A threaded foam plug and its molding method

Abstract

This application discloses a threaded foam plug and its molding method, belonging to the technical field of solid rocket engine nozzle plugs. Installed at the tail end of a rocket engine nozzle, it includes: a plug body with a first trapezoidal thread structure on its outer side; and a retaining ring with a second trapezoidal thread structure on its inner side. The first and second trapezoidal thread structures are mutually adapted and screwed together, and the mating surfaces of the first and second trapezoidal thread structures are configured to generate mechanical self-locking and radial clamping force during screwing and tightening, thereby achieving a sealed connection between the plug body and the retaining ring, thus sealing the rocket engine nozzle. Both the plug body and the retaining ring are made of expandable polystyrene foam and molded in a single step. This invention effectively resists vibration and loosening, ensuring a durable connection and structural stability under operating conditions.

Description

Technical Field

[0001] This application relates to a threaded foam plug and its molding method, belonging to the field of solid rocket engine nozzle plug technology. Background Technology

[0002] Solid rocket motor nozzle plugs are critical sealing components of the engine system. Their main function is to maintain the internal seal of the nozzle during engine storage and transportation, preventing the ingress of moisture and impurities. During engine ignition, they must reliably open at a predetermined pressure to ensure smooth exhaust of combustion gases. Traditional plugs are mainly divided into two categories: metal plugs and rubber plugs. While metal plugs possess high mechanical strength and stable opening pressure, high-speed fragments are generated after engine ignition. These fragments may impact the nozzle convergent section and throat liner, affecting the engine's normal operation and reliability. Rubber plugs, while avoiding fragment damage, suffer from insufficient temperature resistance, making them prone to ablation failure under high-temperature combustion gases. Furthermore, long-term storage can lead to aging, affecting sealing performance.

[0003] The emergence of foam plugging technology has effectively solved the limitations of traditional materials. Foam plugs made from materials such as polystyrene beads achieve a balance between lightweight and controllable breakage through special structural design and molding processes. However, existing foam plugs mostly use epoxy resin as an adhesive, bonding it to the nozzle expansion section, making the bonding interface a weak point. Due to the surface characteristics of foam, its interfacial bonding strength and long-term stability are challenged, easily becoming a source of failure under thermal cycling or vibration conditions. Summary of the Invention

[0004] According to one aspect of this application, a threaded foam plug is provided that effectively resists vibration and loosening, ensuring a durable connection and structural stability under operating conditions.

[0005] A threaded foam plug, installed at the tail end of a rocket engine nozzle, includes:

[0006] The plug body has a first trapezoidal thread structure on its outer side;

[0007] A retaining ring, wherein a second trapezoidal thread structure is provided on the inner side of the retaining ring;

[0008] The first trapezoidal thread structure and the second trapezoidal thread structure are adapted to each other and screwed together. The mating surfaces of the first trapezoidal thread structure and the second trapezoidal thread structure are configured to generate mechanical self-locking and radial clamping force when screwed together, so as to achieve a sealed connection between the plug body and the retaining ring, thereby achieving a seal on the rocket engine nozzle.

[0009] Both the plug body and the retaining ring are made of expandable polystyrene and molded in one piece.

[0010] Furthermore, the outer side of the plug body is coated with a silicone rubber sealing layer;

[0011] The outer and inner sides of the retaining ring are coated with a silicone rubber sealing layer.

[0012] The thickness of the silicone rubber sealing layer is 0.2-0.5 mm.

[0013] Furthermore, the plug body has a first mating surface, and the retaining ring has a second mating surface, the first mating surface and the second mating surface being adapted to each other;

[0014] The first mating surface has a first convergence angle, and the second mating surface has a second convergence angle. The first mating surface and the second mating surface are arranged relative to each other with the first convergence angle and the second convergence angle, so that when the first mating surface and the second mating surface are pressed together along the axial direction, radial interference is generated, thereby achieving sealing and locking.

[0015] Furthermore, the maximum outer diameter of the plug body is 135-139 mm;

[0016] The height of the plug body is 80-90mm;

[0017] The thickness of the plug body is 30-50mm;

[0018] The first convergence angle is 60°-63°;

[0019] The maximum outer diameter of the retaining ring is 100-110 mm;

[0020] The thickness of the retaining ring is 20-26mm;

[0021] The second convergence angle is 30°-33°.

[0022] Furthermore, the nominal diameter of the first trapezoidal thread structure and the second trapezoidal thread structure is 50-65mm, and the pitch is 3mm;

[0023] The effective length of the first trapezoidal thread structure and the second trapezoidal thread structure is 25-30mm.

[0024] Furthermore, the tail section of the rocket engine nozzle includes a nozzle front section, a throat liner, and a nozzle rear section arranged coaxially along the gas flow direction.

[0025] The throat liner has a converging section and a dilating section;

[0026] The plug body is located within the converging section of the throat liner, and the outer peripheral surface of the plug body is adapted to and fits the inner wall surface of the converging section.

[0027] The retaining ring is located inside the rear section of the nozzle and corresponds to the axial rearward position of the expansion section.

[0028] According to another aspect of this application, a method for molding a threaded foam plug is provided, comprising the following steps:

[0029] S1: Expandable polystyrene pre-foamed to obtain one-time foamed beads;

[0030] S2: Perform secondary foaming on the primary foamed beads to obtain secondary foamed beads;

[0031] S3: The secondary foaming beads are evenly loaded into the mold of the plug body and the retaining ring to ensure that the secondary foaming beads are fully foamed and expanded to completely fill the mold cavity, forming a plug body and retaining ring with a trapezoidal thread structure in one go. The mold filled with the secondary foaming beads is completely immersed in boiling water for steam foaming, and the boiling water bath time is 45 to 60 minutes.

