Process for preparing heating body based on powder injection molding method

Abstract

The invention belongs to the technical field of heating bodies, and particularly relates to a process for preparing a heating body based on a powder injection molding method. The preparation method comprises the following steps: mixing metal oxide powder and a dispersing agent, and drying to obtain premixed powder; mixing the premixed powder with a binder, and granulating to obtain injection particles; carrying out injection molding on the injection particles to obtain an injection molding green body; degreasing and sintering the injection molded green body to obtain a heating body; wherein the raw materials of the metal oxide powder comprise lanthanum oxide, strontium carbonate and manganese oxide; the binder is prepared from several raw materials of paraffin, high-density polyethylene, stearic acid, hydrogenated rosin glyceride, polycaprolactone, acetyl tributyl citrate, sorbitan monooleate and an antioxidant; the dispersing agent comprises oleic acid and / or sorbitan monooleate. The heating body prepared by the invention is relatively high in heating rate; the thermal shock resistance is better.

Description

Technical Field

[0001] This invention belongs to the field of heating element technology, specifically relating to a process for preparing heating elements based on powder injection molding. Background Technology

[0002] Currently, heating elements on the market mainly fall into two categories: metal heating wires (such as nickel-chromium alloys and iron-chromium-aluminum alloys) and ceramic heating elements.

[0003] Traditional metal heating wires have several problems: they are prone to oxidation at high temperatures, resulting in a short lifespan; they have poor temperature control accuracy, posing a risk of dry burning; and they may release heavy metal ions, posing a safety hazard.

[0004] Existing ceramic heating elements mostly use an alumina matrix loaded with a metal conductive phase or PTC (positive temperature coefficient) ceramic materials, but they still have shortcomings: the heating rate is slow, affecting the user's immediate experience; the thermal shock resistance is limited, and frequent hot and cold cycles can easily cause cracking. Summary of the Invention

[0005] The purpose of this invention is to provide a process for preparing heating elements based on powder injection molding.

[0006] The above-mentioned objective of the present invention is achieved by the following technical solution: The process for fabricating heating elements based on powder injection molding includes the following steps: Metal oxide powder and dispersant are mixed and dried to obtain premixed powder; the premixed powder is then kneaded with a binder and granulated to obtain injection granules; the injection granules are then injection molded to obtain an injection molded preform; the injection molded preform is then degreased and sintered to obtain a heating element; The raw materials for the metal oxide powder include lanthanum oxide, strontium carbonate, and manganese oxide; the raw materials for the binder include paraffin wax, high-density polyethylene, stearic acid, hydrogenated rosin glycerol ester, polycaprolactone, tributyl acetylacetonate, sorbitan monooleate, and several antioxidants; the dispersant includes oleic acid and / or sorbitan monooleate.

[0007] Furthermore, the preparation method of metal oxide powder includes the following steps: Lanthanum oxide, strontium carbonate, and manganese oxide were weighed to make the molar ratio of La, Sr, and Mn 0.7:0.3:1. The mixture was ball-milled, dried, and then pre-calcined at 900-1100℃ for 3-5 hours in air. The mixture was then cooled to room temperature in the furnace. The pre-calcined lumpy material was ball-milled again and sieved to obtain metal oxide powder with an average particle size D50 of 2-3 μm.

[0008] Further, the metal oxide powder and dispersant are mixed and dried to obtain a premixed powder, including the following steps: Metal oxide powder and dispersant are added to a ball mill. Anhydrous ethanol is used as the medium, and zirconia balls are added. The mixture is ball-milled at 200-300 r / min for 2-4 hours. Then, the mixture is placed in a rotary evaporator and the ethanol is evaporated under reduced pressure at 49-60℃. The resulting powder is then dried in a vacuum drying oven at 70-90℃ for 1-3 hours to obtain the premixed powder.

[0009] Furthermore, the mass of the dispersant is 0.5%-1% of the metal oxide powder; the premixed powder and the binder are mixed at a mass ratio of 1-6:1.

[0010] Furthermore, the method for preparing the adhesive includes the following steps: Weigh out paraffin wax, high-density polyethylene, and stearic acid, add them to a mixer, set the temperature to 120-140℃, turn on the heating and stirring at the same time, with a speed of 20-40 r / min. After heating for 20-40 minutes, increase the speed to 50-70 r / min and continue stirring for 30-50 minutes. After stirring is complete, cool and solidify, then break it into 5-10 mm particles to obtain the binder.

[0011] It should be noted that high-density polyethylene (HDPE) has advantages over low-density polyethylene (LDPE): HDPE has strong chain entanglement ability, which can form a denser polymer network; HDPE crystalline regions provide stronger mechanical support; HDPE and LSMO powder have a small density difference, which can reduce sedimentation and separation; HDPE has a stronger ability to maintain the shape of the green body in the early stage of degreasing.

[0012] During the subsequent compounding process, HDPE molecular chains form an interpenetrating network structure with paraffin and stearic acid. Paraffin acts as the main lubricant and filler component, stearic acid acts as a surfactant to improve the interfacial bonding between the powder and the binder, and HDPE, as the backbone polymer, ensures through its high crystallinity and strong chain entanglement: 1. In the compounding stage, the HDPE network is uniformly dispersed in the paraffin matrix, anchoring the LSMO powder at the polymer network nodes and preventing powder sedimentation and agglomeration. 2. In the molding stage, the high melt strength of HDPE ensures stable flow of the injection feed in the mold cavity, avoiding spraying and delamination. 3. In the debinding stage, after solvent debinding removes the paraffin, the HDPE backbone network maintains the shape of the preform until the HDPE decomposes during the thermal debinding stage.

[0013] Furthermore, the method for preparing the adhesive includes the following steps: Add hydrogenated rosin glycerol ester to a mixer, heat to 80-120℃, start stirring at 40-60 r / min, and after it is completely melted, add polycaprolactone, raise the temperature to 100-140℃, then add acetylated tributyl citrate, sorbitan monooleate, and antioxidant in sequence, then raise the temperature to 120-150℃ and maintain this temperature, increase the stirring speed to 60-100 r / min, and continue stirring for 40-80 minutes. After stirring, degas under vacuum, cool and solidify, and break into 5-10 mm particles to obtain the binder.

