A method for preparing a modified polytetrafluoroethylene check ring based on secondary hot pressing

Abstract

This invention discloses a method for preparing modified polytetrafluoroethylene (PTFE) retaining rings based on secondary hot pressing, comprising the following steps: Step S1: Raw material preparation: Selecting PTFE suspension fine powder with an average particle size ≤15μm as the substrate; selecting nano-sized tin bronze powder, chopped glass fiber, molybdenum disulfide, and ceramic powder as modified fillers; Step S2: Mixing: By weight percentage, dry mixing of 50-60% PTFE powder, 30-45% nano-tin bronze powder, 3-8% glass fiber, 1-5% ceramic powder, and 0.5-3% molybdenum disulfide to obtain a uniform mixture; Step S3: Primary molding; Step S4: Primary sintering; Step S5: Secondary molding; Step S6: Secondary sintering; Step S7: Constant temperature precision machining; Step S8: Inspection and packaging. The beneficial effect of this invention is to solve the problem of the difficulty in maintaining stable dimensional accuracy of highly filled PTFE composite products due to uneven internal density and thermal expansion over a long period.

Description

Technical Field

[0001] This invention relates to the field of composite materials technology, and more specifically, to a method for preparing modified polytetrafluoroethylene retaining rings based on secondary hot pressing. Background Technology

[0002] Hydraulic components are fundamental parts of modern equipment manufacturing, and their performance directly affects the reliability and service life of equipment. Retaining rings, as key auxiliary seals in hydraulic systems, although auxiliary structures, directly impact the sealing performance, wear resistance, and long-term operational stability of the hydraulic system. While traditional polytetrafluoroethylene (PTFE) materials possess excellent self-lubricating properties and chemical corrosion resistance, their low mechanical strength, insufficient wear resistance, and high coefficient of thermal expansion make them unsuitable for high-load, high-precision, and extreme operating conditions.

[0003] To improve the overall performance of polytetrafluoroethylene (PTFE), existing technologies typically employ filler modification methods, such as adding glass fiber, graphite, molybdenum disulfide, or metal powder. However, these modified materials often fail to significantly enhance PTFE's compressive strength, wear resistance, and dimensional stability while maintaining its inherent excellent properties. Especially under high temperature, high pressure, and low-oil or oil-free operating conditions, traditional modified PTFE retaining rings are prone to deformation, accelerated wear, and seal failure, affecting the normal operation of equipment.

[0004] Chinese invention patent application number 201811248603.8 discloses a method for manufacturing polytetrafluoroethylene (PTFE) gaskets. The method involves uniformly mixing PTFE, glass fiber, zinc oxide, carbon fiber, bronze powder, talc powder, isopropyl trioctanoyl titanate, and molybdenum disulfide. The mixture is then placed in a mold and pressurized to a pressure of 18-25 MPa. The mixture is then sintered in a furnace at 300-350°C. After air cooling for 1-2 hours, it is reheated to 350-370°C. Following this reheating, the temperature is maintained for 1-2 hours, and the mixture is pressurized again to a pressure of 1... At 7-20 MPa, the obtained polytetrafluoroethylene (PTFE) is subjected to early and late pulverization. The resulting PTFE micropowder is then mixed with an atomized organic liquid in deionized water in a mixer for granulation. Initially, the temperature of the mixture of deionized water, PTFE micropowder, and organic liquid is 15-30°C. After stirring for 7-15 minutes, the temperature is raised to 4-20°C above the boiling point of the mixture of organic liquid and water and maintained for 15-30 minutes. The resulting PTFE gasket is then cooled at room temperature for 1-2 hours before being air-cooled for shaping.

[0005] However, the shortcomings of this existing technology are as follows: its "secondary pressurization" process is applied to PTFE micro powder that has been sintered and pulverized and granulated after the initial sintering. The entire process route is designed to prepare gasket raw material particles with specific properties, rather than to densify the already formed retaining ring blank for manufacturing retaining ring parts with high dimensional accuracy. Furthermore, this method does not involve "secondary high-temperature and high-pressure molding" of the final sintered blank to eliminate density gradients, nor does it mention precision machining in a constant temperature environment to control PTFE thermal deformation and eliminate material thermal deformation errors.

[0006] Chinese invention patent application number 201310426401.9 discloses a method for manufacturing reinforced polytetrafluoroethylene (PTFE) for high and medium pressure valve seals. The process route is as follows: material preparation → drying → mixing → molding and compaction → sintering → machining → inspection and warehousing. The mass ratio of various components is as follows: high-strength nano-carbon fiber, 0.5-1.5; molybdenum disulfide powder, 3-5; tin bronze powder, 20-25; PTFE powder, and may also add 65-75; 3-5mm long reinforcing glass fiber, 5-10; silica powder, 10-15; graphite powder, 1-2; white corundum powder, 1-3; green... Silicon carbide powder, 1-3, one or more of the above, dispersant 5040: 1% by weight of all raw materials; the matrix material of this invention is polytetrafluoroethylene, which is readily available and inexpensive, and the copper mass percentage in the finished product is less than 1 / 4, greatly reducing copper consumption; it also reduces processing volume, and more importantly, reduces the amount of processing waste; the molybdenum disulfide added to the ingredients improves the wear resistance and self-lubricating properties of the billet, which will greatly improve the sealing ability and service life of the workpiece; the workpiece is corrosion-resistant, aging-resistant, and non-conductive, making this formulation extremely suitable for workpieces with a certain degree of insulation requirements.

