Anti-loosening valve body casting mold

Abstract

The application discloses a shrinkage-preventing valve body casting mold and relates to the technical field of valve body casting molds. The mold comprises an upper mold and a lower mold, and further comprises a shrinkage-preventing mechanism. An upper mold cavity is formed in the upper mold, a lower mold cavity is formed in the lower mold, an upper cooling runner is formed in the upper mold, and a lower cooling runner is formed in the lower mold. The shrinkage-preventing mechanism comprises a gentle curved surface formed in the inner wall of the lower cooling runner. In the application, directional cooling from bottom to top and local temperature control of hot spot positions are adopted to guide the sequential solidification of the casting from the positions far from the riser to the riser direction, ensure the smoothness of the feeding channel, and combine the double pressure feeding mechanism of spring pressurization and cooling liquid storage pressure to upgrade the traditional passive gravity feeding to active pressure feeding, greatly improve the feeding efficiency and feeding pressure, and force the metal liquid to penetrate into the interdendritic shrinkage area, so that the valve body casting with extremely compact internal structure is obtained.

Description

Technical Field

[0001] This invention relates to the field of valve body casting mold technology, and more particularly to a valve body casting mold for preventing shrinkage. Background Technology

[0002] In the field of valve manufacturing, the internal density of the valve body is a key component, which directly affects the valve's pressure-bearing capacity and service life. Due to its complex structure, this type of valve body has many thick cross sections and areas with abrupt changes in wall thickness. During the casting and solidification process, it is very easy to form hot spots, resulting in serious shrinkage cavities and porosity defects, which has become a long-standing quality problem that has plagued the industry.

[0003] Chinese invention patent CN119525431B discloses a casting method for a servo valve body, comprising: casting equipment that sequentially performs three-dimensional modeling design, casting simulation, optimization design, and simulation verification to formulate the casting process; placing feeders for compensating shrinkage, risers for controlling the solidification time of the casting, and chills for controlling the cooling rate on both sides of the valve core and on the upper surface of the mold, respectively; and adopting a combination of "feeders + risers + chills" to effectively reduce shrinkage cavities and porosity inside the casting, optimize the cooling process, thereby reducing defects in the casting quality, and ensuring that the feeding channel forms a temperature gradient smoothly, reducing shrinkage defects, thereby improving the quality and performance of the casting.

[0004] Based on the aforementioned existing technologies, it has been found that in actual use, the feeding effect of risers relies on the gravity of the molten metal, resulting in low feeding efficiency and limited distance. Large risers are often required, leading to low process yield and serious waste of molten metal. Secondly, the effectiveness of external chills is highly dependent on the operator's experience, and the cooling intensity is difficult to control precisely. Excessive chilling can easily cause the casting to develop a hard and brittle chilled structure or even cracks, while insufficient cooling cannot effectively eliminate shrinkage porosity. In addition, during the pouring stage, the molten metal usually fills the mold at a constant rate, making it difficult to balance rapid filling with stable venting. The risk of turbulence and air entrapment is high, and porosity defects are easily generated. Summary of the Invention

[0005] To address the problems mentioned in the background section, the present invention adopts the following technical solution:

[0006] A shrinkage-resistant valve body casting mold includes an upper mold and a lower mold, and also includes a shrinkage-resistant mechanism;

[0007] The upper mold has an upper mold cavity, the lower mold has a lower mold cavity, the upper mold has an upper cooling channel, and the lower mold has a lower cooling channel.

[0008] The anti-shrinkage mechanism includes a gently curved surface formed on the inner wall of the lower cooling channel. Multiple risers are fixedly connected to the upper end of the upper mold. The risers are connected to the upper mold cavity through a shrinkage compensation tube. A pressure plate is slidably connected to the inner wall of the riser. A spring is fixedly connected between the pressure plate and the top of the riser. The upper cooling channel is connected to the riser through a one-way tube. A drain pipe is fixedly connected to the side wall of the riser. A pressure relief valve is installed on the inner wall of the drain pipe.

[0009] Preferably, a plurality of injection pipes are fixedly connected to the side wall of the lower mold, and one end of the plurality of injection pipes is connected to the lower cooling channel, and the other end of the injection pipes is fixedly connected to the external pump body.