[0014] It should be noted that hydrogenated rosin glycerol ester, as the main binder, has the functions of increasing viscosity and improving wettability; polycaprolactone (PCL), as a biodegradable polymer, can act as a support structure, and its decomposition products are also environmentally friendly; acetylthiol tributyl citrate (ATBC), as a plasticizer, reduces the overall viscosity of the binder system, improves mixing flowability and injection molding properties; sorbitan monooleate (Span 80), as a nonionic surfactant, reduces the surface energy of LSMO powder and promotes uniform dispersion of the powder in the binder; antioxidants can prevent thermal oxidative degradation of the material. Hydrogenated rosin glycerol ester and PCL have good compatibility, forming a homogeneous blend system and avoiding phase separation. Moreover, this binder does not require solvent degreasing, all components are thermally decomposable, avoiding the use of organic solvents such as n-hexane, making the production process safer and more environmentally friendly.

[0015] Further, the premixed powder is mixed with a binder, granulated, and then injection molded to obtain injection-produced particles, including the following steps: The premixed powder and binder are added to a twin-screw mixer, and the mixing temperature is set to 80-160℃ and the screw speed is 40-80 r / min. The mixed injection feed is then granulated through a single-screw granulator, with the granulation temperature controlled at 80-160℃, the screw speed at 30-60 r / min, and the particle size range controlled at 0.5-2 mm.

[0016] Further, the injection-molded particles are injection molded to obtain an injection-molded preform, including the following steps: Add the injection granules into the barrel of the injection molding machine, set the barrel temperature to 80-160℃, the injection pressure to 70-120MPa, and the injection speed to 30-50cm. 3 / s, holding pressure is 50-100MPa, holding time is 10-20s, start the injection molding machine, inject the molten injection feed into the mold cavity, after the preform cools and solidifies, open the mold, take out the preform, during the injection molding process, maintain the ambient temperature at 20-30℃ and the relative humidity at 40%-50% to obtain the injection molded preform.

[0017] Preferably, the injection-molded preform is degreased and sintered to obtain the heating element, including the following steps: The injection-molded preform is soaked in n-hexane for 4-6 hours. After solvent degreasing is completed, the preform is removed, the solvent is absorbed, and the solvent-degreased preform is placed in a high-temperature furnace. The temperature is increased from room temperature to 300-500℃ at a rate of 1-2℃ / min and held for 2-3 hours. After degreasing is completed, the preform is allowed to cool naturally to room temperature with the furnace. The degreased green body is placed in a high-temperature sintering furnace and heated from room temperature to 800-900℃ at a heating rate of 3-5℃ / min, and held for 2-3 hours. The temperature is then raised to 1200-1300℃ at a heating rate of 3-5℃ / min and held for 4-6 hours. After sintering, the green body is slowly cooled to room temperature with the furnace, with the cooling rate controlled at 2-10℃ / min, to obtain the heating element.

[0018] Preferably, the injection-molded preform is degreased and sintered to obtain the heating element, including the following steps: Place the injection-molded preform into a high-temperature furnace and raise the temperature from room temperature to 100-200℃ at a rate of 0.5-2℃ / min, hold for 1-2 hours, raise the temperature to 300-500℃ at a rate of 1-2℃ / min, hold for 2-3 hours, and after degreasing, allow it to cool naturally to room temperature with the furnace. The degreased green body is placed in a high-temperature sintering furnace and heated from room temperature to 800-900℃ at a heating rate of 3-5℃ / min, and held for 2-3 hours. The temperature is then raised to 1200-1300℃ at a heating rate of 3-5℃ / min and held for 4-6 hours. After sintering, the green body is slowly cooled to room temperature with the furnace, with the cooling rate controlled at 2-10℃ / min, to obtain the heating element.

[0019] It should be noted that in the prior art, perovskite oxide La 1-x Sr x MnO3 (LSMO) is a class of functional ceramic materials with excellent electrical and thermal properties. When x=0.3, La... 0.7 Sr 0.3 MnO3 has low room temperature resistivity, good high temperature stability and excellent oxidation resistance, making it an ideal candidate material for heating elements.

[0020] However, applying LSMO materials to the manufacture of heating elements faces some technical challenges: 1. Limited molding processes: Traditional dry pressing and casting processes are difficult to prepare heating elements with complex shapes, while powder injection molding technology can achieve complex shapes, but the existing binder system has obvious shortcomings. 2. Complex debinding process.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention features low raw material costs and a simple process: This invention uses a solid-phase reaction method involving oxides and carbonates to prepare metal oxide powders. Compared to wet chemical methods such as sol-gel and co-precipitation, it eliminates the need for expensive organometallic salts or nitrate precursors, reducing raw material costs by 40%-60%. The process is simple, requiring only two main steps: ball milling and calcination, making it easy to scale up production.

[0022] 2. This invention provides two adhesive systems: one is a wax-based adhesive system composed of paraffin wax, high-density polyethylene, and stearic acid; the other is a bio-based composite adhesive system composed of hydrogenated rosin glycerol ester, polycaprolactone, tributyl acetylacetonate, and sorbitan monooleate. The components in both adhesive systems exhibit good synergistic effects, effectively wetting and encapsulating La. 0.7 Sr 0.3 MnO3 metal oxide powder particles enable uniform dispersion of the powder in the binder matrix, avoiding powder agglomeration and thus ensuring uniformity of injection feeding and consistency of the microstructure of the subsequently formed preform.

[0023] Specifically, (1) High-density polyethylene (HDPE) is a backbone polymer with high molecular chain linearity, high crystallinity, and strong chain entanglement ability. It can provide excellent encapsulation and backbone support for metal oxide powders and effectively maintain the structural integrity of the green body during degreasing, which is superior to low-density polyethylene. (2) Hydrogenated rosin glycerol ester provides good wettability and bonding strength. Acetyl tributyl citrate (ATBC) acts as a plasticizer to reduce the viscosity of the system and improve flowability. Sorbitan monooleate (Span 80) acts as a surfactant to further promote powder dispersion. The components work synergistically to form a binder system with excellent comprehensive performance.

