High-stability composite binder MIM feedstock and preparation process thereof

Abstract

The application belongs to the technical field of powder metallurgy and metal injection molding, in particular to a high-stability composite binder MIM feedstock and a preparation process thereof, comprising: metal powder, the volume percentage of which in the high-stability composite binder MIM feedstock is 60% to 70%; the composite binder system, comprising: a multi-block copolymer as a main binder, the mass percentage of which in the composite binder system is 30% to 60%, the multi-block copolymer being a hydroxyl-terminated poly(ethylene-vinyl acetate)-block-poly(caprolactone)-block-poly(ethylene-vinyl acetate) three-block copolymer. The instability problem caused by component difference in the existing composite binder system is fundamentally overcome, the uniformity of component distribution of the feedstock under long-time storage condition is ensured, and the degradation of the feedstock performance is avoided.

Description

Technical Field

[0001] This invention belongs to the field of powder metallurgy and metal injection molding technology, specifically a high-stability composite binder MIM feedstock and its preparation process. Background Technology

[0002] Powder injection molding (MIM) technology, a near-net-shape forming process that combines the advantages of plastic injection molding and powder metallurgy, has demonstrated its unique application value in many high-tech fields such as aerospace, medical devices, electronic information, and automotive parts. This technology is particularly adept at manufacturing small metal parts with complex geometries, high precision, and excellent mechanical properties, effectively compensating for the shortcomings of traditional processing methods in terms of cost and efficiency. Therefore, its development plays a crucial role in promoting the transformation and upgrading of advanced manufacturing industries.

[0003] In the MIM (Metal Injection Molding) process, feedstock preparation is a crucial initial step, directly determining the smooth progress of subsequent injection molding, debinding, and sintering processes, as well as the quality of the final product. In existing technologies, MIM feedstocks are typically composed of a homogeneous mixture of fine metal powder and an organic binder system. Traditional organic binder systems mostly employ multi-component composite structures, aiming to meet the process's comprehensive requirements for feedstock rheological properties, green preform strength, and debinding efficiency through the synergistic effect of different components. Specifically, these composite binders usually include waxes (such as paraffin wax and microcrystalline wax) to provide excellent plasticity and filling properties, polymers (such as polyethylene and polypropylene) to impart sufficient strength and toughness to the green preform, and small amounts of lubricants and dispersants to improve powder wettability and flowability. By precisely adjusting the proportions of each component, existing technologies have, to a certain extent, met the plastic forming capabilities and initial strength requirements of MIM feedstocks, effectively solving the initial forming challenges of complex-shaped metal parts.

[0004] However, with the continuous development of related technologies and the increasingly stringent requirements placed on the performance and mass production stability of MIM components by application scenarios, some inherent characteristics of the aforementioned multi-component composite binder systems at the principle level have gradually revealed deep-seated limitations in addressing new challenges, especially in the key indicator of "high stability." The components in existing composite binder systems exhibit significant differences in chemical structure, molecular weight, polarity, and thermodynamic properties. During the prolonged high-temperature shear mixing, storage, and repeated heating and cooling cycles experienced in feedstock preparation and subsequent injection molding, these differences can easily lead to component separation or microscopic phase transitions within the binder. Specifically, low-molecular-weight wax components may precipitate from the feedstock due to evaporation, migration, or "sweating," while high-molecular-weight polymer components may undergo localized degradation or cross-linking due to localized overheating or uneven shear forces, and even exhibit heterogeneous interfacial bonding with metal powder particles. This inherent physical or chemical instability cannot be fundamentally resolved simply by adjusting the formulation ratio; rather, it is an inherent contradiction of its multi-component nature.

[0005] The consequences of this deep-seated contradiction are multifaceted and interconnected. First, the uneven distribution of binder components within the feedstock directly leads to fluctuations in its rheological properties. During injection molding, this means that the feedstock viscosity may exhibit unpredictable changes with time and temperature, resulting in a narrower injection window and making it prone to forming defects such as under-filling, flash, uneven sintering shrinkage, and cracking of the green preform. Second, the separation or degradation of binder components directly affects their wetting and coating effect on metal powder particles, significantly reducing the overall strength and uniformity of the green preform, increasing the risk of breakage during demolding and handling, and severely impacting production efficiency and product yield. Furthermore, the differences in the thermal decomposition temperature and rate of different components make the subsequent debinding process more complex and difficult to control, especially for complex parts with large variations in wall thickness. This can easily lead to incomplete debinding or localized stress concentration, ultimately resulting in defects such as porosity, deformation, or insufficient strength after sintering. To address these issues, this invention provides a highly stable composite binder MIM feedstock and its preparation process. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, at least one technical problem raised in the background art is solved.

[0007] The technical solution adopted by the present invention to solve its technical problem is: the high-stability composite binder MIM feed of the present invention is composed of metal powder and composite binder system, wherein the composite binder system includes: a multi-block copolymer as the main binder, an auxiliary polymer binder, a sacrificial lubricating wax, an interface-stabilizing nanoparticle, and a dispersing lubricant.

[0008] Specifically, the metal powder has an average particle size D50 ranging from 5 micrometers to 20 micrometers, a particle size distribution D90 of less than 40 micrometers, a spherical or irregular shape, and a surface area greater than 0.5 square meters per gram. The metal powder is selected, for example, from 316L stainless steel powder, iron-based alloy powder, nickel-based alloy powder, or tungsten alloy powder. The volume percentage of the metal powder in the high-stability composite binder MIM feed is 60% to 70%.

