Plastic-based binder for injection molding of low-fluidity iron powder and injection molding method

Abstract

The invention discloses a plastic-based binder for injection molding of low-fluidity iron powder and an injection molding method, and belongs to the technical field of metal injection molding. The plastic-based binder is composed of 70 wt% of polyformaldehyde, 5-15 wt% of a skeleton agent and 5-30 wt% of an additive, and the skeleton agent is selected from high-density polyethylene, ethylene-vinyl acetate copolymer and thermoplastic elastomer; in the additive, the plasticizer is selected from dioctyl phthalate, the lubricant is selected from stearic acid or paraffin, and the stabilizer is selected from stearamide. According to the invention, stearamide is adopted as a stabilizer, so that the stability of the plastic-based binder in banburying and injection is improved. Dioctyl phthalate is used as a plasticizer to improve the rheological property of the feed, and an ethylene-vinyl acetate copolymer is used as a compatibilizer to improve the compatibility of dioctyl phthalate and polyformaldehyde. After the low-fluidity iron powder is subjected to internal mixing and injection molding by using the plastic-based binder, the uniformity of a green body is greatly improved, and the degreasing performance is not obviously changed.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal injection molding, and particularly relates to a plastic-based binder for low-fluidity iron powder injection molding and an injection molding method. BACKGROUND

[0002] The binder for metal injection molding is a core component of the metal injection molding (MIM) technology, which integrates the high efficiency of plastic injection molding and the excellent properties of metal materials to precisely manufacture metal components with complex shapes. The binder is mainly composed of a thermoplastic polymer and a plasticizer, and plays an indispensable role in the MIM process. As a supporting carrier for metal powder, the binder not only maintains the stability of the powder shape, but also can be efficiently removed during the heat treatment process.

[0003] The binder is essentially a composite multi-component polymer mixture, which mainly provides support for the component during the injection molding process and gives the finished product the required structural strength. As a medium specially designed for injection molding, its main function is to integrate and maintain the aggregation structure of metal particles until the sintering process starts. The binder has the following characteristics: excellent fluidity to ensure uniform filling of metal powder in the mold and form the predetermined shape; moderate viscosity to achieve uniform distribution of powder and facilitate injection molding operation; good thermal stability to remain stable during mixing and injection without decomposition of harmful substances and easy removal from the metal part; high removal efficiency, which can be completely removed by pyrolysis, melting or chemical methods in the final stage of metal injection molding.

[0004] In the metal injection molding (MIM) process, the selection of the binder plays a decisive role in ensuring the quality of the final product. The performance indicators of the binder directly affect the geometric size accuracy, surface finish and mechanical performance of the product. Plastic-based binders, as a kind of core binding material, are mainly composed of polyformaldehyde components, which have excellent shape retention performance, rapid molding and debinding efficiency, and low pollution characteristics after debinding. However, traditional plastic-based binders are prone to decomposition during mixing, and have poor rheological properties, resulting in insufficient injection uniformity and other problems. Especially when mixed with metal powder with poor fluidity, the poor rheological properties of the binder will adversely affect the uniformity of the green body during injection molding.

[0005] Therefore, it is an urgent need to develop a new type of high-performance plastic-based binder for low-fluidity metal powder to ensure the stability of the molding process and optimize the quality of the product. SUMMARY

[0006] This invention aims to provide a plastic-based binder for injection molding of low-flow-rate iron powder, addressing issues such as instability during injection and uneven green body composition. By introducing stearamide (SR), the decomposition of polyoxymethylene (POM) is reduced, improving stability during mixing and injection molding. Furthermore, the introduction of dioctyl phthalate (DOP) and ethylene-vinyl acetate copolymer (EVA) improves the rheological properties of the feedstock, ensuring enhanced uniformity of the injection-molded green body.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A low-flowability plastic-based binder for injection molding of iron powder, comprising 70 wt% polyoxymethylene (POM), 5-15 wt% skeleton agent, and 5-30 wt% additives, wherein:

[0009] The skeleton agent is selected from one or more combinations of high-density polyethylene (HDPE), ethylene-vinyl acetate copolymer (EVA), and thermoplastic elastomer (POE);

[0010] The additive consists of one or more of plasticizers, lubricants, and stabilizers;

[0011] The plasticizer is selected from dioctyl phthalate (DOP), the lubricant is selected from stearic acid (SA) or paraffin (PW), and the stabilizer is selected from stearamide (SR).

