Preparation process of composite material plate roller

Abstract

The invention discloses a preparation technology of a composite material plate roller. The preparation technology comprises the following steps that glass fibers pre-coated with resin glue are wound into a glass fiber pipe layer; the glass fiber pipe layer is heated and cured for the first time, and after cooling, a buffer cotton layer is attached to the outer wall of the glass fiber pipe layer; winding a glass fiber layer with a preset thickness on the outer wall of the buffer cotton layer by using glass fibers to prepare a composite inner core; the composite inner core is subjected to secondary heating curing and cooled to the room temperature; turning the two ends of the composite inner core to set sizes; the composite inner core is coaxially arranged in the prefabricated carbon fiber tube layer, then the two ends of the composite inner core and the two ends of the prefabricated carbon fiber tube layer are closed, polyurethane is injected into the interval between the composite inner core and the prefabricated carbon fiber tube layer to form a polyurethane filling layer, and then natural curing is conducted at the room temperature to obtain a blank roller; the outer circle of the blank roller is ground; a plate cutting groove is cut in the outer circle of the blank roller in the axial direction of the blank roller.

Description

Technical Field

[0001] This application relates to the technical field of printing press plate cylinders, and specifically to a preparation process for composite material plate cylinders. Background Art

[0002] The plate cylinder is one of the core components of a printing press, mainly used to carry the printing plate and transfer the graphic information on the printing plate to the substrate. Most of the existing plate cylinders are made of aluminum rollers or nylon. Since aluminum, as a metal material, the plate cylinders made of it still have the disadvantage of large mass on high-speed printing presses, and the temperature resistance of nylon also has deficiencies. Therefore, it is necessary to solve the above problems and propose a preparation process for composite material plate cylinders to prepare composite material plate cylinders for printing presses. Summary of the Invention

[0003] The purpose of this application is to provide a composite material plate cylinder, which solves the problems in the prior art.

[0004] To achieve the above purpose, this application provides the following technical solutions: A preparation process for a composite material plate cylinder, including the following steps: A. Wind the glass fiber pre-coated with resin glue into a glass fiber tube layer 1 with a set inner diameter, wall thickness, and length; B. Conduct the first heating and curing on the glass fiber tube layer 1, and after cooling, attach a buffer cotton layer 2 to the outer wall of the glass fiber tube layer 1; C. Wind a glass fiber layer 3 with a preset thickness on the outer wall of the buffer cotton layer 2 with the glass fiber pre-coated with resin glue to obtain a composite inner core; D. Conduct the second heating and curing on the composite inner core obtained in step C and cool it to room temperature; E. Turn both ends of the composite inner core obtained in step D to set dimensions, and grind the outer circle of the glass fiber layer 3 to a set outer diameter and make its surface reach a preset surface roughness; F. Coaxially install the composite inner core obtained in step E into a prefabricated carbon fiber tube layer 5 and bond the two into one body to obtain a blank roll; G. Conduct grinding treatment on the outer circle of the blank roll; H. Cut a plate cutting groove 9 along the axial direction of the blank roll on its outer circle.

[0005] Preferably, the temperature of both the first heating and curing and the second heating and curing is 130°C to 200°C, and the curing time is 1 to 5 hours.

[0006] Preferably, the step of prefabricating the carbon fiber tube layer 5 includes first winding a preset thickness of T700 carbon fiber and curing it to form a carbon fiber matrix layer 7, and then attaching a layer of carbon cloth layer 6 to the outer wall of the carbon fiber matrix layer 7.