[0009] The primary binder in the composite binder system is a multi-block copolymer, comprising 30% to 60% by mass in the system. The multi-block copolymer has at least two distinct blocks, designed to provide core structural stability, interfacial bonding with the metal powder, and compatibility with other binder components. In a preferred embodiment of the invention, the multi-block copolymer is a hydroxyl-terminated poly(ethylene-vinyl acetate)-block-poly(caprolactone)-block-poly(ethylene-vinyl acetate) (EVA-PCL-EVA) triblock copolymer. The EVA block has a vinyl acetate content of 18% to 28% (mass percentage) and a glass transition temperature (Tg) below -20°C, imparting excellent plasticity and processing flowability to the feedstock at lower temperatures. The weight-average molecular weight of the EVA block ranges from 10,000 g / mol to 30,000 g / mol, and its melt flow index (MFI, at 190°C / 2.16 kg) is 5 g / 10 min to 30 g / 10 min. The PCL block has a weight-average molecular weight range of 20,000 g / mol to 60,000 g / mol and a melting point of 55°C to 65°C. It exhibits good toughness and biodegradability, aiming to provide sufficient strength for green preforms and promote decomposition during subsequent debinding processes. The EVA-PCL-EVA triblock copolymer has a total weight-average molecular weight ranging from 50,000 g / mol to 120,000 g / mol, and its terminals contain hydroxyl functional groups that can interact with metal powder surfaces or interface-stabilized nanoparticles. This triblock structure, by covalently linking blocks with different polarities and thermodynamic properties, ensures good compatibility of the components within the binder system at the molecular level, significantly suppressing phase separation phenomena that may occur under high-temperature shear or long-term storage conditions, thereby maintaining the long-term stability of the feedstock rheological properties. The presence of the PCL block, through its semi-crystalline characteristics and moderate softening point, provides the necessary initial strength for the green preform, while enabling controlled thermal decomposition during subsequent debinding, thereby optimizing debinding efficiency and the quality of the final sintered body.

[0010] The auxiliary polymer binder in the composite binder system comprises 20% to 40% by mass. This auxiliary polymer binder is a thermoplastic polymer designed to further enhance the melt strength of the feedstock and the mechanical strength of the green preform. In a preferred embodiment of the invention, the auxiliary polymer binder is low-density polyethylene (LDPE) with a melt flow index (MFI, at 190°C / 2.16 kg) of 1 g / 10 min to 10 g / 10 min and a weight-average molecular weight ranging from 50,000 g / mol to 150,000 g / mol. The low crystallinity of LDPE ensures good compatibility with EVA blocks, enabling the formation of a homogeneous polymer melt and avoiding rheological fluctuations caused by component incompatibility. Its presence ensures sufficient shear thinning properties in the feedstock during injection molding and, after cooling and solidification, provides additional toughness and impact resistance to the green preform, effectively reducing the risk of breakage during demolding and handling.

[0011] The sacrificial lubricating wax in the composite binder system comprises 10% to 25% by mass. This sacrificial lubricating wax has a defined melting point and thermal decomposition temperature range, designed to provide lubricity of the feedstock at processing temperatures, reduce melt viscosity, and facilitate clean decomposition during degreasing. In a preferred embodiment of the invention, the sacrificial lubricating wax is N,N'-ethylene bis-stearamide (EBS) wax, with a melting point range of 110°C to 140°C and a decomposition temperature range of 300°C to 350°C. In its molten state, the EBS wax can be uniformly dispersed in the polymer matrix, significantly improving feedstock flowability by reducing inter-chain friction, thereby broadening the injection molding process window. Its high melting point ensures that "sweating" does not occur at normal storage temperatures. During subsequent degreasing, the EBS wax can be removed by thermal decomposition or dissolution, and has a low residual carbon content, reducing the formation of sintering defects.

[0012] The interface-stabilizing nanoparticles in the composite binder system comprise 0.5% to 3% by mass. These nanoparticles aim to inhibit the migration and phase separation of binder components through nanoscale physical barriers and chemical bonding, while simultaneously enhancing the interfacial bonding strength between the binder and metal powder particles, thereby stabilizing the rheological behavior of the feedstock and improving the strength of the green preform. In a preferred embodiment of the invention, the interface-stabilizing nanoparticles are epoxy-functionalized fumed silica nanoparticles with an average primary particle size ranging from 7 nm to 30 nm and a specific surface area ranging from 150 m² / g to 380 m² / g. The epoxy-functionalization is achieved by surface modification of the fumed silica nanoparticles with γ-glycidoxypropyltrimethoxysilane (GPTMS). The epoxy groups in GPTMS can chemically react with the terminal hydroxyl functional groups of the multi-block copolymer (EVA-PCL-EVA) to form covalent bonds or strong hydrogen bonds, thereby firmly anchoring the silica nanoparticles in the polymer matrix. Simultaneously, the functionalized nanoparticles form a stable interfacial layer on the surface of the metal powder particles, effectively enhancing the wettability and adhesion between the binder and the metal powder through physical adsorption and potential chemical bonding (e.g., reaction with hydroxyl groups on the oxide layer of the metal powder surface). This microscopic network structure constructed by nanoparticles not only provides a physical barrier to the migration of binder components, but more importantly, it tightly connects different binder components and metal powder particles at the molecular level, fundamentally solving the problems of component separation and insufficient interfacial bonding, and significantly improving the long-term storage stability, rheological stability, and green preform strength of the feedstock.

[0013] The dispersing lubricant in the composite binder system comprises 0.5% to 2% by mass. This dispersing lubricant aims to further improve the uniformity of metal powder dispersion in the binder, reduce melt viscosity, and decrease friction during injection molding. In a preferred embodiment of the invention, the dispersing lubricant is pentaerythritol tetrastearate, with a melting point ranging from 50°C to 65°C. The pentaerythritol tetrastearate forms an adsorption film on the surface of the metal powder through its long-chain alkyl structure, thereby reducing the frictional resistance between powder particles and promoting uniform dispersion of the powder in the composite binder. Its low melting point ensures that it provides lubrication in the early stages of mixing, contributing to more uniform mixing under lower shear forces and reducing binder degradation caused by localized overheating or uneven shear forces.

[0014] This invention also provides a preparation process for a high-stability composite binder MIM feedstock, characterized by comprising the following steps:

[0015] The first step is premixing. The metal powder, the multi-block copolymer main binder, the auxiliary polymer binder, the sacrificial lubricating wax, the interface-stabilizing nanoparticles, and the dispersing lubricant are added to a high-speed mixer according to a predetermined formulation ratio and mixed. The purpose of premixing is to ensure that the components are uniformly distributed on a macroscopic scale. The mixer is, for example, a horizontal paddle mixer with a speed set to 500 rpm to 1500 rpm and a mixing time set to 15 minutes to 30 minutes. The premixing process is carried out at room temperature to ensure that the components do not melt or agglomerate. The premixed material is a homogeneous powder mixture.

