An inorganic 3D printing sand mold and its preparation process

Abstract

This invention relates to an inorganic 3D printing sand mold and its preparation process. The preparation process includes the following steps: Step 1: A layer of sand powder, which is a mixture of silicate curing agent and fine-tuning agent, is spread on the printing platform; Step 2: The print head sprays a binder onto the surface of the sand powder according to the three-dimensional data of the digital model; Step 3: The printing platform descends by one layer thickness, which is 0.2-0.5 mm; Step 4: Steps 1 to 3 are repeated; Step 5: After the molding box is removed from the printing equipment, it is pushed into a microwave + hot air furnace for curing according to the process time; Step 6: After the molding box is removed from the micro furnace, it is transferred to the sand cleaning workbench for sand cleaning treatment, and the inorganic 3D printing sand mold is completed. This invention adjusts the performance of the sand mold to meet the production needs of different working conditions and product structures; solves the problems of thermal deformation and dimensional deviation after sand mold printing; and solves the problems of difficult sand mold storage, moisture absorption during use, and unstable sand mold performance.

Description

Technical Field

[0001] This invention belongs to the field of 3D printing technology and relates to an inorganic 3D printing sand mold and its preparation process. Background Technology

[0002] Inorganic binder sand molds are increasingly widely used in the foundry industry and have made significant contributions to 3D printing and its applications. However, inorganic 3D printing technology is still a world-class advanced technology. Apart from its validation at BMW in Germany, this production technology is currently unavailable in China or even throughout Asia. Therefore, the following technical challenges exist during the production validation process:

[0003] 1. Because inorganic 3D printing sand molds lack strength immediately after printing, the sand box may deform during movement. Furthermore, after the sand mold is dried and hardened by microwave hot air, the product shrinks as the moisture in the binder evaporates and the binder hardens, affecting the final dimensions of the sand mold.

[0004] 2. The inorganic binder is mainly water glass, and the sand mold products produced are prone to moisture absorption. The performance of the products will be affected by the different storage environment and storage time.

[0005] 3. Since it is printed layer by layer by inkjet, the drawing input into the 3D printing device is in STL format. The parameter settings of the format, the placement of the sand mold in the printing equipment, the parameters of the inkjet volume, etc. will also affect the performance of the product.

[0006] 4. 3D printed sand molds are self-hardening, while the inorganic 3D printed sand cores mentioned in this invention require microwave hot air drying to harden. The hardening process also affects the performance of the sand cores. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the above-mentioned problems existing in the prior art and to provide an inorganic 3D printing sand mold and its preparation process.

[0008] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0009] To solve the above-mentioned technical problems, the present invention is implemented using the following technical solution:

[0010] An inorganic 3D printing sand mold preparation process includes the following steps:

[0011] Step 1: Spread a layer of sand powder on the printing platform. The sand powder is a mixture of silicate curing agent and fine-tuning agent.

[0012] Step 2: The print head selectively sprays adhesive onto the surface of the sand powder based on the three-dimensional data of the digital model;

[0013] Step 3: Lower the printing platform by one layer thickness, which is 0.2-0.5mm;

[0014] Step 4: Repeat steps 1 through 3;

[0015] Step 5: After the molding box is removed from the printing equipment, it is pushed into the microwave + hot air oven, the oven door is closed, and it is cured according to the process time.

[0016] Step 6: After the molding box is removed from the micro furnace, it is transferred to the sand cleaning workbench for sand cleaning. The inorganic 3D printing sand mold printing is completed.

[0017] Furthermore, based on the scanning results of the test blocks and products, it was finally determined that the reduction ratio was 0.4%-0.6% for test blocks with a length ≥200mm and 0.2%-0.4% for test blocks with a length ≤200mm.

[0018] Furthermore, the compensation amount for the printing equipment parameters is compensated with negative tolerance in the X and Y axes, with a compensation range of -0.2 to -0.1 mm, while no compensation is required for the Z axis.

[0019] Furthermore, when converting the printing sand mold to STL format, the chord height is less than or equal to the thickness of the printing layer.

