A method for improving black lines in products during MIM injection

Abstract

The present application relates to a method for improving black streaks in products during the MIM injection process, and relates to the technical field of metal powder injection molding. The method comprises: placing metal powder in an aqueous solution containing acrylic resin, heating and dehydrating the mixture to obtain a dried modified metal powder; mixing the modified metal powder with a binder, and extruding the mixture to form an injection feed; injecting the injection feed into a blank, and sequentially performing degreasing, sintering, and heat treatment to obtain a metal injection molded product. The present application uses acrylic resin to modify the surface of the metal powder, thereby improving the wettability and fluidity of the metal powder and achieving better filling and forming properties. At the same time, because acrylic resin has good wettability, it helps the metal powder to better adhere to the binder, improves the bonding strength and adhesion between the powder and the binder, and helps to enhance the stability of the product after injection, making it difficult for the binder to break away from the accumulation and avoiding the formation of black streaks.

Description

Technical Field

[0001] The present application relates to the technical field of metal powder injection molding, and in particular to a method for improving black lines in products during MIM injection. Background Art

[0002] Metal Injection Molding (MIM) is a near-net-shape method suitable for producing small to medium-sized products with high precision and complex shapes. It offers high production efficiency and a high degree of performance adjustability. Metal Injection Molding (MIM) involves mixing the selected powder with a binder, granulating the mixture, and then injection molding the desired shape. The polymer imparts its viscous flow characteristics to the mixture, which aids in forming, cavity filling, and uniform powder loading. After molding, the binder is removed, and the debinded blank is sintered. This process is suitable for the mass production of small, precise, three-dimensional, complex metal parts with special performance requirements.

[0003] For some small or precise products produced by the MIM industry, such as camera brackets for electronic products, watch cases with light and high-strength performance requirements, orthodontic bracket knives for medical products, and internal suture needles, these products are inevitably prone to black lines on the injection blanks due to binder enrichment during the injection molding process. The black lines are the separated binders, and after sintering, the corresponding positions will show obvious marks, depressions and other poor appearances. For products with very high appearance requirements, they are obviously unqualified.

[0004] Therefore, in recent years, measures to address black streaks in MIM injection molding have primarily focused on reducing the occurrence of black streaks by repairing the mold, polishing, or adding texture (localized tanning). However, these methods are only applicable to plastic products and remain difficult to address for metal products. This application is therefore the basis for this proposal. Summary of the Invention

[0005] In order to improve the situation where black lines are formed on products during the MIM injection process, the present application provides a method for improving black lines on products during the MIM injection process. By modifying the surface state of the metal powder, the wettability and adhesion between the metal powder and the binder are enhanced, and the binder is not easily separated from the accumulation, thereby reducing the black line defects on the appearance of the metal product.

[0006] In a first aspect, the present application provides a method for improving black lines in a product during MIM injection, the method comprising:

[0007] placing the metal powder in an aqueous solution containing acrylic resin, heating and dehydrating the solution to obtain a dry modified metal powder;

[0008] After mixing the modified metal powder with the binder, the mixture is extruded to form an injection feed;

[0009] The injection feed is injected into the mold and then degreased, sintered and heat treated in sequence to produce a metal product.

[0010] Furthermore, the mass ratio of the metal powder to the acrylic resin is 100:(0.1-0.3).

[0011] Preferably, the mass ratio of the metal powder to the acrylic resin is 100:0.2.

[0012] Furthermore, the mass ratio of the modified metal powder and the binder during mixing is (52-95):(5-48).

[0013] Furthermore, the metal powder is stainless steel metal powder.

[0014] Furthermore, the stainless steel metal powder is 17-4PH stainless steel powder or 316L stainless steel powder.

[0015] Furthermore, the temperature during the mixing is 190-200°C, and the mixing time is 1-3 hours;

[0016] The adhesive is a plastic-based adhesive.

