Screw head of injection molding machine

Abstract

Provided is a technique capable of achieving an appropriate balance between the fatigue strength of a threaded portion and the static strength of a narrowed portion in a screw head of an injection molding machine. In a screw head (30) attached to the tip of a screw (23) for an injection molding machine (1), a male thread section (37) screwed to the screw (23) and a narrowed section (36) positioned on the tip side of the male thread section (37) and having a smaller diameter (d1) than the bottom diameter (d2) of the male thread section (37) are disposed on a neck lower section (32) positioned on the screw (23) side with respect to a neck upper section (31) positioned on the tip side of the screw head (30). The male thread section (37) has: a first tapered section (41) which is located on the narrowed section (36) side and which has an outer diameter that decreases as the first tapered section (41) advances toward the narrowed section (36) side; a transition part (43) located on the screw (23) side of the first gradual reduction part (41); and a second gradually-decreasing section (42) which is adjacent to the first gradually-decreasing section (41) with the transition section (43) therebetween, and which has an outer diameter that decreases as the first gradually-decreasing section (41) advances toward the narrowed section (43), the average inclination of the first gradually-decreasing section (41) being formed larger than the average inclination of the second gradually-decreasing section (42).

Description

Technical Field

[0001] The present disclosure relates to a screw head for an injection molding machine. Background Art

[0002] Conventionally, a screw head is fixed to the tip of a screw used in injection molding machines to prevent backflow of resin. Pressure during injection, torque during metering, and tensile force during backdraft repeatedly load the screw head throughout the molding cycle. The stress amplitudes generated by these repeated external forces can also cause fatigue failure in the screw head's threads. To prevent this fatigue failure, a narrowing portion with a diameter smaller than the thread root diameter is sometimes provided (see, for example, Patent Document 1).

[0003] By forming a narrowing portion with a diameter smaller than the thread root diameter, the rigidity of the lower portion of the thread neck is intentionally reduced. This reduces the proportion of force borne by the screw head (load distribution coefficient) when subjected to tensile and compressive external forces, thus reducing stress amplitude.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Utility Model Application Laid-Open No. 59-143719 Summary of the Invention

[0007] Problems to be solved by the invention

[0008] However, the narrowed portion has a small diameter due to its structure, making it weak against the torque applied during metering. In situations such as insufficient resin melting, there is a concern that excessive torque could cause static failure. Therefore, it is necessary to strike a balance between the fatigue strength of the threaded portion against tensile and compressive external forces and the static strength of the narrowed portion against torque.

[0009] The present disclosure has been made in view of the above-mentioned problems, and an object of the present disclosure is to provide a technology capable of achieving an appropriate balance between the fatigue strength of the thread portion and the static strength of the constricted portion in a screw head of an injection molding machine.

[0010] Solutions for solving problems

[0011] The present invention discloses a screw head for an injection molding machine, which is a screw head installed at the front end of a screw for an injection molding machine, wherein an external thread portion and a narrowing portion are arranged at a neck lower portion of the screw head located on the screw side relative to the neck upper portion located on the front end side, the external thread portion being screwed together with the screw, the narrowing portion being located on the front end side of the external thread portion and having a diameter smaller than the bottom diameter of the external thread portion, the external thread portion having: a first gradually decreasing portion, which is located on the narrowing portion side and has an outer diameter decreasing as it proceeds toward the narrowing portion side; a transition portion, which is located on the screw side of the first gradually decreasing portion; and a second gradually decreasing portion, which is adjacent to the first gradually decreasing portion across the transition portion and has an outer diameter decreasing as it proceeds toward the narrowing portion side, the average inclination of the first gradually decreasing portion being formed to be larger than the average inclination of the second gradually decreasing portion.

[0012] Effects of the Invention

[0013] According to the present disclosure, it is possible to provide a technology capable of achieving an appropriate balance between the fatigue strength of the threaded portion and the static strength of the constricted portion in a screw head of an injection molding machine. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a schematic diagram showing an example of the structure of an injection molding machine to which the screw head of the first embodiment is applied.

[0015] Figure 2 It is a side view of the screw head of the first embodiment.

