Low-shrinkage precision casting mold material and preparation method thereof

Abstract

The application provides a low-shrinkage precision investment casting mold material and a preparation method thereof. According to weight fractions, the mold material comprises the following components: wax material 40-60 parts, petroleum resin 15-30 parts, polymer 0-10 parts, powder filler 5-15 parts, additive 0-5 parts, and nucleating agent 0-1 part. The mold material has excellent comprehensive performance and can meet the requirements of precision investment casting in the field of aerospace.

Description

Technical Field

[0001] This invention relates to the field of investment casting mold materials, specifically to a low-shrinkage investment casting mold material and its preparation method. Background Technology

[0002] Investment casting, also known as lost-wax casting, is an advanced near-net-shape production process. First, a wax model is made, and a layer of refractory material is coated on its surface. The wax is then heated to melt and flow out, thus obtaining a hollow shell. Molten metal is then poured into the hollow shell, and after it cools, the refractory material is removed to obtain the metal casting.

[0003] In recent years, with rapid economic development and the booming advancement of science and technology, investment casting technology has seen rapid development in the industrial field, especially in the application of precision castings in the aerospace industry. Precision castings are also developing towards larger, more complex, and thinner-walled forms, such as integral casings for aero-engines and guide vanes, which place even higher demands on dimensional accuracy. In the casting process of large, complex, thin-walled parts, shrinkage deformation of the wax pattern and creep deformation of the mold shell can cause dimensional errors in the casting. Wax pattern forming is the first step in the entire investment casting process, and dimensional deviations in the wax pattern account for more than 40% of the final casting dimensional deviations. The dimensional accuracy of the wax pattern directly determines and affects the final accuracy of the casting, and the performance of the molding material is the most important factor affecting the dimensional accuracy of the wax pattern. Therefore, the molding material should have a low linear shrinkage rate and a low coefficient of linear expansion, and excellent overall performance.

[0004] A search of existing technologies revealed Chinese invention patent application number 201810914681.0, which discloses a medium-temperature casting mold material and its preparation method. This patent describes the preparation of the medium-temperature casting mold material by adding lignite wax, polyethylene wax, carnauba wax, ethylene-vinyl acetate copolymer, and stearic acid in a specific ratio and according to certain steps. This invention provides a mold material with high surface hardness and softening point, giving the wax mold a certain strength. However, for some high-precision aerospace castings, the linear shrinkage rate of the medium-temperature mold material provided by this invention is only about 0.9%, which does not meet the dimensional accuracy requirements.

[0005] Chinese invention patent application number 201910992647.X discloses an alloy investment casting precision mold material, its preparation method, and its application. This patent uses wax and natural resin as the main components, adding a certain mass proportion of linear low-density polyethylene to ensure the three components are fully miscible. The resulting medium-temperature mold material has a low linear shrinkage rate. However, the patent's shortcomings include poor thermal stability, which easily causes deformation of the wax mold, and a high ash content, directly affecting the quality of the alloy. Chinese invention patent application number 2015103234868.1 discloses a filled precision casting medium-temperature modulated wax and its preparation method. This patent uses polystyrene as a filler, with a filling ratio of 25%–30%. The prepared modulated wax has moderate hardness and fluidity, facilitating demolding. However, this patent's shortcomings include a large shrinkage rate, high mold material production cost, and a complex preparation process. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the purpose of this invention is to provide a low-shrinkage investment casting mold material and its preparation method.

[0007] According to one aspect of the present invention, a low-shrinkage investment casting mold material is provided, comprising, by weight, 40-60 parts wax, 15-30 parts petroleum resin, 0-10 parts polymer, 5-15 parts powder filler, 0-5 parts additives, and 0-1 parts nucleating agent.

[0008] Optionally, the wax is selected from any one or more of paraffin wax, microcrystalline wax, lignite wax, beeswax, candelilla wax, and carnauba wax.

[0009] Optionally, the petroleum resin is selected from C5 hydrogenated petroleum resin or C9 hydrogenated petroleum resin.

[0010] Optionally, the polymer is selected from any one or more of polystyrene, polyvinyl chloride, and ethylene-vinyl acetate copolymer.

[0011] Optionally, the powder filler is selected from any one or more of cellulose acetate powder, polyethylene terephthalate powder, soybean starch, and short glass fiber powder.