[0012] As a preferred embodiment of the present invention, in the plastic-based binder component, the content of the plasticizer is 0~5wt%, the content of the lubricant is 0~15wt% when stearic acid (SA) is selected, the content of the lubricant is 5~10wt% when paraffin wax (PW) is selected, and the content of the stabilizer is 0~15wt%; the contents of the plasticizer, lubricant and stabilizer are all percentages of the total mass of the binder, and are not all 0.

[0013] As a further preferred technical solution of the present invention, the plastic-based adhesive is composed of 70 wt% polyoxymethylene (POM), 1 wt% ethylene-vinyl acetate copolymer (EVA), 4 wt% thermoplastic elastomer (POE), 5 wt% dioctyl phthalate (DOP), 5 wt% paraffin wax (PW), and 15 wt% stearamide (SR).

[0014] Furthermore, this invention also proposes an injection molding method based on this plastic-based binder, comprising the following steps:

[0015] (1) Raw material preparation: The feed is prepared by mixing 45 vol% metal powder with 55 vol% binder; the metal powder is an ultrafine irregular morphology iron-based alloy powder with a particle size of 2~5 μm.

[0016] (2) Preheating and mixing: Add polyoxymethylene to a mixer and heat and stir at 170~190 ℃, controlling the stirring speed at 45~50 r / min and the heating time at 10~15 min;

[0017] (3) Compound mixing: Add skeleton agent, additives and metal powder to the internal mixer, and continue to heat and stir at 170~190 ℃ for 20~30 min;

[0018] (4) Pressurized mixing: Apply pressure to the mixture and continue to mix for 30-40 minutes. After the material forms a uniform dough, cool it to room temperature and crush it to obtain granular feed.

[0019] (5) Injection molding: After the material preparation is completed, injection molding is performed according to the shape of the metal part. The injection temperature is 160~190 ℃, the injection pressure is 60~90 bar, the injection rate is 45%, the holding pressure is 40 bar, the holding rate is 40%, and the holding time is 2 s to obtain the molded green blank.

[0020] (6) Oxalic acid degreasing: The green body formed in step (5) is degreased with oxalic acid at a temperature of 100~150 ℃, an acid injection rate of 0.1~0.3 g / min, and a degreasing time of 6~10 h to finally obtain metal parts.

[0021] To address the problems of instability and poor uniformity of injection-molded green bodies caused by using plastic binders in the internal mixing of low-flow-rate iron powder, this invention employs stearamide as a stabilizer to improve the stability of the plastic binder during internal mixing and injection molding. Dioctyl phthalate is used as a plasticizer to improve the rheological properties of the feedstock, while ethylene-vinyl acetate copolymer is used as a compatibilizer to improve the compatibility between dioctyl phthalate and polyoxymethylene. After using this plastic binder to internally mix and injection mold low-flow-rate iron powder, the uniformity of the green body is significantly improved, and the degreasing performance remains largely unchanged. Compared with existing technologies, the beneficial effects of this invention are as follows:

[0022] (1) Using stearamide as a stabilizer can not only effectively inhibit the decomposition of polyoxymethylene during mixing and injection molding, but also act as a dispersant to promote the uniform dispersion of metal powder.

[0023] (2) The binder of the present invention has excellent rheological properties, wherein the synergistic addition of paraffin and dioctyl phthalate can significantly improve the rheological properties of the feed system; even when the metal powder with irregular morphology is subjected to intensive mixing, the binder can still ensure that the feed maintains good rheological properties and ensure the uniformity of the green body after injection molding.

