A process for making very large MIM hardware

Abstract

This application discloses a process for fabricating ultra-large MIM hardware, comprising: mixing metal powder and binder in a mixer to form a feedstock; injecting the feedstock into a mold cavity for pressure holding, cooling and demolding to obtain an ultra-large MIM hardware blank; degreasing the ultra-large MIM hardware blank to obtain a blank; and placing the blank in a sintering furnace for partial pressure sintering. The binder comprises, by mass ratio, 85%-91% polyoxymethylene, 2%-5% polyethylene glycol, 2%-3% polypropylene wax, 1%-2% high-density polyethylene, 0.5%-1% nano-SiO2 double-grafted material, 0.5%-1.5% polyethylene wax, 0.5%-1.5% 2,6-di-tert-butyl-p-cresol, and 1%-3% low molecular weight polycaprolactone. The nano-SiO2 double-grafted compound consists of nano-SiO2 first grafted with tributyl citrate and then grafted with polyethylene wax. After grafting, the tributyl citrate segments improve the feed flowability and extensibility, and promote the dispersion of nano-SiO2. The polyethylene wax segments can enhance the interfacial compatibility between nano-SiO2 and non-polar components such as polypropylene wax and high-pressure polyethylene. After degreasing, the nano-SiO2 particles inhibit pore expansion and support atomic diffusion in the early stage of sintering.

Description

Technical Field

[0001] This application belongs to the field of metal injection molding technology, and more specifically, relates to a process for manufacturing ultra-large MIM hardware. Background Technology

[0002] Metal injection molding (MIM) technology, as an advanced manufacturing process combining powder metallurgy and precision injection molding, has been widely used in aerospace, medical devices, new energy and other fields due to its advantages such as being able to directly produce hardware with complex details and shapes close to the final product, high material utilization, and good mass production consistency.

[0003] As industrial equipment develops towards larger size and integration, the demand for ultra-large MIM hardware (structural and functional components weighing over 1000g) is becoming increasingly urgent. These parts not only require one-time forming of complex geometries, but also need to meet stringent performance indicators such as corrosion resistance and chemical stability to adapt to service environments such as humidity, high temperature, and strong media.

[0004] However, multiple companies in the industry have confirmed that while MIM technology can produce parts weighing over 200g, it faces multiple technical challenges. This results in high production difficulty, limited yield, and economic efficiency for parts in this weight range, severely restricting its application in high-requirement fields. Furthermore, with current technology, the maximum weight of hardware that can be formed using MIM technology is approximately 500g. Conventional small and medium-sized MIM parts (≤200g) can have their porosity controlled at 1%-3% through process optimization. For large and ultra-large MIM hardware, due to their significantly increased volume and weight, uneven heat conduction, large internal and external temperature differences, and insufficient metal powder diffusion during sintering easily lead to a large number of residual micropores. The densification difficulty increases with the size of the part, and the porosity increases significantly, often exceeding 5%. These interconnected pores become channels for chemical media penetration, not only accelerating localized corrosion but also greatly reducing the part's resistance to salt spray corrosion. Summary of the Invention

[0005] The purpose of this application is to provide a process for manufacturing ultra-large MIM hardware, so as to solve the technical problem that ultra-large MIM hardware is prone to high porosity in the prior art.

[0006] To achieve the above objectives, the technical solution adopted in this application is: a process for manufacturing ultra-large-scale MIM hardware, comprising the following steps:

[0007] S1. Mix the metal powder and binder in an internal mixer to prepare the feed;

[0008] S2. The feed material is injected into the mold cavity and held under pressure. After holding under pressure, it is cooled and demolded to produce an ultra-large MIM hardware blank.

[0009] S3. Degrease the ultra-large MIM hardware blank to obtain a blank;

[0010] S4. Place the blank in a sintering furnace for partial pressure sintering to obtain an ultra-large MIM hardware finished product.

[0011] The adhesive comprises, by mass ratio, 85%-91% polyoxymethylene, 2%-5% polyethylene glycol, 2%-3% polypropylene wax, 1%-2% high-density polyethylene, 0.5%-1% nano-SiO2 double-grafted material, 0.5%-1.5% polyethylene wax, 0.5%-1.5% 2,6-di-tert-butyl-p-cresol, and 1%-3% low molecular weight polycaprolactone;

[0012] The nano-SiO2 double-grafted compound is nano-SiO2 first grafted with tributyl citrate, and then grafted with polyethylene wax.

[0013] Optionally, the metal powder has a particle size of 0.5-20 μm, and the nano-SiO2 has a particle size of 50-100 nm.

[0014] Optionally, in step S1, the metal powder and binder are mixed in an internal mixer at 160-180°C for 40-60 minutes to form a viscous feed.

[0015] Optionally, the feed comprises 82%-85% metal powder and 15%-18% binder by mass ratio.

[0016] Optionally, the mold adopts a multi-gate and hot runner structure, and the size of the mold cavity is enlarged by 15% according to the size of the ultra-large MIM hardware finished product as compensation for sintering shrinkage.

[0017] Optionally, an injection molding machine is used to inject the material into the mold cavity, with an injection pressure of 80-120MPa, a barrel temperature of 190-210℃, and a holding time of 30-60s.

[0018] Optionally, the ultra-large MIM hardware blank is trimmed before degreasing.

[0019] Optionally, the specific steps of the degreasing process are as follows:

[0020] Pretreatment: The ultra-large MIM hardware blank is placed in a degreasing furnace and vacuumed. Nitrogen gas is then introduced and maintained at a flow rate of 104.5 L / min. The temperature of the vaporization box is 145-150℃ and the temperature inside the degreasing furnace is 80-85℃.

[0021] Catalytic degreasing: Maintain nitrogen flow rate, raise the temperature of the vaporization box to 150-160℃, feed oxalic acid at a rate of 5-10g / min, and raise the temperature in the degreasing furnace in stages to 110-130℃.

[0022] The segmented heating process includes raising the temperature inside the degreasing furnace from 80-85℃ to 90℃ and holding it for 3-4 hours; raising it from 90℃ to 110℃ and holding it for 4-6 hours; and raising it from 110℃ to 130℃ and holding it for 4-6 hours.