[0020] Furthermore, the amount of inorganic binder added is: 0.068-0.070 for inkjet volume and 0.26-0.30 mm for inorganic binder layer thickness.

[0021] Furthermore, the microwave drying hot air temperature is 100-120℃.

[0022] Furthermore, the microwave dehumidification frequency is 20-30Hz.

[0023] Furthermore, the microwave frequency is 15-25 Hz, and the microwave duration is 2500-3000 seconds.

[0024] Furthermore, when the ambient temperature is 20±5℃ and the ambient humidity is 20±10%, the sand mold placement time is ≤3 days.

[0025] An inorganic 3D printing sand mold is prepared using the preparation process described above.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] 1. This invention establishes an inorganic 3D printing sand mold and its preparation process, which differs from the traditional inorganic core process and the process flow of organic 3D printing sand mold. Based on the performance characteristics of the raw materials and the performance requirements of the produced sand mold, a new set of process steps has been formulated.

[0028] 2. Based on the inorganic 3D printing sand molds prepared according to the process steps, compare different process schemes and adjust the performance of the sand molds to meet the production needs of different working conditions and product structures;

[0029] 3. Due to the characteristics of inorganic 3D printing sand mold process, the problems of thermal deformation and dimensional deviation after sand mold printing were solved through process verification;

[0030] 4. Solves problems such as difficulty in storing sand molds, moisture absorption during use, and unstable performance of sand molds. Attached Figure Description

[0031] The invention will now be further described with reference to the accompanying drawings:

[0032] Figure 1 This is a flow chart of the inorganic 3D printing sand mold preparation process described in this invention;

[0033] Figure 2a The first result of a 3D scan of the sand mold printed from the original image;

[0034] Figure 2b The second result of the 3D scanning of the sand mold printed from the original image;

[0035] Figure 3a The result of a 3D scan of a sand mold printed at a scale of 0.6% or more;

[0036] Figure 3b The second result is a 3D scan of a sand mold printed at a scale of 0.6% or more. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of the embodiments of this invention will be described in more detail below with reference to the accompanying drawings. In the drawings, the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The described embodiments are some, but not all, embodiments of this invention. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this invention, and should not be construed as limiting the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. The embodiments of this invention will be described in detail below with reference to the accompanying drawings.

[0038] In the description of this invention, it should be understood that the terms "center", "longitudinal", "lateral", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the scope of protection of this invention.

[0039] The present invention will now be described in detail with reference to the accompanying drawings:

[0040] First, the required conditions for the inorganic 3D printing sand mold preparation process are analyzed and demonstrated:

[0041] 1. Product dimensions:

[0042] ① After printing, the sand mold needs to be microwave-dried to harden and shape the product. However, during the hardening process, the sand mold's dimensions may deviate from the final printed dimensions due to moisture loss and binder cross-linking. Therefore, the 3D data model input before printing needs to be scaled down to the original product dimensions to ensure dimensional accuracy.

[0043] ② In the sand mold 3D printing process, sand is laid layer by layer on the surface of the sand box. Then, the print head sprays binder onto the surface of each layer of sand particles according to the three-dimensional data. However, the binder is fluid, and there are gaps between the sand particles. During the spraying process, non-forming parts can seep into the binder, causing deviations in the sand mold dimensions. Therefore, it is necessary to design a reasonable compensation amount for the equipment based on the forming principle and material properties of 3D printed sand molds to ensure the dimensional accuracy of the sand mold.

[0044] ③ The graphic files input into the 3D printing equipment are in STL format, a universal format for saving 3D models that can adapt to different software and devices. When exporting the STL file, chord height is an important parameter; it represents the maximum deviation between the model's surface and its surface in the actual modeling software. Simply put, chord height is the model's precision, determining the product's accuracy and precision.

[0045] 2. Sand mold properties:

[0046] Printing process: The choice of resolution has a significant impact on the quality and detail of the printed model. Higher resolution allows for the printing of more detailed models, while lower resolution may result in an uneven surface and poor formability.

