Molding device and molding method

Abstract

To provide an apparatus for molding a vibrator.SOLUTION: A molding apparatus includes: a heating coil capable of heating a conductor by high frequency induction; a chamber; a non-conductive base arranged in the chamber; a first conductor arranged in the base and including a cavity having an axial rotation symmetrical shape to the central axis on the upper surface; a conductor support part arranged in the inside of the cavity and having a cylindrical shape around the central axis; and a decompression mechanism capable of decompressing the cavity.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present specification relates to a vibrator molding device and a vibrator molding method. [Background technology]

[0002] Patent Document 1 discloses a Bird-bath Resonator Gyroscope (BRG) that uses fused silica as an oscillator, as a gyro capable of achieving high accuracy. Specifically, a quartz plate is placed in a mold with a hole formed in its surface so as to cover the hole. While reducing the pressure inside the hole, the top surface of the quartz plate is heated with a burner. By melting and deforming the quartz plate so that it fits into the hole, a hemispherical oscillator can be produced. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2018 / 079129 Summary of the Invention [Problem to be solved by the invention]

[0004] Burner heating causes fluctuations in the flame, which leads to variations in temperature distribution and between lots, making it difficult to reproducibly and stably manufacture oscillators of the same shape. [Means for solving the problem]

[0005] One embodiment of a molding device disclosed herein includes a heating coil capable of high-frequency induction heating a conductor. The molding device includes a chamber. The molding device includes a non-conductive base disposed within the chamber. The molding device includes a first conductor portion disposed on the base. The first conductor portion includes a cavity on its upper surface that has an axially rotationally symmetric shape with respect to a central axis. The molding device includes a conductive support portion disposed inside the cavity and having a cylindrical shape centered on the central axis. The molding device includes a pressure reduction mechanism capable of reducing the pressure in the cavity.

[0006] The first conductor and support can be heated by high-frequency induction heating using a heating coil. Compared to heating using a burner, this method eliminates the effects of flame fluctuations, making it possible to suppress variations in temperature distribution and between lots. This also makes it possible to improve processing precision and reproducibility.

[0007] The axisymmetric shape of the cavity may be cylindrical.

[0008] The device may further include a second conductor portion that is disposed opposite the cavity and that is movable along the central axis. Details of the effects will be described in the examples.

[0009] The second conductor may include at least one of a first exhaust hole disposed at a position corresponding to the central axis of the region facing the cavity and a plurality of second exhaust holes disposed at rotationally symmetric positions equidistant from the central axis of the region facing the cavity. The first exhaust hole and the plurality of second exhaust holes may be configured to eject heated inert gas. Details of the effects will be described in the examples.

[0010] A protrusion having an axially rotationally symmetric shape with respect to the central axis may be formed in the region of the second conductor facing the cavity. The outer diameter of the protrusion may be smaller than the inner diameter of the opening of the cavity. Details of the effects will be explained in the examples.

[0011] The device may further include a measuring mechanism capable of monitoring the reduced pressure state within the cavity. Details of the effects will be described in the examples.

[0012] A plurality of first conductors may be provided. The distance between the first conductor and the heating coil may be equal for each of the plurality of first conductors. Details of the effects will be described in the examples.

[0013] The first conductor may include graphite.

[0014] One embodiment of a molding method disclosed herein is a molding method using a molding device. The molding device includes a heating coil capable of high-frequency induction heating a conductor. The molding device includes a chamber. The molding device includes a non-conductive base disposed within the chamber. The molding device includes a first conductor disposed on the base, the first conductor having a cavity on its upper surface that has an axially rotationally symmetric shape with respect to a central axis. The molding device includes a conductive support member disposed inside the cavity and having a cylindrical shape centered on the central axis. The molding device includes a decompression mechanism capable of decompressing the cavity. The molding method includes a placement step of placing a plate-shaped workpiece material on the upper surface of the first conductor member so as to cover the cavity. The molding method also includes a processing step of heating the first conductor member and the support member with a heating coil to melt and deform the workpiece material while decompressing the interior of the cavity using the decompression mechanism. Details of the effects will be described in the examples.

[0015] The molding method may further include a monitoring step of monitoring the reduced pressure state of the cavity during the processing step. The end point of the processing step may be detected based on the monitoring result of the reduced pressure state. Details of the effects will be described in the examples.