[0016] Figure 3 It is an enlarged side view of the external thread portion of the screw head according to the first embodiment.

[0017] Figure 4A This is a side view showing the shape of the screw head of Comparative Example 1.

[0018] Figure 4B It is a side view showing the shape of the screw head of the embodiment.

[0019] Figure 4C This is a side view showing the shape of the screw head of Comparative Example 2.

[0020] Figure 5 This is a graph showing the difference in equivalent stress between the comparative example and the example.

[0021] Figure 6 It is a side view showing the screw head and the internal thread portion of the screw according to the second embodiment.

[0022] Figure 7 It is a side view of the screw head of the third embodiment.

[0023] Figure 8It is a side view of the screw head of the fourth embodiment.

[0024] Figure 9 It is a side view of the screw head of the fifth embodiment. DETAILED DESCRIPTION

[0025] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the description of the second embodiment and thereafter, the same reference numerals are given to the same or identical components as those of the first embodiment, and their description will be omitted as appropriate.

[0026] [First embodiment]

[0027] Figure 1 1 is a schematic diagram showing an example of the structure of the injection molding machine 1 according to the first embodiment. The injection molding machine 1 according to the present embodiment includes an injection unit 2 and a mold clamping unit 3 .

[0028] The injection unit 2 is an injection device comprising a hopper 21, a cylinder 22, and a screw 23. The cylinder 22 is, for example, cylindrical. Resin stored in the hopper 21 is supplied to the cylinder 22. The screw 23 is disposed within the cylinder 22 and rotates to transport the resin toward the front end of the cylinder 22. A screw head 30 is disposed at the front end of the screw 23.

[0029] During resin metering, a check ring (not shown) abuts against the screw head 30, thereby forming a flow path for the resin to flow forward. Furthermore, during resin injection, the check ring abuts against a seal ring (not shown), sealing the flow path and preventing backflow. The structure of the screw head 30 will be described in detail later.

[0030] A heater (not shown) for heating the pellets to melt them is provided in the cylinder 22. By heating the cylinder 22 with the heater, the molten pellets are conveyed to the front end by the screw 23 and injected into the mold 5.

[0031] The mold clamping unit 3 is a device for clamping the mold 5. By clamping the mold 5 by the mold clamping unit 3, a molded product is formed.

[0032] Next, the structure of the screw head 30 according to the first embodiment will be described. Figure 2 1 is a side view of the screw head 30 of the first embodiment. Figure 2 As shown, the screw head 30 of this embodiment includes a neck upper portion 31 that is located on the front end side of the screw head 30 in the axial direction, and a neck lower portion 32 that is located on the screw 23 side in the axial direction.

[0033] The neck upper portion 31 is a portion that comes into contact with the molten resin, and forms a flow path for the molten resin together with other front end components.

[0034] The lower neck portion 32 includes a centering portion 35 , a narrowed portion 36 , and an external thread portion 37 . The centering portion 35 is a portion adjusted so that the axial direction of the screw 23 and the axial direction of the screw head 30 coincide with each other and is located at the front end of the lower neck portion 32 .

[0035] The narrowed portion 36 is located between the centering portion 35 and the external thread portion 37, and its diameter d1 is smaller than the bottom diameter d2 of the external thread portion 37. The narrowed portion 36 reduces the rigidity of the lower neck portion 32 and the load distribution coefficient. This reduces the stress amplitude and improves the fatigue strength of the thread root portion of the external thread portion 37 against tensile and compressive forces.

[0036] The external thread portion 37 is fastened to an internal thread portion (not shown) formed at the front end of the screw rod 23. Figure 3 The structure of the external thread portion 37 will be described. Figure 3 It is an enlarged side view of the external thread portion 37 of the screw head 30 according to the first embodiment.

[0037] like Figure 3 As shown, the external thread portion 37 includes a first gradually decreasing portion 41 , a second gradually decreasing portion 42 , a transition portion 43 and a fully threaded portion 44 .

[0038] The first tapering portion 41 is located on the front end side of the screw head 30 in the external thread portion 37 and is adjacent to the narrowing portion 36. The average inclination g1 of the first tapering portion 41 can be expressed by the following formula 1. In the formula, h1 represents the radial length (height) obtained by subtracting the radius of the front end portion of the screw head 30 from the radius of the rear end portion of the screw head 30 in the axial direction of the first tapering portion 41. l1 is the axial length from the rear end portion to the front end portion of the first tapering portion 41.