[0024] 3. The binder system of the present invention has different thermal decomposition temperature ranges for each component, and can achieve gradient degreasing during the thermal degreasing process. That is, each component decomposes and volatilizes in sequence at different temperature ranges, avoiding concentrated gas release that causes bubbling, cracking or deformation of the green body. (1) Paraffin melts or decomposes first at a lower temperature range, stearic acid decomposes subsequently, and finally high-density polyethylene decomposes completely at a higher temperature range, forming an orderly degreasing gradient. (2) Hydrogenated rosin glycerol ester, ATBC, Span 80 and PCL decompose in sequence at different temperature ranges, achieving more precise gradient degreasing control.

[0025] 4. The heating element prepared by this invention exhibits high density and a uniform microstructure after sintering. The high relative density of the prepared heating element indicates low porosity within the sintered body and tight intergranular bonding. 0.7 Sr 0.3MnO3 forms a stable perovskite crystal structure and a dense microstructure. This high density provides a solid foundation for the electrical, thermal, and mechanical properties of the heating element.

[0026] 5. The heating element prepared by this invention has better electrical properties: its volume resistivity is low. The low volume resistivity indicates that the internal conductive path of the sintered body is continuous and complete, and the electron transport resistance is small, which is beneficial to improving the electro-thermal conversion efficiency.

[0027] 6. The heating element prepared by this invention exhibits excellent heating performance under a 12V DC voltage. The high density and low defect density of the sintered body result in efficient electro-thermal conversion, enabling the heating element to quickly reach the target operating temperature at a lower voltage, meeting the requirements of rapid heating and precise temperature control.

[0028] 7. The heating element prepared by this invention has excellent thermal shock resistance. The sintered body is dense and has a uniform microstructure, without uneven micropores or residual defects, which can effectively resist the thermal stress caused by rapid temperature changes, ensuring that the heating element can work stably for a long time under repeated heating-cooling cycles in actual use, thus extending its service life.

[0029] 8. This invention uses powder injection molding to prepare La 0.7 Sr 0.3 The MnO3 heating element and binder system exhibit good compatibility and mixing uniformity with the metal oxide powder. The prepared injection feed has good flowability and can be molded under reasonable injection temperature and pressure conditions. It can produce complex shapes and high dimensional accuracy blanks in one step without the need for extensive subsequent machining, resulting in high material utilization. It is suitable for industrial mass production and effectively reduces production costs.

[0030] 9. The composite binder system of hydrogenated rosin glycerol ester, PCL, ATBC, and Span 80 can be completely removed by thermal degreasing, eliminating the need for solvent degreasing. This simplifies the process, shortens the production cycle, and avoids the environmental pollution and safety risks associated with using organic solvents such as hexane, aligning with the trend of green manufacturing. Even without solvent degreasing, the heating element prepared using this binder exhibits superior performance in various indicators, including volume resistivity, heating rate, maximum operating temperature, thermal shock resistance, and sintered body density. This demonstrates that the bio-based composite binder system can maintain or even improve the overall performance of the heating element while simplifying the process. Detailed Implementation

[0031] To facilitate understanding of the present invention, a more comprehensive description is provided below, along with preferred embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0033] Unless otherwise specified, the equipment and materials used in the embodiments can be readily obtained from commercial companies.

[0034] Example 1 Prepare the ingredients: Lanthanum oxide: La2O3, purity ≥99.9%.

[0035] Strontium carbonate: SrCO3, purity ≥99.9%.

[0036] Manganese oxide: MnO, purity ≥99.9%.

[0037] The specific steps for preparing metal oxide powder using the above raw materials are as follows: According to the chemical formula La 0.7 Sr 0.3 The stoichiometric ratio of MnO3 was determined by weighing the raw materials using a high-precision electronic balance: lanthanum oxide, strontium carbonate, and manganese oxide were weighed to achieve a molar ratio of La:Sr:Mn of 0.7:0.3:1. These were then added to a planetary ball mill and ball-milled for 6 hours at 250 rpm using anhydrous ethanol as the medium. The homogeneous slurry was dried under reduced pressure at 50°C, then placed in an alumina crucible and pre-calcined at 1000°C for 4 hours in air at a rate of 5°C / min. The pre-calcined lumpy material was then ball-milled again and added to a planetary ball mill and ball-milled for 6 hours at 250 rpm using anhydrous ethanol as the medium. The mixture was then passed through a 200-mesh sieve to obtain metal oxide powder with an average particle size D50 ≈ 2-3 μm.

[0038] Example 2 Prepare the ingredients: Paraffin wax: PW, 58-degree semi-refined paraffin wax, melting point 58-60℃.

[0039] High-density polyethylene (HDPE), grade 5000S, melt index 0.9 g / 10 min (190℃ / 2.16 kg), density 0.954 g / cm³ 3 .

[0040] Stearic acid: SA, industrial grade, purity ≥95%, melting point 67-72℃.

[0041] The specific steps for preparing the adhesive using the above raw materials are as follows: Weigh 650g PW, 250g HDPE, and 100g SA, add them to a double planetary mixer, set the temperature to 130℃, turn on the heating and stirring simultaneously at 30r / min, heat for 30 minutes, then increase the speed to 60r / min and continue stirring for 45 minutes. After stirring is complete, maintain the temperature at 130℃, pour the melt into a stainless steel tray pre-lined with a polytetrafluoroethylene film, and allow it to cool naturally at room temperature for 3 hours until completely solidified. Then, crush it into 5-10mm particles using a crusher to obtain the binder.

[0042] Example 3 Prepare the ingredients: Hydrogenated rosin glycerides: Brand name Foral TM 85-E, Eastman, USA.

[0043] Polycaprolactone (PCL): Grade CAPA 6500, number average molecular weight 50,000, melting point 60°C, Ingevity Corporation.

[0044] Acetyl tributyl citrate (ATBC): 99.2% purity, food grade, colorless and transparent liquid, density 1.046 g / cm³. 3 (20℃), Shanghai McLean Biochemical Technology Co., Ltd.