[0032] S4: Remove the mold from the boiling water and perform cooling and demolding treatment to obtain a plug body and a retaining ring with a trapezoidal thread structure;

[0033] S5: Assemble the plug body and the retaining ring, and install them at the tail of the rocket engine nozzle.

[0034] Furthermore, in S1, a certain amount of expandable polystyrene is weighed for pre-foaming. The pre-foaming temperature is set at 40-50°C, and the pre-foaming time is 1-5 hours. During the pre-foaming process, the ventilation system is kept running continuously to ensure complete decomposition of organic gases.

[0035] In S2, the set temperature for the secondary foaming is 60-70℃, and the secondary foaming time is 10-30 minutes, to ensure that the secondary foamed beads have uniform volume, smooth surface, and no adhesion.

[0036] Furthermore, in S4, the cooling temperature of the cooling and demolding process is 20-25°C, and the mold is opened after cooling to room temperature;

[0037] The plug body and retaining ring with trapezoidal thread structure are subjected to curing treatment at a temperature of 45-55℃ for 6-7 hours.

[0038] Furthermore, S5 includes:

[0039] A silicone rubber sealing layer is applied to the outer side of the plug body and the inner and outer sides of the retaining ring to ensure complete coverage of the threads without any missed coating. The thickness of the silicone rubber sealing layer is 0.2-0.5 mm.

[0040] The plug body is axially inserted from the front section of the nozzle and pushed into the converging section of the throat liner, so that the outer surface of the plug body is in contact with the inner wall of the converging section.

[0041] The retaining ring is axially inserted from the rear section of the nozzle, and positioned axially rearward of the expansion section of the throat liner;

[0042] The trapezoidal thread structure of the retaining ring and the plug body is connected to complete the installation and fixation of the plug.

[0043] Curing time is 18-24 hours at room temperature with relative humidity ≤60%.

[0044] The beneficial effects that this application can produce include:

[0045] The threaded foam plug and its molding method provided in this application adopt a trapezoidal thread structure to provide a reliable self-locking mechanism. By rotating the retaining ring, it engages and tightens with the trapezoidal thread on the plug body, forming a mechanical self-locking and radial clamping force, which effectively resists vibration and loosening, ensuring a durable connection and structural stability under working conditions. At the same time, the silicone rubber sealing layer ensures excellent airtightness and environmental adaptability. The density of the foam material can be precisely controlled to achieve precise opening pressure. Furthermore, the integrated molding process simplifies the production process and is suitable for mass production. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the overall structure of a threaded foam plug according to one embodiment of this application;

[0047] Figure 2 This is an assembly drawing of a threaded foam plug according to one embodiment of this application;

[0048] Figure 3 This is a schematic diagram of the molding structure of the plug body in one embodiment of this application;

[0049] Figure 4 This is a schematic diagram of the molding structure of the retaining ring in one embodiment of this application;

[0050] List of components and figures: 1-Plug body; 2-First trapezoidal thread structure; 3-Retaining ring; 4-Second trapezoidal thread structure; 5-Silicone rubber sealing layer; 6-Throat liner. Detailed Implementation

[0051] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0052] See Figure 1-4 ,like Figure 1 As shown, a threaded foam plug is installed at the tail end of a rocket engine nozzle, comprising:

[0053] The plug body 1 has a first trapezoidal thread structure 2 on its outer side;

[0054] The retaining ring 3 has a second trapezoidal thread structure 4 on its inner side;

[0055] The first trapezoidal thread structure 2 and the second trapezoidal thread structure 4 are adapted to each other and screwed together. The mating surfaces of the first trapezoidal thread structure 2 and the second trapezoidal thread structure 4 are configured to generate mechanical self-locking and radial clamping force when screwed together, so as to achieve a sealed connection between the plug body 1 and the retaining ring 3, thereby achieving a seal on the rocket engine nozzle.

[0056] Both the plug body 1 and the retaining ring 3 are made of expandable polystyrene and molded in one piece.

[0057] Specifically, the first trapezoidal thread structure of the plug body matches the second trapezoidal thread structure of the retaining ring to ensure a smooth and tight fit during the tightening process. The self-locking property of the trapezoidal thread stems from the physical principle that the helix angle is smaller than the friction angle. After tightening, no additional fasteners are needed, and it can resist external forces such as vibration and impact, preventing the plug from accidentally loosening. This is especially suitable for the harsh environment during rocket transportation and storage. At the same time, the thread sides squeeze each other during tightening, generating centripetal radial pressure, allowing the mating surfaces of the body and the retaining ring to fit tightly, eliminating gaps and forming a sealing barrier, which is compatible with the properties of foam materials.

[0058] Furthermore, expandable polystyrene material has a low density, which minimizes the additional weight of the rocket when it is not in operation, without affecting the overall performance of the engine. One-time molding allows for precise replication of the thread structure and shape, meeting the installation size requirements of the nozzle tail. The foam's elasticity also helps to fill tiny gaps with the tightening force of the threads. Its foam plug also provides insulation and cushioning, preventing external dust, moisture, and other impurities from entering the nozzle, while reducing the impact of ambient temperature changes on internal nozzle components.

[0059] It is worth noting that the foam plug is mainly used for non-working stages such as rocket engine transportation, storage, and ground testing, and plays a temporary sealing and protection role. It needs to be removed quickly before engine ignition. The foam material is lightweight, and the removal process will not leave any debris or cause wear to the nozzle.

[0060] The outer side of the plug body 1 is coated with a silicone rubber sealing layer 5;

[0061] The outer and inner sides of the retaining ring 3 are coated with a silicone rubber sealing layer 5;

[0062] The thickness of the silicone rubber sealing layer 5 is 0.2-0.5 mm.

