Inorganic 3D printing sand mold and preparation process thereof

Abstract

The invention relates to an inorganic 3D printing sand mold and a preparation process thereof. The preparation technology comprises the following steps that firstly, a layer of sand powder is laid on a printing platform, and the sand powder is a mixture of a silicate curing agent and a micro-adjusting agent; step 2, the printing head sprays a binder on the surface of the sand powder according to the three-dimensional data of the digital model; thirdly, the printing platform descends by one layer thickness, and the layer thickness ranges from 0.2 mm to 0.5 mm; step 4, repeating the process from step 1 to step 3; 5, the forming box is moved out of the printing equipment and then pushed into a microwave and hot air hearth, and curing is conducted according to the process time; and 6, after being taken out of the micro-thin hearth, the forming box is transferred to a sand removal workbench for sand removal treatment, and printing of the inorganic 3D printing sand mold is completed. The performance of the sand mold is adjusted, and the production requirements of different working conditions and product structures are met; the problems of thermal deformation and size out-of-tolerance after the sand mold is printed are solved; the problems of difficult sand mold storage, moisture absorption in use and unstable sand mold performance are solved.

Description

Technical Field

[0001] The invention belongs to the technical field of 3D printing and relates to an inorganic 3D printing sand mold and a preparation process thereof. Background Art

[0002] Inorganic binder sand molds are increasingly used in the foundry industry, making significant contributions to 3D printing and its application fields. However, inorganic 3D printing technology is still a world-class advanced technology. Except for the verification in BMW, Germany, there is no such production technology in China or even in Asia. Therefore, there are many technical difficulties in the production verification process:

[0003] 1. Since the inorganic 3D printing sand mold has no strength after printing, the sand box may be deformed during the movement. The sand mold is dried and hardened by microwave hot air. As the moisture in the binder evaporates, the product will shrink during the hardening process of the binder, affecting the final size of the sand mold.

[0004] 2. The inorganic binder is mainly water glass, and the sand mold products produced are easy to absorb moisture. Different placement environment and placement time will affect product performance.

[0005] 3. Since it is printed layer by layer by inkjet printing, the drawing input into the 3D printing design 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 and other factors will also affect the performance of the product.

[0006] 4. 3D printing sand mold is a self-hardening method. The inorganic 3D printing sand core mentioned in the present invention needs to be dried by microwave hot air to achieve hardening. The hardening process also affects the performance of the sand core. 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 provide an inorganic 3D printing sand mold and a preparation process thereof.

[0008] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0009] In order to solve the above technical problems, the present invention is implemented by adopting the following technical solutions:

[0010] An inorganic 3D printing sand mold preparation process comprises 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 a binder on the surface of the sand powder according to the three-dimensional data of the digital model;

[0013] Step 3: The printing platform drops one layer thickness, the layer thickness is 0.2-0.5mm;

[0014] Step 4: Repeat the process from step 1 to step 3;

[0015] 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 the molding box is cured according to the process time;

[0016] Step 6. After the molding box is taken out from the micro-thin furnace, it is transferred to the sand cleaning workbench for sand cleaning, and the inorganic 3D printing sand mold is printed.

[0017] Furthermore, based on the test block and product scanning results, it was finally determined that the reduction scale for test block lengths ≥ 200 mm was 0.4%-0.6%, and the reduction scale for test block lengths ≤ 200 mm was 0.2%-0.4%.

[0018] Furthermore, the compensation amount of the printing device parameters is compensated according to the negative tolerance in the X-axis and Y-axis directions, with a compensation range of -0.2 to -0.1 mm, and the Z-axis does not need compensation.

[0019] Furthermore, the chord height when the printed sand mold is converted into Stl format is ≤ the thickness of the printed layer.

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

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

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

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

[0024] Furthermore, when the ambient temperature is 20±5°C 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 present invention has the following beneficial effects:

[0027] 1. The present invention develops an inorganic 3D printing sand mold and its preparation process, which is different from the traditional inorganic core making process and the process flow of organic 3D printing sand mold. According to the performance characteristics of the raw materials and the performance requirements of the produced sand mold, a set of process steps are re-developed;

[0028] 2. Prepare the inorganic 3D printing sand mold according to the process steps, compare different process schemes, and adjust the performance of the sand mold to meet the production requirements of different working conditions and product structures;

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

[0030] 4. Solve the problems of difficult storage of sand molds, moisture absorption during use, and unstable sand mold performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be further described below in conjunction with the accompanying drawings:

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

[0033] Figure 2a The 3D scanning result of the sand mold printed from the original image is 1;

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

[0035] Figure 3a The three-dimensional scanning result of the sand mold printed at a scale of more than 0.6% is shown in the figure 1;

[0036] Figure 3b The second is the 3D scanning result of the sand mold printed at a scale of more than 0.6%. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical scheme and advantages of the implementation of the present invention clearer, the technical scheme in the embodiment of the present invention will be described in more detail below in conjunction with the drawings in the embodiment of the present invention. In the drawings, the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The described embodiments are part of the embodiments of the present invention, not all of the embodiments. The embodiments described below with reference to the drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limitations on the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. The embodiments of the present invention are described in detail below in conjunction with the drawings.

