw head structure capable of suppressing contact between a screw head and a check ring with high reliability from the initial stage of a metering operation to the end of metering, and an injection molding machine including the screw head structure. A screw head included in the screw head structure of the present invention is configured from, in order from the side close to a molding die, a conical portion, a large-diameter portion disposed at the rear section of the conical portion and allowing a resin material to flow through the outer peripheral surface, and a small-diameter portion disposed at the rear section of the large-diameter portion and facing a non-return ring. An overflow groove through which the resin material passing through the annular resin flow path overflows to the outside in the radial direction is formed in the rear outer peripheral surface of the large-diameter section, said rear outer peripheral surface being in contact with the small-diameter section.

Description

Technical Field

[0001] The present invention relates to a screw head structure for extruding a heated and molten resin material into a molding die (mold) for injection molding, and an injection molding machine equipped with the screw head structure. Background Art

[0002] Injection molding machines are known that inject a resin (also referred to as "molten resin") heated and melted in a barrel into a mold at high pressure. In such an injection molding machine, a screw shaft is provided in the barrel so as to be freely rotatable and retractable, and the heated and molten resin is injected into the mold by advancing the screw shaft during injection molding.

[0003] For example, as in Patent Document 1 Figure 1 As described in, for extruding the resin into the die in the barrel, a screw head is provided at the top end of the screw. As an example, such a screw head is attached to the above-mentioned screw shaft by using a known fastening means such as a screw.

[0004] Among them, generally, in injection molding, a large number of molded products are continuously molded, so after injecting molten resin into the mold (injecting molten resin into the mold is also called "shot"), refilling of molten resin for the next injection molding is performed (so-called metering action). In such an injection molding machine, a check ring is used between the screw head and the screw shaft, which slides toward the head side during metering and slides toward the sealing ring side during injection. At this time, as also pointed out in the following patent document, the rear end face of the screw head and the top end face of the check ring (also called "check ring" in the following patent document 2, and these names are collectively referred to as "check ring" in this specification) slide against each other as the screw rotates, so wear and damage occur in the sliding part.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 5-57767

[0008] Patent Document 2: Japanese Patent Application Publication No. 2017-56655 Summary of the invention

[0009] Problems to be solved by the invention

[0010] However, not limited to the above-mentioned patent documents, it can be said that the current technology cannot appropriately meet the needs of the market, and the above-mentioned means have the following problems.

[0011] That is, in the configuration shown in the above patent document, it is proposed to maintain the gap between the screw head and the check ring by the flow of the molten resin moving toward the head tip side by the rotation of the shaft during the above-mentioned metering when the resin flows in the circumferential direction.

[0012] In other words, at the initial stage when metering starts and the molten resin has not yet started to flow in the circumferential direction, it is not possible to completely prevent the molten resin from functioning as a buffer and the check ring from contacting the screw head due to the retreat of the screw head.

[0013] Thus, in the prior art, it is difficult to maintain the gap between the rear end of the screw head and the top end of the check ring from the initial stage of the metering operation to the end of the metering operation, and it can be said that there is still much room for improvement.

[0014] The present invention has been made in view of one example of the above-mentioned problems, and an object of the present invention is to provide a screw head structure capable of suppressing contact between a screw head and a check ring with high reliability from the initial stage of metering operation to the end of metering, and an injection molding machine including the screw head structure.

[0015] Means for solving problems

[0016] In order to solve the above-mentioned problems, a screw head structure involved in one embodiment of the present invention is characterized in that (1) it has: a screw shaft that can be rotatably arranged in a heating barrel for heating and melting a resin material and can move forward and backward in the axial direction in the heating barrel; a screw head installed on the top side of the screw shaft and extruding the molten resin material into a molding die; and a check ring arranged around the rear side of the screw head and capable of forming an annular resin flow path opposite to the outer peripheral surface of the screw head, wherein the screw head is composed of a conical portion, a large diameter portion arranged at the rear section of the conical portion so that the resin material flows on the outer peripheral surface, and a small diameter portion arranged at the rear section of the large diameter portion so as to be opposite to the inner peripheral surface of the check ring, and an overflow groove is formed on the rear outer peripheral surface of the large diameter portion that allows the resin material passing through the annular resin flow path to overflow radially outward.