[0023] Optionally, the specific steps of the partial pressure sintering process are as follows:

[0024] The sintering furnace is pre-evacuated to ≤10Pa and held for 10min; the temperature is then raised from room temperature to 600-800℃, held for 45-60min at a rate of 2-3℃ / min, and nitrogen is introduced at a flow rate of 30-40L / min; the temperature is then raised to 1000-1100℃, held for 60-90min at a rate of 1.5-2.5℃ / min; and the temperature is then raised to 1350℃, held for ≥120min at a rate of 1-2℃ / min.

[0025] The partial pressure sintering is applicable to blanks with a composition of stainless steel 316.

[0026] Optionally, after the blank is sintered, it is cooled to 850°C in the sintering furnace, then cooled to 450°C at a cooling rate of ≥5°C / min, and then slowly cooled to room temperature.

[0027] The beneficial effects of the process provided in this application for manufacturing ultra-large MIM hardware are as follows: Compared with the prior art, the binder used in this application is mainly composed of polyoxymethylene, ensuring the plasticity of the feedstock. Polyethylene glycol in the binder can rapidly seep out during the degreasing stage, opening channels for the discharge of decomposition products of other binder components, effectively solving the problems of slow degreasing and easy residue in ultra-large hardware. Polypropylene wax in the binder works synergistically with high-pressure polyethylene and polyethylene wax to improve feedstock flowability and blank strength, facilitating the molding of complex structures in ultra-large hardware. The tributyl citrate segments of nano-SiO2 double-grafted compounds in the binder can significantly improve the flowability and ductility of the feedstock, ensuring smooth filling of complex mold cavities. In the early stage of degreasing, these segments can improve the compatibility between nano-SiO2 and metal powder, ensuring their uniform dispersion; after degreasing, the uniformly distributed nano-SiO2 particles inhibit excessive agglomeration of metal powder through physical barriers, preventing pore enlargement, and playing a supporting role in the early stage of sintering, facilitating short-range diffusion of metal atoms in the internal low-temperature zone. The other segment of the nano-SiO2 double-grafted compound, polyethylene wax, can enhance the interfacial compatibility between nano-SiO2 and non-polar components such as polypropylene wax and high-pressure polyethylene in the binder, further reducing the viscosity of the feed melt and improving the mixing uniformity of metal powder and binder.

[0028] The segmented catalytic degreasing process employed in this application achieves efficient removal of the binder by controlling the nitrogen flow rate, vaporization chamber temperature, and heating rate, avoiding degreasing dead zones. The partial pressure sintering, through multi-stage coordinated control of pressure and temperature, combined with nitrogen protection, promotes full diffusion of metal powder, reduces porosity, and significantly improves the density and mechanical properties of ultra-large MIM hardware. This effectively solves the performance degradation problems caused by high porosity and binder residue in existing technologies, enabling the product to meet the requirements of harsh service environments such as humidity, high temperature, and strong media. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a flowchart illustrating a process for fabricating ultra-large-scale MIM hardware, provided as an embodiment of this application. Detailed Implementation

[0031] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0032] With the development of industrial equipment towards larger size and integration, there is an urgent need for ultra-large MIM structural and functional components weighing up to 2kg. This requires the one-time forming of complex geometries and the achievement of corrosion resistance and chemical stability requirements under harsh environments such as humidity and high temperature. Although MIM technology can produce parts weighing over 200g, it faces multiple technical challenges. For conventional small and medium-sized MIM parts (≤200g), the porosity can be controlled at 1%-3% through process optimization. However, for ultra-large MIM hardware, due to its significantly increased volume and weight, uneven heat conduction, large internal and external temperature differences, and insufficient diffusion of metal powder during sintering can easily leave a large number of micropores. The densification difficulty increases with the increase in part size, and the porosity increases significantly, often exceeding 5%. These pores become channels for chemical media penetration, accelerating localized corrosion and greatly reducing the part's salt spray corrosion resistance time.

[0033] Please see Figure 1 To address the aforementioned problems, this application provides a process for fabricating ultra-large-scale MIM hardware, comprising the following steps:

[0034] S1. Mix the metal powder and binder in an internal mixer to prepare the feed;

[0035] S2. The feed material is injected into the mold cavity and held under pressure. After holding under pressure, it is cooled and demolded to produce an ultra-large MIM hardware blank.

[0036] S3. Degrease the ultra-large MIM hardware blank to obtain a blank;

[0037] S4. Place the blank in a sintering furnace for partial pressure sintering to obtain an ultra-large MIM hardware finished product.

[0038] The adhesive comprises, by mass ratio, 85%-91% polyoxymethylene, 2%-5% polyethylene glycol, 2%-3% polypropylene wax, 1%-2% high-density polyethylene, 0.5%-1% nano-SiO2 double-grafted material, 0.5%-1.5% polyethylene wax, 0.5%-1.5% 2,6-di-tert-butyl-p-cresol, and 1%-3% low molecular weight polycaprolactone;

[0039] The nano-SiO2 double-grafted compound is nano-SiO2 first grafted with tributyl citrate, and then grafted with polyethylene wax.

[0040] It should be noted that a Banbury mixer is used to evenly disperse the metal powder in the feedstock, preventing agglomeration and ensuring uniform composition of the finished product. The binder (mainly polyoxymethylene) provides good plasticity and flowability, meeting the requirements for filling the mold cavity during injection. Injection pressure is used to fully press the feedstock into the cavity of the ultra-large mold, ensuring that the feedstock fills every detail of the cavity and replicates the mold's designed shape. The holding pressure process replenishes the portion of the feedstock that has cooled and shrunk in the cavity, preventing defects such as material shortages and depressions in the blank. After cooling and solidification, the feedstock yields a blank with the final shape consistent with the ultra-large MIM hardware after demolding, providing a basic form for subsequent processing. Degreasing removes the polyoxymethylene and other binders from the blank, preventing the high-temperature decomposition of the binders during sintering and the generation of gases that could lead to porosity and cracks in the finished product. After degreasing, micropores form inside the blank, providing space for the shrinkage and bonding of the metal powder during sintering, ensuring a smooth sintering process. In a high-temperature environment, the metal powder particles on the surface of the blank melt and diffuse and combine with each other, filling the pores formed after degreasing, thus densifying the blank and significantly improving its mechanical properties such as hardness and strength. Partial pressure sintering solves key problems faced by ultra-large blanks during sintering, such as incomplete removal of degreasing residues, thermal stress concentration, uneven densification, and oxidation risk, by controlling the pressure and temperature in the sintering furnace in stages. The sintering process is accompanied by a certain amount of shrinkage, ultimately yielding ultra-large MIM hardware products with precise dimensions and meeting performance standards.

