An extrusion coating machine for polymer battery production

Abstract

This invention discloses an extrusion coating machine for polymer battery production, relating to the field of coating machine technology. Specifically, it includes a frame with a rotatable front cover, a middle cover, and a rear cover mounted on its top side. A cleaning mechanism for cleaning the coating surface of the separator is installed at the bottom of the front cover, and a drying mechanism for preliminary drying of the slurry on the separator surface is installed at the bottom of the rear cover. Limiting guide rails are installed on both sides of the frame, and a guide frame mechanism is slidably installed between the two limiting guide rails. In this invention, when the feeding belt moves the separator, the limiting bar at the bottom of the feeding belt moves on a limiting groove inside the support platform. With the cooperation of the limiting bar and the limiting groove, and the action of the positioning rollers for extruding the separator, the stability of the separator during movement is maintained, effectively preventing fluctuations in the separator during coating and avoiding uneven distribution of the slurry on the separator surface.

Description

Technical Field

[0001] This invention relates to the field of coating machine technology, and more particularly to an extrusion coating machine for polymer battery production. Background Technology

[0002] Lithium polymer batteries are battery systems in which at least one of the positive electrode, negative electrode, and electrolyte uses polymer materials. In lithium polymer battery systems, polymer materials are mostly used in the positive electrode and electrolyte. The positive electrode material is a conductive polymer or an inorganic compound used in typical lithium-ion batteries. The negative electrode often uses lithium metal or lithium-carbon intercalation compounds. The electrolyte is a solid or colloidal polymer electrolyte or an organic electrolyte. Because lithium polymer batteries do not contain excess electrolyte, they are more reliable and stable.

[0003] In polymer batteries, the electrolyte plays a dual role as both a separator and a liquid electrolyte: on the one hand, it acts like a separator to isolate the positive and negative electrode materials, preventing self-discharge and short circuits inside the battery; on the other hand, it acts like a liquid electrolyte to conduct lithium ions between the positive and negative electrodes. The coating of the slurry is one of the important steps in the polymer battery manufacturing process. Currently, when coating the slurry on polymer batteries, the separator moves on the platform due to friction. During the dragging process, the moving separator above is prone to vibration. When it vibrates, the distance between the separator and the coating head changes, resulting in differences in the thickness of the slurry coating.

[0004] In view of this, the present invention provides an extrusion coating machine for polymer battery production to solve the technical problems existing in the prior art. Summary of the Invention

[0005] Based on the technical problems existing in the background art, the present invention proposes an extrusion coating machine for polymer battery production.

[0006] This invention proposes an extrusion coating machine for polymer battery production, comprising a frame, a rotatable front end cover, a middle end cover, and a rear end cover mounted on the top side of the frame, a cleaning mechanism for cleaning the coating surface of the separator mounted at the bottom of the front end cover, and a drying mechanism for preliminary drying of the slurry on the separator surface mounted at the bottom of the rear end cover, limit guide rails mounted on both sides of the frame, and a guide frame mechanism slidably mounted between the two limit guide rails, a coating mechanism mounted inside the guide frame mechanism, and a slurry storage shell mounted outside the coating mechanism, two positioning rollers for extruding the separator mounted at the bottom of the guide frame mechanism, and an annular groove for passing the slurry in the middle of the positioning roller at the rear end, a retractable coating gate mounted inside the slurry storage shell, and an adjustable-width L-shaped extrusion channel between the coating gate and the bottom end of the slurry storage shell, and a slurry injection pipe for replenishing slurry mounted on the top of the slurry storage shell, with a slurry injection pump and a slurry storage tank connected to the outer end of the slurry injection pipe;

[0007] The frame is equipped with a diaphragm feeding mechanism, which consists of multiple conveying rollers installed inside the frame and a feeding belt sleeved on the outside of the conveying rollers. The frame is also equipped with a drive motor that is connected to the conveying rollers. The diaphragm feeding mechanism also includes a support plate installed inside the frame, and the surface of the support plate is provided with multiple T-shaped limiting grooves. The inner side of the feeding belt is provided with a limiting bar that matches the limiting groove.