[0017] Furthermore, in the screw head structure described in (1), (2) the overflow groove is preferably formed so that the radial cross-sectional area decreases from the inside to the outside with respect to the central axis of the screw shaft.

[0018] In the screw head structure described in (2), (3) preferably, the outer shape of the overflow groove is a semicircular or elliptical shape in which the length Ls in the axial direction and the length Lθ in the circumferential direction are substantially equal.

[0019] In addition, in the screw head structure described in any one of (1) to (3) above, (4) preferably, a circumferential throttling groove is further provided on the rear outer peripheral surface, adjacent to the overflow groove and having a cross-sectional area in the circumferential direction that decreases along the flow direction of the molten resin material.

[0020] In the screw head structure described in (4), (5) it is preferred that a plurality of the overflow grooves are provided at predetermined intervals in the circumferential direction on the rear outer peripheral surface of the large diameter portion.

[0021] In the screw head structure described in (5) above, (6) the circumferential throttling groove is preferably arranged between the overflow grooves adjacent to each other in the circumferential direction.

[0022] In order to solve the above-mentioned problems, an injection molding machine according to one embodiment of the present invention includes the screw head structure described in (1) above, and molten resin material is extruded to a molding die through the screw head structure.

[0023] Effects of the Invention

[0024] According to the present invention, contact between the screw head and the check ring can be suppressed with high reliability throughout the entire process from the initial stage of metering to the end of metering, and wear and damage of the screw head that may occur in the initial stage of metering can be greatly suppressed.

[0025] In addition, due to the flow of resin accompanying the retreat of the screw shaft, when the check ring moves forward, the resin flowing to the rear of the screw head generates a retreat force in the check ring. According to the present invention, the resin pressure flowing in the overflow groove from the initial stage of the metering action is utilized, and since the above-mentioned retreat force occurs early, a stable float effect can be exerted.

[0026] In addition, according to the present invention having such a float effect, a gap is generated between the check ring and the screw head, and the resin pressure generated during the filling of the resin can be received by the entire end surface of the check ring. Therefore, in the present invention, during the above-mentioned filling, due to the high resin pressure, the check ring moves quickly, so the time to close the flow path of the resin is advanced, and the responsiveness of the check ring can be improved.

[0027] Furthermore, according to the present invention, by providing the above-mentioned overflow groove, it is possible to stably ensure the minimum cushion value independently of the molding temperature (resin viscosity) compared to a screw head having a conventional structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a diagram schematically showing a main part of an injection molding machine in an embodiment.

[0029] Figure 2It is a schematic diagram which shows the screw head structure mounted in the injection molding machine in the embodiment.

[0030] Figure 3 for Figure 2 An exploded view of the screw head structure is shown.

[0031] Figure 4 It is a perspective view of the large diameter portion having an overflow groove and a circumferential throttling groove formed in the screw head structure in the embodiment, viewed from obliquely behind.

[0032] Figure 5 This is a side view of the large diameter portion of the screw head structure in which the overflow groove and the circumferential throttle groove are formed in the embodiment, as viewed from the side.

[0033] Figure 6 This is a schematic diagram of the rear end surface of the large diameter portion formed with the overflow groove and the circumferential throttling groove as viewed from the rear.

[0034] Figure 7 It is a schematic diagram showing a cross section of a screw head structure in the embodiment.

[0035] Figure 8 This is a schematic diagram for explaining the flow of molten resin in the initial stage of metering and the effects on the screw head and the check ring.

[0036] Fig. 9 This is a schematic diagram for explaining the flow of the molten resin after the middle stage of the initial metering period and the effects on the screw head and the check ring.