[0007] Preferably, let the outer diameter of the carbon fiber tube layer 5 be D2, the inner diameter of the glass fiber tube layer 1 be D1, the wall thickness of the carbon fiber tube layer 5 be B4, the wall thickness of the glass fiber layer 3 be B3, the wall thickness of the buffer cotton layer 2 be B2, and the wall thickness of the glass fiber tube layer 1 be B1; then, when 75 mm < D2 < 88 mm, in the step F, the inner wall of the carbon fiber tube layer 5 is tightly fitted with the outer wall of the composite inner core and bonded together with glue; when 88 mm < D2 < 192 mm, polyurethane is injected between the inner wall of the carbon fiber tube layer 5 and the outer wall of the composite inner core to form a polyurethane filling layer 4, and the thickness H of the polyurethane filling layer 4 = (D2 - D1) / 2 - (B4 + B3 + B2 + B1), and B3 + B2 + B1 < (D2 - D1) / 2 - B4. The step of injecting polyurethane includes: after coaxially installing the composite inner core into the prefabricated carbon fiber tube layer 5, the two ends of the composite inner core and the prefabricated carbon fiber tube layer 5 need to be closed, and then polyurethane is injected into the space between the composite inner core and the prefabricated carbon fiber tube layer 5 to form the polyurethane filling layer 4. After injection, it is naturally cured at room temperature for 0.5 to 2 hours to obtain the blank roll; Preferably, the end cap 8 for closing the two ends of the composite inner core and the carbon fiber tube layer 5 has a circular flat plate structure. First, install the composite inner core and one end cap 8 on the positioning mandrel, then put the carbon fiber tube layer 5 over the composite inner core, and make the same ends of the composite inner core and the carbon fiber tube layer 5 abut against the end cap 8. Then, put a sleeve 13 with an inner diameter adapted to the outer diameter of the end cap 8 and the outer diameter of the carbon fiber tube layer 5 over the carbon fiber tube layer 5 to keep the composite inner core and the carbon fiber tube layer 5 coaxial. Then, install the other end cap 8 into the sleeve 13 to complete the closing of the spaced open ends of the composite inner core and the carbon fiber tube layer 5. After injecting polyurethane and curing, the end cap 8, the sleeve 13, and the mandrel are removed to obtain the blank roll.

[0008] Preferably, the end cap 8 for closing the two ends of the composite inner core and the carbon fiber tube layer 5 has a circular plate structure with a positioning frustum 12. Let the height of the positioning frustum 12 be H, the length of the carbon fiber tube layer 5 be L, and the length of the composite inner core be N. Then, 2H = L - N; and the outer diameter of the positioning frustum is adapted to the inner diameter of the carbon fiber tube layer 5. During assembly, first install the composite inner core and one end cap 8 on the positioning mandrel, then put the carbon fiber tube layer 5 over the composite inner core, and make the same ends of the composite inner core and the carbon fiber tube layer 5 abut against the end cap 8, where the carbon fiber tube layer 5 is sleeved on the positioning frustum 12. Then, put the other end cap 8 over the positioning mandrel to close the spaced open ends of the composite inner core and the carbon fiber tube layer 5. After injecting polyurethane and curing, the mandrel is removed to obtain the blank roll, and the end cap 8 becomes a part of the composite material plate roll.

[0009] Preferably, an injection hole 10 is provided on one of the end caps 8 for injecting polyurethane, and a plug 11 is also provided for detachably plugging the injection hole 10.

[0010] Advantages of the present application: For the composite material plate roller provided by the present application, the carbon fiber tube layer is set as the surface layer directly bearing the printing plate, which has the advantages of high temperature resistance, light weight, and high dimensional accuracy; the broken fiber tube layer is set as the combined inner layer directly connected to the transmission mechanism, which has the advantages of high connection strength and light weight; at the same time, through the provided composite buffer layer, the vibration energy generated by high-speed printing can be effectively absorbed and reduced, ensuring the smoothness of high-speed printing. Description of the Drawings

[0011] Figure 1 is a three-dimensional view of the composite material plate roller of the present application; Figure 2 is a cross-sectional view of the first embodiment of the composite material plate roller of the present application; Figure 3 is a cross-sectional view of the second embodiment of the composite material plate roller of the present application; In the figure: 1, glass fiber tube layer; 2, buffer cotton layer; 3, glass fiber layer; 4, polyurethane filling layer; 5, carbon fiber tube layer; 6, carbon cloth layer; 7, carbon fiber matrix layer; 8, end cap; 9, plate cutting groove; 10, injection hole; 11, plug; 12, positioning round table; 13, sleeve. Specific Embodiments

