Preparation method of high-flow-rate feed for metal injection molding

Abstract

The invention discloses a preparation method of a high-flow-rate feed for metal injection molding, which comprises the following steps: step 1, bimodal stainless steel powder and a composite binder are weighed, the bimodal stainless steel powder comprises coarse powder and fine powder, the ratio of the coarse powder to the fine powder is 6: 4-7: 3, and the composite binder comprises polyethylene glycol, zinc stearate and paraffin; secondly, vacuum deoxidation and cooling protection are conducted on the double-peak stainless steel powder, screening is conducted on the double-peak stainless steel powder, and drying treatment is conducted on the composite binder; thirdly, the composite binder and the double-peak stainless steel powder are put into an internal mixer to be subjected to internal mixing; and step 4, putting the material obtained by banburying into a granulator for extrusion granulation. According to the preparation method of the high-flow-rate feed for metal injection molding, spherical metal granulation powder with two granularities is adopted as a raw material, coarse powder provides mobility, gaps are filled with fine powder so that the sintering density can be improved, and a composite binder with polyethylene glycol as a main material is adopted, so that the degreasing time is effectively shortened, and the carbon residue is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of metal injection molding, and particularly to a preparation method for high-flow-rate feedstock for metal injection molding. Background Art

[0002] Metal Injection Molding (MIM) technology, as a new type of near-net-shape forming technology, occupies a crucial position in modern manufacturing. This technology combines the advantages of traditional powder metallurgy and plastic injection molding, and can manufacture high-precision and complex-shaped metal parts at a relatively low cost, and is widely used in many fields such as aerospace, automotive manufacturing, electronic information, and medical devices.

[0003] With the continuous development of manufacturing, the requirements for metal injection molding technology are also increasing day by day. High-flow-rate feedstock, as a key factor in metal injection molding technology, plays a crucial role in the development of this technology. High-flow-rate feedstock can significantly improve the efficiency of injection molding, enabling more parts to be produced per unit time and meeting the needs of mass production. High-flow-rate feedstock helps to improve the molding quality of parts. During the injection process, the feedstock can fill the mold cavity more quickly and evenly, reducing defects such as air holes and shrink marks caused by insufficient or uneven filling, thereby improving the dimensional accuracy and surface quality of the parts. For some parts with complex shapes, high-flow-rate feedstock can better fill the fine structures of the mold, ensuring the integrity and accuracy of the parts. High-flow-rate feedstock can also reduce production costs. Improving production efficiency means an increase in output per unit time, thus spreading the costs of equipment, labor, etc.; at the same time, reducing molding defects also reduces the scrap rate, saving raw materials and subsequent processing costs.

[0004] The current trend of miniaturization and complication in the manufacturing of precision parts places higher requirements on the fluidity and debinding efficiency of the feedstock. The limitations of traditional technologies include: 1. Powder selection: Powders with a unimodal distribution have a low packing density and limited fluidity; 2. Binder system: Paraffin-based binders have large fluctuations in high-temperature viscosity, and polyacetal resin has a high debinding residue (literature "Advances in MIM Technology", 2020); 3. Process complexity: The multi-step solvent debinding and dispersion process increases energy consumption.

[0005] Therefore, it is necessary to develop a preparation method for high-flow-rate feedstock for metal injection molding to solve the defects of traditional technologies. Summary of the Invention

[0006] The technical problem to be solved by the present invention is: to provide a preparation method for high-flow-rate feedstock for metal injection molding to increase the melt flow rate, shorten the debinding time, reduce the carbon residue amount, and improve the density.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is as follows: A method for preparing a high-flow feedstock for metal injection molding, comprising the following steps: Step 1, weigh bimodal stainless steel powder and a composite binder. The bimodal stainless steel powder includes coarse powder and fine powder, and the ratio of the coarse powder to the fine powder is 6:4 - 7:3. The composite binder includes polyethylene glycol, a, zinc stearate, and paraffin wax; Step 2, first perform vacuum deoxidation on the bimodal stainless steel powder, then cool and protect it, screen the bimodal stainless steel powder, and dry the composite binder; Step 3, put the composite binder and the bimodal stainless steel powder into a mixer in a certain loading amount and mix them together; Step 4, put the material obtained after mixing into a granulator and extrude and granulate it to obtain granular injection feedstock.

[0008] The beneficial effects of the present invention are as follows: The method for preparing a high-flow feedstock for metal injection molding disclosed by the present invention uses two kinds of spherical metal granulating powders with different particle sizes as raw materials. Among them, the coarse powder provides fluidity, and the fine powder fills the gaps to improve the sintering density. A composite binder mainly composed of polyethylene glycol is used, effectively shortening the debinding time and reducing the carbon residue amount. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is the shear stress-viscosity change curve of the 316L feedstock in the first embodiment of the present invention; Figure 2 It is the shear stress-viscosity change curve of the 17-4PH feedstock in the second embodiment of the present invention; Figure 3 It is the shear stress-viscosity change curve of the 316L feedstock in Comparative Example 1; Figure 4 It is the shear stress-viscosity change curve of the 316L feedstock in Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0010] To describe in detail the technical content, the achieved purpose and the effects of the present invention, the following is described in conjunction with the embodiments and in coordination with the drawings.

[0011] Please refer to Figures 1 to 4 , a method for preparing a high-flow feedstock for metal injection molding, comprising the following steps: Step 1, weigh bimodal stainless steel powder and a composite binder. The bimodal stainless steel powder includes coarse powder and fine powder, and the ratio of the coarse powder to the fine powder is 6:4 - 7:3. The composite binder includes polyethylene glycol, a, zinc stearate, and paraffin wax; Step 2: First, perform vacuum deoxidation on the bimodal stainless steel powder, then cool and protect it, screen the bimodal stainless steel powder, and dry the polyethylene glycol of the composite binder. Step 3: Put the composite binder and the bimodal stainless steel powder into a mixer at a certain loading amount and mix them together. Step 4: Put the material obtained after mixing into a granulator to extrude and granulate, obtaining granular injection feedstock.