[0012] Optionally, the molding material has at least one of the following technical features:

[0013] -The additive is polymethyl methacrylate;

[0014] - The nucleating agent is dibenzyl sorbitol.

[0015] According to another aspect of the present invention, a method for preparing the above-mentioned low-shrinkage investment casting mold material is provided, the method comprising:

[0016] The wax and polymer are blended, heated, and stirred until molten to obtain blend A;

[0017] The blend A was further heated, and petroleum resin and additives were added and stirred to obtain blend B;

[0018] The blend B is heated further, and powdered filler and nucleating agent are added and stirred to make it blend evenly;

[0019] Let it stand, then cool it to room temperature to obtain a low-shrinkage investment casting mold material.

[0020] Optionally, the wax and polymer are blended and then heated and stirred until molten, wherein the heating temperature is 95-120°C and the stirring time is 20-30 min.

[0021] Optionally, the blend A is further heated, and petroleum resin and additives are added and stirred, wherein the heating temperature is 120-140°C and the stirring time is 20-30 min.

[0022] Optionally, the blend B is further heated, and powdered filler and nucleating agent are added and stirred to make it blend evenly, wherein the heating temperature is 110-120°C and the stirring time is at least 60 minutes.

[0023] Compared with the prior art, the present invention has at least one of the following beneficial effects:

[0024] 1. This invention effectively reduces the linear shrinkage rate and coefficient of thermal expansion of the molding compound through polymers and filler powders, thereby improving the dimensional accuracy of castings. Through the synergistic effect of the components, the molding compound has low ash content, reducing the incorporation of impurities into the alloy composition during dewaxing; the molding compound exhibits high strength, moderate viscosity, and high dewaxing efficiency, demonstrating excellent overall performance and meeting the relevant requirements for precision castings in the aerospace field.

[0025] 2. The preparation process of the present invention is simple, which can make the melting and mixing of each component of the molding material more uniform, and each component can give full play to its own characteristics, thereby improving the overall performance of the molding material. Attached Figure Description

[0026] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0027] Figure 1 This is a schematic flowchart of a method for preparing low-shrinkage investment casting mold material according to an embodiment of the present invention. Detailed Implementation

[0028] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0029] This invention provides a low-shrinkage investment casting mold material, which, by weight, comprises: 40-60 parts wax, 15-30 parts petroleum resin, 0-10 parts polymer, 5-15 parts powder filler, 0-5 parts additives, and 0-1 parts nucleating agent.

[0030] In some embodiments, the wax is selected from any one or more of paraffin wax, microcrystalline wax, lignite wax, beeswax, candelilla wax, and carnauba wax. Preferably, the wax is paraffin wax and microcrystalline wax. Paraffin wax is mainly composed of n-alkanes with a relative molecular mass of 300-450 and a melting point of 50-70°C. Its simple straight-chain structure gives it high strength and good fluidity in the molten state, allowing the mold material to fill the cavity smoothly. Microcrystalline wax is mainly composed of cyclic alkanes with long side chains and a relative molecular mass of 450-800. Due to the presence of numerous side chains, microcrystalline wax has higher viscosity and a higher melting point of 70-95°C, which to some extent restricts chain segment movement, reduces mold material shrinkage, and improves the thermal stability of the mold material. The use of paraffin wax and microcrystalline wax allows for complementarity in structure, strength, melting point, and other properties, thereby improving the overall performance of the mold material.

[0031] In some embodiments, the petroleum resin is selected from C5 hydrogenated petroleum resin (C5 petroleum resin) or C9 hydrogenated petroleum resin. Preferably, C5 hydrogenated petroleum resin is used, which is a tackifying resin made from the C5 fraction of ethylene cracking, through cationic polymerization of dienes and monoenes in the C5 component. C5 petroleum resin has a low average molecular weight, with a relative molecular mass of 1000-3000, a low softening point, and a melting point of 90-120°C, which is closer to the molecular weight of the wax components. Therefore, it has good compatibility with other components. Due to its high viscosity and adhesion, adding C5 petroleum resin helps each component to fully utilize its own characteristics.