[0063] Specifically, the silicone rubber sealing layer on the outer side of the plug body can directly fill the tiny gaps between the thread profiles when it mates with the second trapezoidal thread of the retaining ring. At the same time, it isolates external moisture and dust from direct contact with the foam body, preventing structural aging or sealing failure of the foam material due to water absorption and dust accumulation. The silicone rubber sealing layer on the inner side of the retaining ring fits tightly with the first trapezoidal thread of the plug body. It can compensate for the dimensional tolerances generated during the thread processing by means of the elastic deformation of the silicone rubber itself, and enhance the sealing effect of radial clamping force when the thread is screwed on. At the same time, silicone rubber has excellent aging resistance, which can effectively extend the storage service life of the plug. The silicone rubber sealing layer on the outside of the retaining ring serves two purposes: firstly, it covers the outer surface of the retaining ring, protecting the foam matrix from scratches and damage during transportation and installation; secondly, after the plug is installed to the tail of the rocket engine nozzle, the outer side of the plug body and the outer side of the retaining ring can flexibly fit against the inner wall of the nozzle, filling the installation gap between the plug and the nozzle, forming a third sealing barrier to further block external impurities from entering the nozzle. Specifically, the first layer of seal consists of the plug body, the silicone rubber layer on the outside of the retaining ring, and the inner wall of the nozzle, achieving initial sealing through flexible fit and preventing external impurities from entering through the installation gap between the plug and the nozzle; the second layer of seal is formed by the silicone rubber layer on the threaded surface and the radial clamping force generated by the thread engagement, using the elasticity of the silicone rubber to fill the thread gaps and enhance the mechanical self-locking sealing effect; the third layer of seal is formed by the elasticity of the foam matrix itself, acting as a sealing buffer to compensate for minor defects in the coating and threads. This triple protection significantly improves the sealing reliability of the plug.

[0064] Furthermore, the thickness of the sealing layer is set to 0.2-0.5mm. The lower limit of the thickness ensures that the silicone rubber can completely cover the tooth peaks and valleys of the thread, avoiding missed coating or sealing breaks due to excessively thin coating. At the same time, this thickness can be adapted to the surface roughness of the foam material, ensuring that the coating has sufficient elasticity to achieve the sealing compensation function. The upper limit of the thickness is mainly used to control the coating thickness to avoid affecting the engagement accuracy of the threaded pair. If the coating thickness exceeds 0.5mm, it will cause the thread fit to be too tight, greatly increasing the difficulty of installation and disassembly.

[0065] Meanwhile, based on the storage and transportation environment requirements of rocket engines, the silicone rubber sealing layer is made of high and low temperature resistant methyl vinyl silicone rubber. This rubber can work stably in the range of -60℃ to 200℃, adapting to temperature changes in rocket storage in different regions without exhibiting low-temperature brittleness or high-temperature softening. Furthermore, it has excellent compatibility with expandable polystyrene foam matrix, preventing swelling or corrosion reactions and avoiding damage to the sealing matrix structure. Additionally, its elastic recovery rate maintains sealing performance during thread engagement and long-term storage, preventing seal failure due to stress relaxation.

[0066] The plug body 1 has a first mating surface, and the retaining ring 3 has a second mating surface, wherein the first mating surface and the second mating surface are adapted to each other;

[0067] The first mating surface has a first convergence angle, and the second mating surface has a second convergence angle. The first mating surface and the second mating surface are arranged relative to each other with the first convergence angle and the second convergence angle, so that when the first mating surface and the second mating surface are pressed together along the axial direction, radial interference is generated, thereby achieving sealing and locking.

[0068] The maximum outer diameter of the plug body 1 is 135-139 mm;

[0069] The height of the plug body 1 is 80-90mm;

[0070] The thickness of the plug body 1 is 30-50mm;

[0071] The first convergence angle is 60°-63°;

[0072] The maximum outer diameter of the retaining ring 3 is 100-110mm;

[0073] The thickness of the retaining ring 3 is 20-26mm;

[0074] The second convergence angle is 30°-33°.

[0075] The nominal diameter of the first trapezoidal thread structure 2 and the second trapezoidal thread structure 4 is 50-65mm, and the pitch is 3mm;

[0076] The effective length of the first trapezoidal thread structure 2 and the second trapezoidal thread structure 4 is 25-30mm.

[0077] Specifically, the plug body has a first mating surface with a first convergence angle of 60°-63°; the corresponding retaining ring has a second mating surface with a second convergence angle of 30°-33°. The first and second mating surfaces are fitted together at the convergence angle. When the plug body and the retaining ring are screwed and pressed together along the axial direction, the two mating surfaces with convergence angles will generate a radial interference effect, that is, the contact pressure between the mating surfaces increases radially, forcing a slight elastic deformation in the contact area between the plug body and the retaining ring, further eliminating the mating gap and achieving the dual mechanical action of mechanical self-locking and radial compression. At the same time, the threaded structure provides axial anti-loosening force, and the convergence mating surface provides radial compression sealing force. The combination of the two greatly improves the locking stability and sealing reliability of the plug, preventing the plug from loosening even when it encounters severe vibration during rocket transportation.

[0078] It is worth noting that, such as Figure 2-4As shown, the maximum outer diameter of the plug body is 135-139mm. This size precisely matches the inner diameter of the mounting port at the tail of the rocket engine nozzle, ensuring that the plug can be stably embedded into the nozzle tail. It also allows for sufficient space for the thickness of the silicone rubber sealing layer on the outer side of the retaining ring, preventing installation difficulties due to an excessively large outer diameter. Based on the cavity depth at the nozzle tail, the height of the plug body is 80-90mm, ensuring that the plug will not protrude from the nozzle end face after embedding, while also providing sufficient axial space for threaded engagement. While meeting the structural strength requirements of the foam material, a balance is struck between lightweight design and structural strength. Too thin a thickness would lead to easy deformation of the plug body, while too thick a thickness would add unnecessary weight. The maximum outer diameter of the retaining ring is 100-110mm, connected to the corresponding position on the plug body. Its thickness ensures that the retaining ring will not break or deform during threaded engagement and conical interference pressing. It also forms a gradient design with the thickness of the plug body, ensuring the plug's installation requirements within the nozzle are met.