[0041] In the internal mixer, metal powder and binder are mixed. The binder is in a molten or semi-molten state, which ensures uniform dispersion of the metal powder and guarantees feed flowability. Polyoxymethylene (POM) accounts for 86%-91% of the binder. As the main binder phase, its semi-molten state provides high flowability and good moldability, allowing it to uniformly coat the metal powder particles and ensuring that the feed can fill the complex cavity of ultra-large molds during injection. Simultaneously, POM gradually decomposes into gases (mainly formaldehyde) during the subsequent debinding stage, with minimal decomposition product residue, avoiding contamination of the preform and laying the foundation for uniform debinding of ultra-large preforms.

[0042] During mixing in an internal mixer, polyethylene glycol (PEG) melts completely first due to its low melting point, forming a synergistic flow system with the semi-molten polyoxymethylene (POM). PEG's good compatibility reduces the overall viscosity of the binder, further enhancing the feed's wetting ability on the metal powder, especially in difficult-to-fill areas such as deep cavities and thin walls in ultra-large molds, assisting in precise filling. PEG is water-soluble, dissolving in water vapor and detaching directly from ultra-large MIM hardware blanks, forming dense micropore channels within the ultra-large blanks. These channels can subsequently serve as venting channels for formaldehyde. A PEG content of 4%-6% not only helps improve feed flowability during mixing in the internal mixer but also creates sufficient venting channels during debinding.

[0043] Polypropylene wax is a low molecular weight polypropylene homopolymer with low melt viscosity and waxy properties. Its melt flow rate is higher than that of polypropylene, reducing the melt viscosity of the feedstock and making it easier for metal powder to fill deep cavities and thin-walled areas of ultra-large molds during the injection stage. Adding 2%-3% polypropylene wax can lower the feedstock melt viscosity, meeting the filling requirements of deep cavities and thin-walled areas in ultra-large molds. The flexibility of the high-density polyethylene (HDPE) molecular chains can alleviate shrinkage stress during cooling (the temperature difference between the inside and outside of ultra-large blanks is large, and the stress can be 2-3 times that of small parts). If the HDPE content is <1%, the buffering effect is insufficient, and cracks are prone to occur; if the combined polypropylene wax and HDPE content is >5%, the feedstock viscosity will surge due to excessive entanglement of the polymer chains, affecting injection flowability. The decomposition temperature of polypropylene wax and HDPE is higher than that of polyoxymethylene (POM). During the early stages of debinding (when POM decomposes), they maintain structural stability, serving as a temporary skeleton for the blank and preventing collapse due to excessive binder loss.

[0044] Polyethylene wax, as a lubricant, can provide sufficient lubrication without affecting the feed strength, solving the problems of high feed filling resistance in ultra-large molds and difficult demolding of ultra-large MIM hardware blanks. A 1%-2% content of polyethylene wax in the binder can form a uniform lubricating film between the feed and the mold cavity, reducing flow resistance during injection.

[0045] 2,6-Di-tert-butyl-p-cresol (BHT), as a highly effective antioxidant, can inhibit the oxidative degradation of polymeric components such as polyoxymethylene, polyethylene glycol, and polyethylene in binders during mixing and injection molding at an addition level of 1%-2%. In ultra-large MIM feedstocks, the mixing time is longer, and the mold cavity filling path and residence time during injection molding are long. Polymer chains are prone to chain breakage and oxidation due to high temperatures and mechanical shearing, leading to decreased binder fluidity and uneven molecular weight distribution, which in turn causes a decrease in feedstock filling capacity or local performance defects in the blank. BHT stabilizes the molecular structure of the binder by capturing free radicals and terminating the oxidation chain reaction, ensuring stable fluidity and plasticity of the feedstock during long-term processing, providing a foundation for uniform filling of ultra-large mold cavities and the integrity of the blank morphology.

[0046] Due to their large size and high heat capacity, ultra-large MIM hardware is prone to significant internal and external temperature differences during sintering due to long heat transfer paths. This temperature difference causes uneven diffusion dynamics of metal atoms, with sufficient diffusion on the surface and lag in the interior, ultimately leading to porosity due to loose interparticle bonding. Tributyl citrate, as a typical plasticizer, not only effectively lowers the glass transition temperature of main binders such as polyoxymethylene (POM) through its plasticizing effect, but also significantly improves the flowability and ductility of the feedstock. In the injection molding stage, this optimization allows the feedstock to fill the complex structure of the ultra-large mold cavity more smoothly under pressure, reducing localized incomplete filling caused by differences in flow resistance, and reducing the initial porosity of the blank from the source. As a polar plasticizing segment, the tributyl citrate segment forms silicon-oxygen-carbon bonds with the hydroxyl groups on the surface of nano-SiO2. At the same time, due to its good compatibility with polar binder components such as POM and polyethylene glycol, it breaks the agglomeration tendency of nano-SiO2, ensuring its uniform dispersion in the feedstock. Polyethylene wax segments, as non-polar lubricating segments, when grafted onto the outer layer of tributyl citrate, enhance the interfacial compatibility between nano-SiO2 and non-polar components such as polypropylene wax and high-pressure polyethylene in the binder. This further reduces the melt viscosity of the feedstock and improves the smoothness of injection flow, especially addressing the differences in shear resistance in complex flow channels of ultra-large mold cavities, reducing local density fluctuations caused by uneven flow velocity. Simultaneously, during the degreasing stage, the polyethylene wax segments gradually decompose along with the main binder such as polyoxymethylene (POM). The decomposition products are low-molecular-weight hydrocarbons, which are easily discharged through the pre-formed pore channels of polyethylene glycol, avoiding residual contamination. After degreasing is complete, the organic grafted chains are completely removed, leaving uniformly distributed nano-SiO2 particles in the metal matrix. The uniformly distributed nano-SiO2 particles inhibit the excessive migration and agglomeration of metal powder through physical barrier effect, and limit grain growth through "pinning effect" to avoid the expansion of intergranular pores caused by grain coarsening. On the other hand, the stable rigid structure of nano-SiO2 itself plays a supporting role in the early stage of sintering (the stage when metal particles begin to soften but are not fully densified), resisting the excessive collapse of particles caused by surface tension, maintaining the smooth flow of contact points and diffusion channels between powders, and ensuring that even in the low-temperature region inside ultra-large parts, metal atoms can still complete short-range diffusion through a continuous and stable contact interface.