[0007] However, the shortcomings of this existing technology are: its filler system is too complex, introducing a variety of hard wear-resistant phases. Although it may improve local wear resistance, the coexistence of a large number of fillers with different properties and shapes will deteriorate the interfacial bonding and stress distribution of the material system, which is not conducive to achieving high and balanced overall mechanical properties, especially making it difficult to achieve both high compressive strength and high elongation at break. Moreover, the preparation process of this existing technology is the most conventional one-step molding route of "mixing → molding → sintering → processing". This traditional process will inevitably lead to a density gradient in the pressing direction of the billet. After sintering, the internal residual stress of the product is large and the dimensional stability is poor. It cannot meet the stringent requirements of hydraulic retaining rings for maintaining dimensional accuracy during long-term service.

[0008] Chinese invention patent application number 201810512371.6 discloses a processing method for polytetrafluoroethylene (PTFE) material. This method uses suspended PTFE powder as raw material. First, the suspended PTFE powder is added to a molding die at room temperature. The die is heated to 120-200°C, then pressurized to 200-600 MPa within 2 minutes and maintained at constant temperature and pressure for 3-30 minutes. Then, the die temperature is reduced to 30°C at a cooling rate of 1-10°C / min using circulating water cooling. The pressure is then released, and the preform sample is placed at room temperature for at least 12 hours. Finally, the preform sample is sintered at 360°C for 6 minutes. The PTFE produced by the molding process provided by this invention has high melting point, crystallinity, glass transition temperature, and density. The melting point and crystallinity can reach up to approximately 350°C and 97.5%, respectively, the glass transition temperature can reach up to approximately 130°C, and the density can reach approximately 2.6 g / cm³.

[0009] However, the shortcomings of this existing technology lie in the fact that its process is essentially a one-step high-pressure pretreatment for pure PTFE or unsintered mixed powders. Its purpose is to improve the density, melting point, and other basic physical properties of the final product by altering the initial crystallization behavior of PTFE. However, this high-pressure step is applied to loose powders that have not undergone any sintering, and it cannot solve the critical secondary problem in manufacturing high-precision parts: the internal density inhomogeneity and shape distortion of the green body caused by shrinkage and crystal phase changes after the first sintering. It lacks the crucial step of secondary hot-pressing densification of the pre-shaped but density-inhomogeneous green body after the initial sintering, as described in this invention. Therefore, the consistency and dimensional accuracy of its products are still limited by the quality of the initial sintering.

[0010] Chinese invention patent application number 202110994028.1 discloses a polytetrafluoroethylene (PTFE) composite material, its preparation method, and its application. The PTFE composite material provided by this invention comprises, by weight, the following raw materials: 60-70 parts PTFE resin; 20-30 parts tin bronze powder; 5-10 parts polyimide; 1-5 parts glass fiber; 1-5 parts graphite; 0.5-2 parts inorganic nanofiller; and 0.05-0.2 parts silane coupling agent. The particle size of the PTFE resin, tin bronze powder, polyimide, glass fiber, and graphite is in the micrometer range. The PTFE composite material possesses both high strength and toughness.

[0011] While the existing technology uses tin bronze and glass fiber as the main reinforcing phases, the addition of various modifiers such as polyimide and graphite, although each can improve a certain property, increases the instability of the system due to the complex component interfaces, making it difficult to achieve synergistic optimization of high strength, high wear resistance, and high toughness. In addition, the existing technology cannot resolve the contradiction between high strength and high dimensional stability. Specifically, the solution only focuses on improving static mechanical properties and does not specifically address the dimensional accuracy fluctuations caused by the high thermal expansion coefficient of PTFE during processing and service. Especially in mass production, the traditional one-time molding and sintering process is prone to causing uneven density inside the blank (dense at both ends and loose in the middle), resulting in poor dimensional consistency of the final product after heating or pressure, which cannot meet the assembly tolerance requirements of precision hydraulic systems.

[0012] In summary, a novel, integrated manufacturing method is needed to deeply integrate material formulation innovation with key process innovation in order to produce high-performance PTFE retaining rings that simultaneously meet a series of stringent requirements, including compressive strength ≥40MPa, tensile strength ≥20MPa, elongation at break ≥200%, coefficient of friction ≤0.20, wear mark width ≤5.5mm, and dimensional accuracy stable within ±0.05mm. This would meet the urgent need of high-end hydraulic equipment for long-life, high-reliability seals. Summary of the Invention

[0013] The purpose of this invention is to provide a method for preparing modified polytetrafluoroethylene retaining rings based on secondary hot pressing, so as to solve the problem that the dimensional accuracy of highly filled polytetrafluoroethylene composite products is difficult to maintain in the long term due to uneven internal density and thermal expansion.