[0012] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0013] Please refer to Figures 1 - 3 , a preparation process of a composite material plate roller, including the following steps: A. Wind the glass fiber pre-coated with resin glue into a glass fiber tube layer 1 with a set inner diameter, wall thickness, and length; B. First heat-cure the glass fiber tube layer 1, and after cooling, attach a buffer cotton layer 2 to the outer wall of the glass fiber tube layer 1; C. Wind a glass fiber layer 3 with a preset thickness on the outer wall of the buffer cotton layer 2 with the glass fiber pre-coated with resin glue to obtain a composite inner core; D. Second heat-cure the composite inner core obtained in step C and cool it to room temperature; E. Turning both ends of the composite inner core obtained in step D to a set size, and grinding the outer diameter of the glass fiber layer 3 to a set outer diameter and to achieve a preset surface roughness; F. Coaxially inserting the composite inner core obtained in step E into the prefabricated carbon fiber tube layer 5 and bonding the two together to form a blank roller; G. Grinding the outer circle of the blank roller; H. Cut a plate cutting groove 9 on the outer circle of the blank roller along its own axial direction; in this way, after the printing plate is attached to the surface of the carbon fiber tube layer 5, the head and tail positions of the printing plate are located at the plate cutting groove 9. Since there must be dimensional errors when the printing plate is made, when the head and tail edges of the printing plate overlap, the excess part of the printing plate can be cut away along the plate cutting groove 9 to ensure that the printing plate is tightly attached to the surface of the carbon fiber tube layer 5, and the junction is smooth without protrusions, thereby ensuring printing quality and facilitating the replacement of the printing plate, which is conducive to improving the convenience of using the composite material plate roller and thereby improving the production efficiency of the printing machine.

[0014] According to the composite plate roller prepared by the above process provided in this embodiment, the carbon fiber tube layer 5 is set as the surface layer directly bearing the printing plate, which has the advantages of high temperature resistance, light weight and high dimensional accuracy; the broken fiber tube layer 1 is set as a bonding inner layer directly connected to the transmission mechanism, which has the advantages of high connection strength and light weight; at the same time, the buffer cotton layer 2 wrapped around the outer wall of the glass fiber tube layer 1, the polyurethane filling layer 4 connected to the carbon fiber tube layer 5, and the glass fiber layer 3 formed between the buffer cotton layer 2 and the polyurethane filling layer 4 constitute a composite buffer layer, that is, the composite buffer layer constitutes a sandwich-type composite structure, which can ensure better vibration absorption and buffering effect, and avoid excessive elastic shaking between the inner and outer layers. It can effectively absorb and reduce the vibration energy generated by high-speed printing, and ensure the stability of high-speed printing.

[0015] In another embodiment of the present application, the temperature of the first heating curing and the second heating curing are both 130°C~200°C, and the curing time is 1~5 hours; as a preferred embodiment, the temperature of the first heating curing and the second heating curing are preferably 150°C, and the curing time is preferably 2.5 hours. In this way, the glass fiber tube layer 1 and the glass fiber layer 3 can obtain higher strength, and the glass fiber tube layer 1, the buffer cotton layer 2, and the glass fiber layer 3 can be more tightly combined.

[0016] In another embodiment of this application, please refer to Figures 1 to 3 The step of prefabricating the carbon fiber tube layer 5 includes first winding T700 carbon fiber to a preset thickness and curing it to form a carbon fiber matrix layer 7, and then laminating a layer of carbon cloth layer 6 on the outer wall of the carbon fiber matrix layer 7. It can be understood that the outer diameter and thickness of the carbon fiber tube layer 5 can be selected by those skilled in the art according to the specific requirements of the printing press color group, and are not specifically limited here.

[0017] According to the above structure provided in this embodiment, the carbon fiber matrix layer 7 formed by winding and curing carbon fiber in a set manner has the advantage of high strength. And a layer of carbon cloth layer 6 is attached to the carbon fiber matrix layer 7. On the one hand, the pattern with 3D visual characteristics obtained may have strong visual beauty. On the other hand, the surface roughness value is lower, and a smooth support surface is obtained, which is beneficial to improving the printing quality. When using T700 carbon fiber filaments, while obtaining high strength, it has a better cost advantage. Here, the T700 carbon fiber filaments are pre-coated with an adhesive curing agent, and its curing method can adopt the existing technology. The carbon cloth used for the carbon cloth layer 6 can be selected as 3D carbon fiber cloth, that is, carbon fiber three-dimensional fabric.