[0045] Sorbitan monooleate: Span 80, chemically pure, Aladdin Reagent (Shanghai) Co., Ltd.

[0046] Antioxidant: Antioxidant 1010, BASF.

[0047] The specific steps for preparing the adhesive using the above raw materials are as follows: Add 500g of hydrogenated rosin glycerol ester to a double planetary mixer, heat to 100℃, start stirring at 50r / min, and wait until it is completely melted. Then add a total of 300g of PCL in batches, raise the temperature to 120℃, and let the PCL gradually melt. Then add 120g of ATBC, 50g of Span 80, and 30g of antioxidant in sequence, and raise the temperature to 130℃. Maintain this temperature, increase the speed to 80r / min, and continue stirring for 60 minutes. After stirring, degas under vacuum of -0.08MPa for 10 minutes. Pour the molten binder into a stainless steel tray pre-lined with a polytetrafluoroethylene film, and let it cool naturally at room temperature for 2 hours until it is completely solidified. Crush it into 5-10mm particles using a crusher to obtain the binder.

[0048] Example 4 Prepare the ingredients: Dispersant: Oleic acid, chemically pure, purity ≥85%; Metal oxide powder: prepared in Example 1; Adhesive: Prepared in Example 2; Zirconia spheres: a mixture of 5mm and 10mm diameter spheres in a 1:1 mass ratio.

[0049] The specific steps for preparing the heating element using the above-mentioned raw materials based on powder injection molding are as follows: I. Preparation of Injectable Particles: Premixing: 850g of metal oxide powder and 6.8g of dispersant were added to a planetary ball mill. Using 425mL of anhydrous ethanol as the medium, 1275g of zirconia balls were added. The mixture was ball-milled at 250r / min for 3 hours to ensure that the dispersant was evenly coated on the powder surface, thus achieving initial powder dispersion. The mixture was then placed in a rotary evaporator and the ethanol was evaporated under reduced pressure at 50℃. The resulting powder was then placed in an 80℃ vacuum drying oven for 2 hours to obtain the premixed powder.

[0050] Mixing: 850g of premixed powder and 150g of binder are added to a twin-screw mixer. The mixing temperature is set to 150℃ and the screw speed is 60r / min. During the mixing process, the metal oxide powder and binder are mixed to obtain injection feed.

[0051] Granulation: The mixed injection feed is granulated through a single screw granulator. The granulation temperature is controlled at 145℃ and the screw speed is 40r / min to obtain injection granules with a particle size range of 1.5mm.

[0052] II. Preparation of Injection Molded Preform: Mold preparation: Select a suitable injection mold based on the design shape and size of the heating element. Clean the mold with organic solvents such as acetone to remove surface oil and impurities, then wipe it dry with a clean, lint-free cloth to ensure a clean and smooth mold surface, thus guaranteeing the surface quality of the molded preform.

[0053] Process parameter settings: Add injection granules into the barrel of the injection molding machine, set the barrel temperature to 150℃ to ensure complete melting of the granules. Set the injection pressure to 70MPa and the injection speed to 40cm. 3 / s, holding pressure is 50MPa, holding time is 15s.

[0054] Molding operation: Start the injection molding machine and inject the molten injection feed into the mold cavity. After the preform cools and solidifies, open the mold and remove the preform. During the injection molding process, maintain the ambient temperature at 25±2℃ and the relative humidity at 40%-50% to avoid environmental factors affecting the quality of the preform.

[0055] III. Preparation of the heating element: Solvent degreasing: The injection-molded preform is placed in a container with 2500 mL of n-hexane and soaked at room temperature for 5 hours to dissolve and precipitate the paraffin in the preform, removing most of the binder and reducing the difficulty and risk of subsequent thermal degreasing. After solvent degreasing, the preform is removed and the surface is blotted dry with clean filter paper to remove any residual solvent.

[0056] Hot degreasing: The solvent-degreased green body is placed in a high-temperature furnace for hot degreasing. The temperature is increased from room temperature to 400℃ at a rate of 1.0℃ / min, and held at 400℃ for 2.5 hours to allow the remaining binder in the green body to fully decompose and volatilize, thus completing the degreasing process. After degreasing, the green body is allowed to cool naturally to room temperature with the furnace.

[0057] Low-temperature sintering: The degreased green body is placed in a high-temperature sintering furnace and heated from room temperature to 850°C at a heating rate of 4°C / min. It is then held at this temperature for 2 hours to allow the green body to undergo a preliminary solid-phase reaction, thereby eliminating residual trace impurities and gases and improving the density and strength of the green body.

[0058] High-temperature sintering: Continue to raise the temperature to 1250℃ at a heating rate of 4℃ / min, and hold at 1250℃ for 5 hours to allow La to sinter. 0.7 Sr 0.3 The MnO3 reacts fully to form a stable crystal structure and dense microstructure, giving the heating element excellent electrical and thermal properties. After sintering, the element is slowly cooled to room temperature in the furnace at a rate controlled at 2°C / min to prevent cracks or deformation caused by excessively rapid cooling, thus obtaining the heating element.

[0059] Example 5 Prepare the ingredients: Dispersant: Span 80 (sorbitan monooleate), chemically pure, Aladdin Reagent (Shanghai) Co., Ltd.; Metal oxide powder: prepared in Example 1; Adhesive: Prepared in Example 3; Zirconia spheres: a mixture of 5mm and 10mm diameter spheres in a 1:1 mass ratio.

[0060] The specific steps for preparing the heating element using the above-mentioned raw materials based on powder injection molding are as follows: I. Preparation of Injectable Particles: Premixing: 850g of metal oxide powder and 6.8g of dispersant were added to a planetary ball mill. Using 425mL of anhydrous ethanol as the medium, 1275g of zirconia balls were added. The mixture was ball-milled at 250r / min for 3 hours to ensure that the dispersant was evenly coated on the powder surface, thus achieving initial powder dispersion. The mixture was then placed in a rotary evaporator and the ethanol was evaporated under reduced pressure at 50℃. The resulting powder was then placed in an 80℃ vacuum drying oven for 2 hours to obtain the premixed powder.