[0047] The addition of 1%-3% low molecular weight polycaprolactone (PCL) exhibits excellent compatibility with all components of the binder, reducing the viscosity of the feed melt, improving fluidity and wettability, and facilitating the filling of complex cavities in ultra-large molds. It also plays an auxiliary supporting role during the debinding process, preventing the collapse and deformation of ultra-large blanks due to binder loss, and its decomposition products are easily discharged with minimal residue. Furthermore, it optimizes the uniformity of sintering densification, alleviates thermal stress caused by internal and external temperature differences in ultra-large parts, reduces the risk of cracking, and simultaneously helps stabilize the dispersion of metal powder.

[0048] In another embodiment of this application, the metal powder has a particle size of 0.5-20 μm, and the nano-SiO2 has a particle size of 50-100 nm.

[0049] It should be noted that micron-sized metal powders with a particle size of 0.5-20μm have a larger specific surface area, allowing for thorough contact and uniform mixing with polyoxymethylene (POM) binders. This prevents powder agglomeration, significantly improves feed flowability, and ensures smooth filling of ultra-large mold cavities during subsequent injection molding, reducing defects such as material shortages and air bubbles. Furthermore, the smaller gaps between 0.5-20μm fine particles result in denser, more structurally stable green bodies, reducing the risk of cracking and deformation during demolding and transport, while also minimizing degreasing expansion issues. In addition, micron-sized metal powders have shorter atomic diffusion distances during sintering, enabling more efficient densification and significantly reducing the porosity of the finished product. This not only improves mechanical properties such as hardness and tensile strength to meet the structural strength requirements of ultra-large parts but also reduces sintering shrinkage differences, ensuring the dimensional accuracy of ultra-large MIM hardware.

[0050] A particle size range of 50-100nm for nano-SiO2 ensures sufficient dispersion in the feedstock to provide physical barrier and support. However, if the nano-SiO2 particle size is too small (e.g., <50nm), its extremely high specific surface area will lead to a sudden increase in surface energy, making it prone to agglomeration, which in turn increases the viscosity of the feedstock melt and hinders the filling of ultra-large molds. If the particle size is too large (e.g., >100nm), it may become a "rigid impurity," blocking the synergistic flow of metal powder and binder, and even causing local stress concentration during injection due to uneven flow resistance.

[0051] In another embodiment of this application, in step S1, the metal powder and the binder are mixed in an internal mixer at 160-180°C for 40-60 minutes to form a viscous feed.

[0052] It should be noted that a temperature of 160-180℃ allows the binder to fully melt and become fluid, while avoiding excessively high temperatures that could lead to binder aging and decomposition or metal powder oxidation. In terms of mixing time, a mixing duration of 40-60 minutes, combined with the shearing and stirring action of the internal mixer, ensures deep and uniform coating of the molten binder and metal powder, preventing powder agglomeration and ensuring consistent feed composition. The resulting viscous feedstock possesses sufficient fluidity to meet the filling requirements of subsequent ultra-large mold cavities while maintaining a certain degree of shaping ability, preventing issues such as delamination and material shortages during injection. This lays the foundation for preparing high-quality ultra-large MIM hardware blanks.

[0053] In another embodiment of this application, the feed comprises 82%-85% metal powder and 15%-18% binder by mass ratio.

[0054] It should be noted that metal powder accounts for 82%-85% of the feed mass, ensuring a high proportion of metal powder as a skeleton component, laying the foundation for the mechanical properties and dimensional stability of the final product. In addition, 15%-18% binder provides sufficient flowability and plasticity to the feed. The binder can uniformly coat the metal powder particles, meeting the requirement of complete and uniform filling of ultra-large mold cavities.

[0055] In another embodiment of this application, the mold adopts a multi-gate and hot runner structure, and the size of the mold cavity is enlarged by 15% according to the size of the ultra-large MIM hardware finished product as compensation for sintering shrinkage.

[0056] It should be noted that ultra-large mold cavities have large volumes and long filling paths, making it prone to uneven material filling, insufficient material at the end, or significant differences in cooling rates when using a single gate. Multi-gate systems allow for simultaneous injection of material from multiple locations, shortening the single filling path and enabling the material to quickly and evenly fill the entire cavity. This reduces air bubbles, shrinkage cavities, and localized stress concentration, ensuring the integrity and uniformity of the ultra-large MIM hardware blank's shape. Traditional cold runner systems cause the material within the runner to solidify upon cooling, generating substantial gate waste and potentially affecting the stability of the cavity filling pressure. Hot runner systems maintain the material within the runner in a molten flow state through heating, avoiding waste generation, reducing costs, and ensuring consistent material flow during filling. This guarantees stable mold cavity pressure and prevents incomplete filling due to runner cooling. In the MIM process, the blank shrinks during high-temperature sintering due to the bonding of metal powder particles. If the mold cavity is designed according to the dimensions of the ultra-large MIM hardware finished product, the sintered product will shrink and fail to meet dimensional requirements. Enlarging the cavity size by 15% in advance can precisely offset the shrinkage during the sintering process, ensuring that the final ultra-large MIM hardware product size perfectly matches the design specifications and meets the precision requirements of ultra-large components.

[0057] In another embodiment of this application, an injection molding machine is used to inject the feed material into the mold cavity, with an injection pressure of 80-120MPa, a barrel temperature of 190-210℃, and a holding time of 30-60s.