[0024] (3) The adhesive of the present invention uses ethylene-vinyl acetate copolymer as compatibilizer. Through the interaction of the functional groups of ethylene-vinyl acetate copolymer, dioctyl phthalate and polyoxymethylene, the compatibility between polyoxymethylene and dioctyl phthalate is improved, so that dioctyl phthalate can be more effectively dispersed in the polyoxymethylene matrix through the bridging effect of ethylene-vinyl acetate copolymer.

[0025] (4) The plasticizer dioctyl phthalate used in this invention mainly affects the rheology and uniformity of the feed, while having little effect on the degreasing performance. This characteristic provides an important basis for the optimization of the feed formulation. That is, under the premise of ensuring rheology and uniformity, there is no need to over-consider the effect of dioctyl phthalate on the degreasing performance, thereby simplifying the production process and improving production efficiency. Attached Figure Description

[0026] Figure 1 Microscopic morphology diagrams of the iron powder used in the examples and comparative examples (a and b correspond to high and low magnification, respectively).

[0027] Figure 2 Comparison curves of formaldehyde release during the mixing process of binder systems with different additive formulations (a, b, c, and d correspond to Comparative Example 1, Comparative Example 2, Comparative Example 3, and Example 1, respectively).

[0028] Figure 3 Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) curves of different binder systems are shown (a corresponds to POM and POM+DOP, b corresponds to POM+EVA and POM+EVA+DOP; c corresponds to Fourier transform infrared analysis of single component and mixed system).

[0029] Figure 4 The hardness distribution diagrams for the front and back sides of different green blanks are shown in Figure 4 (a corresponds to the hardness distribution diagram for the front and back sides of the green blank prepared in Example 4, b corresponds to the hardness distribution diagram for the front and back sides of the green blank prepared in Example 5, and c corresponds to the hardness distribution diagram for the front and back sides of the green blank prepared in Example 2).

[0030] Figure 5 SEM images of the green body after acid removal (a and b correspond to the high and low magnification SEM images of the green body after acid removal in Example 4, respectively; c and d correspond to the high and low magnification SEM images of the green body after acid removal in Example 5, respectively; e and f correspond to the high and low magnification SEM images of the green body after acid removal in Example 2, respectively). Detailed Implementation

[0031] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0032] The preparation of a low-flowability iron powder injection molding plastic-based binder and the injection molding method based on the plastic-based binder include the following steps:

[0033] (1) Raw material preparation: The feed was prepared by mixing 45 vol% metal powder with 55 vol% binder. The binder composition used in different examples and comparative examples is shown in Table 1. The metal powder is an ultrafine irregular morphology iron-based alloy powder with a particle size of 2~5 μm (Shandong Aofite New Material Technology Co., Ltd.).

[0034] (2) Preheating and mixing: Add polyoxymethylene to a mixer and heat and stir at 180 ℃, controlling the stirring speed to 50 r / min and the heating time to 10 min.

[0035] (3) Compound mixing: Add skeleton agent, additives and metal powder to the internal mixer and continue to heat and stir at 180 °C for 25 min.

[0036] (4) Pressurized mixing: Apply pressure to the mixture and continue to mix for 30 minutes. After the material forms a uniform dough, cool it to room temperature and crush it to obtain granular feed.

[0037] (5) Injection molding: After the material preparation is completed, injection molding is performed according to the shape of the metal part. The injection temperature is 160 ℃, the injection pressure is 80 bar, the injection rate is 45%, the holding pressure is 40 bar, the holding rate is 40%, and the holding time is 2 s to obtain the molded green part.

[0038] (6) Oxalic acid degreasing: The green body formed in step (5) is degreased with oxalic acid at a temperature of 130 °C, an acid injection rate of 0.2 g / min, and a degreasing time of 8 h to finally obtain metal parts.