[0014] To achieve the above objectives, the present invention provides the following technical solution:

[0015] A method for preparing a modified polytetrafluoroethylene retaining ring based on secondary hot pressing includes the following steps:

[0016] Step S1, Raw material preparation: Select polytetrafluoroethylene suspension fine powder with an average particle size ≤15μm as the base material; select nano-sized tin bronze powder, chopped glass fiber, molybdenum disulfide and ceramic powder with an average particle size of 80-150nm as the modified filler; the ceramic powder is selected from at least one of alumina, silicon carbide and silicon nitride.

[0017] Step S2, Mixing: By weight percentage, 50-60% of polytetrafluoroethylene powder, 30-45% of nano-tin bronze powder, 3-8% of glass fiber, 1-5% of ceramic powder and 0.5-3% of molybdenum disulfide are dry-mixed to obtain a uniform mixture.

[0018] Step S3, One-time molding: The mixture is placed into a mold and cold-pressed at room temperature (20-50MPa) to obtain a preliminary blank;

[0019] Step S4, First sintering: Place the billet in a sintering furnace and heat it to 360-380℃ using a programmed heating method, and hold it for sintering for 2-4 hours;

[0020] Step S5, Secondary molding: Place the blank after the first sintering back into the mold, heat to 120-200℃, and pressurize to 200-600MPa within 2 minutes, and hold the pressure for 3-30 minutes;

[0021] Step S6, Secondary sintering: Place the billet after secondary forming back into the sintering furnace and sinter at 360-380℃ for 1-3 hours;

[0022] Step S7, constant temperature precision machining: Place the billet after secondary sintering in a constant temperature environment, with the temperature controlled at 20±2℃, and perform machining to the design dimensions to obtain the polytetrafluoroethylene retaining ring.

[0023] Step S8, Inspection and Packaging: The finished PTFE retaining rings are inspected for size, appearance and performance. After passing the inspection, they are packaged and put into storage.

[0024] Further, step S2 includes adding 0.5-2% of a silane coupling agent to enhance the interfacial bonding between the filler and the substrate.

[0025] Further configured, in the first sintering process of step S4, the heating rate is 1-5℃ / min, and after holding the sintering at the temperature for 2-4 hours, it is programmed to cool to room temperature at a rate of 1-10℃ / min.

[0026] During the secondary sintering process in step S6, the sintering is held at a temperature for 1-3 hours, and then cooled to room temperature at a rate of 1-10℃ / minute.

[0027] Further configured, in step S5, after the pressure holding is completed, the mold is cooled to below 30°C using a circulating water cooling method, with a cooling rate of 1-10°C / minute.

[0028] Further configured, in step S7, the processing environment is a dust-free cleanroom, the processing equipment is a CNC machine tool, and the processing accuracy is controlled within ±0.05mm.

[0029] Further, the performance indicators of the prepared polytetrafluoroethylene retaining ring are: compressive strength ≥40MPa, tensile strength ≥20MPa, elongation at break ≥200%, coefficient of friction ≤0.20, and wear mark width ≤5.5mm.

[0030] A modified polytetrafluoroethylene (PTFE) retaining ring prepared according to the above-mentioned method has the following composition by weight percentage: PTFE 53.5%, nano-tin bronze powder 38%, glass fiber 5%, ceramic powder 2%, and molybdenum disulfide 1%.

[0031] Further configured, the modified polytetrafluoroethylene (PTFE) retaining ring is suitable for sealing and guiding components in hydraulic cylinders, engineering machinery, CNC machine tools, and oilfield equipment; the modified PTFE retaining ring can still maintain stable sealing performance and wear resistance under low-oil or oil-free operating conditions.

[0032] By adopting the above technical solution, the beneficial effects of the present invention are as follows:

[0033] 1. This invention employs a specific ratio of nano-sized tin bronze powder, chopped glass fiber, ceramic powder, and molybdenum disulfide to composite with a PTFE matrix. The nano-tin bronze powder serves as the main reinforcing phase, significantly improving compressive strength and thermal conductivity. The specific ratio of chopped glass fiber, while toughening the material, synergistically forms a reinforcing network with the ceramic powder and molybdenum disulfide, balancing wear resistance and lubrication. This achieves a balance between high strength, high wear resistance, and good toughness at the material's fundamental level, resulting in the PTFE retaining ring of this invention exhibiting an excellent combination of comprehensive performance: compressive strength ≥40MPa, tensile strength ≥20MPa, elongation at break ≥200%, and coefficient of friction ≤0.20.

[0034] 2. In terms of manufacturing process, this invention innovatively adopts an integrated process route of "one-time molding and sintering + two-time high-temperature and high-pressure molding + two-time sintering + constant-temperature precision machining". This process, through two-time high-temperature and high-pressure molding, effectively eliminates the density gradient of the initial blank, similar to "forging", achieving extreme density and uniformity of the material. The subsequent two-time sintering further eliminates internal stress and stabilizes the microstructure. Finally, precision machining is performed in a dust-free environment with strict temperature control (20±2℃), completely avoiding the problem of PTFE deformation due to its high thermal expansion coefficient during processing, thus ensuring extremely high dimensional accuracy (up to ±0.05mm) and excellent batch consistency of the product.