[0008] Preferably, in this invention, the cleaning mechanism includes a cleaning housing, and the interior of the cleaning housing is provided with a dust removal chamber and an exhaust chamber. A matrix fan is installed between the dust removal chamber and the exhaust chamber, and a dust filter is installed at the inlet end of the matrix fan. A transition air duct is provided between the bottom ends of the dust removal chamber and the exhaust chamber, close to the upper surface of the battery separator, and both ends of the transition air duct have a C-shaped structure.

[0009] Preferably, in this invention, the drying mechanism includes a drying chamber, and two electric push rods are installed inside the drying chamber. A baking hood composed of multiple reflective arc plates is installed between the bottom ends of the two electric push rods, and a heater is installed inside the baking hood. An exhaust gas absorber is installed on the top of the drying chamber, and multiple fans are installed at the inlet of the exhaust gas absorber.

[0010] Preferably, in this invention, the guide frame mechanism includes two guide frames, and two movable brackets are rotatably mounted at the ends of the two guide frames. The two positioning rollers are respectively installed between the two movable brackets on the same side, and a stepper motor for driving the positioning rollers is installed on the outer side of the two movable brackets at one end. An adjusting push rod for adjusting the included angle is installed between the two movable brackets at the same end.

[0011] Preferably, in this invention, the slurry storage shell has a homogenization chamber inside, and the coating gate is distributed at the bottom of the homogenization chamber. Two staggered homogenization paddles are installed at the upper end of the homogenization chamber, and the two homogenization paddles rotate in opposite directions. A drive motor that drives the homogenization paddles to rotate is installed at the end of the slurry storage shell.

[0012] Preferably, in this invention, the coating gate is obliquely slidably connected inside the homogenization chamber, and a rack rod extending to the outside of the slurry storage shell is provided at the top of the coating gate. A gearbox that meshes with the rack rod is installed on the outer side of the slurry storage shell, and a servo motor is installed on the side of the gearbox.

[0013] Preferably, in this invention, both sides of the limiting groove are made of strip magnets, and both sides of the limiting bar are made of chain magnets, wherein the sliding surfaces of the strip magnets and the chain magnets are repulsive connections.

[0014] Preferably, in this invention, a magnetic shielding plate is installed at the bottom of the support platform, and a magnetic tensioning member is installed at the bottom of the magnetic shielding plate, with the bottom of the tensioning member repelling the magnetic poles of the chain-like magnetic steel strip.

[0015] Compared with the prior art, the present invention provides an extrusion coating machine for polymer battery production, which has the following advantages:

[0016] In this invention, the two ends of the diaphragm are moved by the unwinding device and the traction device, respectively. When the diaphragm enters the coating machine, it moves at the bottom of the guide frame mechanism and the coating mechanism under the drive of the feeding belt. When the feeding belt moves the diaphragm, the limiting bar at the bottom of the feeding belt moves on the limiting groove inside the support plate. With the cooperation of the limiting bar and the limiting groove, and the action of the positioning roller that squeezes the diaphragm, the stability of the diaphragm during movement is maintained, effectively avoiding fluctuations in the diaphragm during the coating process, which would cause uneven distribution of slurry on the diaphragm surface. Secondly, the slurry is squeezed downward inside the slurry storage shell. The movement of the coating gate controls the width of the L-shaped extrusion channel, and the guide frame mechanism controls the distance between the L-shaped extrusion channel and the diaphragm. The slurry is squeezed out from the L-shaped extrusion channel and evenly coated on the steadily moving diaphragm surface. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of an extrusion coating machine for polymer battery production proposed in this invention;

[0018] Figure 2 This is a partial cross-sectional view of an extrusion coating machine for polymer battery production proposed in this invention.

[0019] Figure 3 This is a schematic diagram of the cleaning mechanism of an extrusion coating machine for polymer battery production proposed in this invention;

[0020] Figure 4 This is a schematic diagram of the drying mechanism of an extrusion coating machine for polymer battery production proposed in this invention;

[0021] Figure 5 This is a schematic diagram of the separator feeding mechanism of an extrusion coating machine for polymer battery production proposed in this invention;

[0022] Figure 6 This is a schematic diagram of the tensioner distribution structure of an extrusion coating machine for polymer battery production according to Embodiment 2 of the present invention;

[0023] Figure 7 This is a schematic diagram of the guide frame mechanism of an extrusion coating machine for polymer battery production proposed in this invention;

[0024] Figure 8This is a schematic diagram of the coating mechanism of an extrusion coating machine for polymer battery production proposed in this invention;

[0025] Figure 9 This is a cross-sectional view of the coating mechanism of an extrusion coating machine for polymer battery production proposed in this invention.