[0039] Table 1: Material composition of adhesives used in each embodiment and comparative example (wt%)

[0040] Component Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 2 Comparative Example 4 Comparative Example 5 Filler - Polyoxymethylene (POM) 70 70 70 70 70 70 70 Skeleton agent - High density polyethylene (HDPE) 15 15 15 15 0 0 0 Skeleton agent - Ethylene-vinyl acetate copolymer (EVA) 0 0 0 0 1 1 1 Skeleton agent - Thermoplastic elastomer (POE) 0 0 0 0 4 4 4 Additive - Plasticizer - Dioctyl phthalate (DOP) 0 0 0 0 5 0 2.5 Additive - Lubricant - Stearic acid (SA) 0 15 10 5 0 0 0 Additive - Lubricant - Paraffin wax (PW) 0 0 0 0 5 10 7.5 Additive - Stabilizer - Stearamide (SR) 15 0 5 10 15 15 15

[0041] I. Analysis of Metal Powder Raw Materials:

[0042] Figure 1 These are images showing the microstructure and particle size distribution of the metal powders used in the examples and comparative examples; Figure 1 It can be seen that the metal powder has an irregular morphology, with a particle size distribution ranging from 2 to 3 μm and a loose packing density of 1.5 g / cm³. 3 The tap density is 3.6 g / cm³. 3The Hausner ratio (HR) is 2.4. According to the powder flowability evaluation standard, when the Hausner ratio is greater than 1.25, it indicates that the powder flowability is poor, which confirms that the metal powder selected in this invention has irregular morphological characteristics and low flowability.

[0043] II. Test Result Analysis:

[0044] 1. Formaldehyde release analysis during the mixing process of the binder system: Figure 2 A comparative curve of formaldehyde release from adhesive systems with different additive formulations; by Figure 2 It can be seen that Comparative Example 1 (15wt% stearic acid) and Comparative Example 2 (10wt% stearic acid + 5wt% stearamide) showed significant formaldehyde release peaks during the mixing process, while the formaldehyde release of Comparative Example 3 (5wt% stearic acid + 10wt% stearamide) and Example 1 (15wt% stearamide) remained at a low level, confirming that stearamide (SR) can effectively inhibit the thermal decomposition of polyoxymethylene. When the amount of stearamide added is ≥10wt%, the formaldehyde release can be stably controlled below the critical value. Considering the requirements of subsequent injection molding process for feed stability, the preferred amount of stearamide added in this invention is 15wt% (Example 2 and Comparative Examples 4 and 5).

[0045] 2. Rheological property testing: The results of the comprehensive rheological factor measurement for different feedstock systems are shown in Table 2 (the comprehensive rheological factor is a characteristic parameter characterizing the rheological properties of the feedstock; a higher value indicates better rheological properties and is more conducive to improving the stability of the injection molding process). As shown in Table 2, the feedstock prepared in Example 2 exhibited the highest comprehensive rheological factor at 165 °C, confirming that it possesses optimal rheological properties and molding adaptability.

[0046] Table 2: Comprehensive rheological factor values ​​for different feeds

[0047] Feedstock type 165℃ Comparative Example 4 3.101 x 10 -6 ]] Comparative Example 5 2.587 x 10 -6 ]] Example 2 3.329 x 10 -6 ]]

[0048] 3. Compatibility verification: The compatibility of the binder system was characterized by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR).

[0049] DSC analysis results ( Figure 3 a and b show: When dioctyl phthalate (DOP) is added alone to a polyoxymethylene (POM) matrix, the melting temperature (T) of the blend system is... m ) and crystallization temperature (T) c The lack of significant change indicates that the plasticizer alone has a limited effect on regulating thermal properties; however, when ethylene-vinyl acetate copolymer (EVA) is introduced into the composite, the T of the system... m With T cSignificant shifts were observed in both, confirming a synergistic effect between ethylene-vinyl acetate copolymer (EVA) and dioctyl phthalate (DOP) on the polyoxymethylene (POM) matrix.

[0050] FTIR spectroscopy analysis ( Figure 3 c) The results show that compared to the single components of polyoxymethylene (POM), ethylene-vinyl acetate copolymer (EVA), and dioctyl phthalate (DOP), the characteristic absorption peaks of the POM+EVA+DOP composite system exhibit a significant shift and a marked change in peak intensity, revealing a stronger interaction between the three components at the molecular level. Therefore, EVA significantly improves the interfacial compatibility between POM and DOP through intermolecular synergy, providing key technical support for optimizing the thermal stability and molding performance of the binder system.