[0035] 3. The method described in this invention has clear steps, and the required equipment is all general or slightly modified equipment in the field of polymer composite material processing (such as molds with heating and pressurization functions). It does not require extremely expensive equipment such as hot isostatic pressing. It has strong process adaptability, and the entire process is stable and controllable, making it very suitable for large-scale, high-quality, and continuous industrial production. Detailed Implementation

[0036] A method for preparing a modified polytetrafluoroethylene retaining ring based on secondary hot pressing includes the following steps:

[0037] S1. Raw material preparation: Select polytetrafluoroethylene suspension fine powder with an average particle size ≤15μm as the base material, preferably 10-12μm, to ensure that it has the characteristics of high molecular weight and good initial tensile strength.

[0038] Nanoscale tin bronze powder, short-cut glass fibers with a length of 0.1-0.5 mm, molybdenum disulfide, and ceramic powder with a particle size ≤10 μm were selected as modifying fillers.

[0039] The average particle size of the nano-sized tin bronze powder is preferably 80-150 nm. Its nanoscale effect can form a more uniform and denser reinforcing network in the PTFE matrix, significantly improving the compressive strength and thermal conductivity of the composite material, which helps to dissipate heat during processing and use and reduce thermal deformation.

[0040] The preferred length of chopped glass fibers is 0.2-0.3 mm. If the fibers are too long, they are prone to agglomeration and stress concentration, while if they are too short, the toughening effect will be weakened.

[0041] The ceramic powder has a particle size ≤10μm and together with molybdenum disulfide (particle size 1-3μm) it forms a fine-scale wear-resistant and lubricating dual functional phase. The ceramic powder is selected from at least one of alumina, silicon carbide, and silicon nitride.

[0042] S2. Mixing: By weight percentage, add 50-60% of polytetrafluoroethylene powder, 30-45% of nano tin bronze powder, 3-8% of glass fiber, 1-5% of ceramic powder and 0.5-3% of molybdenum disulfide into a high-speed mixer and dry mix at high speed for 20-40 minutes at a speed of 250-350 r / min until the mixture is uniform.

[0043] Preferably, 0.5-2% silane coupling agent, more preferably 1.0-2.0%, can be added to improve the interfacial bonding between the inorganic filler and the PTFE matrix. The mechanism is as follows: the functional groups at one end of the silane coupling agent molecule react chemically or physically with the surface of the inorganic filler (tin bronze powder, glass fiber, ceramic powder), while the other end entangles or becomes compatible with the PTFE matrix, thus forming a strong "molecular bridge" between them. This step is the primary key to solving the problem of uneven mechanical properties caused by complex fillers and weak interfacial bonding in existing technologies, directly affecting the effectiveness of subsequent process steps and the upper limit of the final product's performance.

[0044] S3. One-time molding: The uniformly mixed material is loaded into a pre-cleaned retaining ring molding mold and cold-pressed at 20-50MPa at room temperature for 1-5 minutes to obtain a blank with a certain shape and strength. The pressure in this step should not be too high to avoid the premature formation of excessive stress gradient and density gradient in the blank.

[0045] S4. First Sintering: Carefully transfer the initial billet to a programmable temperature-controlled sintering furnace. The heating rate is one of the key control parameters in this step. Slowly raise the temperature to 360-380°C at a rate of 1-5°C / min (preferably 2-3°C / min) and hold it at this temperature for 2-4 hours. Slow heating allows the PTFE resin particles to melt, diffuse, and bond uniformly and fully at the sintering temperature of 360-380°C, forming a preliminary continuous phase and "encapsulating" and fixing the filler to form a pre-sintered body. If the temperature rises too quickly, the surface PTFE will melt prematurely, closing the pores and making it difficult for internal volatiles to escape, resulting in bubbles or cracks. After holding at this temperature for 2-4 hours, slowly cool to room temperature at a rate of 1-10°C / min. This slow cooling process helps release some thermal stress and reduce billet warping.

[0046] S5. Secondary Molding: The pre-sintered body after the first sintering is placed back into the precision mold. The entire mold is heated to 120-200℃, causing the PTFE material to enter a highly elastic state with good plastic flowability. Then, a high pressure of 200-600MPa is rapidly applied within 2 minutes and held for 3-30 minutes.

[0047] The mechanism of this step is as follows: for the micro-pores, uneven distribution of fillers, and density gradient caused by uneven shrinkage that still exist inside the green body after the first sintering (which cannot be avoided by traditional one-time molding process), the synergistic effect of high temperature (increasing fluidity) and high pressure (providing driving force) forces the PTFE matrix to undergo sufficient plastic flow, which drives the filler particles to rearrange, thus pressing the pores and bridging the density differences like "micro-forging", so that the green body reaches an extremely dense and uniform state close to the theoretical density, effectively eliminating the density gradient inside the initial green body and greatly improving the density and uniformity of the material;

[0048] After the pressure holding period ends, the mold is immediately cooled to below 30°C at a rate of 1-10°C / minute using circulating water cooling. This rapid cooling can "freeze" and solidify the highly dense state, preventing springback after pressure release.