[0016] The second step is melt mixing. The premixed material is quantitatively fed into a co-rotating twin-screw extruder for melt mixing. The co-rotating twin-screw extruder has a length-to-diameter ratio (L / D) of 40:1 to 60:1 and is equipped with a screw assembly structure including a conveying section, a plasticizing section, a mixing and dispersing section, a homogenizing section, and a metering section. The extruder is divided into seven independent heating zones, with the temperature distribution set as follows:

[0017] Feeding Zone (Zone 1): Temperature set at 80℃ to 100℃. This zone is mainly responsible for introducing materials and preheating them to prevent blockage at the screw inlet.

[0018] Plasticizing Zones (Zones 2 and 3): Temperature set at 140°C to 160°C. In this zone, the low-melting-point sacrificial lubricating wax and some polymers begin to melt. Under the conveying and initial shearing action of the screw, the material gradually transforms into a plastic state.

[0019] Mixing and Dispersion Zones (Zones 4 and 5): Temperature set at 160°C to 180°C. This zone is equipped with high-shear kneading blocks and reverse-threaded elements designed for intense shearing and mixing of the molten binder system and metal powder. During this process, the multi-block copolymer primary binder, auxiliary polymer binder, and sacrificial lubricating wax are fully melted and mutually dissolved. The interface-stabilized nanoparticles are uniformly dispersed into the polymer matrix by the shearing action of the screw. Simultaneously, the epoxy groups on their surface undergo potential chemical reactions with the hydroxyl groups at the ends of the multi-block copolymer or on the surface of the metal powder, forming a stable interface layer that further promotes the wetting and bonding of the binder and metal powder particles. The screw speed is set at 100 rpm to 500 rpm to provide sufficient shear energy to ensure that the metal powder particles are completely coated with the binder and to achieve molecular-level mixing and stabilization between the binder components.

[0020] Homogenization and degassing zone (zone 6): Temperature set at 170℃ to 190℃. This zone is designed to further homogenize the material and is equipped with a vacuum degassing port. Through vacuuming, volatiles and entrained gases generated during the mixing process are effectively removed, preventing the formation of pores in the final feed and ensuring the density and uniformity of the feed.

[0021] Die head zone (zone 7): Temperature set to 150℃ to 180℃. In this zone, the molten and homogenized feed is extruded into continuous strips through a perforated die head.

[0022] The third step involves cooling and pelletizing. The continuous strip-shaped feed extruded by the extruder is rapidly cooled in a cooling water tank to solidify and form the material. The water temperature in the cooling water tank is maintained between 15°C and 25°C. The cooled solid strip-shaped material is then fed into a pelletizer to be cut into cylindrical or ellipsoidal particles with a diameter of 2 mm to 5 mm and a length of 3 mm to 8 mm, thus obtaining the high-stability composite binder (MIM) feedstock. The size and shape uniformity of the particles should meet the requirements of the injection molding equipment.

[0023] The beneficial effects of this invention are as follows:

[0024] 1. The high-stability composite binder MIM feedstock and its preparation process described in this invention, by introducing an EVA-PCL-EVA main binder with a multi-block structure, achieves intrinsic compatibility between components of different polarities and molecular weights at the molecular level, significantly reducing the phase separation tendency caused by thermodynamic incompatibility between components. Furthermore, epoxy-functionalized fumed silica nanoparticles form an interpenetrating nanoscale stable network through chemical bonding with the main binder and anchoring on the surface of the metal powder, inhibiting the migration and "sweating" phenomenon of binder components (especially sacrificial lubricating wax) from both physical and chemical mechanisms. This synergistic effect fundamentally overcomes the instability problem caused by component differences in existing composite binder systems, ensuring that the uniformity of component distribution is maintained under long-term storage conditions, and avoiding the deterioration of feedstock performance.

[0025] 2. The high-stability composite binder MIM feedstock and its preparation process described in this invention, through an optimized combination of EVA-PCL-EVA multi-block copolymer, LDPE auxiliary polymer binder, and EBS sacrificial lubricant, endows the feedstock with stable melt viscosity at injection temperature and suitable shear-thinning behavior. Epoxy-functionalized silica nanoparticles form a stable weak network structure in the melt, effectively buffering viscosity changes caused by temperature fluctuations or uneven shear forces, ensuring high uniformity and predictability of the feedstock's rheological properties throughout the injection molding process. This significantly broadens the injection molding process window, reduces the sensitivity of injection pressure to feedstock performance, and effectively reduces the frequency of molding defects such as underfill, flash, warpage, and uneven sintering shrinkage, thereby improving product yield and production efficiency.

[0026] 3. The high-stability composite binder MIM feedstock and its preparation process described in this invention, through the PCL blocks in the EVA-PCL-EVA multi-block copolymer and the LDPE auxiliary polymer binder, jointly provide excellent toughness and mechanical strength for the green preform. More importantly, the strong interfacial interaction formed between the epoxy-functionalized silica nanoparticles and the metal powder particles, as well as between the silica nanoparticles and the binder polymer, greatly enhances the binder's ability to coat and bond the metal powder, constructing a denser and more uniform green preform structure. This enhanced interfacial bonding effectively prevents cracking or damage to the green preform due to external stress during demolding, handling, and initial degreasing, significantly reducing the scrap rate of the products and improving the overall operating efficiency of the production line.

[0027] 4. The high-stability composite binder MIM feedstock and its preparation process described in this invention utilizes carefully selected components in the composite binder system, including the PCL block in EVA-PCL-EVA and the EBS sacrificial lubricating wax, which possess controllable thermal decomposition characteristics and low residual carbon content. The moderate decomposition temperature of the PCL block and the relatively high decomposition temperature of the EBS wax form a multi-stage, stepwise decomposition mechanism, ensuring that the binder can be completely removed during degreasing, while avoiding stress concentration and defects in the green blank caused by excessively rapid or uneven binder decomposition. Especially in complex parts with large variations in wall thickness, this staged and uniform degreasing process can effectively avoid the formation of sintering defects such as internal stress, porosity, and deformation, thereby improving the precision and mechanical properties of the final sintered part. The vacuum exhaust section maximizes the removal of potential volatiles and gases during the feedstock preparation stage, further ensuring the purity of subsequent degreasing and sintering. Attached Figure Description

[0028] The invention will now be further described with reference to the accompanying drawings.