[0037] Fig.10 This is a diagram showing the comparison results of wear tests on the screw head structure of the conventional product and the screw head structure of the present embodiment.

[0038] Fig.11 This is a diagram showing the comparison results of the ring responsiveness test in the screw head structure of the conventional product and the screw head structure of the present embodiment.

[0039] Description of Reference Numerals

[0040] 100 Screw head structure

[0041] 10 Screw shaft

[0042] 20 Screw head

[0043] 30 Check ring

[0044] 40 Sealing ring

[0045] 200 Heating barrel

[0046] 300 Injection Molding Machine DETAILED DESCRIPTION

[0047] The embodiment for implementing the present invention is described. It should be noted that, as shown in the following figure, for convenience, the axial direction of the screw shaft 10 is defined as the thrust direction St, and the radial direction intersecting the thrust direction St is defined as the radial direction Rd for description. However, the present invention is not affected by the above-mentioned direction regulations, and the scope of the claims should certainly not be unduly reduced. In addition, for the configuration other than the following detailed description, the structure and element components of the known injection molding machine including the above-mentioned patent document can be appropriately supplemented for implementation.

[0048] <Injection molding machine 300>

[0049] like Figure 1 As shown, the injection molding machine 300 of this embodiment includes a screw head structure 100 for extruding molten resin into a mold, and a heating barrel 200 that accommodates the screw head structure 100. The injection molding machine 300 has a function of extruding the resin material melted in the heating barrel 200 into a molding die (mold) through the screw head structure 100.

[0050] Furthermore, the injection molding machine 300 includes, in addition to the injection unit including the screw head structure 100 and the heating barrel 200, a known clamping unit and a control controller. The heating barrel 200 includes a barrel inner space 201 for accommodating the screw head structure 100. Such a heating barrel 200 can be applied to a known heating barrel connected to a hopper (not shown) as an inlet for feeding a granular resin material (granules) as a raw material.

[0051] <Screw head structure 100>

[0052] Next, refer to Figure 1 to Figure 6 , the detailed structure of the screw head structure 100 in this embodiment is described. As can be understood from these figures, the screw head structure 100 in this embodiment includes a screw shaft 10, a screw head 20, a check ring 30, etc. In addition, there is no particular limitation on the material that can constitute the screw head structure 100, and various known steel materials can be applied.

[0053] The screw shaft 10 is rotatably arranged in a heating barrel 200 that heats and melts a solid resin material such as a granular material (also referred to as "resin pellets"). In addition, the screw shaft 10 is configured to be able to advance and retreat in the axial direction (thrust direction St) in the heating barrel 200 in the injection unit.

[0054] As for such a screw shaft 10, as an example, it can be composed of three parts in sequence from the screw head 20 side described later, namely, a metering part that mixes and transports the raw materials to the front in a uniform manner, a compression part that heats and compresses the transported raw materials, and a supply part that transports the raw materials that fall through a known hopper by their own weight to the front through the rotation of the screw.

[0055] The screw head 20 is attached to the front end side of the screw shaft 10. The screw head 20 has a function of extruding the resin material (molten resin) melted in the front stage into the die.

[0056] As from Figure 3 (a) and the like, the screw head 20 of the present embodiment is composed, in order from the side close to the molding die (mold), of a conical portion 21 constituting the top end of the screw head, a large diameter portion 22 arranged at the rear section of the conical portion 21 (at the rear side of the conical portion 21 in the thrust direction St) so that the resin material (molten resin) flows on the outer peripheral surface, a small diameter portion 23 arranged at the rear section of the large diameter portion 22 and opposite to the check ring 30, and a connecting end 26 connected to the above-mentioned screw shaft 10.