[0058] It should be noted that the barrel temperature needs to be higher than the feed material preparation temperature (160-200℃). The core function is to maintain a stable melt flow dynamic within the barrel for the viscous feed material, preventing it from cooling and solidifying due to excessively low temperatures, clogging the flow channels, or decomposing the binder and oxidizing the metal powder due to excessively high temperatures. A barrel temperature range of 190-210℃ ensures that the feed material always possesses the necessary fluidity to fill the cavity, while also matching the temperature of the subsequent hot runner structure, preventing the feed material from cooling and thickening within the runner, which would affect filling efficiency. For ultra-large mold cavities with large volumes and long filling paths, sufficient pressure is required to overcome the feed material flow resistance. An injection pressure of 80-120MPa can quickly propel the molten feed material to diffuse synchronously from multiple gates, filling every corner of the cavity. This is especially effective for complex or remote areas of the cavity, preventing defects such as incomplete filling, missing corners, and air bubbles caused by insufficient pressure. Meanwhile, the injection pressure of 80-120MPa will not be too high, preventing damage to the mold due to overload or internal stress caused by excessive compression of the blank, which could lead to cracking during subsequent demolding. After the material is injected into the mold cavity, it will gradually shrink due to cooling. If the material is not continuously replenished, it is easy to cause depressions and shrinkage cavities. A holding pressure time of 30-60s allows for continuous pressure application during the initial cooling stage of the material, replenishing the material missing due to shrinkage in the cavity, ensuring the integrity of the blank shape and a smooth surface. At the same time, this duration is suitable for the cooling and shrinkage rate of ultra-large blanks, avoiding defects caused by holding pressure that is too short, or defects caused by holding pressure that is too long, resulting in the blank sticking to the mold, excessive burrs, and increased demolding difficulty.

[0059] In another embodiment of this application, the ultra-large MIM hardware blank is trimmed before degreasing.

[0060] It should be noted that after demolding, the blank often has burrs and excess material. At this time, the blank still retains a certain degree of toughness due to the presence of binder, making it less prone to breakage during trimming. If degreasing is delayed, the blank becomes loose and porous due to the removal of binder, making it fragile. For ultra-large parts, the large size and uneven stress make them highly susceptible to cracking or breakage during trimming. Trimming in advance can completely avoid this problem. Ultra-large MIM hardware has high requirements for dimensional accuracy. If the burrs on the blank are not cleaned in advance, they will shrink synchronously with the blank during degreasing and sintering, eventually turning into dimensional deviations or surface defects (such as protrusions and burrs) in the finished product. Trimming in advance can remove these defects early in the process, preventing defects from being amplified in subsequent processes, ensuring that the finished product dimensions conform to the design specifications, and reducing later correction costs.

[0061] In another embodiment of this application, the specific steps of the degreasing treatment are as follows:

[0062] Pretreatment: The ultra-large MIM hardware blank is placed in a degreasing furnace and vacuumed. Nitrogen gas is then introduced and maintained at a flow rate of 104.5 L / min. The temperature of the vaporization box is 145-150℃, and the temperature inside the degreasing furnace is 80-85℃.

[0063] Catalytic degreasing: Maintain nitrogen flow rate, raise the temperature of the vaporization box to 150-160℃, feed oxalic acid at a rate of 5-10g / min, and raise the temperature in the degreasing furnace in stages to 110-130℃.

[0064] The segmented heating process includes raising the temperature inside the degreasing furnace from 80-85℃ to 90℃ and holding it for 3-4 hours; raising it from 90℃ to 110℃ and holding it for 4-6 hours; and raising it from 110℃ to 130℃ and holding it for 4-6 hours.

[0065] It should be noted that the pretreatment stage uses a degreasing furnace temperature of 80-85℃ and a vaporization box temperature of 145-150℃, combined with a nitrogen flow rate of 104.5L / min. This not only inhibits premature and violent decomposition of the binder through a low-temperature environment, but also ensures an oxygen-free environment with a high flow rate of nitrogen, providing a stable initial degreasing atmosphere for ultra-large blanks. During this stage, polyethylene glycol (the low-melting-point component in the binder) begins to soften slowly, laying the groundwork for subsequent dissolution and volatilization. At the same time, the temperature of 80-85℃ is below the decomposition threshold of polyoxymethylene, avoiding localized stress concentration caused by excessively rapid heating.

[0066] In the catalytic degreasing stage, the temperature of the vaporization chamber is raised to 150-160℃ to match the vaporization requirements of oxalic acid (oxalic acid sublimates in large quantities at 150℃), allowing oxalic acid to fully penetrate the thick-walled areas and complex cavities of the ultra-large blank in gaseous form. The oxalic acid feeding rate of 5-10g / min ensures that the amount of catalyst matches the decomposition requirements of the binder, preventing residual pollution due to excess and affecting decomposition efficiency due to insufficient catalyst. Segmented heating addresses the slow heat transfer characteristic of ultra-large blanks. Through stepped heating and prolonged holding, heat gradually penetrates into the blank, allowing the binder (especially polyoxymethylene) to decompose uniformly under the catalysis of oxalic acid: the 90℃ stage, with slow heating and a sufficient duration of 3-4 hours, promotes the complete volatilization of polyethylene glycol and the formation of exhaust channels; the stepped heating from 90℃ to 110℃, combined with 4-6 hours of holding, accelerates the depolymerization of polyoxymethylene into formaldehyde gas (oxalic acid, as an acidic catalyst, can reduce its decomposition activation energy), avoiding blistering or cracking caused by differences in decomposition rates between the inside and outside; a small increase to 110-130℃ and holding for 4-6 hours ensures that the polyoxymethylene inside the blank also decomposes fully, and the formaldehyde gas produced by decomposition can be discharged in time through the exhaust channels formed in the early stage under the high-flow nitrogen carrier. The long-term holding effectively avoids polyoxymethylene residue caused by insufficient internal temperature of the blank.

[0067] The synergistic effect of the full-process vacuum environment and high-flow nitrogen is particularly crucial: the vacuum reduces the system pressure, promotes the escape of binder decomposition gases, and the nitrogen flow rate of 104.5 L / min serves as the carrier gas and exhaust medium, uniformly delivering gaseous oxalic acid to all areas of the blank, ensuring sufficient catalytic reaction, and promptly removing gases such as formaldehyde to prevent condensation and deposition on the surface.