[0010] Preferably, an electric heating coil is fixedly connected to the bottom of the riser.

[0011] Preferably, the riser sidewall is fixedly connected to an external gating pipe, and the upper mold has multiple cylindrical cavities corresponding to the riser one by one. The cylindrical cavities are connected to the upper mold cavity through the gating port, and the riser is connected to the cylindrical cavities through an internal gating pipe.

[0012] Preferably, both the feeding tube and the inner wall of the ingate are equipped with solenoid valves.

[0013] Preferably, an upper conical cover is fixedly connected to the inner wall of the upper mold cavity, and a lower conical cover is fixedly connected to the inner wall of the lower mold cavity. The turning points of the inner walls of the upper and lower mold cavities are rounded and expanded.

[0014] Preferably, the upper mold has multiple vent holes at its upper end, each vent hole is connected to the upper mold cavity at its lower end, and each vent hole is fixedly connected to an vent plug on its inner wall.

[0015] Preferably, the upper end of the lower mold is fixedly connected with a plurality of guide pillars, and the lower end of the upper mold is provided with a plurality of guide holes that cooperate with the guide pillars.

[0016] Preferably, a guide plate is fixedly connected to the inner wall of the lower cooling channel.

[0017] The present invention has the following beneficial effects:

[0018] 1. By using bottom-up directional cooling and local temperature control design at hot spots, the casting is guided to solidify sequentially from the part away from the riser towards the riser, ensuring the smooth flow of the feeding channel. Combined with the dual pressure feeding mechanism of spring pressurization and coolant pressure storage, the traditional passive gravity feeding is upgraded to active pressure feeding, which improves feeding efficiency and feeding pressure. It can force the molten metal to penetrate into the micro-shrinkage area between dendrites, thereby obtaining a dense valve body casting.

[0019] 2. Through the design of cylindrical cavity and tangential ingate, the continuously injected molten metal is converted into a pulse jet with alternating low and high speeds. The high-speed section ensures rapid filling and prevents cold shuts, while the low-speed section allows sufficient time for the gas in the cavity to be discharged and smoothly discharged through the vent plug. Combined with the rounded corner expansion design at the cavity bend, the risk of turbulence and air entrapment is reduced, and the formation of pores is effectively prevented.

[0020] 3. The use of upper conical cover, lower conical cover and rounded corner expansion in the hot spot of the mold cavity improves the flow state and heat distribution of the molten metal geometrically, reduces the severity of the hot spot, and creates favorable conditions for sequential solidification and effective feeding. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of a shrinkage-resistant valve body casting mold proposed in this invention;

[0022] Figure 2 for Figure 1 A schematic diagram of the structure without pipes installed in the middle section;

[0023] Figure 3 for Figure 1 Cross-sectional view of the middle structure;

[0024] Figure 4 for Figure 3 Enlarged schematic diagram of the structure at point A;

[0025] Figure 5 for Figure 3 Schematic diagram of the upper and lower molds;

[0026] Figure 6 for Figure 5 A cross-sectional view of the upper and middle molds;

[0027] Figure 7 for Figure 6 Enlarged schematic diagram of the structure at point B;

[0028] Figure 8 for Figure 3 Schematic diagram of the structure of riser, feeding tube, one-way tube, drain tube, external gating tube and internal gating tube;

[0029] Figure 9 for Figure 3 Schematic diagram of the upper and lower conical covers;

[0030] Figure 10 for Figure 3 Enlarged schematic diagram of the structure at point C;

[0031] Figure 11 This is a schematic diagram of the thermal distribution of a shrinkage-resistant valve body casting mold proposed in this invention;

[0032] Figure 12 This is a schematic diagram of the flow guide plate guiding the coolant in this invention.