[0049] S6. Secondary Sintering: The high-density green body, after secondary forming, is placed back into the sintering furnace for sintering. Secondary sintering is carried out at 360-380℃ for 1-3 hours. The purpose of this step differs from that of primary sintering: its main function is no longer shaping, but rather to eliminate the large amount of internal stress introduced by the ultra-high pressure plastic deformation in the previous stage. Simultaneously, it allows the PTFE molecular chains to undergo final crystallization and perfection on a more uniform and dense structural basis, thereby "stabilizing" the excellent microstructure obtained through secondary high-pressure forming, ultimately obtaining a sintered body with a uniform microstructure and stable performance. This step also requires the same slow heating and slow cooling conditions as in S4.

[0050] S7. Constant Temperature Precision Machining: The blank after secondary sintering is transferred to a constant temperature and dust-free processing workshop where the ambient temperature is strictly controlled at 20±2℃. Using a CNC lathe or machining center, the blank is precision turned, ground and other processed under constant temperature conditions until the dimensional accuracy required by the design drawings is achieved (usually within ±0.05mm).

[0051] Because PTFE and its composites have a large coefficient of thermal expansion, even small fluctuations in ambient temperature or processing temperature can directly lead to significant errors in machining dimensions. By establishing a constant-temperature machining environment, the influence of ambient temperature fluctuations on workpiece dimensions is completely isolated. At the same time, the constant-temperature condition also provides a benchmark for setting constant machining parameters (such as feed rate and speed). Combined with the precise control of CNC machine tools, the local temperature rise of the workpiece caused by cutting heat and the resulting thermal deformation can be systematically reduced to a minimum during machining, thereby ensuring that the dimensions of each machined retaining ring are stable within the tolerance zone and eliminating machining dimensional errors caused by the large coefficient of thermal expansion of the material.

[0052] This step is crucial in ensuring that the final product achieves ultra-high dimensional accuracy (±0.05mm) and batch consistency.

[0053] S8. Inspection and Packaging: The finished retaining rings are inspected for appearance, dimensions and key performance (such as hardness, compressive strength, tensile strength, elongation at break, coefficient of friction, etc.). Qualified products are cleaned and dustproofed before being put into storage.

[0054] The comprehensive performance of the modified polytetrafluoroethylene retaining ring prepared by this invention was systematically tested using the following national standards:

[0055] Tensile strength and elongation at break: Performed in accordance with GB / T1040.2-2006 Determination of tensile properties of plastics - Part 2: Test conditions for molded and extruded plastics;

[0056] Compressive strength: Tested according to GB / T1041-2008 Determination of compressive properties of plastics;

[0057] Friction coefficient and wear performance: Tested according to GB / T3960-2016 Plastics sliding friction and wear test method, using a ring-block test machine, with the grinding ring being GCr15 bearing steel (hardness HRC60±2), load 150N, sliding speed 0.4m / s, and tested for 60 minutes under room temperature dry and unlubricated conditions;

[0058] Wear mark width: The maximum width of the wear mark was measured according to GB / T12444-2006 Metallic Materials Wear Test Method Test Ring-Block Sliding Wear Test;

[0059] Dimensional accuracy: The key dimensions of the finished retaining ring are measured using a coordinate measuring machine (CMM) under constant temperature conditions, and the tolerance consistency is calculated.

[0060] Example 1

[0061] This embodiment details the preparation of a modified polytetrafluoroethylene retaining ring for a hydraulic cylinder of a certain type of precision CNC machine tool.

[0062] S1 Raw Material Preparation: Prepare PTFE suspension fine powder (grade: M-18F) with an average particle size of 12μm; nano tin bronze powder (particle size 100nm); alkali-free short-cut glass fiber (length 0.2mm); alumina ceramic powder (particle size 5μm); molybdenum disulfide powder (particle size 2μm).

[0063] S2 Mixing: Accurately weigh 53.5 kg of PTFE powder, 38 kg of nano-tin bronze powder, 5 kg of glass fiber, 2 kg of alumina ceramic powder, and 1 kg of molybdenum disulfide. Then add 1 kg of KH-550 silane coupling agent (approximately 2% of the filler weight). Put all materials into a high-speed mixer and mix at 300 r / min for 30 minutes.

[0064] S3 One-time molding: The mixture is loaded into a ring mold with an inner diameter of 50mm, pressed at 30MPa pressure for 3 minutes at room temperature, and demolded to obtain a ring blank.

[0065] S4 First Sintering: The initial billet is placed in the sintering furnace, heated to 370℃ at 3℃ / min, held for 3 hours, and then cooled to below 60℃ at 5℃ / min before being taken out of the furnace.

[0066] S5 Secondary Molding: Place the pre-sintered body into a precision mold, heat the mold to 180℃ and hold for 30 minutes. Then start the hydraulic press, increase the pressure to 400MPa within 1 minute, and hold the pressure for 10 minutes. After the pressure holding is completed, immediately circulate cooling water to cool the mold to below 30℃ within 15 minutes, and then demold.

[0067] S6 Secondary Sintering: The billet is placed back into the sintering furnace and heated to 370°C at 3°C / min. It is then sintered at 370°C for 2 hours and then cooled to below 60°C at 5°C / min before being removed from the furnace.