[0012] As can be seen from the above description, the beneficial effects of the present invention are as follows: A preparation method of a high-flow feedstock for metal injection molding disclosed by the present invention uses two kinds of spherical metal granulating powders with different particle sizes as raw materials. Among them, the coarse powder provides fluidity, and the fine powder fills the gaps to improve the sintering density. And a composite binder mainly composed of polyethylene glycol is used, effectively shortening the debinding time and reducing the carbon residue amount.

[0013] Further, the particle size of the coarse powder of the bimodal stainless steel powder is 18 - 24 μm, and the particle size of the fine powder is 4 - 9 μm.

[0014] Further, a is ethylene-vinyl acetate copolymer or styrene-butadiene copolymer.

[0015] As can be seen from the above description, the components of the composite binder can be selected according to actual production conditions or usage requirements.

[0016] Further, the ratio of polyethylene glycol to a in the composite binder is 2:1 - 3:1.

[0017] Further, when a is ethylene-vinyl acetate copolymer, the mixing temperature in Step 2 is 170 - 180 °C, the rotation speed is 50 - 80 r / min, and the mixing time is 40 - 60 min; when a is styrene-butadiene copolymer, the mixing temperature in Step 2 is 180 - 190 °C, the rotation speed is 50 - 80 r / min, and the mixing time is 40 - 60 min.

[0018] Further, the mass percentage of the bimodal stainless steel powder in the high-flow feedstock is 86% - 92.5%, and the mass percentage of the composite binder in the high-flow feedstock is 7.5% - 14%.

[0019] Further, the mass percentage of polyethylene glycol is 4.0% - 6.0%, the mass percentage of a is 2.0% - 4.0%, the mass percentage of zinc stearate is 0.5% - 1.5%, and the mass percentage of paraffin wax is 1.0% - 2.5%.

[0020] Further, the bimodal stainless steel powder is 316L stainless steel powder or 17 - 4PH stainless steel powder.

[0021] Further, the coarse powder of the bimodal stainless steel powder is a gas atomized powder or a water atomized powder.

[0022] As can be seen from the above description, the type of the coarse powder of the bimodal stainless steel powder can be selected according to actual usage requirements or production conditions.

[0023] Further, the vacuum deoxidation of the bimodal stainless steel powder in step two specifically includes: placing the bimodal stainless steel powder in a vacuum furnace, evacuating to a pressure ≤ 10 -2 Pa in the vacuum furnace, heating at a rate of 5 °C / min to 200 - 250 °C and holding for 1 - 2 hours to remove surface oxides until the oxygen content is less than 0.1 wt%.

[0024] Further, the cooling protection in step two specifically includes: introducing high-purity nitrogen gas (purity ≥ 99.999%) and rapidly cooling to below 50 °C.

[0025] As can be seen from the above description, cooling protection of the bimodal stainless steel powder can effectively prevent secondary oxidation.

[0026] Further, the screening of the bimodal stainless steel powder in step two specifically includes: classifying with a vibrating screen with a mesh number of 200 - 400 meshes, mixing according to a mass ratio of coarse powder:fine powder = 6:4 - 7:3, and ensuring a loose bulk density ≥ 5.2 g / cm 3 .

[0027] Further, the drying treatment of polyethylene glycol in step two specifically includes: placing polyethylene glycol in a vacuum at 80 °C and drying for 4 hours until the water content is less than or equal to 0.05%.

[0028] Further, Step 3 specifically includes: Turn on a twin-screw internal mixer with a capacity of 20 L. After checking and confirming that the internal mixer is clean and can work properly, start the machine, heat the internal mixer to 140 - 160 °C, and preheat for 30 min; sequentially add polyethylene glycol, paraffin, ethylene-vinyl acetate copolymer, and zinc stearate of the composite binder into the preheated internal mixer, close the dust cover and gland, stir at 20 r / min for 10 min until the composite binder is evenly mixed, and then add the bimodal stainless steel powder; close the dust cover and gland, and knead the bimodal stainless steel powder and the composite binder at a kneading temperature of 170 - 180 °C and a rotation speed of 50 - 80 r / min for 40 - 60 min; when the bimodal stainless steel powder and the composite binder are kneaded into a mass, turn off the rotor, open the dust cover and gland, insert the probe of the thermocouple thermometer into the interior of the material. When the material temperature is 175 °C - 195 °C, start kneading, set the rotation speed to 40 - 80 r / min, and the kneading time to 10 - 20 min until the material shows a uniform bright surface state; after kneading is completed, set the temperature of the internal mixer to 140 - 160 °C, turn off the heating, and wait until the measured material temperature drops to 140 - 160 °C, then adjust the screw rotation speed from 40 - 80 r / min to 15 - 20 r / min; when the mixing and kneading reach the predetermined time and quality requirements, stop heating and the rotation of the rotor, open the discharge port of the internal mixer, and discharge the kneaded material into the cooling equipment; cool the material to 50 - 80 °C by air cooling.

[0029] Further, Step 4 specifically includes: Add the kneaded material into the hopper of the granulator. Through the rotation of the granulator screw, the material is conveyed to the heating zone of the extruder. The heating zone of the extruder heats the material to 140 - 180 °C, and then the granulator extrudes and shapes the material and cuts it into feeding pellets. The rotation speed of the cutting tool for cutting is 50 r / min, the diameter of the feeding pellets is 2 - 5 mm, and the length of the feeding pellets is 3 - 6 mm.

[0030] Further, after Step 4, there is also Step 5: Melt flow rate test.

