A method for preparing a low surface roughness MIM feedstock

Abstract

The application belongs to the technical field of metal powder injection molding, and particularly relates to a preparation method of low-surface-roughness MIM feedstock. The method comprises the following steps: mixing stainless steel powder and a binder first, then adding EBS, continuing mixing for 3-6 minutes, discharging and granulating, and the particle size distribution is D50 of 4-5 microns. The technical scheme provided by the application can ensure the necessary flowability of the feedstock in the injection process, the saturation degree of injection and the light flow mark even in the case of a significant reduction in the particle size, and finally the MIM product with Ra<0.5 can be successfully prepared.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal powder injection molding, and particularly relates to a preparation method of MIM feedstock with low surface roughness. BACKGROUND

[0002] Metal powder injection molding technology (MIM) is a new type of powder metallurgy near net shape forming technology by introducing modern plastic injection molding technology into the field of powder metallurgy. The process includes: firstly, uniformly mixing solid powder and organic binder, then injecting into the mold cavity in the plasticized state to solidify and form, and then removing the binder in the formed blank by chemical or thermal decomposition, and finally sintering to obtain the final product. Metal injection molding technology is a better method for mass production of small, complex shape and high strength parts.

[0003] The feedstock of metal powder injection molding refers to the raw material for feeding into the injection molding machine after mixing solid powder and organic binder, and therefore the main physical and chemical properties of the final product are mainly determined by the feedstock. The particle size of the current MIM stainless steel feedstock is generally D50: 9-11 μm. The feedstock with this particle size can be successfully formed in the subsequent injection molding process, but the surface roughness after sintering can only reach Ra: 1.0-1.3.

[0004] Due to product upgrading, the surface roughness of the sintered product needs to be further reduced to Ra<0.5. The reduction of the surface roughness of the sintered product can only be achieved by reducing the particle size of the feedstock. According to experiments, when Ra<0.5, the particle size of the feedstock needs to be reduced to D50: 4-5 μm at least. However, the reduction of the particle size of the feedstock brings problems such as high injection pressure, heavy flow marks and difficult injection, which directly affects the molding of the product, and therefore the feedstock must be optimized to solve the above problems.

[0005] High injection pressure, heavy flow marks and difficult injection are all related to the poor flowability of the feedstock caused by the reduction of the particle size. Injection feedstock is divided into plastic-based system and wax-based system according to the main components of the binder. The flowability of the wax-based system is good, but the subsequent debinding time is very long, and the product is prone to deformation and flash during the debinding process. Therefore, at present, except for some special cases, MIM generally adopts the plastic-based system. To improve the flowability of the injection feedstock of the plastic-based system, the binder is mainly used to solve the problem, that is, by increasing the melt flow index of the main component POM (polyoxymethylene) in the binder. If the high melt index POM is simply used, on the one hand, the cost is too high, and on the other hand, the increase is not obvious.

[0006] The Chinese patent CN109513916A makes the components of the binder well compatible and fluid by adding a compatibility agent. However, the D50 of the particle size distribution of the feedstock is still as high as 9.8-10.8, which is not applicable.

[0007] The Chinese patent CN114210979A has a feedstock particle size of D50: 7-9 μm. The feedstock has good fluidity through customization of the binder system and argon protection. However, the particle size is still much higher than D50: 4-5 μm. SUMMARY

[0008] The application provides a preparation method of low-surface-roughness MIM feedstock to solve the problem that the currently prepared low-particle-size feedstock cannot be processed by MIM.

[0009] To solve the above technical problems, the technical scheme of the application is as follows: the preparation method of the low-surface-roughness MIM feedstock includes the following steps: mixing stainless steel powder and a binder first, then adding EBS (ethylene bis-stearamide), continuing to mix for 3-6 min, discharging and granulating, and the particle size distribution is D50: 4-5 μm.

[0010] EBS can effectively improve the fluidity of the feedstock during injection molding. However, if the mixing time is too long, it will volatilize and crack, thereby losing its effectiveness. If EBS is not mixed but directly mechanically mixed with the feedstock for injection molding, the uniformity is poor, and EBS cannot fully exert its effect of improving the fluidity.