[0061] Mixing: 850g of premixed powder and 150g of binder are added to a twin-screw mixer. The mixing temperature is set to 95℃ and the screw speed to 55r / min. During the mixing process, the metal oxide powder and binder are mixed to obtain injection feed.

[0062] Granulation: The mixed injection feed is granulated through a single screw granulator. The granulation temperature is controlled at 95℃ and the screw speed is 40r / min to obtain injection granules with a particle size range of 1.5mm.

[0063] II. Preparation of Injection Molded Preform: Mold preparation: Select a suitable injection mold based on the design shape and size of the heating element. Clean the mold with organic solvents such as acetone to remove surface oil and impurities, then wipe it dry with a clean, lint-free cloth to ensure a clean and smooth mold surface, thus guaranteeing the surface quality of the molded preform.

[0064] Process parameter settings: Add injection granules into the barrel of the injection molding machine, set the barrel temperature to 100℃ to ensure complete melting of the injection granules. Set the injection pressure to 70MPa and the injection speed to 40cm. 3 / s, holding pressure is 50MPa, holding time is 15s, to ensure that the material can fill the mold cavity and obtain a blank with complete shape and high dimensional accuracy.

[0065] Molding operation: Start the injection molding machine and inject the molten injection feed into the mold cavity. After the preform cools and solidifies, open the mold and remove the preform. During the injection molding process, maintain the ambient temperature at 25±2℃ and the relative humidity at 40%-50% to avoid environmental factors affecting the quality of the preform.

[0066] III. Preparation of the heating element: Thermal debinding: The injection-molded preform is placed in a high-temperature furnace for thermal debinding. The temperature is increased from room temperature to 180°C at a rate of 0.5°C / min and held for 1 hour; then the temperature is increased to 400°C at a rate of 1.0°C / min and held at 400°C for 2.5 hours to allow the binder in the preform to fully decompose and volatilize, completing the debinding process. After debinding, the preform is allowed to cool naturally to room temperature in the furnace.

[0067] Low-temperature sintering: The degreased green body is placed in a high-temperature sintering furnace and heated from room temperature to 850°C at a heating rate of 4°C / min. It is then held at this temperature for 2 hours to allow the green body to undergo a preliminary solid-phase reaction, thereby eliminating residual trace impurities and gases and improving the density and strength of the green body.

[0068] High-temperature sintering: Continue to raise the temperature to 1250℃ at a heating rate of 4℃ / min, and hold at 1250℃ for 5 hours to allow La to sinter. 0.7 Sr 0.3 The MnO3 reacts fully to form a stable crystal structure and dense microstructure, giving the heating element excellent electrical and thermal properties. After sintering, the element is slowly cooled to room temperature in the furnace at a rate controlled at 2°C / min to prevent cracks or deformation caused by excessively rapid cooling, thus obtaining the heating element.

[0069] Comparative Example 1 Prepare the ingredients: Paraffin wax: PW, 58-degree semi-refined paraffin wax, melting point 58-60℃.

[0070] Low-density polyethylene: LDPE, grade LD 40022.LI, ExxonMobil.

[0071] Stearic acid: SA, industrial grade, purity ≥95%, melting point 67-72℃.

[0072] The specific steps for preparing the adhesive using the above raw materials are as follows: Weigh 700g PW, 200g LDPE, and 100g SA, add them to a double planetary mixer, set the temperature to 130℃, turn on the heating and stirring simultaneously at 30r / min, heat for 30 minutes, then increase the speed to 60r / min and continue stirring for 45 minutes. After stirring is complete, maintain the temperature at 130℃, pour the melt into a stainless steel tray pre-lined with a polytetrafluoroethylene film, and allow it to cool naturally at room temperature for 3 hours until completely solidified. Then, crush it into 5-10mm particles using a crusher to obtain the binder.

[0073] Comparative Example 2 Prepare the ingredients: Dispersant: Oleic acid, chemically pure, purity ≥85%; Metal oxide powder: prepared in Example 1; Adhesive: Prepared in Comparative Example 1; Zirconia spheres: a mixture of 5mm and 10mm diameter spheres in a 1:1 mass ratio.

[0074] The specific steps for preparing the heating element using the above-mentioned raw materials based on powder injection molding are as follows: I. Preparation of Injectable Particles: Premixing: 850g of metal oxide powder and 6.8g of dispersant were added to a planetary ball mill. Using 425mL of anhydrous ethanol as the medium, 1275g of zirconia balls were added. The mixture was ball-milled at 250r / min for 3 hours to ensure that the dispersant was evenly coated on the powder surface, thus achieving initial powder dispersion. The mixture was then placed in a rotary evaporator and the ethanol was evaporated under reduced pressure at 50℃. The resulting powder was then placed in an 80℃ vacuum drying oven for 2 hours to obtain the premixed powder.

[0075] Mixing: 850g of premixed powder and 150g of binder are added to a twin-screw mixer. The mixing temperature is set to 150℃ and the screw speed is 60r / min. During the mixing process, the metal oxide powder and binder are mixed to obtain injection feed.

[0076] Granulation: The mixed injection feed is granulated through a single screw granulator. The granulation temperature is controlled at 145℃ and the screw speed is 40r / min to obtain injection granules with a particle size range of 1.5mm.

[0077] II. Preparation of Injection Molded Preform: Mold preparation: Select a suitable injection mold based on the design shape and size of the heating element. Clean the mold with organic solvents such as acetone to remove surface oil and impurities, then wipe it dry with a clean, lint-free cloth to ensure a clean and smooth mold surface, thus guaranteeing the surface quality of the molded preform.

[0078] Process parameter settings: Add injection granules into the barrel of the injection molding machine, set the barrel temperature to 150℃ to ensure complete melting of the granules. Set the injection pressure to 70MPa and the injection speed to 40cm. 3 / s, holding pressure is 50MPa, holding time is 15s.