[0032] In some embodiments, the polymer is a low-crystallinity polymer selected from any one or more of polystyrene, polyvinyl chloride, and ethylene-vinyl acetate copolymers. These low-crystallinity polymers have vinyl groups as the linear chain and other side chain groups, resulting in an asymmetric molecular structure. Due to the large side chain structure, the presence of the side chains in the molten state can, to some extent, hinder chain segment movement, thereby reducing the tendency for linear shrinkage. Preferably, the low-crystallinity polymer is an ethylene-vinyl acetate copolymer, copolymerized from ethylene and vinyl acetate. Because it has large acetate group branches, it alters the original crystalline state, resulting in an irregular structure and thus low crystallinity. Its relative molecular weight is 342, and its melting point is 75°C. The density of the ethylene-vinyl acetate copolymer is closer to that of the wax component, therefore, it is less prone to stratification during blending. Simultaneously, the ethylene-vinyl acetate copolymer has good toughness, maintaining a certain strength while also possessing a certain degree of toughness for the entire blend molding material, reducing the possibility of brittle fracture.

[0033] In some embodiments, the powder filler is selected from any one or more of cellulose acetate powder, polyethylene terephthalate powder, soybean starch, and short glass fiber powder. The particle size of the powder filler affects the surface roughness of the mold material; the smaller the particle size, the lower the surface roughness. More than 80% of the powder filler passes through a 100-mesh standard sieve, and more than 50% passes through a 200-mesh sieve. Powder fillers of 100-200 mesh not only meet the surface roughness requirements of the mold material but also help reduce costs. Preferably, the powder filler is cellulose acetate powder, an organic compound that is easily combustible and therefore has low ash content, with a melting point of 230-300°C. As a thermoplastic resin, cellulose acetate powder has good thermal stability, which can improve the thermal performance of the entire blend system, reduce the coefficient of thermal expansion of the mold material, reduce the linear shrinkage of the mold material, and reduce the possibility of mold shell cracking. Furthermore, the addition of the filler powder can effectively improve the mechanical properties of the mold material, making it less prone to deformation. In addition, cellulose acetate does not remain during the dewaxing process, thus avoiding its incorporation into the alloy.

[0034] For precision casting, linear shrinkage is generally less than 1%. A lower linear shrinkage rate indicates higher dimensional accuracy of the wax mold and better molding quality. In some implementations, a low-shrinkage additive, such as polymethyl methacrylate (PMMA), is used. PMMA has a high heat distortion temperature, good compatibility with resin, and a melting point of 105°C. Because of its low shrinkage rate, it can reduce the shrinkage rate of the resin during processing and molding. The nucleating agent is dibenzyl sorbitol (DMB), which has good compatibility with the polymer, can accelerate the crystallization rate, promote grain refinement, and shorten the molding cycle, thereby altering the crystallization state and thermodynamic properties.

[0035] The low-shrinkage investment casting mold material in this embodiment of the invention uses waxes such as paraffin wax and microcrystalline wax as wax components. Paraffin wax, due to its simple molecular structure, can be used to improve the strength of the mold material. Microcrystalline wax, with its small crystal grains, low brittleness, resistance to breakage, and low ash content, complements the wax in terms of physical properties. Adding petroleum resins with good heat resistance, such as C5 petroleum resin, improves the thermal stability of the blend system, and petroleum resins have good compatibility with the polymer matrix; simultaneously, its moderate melt viscosity can reduce the linear shrinkage rate of the blend system, improving the overall performance of the mold material. Adding low-crystallinity polymers, such as ethylene-vinyl acetate copolymer, for modification, due to its good flexibility, impact strength, and low-temperature resistance, can improve the overall performance of the mold material; at the same time, its low crystallinity reduces the crystallinity during the cooling process of the wax mold, reducing the tendency for volume shrinkage caused by crystallization, thereby reducing the shrinkage rate of the mold material. Adding powdered fillers, such as cellulose acetate powder, as a filler powder, to a certain extent inhibits chain segment movement during the blending process, reducing the shrinkage rate and coefficient of thermal expansion of the system. Additives such as polymethyl methacrylate (PMMA) can be used as weakly polar, low-shrinkage additives because of their low molding shrinkage and low coefficient of linear expansion, which can reduce the overall shrinkage of the blend system. Adding a small amount of dibenzyl sorbitol as a nucleating agent can alter the crystallization behavior of the blend system, refine the grains, shorten the molding cycle, and improve the physical and mechanical properties of the blend.