[0031] As can be seen from the above description, the melt flow rate test step can effectively ensure the feeding yield.

[0032] Further, Step 5 specifically includes: Turn on the melt index tester, set the temperature to 190 °C. After the temperature reaches 190 °C and is stably kept warm for 5 min, take a spoonful of evenly distributed feeding pellets and add them to the melt index tester. The load is 21.6 kg, the cut-off interval time is 1.0 s, record the average cut-off mass of 10 cuts, and calculate to obtain the MFR. Subsequently, use a capillary rheometer (model: Rosand RH2000, capillary diameter 1 mm, length-diameter ratio 30:1) to test the relationship between the viscosity of the feeding at 190 °C and the shear rate.

[0033] A preparation method of a stainless steel product by metal injection molding, using the high-flow feedstock prepared by the above-mentioned high-flow feedstock preparation method for metal injection molding, and formed through an injection process, a debinding process, and a sintering process.

[0034] Further, the injection process in the preparation method of the stainless steel product by metal injection molding specifically includes: using a NEX110ⅢT-type Nissei injection molding machine to perform tensile rod injection on the high-flow feedstock. Before injection, check whether the machine is in good condition, turn on the machine, and turn on the hot oil circulation pump of the mold, and set the temperature to 80-100°C. At the same time, set the five-section barrel temperature of the injection molding machine to 150°C, 160°C, 165°C, 170°C, and 180°C. After the mold and barrel temperatures reach the preset values, start cleaning the screw, add a spoonful of polyethylene into the barrel, turn on the automatic screw cleaning function. After cleaning, add the high-flow feedstock into the barrel for injection. Before injection, set the injection pressure to 80-150 MPa, the holding pressure to 40-80 MPa, the injection speed to 50-100 mm / s, and the cooling time to 10 s. First, inspect the green density, single weight, green cross-section, porosity, etc. of the tensile rod. If not up to standard, fine-tune the injection parameters such as injection pressure until the performance requirements are met.

[0035] Further, the debinding process in the preparation method of the stainless steel product by metal injection molding specifically includes: adopting a two-stage debinding process, using a 480-PRO Hengpu batch furnace for two-stage debinding. Place the green body in an alumina ceramic plate. The first-stage debinding sets the temperature at 200-220°C, the heating rate ≤ 2°C / min, normal pressure, and introduce nitrogen (8-12 L / min), and keep it at 200-220°C for 2 h to remove paraffin and polyethylene glycol; the second-stage debinding process sets the temperature at 450-550°C, the heating rate 1.5°C / min, normal pressure, and introduce hydrogen (4-8 L / min), and keep it at 450-550°C for 4 h. At this stage, ethylene-vinyl acetate copolymer and zinc stearate are pyrolyzed at high temperature, and then cooled to room temperature in a nitrogen atmosphere, and the carbon residue is monitored.

[0036] Further, the sintering process in the preparation method of the stainless steel product by metal injection molding specifically includes: using an XTZ-600L-G Xingteshuo mechanical batch furnace. First, perform pre-sintering. Place the debound brown body on an alumina ceramic plate, cover the cover at the same time, place it on a graphite plate and bring it into the batch furnace, close the furnace door, introduce hydrogen with a flow rate of 8 L / min, and heat the sintering furnace at a heating rate of 2°C / min to 300°C, 600°C, 800°C, 1000°C, 1200-1380°C in sequence, keep it at 1200-1380°C for 3-5 h, and then cool it to 200°C at a cooling rate of 10°C / min in the furnace.

[0037] Example 1 of the present invention is: A preparation method for high-flow feedstock for metal injection molding, comprising the following steps: Step 1: Weigh 10 kg of bimodal 316L stainless steel powder, including coarse powder and fine powder. The particle size of the coarse powder is 18 μm, the particle size of the fine powder is 7 μm, and the mass ratio of the coarse powder to the fine powder is 6:4. The mass percentage of the bimodal 316L stainless steel powder is 91 wt%. The mass percentage of Ni in the bimodal 316L stainless steel powder is 12-14%, the mass percentage of Cr is 16-18%, the mass percentage of Mo is 2-3%, the mass percentage of Mn is less than 2%, the mass percentage of Si is less than 1%, and the balance is Fe. Weigh 1 kg of composite binder, and the mass percentage of the composite binder is 9 wt%. Among them, the mass percentage of polyethylene glycol is 5 wt%, the mass percentage of ethylene-vinyl acetate copolymer is 2.5 wt%, the mass percentage of zinc stearate is 0.8 wt%, and the mass percentage of paraffin is 1.7 wt%. Step 2: Place the bimodal 316L stainless steel powder in a vacuum furnace, evacuate until the air pressure is less than or equal to 10 - 2 Pa, heat it up to 220 °C at a rate of 5 °C / min and keep it warm for 1-2 hours to remove surface oxides until the oxygen content is less than 0.1 wt%. Then, deoxidize it in a vacuum at 220 °C for 1.5 h and cool it with nitrogen. Place polyethylene glycol in a vacuum dryer at 80 °C for 4 h until the moisture content is less than or equal to 0.03%. Step 3: Open a 20L twin-screw internal mixer, check and confirm that the internal mixer is clean and can work normally, and then start the machine. Heat the internal mixer up to 145 °C and preheat it for 30 min. Add polyethylene glycol, paraffin, ethylene-vinyl acetate copolymer, and zinc stearate of the composite binder into the preheated internal mixer in sequence. Close the dust cover and gland, stir at 20 r / min for 10 min until the composite binder is evenly mixed, and then add the bimodal 316L stainless steel powder. Close the dust cover and gland, and knead the bimodal 316L stainless steel powder and the composite binder at a kneading temperature of 170 °C and a rotation speed of 70 r / min for 50 min. Wait until the bimodal 316L stainless steel powder and the composite binder are kneaded into a lump. Turn off the rotor, open the dust cover and gland, insert the probe of the thermocouple thermometer into the interior of the material. When the material temperature is 180 °C, start kneading, set the rotation speed to 60 r / min, and the kneading time to 20 min until the material shows a uniform bright surface state. After kneading is completed, set the temperature of the internal mixer to 140 °C, turn off the heating. After the measured material temperature drops to 140 °C, adjust the screw rotation speed from 60 r / min to 20 r / min. When the mixing and kneading reach the predetermined time and quality requirements, stop heating and rotor rotation, open the discharge port of the internal mixer, and discharge the kneaded material into the cooling equipment. Cool the material to 60 °C by air cooling. Step 4: Add the material after internal mixing into the hopper of the granulator. Through the rotation of the granulator screw, the material is conveyed to the heating zone of the extruder. The heating zone of the extruder heats the material to 160 °C to achieve a good plasticized state and sufficient fluidity. Subsequently, the granulator extrudes and forms the material and cuts it into feeding pellets. The rotational speed of the cutting tool for pelletizing is 50 r / min. The diameter of the feeding pellets is 2 ± 0.2 mm, and the length of the feeding pellets is 3 ± 0.2 mm. Step 5: Turn on the melt flow rate tester and set the temperature to 190 °C. After the temperature reaches 190 °C and is stably maintained for 5 min, take 50 g of the 316L feeding obtained in Step 4 and add it to the melt flow rate tester. The load is 21.6 kg, and the cutting interval time is 1.0 s. Record the average cutting mass of 10 cuts as 3.06 g. Calculate the MFR to be 1836 g / 10 min. Subsequently, use a capillary rheometer (model Rosand RH2000, capillary diameter 1 mm, length-to-diameter ratio 30:1) to test the viscosity of the feeding at 190 °C and a shear rate of 1000 s -1 to be 143.45 Pa·s. The viscosity change curve with respect to the shear rate is as Figure 1 shown.