[0057] In addition, in the screw head 20 of the present embodiment, as shown in FIG. Figure 4 As can be understood from the above, the rear outer peripheral surface 22s of the large diameter portion 22 connected to the small diameter portion 23 is formed with an overflow groove 24 for allowing the resin material (molten resin) that has passed through the annular resin flow path RC to overflow radially outward. Such an overflow groove 24 is formed with a groove-like structure in which a part of the rear outer peripheral surface 22s of the screw head 20 is recessed toward the top end side (St direction). When viewed from the rear end side of the large diameter portion 22, it can be said that a part of the recessed portion is partially missing in a manner that a part of the rear end side extends toward the top end side (St direction) (refer to Figures 4 to 6 wait).

[0058] like Figure 5 As shown, the outer shape of the overflow groove 24 of this embodiment is preferably a semicircular or elliptical shape extending forward (toward the conical portion 21 side) based on the periphery of the rear outer peripheral surface 22s. In other words, as for the overflow groove 24 formed on the periphery of the rear outer peripheral surface 22s, it can be said that it is preferred that the length Ls in the axial direction, i.e., the thrust direction, and the length Lθ in the circumferential direction, i.e., the radial direction Rd, are substantially equal. Furthermore, as for the radial length, i.e., the groove length Lt, of the overflow groove 24, as an example, it can be the length from the outer peripheral surface of the large diameter portion 22 to the outer peripheral surface of the small diameter portion 23.

[0059] Furthermore, as referenced Figure 6 and Figure 7As can be understood, the overflow groove 24 of the present embodiment is formed in such a way that the cross-sectional area in the radial direction decreases from the inside to the outside with respect to the central axis C of the screw shaft 10 and the screw head 20 as a reference. Figure 7 As shown, first, for the area in which the overflow groove 24 is not formed on the rear outer peripheral surface 22s, the gap between the rear outer peripheral surface 22s and the check ring 30 during metering is substantially the same.

[0060] The area of ​​the rear outer peripheral surface 22s where the overflow groove 24 is formed is the inlet side cross-sectional area CS of the molten resin. 1 becomes larger than the outflow side cross-sectional area CS which is the outlet side of the molten resin 2 Thereby, the pressure (resin pressure) of the molten resin can be increased from the inlet side to the outlet side (that is, toward the radially outer side) of the molten resin.

[0061] In addition, if Figure 6 As shown in the figures, as for the overflow grooves 24, it is preferred that a plurality of overflow grooves 24 are provided at predetermined intervals in the circumferential direction (around the thrust direction St or around the center axis C) on the rear outer peripheral surface 22s in the large diameter portion 22. In addition, in the figure, a total of four overflow grooves 24, namely the first overflow groove 24a to the fourth overflow groove 24d, are formed at 90-degree intervals on the rear outer peripheral surface 22s, but the present disclosure is not limited to this example, and a plurality of overflow grooves 24 other than four may be formed on the rear outer peripheral surface 22s. In addition, the plurality of overflow grooves 24 are preferably provided at equal intervals along the circumferential direction.

[0062] Furthermore, in the rear outer peripheral surface 22s of the screw head 20 of the present embodiment, a circumferential throttling groove 25 is formed at a position adjacent to the overflow groove 24 in the circumferential direction (for example, on the downstream side of the overflow groove 24). Figure 4 and Figure 6 As will be understood by others, the circumferential throttling groove 25 of the present embodiment is preferably arranged between adjacent overflow grooves 24 in the circumferential direction.

[0063] Such a circumferential throttle groove 25 is preferably formed adjacent to the above-mentioned overflow groove 24 so that the cross-sectional area in the circumferential direction decreases along the direction of the molten resin flow (more specifically, toward the downstream side). As a result, after the above-mentioned initial metering, the molten resin flowing in the circumferential direction at the top end side of the screw head 20 moves in the circumferential throttle groove 25 by the rotation of the screw shaft 10. At this time, since the cross-sectional area in the circumferential direction of the circumferential throttle groove 25 gradually decreases, the resin pressure in the circumferential direction of the molten resin increases, and the gap between the screw head 20 and the check ring 30 can be maintained.