[0036] In the above embodiments of the present invention, wax is used as the main material, which can improve the strength and softening point of the molding material. Petroleum resin, polymer, powder filler, and additives are used as modifying materials to reduce the shrinkage rate and coefficient of thermal expansion of the molding material. Under the action of polymer and filler powder, the molding material of this formula has a small linear shrinkage rate and good dimensional stability, thus improving the quality of the wax mold. By coordinating the action mechanism of various additives and controlling the ratio between the components, the linear shrinkage rate and coefficient of thermal expansion of the molding material can be reduced, solving the problem of large shrinkage and deformation of wax molds in current investment casting, thereby obtaining a molding material with excellent comprehensive performance. The investment casting molding material in the embodiments of the present invention has a low linear shrinkage rate, a moderate softening point, a certain strength, and good dimensional stability, which can reduce the deformation and shrinkage of wax molds and meet the relevant requirements of investment casting in the aerospace field.

[0037] Based on the same inventive concept, embodiments of the present invention provide a method for preparing the above-mentioned low-shrinkage investment casting mold material, referring to... Figure 1 The method includes:

[0038] S1. After blending the wax and polymer, heat and stir until molten to obtain blend A;

[0039] S2. Continue heating the blend A, add petroleum resin and additives, stir, and obtain blend B;

[0040] S3. Continue heating the blend B, and add the powdered filler and nucleating agent, stirring to make it blend evenly;

[0041] S4. Let stand, then cool to room temperature to obtain a low-shrinkage investment casting mold material.

[0042] In some embodiments, the wax and polymer are blended and then heated and stirred until they reach a molten state. The heating temperature is 95-120°C, preferably 100°C. The stirring is carried out at a temperature higher than the melting point of the wax and polymer for 20-30 minutes to ensure that the components are fully and evenly mixed, thereby better utilizing their respective properties.

[0043] In some embodiments, blend A is further heated, and petroleum resin and additives are added and stirred. The heating temperature is 120°C to 140°C, preferably 120°C. Stirring is carried out at a temperature higher than the melting point of each component for 20 to 30 minutes, so that the components can be fully blended, thereby improving the performance of the entire blend system.

[0044] In some embodiments, blend B is further heated, and powdered filler and nucleating agent are added and stirred to ensure uniform mixing. The heating temperature is 110–120°C, which maintains complete homogeneity of the system, and the stirring time is at least 60 minutes to ensure uniform filler distribution.

[0045] The technical solutions of this application will be further described with more specific embodiments and comparative examples.

[0046] Example 1

[0047] The low-shrinkage investment casting mold material in this embodiment comprises the following components by weight: 30 parts paraffin wax, 30 parts microcrystalline wax, 20 parts petroleum resin, 5 parts ethylene-vinyl acetate copolymer, 10 parts cellulose acetate, 5 parts polymethyl methacrylate, and 0.5 parts dibenzyl sorbitol.

[0048] Example 2

[0049] The low-shrinkage investment casting mold material in this embodiment comprises the following components by weight: 30 parts paraffin wax, 30 parts microcrystalline wax, 20 parts petroleum resin, 10 parts ethylene-vinyl acetate copolymer, 8 parts cellulose acetate, 2 parts polymethyl methacrylate, and 0.5 parts dibenzyl sorbitol.

[0050] Example 3

[0051] The low-shrinkage investment casting mold material in this embodiment comprises the following components by weight: 30 parts paraffin wax, 30 parts microcrystalline wax, 15 parts petroleum resin, 5 parts ethylene-vinyl acetate copolymer, 15 parts cellulose acetate, 5 parts polymethyl methacrylate, and 0.5 parts dibenzyl sorbitol.

[0052] The specific steps for preparing the molding material in Examples 1-3 above are as follows:

[0053] (1) After blending wax and ethylene-vinyl acetate copolymer particles, heat the mixture at 100°C and stir continuously until it melts to obtain blend A.

[0054] (2) Continue heating the blend A obtained in step (1) at 120°C, add petroleum resin and polymethyl methacrylate, and stir continuously to make it evenly mixed with the wax to obtain blend B.

[0055] (3) Continue heating the blend B obtained in step (2) at 110-120°C, slowly add cellulose acetate powder and dibenzyl sorbitol, and continue stirring to make it blend evenly.

[0056] (4) Let stand and cool to room temperature.