[0018] In another embodiment of the present application, please refer to Figures 1 to 3, assuming that the outer diameter of the carbon fiber tube layer 5 is D2, the inner diameter of the glass fiber tube layer 1 is D1, the wall thickness of the carbon fiber tube layer 5 is B4, the wall thickness of the glass fiber layer 3 is B3, the wall thickness of the buffer cotton layer 2 is B2, and the wall thickness of the glass fiber tube layer 1 is B1; then, when 75mm<D2<88mm, such as D2 is 75.867mm, 77.888mm, 79.909mm, 80.920mm, 87.994mm, in step F, the inner wall of the carbon fiber tube layer 5 is tightly matched with the outer wall of the composite inner core and bonded together by glue. At this time, due to the small outer diameter of the entire plate roller, the separation generated by high-speed printing is The centrifugal force and vibration are small, and the composite buffer layer composed of the glass fiber tube layer 1, the buffer cotton layer 2, and the glass fiber layer 3 can achieve better vibration absorption and vibration reduction effects, and the structure is simple and the reliability is good; when 88mm<D2<192mm, such as D2 is 89.005mm, 110.227mm, 118.312mm, 140.545mm, 152.672mm, 162.778mm, 177.937mm, 191.075mm, polyurethane is injected between the inner wall of the carbon fiber tube layer 5 and the outer wall of the composite inner core to form a polyurethane filling layer 4, and the polyurethane filling layer 4 Thickness H=(D2-D1) / 2-(B4+B3+B2+B1), and B3+B2+B1<(D2-D1) / 2-B4, a sandwich-type double composite structure is formed by the glass fiber tube layer 1, the buffer cotton layer 2, the glass fiber layer 3, and the polyurethane filling layer 4, which can ensure a good vibration absorption and buffering effect, and avoid excessive elastic shaking between the inner and outer layers. At the same time, it also significantly reduces the overall weight, thereby ensuring high-speed printing quality; wherein the step of injecting polyurethane includes: after the composite inner core is coaxially loaded into the prefabricated carbon fiber tube layer 5, the composite inner core and the prefabricated carbon fiber need to be connected. The two ends of the tube layer 5 are closed, and then polyurethane is injected into the gap between the composite inner core and the prefabricated carbon fiber tube layer 5 to form a polyurethane filling layer 4. After the injection is completed, it is naturally cured at room temperature for 0.5 to 2 hours to obtain a blank roller. As a preferred embodiment, the natural curing time is 1 hour; here, the outer diameter D2 of the carbon fiber tube layer 5, the inner diameter D1 of the glass fiber tube layer 1, the wall thickness B4 of the carbon fiber tube layer 5, the wall thickness B3 of the glass fiber layer 3, the wall thickness B2 of the buffer cotton layer 2, and the wall thickness B1 of the glass fiber tube layer 1 can be specifically set by technicians in this technical field according to the actual working conditions of the printing color group of the printing press, and are not limited here.

[0019] In another embodiment of this application, please refer to Figures 1 to 2, the end caps 8 used to seal both ends of the composite inner core and the carbon fiber tube layer 5 are of a circular flat plate structure. During assembly, first, the composite inner core and one end cap 8 are installed on a positioning mandrel (not shown), then the carbon fiber tube layer 5 is sleeved outside the composite inner core, and the same ends of the composite inner core and the carbon fiber tube layer 5 are abutted against the end cap 8. Then, a sleeve 13 with an inner diameter adapted to the outer diameters of the end cap 8 and the carbon fiber tube layer 5 respectively is sleeved outside the carbon fiber tube layer 5 to keep the composite inner core and the carbon fiber tube layer 5 coaxial. Then, the other end cap 8 is installed into the sleeve 13 to complete the sealing of the spaced open ends of the composite inner core and the carbon fiber tube layer 5. After injecting polyurethane and curing it, the end cap 8, the sleeve 13, and the mandrel (not shown) are removed to obtain a blank roller.