[0033] In the diagram: 1. Upper mold; 2. Lower mold; 3. Upper mold cavity; 4. Lower mold cavity; 5. Upper cooling channel; 6. Lower cooling channel; 7. Gentle curved surface; 8. Riser; 9. Shrinkage tube; 10. Pressure plate; 11. Spring; 12. One-way tube; 13. Drain pipe; 14. Pressure relief valve; 15. Electric heating coil; 16. External sprue; 17. Cylindrical cavity; 18. Sprue; 19. Internal sprue; 20. Solenoid valve; 21. Upper conical cover; 22. Lower conical cover; 23. Vent hole; 24. Vent plug; 25. Guide post; 26. Guide hole; 27. Injection pipe; 28. Baffle plate. Detailed Implementation

[0034] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0035] Reference Figures 1-12 A shrinkage-resistant valve body casting mold includes an upper mold 1 and a lower mold 2, and also includes a shrinkage-resistant mechanism. The upper end of the lower mold 2 is fixedly connected with a plurality of guide posts 25, and the lower end of the upper mold 1 is provided with a plurality of guide holes 26 that cooperate with the guide posts 25.

[0036] The upper mold 1 has an upper mold cavity 3, the lower mold 2 has a lower mold cavity 4, the upper mold 1 has an upper cooling channel 5, and the lower mold 2 has a lower cooling channel 6.

[0037] Furthermore, when casting the Tain valve, the guide post 25 is first inserted into the guide hole 26, so that the upper mold 1 and the lower mold 2 are joined together, so that a complete casting mold is formed between the upper mold 1 and the lower mold 2, and a complete pouring mold cavity is formed between the upper mold cavity 3 and the lower mold cavity 4, and a complete cooling channel is formed between the upper cooling channel 5 and the lower cooling channel 6.

[0038] The anti-shrinkage mechanism includes a gently curved surface 7 on the inner wall of the lower cooling channel 6. Multiple risers 8 are fixedly connected to the upper end of the upper mold 1. The risers 8 are connected to the upper mold cavity 3 through the shrinkage compensation tube 9. A pressure plate 10 is slidably connected to the inner wall of the riser 8. A spring 11 is fixedly connected between the pressure plate 10 and the top of the riser 8. The upper cooling channel 5 is connected to the riser 8 through a one-way tube 12. A drain pipe 13 is fixedly connected to the side wall of the riser 8. A pressure relief valve 14 is installed on the inner wall of the drain pipe 13.

[0039] Multiple injection pipes 27 are fixedly connected to the side wall of the lower mold 2, and one end of the injection pipes 27 is connected to the lower cooling channel 6, while the other end of the injection pipes 27 is fixedly connected to the external pump body.

[0040] An electric heating coil 15 is fixedly connected to the bottom of the riser 8.

[0041] Furthermore, during the pouring of molten metal, the electric heating coil 15 is simultaneously energized, so that the electric heating coil 15 heats and keeps the molten metal in the riser 8 at a constant temperature, so that the molten metal in the riser 8 will not solidify, thus providing conditions for subsequent feeding.

[0042] The riser 8 is fixedly connected to the side wall of the external gating pipe 16. The upper mold 1 has multiple cylindrical cavities 17 that correspond one-to-one with the riser 8. The cylindrical cavities 17 are connected to the upper mold cavity 3 through the gating port 18. The riser 8 is connected to the cylindrical cavities 17 through the internal gating pipe 19.

[0043] It should be noted that, as Figure 3 As shown, one end of the one-way pipe 12 is connected to the upper space formed by the pressure plate 10 and the riser 8, and one end of the feeding pipe 9, the outer sprue pipe 16 and the inner sprue pipe 19 are all connected to the lower space formed by the pressure plate 10 and the riser 8.

[0044] Solenoid valves 20 are installed on the inner walls of both the feeding tube 9 and the ingate tube 19.

[0045] Furthermore, after the upper mold cavity 3 and the lower mold cavity 4 are filled with molten metal, the outer sprue 16 is closed, and the solenoid valve 20 is simultaneously de-energized. At this time, the solenoid valve 20 on the inner wall of the feeding tube 9 is de-energized and opened, while the solenoid valve 20 on the inner wall of the inner sprue 19 is de-energized and closed. Then, the coolant is injected into the lower cooling channel 6 through the injection pipe 27 by the external pump. The coolant will gradually flow upward in the lower cooling channel 6 and then enter the upper cooling channel 5. Finally, the coolant flows out through the one-way pipe 12. Due to the upward flow of the coolant, the molten metal can slowly solidify from the bottom of the mold upward, so that the solidification sequence is from the far end to the near end of the riser 8, avoiding obstruction of the feeding channel.