[0011] Optionally, the addition amount of EBS is 5-20% wt of the binder, preferably 7-9% wt. If the addition amount of EBS is too small, the required injection fluidity cannot be obtained, and if the addition amount of EBS is too large, the injection will be uneven, the injection pressure will be unstable, and the product forming effect will be affected.

[0012] Optionally, the binder includes the following components by weight:

[0013]

[0014] Optionally, the matrix agent is PE (polyethylene), PP (polypropylene), EVA (ethylene-vinyl acetate copolymer), or EBA (binary copolymer of ethylene (E) and butyl acrylate (BA)), preferably EBA.

[0015] EBA has better compatibility with EBS than EVA, and the same addition amount can more effectively improve the fluidity and injection pressure of the feedstock.

[0016] Optionally, the shrinkage rate of the product after injection molding of the feedstock is 1.162-1.168.

[0017] The shrinkage rate used in the mold design in the MIM industry is generally controlled at 1.162-1.168, and the sintering difficulty of the product produced in this range is controllable, and the strength of the product after sintering is also sufficient.

[0018] Optionally, the stainless steel powder and the binder are in a ratio of, for example, by weight parts as follows:

[0019] The stainless steel powder is 80-90 parts;

[0020] The binder is 10-20 parts;

[0021] Preferably, the stainless steel powder and the binder are in a ratio of, for example, by weight parts as follows:

[0022] The stainless steel powder is 86-88 parts;

[0023] The binder is 12-14 parts.

[0024] After the particle size distribution becomes smaller, the shrinkage rate will become larger, and in order to control the appropriate shrinkage rate, the proportion of the binder needs to be reduced.

[0025] Optionally, the stainless steel powder is 316L.

[0026] Optionally, the mixing process of the stainless steel powder and the binder is as follows: the stainless steel powder is heated to 180-185℃, then the temperature is controlled at 165-170℃, the mixed binder is added, and after the binder and the stainless steel powder are fully mixed at this temperature, the press hammer is pressed to the bottom, and the binder and the stainless steel powder are uniformly mixed by extrusion of the press hammer.

[0027] Optionally, argon gas is continuously introduced and positive pressure protection is provided during the mixing process of the stainless steel powder and the binder.

[0028] Optionally, the melt flow index of the feed is 1500-2500g / min, preferably 1800-2200g / min.

[0029] Optionally, the bulk density of the feed is greater than 2.8g / cm 3 , and the tap density is greater than 4.8g / cm 3 .

[0030] The technical solution provided by the present application combines the lubricating system and the preparation process of the feed, so that even in the case of a significant reduction in particle size, the necessary flowability of the feed during injection, the saturation degree of injection and the light flow marks can still be ensured, and finally the MIM product with Ra<0.5 can be successfully prepared. DETAILED DESCRIPTION

[0031] For the convenience of understanding, the preparation method of the low surface roughness MIM feedstock is described below in combination with examples, and it should be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application.

[0032] The raw materials and reagents used in this example are commercially available general goods, unless otherwise specified. Among them, EBS is a dispersing agent of the Kao brand, model EB-FF, melting point (℃) 141.5-146.5.

[0033] Examples 1-2

[0034] The preparation method of the low surface roughness MIM feedstock includes the following steps: heating the stainless steel powder to 180-185℃, then controlling the temperature at 165-170℃, adding the mixed binder and continuing to heat for 10-15min, then pressing the hammer to the bottom, mixing for 20-25min, then adding EBS and continuing to mix for 3-6min, discharging and granulating.

[0035] The allocation of each component in the above examples is shown in Table 1.

[0036] Table 1

[0037]

[0038] Comparative Examples 1-3

[0039] The difference between Examples 1-2 and Comparative Examples 1-3 is that the stainless steel powder and the binder are mixed and then directly discharged and granulated. Specifically, the stainless steel powder is heated to 180-185℃, then the temperature is controlled at 165-170℃, the mixed binder is added and heated for 10-15min, then the hammer is pressed to the bottom, mixed for 20-25min and directly discharged and granulated.