[0038] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the scope of protection of the present invention.

[0039] The present invention is described in detail below in conjunction with the accompanying drawings:

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

[0041] 1. Product size:

[0042] ① After printing, the sand mold needs to be dried in a microwave oven to achieve hardening and molding of the product. However, during the hardening and molding process, the sand mold will lose water and cross-link the binder, which will cause the sand mold size to deviate from that after printing. Therefore, the 3D data model input before printing needs to increase the process scale based on the original product size to ensure dimensional accuracy.

[0043] ② During the sand mold 3D printing process, sand is spread layer by layer on the surface of the sand box, and then the print head sprays the binder on the surface of each layer of sand grains according to the three-dimensional data. The binder is fluid, and there are gaps between the sand grains. During the spraying process, non-molding parts will penetrate into the binder, resulting in sand mold size deviation. Therefore, it is necessary to design a reasonable compensation amount of the equipment according to the molding principle and material characteristics of the 3D printing sand mold to ensure the size accuracy of the sand mold.

[0044] ③ The graphic file input into the 3D printing device is in STL format, which is a universal format for saving 3D models and can adapt to different software and equipment. When exporting STL files, chord height is an important parameter, which is the maximum deviation between the surface of the model and its surface in the actual modeling software. In simple terms, chord height is the precision of the model, which determines the precision and accuracy of the product.

[0045] 2. Sand mold performance:

[0046] Printing process: The choice of resolution has an important impact on the quality and detail of the printed model. A higher resolution can print a more detailed model, while a lower resolution may result in an uneven surface and poor formability.

[0047] Microwave drying process: Inorganic 3D sand molds are different from organic 3D sand molds. Organic 3D is formed by mixing a curing agent on the surface of sand particles, and then spraying a binder to harden the mold. Inorganic 3D sand molds only have additives and fine-tuning agents on the surface of sand particles, which will not harden the sand mold. Therefore, after the inorganic 3D printing sand mold sprays the binder to form the product outline, the moisture in the binder needs to be heated and dried to achieve hardening of the sand mold.

[0048] Hot air hardening alone not only affects the hardening speed, but also can only fully harden the surface of the sand mold, and has poor penetration. Therefore, it is necessary to introduce microwave technology, which can not only ensure that the inside of the sand mold is fully hardened, but also combine with hot air surface drying to improve the hardening efficiency and dry out the moisture in the sand mold. However, unreasonable microwave and hot air processes will affect the performance of the sand mold.

[0049] Inorganic binder is a water-soluble binder. Inorganic sand molds are highly hydrophilic. As mentioned above, the hardening process of inorganic 3D printing sand molds is the process of drying water. If the sand molds reach the target performance after drying, they cannot be used immediately. If moisture is absorbed during the storage process, the sand core performance will be reduced or even expired. Therefore, the environment and time of sand mold storage are also crucial to the performance of sand molds.

[0050] The 3D printing equipment used in 3D printing technology (3DP technology for short) mainly includes core components such as a work box, a sand spreader, and a print head. The bottom surface of the printing work box can move up and down. Before printing begins, the plane of the work box needs to rise to the top, and then the sand spreader spreads the sand powder in the sand spreader layer by layer on the bottom surface of the work box by vibrating the sand, and then the adhesive (such as inorganic binders or self-hardening resins and other materials) is sprayed layer by layer on the surface of the sand powder through the nozzle of the print head according to the cross-sectional structure of the product drawing. The whole process runs once, completing the printing process of a layer of sand mold. The thickness of each layer of sand mold can be adjusted according to the angle of the sand spreader. Generally, the thickness of each layer of sand mold is 0.2-0.3mm, and the printing time of each layer of sand mold is 10-15 seconds. The height of the printed product determines the number of layers to be printed (such as Figure 1 shown).

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

[0052] The factors affecting the sand mold size mainly occur in the printing stage and in the process of being pushed into the microwave drying oven after printing. In addition, the inorganic binder sand mold containing moisture that has just been printed will shrink in size during the microwave hardening process, which will also cause the product size to deviate from the product image size during the printing process.