[0038] In other possible embodiments, the ethylene-vinyl acetate copolymer in the composite binder can be replaced with a styrene-butadiene copolymer. Correspondingly, the internal mixing temperature in Step 3 is changed to 180 - 190 °C.

[0039] Apply the high-flow feeding prepared by the preparation method of the high-flow feeding for metal injection molding in this Example 1 to the metal injection molding of stainless steel products. The preparation steps and test results of the stainless steel products are as follows: Use a NEX110ⅢT type Nissei injection molding machine to perform tensile bar injection molding on the high-flow feeding. Before injection molding, check whether the machine is in good condition, turn on the machine, and turn on the hot oil circulation pump of the mold. Set the temperature to 80 - 100 °C, and at the same time set the five-section barrel temperature of the injection molding machine to 150 °C, 160 °C, 165 °C, 170 °C, and 180 °C. After the mold and barrel temperatures reach the preset values, start cleaning the screw. Add a spoonful of polyethylene into the barrel, turn on the automatic screw cleaning function. After cleaning, add the high-flow feeding into the barrel for injection. Before injection, set the injection pressure to 100 MPa, the holding pressure to 80 MPa, the injection speed to 60 mm / s, and the cooling time to 10 s. The first inspection density of the tensile bar green body is 5.6 g / cm 3 .

[0040] Adopt a two-stage degreasing process. Use a 480-PRO Hengpu batch furnace for two-stage degreasing. Place the green embryo in an alumina ceramic plate. The set temperature for the first-stage degreasing is 200°C, the heating rate is ≤2°C / min, at normal pressure, introduce nitrogen (8 - 12 L / min), keep it at 200°C for 2 h to remove paraffin and polyethylene glycol; the set temperature for the second-stage degreasing process is 500°C, the heating rate is 1.5°C / min, at normal pressure, introduce hydrogen (4 - 8 L / min), keep it at 500°C for 4 h. At this stage, ethylene-vinyl acetate copolymer and zinc stearate are pyrolyzed at high temperature, and then it is cooled to room temperature under a nitrogen atmosphere. The measured carbon residue is 0.08 wt%.

[0041] Use an XTZ-600L-G Xingteshuo mechanical batch furnace. First, perform pre-sintering. Place the degreased brown embryo on an alumina ceramic plate, cover it with a cover plate at the same time, place it on a graphite plate and bring it into the batch furnace. Close the furnace door, introduce hydrogen with a flow rate of 8 L / min, and heat the sintering furnace to 300°C, 600°C, 800°C, 1000°C, and 1380°C in sequence at a heating rate of 2°C / min. After keeping it at 1380°C for 5 h, cool it in the furnace to 200°C at a cooling rate of 10°C / min. The measured sintered density is 7.89 g / cm³ (98.6% of the theoretical value).