[0039] [Formula 1]

[0040] g1=h1l1 Formula (1)

[0041] The front end side of the second tapering portion 42 is connected to the first tapering portion 41 via the transition portion 43, and the rear end side thereof is connected to the fully threaded portion 44. The average inclination g2 of the second tapering portion 42 can be expressed by the following formula 2. In the formula, h2 represents the radial length (height) obtained by subtracting the radius of the front end side portion of the screw head 30 from the radius of the rear end side portion of the screw head 30 in the axial direction of the second tapering portion 42. l2 is the axial length from the rear end side portion to the front end side portion of the second tapering portion 42.

[0042] [Formula 2]

[0043] g2=h2 / l2 Formula (2)

[0044] As shown in Formula 3, the average inclination g1 of the first gradually decreasing portion 41 is set to be larger than the average inclination g2 of the second gradually decreasing portion 42 .

[0045] [Formula 3]

[0046] g1>g2 Formula (3)

[0047] The transition portion 43 is located between the first gradually decreasing portion 41 and the second gradually decreasing portion 42. In the first embodiment, the transition portion 43 is a portion where the larger inclination and the smaller inclination are switched in the external thread portion 37.

[0048] The perfect thread portion 44 is located on the screw 23 side of the second tapering portion 42. The perfect thread portion 44 is a thread portion that does not taper but extends axially with a constant diameter. In the perfect thread portion 44, the outer diameter of the thread crest is constant.

[0049] Thus, the outer diameter of the external thread portion 37 gradually decreases from the rear end toward the front end of the screw head 30 in the regions of the first tapering portion 41 and the second tapering portion 42. Furthermore, the structure is such that the first tapering portion 41, which has a relatively large inclination, is located on the front end side, and the second tapering portion 42, which has a relatively small inclination, is located on the screw 23 side.

[0050] Next, refer to Figures 4A to 4C and Figure 5 The following describes the results of a stress analysis based on simulations for an example of the screw head 30 having the structure of the first embodiment and comparative examples of screw heads 130 and 131 having different shapes. In the following description, the diameter and pitch of the perfect thread portion 44 are set to the same values ​​in all of the examples, comparative examples 1, and 2. In this example, the perfect thread portion 44 uses an M18×2.5 thread.

[0051] Figure 4A It is a side view showing the shape of the screw head 130 of Comparative Example 1. Figure 4A The screw head 130 of Comparative Example 1 is different from the screw head 30 of the above embodiment in that only one tapering portion 141 having an inclination of 30 degrees is formed.

[0052] Figure 4B : is a side view showing the shape of the screw head 30 of the embodiment. Figure 4B In the screw head 30 of the embodiment, the first tapering portion 41 having an inclination of 30 degrees is formed on the narrowing portion 36 side and on the fully threaded portion 44 side. Furthermore, the axial length of the first tapering portion 41 is shorter than that of the tapering portion 141 of Comparative Example 1. However, due to the presence of the second tapering portion 42, the overall tapering portion length of the embodiment is longer than that of Comparative Example 1.

[0053] Figure 4C It is a side view showing the shape of the screw head 131 of Comparative Example 2. Figure 4C The screw head 131 of Comparative Example 2 differs from the screw head 30 of the above-described embodiment in that it has only one tapering portion 142 with a 6-degree inclination. Because the tapering portion 142 of Comparative Example 2 has a smaller inclination, its axial length is longer than the combined length of the tapering portion 141 of Comparative Example 1 and the first and second tapering portions 41, 42 of the embodiment.

[0054] Figure 5 This is a graph showing the difference in equivalent stress between the comparative example and the example. Figure 5 The graph shown is a graph obtained by analyzing using stress analysis software for each of Comparative Example 1, Example, and Comparative Example 2. In this graph, the maximum stress at the thread bottom of each of Comparative Example 1, Example, and Comparative Example 2 is shown as equivalent stress.