[0047] Microwave drying process: Inorganic 3D sand molds differ from organic 3D sand molds. Organic 3D sand molds have a curing agent mixed on the surface of the sand particles, and then the mold hardens itself after the binder is sprayed. However, inorganic 3D sand molds only have additives and fine-tuning agents on the surface of the sand particles, and the sand mold itself does not harden. Therefore, after the product outline is formed by spraying the binder, the moisture in the binder needs to be heated and dried to harden the sand mold.

[0048] Hot air curing alone not only affects the curing speed, but also only achieves sufficient curing on the surface of the sand mold, with poor penetration. Therefore, it is necessary to introduce microwave technology, which not only ensures sufficient curing inside the sand mold, but also improves curing efficiency by combining it with hot air surface drying, effectively drying the moisture in the sand mold. However, improper use of microwave and hot air processes can affect the properties of the sand mold.

[0049] Inorganic binders are water-soluble binders, and inorganic sand molds have strong hydrophilicity. As mentioned above, the hardening process of inorganic 3D printing sand molds is essentially a moisture drying process. If the dried sand mold reaches the target performance but cannot be used immediately, and moisture absorption occurs during storage, it will lead to a decline in the performance of the sand core or even aging. Therefore, the storage environment and time of the sand mold have a crucial impact on its performance.

[0050] 3D printing technology (3DP) utilizes equipment that primarily consists of a work chamber, a sand spreader, and a print head. The work chamber's bottom can move vertically. Before printing begins, the work chamber must be raised to its highest point. The sand spreader then vibrates to spread sand powder layer by layer onto the bottom of the work chamber. Next, the print head's nozzles spray a binder (such as inorganic binders or self-hardening resins) layer by layer onto the sand powder surface, following the cross-sectional structure of the product drawing. Each cycle completes one layer of the sand mold. The thickness of each layer can be adjusted according to the angle of the sand spreader, typically 0.2-0.3 mm. Each layer takes 10-15 seconds to print. The height of the printed product determines the number of layers (e.g., ...). Figure 1 (As shown).

[0051] The inorganic binder used in inorganic 3D printing sand molds is modified water glass. After printing, the sand mold cannot harden on its own to generate strength. The printing work box needs to be pushed out of the printing equipment and put into a microwave drying oven to dry and evaporate the moisture of the water glass binder before it can harden and form.

[0052] The factors affecting the size of the sand mold mainly occur during the printing stage and the process of pushing it into the microwave drying oven after printing. In addition, the inorganic binder sand mold containing moisture will shrink in size during the microwave curing process, which will also cause the size of the product to deviate from the size of the product drawing in the printing process.

[0053] Factors affecting dimensional deviations during printing: the distance between the sand spreader and the sand mold surface, the amount of sand box sinking after each layer is printed, and the distance between the print head and the sand mold surface.

[0054] Factors affecting dimensional deviations after printing: the transfer track from the printing equipment to the microwave oven, the maximum outline size of the product, and the shrinkage ratio of the sand mold.

[0055] After identifying the above influencing factors, the optimal solution was determined through factor-level experiments, and the best preparation process was determined through continuous and repeated verification of stability.

[0056] The key factors affecting the strength of sand molds are mainly: the amount of binder added to inorganic 3D printed sand molds directly affects their strength; furthermore, the sand molds require microwave curing after printing, and the curing process also affects their strength and performance. Additionally, inorganic binders are highly hygroscopic and therefore quite sensitive to environmental conditions.

[0057] Factors affecting the amount of inorganic binder added: inkjet resolution and layup thickness.

[0058] Factors affecting microwave curing include: microwave duration, microwave duration distribution, dehumidification frequency, microwave frequency, and hot air drying temperature.

[0059] Environmental factors: humidity and temperature.

[0060] Based on the above analysis, an embodiment of the present invention is provided, a process for preparing inorganic 3D printing sand molds, comprising the following steps:

[0061] Step 1: Spread a layer of sand powder on the printing platform. The sand powder is a mixture of silicate curing agent and fine-tuning agent prepared before printing. Step 2: The print head selectively sprays binder onto the sand powder surface based on the 3D data of the digital model. Step 3: The printing platform descends by one layer thickness, 0.2-0.5mm. Step 4: Repeat steps 1 to 3. Step 5: After the molding box is removed from the printing equipment, it is pushed into the microwave + hot air furnace. The furnace door is closed, and curing is performed according to the process time. Step 6: After the molding box is removed from the micro-furnace, it is transferred to the sand-cleaning worktable for sand removal. The inorganic 3D printing sand mold printing is complete.