[0042] Example two of the present invention is as follows: A preparation method for a high-flow feedstock for metal injection molding, comprising the following steps: Step one, weigh 10 kg of bimodal 17-4PH stainless steel powder, including coarse powder and fine powder. The particle size of the coarse powder is 20 μm, the particle size of the fine powder is 8 μm, and the mass ratio of the coarse powder to the fine powder is 7:3. The mass percentage of the bimodal 17-4PH stainless steel powder is 92 wt%; the mass percentage of Cu in the bimodal 17-4PH stainless steel powder is 3 - 5%, the mass percentage of Ni is 3 - 5%, the mass percentage of Nb is 0.15 - 0.45%, the mass percentage of Cr is 15 - 17.5%, the mass percentage of Mn is less than 1%, the mass percentage of Si is less than 1%, and the balance is Fe; weigh 0.87 kg of composite binder, and the mass percentage of the composite binder is 8 wt%. Among them, the mass percentage of polyethylene glycol is 4.5 wt%, the mass percentage of ethylene-vinyl acetate copolymer is 2 wt%, the mass percentage of zinc stearate is 1 wt%, and the mass percentage of paraffin is 0.5 wt%. Step two, place the bimodal 17-4PH stainless steel powder in a vacuum furnace, evacuate until the air pressure is less than or equal to 10 - 2Pa was heated to 220 °C at a rate of 5 °C / min and held for 1 - 2 hours to remove surface oxides until the oxygen content was less than 0.1 wt%. Subsequently, it was deoxidized in vacuo at 220 °C for 1.5 h and cooled with nitrogen. Polyethylene glycol was placed in a vacuum oven at 80 °C and dried for 4 h until the moisture content was less than or equal to 0.05%. Step 3: Turn on a 20 L twin-screw internal mixer, check and confirm that the internal mixer is clean and can operate normally, then start the machine. Heat the internal mixer to 150 °C and preheat for 20 min. Sequentially add polyethylene glycol, paraffin, ethylene-vinyl acetate copolymer, and zinc stearate of the composite binder into the preheated internal mixer. Close the dust cover and gland, and stir at 20 r / min for 10 min until the composite binder is uniformly mixed, then add duplex 17-4PH stainless steel powder. Close the dust cover and gland, and knead the duplex 17-4PH stainless steel powder and the composite binder at a kneading temperature of 170 °C and a rotation speed of 60 r / min for 60 min. When the duplex 17-4PH stainless steel powder and the composite binder are kneaded into a mass, turn off the rotor, open the dust cover and gland, insert the probe of the thermocouple thermometer into the interior of the material. When the material temperature is 175 °C, start kneading, set the rotation speed to 60 r / min, and the kneading time to 30 min until the material shows a uniform bright surface state. After kneading is completed, set the temperature of the internal mixer to 140 °C, turn off the heating. After the measured material temperature drops to 140 °C, adjust the screw rotation speed from 60 r / min to 20 r / min. When the mixing and kneading reach the predetermined time and quality requirements, stop heating and rotor rotation, open the discharge port of the internal mixer, and discharge the kneaded material into the cooling equipment. Air-cool the material to 60 °C. Step 4: Add the kneaded material into the hopper of the granulator. The material is conveyed to the heating zone of the extruder by the rotation of the granulator screw. The heating zone of the extruder heats the material to 160 °C to achieve a good plasticized state and sufficient fluidity. Subsequently, the granulator extrudes and shapes the material and cuts it into feed pellets. The rotational speed of the cutting tool for cutting is 50 r / min. The diameter of the feed pellets is 2 ± 0.2 mm, and the length of the feed pellets is 3 ± 0.2 mm. Step 5: Turn on the melt flow rate tester, set the temperature to 190 °C. After the temperature reaches 190 °C and is held stably for 5 min, take 50 g of the 17-4PH feed obtained in Step 4 and add it to the melt flow rate tester. The load is 21.6 kg, and the cut-off interval time is 1.0 s. Record the average cut-off mass of the cuttings as 2.51 g, and calculate the MFR to be 1506 g / 10 min. Subsequently, use a capillary rheometer (model Rosand RH2000, capillary diameter 1 mm, length-to-diameter ratio 30:1) to measure the viscosity of the feed at 190 °C and a shear rate of 1000 s -1 to be 175.03 Pa·s. The viscosity change curve with respect to the shear rate is as Figure 2as shown

[0043] In other possible embodiments, the ethylene-vinyl acetate copolymer in the composite binder can be replaced with a styrene-butadiene copolymer. Correspondingly, the kneading temperature in Step 3 is changed to 180-190°C.

[0044] The high-flow feed prepared by the method for preparing a high-flow feed for metal injection molding of this Second Embodiment is applied to a metal injection molded stainless steel product. The preparation steps and test results of the stainless steel product are as follows: Use a NEX110ⅢT type Nissei injection molding machine to perform tensile bar injection molding on the high-flow feed. Before injection molding, check whether the machine is in good condition, turn on the machine, turn on the hot oil circulation pump of the mold, set the temperature to 80-100°C, and at the same time set the five-section barrel temperature of the injection molding machine to 150°C, 160°C, 165°C, 170°C, 180°C. After the mold and barrel temperatures reach the preset values, start cleaning the screw, add a spoonful of polyethylene into the barrel, turn on the automatic screw cleaning function. After cleaning, add the high-flow feed into the barrel for injection. Before injection, set the injection pressure to 100 MPa, the holding pressure to 80 MPa, the injection speed to 80 mm / s, and the cooling time to 10 s. The density of the first-inspected tensile bar green body is 5.4 g / cm 3 。

[0045] Adopt a two-stage debinding process, use a 480-PRO Hengpu batch furnace for two-stage debinding. Place the green body in an alumina ceramic plate. The set temperature of the first-stage debinding is 200°C, the heating rate ≤ 2°C / min, normal pressure, and nitrogen (8-12 L / min) is introduced. Keep it at 200°C for 2 h to remove paraffin and polyethylene glycol; the set temperature of the second-stage debinding process is 500°C, the heating rate is 1.5°C / min, normal pressure, and hydrogen (4-8 L / min) is introduced. Keep it at 500°C for 4 h. At this stage, the ethylene-vinyl acetate copolymer and zinc stearate are pyrolyzed at high temperature, and then cooled to room temperature under a nitrogen atmosphere. The measured carbon residue is 0.10 wt%.

[0046] Use an XTZ-600L-G Xingteshuo mechanical batch furnace. First, pre-sinter. Place the debound brown body on an alumina ceramic plate, cover it with a cover plate at the same time, place it on a graphite plate and bring it into the batch furnace. Close the furnace door, introduce hydrogen at a flow rate of 8 L / min, and heat the sintering furnace to 300°C, 600°C, 800°C, 1000°C, 1300°C in sequence at a heating rate of 2°C / min. After keeping it at 1300°C for 5 h, cool it to 200°C at a cooling rate of 10°C / min in the furnace. The measured sintered density is 7.75 g / cm³ (98.2% of the theoretical value).