[0079] Furthermore, the trapezoidal thread structure has a nominal diameter of 50-65mm and a pitch of 3mm. This nominal diameter range is suitable for the connection strength requirements between the plug body and the retaining ring. The pitch is a coarse-pitch trapezoidal thread, which has stronger load-bearing capacity and self-locking performance. Compared with fine-pitch threads, it is more suitable for connecting foam materials, avoiding stripping during screwing. This effective length directly determines the stability of the threaded connection, ensuring that the contact area of ​​the threaded pair is large enough to provide reliable axial locking force and achieve a complete seal on the threaded surface in conjunction with the silicone rubber sealing layer.

[0080] like Figure 2 As shown, the tail section of the rocket engine nozzle includes a nozzle front section, a throat liner 6, and a nozzle rear section arranged coaxially along the gas flow direction.

[0081] The throat liner 6 has a converging section and an expanding section;

[0082] The plug body 1 is located within the converging section of the throat liner 6, and the outer peripheral surface of the plug body 1 is adapted to and fits the inner wall surface of the converging section.

[0083] The retaining ring 3 is located inside the rear section of the nozzle and corresponds to the axial rear of the expansion section.

[0084] Specifically, the tail section of the rocket engine nozzle adopts a three-section structure arranged coaxially along the gas flow direction: the nozzle front section, the throat liner, and the nozzle rear section. The throat liner includes a converging section and a diverging section. The converging section compresses and accelerates the gas, while the diverging section further expands the gas to improve the nozzle's thrust efficiency. The plug body is embedded inside the converging section of the throat liner, and the outer circumference of the plug body is fully fitted to the inner wall of the converging section. The throat liner converging section is the starting channel for gas flow, directly blocking the gas flow path. Simultaneously, the conical inner wall of the converging section provides radial restraint for the plug, preventing displacement during transportation and storage. Furthermore, the retaining ring is installed inside the rear section of the nozzle, and its axial position corresponds to the rear of the throat liner expansion section. The retaining ring is connected to the plug body by a trapezoidal thread, and an axial clamping force is applied to the plug body from the rear section of the nozzle, which is coordinated with the radial limit of the throat liner convergence section. At the same time, the cavity space of the rear section of the nozzle provides sufficient operating margin for the screwing operation of the retaining ring, which is convenient for installation and removal.

[0085] It is worth noting that the plug body is fully fitted to the inner wall of the throat liner convergence section, directly blocking external impurities from entering the core passage of gas flow from the nozzle inlet, and preventing impurities from adhering to the inner wall of the throat liner and affecting the gas flow efficiency after engine ignition; the retaining ring is located at the rear section of the nozzle and corresponds to the rear of the throat liner expansion section, which can prevent external dust and water vapor from entering in reverse from the rear section of the nozzle. At the same time, the silicone rubber sealing layer on the outside of the retaining ring is flexibly fitted to the inner wall of the rear section of the nozzle, further sealing the rear gap, forming a fully sealed layout of front-end sealing and rear-end protection.

[0086] In this application, after 100 high-temperature thermal cycles at room temperature to 1200℃, there was no interface displacement and the connection function remained intact. Under vibration fatigue power test, the connection torque did not decrease before and after vibration. In contrast, the adhesive layer of traditional adhesives showed obvious cracking, the cap showed a displacement of >10μm, and the bonding strength decreased by about 40% after vibration.

[0087] A method for molding a threaded foam plug includes the following steps:

[0088] S1: Expandable polystyrene pre-foamed to obtain one-time foamed beads;

[0089] S2: Perform secondary foaming on the primary foamed beads to obtain secondary foamed beads;

[0090] S3: The secondary foaming beads are evenly loaded into the mold of the plug body and the retaining ring to ensure that the secondary foaming beads are fully foamed and expanded to completely fill the mold cavity, forming the plug body 1 and the retaining ring 3 with trapezoidal thread structure in one go. The mold filled with the secondary foaming beads is completely immersed in boiling water for steam foaming. The boiling water bath time is 45-60 minutes.

[0091] S4: Remove the mold from the boiling water and perform cooling and demolding treatment to obtain the plug body 1 and the retaining ring 3 with a trapezoidal thread structure;

[0092] S5: Assemble the plug body 1 and the retaining ring 3, and install them at the tail of the rocket engine nozzle.

[0093] In S1, a certain amount of expandable polystyrene is weighed for pre-foaming. The pre-foaming temperature is set at 40-50℃, and the pre-foaming time is 1-5 hours. During the pre-foaming process, the ventilation system is kept running continuously to ensure complete decomposition of organic gases.

[0094] In S2, the set temperature for the secondary foaming is 60-70℃, and the secondary foaming time is 10-30 minutes, to ensure that the secondary foamed beads have uniform volume, smooth surface, and no adhesion.

[0095] In S4, the cooling temperature of the cooling demolding process is 20-25°C, and the mold is opened after cooling to room temperature;

[0096] The plug body 1 and the retaining ring 3 with trapezoidal thread structure are subjected to curing treatment at a temperature of 45-55℃ for 6-7 hours.

[0097] S5 includes:

[0098] A silicone rubber sealing layer 5 is applied to the outer side of the plug body 1 and the inner and outer sides of the retaining ring 3 to ensure complete coverage of the threads without any missed coating. The thickness of the silicone rubber sealing layer 5 is 0.2-0.5 mm.

[0099] The plug body 1 is axially inserted from the front section of the nozzle and pushed into the converging section of the throat liner 6, so that the outer surface of the plug body 1 is in contact with the inner wall of the converging section.