[0068] In another embodiment of this application, the specific steps of the partial pressure sintering process are as follows:

[0069] The sintering furnace is pre-evacuated to ≤10Pa and held for 10 min; the temperature is then raised from room temperature to 600-800℃, held for 45-60 min at a rate of 2-3℃ / min, and nitrogen is introduced at a flow rate of 30-40L / min; the temperature is then raised to 1000-1100℃, held for 60-90 min at a rate of 1.5-2.5℃ / min; and the temperature is then raised to 1350℃, held for ≥120 min at a rate of 1-2℃ / min.

[0070] The partial pressure sintering is applicable to blanks with a composition of stainless steel 316.

[0071] It should be noted that by pre-evacuating and strictly controlling the vacuum level to ≤10Pa, the residual oxygen content in the sintering furnace can be reduced to below 0.1%, thus preventing the oxidation of Cr and Mo elements in 316 stainless steel during subsequent heating. The 10-minute pressure holding step verifies the furnace's sealing performance (if the vacuum level rises above the standard, it indicates a micro-leakage), preventing air from seeping in during subsequent nitrogen introduction and ensuring the purity of the inert atmosphere. This is crucial for the overall oxidation protection of ultra-large blanks.

[0072] Large-scale MIM blanks are bulky and may have uneven wall thickness, resulting in slow heat conduction. Slowly raising the temperature from room temperature to 600-800℃ at a gentle heating rate of 2-3℃ / min allows heat to gradually penetrate from the surface of the blank to its interior, ensuring a uniform temperature rise. Although degreasing has been performed previously, trace amounts of non-volatile binders may remain inside the large-scale blanks. Gradual heating below 600-800℃ allows these residual components to gradually volatilize at lower temperatures, preventing blistering or porosity caused by sudden volatilization at higher temperatures. Introducing nitrogen at a flow rate of 32L / min creates an inert atmosphere. The stable flow of nitrogen effectively removes residual non-volatile binder decomposition products, trace amounts of oil, and other volatile components from the blank, preventing their accumulation in the furnace or re-adhesion to the blank surface, thus reducing defects such as porosity and inclusions.

[0073] The sintering initiation temperature of 1000-1100℃ for 316 stainless steel billets is reached, and the surface atoms of the metal powder become active, with necking gradually forming in the contact areas between particles. The heating rate of 1.5-2.5℃ / min is slower than in the first stage, which can avoid abnormal grain growth caused by excessively high local temperatures. The holding time of 60-90min allows the billet to achieve uniform necking between particles and slow grain growth under stable temperature and pressure, forming a fine and uniform initial grain boundary structure, providing a good grain foundation for densification optimization in the subsequent high-temperature stage.

[0074] The 1130-1350℃ range is the efficient sintering temperature range for metal powders, which significantly improves the diffusion rate of metal atoms. The low heating rate of 1-2℃ / min can completely avoid the internal and external temperature differences caused by the size effect of ultra-large blanks, ensuring that the interior and surface of the blank reach the high-temperature sintering state simultaneously and preventing insufficient local densification. The ultra-long holding time of 120min or more can, on the one hand, allow the remaining voids inside the blank to be fully filled by atoms to achieve deep densification, and on the other hand, promote grain boundary diffusion and grain homogenization, eliminate microscopic defects that may be generated in the early sintering stage, and further optimize the uniformity of grain structure and mechanical properties.

[0075] In another embodiment of this application, after the blank is sintered, it is cooled to 850°C in the sintering furnace, then cooled to 450°C at a cooling rate of ≥5°C / min, and then slowly cooled to room temperature.

[0076] It should be noted that the cooling process for ultra-large 316 stainless steel MIM billets is designed as follows: first, the billets are cooled to 850℃ in the sintering furnace to release residual high-temperature stress and achieve temperature homogenization; then, they are cooled at a rate of ≥5℃ / min to 450℃ to rapidly overcome the carbide precipitation zone. The corrosion resistance of 316 stainless steel relies primarily on the continuous passivation film (Cr2O3) formed by Cr on the surface. However, if the cooling rate is too slow at high temperatures, the carbon in the billet will combine with Cr to form Cr2O3. 23 C6 carbides preferentially precipitate at grain boundaries, leading to a significant reduction in Cr content near the grain boundaries (i.e., Cr depletion). This causes discontinuous breakage of the passivation film, greatly increasing the risk of intergranular corrosion and directly affecting the service life of parts in humid and harsh environments. Rapid cooling at a rate of ≥5℃ / min can significantly suppress the precipitation kinetics of carbides, i.e., rapidly passing through the temperature range where carbides are most likely to precipitate (850-450℃), reducing the chance of Cr combining with C, ensuring sufficient Cr content in the grain boundaries and matrix, and ensuring the continuity and integrity of the passivation film. At 450℃, the driving force for carbide precipitation has been greatly reduced, but there is still a certain temperature difference between the inside and outside of the ultra-large blank (the surface cools faster than the inside). If rapidly cooled from 450℃ to room temperature, thermal stress will be generated again, and long-term residual stress may cause deformation of the part during subsequent use. Slow cooling allows the temperature inside and outside the blank to gradually drop to room temperature simultaneously, completely releasing the trace residual stress generated during the second stage of rapid cooling.

[0077] The process described in this application can be used to prepare ultra-large, high-density MIM hardware weighing up to 1000g. By optimizing the feed formulation, mold structure, process parameters, and debinding and sintering process, it achieves uniform mixing of metal powder and binder, complete filling of ultra-large mold cavity, thorough debinding of binder, and dense sintering of metal powder. The prepared hardware has excellent mechanical properties while ensuring dimensional accuracy, which can meet the application requirements of ultra-large precision metal parts.

[0078] The present invention will be further illustrated below through specific embodiments:

[0079] Example 1

[0080] S1. The particle size of the 316 stainless steel powder is 10μm. The 10μm 316 stainless steel powder and binder are mixed in a Banbury mixer at 180°C for 50 minutes to produce a viscous feedstock. The binder includes 89% polyoxymethylene, 4% polyethylene glycol, 2% polypropylene wax, 1% high-density polyethylene, 1% nano-SiO2 double-grafted compound, 1% polyethylene wax, 1% 2,6-di-tert-butyl-p-cresol, and 1% low molecular weight polycaprolactone. The feedstock, by mass ratio, comprises 84% ​​metal powder and 16% binder.