[0017] Furthermore, when the injection feed is injected into the blank, the injection temperature is 170-210°C and the injection speed is 35-85cm 3 / s, injection pressure P>60MPa.

[0018] Furthermore, the degreasing temperature is 500-650°C.

[0019] Furthermore, the sintering includes a primary sintering and a secondary sintering, and the temperature of the primary sintering is the same as the temperature during degreasing;

[0020] The secondary sintering temperature is 1050-1360° C., the secondary sintering time is 1-3 hours, and the secondary sintering is performed under a protective atmosphere.

[0021] Furthermore, the heat treatment includes solution treatment and aging treatment, wherein the temperature of the solution treatment is 950-1050° C. and the time is 0.5-1.5 h;

[0022] The aging temperature during aging treatment is 400-550°C and the time is 0.5-6h.

[0023] In summary, this application includes at least one of the following beneficial technical effects:

[0024] 1. This application uses acrylic resin to modify the surface of metal powder. Acrylic resin can form a lubricating film on the metal powder surface, reducing the friction between powder particles, thereby improving the wettability and fluidity of the metal powder, achieving better filling and formability. At the same time, due to the good wettability of acrylic resin, it helps the metal powder to better adhere to the binder, improving the bonding strength and adhesion between the powder and the binder, helping to enhance the stability of the product after injection, making the binder less likely to fall off and avoiding the formation of black streaks.

[0025] 2. The method for modifying and treating metal powder in the present application only requires heating and stirring the metal powder and acrylic resin, with fewer reaction conditions and fewer operating steps. The modification and treatment of metal powder has a high fault tolerance rate and is more operable. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a metallographic diagram of the modified metal powder after sintering and heat treatment in Example 1 of the present application;

[0027] Figure 2 1 is a comparison diagram of the appearance of the injection molded products in Example 1 and Comparative Example 1 of the present application;

[0028] Figure 3 It is a comparison diagram of the appearance of the sintered blanks in Example 1 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0029] The present application is further described in detail with reference to the following examples. It should be noted that if no specific conditions are specified in the following examples, the experiments were carried out according to conventional conditions or those recommended by the manufacturer; unless otherwise specified, the raw materials used in the following examples can be obtained from common commercial sources.

[0030] The present application provides a method for improving the black lines of products during the MIM injection process. Metal injection molding, referred to as MIM (metal injection molding), is a method of mixing metal powder and adhesive for injection molding. The process flow is: first, the selected powder is mixed with the binder, and then the mixture is granulated and injection molded into the desired shape. The binder is removed by degreasing and sintering to obtain the desired metal product. However, during the MIM injection process, there is a black line phenomenon. Black lines refer to black stripes that appear on the surface of the plastic part. When the injection speed is too high, the mold temperature is too high, or the product structure is too complex, the adhesive accumulates here during the injection process, resulting in concave marks after catalytic sintering at the corresponding position. Severe black line defects in the injection blank will show obvious marks, depressions and other poor appearances on the sintered parts during sintering. Black lines are a major appearance difficulty in the process of MIM injection molding complex structural parts.

[0031] Prior art methods for improving black streaks often focus on designing and modifying the mold structure, or employing methods that partially texture the mold. Specifically, this method involves preparing the mold, applying adhesive tape to the surface where texture is not desired, soaking the mold in a chemical solution (which will cause the textured area to appear dark), evenly applying wax, and applying wax paper to the desired textured area. The principle behind this improvement approach is that local texture in the mold slows the flow rate of the melt surface, while maintaining the overall flow rate, making it less likely to produce black streaks on the product surface. However, this improvement method is cumbersome, has poor operability, requires multiple reagents, and is costly. Furthermore, this method cannot fundamentally address the black streak problem on all products, and requires a long verification cycle.