[0026] Figure 10 This is a schematic diagram of the homogenizing chamber structure of an extrusion coating machine for polymer battery production proposed in this invention;

[0027] Figure 11 This is a schematic diagram of the coating gate structure of an extrusion coating machine for polymer battery production proposed in this invention.

[0028] In the diagram: 1. Frame, 2. Front cover, 3. Cleaning mechanism, 31. Cleaning housing, 32. Dust removal chamber, 33. Exhaust chamber, 34. Transition duct, 35. Matrix fan, 36. Dust filter, 4. Middle cover, 5. Drying mechanism, 51. Drying chamber, 52. Fan, 53. Waste gas absorber, 54. Electric push rod, 55. Baking hood, 56. Heater, 6. Rear cover, 7. Diaphragm feeding mechanism, 71. Conveyor roller, 72. Feeding belt, 73. Support plate, 7 4. Limiting chute, 75. Limiting bar, 76. Strip magnet, 77. Chain magnet, 78. Magnetic shielding plate, 79. Tensioning component, 8. Guide frame mechanism, 81. Guide frame, 82. Movable support, 83. Positioning roller, 84. Adjusting push rod, 85. Stepper motor, 9. Coating mechanism, 91. Slurry storage shell, 92. Coating gate, 93. Rack and pinion, 94. Gearbox, 95. Homogenizing slurry plate, 96. L-shaped extrusion channel, 97. Grouting pipe, 98. Homogenizing cavity. Detailed Implementation

[0029] The embodiments of this patent are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this patent, and should not be construed as limiting this patent.

[0030] Example 1:

[0031] Reference Figure 1-5 and Figure 7-11An extrusion coating machine for polymer battery production includes a frame 1. A rotatable front cover 2, a middle cover 4, and a rear cover 6 are mounted on the top side of the frame 1. A cleaning mechanism 3 for cleaning the coating surface of the separator is mounted at the bottom of the front cover 2. A drying mechanism 5 for preliminary drying of the slurry on the separator surface is mounted at the bottom of the rear cover 6. Limiting guide rails are mounted on both sides of the frame 1, and a guide frame mechanism 8 is slidably mounted between the two limiting guide rails. A coating mechanism 9 is installed inside the guide frame mechanism 8. The outer side of the structure 9 is provided with a slurry storage shell 91. The bottom of the guide frame mechanism 8 is provided with two positioning rollers 83 for extruding diaphragms. The middle of the positioning roller 83 at the rear end is provided with an annular groove for passing the slurry. The inside of the slurry storage shell 91 is installed with a telescopic coating gate 92. An adjustable L-shaped extrusion channel 96 is provided between the coating gate 92 and the bottom end of the slurry storage shell 91. The top of the slurry storage shell 91 is provided with a grouting pipe 97 for replenishing slurry. The outer end of the grouting pipe 97 is connected to a grouting pump and a slurry storage tank.

[0032] The frame 1 is equipped with a diaphragm feeding mechanism 7, which consists of multiple conveying rollers 71 installed inside the frame 1 and a feeding belt 72 sleeved on the outside of the conveying rollers 71. The frame 1 is equipped with a drive motor that is connected to the conveying rollers 71. The diaphragm feeding mechanism 7 also includes a support plate 73 installed inside the frame 1. The surface of the support plate 73 is provided with multiple T-shaped limiting grooves 74. The inner side of the feeding belt 72 is provided with a limiting bar 75 that is adapted to the limiting grooves 74.