[0029] Figure 1 This is a flowchart of the preparation process in this invention. Detailed Implementation

[0030] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0031] In one specific embodiment, the high-stability composite binder MIM feed mainly consists of metal powder and a composite binder system. The average particle size D50 of the metal powder is controlled between 5 micrometers and 20 micrometers to balance good flowability and sintering density. Its particle size distribution D90 is less than 40 micrometers, ensuring the uniformity of the powder system and reducing the adverse effects of coarse particles on the injection molding and sintering processes. The powder shape can be spherical or irregular, with spherical powder generally providing better flowability, while irregularly shaped powder may enhance the formation of sintering necks and the strength of green blanks in certain applications. The specific surface area of ​​the metal powder is greater than 0.5 m² / g to ensure sufficient contact area and interfacial bonding potential with the composite binder system. As an example of the invention, the metal powder can be 316L stainless steel powder, which has excellent corrosion resistance and biocompatibility, or iron-based alloy powder, nickel-based alloy powder, or tungsten alloy powder, etc., to meet the material performance requirements of different application fields. The metal powder in the high-stability composite binder MIM feed has a volume percentage of 60% to 70%. This range ensures that the feed has a high metal solid content while maintaining good flowability, which is beneficial for obtaining a dense final product in the subsequent sintering process.

[0032] The composite binder system is one of the core technical features of this invention, designed to provide excellent rheological properties of the feedstock during injection molding, sufficient strength of the green preform, and clean removal during degreasing. The composite binder system comprises five key components: a multi-block copolymer as the primary binder, an auxiliary polymer binder, a sacrificial lubricating wax, interface-stabilizing nanoparticles, and a dispersing lubricant.

[0033] Specifically, the main binder in the composite binder system is a multi-block copolymer, which accounts for 30% to 60% of the total mass of the composite binder system. This multi-block copolymer, through its unique multi-block structure, provides core structural stability to the entire binder system, enhances interfacial bonding with metal powders, and ensures compatibility with other binder components. In a preferred embodiment of the invention, the multi-block copolymer is a hydroxyl-terminated poly(ethylene-vinyl acetate)-block-poly(caprolactone)-block-poly(ethylene-vinyl acetate) (EVA-PCL-EVA) triblock copolymer. The vinyl acetate (VA) content of the EVA block is controlled between 18% and 28% (mass percentage). This content range imparts suitable polarity to the EVA block, enabling it to maintain good compatibility with non-polar or weakly polar polymer components while providing flexibility. The EVA block has a glass transition temperature (Tg) below -20°C, meaning that it remains in a highly elastic state within the processing temperature range of the MIM feedstock, thus imparting excellent plasticity and flowability to the feedstock at lower temperatures. The weight-average molecular weight of the EVA block ranges from 10,000 g / mol to 30,000 g / mol, and its melt flow index (MFI, at 190°C / 2.16 kg) is from 5 g / 10 min to 30 g / 10 min, indicating suitable processing flowability.

[0034] The PCL blocks have a weight-average molecular weight ranging from 20,000 g / mol to 60,000 g / mol and a melting point of 55°C to 65°C. The introduction of PCL blocks, utilizing their semi-crystalline properties and moderate softening point, provides the necessary initial strength and toughness for the green preform after cooling and solidification, effectively reducing the risk of breakage during demolding and handling. PCL also exhibits good biodegradability, capable of controlled thermal decomposition during degreasing with low residual carbon content, thereby optimizing degreasing efficiency and reducing the defect rate of the final sintered body. The total weight-average molecular weight of the EVA-PCL-EVA triblock copolymer ranges from 50,000 g / mol to 120,000 g / mol, ensuring sufficient mechanical strength and processing viscosity. Furthermore, the EVA-PCL-EVA triblock copolymer contains hydroxyl functional groups at its ends that can interact with the surface of metal powder or interface-stabilized nanoparticles. This triblock structure covalently links EVA and PCL blocks with different polarities and thermodynamic properties, constructing a polymer network with high intrinsic compatibility at the molecular level. This network structure significantly suppresses phase separation that may occur under high-temperature shear or long-term storage conditions, thereby maintaining the long-term stability of the feed rheological properties.

[0035] The auxiliary polymer binder in the composite binder system comprises 20% to 40% by mass. This auxiliary polymer binder, a thermoplastic polymer, is designed to further enhance the melt strength of the feedstock and the mechanical strength of the green preform. In a preferred embodiment of the invention, the auxiliary polymer binder is low-density polyethylene (LDPE). The LDPE has a melt flow index (MFI, at 190°C / 2.16 kg) of 1 g / 10 min to 10 g / 10 min and a weight-average molecular weight ranging from 50,000 g / mol to 150,000 g / mol. The low crystallinity of LDPE ensures good compatibility with EVA blocks, enabling it to form a homogeneous polymer melt with the main binder, avoiding rheological fluctuations caused by component incompatibility. The presence of LDPE ensures sufficient shear thinning properties in the feedstock during injection molding, which is crucial for filling complex-shaped parts. Meanwhile, after cooling and curing, LDPE provides green preforms with additional toughness and impact resistance, further reducing the risk of breakage during demolding and handling.