[0064] The check ring 30 is disposed around the rear side of the screw head 20 and is configured to form an annular resin flow path relative to the outer peripheral surface of the screw head 20. More specifically, Figure 3As shown in FIGS. 1 and 10 , the check ring 30 of the present embodiment is provided so as to cover the periphery of the small diameter portion 23 of the screw head 20 .

[0065] As a specific structure of the check ring 30 , various known check rings and retaining rings exemplified in Patent Documents 1 and 2 may be applied as long as the above-mentioned resin flow path can be formed.

[0066] The seal ring 40 is arranged on the rear end side of the check ring 30 in the screw head 20 (the side opposite to the conical portion 21 based on the check ring 30), and cooperates with the check ring 30 during injection into the mold to prevent the backflow of the molten resin. More specifically, when the molten resin is injected into the mold, the check ring 30 retreats in the thrust direction St due to its reaction and abuts against the seal ring 40, thereby exerting a sealing effect.

[0067] As a specific structure of the seal ring 40 , any of various known seal rings exemplified in Patent Documents 1 and 2 may be applied as long as the seal ring 40 cooperates with the check ring 30 to perform a sealing function.

[0068] <Flow of molten resin during metering and the role of the screw head>

[0069] Reference Figures 7 to 9 , the flow of molten resin during metering and the role of the screw head in injection molding performed in this embodiment are described.

[0070] As an example, during injection molding, the screw shaft 10 rotates about its axis, and pellets as a raw material are fed into the barrel through a known hopper in the supply portion of the heating barrel 200 .

[0071] Next, the pellets are heated and compressed in the compression section of the heating barrel 200, and are kneaded to form molten resin. Then, the molten resin is kneaded uniformly without unevenness in the metering section of the heating barrel 200 and conveyed forward.

[0072] In this way, during the metering operation of the molten resin, the screw shaft 10 rotates around the axis and simultaneously stores a predetermined amount of molten resin in the resin storage space RAC (see Figure 1 ). At this time, if Figure 7 As shown, the molten resin flows through the resin flow path RC formed between the small diameter portion 23 and the check ring 30 in the screw head structure 100 and moves to the front resin retention space RAC.

[0073] After a predetermined amount of molten resin is stored in the resin retention space RAC, the screw shaft 10 is moved toward the mold, thereby injecting the predetermined amount of molten resin into the mold. That is, since the check ring 30 is stationary around the small diameter portion 23, it is relatively separated from the conical portion 21 and the large diameter portion 22, and contacts the sealing ring 40. Thus, while the sealing ring 40 suppresses the backward movement of the molten resin, the molten resin in the resin retention space RAC is injected into the mold through the conical portion 21. Then, after the injected molten resin is cooled in the mold for a predetermined time, the finished product is taken out of the mold.

[0074] Usually, a predetermined amount of finished products are injection molded by repeating the above-mentioned process several times. Therefore, in the injection molding after the second time, the above-mentioned metering action is performed again. That is, the distance between the check ring 30 and the large diameter part 22 is reduced by reversing the screw head 20 (conical part 21, large diameter part 22, etc.) moving in the direction of the mold. At this time, although the screw shaft 10 starts to rotate around the axis again, especially in the initial stage of the metering action, there is almost no flow of molten resin around the axis, and the flow of molten resin begins along the thrust direction St.

[0075] Furthermore, in the screw head of the conventional structure, as exemplified in the above patent document, since the interval between the large diameter portion 22 and the check ring 30 is maintained by the flow of the molten resin around the shaft, the rear end of the large diameter portion 22 contacts the front end of the check ring 30, especially at the initial stage of the metering operation. Moreover, by repeating such metering operation multiple times, at least one of the rear end of the large diameter portion 22 and the front end of the check ring 30 is damaged by wear, damage, or scratches. If such damage occurs, problems such as the inability to properly supply the molten resin flowing in the resin flow path RC to the resin retention space RAC may occur.