[0047] Comparative Example 1 A method for preparing a high-flow feed for metal injection molding, comprising the following steps: Step 1: Weigh 10 kg of single-peak 316L stainless steel powder with a particle size of 15 μm, and the mass percentage of the single-peak 316L stainless steel powder is 91 wt%; weigh 1 kg of composite binder, and the mass percentage of the composite binder is 9 wt%, among which the mass percentage of polyethylene glycol is 5 wt%, the mass percentage of ethylene-vinyl acetate copolymer is 2.5 wt%, the mass percentage of zinc stearate is 0.8 wt%, and the mass percentage of paraffin wax is 1.7 wt%. Step 2: Place the single-peak 316L stainless steel powder in a vacuum furnace, evacuate until the air pressure is less than or equal to 10 - 2 Pa, heat it up to 220 °C at a rate of 5 °C / min and hold for 1 - 2 hours to remove surface oxides until the oxygen content is less than 0.1 wt%, then deoxidize in vacuum at 220 °C for 1.5 h, and cool with nitrogen; place polyethylene glycol in a vacuum dryer at 80 °C for 4 h until the moisture content is less than or equal to 0.03%. Step 3: Open a 20 L twin-screw internal mixer, check and confirm that the internal mixer is clean and can work normally, then start the machine, heat the internal mixer to 150 °C, and preheat for 20 min; add polyethylene glycol, paraffin wax, ethylene-vinyl acetate copolymer, and zinc stearate of the composite binder into the preheated internal mixer in sequence, close the dust cover and gland, stir at 20 r / min for 10 min until the composite binder is evenly mixed, and then add the single-peak 316L stainless steel powder; close the dust cover and gland, and knead the single-peak 316L stainless steel powder and the composite binder at a kneading temperature of 170 °C and a rotation speed of 60 r / min for 60 min; wait until the single-peak 316L stainless steel powder and the composite binder are kneaded into a mass, turn off the rotor, open the dust cover and gland, insert the thermocouple thermometer probe into the interior of the material, when the material temperature is 175 °C, start kneading, set the rotation speed to 60 r / min, and the kneading time to 30 min until the material shows a uniform bright surface state; after kneading is completed, set the temperature of the internal mixer to 140 °C, turn off the heating, and after the measured material temperature drops to 140 °C, adjust the screw rotation speed from 60 r / min to 20 r / min; when the mixing and kneading reach the predetermined time and quality requirements, stop heating and rotor rotation, open the discharge port of the internal mixer, and discharge the kneaded material into the cooling equipment; cool the material to 60 °C by air cooling. Step 4: Add the kneaded material into the hopper of the granulator, and convey the material to the heating zone of the extruder through the rotation of the granulator screw. The heating zone of the extruder heats the material to 160 °C to make it reach a good plasticized state and have sufficient fluidity. Then the granulator extrudes and shapes the material and cuts it into feeding pellets. The rotational speed of the cutting tool for cutting is 50 r / min, the diameter of the feeding pellets is 2 ± 0.2 mm, and the length of the feeding pellets is 3 ± 0.2 mm.

[0048] Step 5: Turn on the melt flow rate tester and set the temperature to 190°C. After the temperature reaches 190°C and is stably maintained for 5 minutes, add 50 g of the 316L feedstock obtained in Step 4 to the melt flow rate tester, with a load of 21.6 kg and a cut-off interval time of 1.0 s. Record the average cut-off mass of 10 cuts as 1.68 g, and calculate the MFR to be 1008 g / 10 min. Subsequently, use a capillary rheometer (model Rosand RH2000, capillary diameter 1 mm, length-to-diameter ratio 30:1) to test the viscosity of the feedstock at 190°C and a shear rate of 1000 s -1 The viscosity at a shear rate of 1000 s is 340.23 Pa·s, and the viscosity change curve with respect to the shear rate is as Figure 3 shown.

[0049] In other possible embodiments, the ethylene-vinyl acetate copolymer in the composite binder can be replaced with a styrene-butadiene copolymer. Correspondingly, the kneading temperature in Step 3 is changed to 180 - 190°C.

[0050] Apply the high-flow feedstock prepared by the preparation method of the high-flow feedstock for metal injection molding of this Comparative Example 1 to a metal injection molded stainless steel product. The preparation steps and test results of the stainless steel product are as follows: Use a NEX110ⅢT-type Nissei injection molding machine to perform tensile bar injection molding on the high-flow feedstock. Before injection molding, check whether the machine is in good condition, turn on the machine, and turn on the hot oil circulation pump of the mold. Set the temperature to 80 - 100°C, and at the same time set the five-section barrel temperature of the injection molding machine to 150°C, 160°C, 165°C, 170°C, and 180°C. After the mold and barrel temperatures reach the preset values, start cleaning the screw. Add a spoonful of polyethylene to the barrel, turn on the automatic screw cleaning function. After cleaning, add the high-flow feedstock to the barrel for injection. Before injection, set the injection pressure to 100 MPa, the holding pressure to 80 MPa, the injection speed to 60 mm / s, and the cooling time to 10 s. The first inspection density of the tensile bar green body is 4.9 g / cm 3 Moreover, flash and porosity phenomena occurred in Comparative Example 1 after injection.

[0051] Adopt a two-stage debinding process and use a 480-PRO Hengpu batch furnace for two-stage debinding. Place the green body on an alumina ceramic plate. The first-stage debinding is set at a temperature of 200°C, a heating rate of ≤2°C / min, normal pressure, and nitrogen is introduced (8 - 12 L / min). Hold at 200°C for 2 h to remove paraffin and polyethylene glycol; the second-stage debinding process is set at a temperature of 500°C, a heating rate of 1.5°C / min, normal pressure, and hydrogen is introduced (4 - 8 L / min). Hold at 500°C for 4 h. At this stage, the ethylene-vinyl acetate copolymer and zinc stearate are pyrolyzed at high temperature. Subsequently, cool to room temperature under a nitrogen atmosphere, and the measured carbon residue is 0.18 wt%.