[0036] The sacrificial lubricating wax in the composite binder system comprises 10% to 25% by mass. This sacrificial lubricating wax has a defined melting point and thermal decomposition temperature range. Its main functions are to provide lubrication for the feedstock at processing temperatures, reduce melt viscosity, and clean decomposition during degreasing. In a preferred embodiment of the invention, the sacrificial lubricating wax is N,N'-ethylene bis-stearamide (EBS) wax. The EBS wax has a melting point range of 110°C to 140°C and a decomposition temperature range of 300°C to 350°C. In its molten state, the EBS wax can be uniformly dispersed in the polymer matrix. By reducing friction between polymer molecular chains and metal powder particles, as well as friction between polymer chain segments, it significantly improves the flowability of the feedstock, thereby widening the injection molding process window and reducing injection pressure. Its high melting point ensures that the EBS wax remains solid at normal storage temperatures, preventing "sweating" due to component migration and thus maintaining the long-term storage stability of the feedstock. In the subsequent degreasing process, the EBS wax can be removed by thermal decomposition or dissolution, and has a low residual carbon content, which is crucial for reducing the formation of porosity and defects in the final sintered body.

[0037] The interface-stabilizing nanoparticles in the composite binder system constitute 0.5% to 3% of the total mass. These nanoparticles effectively inhibit the migration and phase separation of binder components through their nanoscale physical barrier effect and potential chemical bonding ability, while significantly enhancing the interfacial bonding strength between the binder and the metal powder particles, thereby stabilizing the rheological behavior of the feedstock and improving the strength of the green preform. In a preferred embodiment of the invention, the interface-stabilizing nanoparticles are epoxy-functionalized fumed silica nanoparticles. The average primary particle size of these nanoparticles ranges from 7 nm to 30 nm, and they possess a high specific surface area, ranging from 150 m² / g to 380 m² / g. The epoxy-functionalization is achieved by surface modification of the fumed silica nanoparticles with γ-glycidoxypropyltrimethoxysilane (GPTMS). Specifically, the GPTMS molecule contains an epoxy group at one end and a trimethoxysilyl group at the other end. During surface modification, the trimethoxysilane group can undergo hydrolysis-condensation reaction with the abundant hydroxyl groups on the surface of fumed silica nanoparticles to form stable siloxane bonds, thereby covalently attaching GPTMS molecules to the surface of silica nanoparticles. In this way, silica nanoparticles are endowed with more reactive epoxy groups.

[0038] The epoxy groups in the GPTMS can undergo ring-opening reactions with the terminal hydroxyl functional groups of the multi-block copolymer (EVA-PCL-EVA) under the high temperature and shearing action of melt mixing, forming covalent bonds or strong hydrogen bonds. This firmly anchors the silica nanoparticles in the polymer matrix, constructing a molecular-level interpenetrating network structure. Simultaneously, the functionalized nanoparticles can form a stable interfacial layer on the surface of the metal powder particles through physical adsorption and potential chemical bonding (e.g., reaction with hydroxyl groups on the oxide layer of the metal powder surface), effectively enhancing the wettability and adhesion between the binder and the metal powder. This microscopic network structure constructed by nanoparticles not only provides a physical barrier to the migration of binder components (especially low-molecular-weight sacrificial lubricants), but more importantly, it tightly connects different binder components and metal powder particles at the molecular level, fundamentally solving the problems of component separation and insufficient interfacial bonding, and significantly improving the long-term storage stability, rheological stability, and green preform strength of the feedstock.

[0039] The dispersing lubricant in the composite binder system comprises 0.5% to 2% by mass. This dispersing lubricant aims to further improve the uniformity of metal powder dispersion in the binder, reduce melt viscosity, and decrease mold friction during injection molding. In a preferred embodiment of the invention, the dispersing lubricant is pentaerythritol tetrastearate, with a melting point ranging from 50°C to 65°C. The pentaerythritol tetrastearate forms an adsorption film on the surface of the metal powder through its long-chain alkyl structure, thereby reducing the frictional resistance between powder particles and promoting uniform dispersion of the powder in the composite binder. Its low melting point ensures that it melts and provides lubrication in the early stages of mixing, contributing to more uniform mixing under lower shear forces and reducing binder degradation caused by localized overheating or uneven shear forces.

[0040] This invention also provides a preparation process for a high-stability composite binder MIM feedstock, characterized by comprising the following steps, the preparation process flow of which can be referred to Figure 1 As shown.

[0041] The first step is premixing. The metal powder, the multi-block copolymer main binder, the auxiliary polymer binder, the sacrificial lubricating wax, the interface-stabilizing nanoparticles, and the dispersing lubricant are precisely weighed according to a predetermined formula ratio and then added to a high-speed mixer for mixing. The purpose of premixing is to ensure that the components are uniformly distributed on a macroscopic scale, providing a homogeneous initial material for subsequent melt mixing. The mixer is preferably a horizontal paddle mixer, whose unique paddle structure and mixing mode can effectively and uniformly disperse the powdered material. The mixer speed is set to 500 rpm to 1500 rpm, and the mixing time is set to 15 minutes to 30 minutes to ensure that all components are fully mixed without phase change. The premixing process is carried out at room temperature to prevent any component from melting or agglomerating during the mixing stage, thereby maintaining the powder's flowability and dispersibility. The premixed material presents as a homogeneous powdered mixture.

[0042] The second step is melt mixing. The premixed material is fed into a co-rotating twin-screw extruder at a stable rate via a metering device for melt mixing. The co-rotating twin-screw extruder has a length-to-diameter ratio (L / D) of 40:1 to 60:1 to ensure sufficient residence time for the material within the extruder, allowing for adequate shearing, mixing, and heat exchange. The extruder is equipped with an optimized screw assembly structure comprising a conveying section, a plasticizing section, a mixing and dispersing section, a homogenizing section, and a metering section. These screw components work together to ensure efficient material transport while achieving refined material processing. The extruder is divided into seven independent heating zones with precisely set temperature distributions to accommodate the melting characteristics and reaction requirements of each component.

[0043] In the first feeding zone, the temperature is set between 80°C and 100°C. This section is mainly responsible for the introduction and initial preheating of materials. Its temperature design aims to prevent condensation or blockage of materials at the screw inlet, ensuring smooth feeding.

[0044] In the second and third plasticizing zones, the temperature is set between 140°C and 160°C. In this zone, the sacrificial lubricating wax with a lower melting point and some polymer components begin to melt. Under the conveying and initial shearing action of the screw, the material gradually transforms from a powdery state to a plastic state, forming the initial molten matrix.