[0081] S2. The mold adopts a multi-gate and hot runner structure. The size of the mold cavity is 15% larger than the finished size of the ultra-large MIM hardware to compensate for sintering shrinkage. The material is injected into the mold cavity using an injection molding machine at an injection pressure of 100MPa, a barrel temperature of 200℃, and a holding time of 60s. After cooling, the material is demolded to form an ultra-large MIM hardware blank, which is then trimmed.

[0082] S3. Place the ultra-large MIM hardware blank into a degreasing furnace and perform vacuum treatment. Then, introduce nitrogen gas and maintain a flow rate of 104.5 L / min. The vaporization chamber temperature is 145℃, and the degreasing furnace temperature is 83℃. Subsequently, the vaporization chamber temperature is raised to 157℃, the oxalic acid injection rate is 8 g / min, and the degreasing furnace temperature is increased in stages to 110-120℃. The staged temperature increase includes raising the degreasing furnace temperature from 83℃ to 90℃ and holding for 3 hours; raising it from 90℃ to 110℃ and holding for 4 hours; and raising it from 110℃ to 130℃ and holding for 4 hours.

[0083] S4. The blank is placed in a sintering furnace for sintering. The sintering furnace is pre-evacuated to ≤10Pa and held for 10 minutes. The temperature is then increased from room temperature to 600℃, held for 45 minutes at a rate of 3℃ / min, and nitrogen gas is introduced at a flow rate of 33L / min. The temperature is then increased to 1000℃, held for 60 minutes at a rate of 2℃ / min, and finally increased to 1350℃, held for 150 minutes at a rate of 1℃ / min. After sintering, a 2kg ultra-large MIM hardware product is obtained.

[0084] Example 2

[0085] S1. Specifically, the same as in Example 1, the binder includes 90% polyoxymethylene, 3% polyethylene glycol, 2% polypropylene wax, 1% high-density polyethylene, 1% nano-SiO2 double-grafted material, 1% polyethylene wax, 1% 2,6-di-tert-butyl-p-cresol, and 1% low molecular weight polycaprolactone. The feedstock includes 83.5% metal powder and 16.5% binder by mass ratio.

[0086] S2, specifically the same as in Example 1.

[0087] S3. Specifically, the process is the same as in Example 1, with an oxalic acid infusion rate of 9 g / min and the temperature inside the defatting furnace increased in stages to 117°C. The staged temperature increase includes raising the temperature inside the defatting furnace from 83°C to 90°C and holding it for 4 hours; raising it from 90°C to 110°C and holding it for 5 hours; and raising it from 110°C to 130°C and holding it for 5 hours.

[0088] S4. The blank is placed in a sintering furnace for sintering. The sintering furnace is pre-evacuated to ≤10Pa and held for 10 minutes. The temperature is then raised from room temperature to 600℃, held for 50 minutes at a rate of 3℃ / min, and nitrogen is introduced at a flow rate of 35L / min. The temperature is then raised to 1005℃, held for 70 minutes at a rate of 2℃ / min, and finally raised to 1350℃, held for 180 minutes at a rate of 1℃ / min. After sintering, a 2kg ultra-large MIM hardware product is obtained.

[0089] Example 3

[0090] S1. Specifically, the same as in Example 1, the binder includes 91% polyoxymethylene, 2% polyethylene glycol, 2% polypropylene wax, 1% high-density polyethylene, 1% nano-SiO2 double-grafted compound, 1% polyethylene wax, 1% 2,6-di-tert-butyl-p-cresol, and 1% low molecular weight polycaprolactone. The feedstock includes 84% ​​metal powder and 17% binder by mass ratio.

[0091] S2, specifically the same as in Example 1.

[0092] S3. Specifically, the process is the same as in Example 1, with an oxalic acid infusion rate of 10 g / min and the temperature inside the defatting furnace increased in stages to 120°C. The staged temperature increase includes raising the temperature inside the defatting furnace from 85°C to 90°C and holding it for 4 hours; raising it from 90°C to 110°C and holding it for 6 hours; and raising it from 110°C to 130°C and holding it for 6 hours.

[0093] S4. The blank is placed in a sintering furnace for sintering. The sintering furnace is pre-evacuated to ≤10Pa and held for 10 minutes. The temperature is then raised from room temperature to 800℃, held for 55 minutes at a rate of 3℃ / min, and nitrogen is introduced at a flow rate of 37L / min. The temperature is then raised to 1100℃, held for 80 minutes at a rate of 2.5℃ / min, and finally raised to 1350℃, held for 200 minutes at a rate of 2℃ / min. After sintering, a 2kg ultra-large MIM hardware product is obtained.

[0094] Comparative Example 1: The difference between this comparative example and Example 1 is that the adhesive used is 100% polyoxymethylene.

[0095] Porosity test

[0096] Test principle: Based on Archimedes' principle, the sample's mass in air and in liquid are measured, and the sample volume is calculated using the difference between the two mass measurements, thus yielding the material density.

[0097] Test standard: GB / T 3850-2015.

[0098] Instruments and materials: electronic precision balance (accurate to 0.01g), suspension wire, container (for holding the test liquid).

[0099] Test liquid: 20℃ deionized water, with 2 drops of wetting agent added to the test liquid.

[0100] Test samples: One sample each from the surface and the interior of each laser-cut sample was taken, with a sample mass of 25.00g.

[0101] Test Procedure: Ensure the sample surface is clean and free of oil or impurities; place the sample on an electronic precision balance and weigh it in air (recorded as m1), accurate to 0.01g; suspend the sample in the liquid using a hanger (ensuring each sample is completely submerged and does not touch the container wall / bottom, with no air bubbles adhering), and weigh it in the liquid (recorded as m2), accurate to 0.01g; simultaneously measure the temperature of the liquid during the test, and find the density of the liquid at that temperature (recorded as ρ). 液 ).