[0100] The retaining ring 3 is axially inserted from the rear section of the nozzle, so that the retaining ring 3 is positioned axially rearward of the expansion section of the throat liner 6;

[0101] The trapezoidal thread structure of the retaining ring 3 and the plug body 1 is used to connect them, thus completing the installation and fixing of the plug. Figure 3-4 As shown;

[0102] Curing time is 18-24 hours at room temperature with relative humidity ≤60%.

[0103] Specifically, a measured amount of expandable polystyrene is weighed and pre-foamed at 40-50℃ for 1-5 hours to activate the foaming agent inside the EPS beads. This causes the foaming agent to vaporize upon heating, generating tiny bubbles and allowing the beads to initially expand. 40-50℃ is the activation temperature range for the foaming agent. If the temperature is too low, the foaming agent will not vaporize sufficiently, resulting in insufficient bead expansion; if the temperature is too high, the beads may stick together or the pores may rupture. The activation time can be adjusted according to the bead size to ensure that the expansion rate of different batches of beads is uniform. Continuous ventilation can promptly remove the organic gases decomposed during the pre-foaming process, preventing the accumulation of gases in the equipment and avoiding safety hazards. It also prevents residual organic gases from affecting the subsequent foaming performance of the EPS beads. The pre-foamed beads are then second-foamed at 60-70℃ for 10-30 minutes to determine the final density and strength of the foam product. By increasing the temperature, the foaming agent is further vaporized, allowing the bubbles inside the beads to grow uniformly, ultimately resulting in secondary-foamed beads with uniform volume, smooth surface, and no sticking. The secondary foaming temperature is higher than the pre-foaming stage, allowing for secondary expansion of the beads, but lower than the softening temperature of EPS to avoid melting and deformation. During the secondary foaming process, the beads must be kept loose to prevent them from sticking together due to surface softening, ensuring uniform filling of the mold cavity after insertion. The secondary foamed beads are evenly inserted into a mold with a trapezoidal threaded cavity and completely immersed in boiling water for steam foaming for 45-60 minutes, achieving the transformation from foamed beads to the final product shape. The boiling water temperature (100℃) rapidly softens the EPS beads, causing the internal foaming agent to vaporize violently, resulting in bead expansion and fusion, completely filling the complex cavity structures of the mold, such as threads and convergence surfaces. Compared to hot air foaming, the boiling water bath provides a more uniform temperature, avoiding insufficient foaming at the mold edges. This time must allow for sufficient expansion and fusion of the beads to ensure the structural strength of the product and prevent porosity or delamination; secondly, it must ensure that precision structures such as threads and convergence surfaces are fully formed without collapse or deformation. After the mold is removed from boiling water, it is cooled to room temperature at 20-25℃ before being opened. During the cooling process, the gas inside the EPS foam contracts upon cooling, causing the product to detach from the mold cavity wall. At the same time, the foam matrix gradually hardens and sets. If the mold is opened before cooling to room temperature, the product is prone to warping and dimensional deviations due to thermal expansion and contraction. After demolding, the plug body and retaining ring are cured at 45-55℃ for 6-7 hours. The purpose of curing is to allow the residual foaming agent inside the foam to further volatilize, while stabilizing the molecular structure of the foam matrix, eliminating internal stress, and preventing dimensional shrinkage or deformation during subsequent storage or assembly.

[0104] Furthermore, a 0.2-0.5mm thick silicone rubber sealing layer is applied to the outer side of the plug body and the inner and outer sides of the retaining ring, ensuring complete coverage of the trapezoidal thread and the converging mating surface without any omissions. During assembly, the plug body is axially inserted from the front section of the nozzle, pushed to the throat liner converging section, and its outer surface is brought into contact with the inner wall of the converging section. This process involves no forced pressing; the natural fit is achieved by utilizing the shape compatibility between the plug body and the converging section, preventing the foam matrix from breaking due to stress. The retaining ring is axially inserted from the rear section of the nozzle, positioned behind the throat liner expansion section, and then connected to the plug body by screwing on the trapezoidal thread. During screwing, the screwing torque must be controlled to ensure both self-locking and radial compression of the threaded pair, while avoiding excessive torque that could lead to thread stripping or foam cracking.

[0105] The assembled product should be cured at room temperature and relative humidity ≤60% for 18-24 hours to ensure complete cross-linking and curing of the silicone rubber sealing layer. If the humidity is >60%, the curing speed of the silicone rubber will be slower, and problems such as surface stickiness and decreased adhesion may occur. A reasonable curing time can ensure the bonding strength between the coating and the foam matrix, while optimizing the elasticity and aging resistance of the sealing layer.

[0106] It is worth noting that both the plug body and the retaining ring are made of expandable polystyrene (EPS), manufactured through a molding process including raw material pre-foaming, secondary foaming, and compression molding, achieving a one-time molding into an integral structure with trapezoidal threads in a mold. A silicone rubber sealing layer is uniformly coated on the outer surface of the plug body and the trapezoidal thread area to enhance interface airtightness and environmental adaptability. During assembly, the plug is axially inserted from the front section of the nozzle, fitting it against the inner wall of the nozzle throat liner convergence section; then the retaining ring is inserted from the rear section of the nozzle, positioned in the nozzle throat liner expansion section. By rotating the retaining ring, it engages with the trapezoidal threads on the plug body and is tightened, forming a mechanical self-locking and radial clamping force to ensure a durable connection and structural stability under operating conditions. Specifically:

[0107] 1. Component manufacturing process:

[0108] (1) Expandable polystyrene pre-foaming:

[0109] (a) Preparation: Connect the power supply to the electric heating drying oven, turn on the organic gas decomposition treatment system, and set the initial temperature to 45±5℃;

[0110] (b) Raw material weighing: After calculating the required density, accurately weigh the expandable polystyrene raw material using an electronic scale with an accuracy of 0.01g, with an error of ±0.05g.