[0032] Based on this, the present application changes its perspective and starts from the raw materials themselves. The two most important raw materials in the MIM injection process are metal powder and binder, and the quality of the product mainly depends on the interaction between the metal powder and the binder. Therefore, the present application adopts acrylic resin to modify the state of the metal surface and enhance the interaction between the metal powder and the binder. The interaction specifically refers to wettability, fluidity and surface bonding force. After improving the fluidity of the feed, it is easier to disperse evenly during injection, and the black line defects on the appearance of the injection blank can be significantly improved. The appearance of the corresponding product after sintering is also well improved. In addition, the improvement method is easy to operate, low in cost, shorter in verification cycle, and more efficient. It can be applied to metal products and plastic products. At the same time, acrylic resin has better light resistance, stability, small volume shrinkage, low volatility, no discoloration, and no deformation. The mechanical properties of the metal powder modified by it are at least the same as before modification, and remain consistent, or even higher.

[0033] A method for improving black lines in products during MIM injection includes:

[0034] Step S1: placing metal powder in an aqueous solution containing acrylic resin, heating and dehydrating the solution to obtain dry modified metal powder.

[0035] The metal powder can be stainless steel metal powder, nickel alloy metal powder, aluminum alloy metal powder, copper alloy metal powder, etc. Preferably, the metal powder is stainless steel metal powder, and more preferably, the stainless steel metal powder is 17-4PH stainless steel powder or 316L stainless steel powder.

[0036] The 17-4PH stainless steel powder comprises the following components by weight: 15.5-17.5% chromium, 3-5% copper, 3-5% nickel, 0.15-0.45% niobium + tantalum, ≤1.0% manganese, ≤1.0% silicon, ≤0.07% carbon, and the balance is iron. The purity of the 17-4PH stainless steel powder is ≥99.5%.

[0037] The 316L stainless steel powder includes the following components in percentage by weight: 16.0-18.0% chromium, 10.0-14.0% nickel, 2.0-3.0% molybdenum, ≤2.0% manganese, ≤1.0% silicon, ≤0.03% carbon, and the balance is iron. The purity of the 316L stainless steel powder is ≥99.5%.

[0038] Both 17-4PH stainless steel powder and 316L stainless steel powder have good corrosion resistance, mechanical properties and heat treatment capabilities, and are more suitable for metal injection molding.

[0039] Further preferably, the stainless steel metal powder is 17-4PH stainless steel powder, and the mass ratio of acrylic resin to 17-4PH stainless steel powder is (0.1-0.3):100, and the specific mass ratios of acrylic resin to 17-4PH stainless steel powder are 0.1:100, 0.2:100, and 0.3:100.

[0040] Before modifying the 17-4PH stainless steel powder, it is necessary to prepare an aqueous solution of acrylic resin. During the preparation, add the tough acrylic resin to pure water and stir for more than 30 minutes until it is uniform. A uniform aqueous solution of acrylic resin is obtained. Then add the 17-4PH stainless steel metal powder and heat while stirring until all the water evaporates and the powder becomes dry to obtain dry modified metal powder.

[0041] Step S2: Pour the dried modified metal powder and the binder into a Σ-type kneader at a mass ratio of (52-95): (5-48) and mix for 1-3 hours at a mixing temperature of 190-200°C. After mixing, pour the feed into a mixing extruder and extrude it into an injection feed. The mixing time can be 1 hour, 1.1 hours, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, or 3 hours; the mixing temperature can be 190°C, 191°C, 192°C, 193°C, 194°C, 195°C, 196°C, 197°C, 198°C, 199°C, or 200°C.

[0042] The above-mentioned binder is a plastic-based binder, which is an organic binder used in the metal injection molding (MIM) process. It is made of a high molecular polymer and has good plasticity and bonding properties. It can bond with the modified metal powder to form an injection molded embryo. During the subsequent thermal degreasing and sintering process, the plastic-based binder will be catalytically acid-desorbed and thermally evaporated, thereby removing it from the modified metal powder, further strengthening the bonding between the modified metal powder particles and forming a dense metal part. The main components of plastic-based binders include polyoxymethylene (POM), polypropylene (PP), polyethylene (PE), polystyrene (PS), etc. Different plastic-based binders have different characteristics and thermal properties, so when selecting, they need to be evaluated and selected according to the specific material and process requirements.