[0046] An upper conical cover 21 is fixedly connected to the inner wall of the upper mold cavity 3, and a lower conical cover 22 is fixedly connected to the inner wall of the lower mold cavity 4. The turning points of the inner walls of the upper mold cavity 3 and the lower mold cavity 4 are rounded and expanded.

[0047] Furthermore, due to the presence of a gently curved surface 7, and the location of the gently curved surface 7 at two hot spots (e.g. Figure 2 and Figure 10As shown in the diagram, when the coolant flows to the gently curved surface 7, the flow rate of the coolant decreases, thereby increasing the heat absorption effect at the hot spot. This allows the hot spot to solidify preferentially compared to other parts, thus enabling effective shrinkage compensation and preventing shrinkage porosity at the hot spot. In addition, the upper conical cover 21 increases the wall thickness at the hot spot, reducing the accumulation of molten metal and making the wall thickness transition smoothly, thereby reducing the formation of the hot spot. Furthermore, the hot spot in the lower mold cavity 4 undergoes rounded corner expansion treatment, which reduces the flow rate of molten metal at this location, thereby reducing the heat brought into the area per unit time and preventing the initial temperature of the area from being too high, thus reducing the severity of the hot spot. Therefore, by controlling the solidification sequence and reducing the formation of hot spots, shrinkage porosity in the Tain valve body can be prevented. Compared to the existing method of using external chills, the cooling effect is uncontrollable, and overuse can lead to excessively hard castings, cracks, and chilling structures. Moreover, the location and amount of external chills used depend entirely on manual experience.

[0048] It is worth mentioning that when the coolant enters the riser 8 through the one-way pipe 12, the coolant will accumulate in the riser 8. When the pressure in the riser 8 exceeds the pressure of the pressure relief valve 14, the pressure relief valve 14 will open, and the coolant will be discharged and collected through the drain pipe 13. At this time, the coolant in the riser 8 will exert downward pressure on the pressure plate 10, and the pressure plate 10 will squeeze the molten metal in the riser 8. At this time, the molten metal will enter the feeding pipe 9 and enter the upper mold cavity 3 under pressure. This pressure will act directly on the solidification front of the casting through the unsolidified feeding channel, forcibly squeezing and penetrating the shrinkage porosity area that is forming between dendrites, thereby obtaining a dense Tyne valve body. Compared with the existing gravity feeding or increasing the volume of the riser 8, the traditional gravity feeding can be transformed into active pressure feeding, which improves the feeding efficiency and feeding distance. Moreover, compared with using a larger volume riser 8, the waste of molten metal can be significantly reduced.

[0049] The upper mold 1 has multiple vent holes 23 at its upper end. The lower end of each vent hole 23 is connected to the upper mold cavity 3, and each vent hole 23 has a vent plug 24 fixedly connected to its inner wall.

[0050] Furthermore, molten metal is continuously and uniformly injected into riser 8 through outer gating pipe 16 by an external pouring pump. At this time, solenoid valve 20 is energized, solenoid valve 20 on the inner wall of inner gating pipe 19 is energized and opened, while solenoid valve 20 on the inner wall of feeding pipe 9 is energized and closed. At this time, the molten metal in riser 8 will enter the cylindrical cavity 17 tangentially through inner gating pipe 19. The molten metal will rotate along the inner wall of cylindrical cavity 17, forming a vortex. At this time, the kinetic energy of the molten metal is mainly used to maintain rotation, so the flow rate of molten metal at the central pouring port 18 is slower. The molten metal enters the upper mold cavity 3 at a low speed. When the cylindrical cavity 17 is filled with molten metal, the rotational kinetic energy reaches its maximum value. At this time, the pressure at the center of the vortex is the lowest, which will form a strong pressure gradient. This causes the molten metal to be injected into the upper mold cavity 3 at high speed from the pouring port 18 like a tornado. After one injection, the pressure in the cylindrical cavity 17 is released, and the flow rate of the molten metal will slow down again. The molten metal will start to be injected tangentially again to store rotational energy. This cycle continues. During the entire pouring process, the molten metal will be injected into the upper mold cavity 3 at low speed, high speed, and low speed in a cycle, similar to pulse injection.