[0016] The molding device may further include a second conductor disposed opposite the cavity and movable along the central axis. In the processing step, the second conductor heated by the heating coil may be brought into contact with the workpiece. Details of the effects will be described in the examples.

[0017] The second conductor may include at least one of a first exhaust hole disposed at a position corresponding to the central axis of the region facing the cavity and a plurality of second exhaust holes disposed at rotationally symmetric positions equidistant from the central axis of the region facing the cavity. In the processing step, heated inert gas may be ejected from the first exhaust hole and the plurality of second exhaust holes. Details of the effects will be described in the examples. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view of a molding apparatus 1 of a first embodiment. [Figure 2] FIG. 2 is a top view taken along the line II-II in FIG. [Figure 3] FIG. 10 is a cross-sectional view after melt deformation. [Figure 4] 10 is a graph showing monitored pressure. [Figure 5] FIG. 10 is a cross-sectional view of a molding apparatus 201 according to a second embodiment. [Figure 6] FIG. 10 is a cross-sectional view of a molding apparatus 301 according to a third embodiment. [Figure 7] FIG. 10 is a cross-sectional view of a molding apparatus 401 according to a fourth embodiment. [Figure 8] FIG. 8 is a top view of the cross section taken along line VIII-VIII in FIG. 7. [Figure 9] FIG. 10 is a diagram showing a modified molding device 501. DETAILED DESCRIPTION OF THE INVENTION [Example]

[0019] 1 and 2 show a molding apparatus 1 of Example 1. FIG. 1 is a cross-sectional view. FIG. 2 is a top view of the cross section taken along line II-II in FIG. 1. Note that FIG. 2 omits the illustration of a chamber 10 and a heating coil 11. The molding apparatus 1 includes a chamber 10, a heating coil 11, a susceptor 13, a pressure reducing mechanism 14, a measuring mechanism 15, a base mold 20, a first conductor 21, a support 23, and a quartz plate 30.

[0020] The chamber 10 is a sealed container that houses the susceptor 13 and the base mold 20. Heated inert gas (e.g., nitrogen or argon) is supplied from the intake holes 10i of the chamber 10. The heating coil 11 is a component for high-frequency induction heating the first conductor portion 21 and the support 23, which are conductors. The heating coil 11 is arranged around the chamber 10.

[0021] The susceptor 13 is a circular platform on which the base mold 20 is placed. The shape of the susceptor 13 is not limited to the circle shown in FIG. 2. For example, it may be square. The material of the susceptor 13 may be high-heat-resistant ceramics. Examples of materials include alumina, zirconia, SiC, Si3N4, and BN. From the viewpoint of material cost, alumina is preferable. The susceptor 13 has an exhaust pipe 13e in the center of the bottom surface 13b. The exhaust pipe 13e is connected to a pressure reduction mechanism 14 and a measurement mechanism 15. The pressure reduction mechanism 14 is a mechanism capable of vacuuming, such as a vacuum pump. The measurement mechanism 15 is a mechanism capable of monitoring the pressure reduction state, such as a pressure gauge.

[0022] The base mold 20 is placed on the susceptor 13 and has a disk shape. A material having high insulating properties and high heat resistance can be used for the base mold 20. Examples of the material include BN (boron nitride), alumina, zirconia, SiC, and Si3N4. BN is preferred because of its ease of processing.

[0023] A hole 20a having a cylindrical shape with a central axis CA1 is formed in the surface 20s of the base mold 20. A first conductor 21 and a support 23 are disposed inside the hole 20a. The first conductor 21 and the support 23 can be made of a conductive material with a high melting point. Examples of such materials include isotropic graphite and high-melting-point metals such as Ta, Nb, and Mo. Isotropic graphite is preferred because of its ease of processing.

[0024] The first conductor 21 is a cylindrical member having a cylindrical shape centered on the central axis CA1. The inner peripheral surface of the hole 20a and the outer peripheral surface of the first conductor 21 may be fitted together without any gap, or a certain gap may be formed. A cavity 21c, which is a deformation space for the quartz plate 30 to melt and deform, is formed inside the first conductor 21. In other words, the first conductor 21 has the cavity 21c, which has an axial rotation-symmetric shape (cylindrical shape) with respect to the central axis CA1, on its upper surface.