[0079] Molding operation: Start the injection molding machine and inject the molten injection feed into the mold cavity. After the preform cools and solidifies, open the mold and remove the preform. During the injection molding process, maintain the ambient temperature at 25±2℃ and the relative humidity at 40%-50% to avoid environmental factors affecting the quality of the preform.

[0080] III. Preparation of the heating element: Solvent degreasing: The injection-molded preform is placed in a container with 2500 mL of n-hexane and soaked at room temperature for 5 hours to dissolve and precipitate the paraffin in the preform, removing most of the binder and reducing the difficulty and risk of subsequent thermal degreasing. After solvent degreasing, the preform is removed and the surface is blotted dry with clean filter paper to remove any residual solvent.

[0081] Hot degreasing: The solvent-degreased green body is placed in a high-temperature furnace for hot degreasing. The temperature is increased from room temperature to 400℃ at a rate of 1.0℃ / min, and held at 400℃ for 2.5 hours to allow the remaining binder in the green body to fully decompose and volatilize, thus completing the degreasing process. After degreasing, the green body is allowed to cool naturally to room temperature with the furnace.

[0082] Low-temperature sintering: The degreased green body is placed in a high-temperature sintering furnace and heated from room temperature to 850°C at a heating rate of 4°C / min. It is then held at this temperature for 2 hours to allow the green body to undergo a preliminary solid-phase reaction, thereby eliminating residual trace impurities and gases and improving the density and strength of the green body.

[0083] High-temperature sintering: Continue heating at a rate of 4℃ / min to reach 1250℃, and hold at 1250℃ for 5 hours. After sintering, slowly cool the furnace to room temperature at a rate of 2℃ / min to obtain the heating element.

[0084] Comparative Example 3 Prepare the ingredients: Dispersant: Span 80 (sorbitan monooleate), chemically pure, Aladdin Reagent (Shanghai) Co., Ltd.; Metal oxide powder: prepared in Example 1; Binder: Polycaprolactone (PCL): Grade CAPA 6500, number average molecular weight 50,000, melting point 60°C, Ingevity Corporation; Zirconia spheres: a mixture of 5mm and 10mm diameter spheres in a 1:1 mass ratio.

[0085] The specific steps for preparing the heating element using the above-mentioned raw materials based on powder injection molding are as follows: I. Preparation of Injectable Particles: Premixing: 850g of metal oxide powder and 6.8g of dispersant were added to a planetary ball mill. Using 425mL of anhydrous ethanol as the medium, 1275g of zirconia balls were added. The mixture was ball-milled at 250r / min for 3 hours to ensure that the dispersant was evenly coated on the powder surface, thus achieving initial powder dispersion. The mixture was then placed in a rotary evaporator and the ethanol was evaporated under reduced pressure at 50℃. The resulting powder was then placed in an 80℃ vacuum drying oven for 2 hours to obtain the premixed powder.

[0086] Mixing: Add 850g of premixed powder and 150g of binder to a twin-screw mixer, set the mixing temperature to 100℃ and the screw speed to 55r / min, and mix. During the mixing process, the metal oxide powder and binder are mixed to obtain injection feed.

[0087] Granulation: The mixed injection feed is granulated through a single screw granulator. The granulation temperature is controlled at 95℃ and the screw speed is 40r / min to obtain injection granules with a particle size range of 1.5mm.

[0088] II. Preparation of Injection Molded Preform: Mold preparation: Select a suitable injection mold based on the design shape and size of the heating element. Clean the mold with organic solvents such as acetone to remove surface oil and impurities, then wipe it dry with a clean, lint-free cloth to ensure a clean and smooth mold surface, thus guaranteeing the surface quality of the molded preform.

[0089] Process parameter settings: Add injection granules into the barrel of the injection molding machine, set the barrel temperature to 100℃ to ensure complete melting of the granules. Set the injection pressure to 70MPa and the injection speed to 40cm. 3 / s, holding pressure is 50MPa, holding time is 15s, to ensure that the material can fill the mold cavity and obtain a blank with complete shape and high dimensional accuracy.

[0090] Molding operation: Start the injection molding machine and inject the molten injection feed into the mold cavity. After the preform cools and solidifies, open the mold and remove the preform. During the injection molding process, maintain the ambient temperature at 25±2℃ and the relative humidity at 40%-50% to avoid environmental factors affecting the quality of the preform.

[0091] III. Preparation of the heating element: Thermal debinding: The injection-molded preform is placed in a high-temperature furnace for thermal debinding. The temperature is increased from room temperature to 180°C at a rate of 0.5°C / min and held for 1 hour; then the temperature is increased to 400°C at a rate of 1.0°C / min and held at 400°C for 2.5 hours to allow the binder in the preform to fully decompose and volatilize, completing the debinding process. After debinding, the preform is allowed to cool naturally to room temperature in the furnace.

[0092] Low-temperature sintering: The degreased green body is placed in a high-temperature sintering furnace and heated from room temperature to 850°C at a heating rate of 4°C / min, and held at that temperature for 2 hours.

[0093] High-temperature sintering: Continue heating at a rate of 4℃ / min to reach 1250℃, and hold at 1250℃ for 5 hours. After sintering, slowly cool the furnace to room temperature at a rate of 2℃ / min to obtain the heating element.

[0094] To evaluate the performance of the heating element, the heating elements prepared in Examples 4 and 5, as well as Comparative Examples 2 and 3, were all molded into standard specimens of uniform specifications using the same mold: 1. Shape: rectangular sheet. 2. Dimensions: length × width × height = 50mm × 5mm × 1.5mm.

[0095] 1. Volume resistivity: The resistance of the heating element was measured at room temperature using the four-probe method, and the volume resistivity (unit: Ω·cm) was calculated based on the dimensions. Five samples were measured for each example and comparative example, and the average value was taken.

[0096] The test results show that the volume resistivity of Example 4 is 3.2 × 10⁻⁶. -3 Ω·cm; The volume resistivity of Example 5 is 2.8 × 10⁻⁶. -3 Ω·cm; The volume resistivity of Comparative Example 2 is 4.6 × 10⁻⁶ Ω·cm. -3 Ω·cm; The volume resistivity of Comparative Example 3 is 8.5 × 10⁻⁶ Ω·cm. -3 Ω·cm.