[0051] III. Characterization of green body uniformity:

[0052] The hardness distribution of the green compact cross section was tested using a microhardness tester, and the results are as follows: Figure 4 As shown in Table 3 (the uniformity of hardness distribution is characterized by the difference in the average hardness of the front and back sides and the standard deviation; the smaller the difference and the lower the standard deviation, the more uniform the green body structure).

[0053] Table 3: Hardness (HD) of different green bodies (front and back sides)

[0054] Feedstock type Hardness average (front) Hardness average (back) Hardness value standard deviation (front) Hardness value standard deviation (back) Comparative Example 4 68.65 72.44 1.79 1.09 Comparative Example 5 71.45 73.91 0.79 0.87 Example 2 72.81 73.19 0.95 0.55

[0055] The green body of Comparative Example 4 exhibited the lowest average hardness (68.65 HD), and the powder particles showed significant aggregation in the injection port area (e.g., Figure 4 (as shown in a); the average hardness of the front side (68.65 HD) differs from that of the back side (72.44 HD) by 3.79 HD, and the standard deviations of the hardness of the front and back sides are 1.79 and 1.09, respectively, confirming that the green blank has serious internal stress unevenness and structural defects.

[0056] Comparative Example 5 showed a significant improvement in green uniformity after the introduction of DOP: the difference in average hardness between the front and back sides was reduced by 35% compared to Comparative Example 4, the standard deviation of hardness on the front side was reduced to 0.79, and on the back side it was reduced to 0.87.

[0057] Example 2 exhibits the best green uniformity: compared to Comparative Example 5, the difference in average hardness between the front and back sides is further reduced by 84% (the difference between the average hardness of the front side and the average hardness of the back side is only 0.38 HD); the standard deviation of hardness on the front side is reduced by a maximum of 47% (down to 0.95) compared to Comparative Example 4, and the maximum reduction on the back side is 49% (down to 0.55), confirming that its internal structure has reached a highly uniform state.

[0058] IV. Analysis of Regulatory Mechanism and Characterization of Degreasing Performance:

[0059] 1. Mechanism for regulating structural uniformity: The above-mentioned differences in green body uniformity are attributed to the synergistic compatibilizing effect of the binder system. Ethylene-vinyl acetate copolymer (EVA) improves the interfacial compatibility between polyoxymethylene (POM) and dioctyl phthalate (DOP) through molecular chain entanglement, significantly inhibiting binder phase separation and powder particle segregation during injection molding, thereby improving the internal structural uniformity and mechanical property stability of the green body.

[0060] 2. Degreasing performance test: The experiment was repeated 5 times according to the proportions of Example 2, Comparative Example 4 and Comparative Example 5. The degreasing rate of the green body was then determined by the weight loss method. The results are shown in Table 4. The data show that the content of dioctyl phthalate (DOP) has no significant effect on the degreasing rate of the green body (all maintained at 9.85~10.52wt%).

[0061] Table 4: Degreasing rate of different samples

[0062] Feedstock type 1# sample 2# sample 3# sample 4# sample 5# sample Comparative Example 4 10.22 10.15 10.06 10.22 10.11 Comparative Example 5 10.21 10.03 10.13 9.85 10.22 Example 2 10.18 10.19 10.39 10.16 10.52

[0063] 3. Pore structure characterization: The cross-sectional pore morphology of the green body after acid defatting was observed using scanning electron microscopy (SEM). Figure 5 As shown in the figure, the pore structures are mainly divided into two categories:

[0064] ① Regular circular pores: average pore diameter 1~2 μm (marked by red circles in the figure), formed by the escape of thermal decomposition products of polyoxymethylene (POM) continuous phase.

[0065] ② Irregular slit-like pores: average pore size 2~5 μm (marked by blue box in figure), originating from the difference in thermal decomposition between polyoxymethylene (POM) and the dispersed phase interface.