[0055] When tightening a thread, the axial force generated is shared among the thread crests. However, in typical threads, a significant amount of the load is concentrated on the first thread crest, where it begins to mesh with the internal thread. This results in significant stress concentration at the first thread root, which reduces strength. Therefore, by evenly distributing the load across the thread crests, stress concentration at the first thread root can be mitigated. It is known that by gradually increasing the degree of engagement between the external and internal threads over several pitches, the load concentrated on the first thread crest can be reduced, making the load distribution ratio more even.

[0056] In Comparative Example 1, the inclination of the tapered portion 141 is too large, so the length of engagement between the tapered portion 141 and the internal thread portion 50 of the screw 23 is short, less than one pitch. Consequently, the load distributed at the first thread crest is not distributed across the other thread crests, resulting in a very large equivalent stress at the first thread root.

[0057] The value of the equivalent stress of Comparative Example 2 is lower than the value of the equivalent stress of Comparative Example 1. This is because the amount of engagement between the tapering portion 142 with a smaller inclination and the internal thread portion 50 of the screw 23 gradually increases over several pitches, thereby making the load distribution ratio uniform. On the other hand, the value of the equivalent stress of Comparative Example 2 is larger than that of the embodiment. This is because the thread portion tapers at a constant smaller inclination due to the connection portion from the narrowing portion 36 with a smaller diameter, so the number of thread crests that are not engaged with the internal thread is larger on the narrowing portion 36 side, and the distributed load of the engaged thread crests is larger. The reduction in the shear area caused by the reduction in the number of effective thread crests engaged with the internal thread also causes shear failure of the thread crests.

[0058] The value of the equivalent stress of the embodiment is lower than the value of the equivalent stress of the comparative example 2. This is because the first tapering portion 41 prevents the number of effective thread crests that engage with the internal thread on the narrowing portion 36 side from decreasing, and the amount of gradual engagement with the internal thread is increased with a smaller inclination of the second tapering portion 42, thereby achieving an effect of equalizing the load distribution ratio. That is, the effect of equalizing the load distribution ratio can be achieved by utilizing the second tapering portion 42 having a smaller inclination, and the first tapering portion 41 is provided to prevent a decrease in the number of effective thread crests. In this way, the screw head 30 of the injection molding machine 1 according to the first embodiment can achieve the following effects.

[0059] In the screw head 30 attached to the front end of the screw 23 for the injection molding machine 1 of the first embodiment, an external threaded portion 37 that is screwed with the screw 23 is arranged on the neck lower portion 32 located on the screw 23 side relative to the neck upper portion 31 located on the front end side of the screw head 30, and a narrowing portion 36 located on the front end side of the external threaded portion 37 and having a diameter d1 smaller than the bottom diameter d2 of the external threaded portion 37. The external threaded portion 37 has: a first tapered portion 41 located on the narrowing portion 36 side, the outer diameter of which decreases as it proceeds to the narrowing portion 36 side; a transition portion 43 located on the screw 23 side of the first tapered portion 41; and a second tapered portion 42 adjacent to the first tapered portion 41 across the transition portion 43, the outer diameter of which decreases as it proceeds to the tapered portion 36 side, and the average inclination of the first tapered portion 41 is formed to be larger than the average inclination of the second tapered portion 42. As a result, the load distribution ratio of each thread crest of the external thread portion 37 is made uniform, and the significant stress concentration that often occurs at the thread root due to tightening to the screw 23 can be alleviated. Therefore, the fatigue strength of the thread root relative to tensile and compressive external forces can be improved. By improving the fatigue strength, it is also possible to prevent the narrowing portion 36 from being formed too thin, and the static strength of the narrowing portion 36 relative to torque can be ensured. A first tapering portion 41 with a larger inclination is arranged near the narrowing portion 36, and a second tapering portion 42 with a smaller inclination is arranged adjacent to the first tapering portion 41. This can suppress the reduction in the effective thread crest and achieve a high level of load distribution ratio uniformity. That is, through the structure of the first embodiment, an appropriate balance between the fatigue strength of the threaded portion and the static strength of the narrowing portion can be achieved.