[0025] The support pillar 23 has a cylindrical shape centered on the central axis CA1. An engagement groove 20t is formed in the bottom surface 20b of the hole 20a. The lower surface of the support pillar 23 engages with the engagement groove 20t. In FIG. 1, the position in the z direction of the upper surface of the support pillar 23 is lower than the position in the z direction of the surface 20s. Therefore, the upper surface of the support pillar 23 does not contact the lower surface of the quartz plate 30. As a variant, the positions in the z direction of the upper surface of the support pillar 23 and the surface 20s may be the same, and the upper surface of the support pillar 23 may contact the lower surface of the quartz plate 30.

[0026] A rear surface inner peripheral portion 20r1 of the base mold 20 is recessed relative to a rear surface outer peripheral portion 20r2. A space 20o is formed between the rear surface inner peripheral portion 20r1 and the bottom surface 13b of the susceptor 13. The space 20o is connected to the pressure reducing mechanism 14 via an exhaust pipe 13e.

[0027] The bottom surface 20b of the base mold 20 is provided with an exhaust port 20e and intake holes 20v1 and 20v2 that penetrate to the inner circumferential portion 20r1 of the back surface. The exhaust port 20e connects the cavity 21c to the space 20o. With the upper portion of the cavity 21c blocked by the quartz plate 30, the cavity 21c can be placed in a reduced-pressure state by evacuating the space 20o. The upper end of the intake hole 20v1 is in contact with the back surface of the first conductor 21. The upper end of the intake hole 20v2 is in contact with the back surface of the support 23. As a result, the first conductor 21 and the support 23 can be adsorbed and fixed to the base mold 20 by evacuating the space 20o.

[0028] A quartz plate 30 is disposed on the surface 20s so as to cover the hole 20h. The quartz plate 30 is a material to be processed to form the vibrator. The thickness of the quartz plate 30 is, for example, 100 μm or less. In this embodiment, the quartz plate 30 is square, but it may also be rectangular or circular.

[0029] (manufacturing process) The vibrator manufacturing process will be described with reference to Figures 1 and 3. In step S1, a base mold 20 is placed on a susceptor 13. In step S2, a quartz plate 30 is placed on the surface 20s of the base mold 20 so as to cover the cavity 21c. In step S3, an inert gas preheated to 1000°C or higher is supplied from the intake port 10i of the chamber 10. By replacing the atmosphere in the chamber with the inert gas, oxidation of the graphite components (first conductor 21, support 23) can be prevented. In addition, the cavity 21c is decompressed by the decompression mechanism 14. The quartz plate 30 is fixed by adsorption to the surface 20s. This results in the state shown in Figure 1.

[0030] In step S4, a monitoring step is started in which the measurement mechanism 15 monitors the reduced pressure state of the cavity 21c, thereby making it possible to control the degree of vacuum in the cavity 21c to be constant.

[0031] In step S5, the processing process begins. Specifically, the first conductor 21 and the support 23 are high-frequency induction heated by the heating coil 11. Because the heat-generating conductor is present only in the portion that forms the cavity 21c, only the quartz plate 30 near the cavity 21c can be selectively heated.

[0032] In step S6, the end point of the processing process is detected based on the results of monitoring the reduced pressure state. Specifically, as shown in FIG. 3, the quartz plate 30 rapidly melts and deforms, causing a sudden decrease in the volume of the space formed by the cavity 21c and the quartz plate 30. This causes a spike-like pressure fluctuation in the pressure monitored by the measurement mechanism 15, as shown in FIG. 4. The processing end point can be determined to be time t1, which is a certain time pt after the start time t0 of this pressure fluctuation. Since the amount of heat applied to the quartz plate 30 can be kept constant, it is possible to suppress lot-to-lot variation in the processed shape.

[0033] In step S7, the high-frequency induction heating and the preheating of the introduced inert gas are terminated in response to the detection of the processing end point. After the quartz plate 30 has cooled to the desired temperature, the cavity 21c is opened to the atmosphere. The melt-processed quartz plate 30 is removed from the chamber 10. Note that steps S6 and S7 may be omitted. In this case, the heating may be terminated in response to the passage of a predetermined time from the start of heating. In step S8, the unformed region UR (see FIG. 3) of the quartz plate 30 is removed by a CMP method or the like, thereby completing the vibrator.