[0062] The specific implementation includes the following:

[0063] 1. First, print a 300×200×100mm test block according to the original dimensions. After printing, use calipers to measure the dimensional accuracy in all directions. Simultaneously, compare the accuracy of the sand mold printed according to the three-dimensional dimensions with the original dimensions. Figure 2a , Figure 2b ).

[0064] 2. Based on the scanning results of the test blocks and products, the final reduction is determined to be 0.4%-0.6% for lengths ≥200mm and 0.2%-0.4% for lengths ≤200mm. The compensation for equipment parameters is as follows: negative tolerance compensation is applied to the X and Y axes, with a compensation range of -0.2 to -0.1mm; when no compensation is required for the Z axis, the dimensional accuracy can reach ±0.3mm.

[0065] 3. After determining the scale and final compensation amount, repeat printing is performed, followed by 3D scanning measurement for comparison and verification. Figure 3a , Figure 3b This can meet the accuracy requirements.

[0066] 4. Influence of Printing Parameters on Tensile Strength: Since only two factors affect strength during the printing process, tensile strength results were tested based on the adjustment range of different printing parameters. The printer resolution was set with a lower limit of 0.06, an upper limit of 0.072, and a middle limit of 0.066; layer thicknesses were fixed at 0.25mm, 0.30mm, and 0.35mm. Through repeatable analysis, when the resolution was ≤0.068 and the layer thickness was ≤0.26mm, the strength did not increase regardless of the microwave time and temperature adjustments; instead, it decreased. The optimal strength was achieved at a resolution of 0.07 and a layer thickness of 0.30mm. Increasing the resolution and layer thickness resulted in unstable strength (Table 1).

[0067] 5. To determine the influence of microwave parameters on tensile strength, an orthogonal analysis table of microwave parameters was developed, as shown in Tables 2-1 and 2-2. A four-factor, three-level experimental scheme was formulated. The "8"-shaped pattern was laid out according to GB / T2684. Inorganic 3D printing was performed with a resolution of 0.07% and a layer thickness of 0.30 mm. The tensile strength output results under different microwave process parameters were compared to determine the optimal microwave scheme.

[0068] 6. The optimal process scheme was finally determined: hot air temperature of 100℃, dehumidification frequency of 30Hz, microwave frequency of 15Hz, and microwave duration of 3000 seconds yielded the best performance. Repeatability tests were conducted, and this scheme achieved a tensile strength of 2.28MPa. According to the orthogonal analysis table, the dehumidification frequency and microwave frequency had the greatest impact on the sand mold. Adjusting the level data of each factor revealed that when the tensile strength dropped below 1.5MPa, production requirements could not be met. The final microwave process parameter range was determined when the tensile strength was ≥1.5MPa.

[0069] 7. Verify the influence of different ambient temperatures and storage times on the tensile strength of sand molds. A three-factor, two-level analysis scheme was developed, considering ambient humidity, temperature, and storage time (Tables 3-1, 3-2, 3-3, and 3-4). Through experiments, the optimal control scheme was determined to be 10% relative humidity, 15℃ ambient temperature, and 3 days of use. When storage time and ambient temperature and humidity were increased, as mentioned in point 6, the tensile strength ≤1.5MPa could not meet production requirements, thus establishing a reasonable process range.

[0070] Table 1. Orthogonal analysis table of the influence of printing parameters on strength.