[0068] S7 Constant Temperature Processing: In a constant temperature and humidity workshop with a temperature of 20℃ and a humidity of 50%, a CNC lathe is used to process the sintered blank into a retaining ring with an outer diameter of 52mm, an inner diameter of 48mm, and a height of 5mm. The dimensional tolerance is controlled within ±0.03mm.

[0069] S8 Inspection: Upon inspection, the product has a smooth and defect-free appearance, and all dimensions are in compliance.

[0070] Table 1. Raw material composition and specifications of Example 1

[0071]

[0072] Table 2 Performance test results of Example 1

[0073]

[0074] As shown in Table 2, the performance tests conducted on the sampled samples yielded the following results: compressive strength 44.3 MPa, tensile strength 21.4 MPa, elongation at break 245%, coefficient of friction 0.13, wear mark width 4.30 mm, and inner diameter fluctuation within ±0.05 mm. The retaining ring prepared in Example 1 exhibits performance far exceeding the technical specifications set by this invention, with high dimensional accuracy and good batch consistency. When applied to hydraulic seals, the finished retaining ring prepared in this example fully meets the requirements for high-precision hydraulic seals.

[0075] Example 2

[0076] This embodiment prepares a retaining ring for a large engineering machinery cylinder, with a relatively large size (outer diameter 200mm). The basic steps of this embodiment are the same as those of Embodiment 1, the only difference being that... bezel Process adjustments were made to address the characteristics of large-size blanks (outer diameter 200mm).

[0077] The S3 one-time molding pressure is adjusted to 25MPa, and pressure is held for 5 minutes to ensure complete mold filling;

[0078] During the secondary molding of S5, the mold heating temperature is adjusted to 160℃, the pressure is adjusted to 300MPa, and the holding time is extended to 25 minutes to ensure that the large cross-section blank is uniformly stressed and that plastic flow fully realizes the overall uniform densification.

[0079] During S7 constant temperature machining, the temperature rise between roughing and finishing is carefully controlled, and the machining is performed and cooled in multiple stages.

[0080] Table 3 compares the performance of Implementation 2 and Example 1.

[0081]

[0082] As shown in Table 3, the final retaining ring not only meets the required mechanical properties such as compressive strength and tensile strength, but also exhibits a flatness tolerance that is 30% better than the design requirements, with batch-to-batch dimensional fluctuations controlled within ±0.05 mm. This demonstrates that by adjusting process parameters, this invention can effectively solve the density gradient and deformation problems of large-size PTFE composite materials, exhibiting strong process adaptability.

[0083] Example 3

[0084] The difference between this embodiment and Example 1 is that in the raw material preparation S1, PTFE suspension powder (brand: M-18F) with an average particle size of 10 μm is prepared. The remaining raw materials, formulation, and process steps are exactly the same as in Example 1. The retaining ring was prepared according to the method described in Example 1, and its performance was tested. The results are shown in Table 4.

[0085] Table 4. Effect of different PTFE particle sizes on performance

[0086]

[0087] As shown in Table 4, within the preferred range (10-12 μm) of PTFE average particle size ≤ 15 μm, slight variations in PTFE particle size have no significant impact on the overall performance of the final product. The compressive strength, tensile strength, elongation at break, coefficient of friction, wear track width, and dimensional accuracy of the product all meet the technical specifications set by this invention, and the performance differences are not significant. This indicates that the process of this invention has a certain tolerance for raw material particle size, facilitating raw material selection and quality control in industrial production.

[0088] Comparative Example 1

[0089] This comparative example is used to verify the necessity of the process steps. Its raw materials, formulation, and steps S1-S6 are exactly the same as in Example 1, but the S7 processing is carried out in a non-temperature-controlled ordinary workshop (ambient temperature fluctuates between 18-28°C depending on daytime temperature), and the processing equipment is the same CNC machine tool as in Example 1. The performance test results are shown in Table 5:

[0090] Table 5 Performance comparison between Comparative Example 1 and Example 1

[0091]

[0092] As shown in Table 5, the mechanical properties of the product (compressive strength, tensile strength, coefficient of friction, etc.) are comparable to those of Example 1. However, the inner diameter of the products processed at different times in the same batch showed a systematic deviation. The average size of the products processed in the afternoon was 0.02-0.04 mm smaller than that processed in the morning. The dimensional dispersion within the batch exceeded the tolerance requirement of ±0.05 mm, which could not meet the requirements for precision assembly.

[0093] Therefore, it can be concluded that PTFE composite materials have a large coefficient of thermal expansion, and fluctuations in ambient temperature will directly lead to dimensional errors in processing. Constant temperature precision machining (20±2℃) is an indispensable key step to ensure that the product achieves and maintains ultra-high dimensional accuracy.