[0060] [Second embodiment]

[0061] Next, the screw head 30a according to the second embodiment will be described. Figure 6 : is a side view showing the screw head 30a and the internal thread portion 50 of the screw according to the second embodiment. Figure 6 1 and 2 show the cross-section of the screw head 30 a and the internal thread portion 50 of the screw 23 according to the second embodiment.

[0062] In addition, when the transition (change in inclination) from the first tapered portion 41 to the second tapered portion 42 is too early, the reduction amount of the effective thread crest on the narrowing portion 36 side becomes larger, and the number of thread crests meshing with the internal thread portion 50 of the screw decreases. As a result, not only does the effect of alleviating the load distribution ratio of the first thread crest become weaker, but the risk of shear failure of the thread crest also becomes higher. In addition, the second tapered portion 42 is used to equalize the load distribution ratio of the thread crest by gradually meshing with the internal thread portion 50 of the screw, and even if it exists in the unmeshed portion, it is meaningless. Therefore, in order to prevent the reduction amount of the effective thread crest from becoming unnecessarily large, it is only necessary to make the outer diameter of the transition portion 43 larger than the inner diameter of the internal thread portion 50.

[0063] On the other hand, when the transition (change in inclination) from the first tapered portion 41 to the second tapered portion 42 is too late, in the transition portion 43, the external thread has meshed with the internal thread relatively more, and the meshing length between the second tapered portion 42 and the internal thread portion 50 of the screw becomes shorter. In this case, the task of equalizing the load distribution ratio that the second tapered portion should originally bear is taken over by the first tapered portion with a larger inclination, and its effect becomes weaker. In order to prevent the meshing length between the second tapered portion 42 and the internal thread portion 50 of the screw from becoming too short, it is only necessary to make the outer diameter of the transition portion 43 smaller than the effective diameter of the internal thread portion 50.

[0064] Therefore, in the second embodiment, in order to achieve a balance between the reduction amount of the thread crest and the equalization of the load distribution ratio, the screw head 30a is configured to satisfy the relationship of Equation 4. In addition, in Equation 4, d m represents the outer diameter of the transition portion 43 between the first tapered portion 41 and the second tapered portion 42, D1 represents the inner diameter of the internal thread portion 50, and D2 represents the effective diameter of the internal thread portion 50. By satisfying the relationship of Equation 4, the position of the transition portion 43 transitioning from the first tapered portion 41 to the second tapered portion 42 can be set appropriately and easily.

[0065] [Equation 4]

[0066] D1 < d m < D2 Equation (4)

[0067] As described above, in the second embodiment, the outer diameter d of the transition portion 43 m is formed larger than the inner diameter D1 of the internal thread portion 50 provided at the front end of the screw 23. As a result, it is possible to effectively suppress the weakening of the effect of alleviating the load distribution ratio of the first thread crest due to the reduction in the number of effective thread crests and the decrease in the shear strength of the thread crest.

[0068] In addition, in the second embodiment, the outer diameter d of the transition portion 43 mIt is formed smaller than the effective diameter D2 of the internal thread portion 50 provided on the front end side of the screw 23. This effectively prevents the first tapered portion with a large inclination from meshing with the internal thread and reducing the effect of equalizing the load distribution ratio.

[0069] [Third embodiment]

[0070] Next, the screw head 30b of the third embodiment will be described. Figure 7 3. This is a side view of the screw head 30b of the third embodiment. In the third embodiment, the structure of the second tapering portion 42a is different from that of the above-described embodiment.

[0071] like Figure 7 As shown, the inclination of the second tapering portion 42a is not constant, but becomes smaller on the front end (transition portion 43) side and larger on the screw 23 (complete thread portion 44) side. In this example, the inclination of the second tapering portion 42a is formed into an arc shape that rises from bottom to top.

[0072] As described above, in the third embodiment, the inclination of the second tapering portion 42a is configured to increase as it moves from the transition portion 43 toward the screw 23. While the effect of equalizing the load distribution ratio increases with decreasing inclination, the effect is stronger. However, this also increases the reduction in the shear area of ​​the thread crest, increasing the risk of shear failure. Therefore, with this configuration, the inclination of the second tapering portion 42a can be minimized at the portion where the male and female threads begin to mesh, which has a greater impact on equalizing the load distribution ratio, while the inclination can be increased at the rear end, where the impact on equalizing the load distribution ratio is less. This allows for a screw head 30b configuration that achieves a high load distribution ratio equalization effect while suppressing the reduction in the shear area of ​​the threads.