[0051] When molten metal is injected into the upper mold cavity 3 at high speed, it can quickly fill the upper mold cavity 3 and the lower mold cavity 4 before cooling. The rapid filling of the upper mold cavity 3 and the lower mold cavity 4 by the molten metal reduces the total surface area and time of contact between the molten metal and the mold, thereby reducing overall heat loss. In addition, during high-speed filling, the air in the upper mold cavity 3 and the lower mold cavity 4 will be chased and encapsulated by the molten metal, forming pores. When entering low-speed injection, the speed and pressure at the front end of the molten metal decrease, giving these gases enough time to be smoothly discharged through the vent hole 23 and the vent plug 24. The smooth low-speed flow can reduce the turbulence and eddies caused by high speed, thereby reducing the risk of gas being drawn into the interior of the molten metal. Furthermore, this high-speed and low-speed speed cycle is smooth and continuous, which can avoid the sudden start and stop of the molten metal flow, thereby reducing the destructive force on the upper mold cavity 3 and the lower mold cavity 4 due to inertial impact and reducing mold wear.

[0052] It is worth mentioning that when the molten metal enters the direct flow section of the upper mold cavity 3, the upper conical cover 21 causes the direct flow section of the upper mold cavity 3 to exhibit a gradient contraction. According to Bernoulli's principle, with a constant flow rate, a decrease in cross-sectional area will increase the flow velocity. Therefore, the flow velocity of the molten metal will gradually increase in the longer direct flow section, which can shorten the flow time of the molten metal, thereby reducing heat loss and effectively preventing cold shut defects caused by excessively low temperature at the front end of the molten metal. The principle of the lower conical cover 22 in the lower mold cavity 4 is the same as above, which can increase the flow velocity of the molten metal in the direct flow section of the lower mold cavity 4.

[0053] It is worth mentioning that existing methods using uniform pouring of molten metal can lead to air entrapment if the speed is too high, and cold shuts if the speed is too low. However, the high-speed, low-speed circulating pouring method utilizes the high kinetic energy of the molten metal during the high-speed phase to ensure rapid filling of the far ends and thin-walled areas of the mold cavity before cooling, effectively preventing cold shuts and incomplete pouring defects caused by excessive heat loss. During the low-speed phase, the advancing speed of the molten metal front slows down, allowing sufficient time for compressed air within the mold cavity to escape. The gas can then smoothly exit through the micropores of the vent plug 24, rather than being trapped by the molten metal and forming pores. Simultaneously, the low-speed flow promotes a stable laminar flow, further reducing the risk of entraining gas and oxide scale into the molten metal. Furthermore, the rhythmic flow... It has a tearing effect on the oxide scale on the surface of the molten metal, which is conducive to the aggregation and floating of inclusions, thereby purifying the molten metal and improving the purity of the casting. In addition, compared with the uniform injection of molten metal at a speed between low and high speed, the uniform injection speed is too fast, which will generate air entrapment, and the injection speed is too slow, which will generate cold shut. It is impossible to achieve both at the same speed. Moreover, the flow pattern is fixed and cannot be changed when injecting at a uniform speed. If the cavity structure is complex, stable eddies and turbulence will inevitably be generated in certain areas, continuously entraining gas into the interior. The pulsed injection of molten metal can disrupt the formation conditions of stable eddies. Furthermore, high-speed injection fills the mold quickly, while low-speed injection exhausts the air quickly. The two complement each other, which can prevent the occurrence of cold shut and avoid the appearance of porosity in the casting.

[0054] The bends in the upper mold cavity 3 and the lower mold cavity 4 are rounded, allowing for smooth expansion of this area and increasing the cross-sectional area of ​​the bend. This reduces the flow velocity of the molten metal when turning, weakening the centrifugal force effect. With the flow velocity reduced, the molten metal can pass through the bend more smoothly, greatly reducing the generation of eddies and turbulence. This effectively prevents gas from being drawn into the molten metal and prevents the formation of pores in the Tain valve body. In addition, the larger space provided by the expansion allows the molten metal to change direction smoothly, reducing the impact and energy loss on the inner walls of the upper mold cavity 3 and the lower mold cavity 4.