[0034] (effect) When a quartz plate is heated with a burner as in conventional technology, flame fluctuations result in variations in temperature distribution and temperature variations between lots. It is difficult to create a vibrator with a uniform hemispherical shape or to reproducibly and stably manufacture vibrators of the same shape. With the technology of this specification, the first conductor 21 and the support 23 can be heated by high-frequency induction heating using a heating coil 11. Compared to heating using a burner, the effects of flame fluctuations can be eliminated, making it possible to suppress variations in temperature distribution and temperature variations between lots. This makes it possible to improve the precision and reproducibility of vibrator processing. [Example]

[0035] Fig. 5 shows a cross-sectional view of a molding apparatus 201 of Example 2. Example 2 differs from Example 1 in that it further includes a second conductor 22. Components common to the molding apparatus 1 of Example 1 are given the same reference numerals and will not be described again. Note that in Fig. 5, the chamber 10 and susceptor 13 are not shown.

[0036] The second conductor 22 is a cylindrical member. The second conductor 22 is disposed opposite the cavity 21c and is movable up and down along the central axis CA1. As with the first conductor 21, the second conductor 22 may be made of a conductive, high-melting-point material. In this embodiment, the second conductor 22 is made of isotropic graphite. The diameter D1 of the second conductor 22 is preferably larger than the inner diameter D2 of the opening of the cavity 21c. Preferably, the relationship "D2≦D1≦1.5×D2" is satisfied.

[0037] A protrusion 22p is formed in a region of the second conductor 22 facing the cavity 21c, protruding in a dome shape with the central axis CA1 as its apex. That is, the protrusion 22p has an axially rotationally symmetric shape with respect to the central axis CA1. The outer diameter D3 of the protrusion 22p is equal to or less than the inner diameter D2 of the cavity 21c. Preferably, the relationship "0.8 × D2 ≦ D3 ≦ D2" is satisfied. The height of the protrusion 22p from the lower surface 22r of the second conductor 22 is defined as H1. The distance from the surface 21s of the first conductor 21 to the upper surface 23t of the support 23 is defined as R1. The height H1 is equal to or less than the distance R1. Preferably, the relationship "0.8 × R1 ≦ H1 ≦ R1" is satisfied. This size relationship allows the protrusion 22p to enter the cavity 21c. That is, the protrusion 22p and the cavity 21c can engage with each other.

[0038] First exhaust hole 22e1 is disposed at a position corresponding to central axis CA1 of the region facing the cavity of second conductor 22. First exhaust hole 22e1 is a hole through which heated inert gas can be ejected. In addition, the diameter of first exhaust hole 22e1 is preferably smaller than diameter D4 of support 23.

[0039] The distance R1 may be zero. In this case, the protrusion 22p may not be disposed, and the height H1 may be zero. (That is, the area of the second conductor 22 facing the cavity 21c may be flat.)

[0040] (manufacturing process) Only the differences from the manufacturing process of Example 1 will be explained. In the processing step of step S5, the second conductor 22 is high-frequency induction heated by the heating coil 11. The heated second conductor 22 is then moved downward along the central axis CA1. This brings the protrusion 22p into contact with the quartz plate 30. Also, an inert gas heated to 1000°C or higher is ejected from the first exhaust hole 22e1.

[0041] (effect) In addition to heating the quartz plate 30 from the bottom surface by the first conductors 21 and the supports 23, the quartz plate 30 can be heated from the top surface by the second conductors 22. This increases the heating efficiency, thereby shortening the processing time.

[0042] The quartz plate 30 can be heated by heat conduction caused by contact with the second conductor 22. Since the thermal conductivity can be increased compared to heat conduction via inert gas, variations in temperature distribution can be suppressed. This makes it possible to improve processing accuracy and processing reproducibility.