[0097] Analysis: Comparative Example 2 used LDPE instead of HDPE. Due to the high branching degree and weak chain entanglement ability of LDPE molecular chains, its ability to encapsulate powder and provide skeletal support is not as good as HDPE. This results in poor structure retention of the green body during debinding, uneven distribution of micropores after sintering, and a slight increase in resistivity. Comparative Example 3 used pure PCL as a binder. Lacking the synergistic effect of hydrogenated rosin glycerol ester as a thickener and ATBC as a plasticizer, the powder dispersion was uneven. During mixing, powder agglomeration occurred in some areas. During debinding, microcracks and local pores were generated due to the concentrated decomposition temperature range, resulting in more defects inside the sintered body and a significant increase in resistivity.

[0098] 2. Heating rate and maximum operating temperature: Apply a 12V DC voltage to both ends of the heating element and use an infrared thermal imager to record the time it takes for its surface temperature to rise from room temperature to 200℃ and the final maximum temperature reached.

[0099] The test results are as follows: Example 4 had a heating rate of 18.5 ℃ / s to 200℃ and a maximum operating temperature of 385℃; Example 5 had a heating rate of 22.3 ℃ / s to 200℃ and a maximum operating temperature of 420℃; Comparative Example 2 had a heating rate of 15.8 ℃ / s to 200℃ and a maximum operating temperature of 355℃; Comparative Example 3 had a heating rate of 12.6 ℃ / s to 200℃ and a maximum operating temperature of 340℃.

[0100] Analysis: The heating rate and maximum operating temperature are related to the density and internal defects of the sintered body. Higher density and fewer internal defects result in higher electro-thermal conversion efficiency, faster heating, and a higher maximum temperature. In Comparative Example 3, the debinding defects of the pure PCL binder led to numerous pores and microcracks inside the sintered body, obstructing the current conduction path and reducing heating efficiency.

[0101] 3. Thermal shock resistance: Heat the heating element to 200°C in the air and keep it at that temperature for 10 minutes. Then quickly immerse it in cold water at 25°C. Repeat this cycle 10 times and observe whether cracks or fissures appear on the surface of the sample.

[0102] The test results were as follows: Example 4 showed no cracks; Example 5 showed no cracks; Comparative Example 2 showed microcracks after the 8th cycle; Comparative Example 3 showed cracks after the 5th cycle.

[0103] Analysis: Thermal shock resistance depends on the density, uniformity, and internal defect state of the sintered body. In Examples 4 and 5, due to the reasonable design of the binder system, uniform powder dispersion, and smooth debinding process, the sintered bodies are dense and have a uniform microstructure, enabling them to withstand repeated thermal shocks. In Comparative Example 2, the LDPE skeleton lacks sufficient support, and the sintered body contains uneven micropore distribution, becoming weak points for crack initiation under thermal stress. In Comparative Example 3, microcracks were generated during the debinding stage. Although some healed after sintering, residual defects remained, which rapidly propagated into cracks under thermal shock conditions.

[0104] 4. Density of sintered body: The density of the sintered heating element was measured using Archimedes' displacement method and compared with the theoretical density (La). 0.7 Sr 0.3 The theoretical density of MnO3 is approximately 6.45 g / cm³. 3 By comparison, the relative density is calculated.

[0105] The test results were as follows: the relative density of Example 4 was 96.8%; the relative density of Example 5 was 98.2%; the relative density of Comparative Example 2 was 95.1%; and the relative density of Comparative Example 3 was 92.5%.

[0106] Analysis: Example 5 exhibited the highest relative density, attributed to the synergistic effect of the hydrogenated rosin glycerol ester / PCL / ATBC / Span 80 composite binder system. Hydrogenated rosin glycerol ester provided excellent wettability and bonding strength, ATBC reduced system viscosity and improved flowability, and Span 80, acting as a surfactant, promoted powder dispersion. The components decomposed sequentially at different temperature ranges, achieving gradient degreasing and preventing deformation and cracking of the green body. In Comparative Example 2, the crystallinity and molecular weight distribution of LDPE differed from HDPE, resulting in less than ideal thermal decomposition behavior during degreasing, leading to a slightly lower density in the sintered body. In Comparative Example 3, the pure PCL binder had a single component and a narrow decomposition temperature range, mainly concentrated between 300-400℃. During degreasing, concentrated gas release caused large pores and cracks inside the green body, which were difficult to completely eliminate after sintering, resulting in the lowest relative density.

[0107] It is worth noting that, comparing the test results of Examples 4 and 5, the present invention uses a bio-based adhesive system (Examples 3 and 5) with hydrogenated rosin glycerol ester as the main component, polycaprolactone as the backbone, acetylsicitrin tributyl ester as the plasticizer, and sorbitan monooleate as the surfactant, to replace the paraffin and high-density polyethylene-based adhesive system (Examples 2 and 4), further solving the technical problem that the adhesive needs to be processed at high temperatures and relies on solvent degreasing.

[0108] This invention introduces hydrogenated rosin glycerol ester into the field of metal powder injection molding binders. Utilizing its good thermoplasticity, moderate melting temperature, excellent wettability with metal oxide powders, and extremely low thermal decomposition carbon residue, combined with the skeletal support of polycaprolactone and the plasticizing effect of tributyl acetylacetic acid, a bio-based binder system is constructed that can be completely removed by low-temperature processing at 100℃ without solvent degreasing, using only pure thermal degreasing. La prepared using this system... 0.7 Sr 0.3 The MnO3 heating element has achieved further improvements in density, mechanical strength, electrical properties, heating uniformity, and cycle life, while the process is more environmentally friendly and does not require solvent degreasing.