[0033] In this invention, the two ends of the diaphragm are moved by the unwinding device and the traction device, respectively. When the diaphragm enters the coating machine, it moves at the bottom of the guide frame mechanism 8 and the coating mechanism 9 under the drive of the feeding belt 72. When the feeding belt 72 moves the diaphragm, the limiting bar 75 at the bottom of the feeding belt 72 moves on the limiting groove 74 inside the support plate 73. With the cooperation of the limiting bar 75 and the limiting groove 74, and the action of the positioning roller 83 that squeezes the diaphragm, the stability of the diaphragm during movement is maintained, effectively avoiding fluctuations in the diaphragm during the coating process, which would cause uneven distribution of slurry on the diaphragm surface. Secondly, the slurry is squeezed downward inside the slurry storage shell 91. The movement of the coating gate 92 controls the width of the L-shaped extrusion channel 96, and the guide frame mechanism 8 controls the distance between the L-shaped extrusion channel 96 and the diaphragm. The slurry is squeezed out from the L-shaped extrusion channel 96 and evenly coated on the steadily moving diaphragm surface.

[0034] As a further embodiment of the present invention, the cleaning mechanism 3 includes a cleaning housing 31, and the interior of the cleaning housing 31 is provided with a dust removal chamber 32 and an exhaust chamber 33. A matrix fan 35 is installed between the dust removal chamber 32 and the exhaust chamber 33, and a dust filter 36 is installed at the inlet end of the matrix fan 35. A transition air duct 34 close to the upper surface of the battery separator is provided between the bottom ends of the dust removal chamber 32 and the exhaust chamber 33, and both ends of the transition air duct 34 have a C-shaped structure. In the present invention, the air inside the transition air duct 34, the dust removal chamber 32 and the exhaust chamber 33 circulates under the action of the rectangular fan 35. When the battery separator passes through the cleaning mechanism 3, the transition air duct 34 is close to the upper surface of the battery separator, and the air blows obliquely from one side against the direction of the upper surface of the separator, while the other side quickly draws in the blowing air, thereby using the airflow to quickly remove any impurities that may exist on the surface of the separator. Secondly, the airflow is used to smooth the upper surface of the separator, thereby improving the quality of subsequent slurry coating.

[0035] As a further embodiment of the present invention, the drying mechanism 5 includes a drying chamber 51, and two electric push rods 54 are installed inside the drying chamber 51. A baking hood 55 composed of multiple reflective arc plates is installed between the bottom ends of the two electric push rods 54, and a heater 56 is installed inside the baking hood 55. An exhaust gas absorber 53 is installed on the top of the drying chamber 51, and multiple fans 52 are installed at the inlet of the exhaust gas absorber 53. In the present invention, after the slurry is coated on the upper surface of the diaphragm, it passes through the drying mechanism 5. Under the action of the fans 52, the bottom opening of the drying mechanism 5 is in a slightly negative pressure state. The heater 56 and the baking hood 55 form a heat source on the diaphragm surface for drying, which accelerates the evaporation rate of the solvent in the slurry, accelerates the slurry setting effect, and effectively improves the stability of slurry adhesion and setting.

[0036] As a further embodiment of the present invention, the guide frame mechanism 8 includes two guide frames 81, and two movable supports 82 are rotatably mounted at the ends of the two guide frames 81. Two positioning rollers 83 are respectively installed between the two movable supports 82 on the same side. Stepper motors 85 for driving the positioning rollers 83 are installed on the outer surfaces of the two movable supports 82 at one end. An adjusting push rod 84 for adjusting the included angle is installed between the two movable supports 82 at the same end. In the present invention, the coating mechanism 9 is supported by the two positioning rollers 83 at the bottom of the guide frame mechanism 8. In application, the distance between the two positioning rollers 83 is controlled by adjusting the push rod 84, thereby adjusting the distance between the coating mechanism 9 and the bottom diaphragm. During the coating operation, the two ends of the coating area of ​​the diaphragm are expanded and flattened by the positioning rollers 83, so that the diaphragm is fully in contact with the surface of the feeding belt 72, further improving the uniformity and stability of the slurry coating process.

[0037] As a further embodiment of the present invention, a homogenizing cavity 98 is provided inside the slurry storage shell 91, and a coating gate 92 is distributed at the bottom of the homogenizing cavity 98. Two staggered homogenizing paddles 95 are installed at the upper end of the homogenizing cavity 98, and the two homogenizing paddles 95 rotate in opposite directions. A drive motor that drives the homogenizing paddles 95 to rotate is installed at the end of the slurry storage shell 91. In the present invention, the two homogenizing paddles 95 move synchronously in opposite directions. After the slurry enters the slurry storage shell 91, it is quickly dispersed by the two rotating homogenizing paddles 95. When the two homogenizing paddles 95 move downward, they provide downward pushing pressure to the slurry, so that the slurry pressure distribution in the area where the coating gate 92 is located is uniform, thereby ensuring that the thickness and speed distribution of the slurry extruded in the L-shaped extrusion channel 96 are uniform, and improving the uniformity of slurry distribution.