[0020] In another embodiment of the present application, please refer to Figure 1 and Figure 3 , the end cap 8 used to seal both ends of the composite inner core and the carbon fiber tube layer 5 is a circular plate structure with a positioning frustum 12. Let the height of the positioning frustum 12 be H, the length of the carbon fiber tube layer 5 be L, and the length of the composite inner core be N. Then, 2H = L - N; and the outer diameter of the positioning frustum is adapted to the inner diameter of the carbon fiber tube layer 5. During assembly, first, the composite inner core and one end cap 8 are installed on a positioning mandrel (not shown), then the carbon fiber tube layer 5 is sleeved outside the composite inner core, and the same ends of the composite inner core and the carbon fiber tube layer 5 are abutted against the end cap 8, where the carbon fiber tube layer 5 is sleeved on the positioning frustum 12. Then, the other end cap 8 is sleeved on the positioning mandrel (not shown) to seal the spaced open ends of the composite inner core and the carbon fiber tube layer 5. After injecting polyurethane and curing it, the mandrel (not shown) is removed to obtain a blank roller, and the end cap 8 becomes a part of the composite material plate roller. According to the above structure provided in this embodiment, the height H of the frustum 12 usually takes 0.5 - 3 mm, preferably H takes 2 mm. The length L of the carbon fiber tube layer 5 can be determined according to the requirements of the printing width of the printing press. After the length L of the carbon fiber tube layer 5 and the height H value of the frustum 12 are determined, the length N of the composite inner core can be obtained. The end cap 8 in this embodiment is made of aluminum alloy material, which is beneficial to enhancing the overall strength of the composite material plate roller and improving the high-speed printing performance of the printing press. Here, it can be understood that the mandrel can adopt the existing technical structure in this technical field. It can be understood that the end cap 8 has a through hole adapted to the inner diameter of the glass fiber tube layer 1 for the mandrel to pass through, and the outer diameter of the mandrel is also adapted to the glass fiber tube layer 1.

[0021] Preferably, an injection hole 10 for injecting polyurethane is formed in one end cap 8, and a plug 11 is further provided which is detachable and used to block the injection hole 10. According to the above structure provided by the present application, after the polyurethane injection is completed, the injection hole 10 can be effectively blocked with the plug 11 in time, which can effectively prevent the polyurethane from escaping and ensure the integrity of the polyurethane filling layer 4 formed after filling. At the same time, in the embodiment where the end cap 8 becomes a part of the composite material plate roller, the plug 11 is also used to make up the weight at the injection hole 10 to ensure the dynamic balance performance of the finished plate roller.

[0022] It should be noted that in this text, the term "including", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or further includes elements inherent to such process, method, article or device.

[0023] In the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "inner", "outer", "both ends", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present application.

[0024] At the same time, the content not described in detail in this specification belongs to the prior art well known to those skilled in the art.

[0025] Although the embodiments of the present application have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principle and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A preparation process for a composite plate roller, characterized in that: It includes the following steps: A. Wind the glass fiber pre-coated with resin glue into a glass fiber tube layer (1) with a set inner diameter, wall thickness and length; B. Conduct the first heating and curing on the glass fiber tube layer (1), and after cooling, attach a buffer cotton layer (2) to the outer wall of the glass fiber tube layer (1); C. Wind a glass fiber layer (3) with a preset thickness on the outer wall of the buffer cotton layer (2) with the glass fiber pre-coated with resin glue to obtain a composite inner core; D. Conduct the second heating and curing on the composite inner core obtained in step C and cool it to room temperature; E. Turn the two ends of the composite inner core obtained in step D to set dimensions, and grind the outer circle of the glass fiber layer (3) to a set outer diameter and make its surface reach a preset surface roughness; F. Coaxially install the composite inner core obtained in step E into a prefabricated carbon fiber tube layer (5) and bond the two together to obtain a blank roll; G. Conduct grinding treatment on the outer circle of the blank roll; H. Cut a cutting plate groove (9) along the axial direction of the blank roll on its outer circle.

2. The preparation process of the composite material plate roll according to claim 1, characterized in that: The temperatures of the first heating and curing and the second heating and curing are both 130°C to 200°C, and the curing time is 1 to 5 hours.