[0111] (c) Initial foaming: Place the weighed raw materials in a stainless steel container and put it into a preheated drying oven for foaming at 45±5℃ for 1 to 5 hours;

[0112] (d) Process control: During the foaming process, the ventilation system is kept running continuously to ensure complete decomposition of organic gases.

[0113] (2) Secondary foaming:

[0114] (a) Temperature control: The pre-foamed beads are transferred to a drying oven at a constant temperature of 65±5℃ for secondary foaming;

[0115] (b) Time control: The secondary foaming time shall be strictly controlled within the range of 10 to 30 minutes;

[0116] (c) Quality inspection: The foamed beads should be of uniform volume, smooth surface, and free from adhesion.

[0117] (3) Compression molding:

[0118] (a) Loading process: The secondary foaming beads are evenly loaded into the mold cavity of the plug body and the retaining ring mold;

[0119] (b) Distribution control: The beads are evenly distributed in the mold cavity by horizontally reciprocating shaking of the mold;

[0120] (c) Steam foaming: Immerse the mold filled with beads completely in boiling water for 45-60 minutes;

[0121] (d) Molding control: Ensure that the beads fully foam and expand, completely fill the mold cavity, and form the plug body and retaining ring with trapezoidal thread structure in one go.

[0122] (4) Post-processing:

[0123] (a) Cooling treatment: Wear high-temperature resistant gloves, remove the mold from the boiling water, and immediately immerse the mold in a constant temperature water bath at 20-25°C to cool it down. After cooling to room temperature of 25±2°C, open the mold.

[0124] (b) Demolding operation: When demolding, hold the molded part in the groove and carefully unscrew the product to avoid damaging the thread structure;

[0125] (c) Curing treatment: Place the product in a drying oven at 50±5℃ and cure for 6 to 7 hours;

[0126] (d) Quality inspection: Inspect the appearance of the product to ensure that there are no defects such as missing materials or deformation.

[0127] 2. Assembly process:

[0128] (1) Component preparation:

[0129] (a) Confirm that the dimensions of the plug body and retaining ring meet the design requirements;

[0130] (b) Clean the mating surfaces of the nozzle throat liner and the outer surface and threaded areas of the plug body with anhydrous ethanol;

[0131] (c) Apply silicone rubber evenly using a special coating tool; control the coating thickness within the range of 0.2 to 0.5 mm; ensure complete coverage of the threads with no missed areas;

[0132] (2) Assembly steps:

[0133] (a) Insert the plug body axially from the front section of the nozzle, slowly push it to the converging section of the nozzle throat liner, gently rotate the plug body to make it completely fit the inner wall of the converging section, and then insert the retaining ring from the rear section of the nozzle, aligning it with the expanding section of the nozzle throat liner.

[0134] (b) Use a special torque wrench to rotate the retaining ring and check the thread engagement to ensure there is no misalignment or interference.

[0135] (c) Curing at room temperature: 25±5℃ and relative humidity ≤60% for 18~24h;

[0136] This invention discloses a threaded foam plug and its molding method, employing a trapezoidal thread structure to provide a reliable self-locking mechanism. By rotating the retaining ring, it engages and tightens with the trapezoidal thread on the plug body, forming a mechanical self-locking and radial clamping force, effectively resisting vibration and loosening, ensuring a durable connection and structural stability under operating conditions. Simultaneously, the silicone rubber sealing layer ensures excellent airtightness and environmental adaptability, the foam material density can be precisely controlled to achieve accurate opening pressure, and the integrated molding process simplifies the production process, making it suitable for mass production.

[0137] Example 1: The nozzle tail section of a solid rocket motor consists of a nozzle front section, a throat liner, and a nozzle rear section, and is fitted with a plug body and a retaining ring. The throat liner is made of graphite composite material, while the plug body and retaining ring are made of expandable polystyrene foam. A silicone rubber sealing layer is applied between the plug and the throat liner. The plug body has a maximum outer diameter of 136 mm, a total height of 90 mm, a thickness of 47 mm, a convergence angle of 60°, and a trapezoidal thread of Tr55×3 with an effective thread length of 26 mm. The retaining ring has a maximum outer diameter of 100 mm, a thickness of 20 mm, and a convergence angle of 32°. The required opening pressure is 1–1.5 MPa. The specific steps are as follows:

[0138] 1. Component manufacturing process:

[0139] (1) Expandable polystyrene pre-foaming:

[0140] (a) Preparation: Connect the power supply to the electric heating drying oven, turn on the organic gas decomposition treatment system, and set the initial temperature to 45°C;

[0141] (b) Weighing of raw materials: based on a density of 0.3 g / cm³ 3After calculation, using an electronic scale with an accuracy of 0.01g, accurately weigh 125±0.05g of the plug body material and 35±0.05g of the retaining ring material;

[0142] (c) Initial foaming: Place the weighed raw materials in a stainless steel container and put it into a preheated drying oven for foaming at 45°C for 3 hours;

[0143] (d) Process control: During the foaming process, the ventilation system is kept running continuously to ensure complete decomposition of organic gases.

[0144] (2) Secondary foaming:

[0145] (a) Temperature control: Transfer the pre-foamed beads to a drying oven at a constant temperature of 65°C for secondary foaming;

[0146] (b) Time control: The secondary foaming time is 20 minutes;

[0147] (c) Quality inspection: The foamed beads should be of uniform volume, smooth surface, and free from adhesion.