[0043] In the embodiment of the present application, the plastic-based binder is preferably a plastic-based binder containing POM. The reason is that the plastic-based binder containing POM has high injection molding strength, and the acid-catalyzed process has the advantage of being convenient for large-scale continuous production.

[0044] Step S3: Inject the injection feed into the mold at an injection temperature of 170-210°C and an injection speed of 35-85 cm 3 / s and injection pressure P>60MPa, injection molding injection embryos are obtained by injection. Specifically, the injection temperature is 170℃, 180℃, 190℃, 200℃, 210℃; the injection speed is 35cm 3 / s、40cm 3 / s、45cm 3 / s、50cm 3 / s、55cm 3 / s、60cm 3 / s、65cm 3 / s、70cm 3 / s、75cm 3 / s、80cm 3 / s、85cm 3 / s.

[0045] Step S4: The injection molded injection molded blank is subjected to acid stripping. The acid solution used for acid stripping can be either oxalic acid or nitric acid. The injection molded blank after acid stripping is placed in an environment of 500-650°C for thermal debinding and sintering. The sintering includes primary sintering and secondary sintering. After thermal debinding, the primary sintering is first performed to obtain a pre-sintered blank. The specific temperature of the thermal debinding and the primary sintering is the same, which can be 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, or 650°C.

[0046] Step S4: Arrange the pre-sintered blanks neatly and place them into a sintering furnace for secondary sintering at a temperature of 1050-1360°C for 1-3 hours. The secondary sintering is carried out under a protective atmosphere. Specifically, vacuum sintering or the introduction of reducing gas or inert gas for protection can be adopted. The gas used can be nitrogen, argon, helium, etc. The specific secondary sintering temperatures are 1050°C, 1100°C, 1150°C, 1200°C, 1250°C, 1270°C, 1290°C, 1310°C, 1330°C, 1350°C, and 1360°C; the secondary sintering times are 1 hour, 1.5 hours, 2 hours, 2.5 hours, and 3 hours.

[0047] The temperature of the secondary sintering varies depending on the stainless steel metal powder selected. If 17-4PH stainless steel powder is selected, the temperature of the secondary sintering is 1050-1270°C. If 316L stainless steel powder is selected, the temperature of the secondary sintering is 1050-1360°C.

[0048] Step S5: The sintered product is placed in a heat treatment furnace for solution treatment and aging treatment. The temperature for solution treatment is 950-1050°C and the time is 0.5-1.5 hours. The temperature for solution treatment is 950°C, 1000°C, and 1050°C, and the time is 0.5 hour, 0.8 hour, 1 hour, 1.2 hour, and 1.5 hour. The temperature for aging treatment is 400°C, 450°C, 500°C, and 550°C, and the time is 0.5 hour, 1 hour, 1.5 hour, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, and 6 hours.

[0049] Example

[0050] Implementation 1

[0051] Preparation of a metal injection molded product:

[0052] 1. Weigh 17-4PH stainless steel powder and tough acrylic resin in a mass ratio of 100:0.2. Place the tough acrylic resin in pure water and stir for 40 minutes to obtain an aqueous solution of acrylic resin with a concentration of 0.2%;

[0053] 2. Place the 17-4PH stainless steel powder in an aqueous solution of acrylic resin, heat and stir until all the water evaporates to obtain dry modified metal powder;

[0054] 3. Add the dried modified metal powder and plastic-based binder in a mass ratio of 60:40 into a Σ-type kneader and mix for 2 hours at a mixing temperature of 200°C. After mixing, pour the feed into a mixing extruder and extrude it to obtain injection feed;

[0055] 4. Injection feed is placed at 190℃ and 70MPa at 60cm3 / s injection speed is injected into the embryo material to obtain an injected embryo;

[0056] 5. After the injection mold is acid-degreased, it is thermally degreased and sintered once at 600°C to obtain a pre-sintered mold;

[0057] 6. Arrange the pre-sintered embryos neatly and place them into the sintering furnace. Carry out secondary sintering at 1160℃ and protective atmosphere. After secondary sintering for 2 hours, place the product into the heat treatment furnace and carry out solution treatment at 1000℃ for 1 hour. Then carry out aging treatment at 500℃ for 3.5 hours. After the treatment, the metal product is obtained.