3. The preparation process of the composite material plate roller according to claim 1, characterized in that: The step of prefabricating the carbon fiber tube layer (5) includes first winding a preset thickness with T700 carbon fiber and curing to form a carbon fiber matrix layer (7), and then attaching a layer of carbon cloth layer (6) to the outer wall of the carbon fiber matrix layer (7).

4. The preparation process of the composite material plate roller according to claim 1, characterized in that: Let the outer diameter of the carbon fiber tube layer (5) be D2, the inner diameter of the glass fiber tube layer (1) be D1, the wall thickness of the carbon fiber tube layer (5) be B4, the wall thickness of the glass fiber layer (3) be B3, the wall thickness of the buffer cotton layer (2) be B2, and the wall thickness of the glass fiber tube layer (1) be B1; then, when 75mm < D2 < 88mm, in step F, the inner wall of the carbon fiber tube layer (5) is tightly fitted with the outer wall of the composite inner core and bonded together by glue; when 88mm < D2 < 192mm, polyurethane is injected between the inner wall of the carbon fiber tube layer (5) and the outer wall of the composite inner core to form a polyurethane filling layer (4), and the thickness H of the polyurethane filling layer (4) = (D2 - D1) / 2 - (B4 + B3 + B2 + B1), and B3 + B2 + B1 < (D2 - D1) / 2 - B4. The step of injecting polyurethane includes: after coaxially installing the composite inner core into the prefabricated carbon fiber tube layer (5), the two ends of the composite inner core and the prefabricated carbon fiber tube layer (5) need to be sealed, and then polyurethane is injected into the space between the composite inner core and the prefabricated carbon fiber tube layer (5) to form the polyurethane filling layer (4). After injection, it is naturally cured at room temperature for 0.5 to 2 hours to obtain the blank roll.

5. The preparation process of the composite material plate roller according to claim 4, characterized in that: The end caps (8) used to seal both ends of the composite inner core and the carbon fiber tube layer (5) are of a circular flat plate structure. First, the composite inner core and one end cap (8) are installed on a positioning mandrel. Then, the carbon fiber tube layer (5) is sleeved outside the composite inner core, and the composite inner core and the carbon fiber tube layer (5) are abutted against the end cap (8) at the same end. Next, a sleeve (13) with an inner diameter adapted to the outer diameter of the end cap (8) and the outer diameter of the carbon fiber tube layer (5) is sleeved outside the carbon fiber tube layer (5) to keep the composite inner core and the carbon fiber tube layer (5) coaxial. Then, the other end cap (8) is inserted into the sleeve (13) to complete the sealing of the spaced open ends of the composite inner core and the carbon fiber tube layer (5). After injecting and curing the polyurethane, the end cap (8), the sleeve (13), and the mandrel are removed to obtain the blank roll.

6. The preparation process of the composite material plate roller according to claim 4, characterized in that: The end caps (8) used to seal both ends of the composite inner core and the carbon fiber tube layer (5) are of a circular plate structure with a positioning frustum (12). Let the height of the positioning frustum (12) be H, the length of the carbon fiber tube layer (5) be L, and the length of the composite inner core be N. Then, 2H = L - N; and the outer diameter of the positioning frustum (12) is adapted to the inner diameter of the carbon fiber tube layer (5). During assembly, first, the composite inner core and one end cap (8) are installed on a positioning mandrel. Then, the carbon fiber tube layer (5) is sleeved outside the composite inner core, and the composite inner core and the carbon fiber tube layer (5) are abutted against the end cap (8) at the same end, where the carbon fiber tube layer (5) is sleeved on the positioning frustum (12). Then, the other end cap (8) is sleeved on the positioning mandrel to seal the spaced open ends of the composite inner core and the carbon fiber tube layer (5). After injecting and curing the polyurethane, the mandrel is removed to obtain the blank roll, and the end cap (8) becomes part of the composite material plate roll.

7. The preparation process of the composite material plate roller according to any one of claims 5 to 6, characterized in that: One of the end caps (8) is provided with an injection hole (10) for injecting polyurethane and a plug (11) that is detachable and used to block the injection hole (10).

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