[0043] 3, even after the quartz plate 30 is melted and deformed and separated from the protrusion 22p, the quartz plate 30 can be heated by the inert gas ejected from the first exhaust hole 22e1. This makes it possible to efficiently perform finishing processing of the quartz plate 30. [Example]

[0044] 6 shows a cross-sectional view of a molding device 301 of Example 3. Example 3 differs from Example 2 in the shape of the protrusions of the second conductors 22. Portions common to Example 2 are given the same reference numerals, and descriptions thereof will be omitted.

[0045] A protrusion 322p is formed in a region of the second conductor 22 facing the cavity 21c, protruding in a doughnut shape with the central axis CA1 as its apex. That is, the protrusion 322p has a shape that is concentric and rotationally symmetrical with respect to the central axis CA1. The outer diameter D5 of the protrusion 322p is equal to or smaller than the inner diameter D2 of the cavity 21c. It is preferable that the relationship "0.8 × D2 ≦ D5 ≦ D2" is satisfied. Furthermore, it is preferable that the inner diameter D6 of the protrusion 322p is larger than the diameter D4 of the support 23. This size relationship allows the protrusion 322p to enter the cavity 21c.

[0046] (effect) 3, after melting and deforming, the quartz plate 30 has the greatest amount of deformation in the region between the upper surface 23t of the support 23 and the surface 21s of the first conductor 21. The protrusions 322p have concentric protrusions corresponding to the region where the deformation is greatest. This allows the quartz plate 30 to be melted and deformed more efficiently by contacting the protrusions 322p. [Example]

[0047] 7 and 8 show a molding apparatus 401 of Example 4. FIG. 7 is a cross-sectional view. FIG. 8 is a top view at the cross section taken along line VIII-VIII in FIG. 7. Note that the quartz plate 430 is not shown in FIG. 8. Example 4 has a structure including a plurality of first conductors 21 and second conductors 22 of Example 2. Parts common to those of the molding apparatus 201 of Example 2 are given the same reference numerals, and description thereof will be omitted.

[0048] The chamber 10 has a cylindrical shape centered on a central axis CA2. The heating coil 11 is arranged concentrically around the chamber 10, centered on the central axis CA2. The susceptor 13 and the base mold 420 are disk-shaped, and their centers coincide with the central axis CA2. The base mold 420 includes a plurality of first conductors 21. The central axes CA1 of the plurality of first conductors 21 are located on a circle CC centered on the central axis CA2. That is, the plurality of first conductors 21 are arranged equidistant from the central axis CA2. Therefore, the distance CD between the first conductors 21 and the heating coil 11 is equal for each of the plurality of first conductors 21. This reduces temperature variation among the plurality of first conductors 21. Shape variation among the vibrators can be suppressed, thereby improving yield.

[0049] Furthermore, a jig 24 and a plurality of second conductors 22 are arranged inside the chamber 10. The jig 24 is a component for holding the plurality of second conductors 22. The jig 24 is disk-shaped, and its center coincides with the central axis CA2. The jig 24 is movable up and down along the central axis CA2. The jig 24 may be made of a material with high insulating properties and high heat resistance. In this embodiment, the jig 24 is made of alumina. A plurality of second conductors 22 are fixed to the lower surface of the jig 24 in correspondence with the plurality of first conductors 21. The structures of the first conductors 21 and the second conductors 22 are the same as those in Example 2, and therefore will not be described here. A circular quartz plate 430 is arranged on the surface 420s of the base mold 420 so as to cover the plurality of cavities 21c.

[0050] (effect) It is possible to simultaneously manufacture multiple vibrators in a single processing step, greatly improving vibrator production efficiency.

[0051] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0052] (Variation) In the fourth embodiment, the plurality of second conductors 22 may not be provided.

[0053] In the fourth embodiment, the position and number of exhaust holes provided in the second conductor 22 may be various. For example, as shown in a modified molding apparatus 501 of FIG. 9, multiple second exhaust holes 22e2 may be provided. FIG. 9(A) is a view of the bottom surface of the second conductor 22 as viewed from the -z direction. FIG. 9(B) is a cross-sectional view of the molding apparatus 501. In the region of the second conductor 22 facing the cavity 21c, four second exhaust holes 22e2 are arranged at rotationally symmetric positions equidistant from the central axis CA1. The heated inert gas ejected from the four second exhaust holes 22e2 can heat the quartz plate 30 more efficiently. Note that the number of second exhaust holes 22e2 is not limited to four and can be any number. Furthermore, the shape of the region where the second exhaust holes 22e2 are formed is not limited to the shape of the protrusion 22p of FIG. 9 and may be various. For example, the second exhaust holes 22e2 may be formed in the protrusion 322p shown in the molding apparatus 301 (FIG. 6). Alternatively, both the first exhaust hole 22e1 (FIGS. 5 and 6) and the second exhaust hole 22e2 (FIG. 9) may be provided.