[0045] In the fourth and fifth mixing and dispersion zones, the temperature is set to 160°C to 180°C. This section is equipped with a high-shear kneading block and a reverse screw element, designed to subject the molten binder system and metal powder to intense shearing, stretching, and mixing. During this process, the multi-block copolymer primary binder, auxiliary polymer binder, and sacrificial lubricating wax fully melt and dissolve in each other to form a homogeneous polymer melt. The interface-stabilized nanoparticles are uniformly dispersed into the polymer matrix by the high shear force of the screw. Simultaneously, the epoxy groups on their surface undergo potential chemical reactions with the hydroxyl groups at the ends of the multi-block copolymer or on the oxide layer of the metal powder surface, forming stable covalent bonds or strong hydrogen bonds, thereby constructing a stable interface layer and further promoting the wetting and bonding of the binder and metal powder particles. The screw speed is set to 100 rpm to 500 rpm to provide sufficient shear energy to ensure that the metal powder particles are completely coated by the binder and to achieve molecular-level mixing and stabilization between the binder components, preventing powder agglomeration and binder phase separation.

[0046] In the sixth homogenization and venting zone, the temperature is set between 170°C and 190°C. This section aims to further homogenize the composition and structure of the material and is equipped with a vacuum vent. A vacuum pump is connected to the vent to effectively remove volatiles (such as water vapor and small molecule degradation products) and entrained gases that may be generated during mixing. This step is crucial for ensuring the density and uniformity of the final feed, significantly reducing the risk of porosity defects during injection molding and sintering.

[0047] In the seventh die section, the temperature is set between 150°C and 180°C. This section extrudes the homogenized molten feed into continuous strips through a perforated die. Precise control of the die temperature helps maintain stable melt flow, preventing degradation or expansion during extrusion.

[0048] The third step involves cooling and pelletizing. The continuous strip-shaped feed extruded by the extruder immediately enters a cooling water tank for rapid cooling after passing through the die, allowing it to solidify quickly. The water temperature in the cooling water tank is maintained between 15°C and 25°C, a temperature range that ensures rapid solidification of the feed strip without generating internal stress or deformation. The cooled solidified strip-shaped material is then fed into a pelletizer, cut into cylindrical or ellipsoidal particles with a diameter of 2 mm to 5 mm and a length of 3 mm to 8 mm, thus obtaining the high-stability composite binder (MIM) feedstock. The size and shape uniformity of the particles are strictly controlled to meet the feeding requirements of downstream injection molding equipment, ensuring stable and continuous production.

[0049] Example 1

[0050] This embodiment aims to elaborate in detail a typical formulation and preparation process of using the high-stability composite binder MIM feed described in this invention, and to evaluate its performance.

[0051] Feeding formula

[0052] Metal powder: 316L stainless steel powder, D50=12.5 micrometers, D90=28 micrometers, spherical, specific surface area of ​​0.85 square meters / gram. Volume percentage: 65%.

[0053] Composite binder system (35% of total feed volume):

[0054] Main binder: hydroxyl-terminated EVA-PCL-EVA triblock copolymer (EVA block VA content 22%), EVA block Mw = 20,000 g / mol, PCL block Mw = 40,000 g / mol, total Mw = 90,000 g / mol, MFI (190℃ / 2.16 kg) is 18 g / 10 min. Mass percentage in the composite binder system: 45%.

[0055] Auxiliary polymer binder: LDPE, MFI (190℃ / 2.16kg) 5g / 10min, Mw=100,000g / mol. Mass percentage in the composite binder system: 30%.

[0056] Sacrificial lubricant: EBS wax, melting point 128℃, decomposition temperature 320℃. Percentage by mass in the composite binder system: 15%.

[0057] Interface-stabilized nanoparticles: epoxy-functionalized fumed silica nanoparticles (modified with GPTMS), with an average primary particle size of 15 nm and a specific surface area of ​​250 m² / g. Mass percentage in the composite binder system: 1.5%.

[0058] Dispersant lubricant: Pentaerythritol tetrastearate, melting point 58°C. Mass percentage in the composite binder system: 1.0%.

[0059] The total of the above components is 100%, of which metal powder accounts for 65 vol and composite binder system accounts for 35 vol.

[0060] Preparation process

[0061] Premixing: Accurately weighed 316L stainless steel powder, EVA-PCL-EVA, LDPE, EBS wax, epoxy-functionalized silica nanoparticles, and pentaerythritol tetrastearate were added to a horizontal paddle mixer. The mixer speed was set to 1000 rpm, and the mixing time was 20 minutes, at room temperature (25°C).

[0062] Melt mixing: The premixed material is quantitatively fed into a co-rotating twin-screw extruder with an L / D ratio of 48:1. The screw speed is set to 250 rpm. The temperature settings for the seven heating zones of the extruder are as follows:

[0063] Feeding area (zone 1): 90℃

[0064] Plasticizing zones (zones 2 and 3): 150℃

[0065] Mixed dispersion zone (zones 4 and 5): 170℃

[0066] Homogenization exhaust zone (zone 6): 180℃ (vacuum degree -0.08MPa)

[0067] Die head area (zone 7): 165℃

[0068] Cooling and pelletizing: The extruded strip feed is rapidly cooled and solidified in a 18°C ​​cooling water bath, and then fed into a pelletizer to be cut into cylindrical pellets with a diameter of 3 mm and a length of 5 mm.

[0069] Performance evaluation

[0070] Long-term storage stability: The prepared feed was stored at 50℃ and 70% relative humidity for 6 months. Its stability was evaluated by visual observation and MFI test. After 6 months of storage, the feed showed no obvious "sweating" phenomenon, and the surface particles did not stick together. The initial MFI was 15.2 g / 10 min, and the MFI after storage was 14.8 g / 10 min, with an MFI change rate of only -2.6%.

[0071] Rheological stability: The feedstock was tested at different shear rates at 170℃ using a capillary rheometer. At a shear rate of 100 s⁻¹, the feedstock viscosity was 280 Pa·s, exhibiting good shear-thinning behavior. During injection molding, 500 standard samples were continuously injected, and the batch-to-batch coefficient of variation (CV) of each sample weight was measured. The result was 0.8%, indicating that the feedstock rheological properties were highly stable.