[0073] [Fourth embodiment]

[0074] Next, a screw head 30c according to a fourth embodiment will be described. Figure 8 1 is a side view showing a screw head 30c according to a fourth embodiment. In the fourth embodiment, the structure of a transition portion 43a is different from that of the above-described embodiment.

[0075] The transition portion 43a in the external thread portion 37b of the fourth embodiment differs from the transition portion 43 in the above-described embodiment in that it has a constant length in the axial direction. Furthermore, the transition portion 43a is formed into a cylindrical or cylindrical shape having an outer diameter greater than the inner diameter D1 of the internal thread portion 50 and the same outer diameter, and has a circumferential surface 45. Alternatively, the transition portion 43a may include a thread crest and a thread root.

[0076] As described above, in the fourth embodiment, the transition portion 43a is formed to extend in the axial direction with the same outer diameter, and the outer diameter of the transition portion 43a is formed to be larger than the inner diameter D1 of the internal thread portion 50 of the screw 23. As a result, the outer diameter dimension of the transition portion 43a, which is very important in equalizing the load distribution ratio, can be accurately measured. Therefore, dimensional management can be performed more appropriately, and the effect of increasing the strength of the external thread portion 37b can be stably obtained. In addition, since the outer diameter of the transition portion 43a is larger than the diameter at which engagement with the internal thread portion 50 begins, that is, the inner diameter D1, the transition portion 43a itself can also contribute to equalizing the load distribution ratio together with the second tapering portion 42.

[0077] [Fifth embodiment]

[0078] Next, a screw head 30d according to a fifth embodiment will be described. Figure 9 30d is a side view showing a screw head according to a fifth embodiment. In the fifth embodiment, the axial length of the transition portion 43b is different from that of the fourth embodiment.

[0079] like Figure 9 As shown, in the fifth embodiment, the axial length d3 of the transition portion 43 in the external thread portion 37c is set to be greater than the thread pitch d4. The thread pitch d4 is between the thread crests or between the thread roots in the fully threaded portion 44.

[0080] That is, in the fifth embodiment, the axial length of the transition portion 43b is equal to or greater than the thread pitch d4. This allows for highly reliable dimensional measurement using a measuring instrument such as a micrometer, allowing for more appropriate management of the outer diameter of the transition portion 43b and more stable improvement in the strength of the threaded portion.

[0081] Furthermore, in the above embodiments, the inclination is gradually reduced in two stages, namely, the first and second gradually reducing sections. However, this is not a limitation. For example, the inclination of the external thread portion may vary in three or more stages. Thus, the variation in inclination can be appropriately modified depending on the circumstances.

[0082] The present disclosure has been described in detail, but the present disclosure is not limited to the above-mentioned embodiments. These embodiments can be variously added, replaced, changed, partially deleted, etc. without departing from the scope of the gist of the present disclosure, or without departing from the scope of the gist of the present disclosure derived from the contents recorded in the claims and their equivalents. In addition, these embodiments can also be implemented in combination. For example, in the above-mentioned embodiments, the order of each action and the order of each processing are shown as an example, and are not limited to the order of each action and the order of each processing. In addition, the same applies to the numerical values ​​or formulas used in the description of the above-mentioned embodiments.

[0083] The following supplementary notes are further disclosed regarding the above-mentioned embodiment and modifications.

[0084] (Note 1)

[0085] A screw head (30, 30a, 30b, 30c, 30d) is mounted on the front end of a screw (23) for an injection molding machine (1), wherein:

[0086] An external threaded portion (37, 37a, 37b, 37c) and a narrowing portion (36) are provided on the lower neck portion (32) of the screw head (30, 30a, 30b, 30c, 30d) located on the side of the screw (23) relative to the upper neck portion (31) located on the front end side. The external threaded portion is screwed into the screw (23). The narrowing portion is located on the front end side of the external threaded portion (37, 37a, 37b, 37c) and has a diameter (d1) smaller than a bottom diameter (d2) of the external threaded portion (37, 37a, 37b, 37c).