[0076] In the screw head structure 100 of the present embodiment, the rear outer peripheral surface 22s of the large diameter portion 22 that is in contact with the small diameter portion 23 is provided with the above-mentioned overflow groove 24. Figure 8 As shown, even in the early stage of the metering action of the flow of the molten resin in the circumferential direction, the molten resin moving in the thrust direction St can move in the radial direction (radial direction Rd) by increasing the resin pressure via the overflow groove 24. At this time, a part of the molten resin overflowing from the overflow groove 24 to the radial outside acts as a buffer between the large diameter portion 22 and the check ring 30, and acts in a manner of pushing back the front end of the check ring 30.

[0077] Therefore, even in the initial stage of the metering action using the screw head structure 100 of this embodiment, the contact between the rear end of the large diameter portion 22 and the front end of the check ring 30 can be suppressed by the action of the molten resin flowing radially outward from the center axis C through the above-mentioned overflow groove 24, and as a result, their spacing can be maintained.

[0078] Furthermore, in the screw head structure 100 of the present embodiment, the above-mentioned circumferential throttling groove 25 is provided adjacent to the overflow groove 24 (in this example, on the downstream side of the overflow groove 24 in the circumferential direction). Fig. 9 As shown, the screw shaft 10 revolves around the axis and starts to rotate again. Even after the middle stage (after the initial stage) of the metering action of the molten resin flowing in the circumferential direction, the molten resin moving around the central axis C can increase the resin pressure and move through the circumferential throttle groove 25. At this time, a part of the molten resin moving in the circumferential direction from the circumferential throttle groove 25 becomes a buffer between the large diameter portion 22 and the check ring 30, and acts in a manner of pushing back the front end of the check ring 30.

[0079] Therefore, even after the middle stage of the metering operation using the screw head structure 100 of the present embodiment, the contact between the rear end of the large diameter portion 22 and the front end of the check ring 30 is suppressed by the action of the molten resin flowing around the central axis C through the circumferential throttle groove 25, and the interval between them can be continuously maintained. In this way, in the screw head structure 100 of the present embodiment, the extrusion action of the molten resin on the check ring 30 can be exerted in the two stages of the initial stage and after the middle stage of the metering operation, and the number of contacts between the screw head and the check ring during the metering operation can be greatly suppressed with high reliability.

[0080] <Comparative experiment between conventional products and the product of the present invention and their effects>

[0081] exist Fig.10 The experimental results of the wear test comparing the conventional product without the overflow groove 24 and the product of the present invention with the overflow groove 24 are shown in FIG. In this experiment, 500 injection purges were performed, and the weight of the screw head was measured before and after the purge to compare the wear amount. Then, after the purge was completed, the distance between the deepest part of the sliding part and the non-sliding part was measured at 4 points (A to D in the figure) around the shaft, and the average value was calculated.

[0082] During injection molding, an injection molding machine (SE75EV-A) manufactured by Sumitomo Heavy Industries, Ltd. was used. In addition, as a screw head equivalent to an existing product, a φ25 standard screw head installed as a standard product was used. In addition, as an example, the above-mentioned purging conditions are as follows.

[0083] Measuring: 50mm

[0084] Barrel temperature: 220℃

[0085] Speed: 200rpm

[0086] Back pressure: 2MP

[0087] Injection speed: 50m / s

[0088] As shown in the test results attached to the figure, when comparing the product of the present invention with the conventional product, the product of the present invention achieves a reduction in wear of about 50% compared with the conventional product. That is, the product of the present invention can exert the advantageous float effect described above compared with the conventional product.

[0089] In addition, Fig.11 The results of the ring responsiveness test comparing the conventional product (the standard screw head) without the overflow groove 24 and the product of the present invention with the overflow groove 24 are shown in FIG. As an example, in this test, the same injection molding machine as the wear test is used, and 30 shots after the molding is stabilized are continuously molded under the following molding conditions, and the minimum buffer amount and metering time are recorded respectively. That is, in this embodiment, the ring responsiveness of the check ring is evaluated based on these minimum buffer amounts and metering times.