[0066] The residual binder (mainly thermoplastic elastomer (POE) and ethylene-vinyl acetate copolymer (EVA)) forms a continuous three-dimensional network structure, which can effectively maintain the shape stability of the green body after degreasing. Comparison of the pore structure parameters of the three samples shows that the dioctyl phthalate (DOP) content has no significant regulatory effect on the size distribution and morphology of the degreasing pores.

Claims

1. A low-flowability iron powder injection molding plastic-based binder, characterized in that, It is composed of 70wt% polyoxymethylene (POM), 5-15wt% skeleton agent, and 5-30wt% additives, wherein: The skeleton agent is selected from one or more combinations of high-density polyethylene (HDPE), ethylene-vinyl acetate copolymer (EVA), and thermoplastic elastomer (POE); The additive consists of one or more of plasticizers, lubricants, and stabilizers; The plasticizer is selected from dioctyl phthalate (DOP), the lubricant is selected from stearic acid (SA) or paraffin (PW), and the stabilizer is selected from stearamide (SR).

2. The low-flowability iron powder injection molding plastic-based binder as described in claim 1, characterized in that, The plasticizer content is 0~5wt%, the lubricant content is 0~15wt% when stearic acid (SA) is selected, the lubricant content is 5~10wt% when paraffin wax (PW) is selected, and the stabilizer content is 0~15wt%. The contents of plasticizer, lubricant and stabilizer are all percentages of the total mass of the binder, and are not all 0.

3. The low-flowability iron powder injection molding plastic-based binder as described in claim 2, characterized in that, It is composed of 70 wt% polyoxymethylene (POM), 1 wt% ethylene-vinyl acetate copolymer (EVA), 4 wt% thermoplastic elastomer (POE), 5 wt% dioctyl phthalate (DOP), 5 wt% paraffin wax (PW), and 15 wt% stearamide (SR).

4. An injection molding method based on the plastic-based binder according to claim 1, 2, or 3, characterized in that, Includes the following steps: (1) Raw material preparation: The feed is prepared by mixing metal powder and binder; the metal powder is an ultrafine irregular morphology iron-based alloy powder with a particle size of 2~5 μm; (2) Preheating and mixing: Add polyoxymethylene to a mixer and heat and stir; (3) Compound mixing: Add skeleton agent, additives and metal powder to the internal mixer and continue heating and stirring; (4) Pressure mixing: Apply pressure to the mixture and continue mixing until the material forms a uniform dough. Then cool it to room temperature and crush it to obtain granular feed. (5) Injection molding: After the material preparation is completed, injection molding is performed according to the shape of the metal part to obtain the molded blank; (6) Oxalic acid degreasing: The green blank formed in step (5) is degreased with oxalic acid to finally obtain metal parts.

5. The injection molding method as described in claim 4, characterized in that, In step (1), the feed is prepared in a ratio of 45 vol% metal powder to 55 vol% binder.

6. The injection molding method as described in claim 4, characterized in that, In step (2), polyoxymethylene is added to a mixer and heated and stirred at 170~190 ℃, with the stirring speed controlled at 45~50 r / min and the heating time at 10~15 min.

7. The injection molding method as described in claim 4, characterized in that, In step (3), add skeleton agent, additives and metal powder to the internal mixer and continue to heat and stir at 170~190 ℃ for 20~30 min.

8. The injection molding method as described in claim 4, characterized in that, In step (4), pressure is applied to the mixture and kneading is continued for 30-40 minutes. After the material forms a uniform dough, it is cooled to room temperature and crushed to obtain granular feed.

9. The injection molding method as described in claim 4, characterized in that, In step (5), injection molding is performed according to the shape of the metal part, wherein the injection temperature is 160~190 ℃, the injection pressure is 60~90 bar, the injection rate is 45%, the holding pressure is 40 bar, the holding rate is 40%, and the holding time is 2 s to obtain the molded green blank.

10. The injection molding method as described in claim 4, characterized in that, In step (6), the formed green body is degreased with oxalic acid at a temperature of 100~150 ℃, an acid injection rate of 0.1~0.3 g / min, and a degreasing time of 6~10 h, and finally metal parts are obtained.

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