[0058] Comparative Example 1

[0059] A comparative example 1 was set up according to Example 1. The difference between comparative example 1 and Example 1 was that comparative example 1 directly used 17-4PH stainless steel powder in combination with a binder without modification. The specific steps were the same as steps 3-6 in Example 1.

[0060] The black lines of the metal injection molded products prepared in Example 1 and Comparative Example 1 were examined. Figure 1 、 Figure 2 as well as Figure 3 .

[0061] First reference Figure 1 It can be seen that the grains of the improved metal powder are finer, the strength and hardness of the modified metal powder are higher, and the plasticity and toughness are also better.

[0062] refer to Figure 2 In the case of high physical properties of injection molded products, the black line defects of products obtained by injection molding with modified metal powder are significantly reduced compared with those without modification, and the product appearance is neat and beautiful, while the unmodified product has a rough appearance and obvious black lines. Figure 3 , Figure 3 The left image in the center shows a product injection-molded with modified metal powder, while the right image shows a product injection-molded with unmodified metal powder. The gravure on the left is lighter than the right, and there is one less gravure at the bottom (the injection black streaks improved in this embodiment appear as gravures on the sintered blank). Therefore, the method of Example 1 can better address the problem of black streaks, shorten the verification cycle, increase efficiency, and be easy to operate and low-cost.

[0063] Comparative Example 2

[0064] According to Example 1, a comparative example 2 was set up. The difference between comparative example 2 and Example 1 was that comparative example 2 used a silane coupling agent instead of acrylic resin to modify the 17-4PH stainless steel powder, and the effects of modified metal powders prepared by different modifiers on the black streaks of the product were investigated.

[0065] The metal powder modification steps are as follows, and the remaining steps are consistent with Example 1:

[0066] 1. Weigh 17-4PH stainless steel powder and silane coupling agent in a mass ratio of 100:0.2, first place the silane coupling agent in pure water, and stir for 40 minutes to obtain an aqueous solution of silane coupling agent with a concentration of 0.2%;

[0067] 2. Place the 17-4PH stainless steel powder in the aqueous solution of the silane coupling agent, heat and stir until all the water evaporates to obtain dry modified metal powder;

[0068] The improved metal powders obtained in Example 1 and Comparative Example 2 were examined for black lines on the products after injection. The results are shown in Table 1.

[0069] Table 1. Black lines on metal products (quantity)

[0070] Black lines (number) Example 1 No obvious black lines Example 2 There are a few black lines

[0071] According to the results of the investigation, the effect of using silane coupling agent to modify metal powder is poor, and the metal products obtained by injection still have black lines. Therefore, it is preferred to modify the metal powder with acrylic resin.

[0072] Example 2

[0073] 1. Weigh 316L stainless steel powder and tough acrylic resin in a mass ratio of 100:0.2. Place the tough acrylic resin in pure water and stir for 40 minutes to obtain an aqueous solution of acrylic resin with a concentration of 0.2%;

[0074] 2. Place the 17-4PH stainless steel powder in an aqueous solution of acrylic resin, heat and stir until all the water evaporates to obtain dry modified metal powder;

[0075] 3. Add the dried modified metal powder and plastic-based binder in a mass ratio of 70:30 into a Σ-type kneader and mix for 2 hours at a mixing temperature of 195°C. After mixing, pour the feed into a mixing extruder and extrude it to obtain injection feed;

[0076] 4. Injection feed is placed at 180℃ and 70MPa at 75cm 3 / s injection speed is injected into the embryo material to obtain an injected embryo;

[0077] 5. After the injection mold is acid-degreased, it is thermally degreased and sintered once at 650°C to obtain a pre-sintered mold;

[0078] 6. Arrange the pre-sintered embryos neatly and place them into the sintering furnace. Carry out secondary sintering at 1320℃ and protective atmosphere. After secondary sintering for 2 hours, place the product into the heat treatment furnace and carry out solution treatment at 1000℃ for 1 hour. Then carry out aging treatment at 500℃ for 3.5 hours. After the treatment, the metal product is obtained.