[0102] Given that the theoretical density (ρ) of stainless steel 316 is... t The concentration of the substance is approximately 7.93 g / cm³.

[0103] The bulk density (ρ) of a material is calculated using the following formula: ρ = m1 / m1 - m2 × ρ 液 ;

[0104] Total porosity P is calculated using the following formula: P=(1-ρ / ρ t )×100%.

[0105] Table 1. Data related to porosity testing

[0106]

[0107] As clearly shown in Table 1, Examples 1-3, which utilize the optimized formulation and process of this application, exhibit significantly lower surface and internal porosity of the 316 stainless steel ultra-large MIM hardware compared to Comparative Example 1, which uses only pure polyoxymethylene binder. Example 2 demonstrates the optimal combination of process parameters, achieving a densification effect close to the theoretical density. This is attributed to the synergistic effect of the multiple components in the binder—the gradient decomposition characteristics of polyethylene glycol and polypropylene wax, combined with the interface modification effect of the nano-SiO2 double-grafted material, effectively suppressing the generation of defects during the degreasing process. Specifically, the polyethylene wax segments enhance the interfacial compatibility between nano-SiO2 and non-polar components such as polypropylene wax and high-pressure polyethylene in the binder, further reducing the melt viscosity of the feed and improving the mixing uniformity of the metal powder and binder. The tributyl citrate segments improve the compatibility between nano-SiO2 and metal powder. Thus, the physical barrier and support provided by the uniformly distributed nano-SiO2 effectively inhibits the excessive agglomeration and collapse of internal metal particles, maintains unobstructed diffusion channels, and allows for more complete atomic diffusion in the internal low-temperature zone, thereby narrowing the difference in porosity between the inside and outside. In contrast, Comparative Example 1, lacking auxiliary binder components, easily forms interconnected pores when polyoxymethylene decomposes rapidly in large quantities, leading to a significant increase in porosity after sintering.

[0108] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A process for manufacturing ultra-large MIM hardware, wherein the ultra-large MIM hardware is a structural or functional component weighing over 1000g, characterized in that, The process includes the following steps: S1. Mix the metal powder and binder in an internal mixer to prepare the feed; S2. The feed material is injected into the mold cavity and held under pressure. After holding under pressure, it is cooled and demolded to produce an ultra-large MIM hardware blank. S3. Degrease the ultra-large MIM hardware blank to obtain a blank; S4. Place the blank in a sintering furnace for pressure sintering to obtain an ultra-large MIM hardware finished product; The adhesive comprises, by weight percentage, 85%-91% polyoxymethylene, 2%-5% polyethylene glycol, 2%-3% polypropylene wax, 1%-2% high-density polyethylene, 0.5%-1% nano-SiO2 double-grafted compound, 0.5%-1.5% polyethylene wax, 0.5%-1.5% 2,6-di-tert-butyl-p-cresol, and 1%-3% low molecular weight polycaprolactone; The nano-SiO2 double-grafted compound is nano-SiO2 first grafted with tributyl citrate, and then grafted with polyethylene wax.

2. The process for fabricating ultra-large-scale MIM hardware as described in claim 1, characterized in that, The metal powder has a particle size of 0.5-20 μm, and the nano-SiO2 has a particle size of 50-100 nm.

3. The process for fabricating ultra-large-scale MIM hardware as described in claim 1, characterized in that, In step S1, the metal powder and binder are mixed in an internal mixer at 160-180°C for 40-60 minutes to form a viscous feed.

4. The process for fabricating ultra-large-scale MIM hardware as described in claim 1, characterized in that, The feed comprises 82%-85% metal powder and 15%-18% binder by mass percentage.

5. The process for fabricating ultra-large-scale MIM hardware as described in claim 1, characterized in that, The mold adopts a multi-gate and hot runner structure. The size of the mold cavity is enlarged by 15% according to the size of the ultra-large MIM hardware finished product as compensation for sintering shrinkage.

6. The process for fabricating ultra-large-scale MIM hardware as described in claim 1, characterized in that, The material is injected into the mold cavity using an injection molding machine with an injection pressure of 80-120MPa, a barrel temperature of 190-210℃, and a holding time of 30-60s.

7. The process for fabricating ultra-large-scale MIM hardware as described in claim 1, characterized in that, The ultra-large MIM hardware blank undergoes edge trimming before degreasing.

8. The process for fabricating ultra-large-scale MIM hardware as described in claim 1, characterized in that, The specific steps of the degreasing process are as follows: Pretreatment: The ultra-large MIM hardware blank is placed in a degreasing furnace and vacuumed. Nitrogen gas is then introduced and maintained at a flow rate of 104.5 L / min. The temperature of the vaporization box is 145-150℃ and the temperature inside the degreasing furnace is 80-85℃. Catalytic degreasing: Maintain nitrogen flow rate, raise the temperature of the vaporization box to 150-160℃, introduce oxalic acid into the vaporization box at a rate of 5-10g / min, and raise the temperature in the degreasing furnace in stages to 110-130℃; The segmented heating process includes raising the temperature inside the degreasing furnace from 80-85℃ to 90℃ and holding it for 3-4 hours; raising it from 90℃ to 110℃ and holding it for 4-6 hours; and raising it from 110℃ to 130℃ and holding it for 4-6 hours.

9. The process for fabricating ultra-large-scale MIM hardware as described in claim 1, characterized in that, The specific steps of the partial pressure sintering process are as follows: The sintering furnace is pre-evacuated to ≤10Pa and held for 10 min; the temperature is then raised from room temperature to 600-800℃, held for 45-60 min at a rate of 2-3℃ / min, and nitrogen is introduced at a flow rate of 30-40L / min; the temperature is then raised to 1000-1100℃, held for 60-90 min at a rate of 1.5-2.5℃ / min; and the temperature is then raised to 1350℃, held for ≥120 min at a rate of 1-2℃ / min. The partial pressure sintering is applicable to blanks with a composition of stainless steel 316.

10. The process for fabricating ultra-large-scale MIM hardware as described in claim 9, characterized in that, After sintering, the blank is cooled to 850°C in the sintering furnace, then cooled to 450°C at a cooling rate of ≥5°C / min, and then slowly cooled to room temperature.

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