[0090] In the injection molding in the ring responsiveness test, the molding temperature was set to two types (220° C. and 240° C.), the viscosity of the resin was changed, and the results were recorded for each type.

[0091] Measuring: 20mm

[0092] Speed: 50rpm

[0093] Speed: 200rpm

[0094] Injection speed: 30m / s

[0095] As can be seen from the test results attached to the figure, when comparing the product of the present invention with the existing product, it can be confirmed that the product of the present invention shows a stable ring responsiveness that is independent of the molding temperature compared to the existing product. In other words, the product of the present invention having the above-mentioned overflow groove can stably ensure the minimum buffer value independent of the molding temperature (resin viscosity) compared to the existing structure.

[0096] As described above, according to the screw head structure and the injection molding machine of the present disclosure, wear and damage of the screw head can be significantly suppressed throughout the entire process from the initial stage of the metering operation to the end of the metering operation.

[0097] The preferred embodiments of the present invention and its variants are described in detail with reference to the accompanying drawings, but the present invention is not limited to these examples. A person skilled in the art of the present invention can think of various variants or modifications within the scope of the technical ideas described in the claims, and it can be understood that these certainly belong to the technical scope of the present invention.

[0098] For example, Figure 6 In the embodiment, the width of the overflow groove 24 viewed from the rear end side of the head (connection end 26 side) is roughly the same from the radial inside to the outside, but the width may be narrowed in a manner that it narrows from the radial inside to the outside. Thus, the molten resin moving in the thrust direction St can also move in the radial direction (radial direction Rd) through the overflow groove 24 by increasing the resin pressure.

[0099] Industrial Applicability

[0100] The screw head structure according to the present invention is suitable for realizing an injection molding machine having a screw head having excellent durability.

Claims

1. A screw head structure, characterized in that: The invention comprises: a screw shaft rotatably arranged in a heating barrel for heating and melting a resin material and capable of advancing and retreating axially in the heating barrel; a screw head installed on the top end side of the screw shaft and extruding the molten resin material into a molding die; and a check ring arranged around the rear side of the screw head and capable of forming an annular resin flow path opposite to the outer peripheral surface of the screw head, wherein the screw head is composed of, from the side close to the molding die, a conical portion, a large diameter portion arranged in the rear section of the conical portion so that the resin material flows on the outer peripheral surface, and a small diameter portion arranged in the rear section of the large diameter portion so as to be opposite to the inner peripheral surface of the check ring, and an overflow groove is formed on the rear outer peripheral surface of the large diameter portion that allows the resin material passing through the annular resin flow path to overflow radially outward.

2. The screw head structure according to claim 1, wherein: The overflow groove is formed such that a radial cross-sectional area decreases from the inside to the outside with respect to the central axis of the screw shaft.

3. The screw head structure according to claim 2, wherein: The outer shape of the overflow groove is a semicircular shape or an elliptical shape in which the axial length Ls and the circumferential length Lθ are substantially equal.

4. The screw head structure according to any one of claims 1 to 3, wherein The rear outer peripheral surface is further provided with a circumferential throttling groove adjacent to the overflow groove and having a cross-sectional area in the circumferential direction reduced along a flow direction of the molten resin material.

5. The screw head structure according to claim 4, wherein: The overflow grooves are provided in plurality at predetermined intervals in the circumferential direction on the rear outer peripheral surface of the large diameter portion.

6. The screw head structure according to claim 5, wherein: The circumferential throttle groove is arranged between the overflow grooves adjacent to each other in the circumferential direction.

7. Injection molding machine, characterized in that, The invention comprises the screw head structure according to claim 1, wherein the resin material melted in the heating barrel is extruded to the molding die through the screw head structure.

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