[0079] Examples 3-4

[0080] The difference between Example 3-4 and Example 1 is that the mass ratio of 17-4PH stainless steel powder and acrylic resin is different, as shown in Table 2 below.

[0081] Table 2. Mass ratio of 17-4PH stainless steel powder and acrylic resin

[0082] Mass ratio Example 1 100:0.2 Example 3 100:0.1 Example 4 100:0.3 Comparative Example 2 100:0.01 Comparative Example 3 100：1.0

[0083] The mass ratio of stainless steel powder and acrylic resin has a certain influence on the modification degree of the modified metal powder, and the modification degree of the modified metal powder further affects the occurrence of black lines after injection of the product. The results of the investigation are shown in Table 3 below.

[0084] Table 3. Degree of black streaks

[0085]

[0086]

[0087] Conclusion: According to Table 3, the optimal acrylic resin addition level is 0.1-0.3, resulting in virtually no black streaks in the injected metal parts. Too little acrylic resin results in poor modification and insufficient improvement of black streaks. Too much acrylic resin introduces new impurities, leading to uneven metal powder flow and the formation of black streaks. Therefore, a ratio of 0.2:100 of acrylic resin mass to metal powder mass is optimal.

[0088] The above are all preferred embodiments of the present application, and are not intended to limit the scope of protection of the present application. Therefore, any equivalent changes made based on the structure, shape, and principle of the present application should be included in the scope of protection of the present application.

Claims

1. A method for improving black lines in products during MIM injection, characterized in that: include: placing the metal powder in an aqueous solution containing acrylic resin, heating and dehydrating the solution to obtain a dry modified metal powder; After mixing the modified metal powder with a binder, the mixture is extruded to prepare an injection feed; The injection feed is injected into a mold, and then degreased, sintered, and heat-treated in sequence to obtain a metal product; The mass ratio of the metal powder to the acrylic resin is 100:(0.1-0.3); The mass ratio of the modified metal powder and the binder when mixed is (52-95): (5-48); The metal powder is stainless steel metal powder.

2. The method for improving black lines in products during MIM injection according to claim 1, characterized in that: The stainless steel metal powder is 17-4PH stainless steel powder or 316L stainless steel powder.

3. The method for improving black lines in products during MIM injection according to claim 1, characterized in that: The temperature during the mixing is 190-200° C., and the mixing time is 1-3 hours. The adhesive is a plastic-based adhesive.

4. The method for improving black lines in products during MIM injection according to claim 1, characterized in that: When the injection feed is injected into the blank, the injection temperature is 170-210°C and the injection speed is 35-85 cm 3 / s, injection pressure P>60MPa.

5. The method for improving black lines in products during MIM injection according to claim 1, characterized in that: The degreasing temperature is 500-650°C.

6. The method for improving black lines in products during MIM injection according to claim 1, characterized in that: The sintering includes primary sintering and secondary sintering. The temperature of the primary sintering is the same as the temperature during degreasing. The temperature of the secondary sintering is 1050-1360° C. The time of the secondary sintering is 1-3 hours, and the secondary sintering is performed under a protective atmosphere.

7. The method for improving black lines in products during MIM injection according to claim 1, characterized in that: The heat treatment includes solution treatment and aging treatment. The temperature of the solution treatment is 950-1050° C. and the time is 0.5-1.5 hours. The aging treatment temperature is 400-550° C. and the time is 0.5-6 hours.

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