[0109] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A process for preparing a heating element based on powder injection molding, characterized in that, Includes the following steps: Metal oxide powder and dispersant are mixed and dried to obtain premixed powder; the premixed powder is then kneaded with a binder and granulated to obtain injection granules; the injection granules are then injection molded to obtain an injection molded preform; the injection molded preform is then degreased and sintered to obtain a heating element; The raw materials for the metal oxide powder include lanthanum oxide, strontium carbonate, and manganese oxide. The raw materials for the adhesive include several of the following: paraffin wax, high-density polyethylene, stearic acid, hydrogenated rosin glycerol ester, polycaprolactone, acetylglucosyl tributyl ester, sorbitan monooleate, and antioxidants. Dispersants include oleic acid and / or sorbitan monooleate.

2. The process for preparing a heating element based on powder injection molding according to claim 1, characterized in that, The method for preparing metal oxide powder includes the following steps: Lanthanum oxide, strontium carbonate, and manganese oxide were weighed to make the molar ratio of La, Sr, and Mn 0.7:0.3:

1. The mixture was ball-milled, dried, and then pre-calcined at 900-1100℃ for 3-5 hours in air. The mixture was then cooled to room temperature in the furnace. The pre-calcined lumpy material was ball-milled again and sieved to obtain metal oxide powder with an average particle size D50 of 2-3 μm.

3. The process for preparing a heating element based on powder injection molding according to claim 1, characterized in that, The metal oxide powder and dispersant are mixed and dried to obtain a premixed powder, including the following steps: Metal oxide powder and dispersant are added to a ball mill, anhydrous ethanol is used as the medium, and zirconium oxide balls are added. The mixture is ball-milled at 200-300 r / min for 2-4 hours. Then, it is placed in a rotary evaporator and the ethanol is evaporated under reduced pressure at 49-60℃. The resulting powder is then placed in a vacuum drying oven at 70-90℃ for 1-3 hours to obtain the premixed powder.

4. The process for preparing a heating element based on powder injection molding according to claim 1, characterized in that, The mass of the dispersant is 0.5%-1% of the metal oxide powder; the premixed powder and the binder are mixed at a mass ratio of 1-6:

1.

5. The process for preparing a heating element based on powder injection molding according to claim 1, characterized in that, The method for preparing the adhesive includes the following steps: Weigh out paraffin wax, high-density polyethylene, and stearic acid, add them to a mixer, set the temperature to 120-140℃, turn on the heating and stirring at the same time, with a speed of 20-40 r / min. After heating for 20-40 minutes, increase the speed to 50-70 r / min and continue stirring for 30-50 minutes. After stirring is complete, cool and solidify, then break it into 5-10 mm particles to obtain the binder.

6. The process for preparing a heating element based on powder injection molding according to claim 1, characterized in that, The method for preparing the adhesive includes the following steps: Add hydrogenated rosin glycerol ester to a mixer, heat to 80-120℃, start stirring at 40-60 r / min, and after it is completely melted, add polycaprolactone, raise the temperature to 100-140℃, then add acetylated tributyl citrate, sorbitan monooleate, and antioxidant in sequence, then raise the temperature to 120-150℃ and maintain this temperature, increase the stirring speed to 60-100 r / min, and continue stirring for 40-80 minutes. After stirring, degas under vacuum, cool and solidify, and break into 5-10 mm particles to obtain the binder.

7. The process for preparing a heating element based on powder injection molding according to claim 1, characterized in that, The premixed powder is mixed with a binder, granulated, and then injection molded to obtain injection granules, including the following steps: The premixed powder and binder are added to a twin-screw mixer, and the mixing temperature is set to 80-160℃ and the screw speed is 40-80 r / min. The mixed injection feed is then granulated through a single-screw granulator, with the granulation temperature controlled at 80-160℃, the screw speed at 30-60 r / min, and the particle size range controlled at 0.5-2 mm.

8. The process for preparing a heating element based on powder injection molding according to claim 1, characterized in that, The injection molding process involves molding the injection-molded particles to obtain an injection-molded preform, including the following steps: Add the injection granules into the barrel of the injection molding machine, set the barrel temperature to 80-160℃, the injection pressure to 70-120MPa, and the injection speed to 30-50cm. 3 / s, holding pressure is 50-100MPa, holding time is 10-20s, start the injection molding machine, inject the molten injection feed into the mold cavity, after the preform cools and solidifies, open the mold, take out the preform, during the injection molding process, maintain the ambient temperature at 20-30℃ and the relative humidity at 40%-50% to obtain the injection molded preform.

9. The process for preparing a heating element based on powder injection molding according to claim 5, characterized in that, The injection-molded preform is degreased and sintered to obtain the heating element, including the following steps: Soak the injection-molded preform in n-hexane for 4-6 hours. After solvent degreasing is completed, remove the preform, absorb the solvent, and place the solvent-degreased preform into a high-temperature furnace. Raise the temperature from room temperature to 300-500℃ at a rate of 1-2℃ / min and hold for 2-3 hours. After degreasing is completed, allow it to cool naturally to room temperature with the furnace. The degreased green body is placed in a high-temperature sintering furnace and heated from room temperature to 800-900℃ at a heating rate of 3-5℃ / min, and held for 2-3 hours. The temperature is then raised to 1200-1300℃ at a heating rate of 3-5℃ / min and held for 4-6 hours. After sintering, the green body is slowly cooled to room temperature with the furnace, with the cooling rate controlled at 2-10℃ / min, to obtain the heating element.

10. The process for preparing a heating element based on powder injection molding according to claim 6, characterized in that, The injection-molded preform is degreased and sintered to obtain the heating element, including the following steps: Place the injection-molded preform into a high-temperature furnace and raise the temperature from room temperature to 100-200℃ at a rate of 0.5-2℃ / min, hold for 1-2 hours, raise the temperature to 300-500℃ at a rate of 1-2℃ / min, hold for 2-3 hours, and after degreasing, allow it to cool naturally to room temperature with the furnace. The degreased green body is placed in a high-temperature sintering furnace and heated from room temperature to 800-900℃ at a heating rate of 3-5℃ / min, and held for 2-3 hours. The temperature is then raised to 1200-1300℃ at a heating rate of 3-5℃ / min and held for 4-6 hours. After sintering, the green body is slowly cooled to room temperature with the furnace, with the cooling rate controlled at 2-10℃ / min, to obtain the heating element.