[0038] As a further embodiment of the present invention, the coating gate 92 is obliquely slidably connected inside the homogenization chamber 98. The top of the coating gate 92 is provided with a rack rod 93 extending to the outside of the slurry storage shell 91. A gearbox 94 that meshes with the rack rod 93 is installed on the outer side of the slurry storage shell 91, and a servo motor is installed on the side of the gearbox 94. In this invention, the coating gate 92 is obliquely slidably connected inside the homogenization chamber 98, and the coating gate 92 with its movable shell and the bottom end of the slurry storage shell 91 form an L-shaped extrusion channel 96 with adjustable width. This ensures that the slurry is always tangentially extruded while adhering to the upper surface of the diaphragm, quickly covering the upper surface of the diaphragm. The angle between the coating gate 92 and the slurry storage shell 91 forms multiple extrusion spaces with gradually decreasing widths. When the slurry moves downward and is extruded, the pressure is quickly dispersed during the flow process, improving the stability of the slurry extrusion pressure.

[0039] Example 2:

[0040] Reference Figure 1-11 An extrusion coating machine for polymer battery production is disclosed in this embodiment. Based on Embodiment 1, both sides of the limiting groove 74 are composed of strip magnets 76, and both sides of the limiting bar 75 are composed of chain magnets 77. The sliding surfaces of the strip magnets 76 and the chain magnets 77 are repulsive. In this invention, when the feeding belt 72 moves above the support plate 73, the chain magnets 77 enter below the strip magnets 76. Under the magnetic repulsion, the chain magnets 77 pull the limiting bar 75 and the feeding belt 72 to maintain a downward pull, thereby making the feeding belt 72 closely adhere to the upper surface of the support plate 73. When fluctuations occur, they are quickly eliminated under the repulsive force, further improving the stability of the separator movement in the coating operation area.

[0041] As a further embodiment of the present invention, a magnetic shielding plate 78 is installed at the bottom of the support plate 73, and a magnetic tensioning member 79 is installed at the bottom of the magnetic shielding plate 78. The bottom of the tensioning member 79 repels the magnetic poles of the chain magnetic steel strip 77. In the present invention, the bottom of the tensioning member 79 repels the magnetic poles of the chain magnetic steel strip 77, and thus a force is provided below the support plate 73 that always pushes the chain magnetic steel strip 77 and the feeding belt 72 downward, thereby keeping the feeding belt 72 in a taut state and maintaining the stability of the upper surface of the feeding belt 72.

[0042] When in use, the diaphragm enters the coating machine and moves at the bottom of the guide frame mechanism 8 and coating mechanism 9 under the drive of the feeding belt 72. When the feeding belt 72 moves with the diaphragm, the limiting bar 75 at the bottom of the feeding belt 72 moves on the limiting groove 74 inside the support plate 73. Then, the slurry is squeezed downward inside the slurry storage shell 91. The movement of the coating gate 92 controls the width of the L-shaped extrusion channel 96, and the guide frame mechanism 8 controls the distance between the L-shaped extrusion channel 96 and the diaphragm. The slurry is squeezed out from the L-shaped extrusion channel 96 and evenly coated on the steadily moving diaphragm surface.