[0040] The allocation of each component in the above examples is shown in Table 2.

[0041] Table 2

[0042]

[0043] Performance test

[0044] Examples 1-2 and Comparative Examples 1-3 were tested for the following properties, and the test results are shown in Table 3:

[0045] Powder particle size test method: laser particle size analysis was used to detect the same sample for 5 times, and the experimental results were the interval of the detected D50 data.

[0046] Flowability test method: reference ISO 1133:2005 Plastics. Determination of the melt mass-flow rate (MFR) and the melt volume-flow rate (MVR) of thermoplastics was used for testing.

[0047] Shrinkage is the ratio of the size of the mold cavity and the size of the product after sintering for 24 hours.

[0048] Injection pressure is the actual value used on the injection machine.

[0049] The test method for roughness after sintering: the surface of the product after sintering is detected 5 times by a contact roughness meter and then the average value is taken.

[0050] Table 3

[0051]

[0052] According to the examples 1 and 2 shown in Table 3, it can be found that the feed of example 1 has better flowability during injection and smaller injection pressure because of better compatibility of EBA and EBS, and no flow marks. The performance index of the feed of comparative example 1, which is a conventional particle size and conventional formula, after injection and sintering can be found that its roughness exceeds 1, which cannot meet our needs, while in comparative example 2, the particle size distribution is reduced and the final sintering roughness is indeed less than 0.5, but the flowability during injection is poor, the injection pressure is large, the flow marks are serious, and the product is unqualified. Although EBS is added in comparative example 3, it is added together with the binder, although the flowability and injection pressure are improved compared with comparative example 2 under the condition of reducing the particle size, but they are still high, and there is still a problem of slight flow marks on the surface.

[0053] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit it. Although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features, and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method of making a low surface roughness MIM feedstock, characterized by, The method comprises the following steps: mixing stainless steel powder and adhesive, then adding EBS and mixing for 3-6 minutes, discharging and granulating, and the particle size distribution is D50 of 4-5 μm. The adhesive comprises the following components in parts by weight: POM 8-12 parts; SA 1-3 parts; PW 0.3-1.0 parts; Framework agent 0.8-1.2 parts; The framework agent is EBA.

2. The method of claim 1, wherein the low surface roughness MIM feedstock is prepared by the steps of: The added amount of EBS is 5-20 wt% of the adhesive.

3. The method of claim 2, wherein the low surface roughness MIM feedstock is prepared by the steps of: The added amount of EBS is 7-9 wt% of the adhesive.

4. The method of claim 1, wherein the low surface roughness MIM feedstock is prepared by the steps of: The stainless steel powder and the adhesive are mixed in the following proportions in parts by weight: Stainless steel powder 80-90 parts; Adhesive 10-20 parts.

5. The method of claim 4, wherein the low surface roughness MIM feedstock is prepared by the steps of: The stainless steel powder and the adhesive are mixed in the following proportions in parts by weight: Stainless steel powder 86-88 parts; Adhesive 12-14 parts.

6. The method of claim 1, wherein the low surface roughness MIM feedstock is prepared by the steps of: The stainless steel powder is 316L.

7. The method for preparing low surface roughness MIM feedstock according to claim 1, characterized in that, The mixing process of the stainless steel powder and the adhesive is as follows: the stainless steel powder is heated to 180-185 ℃, then the temperature is controlled at 165-170 ℃, the mixed adhesive is added and heated for 10-15 minutes, then the press hammer is pressed to the bottom, and the mixing is carried out for 20-25 minutes.

8. The method of claim 1, wherein the low surface roughness MIM feedstock is prepared by the steps of: Argon gas is continuously introduced and positive pressure protection is provided during the mixing process of the stainless steel powder and the adhesive.

9. The method of claim 1, wherein the low surface roughness MIM feedstock is prepared by the steps of: The melt flow index of the feed is 1500-2500 g / min.

10. The method of claim 9, wherein the low surface roughness MIM feedstock is prepared by, The melt flow index of the feed is 1800-2200 g / min.

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