[0071]

[0072] Table 2-1 Initial Orthogonal Analysis Table and Visual Analysis Table of Microwave Parameters (I)

[0073]

[0074] Table 2-2 Initial Orthogonal Analysis Table and Visual Analysis Table of Microwave Parameters (II)

[0075]

[0076]

[0077] Table 3-1 Effects of Different Storage Environments on Sand Mold Properties (Part 1)

[0078]

[0079] Table 3-2 The Influence of Different Storage Environments on Sand Mold Properties (Part 2)

[0080]

[0081] Table 3-3 The Influence of Different Storage Environments on Sand Mold Properties (Part 3)

[0082]

[0083] Table 3-4 The Influence of Different Storage Environments on Sand Mold Properties (Part 4)

[0084]

[0085] The present invention provides another embodiment of an inorganic 3D printing sand mold, which is prepared using the aforementioned inorganic 3D printing sand mold preparation process.

[0086] Step 1: Spread a layer of powder on the printing platform. The powder is a mixture of silicate curing agent and fine-tuning agent.

[0087] Step 2: The print head selectively sprays adhesive based on the three-dimensional data of the digital model;

[0088] Based on the scanning results of the test blocks and products, the reduction ratio was ultimately determined to be 0.4%-0.6% for test blocks with a length ≥200mm and 0.2%-0.4% for test blocks with a length ≤200mm.

[0089] The compensation for equipment parameters is based on negative tolerances in the X and Y axes, with a compensation range of -0.2 to -0.1 mm. No compensation is required for the Z axis.

[0090] Inorganic binder addition: inkjet volume 0.068-0.070, layer thickness 0.26-0.30mm.

[0091] Step 3: Lower the printing platform by one layer thickness, which is 0.2-0.5mm;

[0092] When converting a sand pattern to STL format, the chord height is less than or equal to the thickness of the printed layer.

[0093] Step 4: Repeat steps 1 through 3;

[0094] Step 5: Microwave curing;

[0095] Microwave drying hot air temperature is 100-120℃.

[0096] Microwave dehumidification frequency: 20-30Hz.

[0097] Microwave frequency 15-25Hz, microwave duration 2500-3000 seconds.

[0098] Step 6: Cleaning complete.

[0099] When the ambient temperature is 20±5℃ and the ambient humidity is 20±10%, the sand core should be placed for ≤3 days.

[0100] The required sand mold is obtained through the above preparation process.

[0101] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention, and within the spirit and principles of the present invention, should be included within the scope of protection of the present invention. Furthermore, all content not described in detail in this specification is prior art known to those skilled in the art.

Claims

1. An inorganic 3D printing sand mold preparation process, characterized in that, Includes the following steps: Step 1: Spread a layer of sand powder on the printing platform. The sand powder is a mixture of silicate curing agent, fine-tuning agent and sand particles. Step 2: The print head selectively sprays adhesive onto the surface of the sand powder based on the three-dimensional data of the digital model; Step 3: Lower the printing platform by one layer thickness; Step 4: Repeat steps 1 through 3; Step 5: After the molding box is removed from the printing equipment, it is pushed into the microwave + hot air oven, the oven door is closed, and it is cured according to the process time. Step 6: After the molding box is removed from the microwave oven chamber, it is transferred to the sand cleaning workbench for sand cleaning. The inorganic 3D printing sand mold printing is completed. For dimensions with a length ≥ 200mm, set the scaling factor to 0.4%-0.6%; for dimensions with a length ≤ 200mm, set the scaling factor to 0.2%-0.4%. The compensation amount for the printing equipment parameters is compensated with negative tolerance in the X and Y axes, with a compensation range of -0.2 to -0.1 mm. No compensation is required for the Z axis. When converting a sand pattern to STL format, the chord height is less than or equal to the thickness of the printed layer. The printing parameters are: resolution 0.068-0.070, layer thickness 0.26-0.30mm; Drying hot air temperature: 100-120℃; Dehumidification frequency 20-30Hz; Microwave frequency 15-25Hz, microwave duration 2500-3000 seconds.

2. The inorganic 3D printing sand mold preparation process according to claim 1, characterized in that: When the ambient temperature is 20±5℃ and the ambient humidity is 20±10%, the sand mold should be left to stand for ≤3 days.

3. An inorganic 3D printing sand mold, characterized in that: Prepared using the preparation process described in either claim 1 or 2.

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