[0043] The above are merely preferred embodiments 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. An extrusion coating machine for polymer battery production, comprising a frame (1), wherein a rotatable front end cover (2), a middle end cover (4), and a rear end cover (6) are mounted on the top side of the frame (1), characterized in that, The bottom of the front cover (2) is equipped with a cleaning mechanism (3) for cleaning the coating surface of the diaphragm, and the bottom of the rear cover (6) is equipped with a drying mechanism (5) for preliminary drying of the slurry on the diaphragm surface. Limiting guide rails are installed on both sides of the frame (1), and a guide frame mechanism (8) is slidably installed between the two limiting guide rails. A coating mechanism (9) is installed inside the guide frame mechanism (8), and a slurry storage shell (91) is provided on the outside of the coating mechanism (9). The bottom of the guide frame mechanism (8) is... The positioning roller (83) is provided with two extrusion diaphragms, and the positioning roller (83) at the rear end is provided with an annular groove for passing through the slurry. The slurry storage shell (91) is equipped with a telescopic coating gate (92), and an adjustable L-shaped extrusion channel (96) is provided between the coating gate (92) and the bottom end of the slurry storage shell (91). The top of the slurry storage shell (91) is equipped with a grouting pipe (97) for replenishing slurry, and the outer end of the grouting pipe (97) is connected to a grouting pump and a slurry storage tank. The frame (1) is equipped with a diaphragm feeding mechanism (7), which consists of multiple conveying rollers (71) installed inside the frame (1) and a feeding belt (72) sleeved on the outside of the conveying rollers (71). The frame (1) is equipped with a drive motor that is connected to the conveying rollers (71). The diaphragm feeding mechanism (7) also includes a support plate (73) installed inside the frame (1). The surface of the support plate (73) is provided with multiple T-shaped limiting grooves (74). The inner side of the feeding belt (72) is provided with a limiting bar (75) that is compatible with the limiting grooves (74). The guide frame mechanism (8) includes two guide frames (81), and two movable brackets (82) are rotatably mounted at the ends of the two guide frames (81). The two positioning rollers (83) are respectively installed between the two movable brackets (82) on the same side. The outer surfaces of the two movable brackets (82) at one end are each equipped with a stepper motor (85) for driving the positioning rollers (83). An adjusting push rod (84) for adjusting the included angle is installed between the two movable brackets (82) at the same end.

2. The extrusion coating machine for polymer battery production according to claim 1, characterized in that, The cleaning mechanism (3) includes a cleaning housing (31), and the interior of the cleaning housing (31) is provided with a dust removal chamber (32) and an exhaust chamber (33). A matrix fan (35) is installed between the dust removal chamber (32) and the exhaust chamber (33), and a dust filter (36) is installed at the inlet end of the matrix fan (35). A transition air duct (34) close to the upper surface of the battery separator is provided between the bottom ends of the dust removal chamber (32) and the exhaust chamber (33), and both ends of the transition air duct (34) present a C-shaped structure.

3. The extrusion coating machine for polymer battery production according to claim 1, characterized in that, The drying mechanism (5) includes a drying chamber (51), and two electric push rods (54) are installed inside the drying chamber (51). A baking hood (55) composed of multiple reflective arc plates is installed between the bottom ends of the two electric push rods (54), and a heater (56) is installed inside the baking hood (55). An exhaust gas absorber (53) is installed on the top of the drying chamber (51), and multiple fans (52) are installed at the inlet of the exhaust gas absorber (53).

4. The extrusion coating machine for polymer battery production according to claim 1, characterized in that, The slurry storage shell (91) has a homogenization chamber (98) inside, and a coating gate (92) is distributed at the bottom of the homogenization chamber (98). Two staggered homogenization paddles (95) are installed at the upper end of the homogenization chamber (98), and the two homogenization paddles (95) rotate in opposite directions. A drive motor that drives the homogenization paddles (95) to rotate is installed at the end of the slurry storage shell (91).

5. The extrusion coating machine for polymer battery production according to claim 4, characterized in that, The coating gate (92) is obliquely slidably connected inside the homogenization chamber (98). The top of the coating gate (92) is provided with a rack rod (93) extending to the outside of the slurry storage shell (91). A gearbox (94) that meshes with the rack rod (93) is installed on the outer side of the slurry storage shell (91), and a servo motor is installed on the side of the gearbox (94).

6. The extrusion coating machine for polymer battery production according to claim 1, characterized in that, Both sides of the limiting groove (74) are composed of strip magnets (76), and both sides of the limiting bar (75) are composed of chain magnets (77). The sliding surfaces of the strip magnets (76) and the chain magnets (77) are repulsive.

7. The extrusion coating machine for polymer battery production according to claim 6, characterized in that, The bottom of the support plate (73) is equipped with a magnetic shielding plate (78), and the bottom of the magnetic shielding plate (78) is equipped with a magnetic tensioning member (79), the bottom of the tensioning member (79) repelling the magnetic poles of the chain-shaped magnetic steel strip (77).

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