[0072] Green preform strength: Standard dog bone-shaped specimens were injection molded, and their three-point bending strength was measured. The average three-point bending strength of the green preform was 32 MPa, and the elongation at break was 6%.

[0073] Degreasing residual carbon rate: Thermogravimetric analysis (TGA) was used to simulate the degreasing process of the green billet. Under an air atmosphere at 550℃ and a heating rate of 5℃ / min, the final residual carbon rate was 0.015%.

[0074] Comparative Example 1

[0075] This comparative example aims to demonstrate a MIM feed formulation and its performance that does not possess the key features of this invention, so as to illustrate the technical advantages of this invention.

[0076] Feeding formula

[0077] Metal powder: 316L stainless steel powder, D50=12.5 micrometers, D90=28 micrometers, spherical, specific surface area of ​​0.85 square meters / gram. Volume percentage: 65%.

[0078] Composite binder system (35% of total feed volume):

[0079] Primary binder: Polyethylene wax (PE wax), Mw = 5,000 g / mol. Mass percentage in the composite binder system: 45%.

[0080] Auxiliary polymer binder: LDPE, MFI (190℃ / 2.16kg) 5g / 10min, Mw=100,000g / mol. Mass percentage in the composite binder system: 30%.

[0081] Sacrificial lubricant: Paraffin wax, melting point 60℃, decomposition temperature 280℃. Percentage by mass in the composite binder system: 15%.

[0082] Interface-stabilized nanoparticles: Unfunctionalized fumed silica nanoparticles with an average primary particle size of 15 nm and a specific surface area of ​​250 m² / g. Mass percentage in the composite binder system: 1.5%.

[0083] Dispersant lubricant: Pentaerythritol tetrastearate, melting point 58°C. Mass percentage in the composite binder system: 1.0%.

[0084] Note: In this comparative example, the main binder is a traditional low molecular weight PE wax, the sacrificial lubricant is a paraffin wax with a low melting point, and the silica nanoparticles are not functionalized.

[0085] Preparation process

[0086] Premixing: Same as in Example 1.

[0087] Melt mixing: Same as in Example 1, but due to component differences, the temperature of the mixing and dispersion zones (zones 4 and 5) is adjusted to 160°C to accommodate the low melting point of paraffin wax, and the screw speed is adjusted to 200 rpm to avoid shear overheating that could lead to degradation of low molecular weight polymers. The temperature settings for other zones remain consistent.

[0088] Cooling and pelletizing: Same as in Example 1.

[0089] Performance evaluation

[0090] Long-term storage stability: Stored for 6 months at 50℃ and 70% relative humidity. During storage, in the second month, obvious oily exudate ("sweating") was observed on the feed surface, and slight adhesion occurred between particles. The initial MFI was 18.5 g / 10 min, and after storage, the MFI was 24.3 g / 10 min, with an MFI change rate as high as +31.4%, indicating that the binder component underwent significant migration and decomposition.

[0091] Rheological stability: at 170℃ and a shear rate of 100 s⁻¹ -1 At that time, the feed viscosity was 220 Pa·s. After continuous injection of 500 standard samples, the batch-to-batch coefficient of variation (CV value) of the weight of each sample was measured to be 3.5%, indicating that the rheological properties of the feed fluctuated greatly.

[0092] Strength of green blanks: The average three-point bending strength of green blanks is 18 MPa, and the elongation at break is 3.5%.

[0093] Degreasing residual carbon rate: Under air atmosphere at 550℃ and heating rate of 5℃ / min, the final residual carbon rate is 0.058%.

[0094] Performance Comparison Summary

[0095] The table below compares the key performance indicators of Example 1 and Comparative Example 1:

[0096] Performance indicators Example 1 Comparative Example 1 Long-term storage stability No obvious "sweating" phenomenon, and no particles sticking together; MFI change rate: -2.6% "Sweating" appeared after 2 months, with slight adhesion of particles; MFI change rate: +31.4% Rheological stability (MFI) Initial MFI: 15.2g / 10min Initial MFI: 18.5g / 10min Rheological homogeneity (CV value of injected sample weight) 0.8% 3.5% Three-point bending strength of green blank 32MPa 18MPa Green blank elongation at break 6% 3.5% Degreasing residual carbon rate 0.015% 0.058%

[0097] The comparative data above show that the high-stability composite binder MIM feedstock provided by this invention, through the introduction of multi-block copolymers as the main binder and epoxy-functionalized interface-stabilizing nanoparticles, exhibits significant improvements in long-term storage stability, injection molding rheological stability, green preform strength, and cleanliness during degreasing. The feedstock shown in Example 1, after 6 months of storage, showed an extremely low MFI change rate, far superior to the 31.4% MFI increase in Comparative Example 1, and no "sweating" phenomenon was observed. This directly demonstrates the effectiveness of this invention in inhibiting binder component migration and maintaining long-term component uniformity. Regarding rheological stability, the injection sample weight CV value of Example 1 was only 0.8%, indicating highly stable rheological behavior during continuous injection molding, significantly reducing the risk of molding defects. In contrast, the CV value of Comparative Example 1 was as high as 3.5%, showing poor rheological uniformity.

[0098] Furthermore, the green preforms prepared by the feedstock of this invention exhibit higher mechanical strength and toughness. Example 1 shows a three-point bending strength of 32 MPa and an elongation at break of 6%, significantly improved compared to Comparative Example 1's 18 MPa and 3.5%. This is attributed to the strong interfacial bonding between the functionalized nanoparticles, the binder polymer, and the metal powder, as well as the robust polymer network constructed by the multi-block copolymer and LDPE. Regarding debinding, the debinding residual carbon rate of the feedstock of this invention is only 0.015%, far lower than the 0.058% of Comparative Example 1, indicating that the composite binder system of this invention can achieve more thorough and cleaner thermal decomposition, thereby effectively reducing sintering defects and facilitating the acquisition of high-density, high-performance final sintered parts.