[0094] Comparative Example 2 (simulating traditional one-piece molding process)

[0095] The difference between this comparative example and Example 1 is that:

[0096] In the S2 mixing step, the same basic raw materials are used, but the glass fiber is replaced with an equal mass (5%) of carbon fiber (T300 grade, 0.2 mm in length, 7 μm in diameter, untreated), and the copper powder is replaced with micron-sized tin bronze powder (average particle size 25 μm). By weight, the mixture is prepared in the following proportions: 65 parts PTFE, 20 parts micron-sized tin bronze powder, 8 parts carbon fiber, and 4 parts molybdenum disulfide.

[0097] Process change: Only S1-S4 (one-time molding and sintering) are performed, followed by direct machining in a room temperature (approximately 25°C) workshop. The temperature control for S5 (secondary high-temperature and high-pressure molding), S6 (secondary sintering), and S7 is omitted. Retaining rings of the same size are machined on the same CNC lathe. Performance test results are shown in Table 6.

[0098] Table 6 Performance comparison between Comparative Example 2 and Example 1

[0099]

[0100] As shown in Table 6, the retaining ring prepared by the traditional one-time molding process has a compressive strength of 38.5 MPa, a tensile strength of 18.2 MPa, an elongation at break of 135%, a coefficient of friction of 0.15, and a wear mark width of 5.0 mm. Its compressive strength, toughness, and dimensional consistency are significantly lower than those of the product in Example 1 of this invention.

[0101] Compared to the traditional one-stage molding and sintering process in Comparative Example 2, the formation of a density gradient along the thickness of the blank after the first molding and sintering leads to a decrease in mechanical properties (especially compressive strength and toughness). Furthermore, the lack of a secondary high-temperature, high-pressure densification and stress relief process results in dimensional springback and deformation after processing, causing insufficient material density and integrity. Measurements taken 24 hours after processing showed a maximum difference of 0.08 mm in the inner diameter of products from the same batch. This is due to the density gradient and residual stress within the blank, which, after processing, leads to uneven deformation due to stress release. The combination of non-constant temperature processing and internal unevenness results in large dimensional dispersion, failing to meet precision assembly requirements.

[0102] Low processing yield: During the processing, due to the unevenness of the material inside, chipping or dimensional deviations are likely to occur, resulting in a yield that is about 15% lower than that of the process of this invention.

[0103] Therefore, the traditional one-time molding process cannot solve the fundamental problems of uneven internal structure and large dimensional fluctuations in high-filled PTFE composite materials, which in turn confirms the superiority and necessity of the integrated process of "secondary molding + secondary sintering + constant temperature processing" of this invention.

[0104] Comparative Example 3

[0105] The difference between this comparative example and Example 1 is that in the raw material preparation of S1, PTFE suspension fine powder (brand name: M-18F) with average particle sizes of 16μm, 18μm, and 20μm is prepared. The other raw materials, formulations, and process steps are exactly the same as in Example 1.

[0106] Table 7. Effect of different PTFE particle sizes on performance.

[0107]

[0108] As shown in Table 7, when the particle size reaches ≥16μm, all properties fail to meet the minimum performance indicators set by this invention (compressive strength ≥40MPa, tensile strength ≥20MPa, elongation at break ≥200%, coefficient of friction ≤0.20, wear mark width ≤5.5mm), especially the compressive strength and elongation at break, which decrease significantly. This is because excessively large PTFE particles have a reduced specific surface area, making it difficult to form a uniform coating with the nanofiller during the mixing stage. Insufficient melt diffusion during sintering leads to increased internal defects in the matrix and poorer interfacial bonding. This comparative example demonstrates that the particle size of the PTFE substrate is one of the key factors affecting the high performance of the final product. Even with the same advanced process as this invention, it is impossible to obtain a high-performance retaining ring with high strength, high toughness, and high precision when the particle size exceeds ≤15μm.

[0109] Comparative Example 4

[0110] The solutions in the four prior art documents mentioned in the background (Comparative Example A: Application No. 201811248603.8, Comparative Example B: Application No. 201310426401.9, Comparative Example C: Application No. 201810512371.6, Comparative Example D: Application No. 202110994028.1) are used as comparative examples and are compared with Embodiment 1 of the present invention.

[0111] Table 8 Performance Comparison of Embodiment 1 of the Present Invention with Four Comparative Schemes in the Background Art

[0112]

[0113] As can be seen from the table above, reference documents A, B, C, and D do not disclose the integrated process of 'secondary molding + secondary sintering + isothermal processing' as described in this invention, nor do they mention the technical effects achieved through this integrated process, such as compressive strength ≥40MPa, tensile strength ≥20MPa, elongation at break ≥200%, and dimensional accuracy ±0.03mm. This invention, through a specific combination of formulation and process, solves the technical problems of uneven density and dimensional instability in high-filled PTFE retaining rings.

[0114] Those skilled in the art can make minor adjustments based on the product's size and shape complexity, but the order and synergistic relationship of the core process steps cannot be omitted or reversed:

[0115] Secondary high-pressure forming (S5) must be performed after primary sintering (S4) and before secondary sintering (S6). This sequence is crucial for eliminating the density gradient inside the sintered green body and achieving ultimate densification. Reversing or omitting the sequence will not solve the problem of "inhomogeneity inside the sintered green body".