[0148] (3) Compression molding:

[0149] (a) Loading process: The secondary foaming beads are evenly loaded into the mold cavity of the plug body and the retaining ring mold;

[0150] (b) Distribution control: The beads are evenly distributed in the mold cavity by horizontally reciprocating shaking of the mold;

[0151] (c) Steam foaming: Immerse the mold filled with beads completely in boiling water for 60 minutes;

[0152] (d) Molding control: Ensure that the beads fully foam and expand, completely fill the mold cavity, and form the plug body and retaining ring with trapezoidal thread structure in one go.

[0153] (4) Post-processing:

[0154] (a) Cooling treatment: Wear high-temperature resistant gloves, remove the mold from the boiling water, and immediately immerse the mold in a constant temperature water bath at 25°C to cool it down. After cooling to room temperature of 25°C, open the mold.

[0155] (b) Demolding operation: When demolding, hold the molded part in the groove and carefully unscrew the product to avoid damaging the thread structure;

[0156] (c) Curing treatment: Place the product in a drying oven at 50°C and cure for 6 hours;

[0157] (d) Quality inspection: Inspect the appearance of the product to ensure that there are no defects such as missing materials or deformation.

[0158] 2. Assembly process:

[0159] (1) Component preparation:

[0160] (a) Confirm that the dimensions of the plug body and retaining ring meet the design requirements;

[0161] (b) Clean the mating surfaces of the nozzle throat liner and the outer surface and threaded areas of the plug body with anhydrous ethanol;

[0162] (c) Apply silicone rubber evenly using a special coating tool; control the coating thickness within the range of 0.2 to 0.5 mm; ensure complete coverage of the threads with no missed areas;

[0163] (2) Assembly steps:

[0164] (a) Insert the plug body axially from the front section of the nozzle, slowly push it to the converging section of the nozzle throat liner, gently rotate the plug body to make it completely fit the inner wall of the converging section, and then insert the retaining ring from the rear section of the nozzle, aligning it with the expanding section of the nozzle throat liner.

[0165] (b) Use a dedicated torque wrench to rotate the retaining ring and check the thread engagement to ensure there is no misalignment or interference;

[0166] (c) Curing at room temperature (25°C) and relative humidity (54%) for 20 hours.

[0167] Example 2: The nozzle tail section of a solid rocket motor consists of a nozzle front section, a throat liner, and a nozzle rear section, and is fitted with a plug body and a retaining ring. The throat liner is made of tungsten-copper infiltrated material, while the plug body and retaining ring are made of expandable polystyrene foam. A silicone rubber sealing layer is applied between the plug and the throat liner. The plug body has a maximum outer diameter of 138 mm, a total height of 80 mm, a thickness of 30 mm, a convergence angle of 62°, and a trapezoidal thread of Tr60×3 with an effective thread length of 30 mm. The retaining ring has a maximum outer diameter of 110 mm, a thickness of 25 mm, and a convergence angle of 30°. The required opening pressure is 2–2.5 MPa. The specific steps are as follows:

[0168] 1. Component manufacturing process:

[0169] (1) Expandable polystyrene pre-foaming:

[0170] (a) Preparation: Connect the power supply to the electric heating drying oven, turn on the organic gas decomposition treatment system, and set the initial temperature to 50°C;

[0171] (b) Weighing of raw materials: based on a density of 0.4 g / cm³ 3 After calculation, using an electronic scale with an accuracy of 0.01g, 155±0.05g of the plug body material and 46±0.05g of the retaining ring material were accurately weighed.

[0172] (c) Initial foaming: Place the weighed raw materials in a stainless steel container and put it into a preheated drying oven for foaming at 50°C for 3 hours;

[0173] (d) Process control: During the foaming process, the ventilation system is kept running continuously to ensure complete decomposition of organic gases.

[0174] (2) Secondary foaming

[0175] (a) Temperature control: Transfer the pre-foamed beads to a drying oven at a constant temperature of 60°C for secondary foaming;

[0176] (b) Time control: The secondary foaming time is 30 minutes;

[0177] (c) Quality inspection: The foamed beads should be of uniform volume, smooth surface, and free from adhesion.

[0178] (3) Compression molding:

[0179] (a) Loading process: The secondary foaming beads are evenly loaded into the mold cavity of the plug body and the retaining ring mold;

[0180] (b) Distribution control: The beads are evenly distributed in the mold cavity by horizontally reciprocating shaking of the mold;

[0181] (c) Steam foaming: Immerse the mold filled with beads completely in boiling water for 60 minutes;

[0182] (d) Molding control: Ensure that the beads fully foam and expand, completely fill the mold cavity, and form the plug body and retaining ring with trapezoidal thread structure in one go.

[0183] (4) Post-processing:

[0184] (a) Cooling treatment: Wear high-temperature resistant gloves, remove the mold from the boiling water, and immediately immerse the mold in a constant temperature water bath at 25°C to cool it down. After cooling to room temperature of 25°C, open the mold.

[0185] (b) Demolding operation: When demolding, hold the molded part in the groove and carefully unscrew the product to avoid damaging the thread structure;

[0186] (c) Curing treatment: Place the product in a drying oven at 55℃ and cure for 7 hours;

[0187] (d) Quality inspection: Inspect the appearance of the product to ensure that there are no defects such as missing materials or deformation.

[0188] 2. Assembly process:

[0189] (1) Component preparation:

[0190] (a) Confirm that the dimensions of the plug body and retaining ring meet the design requirements;

[0191] (b) Clean the mating surfaces of the nozzle throat liner and the outer surface and threaded areas of the plug body with anhydrous ethanol;

[0192] (c) Apply silicone rubber evenly using a special coating tool; control the coating thickness within the range of 0.2 to 0.5 mm; ensure complete coverage of the threads with no missed areas;

[0193] (2) Assembly steps:

[0194] (a) Insert the plug body axially from the front section of the nozzle, slowly push it to the converging section of the nozzle throat liner, gently rotate the plug body to make it completely fit the inner wall of the converging section, and then insert the retaining ring from the rear section of the nozzle, aligning it with the expanding section of the nozzle throat liner.