[0052] Use XTZ-600L-G Xingte Shuo mechanical batch furnace, first pre-sintering, place the degreased brown embryo on the alumina ceramic plate, cover it with a cover plate, place it on the graphite plate and bring it into the batch furnace, close the furnace door, introduce hydrogen at a flow rate of 8L / min, and heat the sintering furnace to 300℃, 600℃, 800℃, 1000℃, and 1380℃ in sequence at a heating rate of 2℃ / min. After keeping at 1380℃ for 5h, cool it to 200℃ at a cooling rate of 10℃ / min. The sintered density is measured to be 7.59g / cm³ (94.8% of the theoretical value).

[0053] Comparative Example 2 A method for preparing high-flow rate feed for metal injection molding comprises the following steps: Step 1, prepare 10kg of bimodal 316L stainless steel powder, including coarse powder and fine powder, wherein the coarse powder particle size is 18μm, the fine powder particle size is 7μm, the mass ratio of coarse powder to fine powder is 6:4, and the mass percentage of bimodal 316L stainless steel powder is 91wt%; the mass percentage of Ni in the bimodal 316L stainless steel powder is 12-14%, the mass percentage of Cr is 16-18%, the mass percentage of Mo is 2-3%, the mass percentage of Mn is less than 2%, the mass percentage of Si is less than 1%, and the balance is Fe; prepare 1kg of binder, the mass percentage of the binder is 9wt%, the binder includes polyethylene glycol, ethylene-vinyl acetate copolymer and paraffin, wherein the mass percentage of polyethylene glycol is 3wt%, the mass percentage of ethylene-vinyl acetate copolymer is 4wt%, and the mass percentage of paraffin is 3wt%; Step 2: Place the bimodal 316L stainless steel powder in a vacuum furnace and evacuate until the pressure is less than or equal to 10 - 2 Pa, heating to 220°C at a rate of 5°C / min and keeping for 1-2 hours to remove surface oxides until the oxygen content is less than 0.1wt%, then deoxidizing in vacuum at 220°C for 1.5h and cooling with nitrogen; drying the polyethylene glycol in vacuum at 80°C for 4h until the moisture content is less than or equal to 0.05%; Step 3: Turn on the twin-screw internal mixer with a capacity of 20 L. After checking and confirming that the internal mixer is clean and can operate normally, start the machine, heat the internal mixer to 150 °C, and preheat for 20 min. Add polyethylene glycol, paraffin, ethylene-vinyl acetate copolymer, and zinc stearate of the composite binder to the preheated internal mixer in sequence. Close the dust cover and gland, and stir at 20 r / min for 10 min until the composite binder is evenly mixed, then add the duplex 316L stainless steel powder. Close the dust cover and gland, and knead the duplex 316L stainless steel powder and the composite binder at a kneading temperature of 170 °C and a rotation speed of 80 r / min for 40 min. When the duplex 316L stainless steel powder and the composite binder are kneaded into a mass, turn off the rotor, open the dust cover and gland, insert the probe of the thermocouple thermometer into the interior of the material. When the material temperature is 180 °C, start kneading, set the rotation speed to 60 r / min, and the kneading time to 20 min until the material shows a uniform bright surface state. After kneading is completed, set the temperature of the internal mixer to 140 °C, turn off the heating. After the measured material temperature drops to 140 °C, adjust the screw rotation speed from 60 r / min to 20 r / min. When the mixing and kneading reach the predetermined time and quality requirements, stop heating and rotor rotation, open the discharge port of the internal mixer, and discharge the kneaded material into the cooling equipment. Cool the material to 60 °C by air cooling; Step 4: Add the kneaded material into the hopper of the granulator. Through the rotation of the granulator screw, convey the material to the heating zone of the extruder. The heating zone of the extruder heats the material to 160 °C to make it reach a good plasticized state and have sufficient fluidity. Subsequently, the granulator extrudes and shapes the material and cuts it into feeding pellets. The rotation speed of the cutting tool for cutting is 50 r / min, the diameter of the feeding pellets is 2 ± 0.2 mm, and the length of the feeding pellets is 3 ± 0.2 mm; Step 5: Turn on the melt flow rate tester, set the temperature to 190 °C. After the temperature reaches 190 °C and is stably kept warm for 5 min, take 50 g of the 316L feeding obtained in Step 4 and add it to the melt flow rate tester. The load is 21.6 kg, and the cut-off interval time is 1.0 s. Record that the average cut-off mass for 10 cuts is 1.48 g, and calculate the MFR to be 888 g / 10 min. Subsequently, use a capillary rheometer (model Rosand RH2000, capillary diameter 1 mm, length-to-diameter ratio 30:1) to test the viscosity of the feeding at 190 °C and a shear rate of 1000 s -1 to be 360.56 Pa·s. The viscosity change curve with respect to the shear rate is as Figure 4 shown.

[0054] In other possible embodiments, the ethylene-vinyl acetate copolymer in the composite binder can be replaced with a styrene-butadiene copolymer. Correspondingly, the kneading temperature in Step 3 is changed to 180 - 190 °C.