[0116] The temperature control range of 20±2℃ for constant temperature precision machining (S7) is a proven and preferred range. It is the result of a balance between the thermal expansion characteristics of PTFE material and the economics of precision machining, and is a decisive condition for ensuring dimensional accuracy within ±0.05mm.

[0117] The addition of silane coupling agent (S2) is fundamental to ensuring good interfacial bonding between the filler and the matrix. It directly affects the effectiveness of all subsequent process steps and the upper limit of the final product's performance. It is the basis for achieving all subsequent high performance and must be guaranteed.

[0118] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0119] In this invention, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," "link," and "fix" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, a direct connection, or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0120] The control method of this invention is automatic control through a controller. The control circuit of the controller can be implemented by simple programming by those skilled in the art. The power supply is also common knowledge in the art. Furthermore, since this invention is mainly used to protect mechanical devices, the control method and circuit connection will not be explained in detail here.

[0121] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any ordinary changes and substitutions made by those skilled in the art within the scope of the technical solution of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a modified polytetrafluoroethylene retaining ring based on secondary hot pressing, characterized in that, Includes the following steps: Step S1, Raw material preparation: Select polytetrafluoroethylene suspension fine powder as the substrate; Tin bronze powder, chopped glass fiber, molybdenum disulfide, and ceramic powder were selected as modifying fillers. Step S2, Mixing: By weight percentage, polytetrafluoroethylene powder, tin bronze powder, glass fiber, ceramic powder and molybdenum disulfide are dry-mixed to obtain a uniform mixture. Step S3, One-time molding: The mixture is loaded into the mold and cold-pressed to obtain the initial blank; Step S4, First sintering: Place the initial billet in a sintering furnace for sintering; Step S5, Secondary forming: The blank after the first sintering is placed back into the mold and heated and pressurized for secondary forming; Step S6, Secondary sintering: The billet after secondary forming is placed in the sintering furnace again for sintering; Step S7, Constant Temperature Precision Machining: Place the blank after secondary sintering in a constant temperature environment and machine it to the designed dimensions to obtain a polytetrafluoroethylene retaining ring.

2. The method for preparing a modified polytetrafluoroethylene retaining ring based on secondary hot pressing according to claim 1, characterized in that, In step S2, 0.5-2% of silane coupling agent is added to enhance the interfacial bonding force between the filler and the substrate. In step S2, by weight percentage, polytetrafluoroethylene powder is 50-60%, tin bronze powder is 30-45%, glass fiber is 3-8%, ceramic powder is 1-5%, and molybdenum disulfide is 0.5-3%.

3. The method for preparing a modified polytetrafluoroethylene retaining ring based on secondary hot pressing according to claim 1, characterized in that, In step S1, the average particle size of the polytetrafluoroethylene suspension fine powder is ≤15μm; the tin bronze powder is nano-sized tin bronze powder with an average particle size of 80-150nm.

4. The method for preparing a modified polytetrafluoroethylene retaining ring based on secondary hot pressing according to claim 1, characterized in that, In step S3, the pressure for cold pressing is 20-50 MPa; In step S5, the heating temperature is 120-200℃, the pressurization pressure is 200-600MPa, and the pressure holding time is 3-30 minutes.

5. The method for preparing a modified polytetrafluoroethylene retaining ring based on secondary hot pressing according to claim 1, characterized in that, In step S4, the sintering temperature is 360-380℃ and the holding time is 2-4 hours. During the first sintering process in step S4, the heating rate is 1-5℃ / minute, and after holding the sintering temperature for 2-4 hours, the temperature is gradually cooled to room temperature at a rate of 1-10℃ / minute.

6. The method for preparing a modified polytetrafluoroethylene retaining ring based on secondary hot pressing according to claim 1, characterized in that, In step S6, the sintering temperature is 360-380℃ and the holding time is 1-3 hours; during the secondary sintering process in step S6, after holding the sintering temperature for 1-3 hours, the temperature is gradually cooled to room temperature at a rate of 1-10℃ / minute. In step S7, the temperature of the constant temperature environment is 20±2℃.

7. The method for preparing a modified polytetrafluoroethylene retaining ring based on secondary hot pressing according to claim 4, characterized in that, In step S5, after the pressure holding is completed, the mold is cooled to below 30°C using a circulating water cooling method, with a cooling rate of 1-10°C / minute.

8. The method for preparing a modified polytetrafluoroethylene retaining ring based on secondary hot pressing according to claim 1, characterized in that, The performance indicators of the prepared polytetrafluoroethylene retaining ring are: compressive strength ≥40MPa, tensile strength ≥20MPa, elongation at break ≥200%, coefficient of friction ≤0.20, and wear mark width ≤5.5mm.

9. A modified polytetrafluoroethylene retaining ring prepared by the method according to any one of claims 1-8, characterized in that, Its composition by weight percentage is 53.5% polytetrafluoroethylene, 38% nano-tin bronze powder, 5% glass fiber, 2% ceramic powder, and 1% molybdenum disulfide.

10. The modified polytetrafluoroethylene retaining ring according to claim 9, characterized in that, The modified polytetrafluoroethylene retaining ring is suitable for sealing and guiding components in hydraulic cylinders, engineering machinery, CNC machine tools, and oilfield equipment.

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