[0053] Factors that affect dimensional deviation during printing: the distance between the sand spreader and the sand mold surface, the size of the sand box sinking after each layer of printing is completed, and the distance between the print head and the sand mold surface.

[0054] Factors that affect dimensional deviation after printing are: the transfer track of the work box 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 is determined through the factor level test method, and then the best preparation process is determined through continuous and repeated stability verification.

[0056] The key factors affecting the strength of the sand mold are that the amount of binder added to the inorganic 3D printing sand mold directly affects the strength of the sand mold, and the sand mold needs to be cured by microwave after printing, and the curing process will also affect the strength and performance of the sand mold. In addition, the inorganic binder has strong moisture absorption and is sensitive to the environment.

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

[0058] Factors affecting microwave hardening: microwave time, microwave time distribution, dehumidification frequency, microwave frequency and hot air drying temperature.

[0059] Environmental factors: humidity and temperature.

[0060] In combination with the above analysis, an embodiment of the present invention is provided, a process for preparing an inorganic 3D printing sand mold, 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 a binder on the surface of the sand powder according to the three-dimensional data of the digital model; Step 3: The printing platform drops a layer thickness of 0.2-0.5mm; Step 4: Repeat the process from step 1 to step 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 it is cured according to the process time; Step 6: After the molding box is taken out of the micro-thin furnace, it is transferred to the sand cleaning workbench for sand cleaning, and the inorganic 3D printing sand mold is printed;

[0062] The specific implementation includes the following:

[0063] 1. First, print a 300×200×100mm test block according to the original size. After printing, use a caliper to measure the dimensional accuracy in all directions. At the same time, compare the original size sand mold accuracy based on the 3D size printing ( Figure 2a , Figure 2b ).

[0064] 2. According to the scanning results of the test block and the product, it is finally determined that when the length is ≥200mm, the scale is 0.4%-0.6%, and when the length is ≤200mm, the scale is 0.2%-0.4%. The compensation amount of the equipment parameters, the X-axis and Y-axis directions are compensated according to the negative tolerance, the compensation range is -0.2~-0.1mm, and when the Z-axis does not need compensation, the dimensional accuracy can reach ±0.3mm.

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

[0066] 4. The influence of printing parameters on tensile strength: Since there are only two factors that affect the strength during the printing process. Therefore, the tensile strength results are tested according to the adjustment range of different printing parameters. The lower limit of the printer's resolution is 0.06, the upper limit is 0.072, and the middle limit is set to 0.066; the layer thickness is fixed at 0.25mm, 0.30mm, and 0.35mm for adjustment tests. Through repetitive analysis tests, when the resolution is ≤0.068 and the layer thickness is ≤0.26mm, no matter how the microwave time and temperature are adjusted, the intensity will not increase, but will decrease; when the resolution is 0.07 and the layer thickness is 0.30mm, the optimal level can be achieved. Increasing the resolution and increasing the layer thickness will cause unstable strength (Table 1)

[0067] 5. In order to determine the influence of microwave parameters on tensile strength, an orthogonal analysis table of microwave parameters was developed, as shown in Table 2-1 and Table 2-2. A four-factor three-level test plan was developed, and the layout was carried out according to the "8" block specified in GB / T2684. The inorganic 3D printing process parameters were printed according to the resolution of 0.07% and the layer thickness of 0.30mm. The tensile strength output results under different microwave process parameters were compared to determine the best microwave solution.

[0068] 6. Finally determine the best process plan: hot air temperature 100℃, dehumidification frequency 30HZ, microwave frequency 15HZ, microwave time 3000 seconds, the performance is the best; and repeat the time, the tensile strength of this plan can reach 2.28MPa. According to the orthogonal analysis table, it is confirmed that the dehumidification frequency and microwave frequency have the greatest impact on the sand mold. Adjust the horizontal data of each factor. When the tensile strength drops below 1.5MPa, it cannot meet the production requirements. Finally determine the range of microwave process parameters when the tensile strength is ≥1.5MPa.

[0069] 7. Verify the influence of different ambient temperatures and storage time on the tensile strength of sand molds. Develop a 3-factor 2-level analysis plan for ambient humidity, temperature and storage time (Table 3-1, Table 3-2, Table 3-3, Table 3-4). Through experiments, it is finally determined that when the relative humidity of the environment is 10%, the ambient temperature is 15°C, and the best control plan is used after 3 days. When the storage time and ambient temperature and humidity are increased, as described in 6, the tensile strength is ≤1.5MPa, which can no longer meet the production requirements, and the final reasonable process range.