[0054] The support pillars 23 are not limited to being conductive, and may be made of a non-conductive material.

[0055] The material of the vibrator is not limited to the quartz plate 30. Any dielectric material that melts and deforms can be used.

[0056] Furthermore, the technical elements described in this specification or drawings may exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technologies illustrated in this specification or drawings may achieve multiple objectives simultaneously, and achieving one of those objectives alone is technically useful. [Explanation of symbols]

[0057] 1, 201, 301, 401: molding device 10: chamber 11: heating coil 14: pressure reducing mechanism 15: measuring mechanism 20: base mold 21: first conductor part 21c: cavity 22: second conductor part 22e: exhaust hole 22p: protrusion 23: support

Claims

1. a heating coil capable of high-frequency induction heating a conductor; a chamber; a non-conductive base disposed within the chamber; a first conductor disposed on the base, the first conductor having a cavity on its upper surface that has an axial rotation symmetric shape with respect to a central axis; a conductive support portion that is disposed inside the cavity and has a cylindrical shape centered on the central axis; a pressure reducing mechanism capable of reducing the pressure in the cavity; a second conductor portion disposed opposite the cavity and movable along the central axis; A molding device comprising:

2. the second conductor portion includes at least one of a first exhaust hole arranged at a position corresponding to the central axis of the region facing the cavity, and a plurality of second exhaust holes arranged at rotationally symmetric positions equidistant from the central axis of the region facing the cavity, The molding apparatus according to claim 1 , wherein the first exhaust hole and the plurality of second exhaust holes are configured to be able to eject heated inert gas.

3. a protrusion having an axial rotation symmetric shape with respect to the central axis is formed in a region of the second conductor facing the cavity, The molding apparatus according to claim 1 or 2, wherein an outer diameter of the protrusion is smaller than an inner diameter of the opening of the cavity.

4. The molding apparatus according to any one of claims 1 to 3, further comprising a measuring mechanism capable of monitoring a reduced pressure state within the cavity.

5. a plurality of the first conductor portions are provided, The molding device according to any one of claims 1 to 4, wherein the distance between the first conductor and the heating coil is equal for each of the plurality of first conductors.

6. The molding device according to any one of claims 1 to 5, wherein the first conductor portion contains graphite.

7. a heating coil capable of high-frequency induction heating a conductor; a chamber; a non-conductive base disposed within the chamber; a first conductor disposed on the base, the first conductor having a cavity on its upper surface that has an axial rotation symmetric shape with respect to a central axis; a conductive support portion that is disposed inside the cavity and has a cylindrical shape centered on the central axis; a pressure reducing mechanism capable of reducing the pressure in the cavity; a second conductor portion disposed opposite the cavity and movable along the central axis; A molding method using a molding apparatus comprising: a placement step of placing a plate-shaped workpiece on the upper surface of the first conductor portion so as to cover the cavity; a processing step of heating the first conductor portion and the support portion with the heating coil while reducing the pressure inside the cavity with the pressure reducing mechanism to melt and deform the workpiece material; Equipped with In the processing step, the second conductor portion heated by the heating coil is brought into contact with the workpiece. Molding method.

8. a monitoring step of monitoring a reduced pressure state of the cavity during the processing step, The molding method according to claim 7 , further comprising detecting an end point of the processing step based on a result of monitoring the reduced pressure state.

9. the second conductor portion includes at least one of a first exhaust hole arranged at a position corresponding to the central axis of the region facing the cavity, and a plurality of second exhaust holes arranged at rotationally symmetric positions equidistant from the central axis of the region facing the cavity, The molding method according to claim 7 , wherein in the processing step, heated inert gas is ejected from the first exhaust hole and the plurality of second exhaust holes.

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