[0055] The high-flow feedstock prepared by the preparation method of high-flow feedstock for metal injection molding in Comparative Example 2 was applied to the metal injection molding of stainless steel products. The preparation steps and test results of the stainless steel products are as follows: The high-flow feedstock was injection molded into tensile bars using a NEX110ⅢT-type Nissei injection molding machine. Before injection molding, check whether the machine is in good condition, turn on the machine, and turn on the hot oil circulation pump of the mold. Set the temperature to 80 - 100°C, and at the same time, set the five-section barrel temperature of the injection molding machine to 150°C, 160°C, 165°C, 170°C, and 180°C. After the mold and barrel temperatures reach the preset values, start cleaning the screw. Add a spoonful of polyethylene into the barrel, turn on the automatic screw cleaning function. After cleaning, add the high-flow feedstock into the barrel for injection. Before injection, set the injection pressure to 100 MPa, the holding pressure to 80 MPa, the injection speed to 50 - 100 mm / s, and the cooling time to 10 s. The density of the first-inspected tensile bar green body was 5.3 g / cm 3 。

[0056] A two-stage debinding process was adopted, and a 480-PRO Hengpu batch furnace was used for two-stage debinding. The green body was placed on an alumina ceramic plate. The first-stage debinding was set at a temperature of 200°C, a heating rate of ≤2°C / min, normal pressure, and nitrogen gas (8 - 12 L / min) was introduced. It was held at 200°C for 3 h to remove paraffin and polyethylene glycol; the second-stage debinding process was set at a temperature of 500°C, a heating rate of 1.5°C / min, normal pressure, and hydrogen gas (4 - 8 L / min) was introduced. It was held at 500°C for 6 h. At this stage, ethylene-vinyl acetate copolymer and zinc stearate were pyrolyzed at high temperature. Subsequently, it was cooled to room temperature in a nitrogen atmosphere. The measured carbon residue was 0.38 wt%, and partial debinding cracking occurred.

[0057] Using an XTZ-600L-G Xingteshuo mechanical batch furnace, first pre-sinter. Place the debound brown body on an alumina ceramic plate, cover it with a cover plate at the same time, place it on a graphite plate and bring it into the batch furnace, close the furnace door, introduce hydrogen gas at a flow rate of 8 L / min, and heat the sintering furnace to 300°C, 600°C, 800°C, 1000°C, and 1300°C in sequence at a heating rate of 2°C / min. After holding at 1300°C for 5 h, furnace cool to 200°C at a cooling rate of 10°C / min. The measured sintered density was 7.52 g / cm³ (94.0% of the theoretical value).

[0058] In summary, the preparation method of high-flow feedstock for metal injection molding provided by the present invention uses spherical metal granulating powders with two different particle sizes as raw materials. The coarse powder provides fluidity, and the fine powder fills the gaps to improve the sintered density. A composite binder mainly composed of polyethylene glycol is used, which effectively shortens the debinding time and reduces the carbon residue amount.

[0059] The above are only embodiments of the present invention, and thus do not limit the patent scope of the present invention. Any equivalent transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied in related technical fields, shall be similarly included in the patent protection scope of the present invention.

Claims

1. A preparation method of high-flow feedstock for metal injection molding, characterized in that, It includes the following steps: Step 1: Weigh bimodal stainless steel powder and a composite binder. The bimodal stainless steel powder includes coarse powder and fine powder, and the ratio of the coarse powder to the fine powder is 6:4 - 7:

3. The composite binder includes polyethylene glycol, a, zinc stearate, and paraffin wax; Step 2: First, conduct vacuum deoxidation on the bimodal stainless steel powder, then cool and protect it. Screen the bimodal stainless steel powder and dry the polyethylene glycol; Step 3: Put the composite binder and the bimodal stainless steel powder into a mixer at a certain loading amount and mix them together; Step 4: Put the material obtained after mixing into a granulator to extrude and granulate to obtain granular injection feedstock.

2. A preparation method of a high-flow feedstock for metal injection molding according to claim 1, characterized in that: The particle size of the coarse powder of the bimodal stainless steel powder is 18 - 24 μm, and the particle size of the fine powder is 4 - 9 μm.

3. A preparation method for a high-flow feedstock for metal injection molding according to claim 1, characterized in that: a is ethylene - vinyl acetate copolymer or styrene - butadiene copolymer.

4. A preparation method for a high-flow feedstock for metal injection molding according to claim 3, characterized in that: The ratio of polyethylene glycol to a in the composite binder is 2:1 - 3:

1.

5. A preparation method of a high-flow feedstock for metal injection molding according to claim 3, characterized in that: When a is ethylene - vinyl acetate copolymer, the mixing temperature in Step 2 is 170 - 180 °C, the rotation speed is 50 - 80 r / min, and the mixing time is 40 - 60 min; when a is styrene - butadiene copolymer, the mixing temperature in Step 2 is 180 - 190 °C, the rotation speed is 50 - 80 r / min, and the mixing time is 40 - 60 min.

6. A preparation method of a high-flow feedstock for metal injection molding according to claim 3, characterized in that: The mass percentage of the bimodal stainless steel powder in the high - flow feedstock is 86% - 92.5%, and the mass percentage of the composite binder in the high - flow feedstock is 7.5% - 14%.

7. A preparation method of a high-flow feedstock for metal injection molding according to claim 6, characterized in that: The mass percentage of polyethylene glycol is 4.0% - 6.0%, the mass percentage of a is 2.0% - 4.0%, the mass percentage of zinc stearate is 0.5% - 1.5%, and the mass percentage of paraffin wax is 1.0% - 2.5%.

8. A preparation method for a high-flow feedstock for metal injection molding according to claim 1, characterized in that: The bimodal stainless steel powder is 316L stainless steel powder or 17 - 4PH stainless steel powder.

9. A preparation method for a high-flow feedstock for metal injection molding according to claim 1, characterized in that: The coarse powder of the bimodal stainless steel powder is gas - atomized powder or water - atomized powder.

10. A preparation method for a high-flow feedstock for metal injection molding according to claim 1, characterized in that, After Step 4, it also includes Step 5: Melt flow rate test.

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