[0055] refer to Figure 11 A guide plate 28 is fixedly connected to the inner wall of the lower cooling channel 6, and the cross-section of the guide plate 28 is J-shaped.

[0056] Furthermore, after the coolant flows past the hot spot of the lower cooling channel 6, it enters the crossflow section of the lower cooling channel 6. At this time, some coolant flows through the guide plate 28 and tends to return, thus creating a strong convection area at the hot spot of the lower cooling channel 6 (such as...). Figure 11As shown, this can reduce the flow rate of the coolant at this point, increase the heat absorption time of the coolant, and because of the strong convection, it will cause the coolant at this point to swirl, thereby causing the coolant near the lower mold cavity 4 and the coolant far from the lower mold cavity 4 to circulate and convect, further improving the cooling efficiency, thereby increasing the solidification rate of the molten metal at this hot spot. In contrast, weak convection occurs in the crossflow section, where the coolant flow rate is faster and the heat absorption efficiency is reduced, which can reduce the solidification rate of the molten metal at this point. This makes the solidification rate of the molten metal in the crossflow section slower than that at the hot spot, preventing the metal in the crossflow section from solidifying first and causing blockage of the feeding channel.

[0057] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A shrinkage-resistant valve body casting mold, comprising an upper mold (1) and a lower mold (2), characterized in that, It also includes a shrinkage prevention mechanism; The upper mold (1) has an upper mold cavity (3), the lower mold (2) has a lower mold cavity (4), the upper mold (1) has an upper cooling channel (5), and the lower mold (2) has a lower cooling channel (6). The anti-shrinkage mechanism includes a gently curved surface (7) on the inner wall of the lower cooling channel (6). The upper mold (1) is fixedly connected to a plurality of risers (8). The risers (8) are connected to the upper mold cavity (3) through a shrinkage compensation pipe (9). The inner wall of the riser (8) is sealed and slidably connected to a pressure plate (10). A spring (11) is fixedly connected between the pressure plate (10) and the top of the riser (8). The upper cooling channel (5) is connected to the riser (8) through a one-way pipe (12). The side wall of the riser (8) is fixedly connected to a drain pipe (13). A pressure relief valve (14) is installed on the inner wall of the drain pipe (13). The lower mold (2) has multiple injection pipes (27) fixedly connected to its side wall, and one end of each injection pipe (27) is connected to the lower cooling channel (6), and the other end of each injection pipe (27) is fixedly connected to the external pump body. The lower cooling channel (6) has a guide plate (28) fixedly connected to its inner wall. The riser (8) is fixedly connected to the side wall of the external gating pipe (16). The upper mold (1) has multiple cylindrical cavities (17) that correspond one-to-one with the riser (8). The cylindrical cavities (17) are connected to the upper mold cavity (3) through the pouring port (18). The riser (8) is connected to the cylindrical cavities (17) through the internal gating pipe (19). The upper mold cavity (3) is fixedly connected to the inner wall of the upper conical cover (21), and the lower mold cavity (4) is fixedly connected to the inner wall of the lower conical cover (22). The turning points of the inner walls of the upper mold cavity (3) and the lower mold cavity (4) are rounded and expanded.

2. The anti-shrinkage valve body casting mold according to claim 1, characterized in that, An electric heating coil (15) is fixedly connected to the bottom of the riser (8).

3. The anti-shrinkage valve body casting mold according to claim 1, characterized in that, Solenoid valves (20) are installed on the inner walls of both the feeding tube (9) and the ingate tube (19).

4. The anti-shrinkage valve body casting mold according to claim 3, characterized in that, The upper mold (1) has multiple vent holes (23) at its upper end. The lower end of each vent hole (23) is connected to the upper mold cavity (3), and each vent hole (23) has a vent plug (24) fixedly connected to its inner wall.

5. The anti-shrinkage valve body casting mold according to claim 1, characterized in that, The lower mold (2) is fixedly connected to a plurality of guide posts (25) at its upper end, and the upper mold (1) is provided with a plurality of guide holes (26) that cooperate with the guide posts (25) at its lower end.

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