[0070] Table 1 Orthogonal analysis of the effect of printing parameters on strength

[0071]

[0072] Table 2-1 Initial orthogonal analysis table and intuitive analysis table of microwave parameters (I)

[0073]

[0074] Table 2-2 Initial orthogonal analysis table and intuitive analysis table of microwave parameters (II)

[0075]

[0076]

[0077] Table 3-1 Effects of different storage environments on sand mold performance (I)

[0078]

[0079] Table 3-2 Effects of different storage environments on sand mold performance (II)

[0080]

[0081] Table 3-3 Effects of different storage environments on sand mold performance (III)

[0082]

[0083] Table 3-4 Effects of different storage environments on sand mold performance (IV)

[0084]

[0085] The present invention provides another embodiment, an inorganic 3D printing sand mold, which is prepared using the 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 the adhesive according to the three-dimensional data of the digital model;

[0088] According to the scanning results of the test block and product, it is finally determined that the reduction scale is 0.4%-0.6% when the test block length is ≥200mm, and the reduction scale is 0.2%-0.4% when the test block length is ≤200mm.

[0089] The compensation amount of the equipment parameters is compensated according to the negative tolerance in the X-axis and Y-axis directions, with a compensation range of -0.2 to -0.1mm, and the Z-axis does not require compensation.

[0090] The amount of inorganic binder added: the inkjet amount is 0.068-0.070, and the layer thickness is 0.26-0.30mm.

[0091] Step 3: The printing platform drops one layer thickness, the layer thickness is 0.2-0.5mm;

[0092] The chord height when the printed sand mold is converted to Stl format is ≤ the thickness of the printed layer.

[0093] Step 4: Repeat the process from step 1 to step 3;

[0094] Step 5: Microwave curing;

[0095] The hot air temperature of microwave drying is 100-120℃.

[0096] Microwave dehumidification frequency is 20-30HZ.

[0097] Microwave frequency 15-25HZ, microwave time 2500-3000.

[0098] Step 6. Cleaning is complete.

[0099] When the ambient temperature is 20±5℃ and the ambient humidity is 20±10%, the sand core placement time is ≤3 days.

[0100] Through the above preparation process, the required sand mold is prepared.

[0101] The above is only a specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any modification, equivalent replacement and improvement made by any person skilled in the art within the technical scope disclosed by the present invention and within the spirit and principle of the present invention shall be covered within the protection scope of the present invention. At the same time, the contents not described in detail in this specification belong to the prior art known to those skilled in the art.

Claims

1. An inorganic 3D printing sand mold preparation process, characterized in that: The following steps are involved: 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. Step 2: The print head selectively sprays a binder on the surface of the sand powder according to the three-dimensional data of the digital model; Step 3: The printing platform drops one layer thickness, the layer thickness is 0.2-0.5mm; Step 4: Repeat the process from step 1 to step 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 the molding box is cured according to the process time; Step 6. After the molding box is taken out from the micro-thin furnace, it is transferred to the sand cleaning workbench for sand cleaning, and the inorganic 3D printing sand mold is printed.

2. The process for preparing an inorganic 3D printing sand mold according to claim 1, characterized in that: According to the scanning results of the test block and product, it is finally determined that the reduction scale is 0.4%-0.6% when the test block length is ≥200mm, and the reduction scale is 0.2%-0.4% when the test block length is ≤200mm.

3. The process for preparing an inorganic 3D printing sand mold according to claim 1, characterized in that: Print the compensation amount of the equipment parameters, the X-axis and Y-axis directions are compensated according to the negative tolerance, the compensation range -0.2~-0.1mm, Z axis does not require compensation.

4. The inorganic 3D printing sand mold preparation process according to claim 1, characterized in that: The chord height when the printed sand mold is converted to Stl format is ≤ the thickness of the printed layer.

5. The inorganic 3D printing sand mold preparation process according to claim 1, characterized in that: The amount of inorganic binder added: the inkjet amount is 0.068-0.070, and the layer thickness of the inorganic binder is 0.26-0.30mm.

6. The process for preparing an inorganic 3D printing sand mold according to claim 1, characterized in that: The hot air temperature of microwave drying is 100-120℃.

7. The process for preparing an inorganic 3D printing sand mold according to claim 1, characterized in that: Microwave dehumidification frequency is 20-30HZ.

8. The process for preparing an inorganic 3D printing sand mold according to claim 1, characterized in that: Microwave frequency is 15-25HZ, and microwave time is 2500-3000 seconds.

9. 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 placement time is ≤3 days.

10. An inorganic 3D printing sand mold, characterized in that: The method is prepared by the preparation process described in any one of claims 1 to 9.

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