[0057] The preparation method is characterized in that the stirring time in steps (1) and (2) is 20-30 min, and the stirring time in step (3) is at least 60 min.

[0058] Comparative Example 1

[0059] This comparative example uses a commonly used medium-temperature investment casting mold material, which is commercially available and is designated as model 996D.

[0060] Comparative Example 2

[0061] The mold material composition in this comparative example includes: 30 parts paraffin wax, 30 parts microcrystalline wax, 15 parts petroleum resin, 5 parts ethylene-vinyl acetate copolymer, 15 parts cellulose acetate, and 0.5 parts dibenzyl sorbitol. The preparation process is as follows:

[0062] (1) After blending wax and ethylene-vinyl acetate copolymer particles, heat the mixture at 100°C and stir continuously until it melts to obtain blend A.

[0063] (2) Continue heating the blend A obtained in step (1) at 120°C, add petroleum resin and stir continuously to make it evenly mixed with the wax to obtain blend B.

[0064] (3) Continue heating the blend B obtained in step (2) at 110-120°C, slowly add cellulose acetate powder and dibenzyl sorbitol, and continue stirring to make it blend evenly.

[0065] (4) Let stand and cool to room temperature.

[0066] Comparative Example 3

[0067] In this comparative example, paraffin wax, microcrystalline wax, petroleum resin, ethylene-vinyl acetate, cellulose acetate, and dibenzyl sorbitol were simultaneously added to a stirrer and stirred continuously at 120°C until they reached a molten state to obtain a new molding material. The content of each component was the same as in Example 1.

[0068] The performance of the mold materials in the above embodiments and comparative examples was tested, and the results are shown in Table 1.

[0069] Table 1. Molding material test results in the examples and comparative examples.

[0070]

[0071] As can be seen from the mold material performance test results in Table 1, compared with the comparative example, the mold material provided by the embodiments of the present invention has a significantly reduced linear shrinkage rate, all less than 0.3%, which can meet the relevant dimensional accuracy requirements of precision castings in the aerospace field. The softening point of the mold material in the comparative example is slightly lower than that in the embodiments, while the softening point in the embodiments of the present invention is maintained at around 60℃, making the mold material less prone to thermal deformation and easier to demold; the lower ash content makes it less likely for impurities to be added to the alloy, improving the quality of the casting. Penetration refers to the depth to which a 100g standard needle penetrates the surface of the mold material within 5 seconds at 25℃. A smaller penetration value indicates a higher surface hardness and mechanical strength. The penetration values ​​of the embodiments are generally smaller than those of the comparative example, indicating that the mold material in the embodiments has a greater surface hardness and higher strength, especially higher than the penetration value of the commonly used medium-temperature mold material in Comparative Example 1, demonstrating the advantages of the mold material provided by the embodiments of the present invention in terms of mechanical strength.

[0072] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention. The above preferred features can be used in any combination without conflict.

Claims

1. A method for preparing a low-shrinkage investment casting mold material, characterized in that, include: The wax and polymer are blended and heated, and stirred until they reach a molten state to obtain blend A; wherein: the heating temperature is 95-120℃, and the stirring time is 20-30 min; The blend A is further heated, and petroleum resin and additives are added and stirred to obtain blend B; wherein: the heating temperature is 120-140℃, and the stirring time is 20-30min; The blend B is further heated, and powdered filler and nucleating agent are added and stirred to make it blend evenly; wherein: the heating temperature is 110-120℃, and the stirring time is at least 60min; Let it stand, then cool it to room temperature to obtain a low-shrinkage investment casting mold material; By weight, the molding material comprises: 40-60 parts wax, 15-30 parts petroleum resin, 5-10 parts polymer, 5-15 parts powder filler, 5 parts additives, and 0.5-1 part nucleating agent. The wax is selected from any one or more of microcrystalline wax, candelilla wax and carnauba wax; The polymer is an ethylene-vinyl acetate copolymer; The additive is polymethyl methacrylate; The powder filler is cellulose acetate powder; The nucleating agent is dibenzyl sorbitol; The linear shrinkage rate of the mold material is less than 0.3%, and the mold material is used in precision casting in the aerospace field.

2. The method for preparing low-shrinkage investment casting mold material according to claim 1, characterized in that, The petroleum resin is selected from C5 hydrogenated petroleum resin or C9 hydrogenated petroleum resin.

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