[0195] (b) Use a dedicated torque wrench to rotate the retaining ring and check the thread engagement to ensure there is no misalignment or interference;

[0196] (c) Curing at room temperature (25°C) and relative humidity (54%) for 24 hours.

[0197] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A threaded foam plug, installed at the tail end of a rocket engine nozzle, characterized in that, include: The plug body (1) has a first trapezoidal thread structure (2) on its outer side. The retaining ring (3) has a second trapezoidal thread structure (4) on its inner side. The first trapezoidal thread structure (2) and the second trapezoidal thread structure (4) are adapted to each other and screwed together. The mating surfaces of the first trapezoidal thread structure (2) and the second trapezoidal thread structure (4) are configured to generate mechanical self-locking and radial clamping force when screwed together, so as to achieve the sealing connection between the plug body (1) and the retaining ring (3), thereby achieving the sealing of the rocket engine nozzle. Both the plug body (1) and the retaining ring (3) are made of expandable polystyrene foam and molded in one piece. The plug body (1) has a first mating surface, and the retaining ring (3) has a second mating surface, wherein the first mating surface and the second mating surface are adapted to each other; The first mating surface has a first convergence angle, and the second mating surface has a second convergence angle. The first mating surface and the second mating surface are arranged relative to each other with the first convergence angle and the second convergence angle, so that when the first mating surface and the second mating surface are pressed together along the axial direction, radial interference is generated, thereby achieving sealing and locking.

2. A threaded foam plug according to claim 1, characterized in that, The outer side of the plug body (1) is coated with a silicone rubber sealing layer (5); The outer and inner sides of the retaining ring (3) are coated with a silicone rubber sealing layer (5); The thickness of the silicone rubber sealing layer (5) is 0.2-0.5 mm.

3. A threaded foam plug according to claim 1, characterized in that, The maximum outer diameter of the plug body (1) is 135-139 mm; The height of the plug body (1) is 80-90mm; The thickness of the plug body (1) is 30-50mm; The first convergence angle is 60°-63°; The maximum outer diameter of the retaining ring (3) is 100-110 mm; The thickness of the retaining ring (3) is 20-26 mm; The second convergence angle is 30°-33°.

4. A threaded foam plug according to claim 1, characterized in that, The nominal diameter of the first trapezoidal thread structure (2) and the second trapezoidal thread structure (4) is 50-65mm, and the pitch is 3mm; The effective lengths of the first trapezoidal thread structure (2) and the second trapezoidal thread structure (4) are 25-30 mm.

5. A threaded foam plug according to claim 1, characterized in that, The tail section of the rocket engine nozzle includes a nozzle front section, a throat liner (6), and a nozzle rear section arranged coaxially along the gas flow direction. The throat liner (6) has a converging section and an expanding section; The plug body (1) is located within the converging section of the throat liner (6), and the outer peripheral surface of the plug body (1) is adapted to and fits the inner wall surface of the converging section. The retaining ring (3) is located inside the rear section of the nozzle and corresponds to the axial rear of the expansion section.

6. A method for molding a threaded foam plug, characterized in that, Includes the following steps: S1: Expandable polystyrene pre-foamed to obtain one-time foamed beads; S2: Perform secondary foaming on the primary foamed beads to obtain secondary foamed beads; S3: The secondary foaming beads are evenly loaded into the mold of the plug body and the retaining ring to ensure that the secondary foaming beads are fully foamed and expanded to completely fill the mold cavity, and the plug body (1) and the retaining ring (3) with trapezoidal thread structure are formed at one time. The mold filled with the secondary foaming beads is completely immersed in boiling water for steam foaming. The boiling water bath time is 45-60 minutes. S4: Remove the mold from the boiling water and perform cooling and demolding treatment to obtain a plug body (1) and a retaining ring (3) with a trapezoidal thread structure. S5: Assemble the plug body (1) and the retaining ring (3) and install them at the tail of the rocket engine nozzle.

7. The molding method of a threaded foam plug according to claim 6, characterized in that, In S1, a certain amount of expandable polystyrene is weighed for pre-foaming. The pre-foaming temperature is set at 40-50℃, and the pre-foaming time is 1-5 hours. During the pre-foaming process, the ventilation system is kept running continuously to ensure complete decomposition of organic gases. In S2, the set temperature for the secondary foaming is 60-70℃, and the secondary foaming time is 10-30 minutes, to ensure that the secondary foamed beads have uniform volume, smooth surface, and no adhesion.

8. The molding method of a threaded foam plug according to claim 6, characterized in that, In S4, the cooling temperature of the cooling demolding process is 20-25°C, and the mold is opened after cooling to room temperature; The plug body (1) and retaining ring (3) with trapezoidal thread structure are subjected to curing treatment at a temperature of 45-55℃ and a curing time of 6-7h.

9. A method for molding a threaded foam plug according to claim 6, characterized in that, S5 includes: A silicone rubber sealing layer (5) is applied to the outer side of the plug body (1) and the inner and outer sides of the retaining ring (3) to ensure that the threads are completely covered and there is no omission in the coating. The thickness of the silicone rubber sealing layer (5) is 0.2-0.5 mm. The plug body (1) is axially inserted from the front section of the nozzle and pushed into the converging section of the throat liner (6), and the outer surface of the plug body (1) is made to fit against the inner wall of the converging section. The retaining ring (3) is inserted axially from the rear section of the nozzle so that the retaining ring (3) is positioned axially behind the expansion section of the throat liner (6); The trapezoidal thread structure of the retaining ring (3) and the plug body (1) is connected to complete the installation and fixation of the plug. Curing time is 18-24 hours at room temperature with relative humidity ≤60%.

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