[0087] The external threaded portion (37, 37a, 37b, 37c) has:

[0088] a first gradually decreasing portion (41) located on the side of the narrowing portion (36) and having an outer diameter that decreases as it advances toward the narrowing portion (36); a transition portion (43, 43a, 43b) located on the side of the screw (23) of the first gradually decreasing portion (41); and a second gradually decreasing portion (42, 42a) adjacent to the first gradually decreasing portion (41) across the transition portion (43, 43a, 43b) and having an outer diameter that decreases as it advances toward the narrowing portion (36).

[0089] The average inclination of the first gradually decreasing portion (41) is formed to be larger than the average inclination of the second gradually decreasing portion (42, 42a).

[0090] (Note 2)

[0091] In the screw head (30a) of the above-mentioned injection molding machine (1),

[0092] The outer diameter (d m ) is formed to be larger than the inner diameter (D1) of the internal thread portion (50) provided on the front end side of the screw (22).

[0093] (Note 3)

[0094] In the screw head (30a) of the above-mentioned injection molding machine (1) or (2),

[0095] The outer diameter (d m) is formed to be smaller than the effective diameter (D2) of the internal thread portion (50) provided on the front end side of the screw (22).

[0096] (Note 4)

[0097] In the screw head (30b) of the above-mentioned injection molding machine (1),

[0098] The inclination of the second gradually decreasing portion (42a) is configured to increase as it moves from the transition portion (43) side toward the screw (22) side.

[0099] (Note 5)

[0100] In the screw head (30c) described in any one of the supplementary notes of the above-mentioned injection molding machines (1) to (4),

[0101] The transition portions (43a, 43b) are formed to extend in the axial direction with the same outer diameter.

[0102] The outer diameter of the transition portion (43a, 43b) is formed to be larger than the inner diameter of the internal thread portion of the screw (22).

[0103] (Note 6)

[0104] In the screw head (30d) of the above-mentioned injection molding machine (1),

[0105] The axial length of the transition portion (43b) is greater than the pitch of the thread.

[0106] Description of Reference Numerals

[0107] 1. Injection molding machine; 30, 30a, 30b, 30c, 30d, screw head; 36, narrowing portion; 37, 37a, 37b, 37c, external threaded portion; 41, first tapering portion; 42, 42a, second tapering portion; 43, 43a, 43b, transition portion.

Claims

1. A screw head for an injection molding machine, which is a screw head mounted on the front end of a screw for an injection molding machine, wherein: An external thread portion and a narrowing portion are provided at a neck lower portion located on the screw side relative to an upper neck portion located on the front end side of the screw head. The external thread portion is screwed into the screw, and the narrowing portion is located on the front end side of the external thread portion and has a diameter smaller than the bottom diameter of the external thread portion. The external thread portion has: a first tapering portion, located on the narrowed portion side, the outer diameter of which decreases as it advances toward the narrowed portion side; a transition portion located on the screw side of the first tapering portion; and a second tapering portion, adjacent to the first tapering portion across the transition portion, and having an outer diameter that decreases as it advances toward the narrowing portion; The average inclination of the first gradually decreasing portion is formed to be larger than the average inclination of the second gradually decreasing portion.

2. The screw head according to claim 1, wherein The outer diameter of the transition portion is formed to be larger than the inner diameter of the internal thread portion provided on the front end side of the screw.

3. The screw head of the injection molding machine according to claim 1 or 2, wherein: The outer diameter of the transition portion is formed to be smaller than the effective diameter of the internal thread portion provided on the front end side of the screw.

4. The screw head of the injection molding machine according to any one of claims 1 to 3, wherein The inclination of the second gradually decreasing portion is configured to increase from the transition portion side toward the screw side.

5. The screw head according to any one of claims 1 to 4, wherein The transition portion is formed to extend in the axial direction with the same outer diameter, The outer diameter of the transition portion is formed to be larger than the inner diameter of the internal thread portion of the screw.

6. The screw head of the injection molding machine according to claim 5, wherein The axial length of the transition portion is greater than the pitch of the thread.

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