[0099] In summary, this invention, by constructing a structurally sophisticated, synergistically composed, and interfacially stable composite binder system and employing a precisely controlled preparation process, fundamentally resolves the deep-seated instability issues of existing MIM feedstocks. This provides a solid technical guarantee for the industrial application of MIM technology in manufacturing high-precision, complex-shaped metal parts. The implementation details of the described technical solution and the performance indicators achieved demonstrate that it can provide those skilled in the art with a complete and reproducible solution for preparing MIM feedstocks that meet high-performance requirements.

[0100] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A high-stability composite binder MIM feedstock, characterized in that, include: Metal powder, which constitutes 60% to 70% by volume in the high-stability composite binder MIM feed; Composite adhesive systems, including: A multiblock copolymer is used as the main binder, and its mass percentage in the composite binder system is 30% to 60%. The multiblock copolymer is a hydroxyl-terminated poly(ethylene-vinyl acetate)-block-poly(caprolactone)-block-poly(ethylene-vinyl acetate) triblock copolymer. The multiblock copolymer has hydroxyl functional groups at its ends, and these hydroxyl functional groups can interact with the surface of metal powder or interface-stabilized nanoparticles. An auxiliary polymer binder, wherein the mass percentage of the composite binder system is 20% to 40%; A sacrificial lubricating wax, comprising 10% to 25% by mass in the composite binder system; An interface-stabilized nanoparticle, comprising 0.5% to 3% by mass in the composite binder system, wherein the interface-stabilized nanoparticle is epoxy-functionalized fumed silica nanoparticle, wherein the epoxy-functionalization is achieved by surface modification of the fumed silica nanoparticle with γ-epoxypropoxypropyltrimethoxysilane. A dispersing lubricant, comprising 0.5% to 2% by mass in the composite binder system.

2. The high-stability composite binder MIM feed according to claim 1, characterized in that, The average particle size D50 of the metal powder ranges from 5 micrometers to 20 micrometers, its particle size distribution D90 is less than 40 micrometers, its shape is spherical or irregular, and the specific surface area of ​​the metal powder is greater than 0.5 square meters / gram; the metal powder is selected from any one of 316L stainless steel powder, iron-based alloy powder, nickel-based alloy powder or tungsten alloy powder.

3. The high-stability composite binder MIM feed according to claim 1, characterized in that, The poly(ethylene-vinyl acetate) block contained in the multi-block copolymer main binder has a vinyl acetate content of 18% to 28% based on the mass of the poly(ethylene-vinyl acetate) block itself, a glass transition temperature of less than -20°C, a weight-average molecular weight range of 10,000 g / mol to 30,000 g / mol, and a melt flow index of 5 g / 10 min to 30 g / 10 min.

4. The high-stability composite binder MIM feed according to claim 1, characterized in that, The poly(caprolactone) block in the main binder of the multi-block copolymer has a weight-average molecular weight ranging from 20,000 g / mol to 60,000 g / mol, and the melting point of the poly(caprolactone) block is 55°C to 65°C; and the total weight-average molecular weight of the hydroxyl-terminated poly(ethylene-vinyl acetate)-block-poly(caprolactone)-block-poly(ethylene-vinyl acetate) triblock copolymer ranges from 50,000 g / mol to 120,000 g / mol.

5. The high-stability composite binder MIM feed according to claim 1, characterized in that, The auxiliary polymer binder is low-density polyethylene with a melt flow index of 1 g / 10 min to 10 g / 10 min and a weight-average molecular weight range of 50,000 g / mol to 150,000 g / mol.

6. The high-stability composite binder MIM feed according to claim 1, characterized in that, The sacrificial lubricating wax is N,N'-ethylene bis-stearamide wax, with a melting point range of 110°C to 140°C and a decomposition temperature range of 300°C to 350°C.

7. The high-stability composite binder MIM feed according to claim 6, characterized in that, The average primary particle size of the interface-stabilized nanoparticles ranges from 7 nanometers to 30 nanometers, and the specific surface area ranges from 150 square meters / gram to 380 square meters / gram.

8. The high-stability composite binder MIM feeder according to claim 1, characterized in that, The epoxy groups in the epoxy-functionalized fumed silica nanoparticles can chemically react with the hydroxyl functional groups at the ends of the multi-block copolymer, and can form a stable interface layer on the surface of the metal powder particles, thereby enhancing the wettability and adhesion between the binder and the metal powder.

9. The high-stability composite binder MIM feed according to claim 1, characterized in that, The dispersing lubricant is pentaerythritol tetrastearate, with a melting point range of 50°C to 65°C.

10. A high-stability composite binder MIM feed preparation process, applicable to the high-stability composite binder MIM feed according to any one of claims 1-9, characterized in that, Includes the following steps: S1. Premixing: Metal powder, multiblock copolymer main binder, auxiliary polymer binder, sacrificial lubricating wax, interface-stabilized nanoparticles and dispersing lubricant are put into a high-speed mixer and mixed. At room temperature, the speed is set to 500 rpm to 1500 rpm and the mixture is stirred for 15 to 30 minutes to obtain the premixed material. S2. Melt Mixing: The premixed material is quantitatively fed into a co-rotating twin-screw extruder. This co-rotating twin-screw extruder is equipped with a screw assembly structure including a conveying section, a plasticizing section, a mixing and dispersing section, a homogenizing section, and a metering section. The temperature distribution of the multiple independent heating sections of the extruder is set as follows: feeding zone 80℃ to 100℃; plasticizing zone 140℃ to 160℃; mixing and dispersing zone 160℃ to 180℃, with the screw speed set to 100rpm to 500rpm; homogenizing and venting zone 170℃ to 190℃ with a vacuum vent; and die zone 150℃ to 180℃. The melted and homogenized feed is extruded into a continuous strip feed through a multi-hole die. S3. Cooling and pelletizing: The continuous strip feed extruded by the extruder is rapidly cooled by a cooling water tank with the water temperature maintained at 15°C to 25°C. The cooled solid strip material is fed into a pelletizer and cut into cylindrical or ellipsoidal particles with a diameter of 2 mm to 5 mm and a length of 3 mm to 8 mm, thus obtaining the high-stability composite binder MIM feed.

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