A 3D printing sand mold printing parameter determination method

Abstract

The present application relates to a kind of 3D printing sand mould printing parameter determination method, belong to foundry technology field, solve the problem that existing technology cannot accurately evaluate and accept sand mould strength and the problem that it is difficult to obtain the best printing parameter quickly.The method includes: step S1, sample preparation is detected;Step S2, the tensile strength ratio of test block 1 and test block 2 is used to determine the anisotropy of sand mould;Step S3, the mathematical model of the relationship between sand mould strength and printing parameter is established;Step S4, based on single factor control variable method, obtain the linear regression equation of sand mould different direction strength and printing parameter;Step S5, based on the linear regression equation determined in step S4, determine the best printing parameter;Step S6, according to the best printing parameter determined in step S5, verify the reliability of linear regression equation.The present application realizes the anisotropy of the mechanical properties of sand mould, improves the controllability of sand mould strength, and can conveniently, quickly and reliably obtain the best process parameter.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of casting, in particular to a 3D printing sand mold printing parameter determination method. BACKGROUND

[0002] 3D printing (also known as additive manufacturing) is a new and transformative manufacturing technology, which is a process of making objects based on three-dimensional model data, layer by layer. 3D printing does not require a mold, can quickly shape, can manufacture complex geometric configurations, and has the characteristics of integrating material preparation process and part shaping process. In recent years, 3D printing sand mold has been increasingly widely used in casting production.

[0003] However, unlike the traditional sand mold forming method, the 3D printing sand mold is formed by layer-by-layer accumulation, resulting in anisotropy of the sand mold strength, that is, the sand mold printed by the same parameters has different strengths in different directions. The traditional molding method adopts the method of overall solidification forming, so the strengths in different directions are consistent.

[0004] As can be seen from the above, the strength of the 3D printed sand mold has the characteristic of anisotropy, and the existing sand mold strength acceptance process and evaluation method does not consider the difference in sand mold strength in different printing directions. When evaluating the strength of the 3D printed sand mold, the accuracy is low, it cannot guide the selection of the best process parameters of 3D printing, and it cannot quickly and accurately determine the appropriate printing parameters to realize the isotropy of the sand mold strength; In actual application, the non-uniformity and uncontrollability of the strength of the sand mold in different directions will directly lead to the instability of the quality, and problems such as internal defects and fracture are prone to occur. SUMMARY

[0005] In view of the above analysis, the present application aims to provide a 3D printing sand mold printing parameter determination method to solve the technical problems that the existing technology cannot accurately evaluate and accept the strength of the 3D printed sand mold, and it is difficult to quickly obtain the best printing parameters.

[0006] The purpose of the present application is mainly realized by the following technical solutions:

[0007] The present application provides a 3D printing sand mold printing parameter determination method, comprising the following steps:

[0008] Step S1, sample preparation detection: print a sand mold sample, and print two eight-shaped test blocks, namely test block 1 and test block 2;

[0009] Step S2, determining the anisotropy of the sand mold according to the tensile strength ratio of test block 1 and test block 2;

[0010] Step S3, establishing a mathematical model of the relationship between the sand mold strength and the printing parameters;

[0011] Step S4, based on the single factor control variable method, the data of the strength of the sand mold under different printing parameters are obtained by experiment, the regression parameter value of the mathematical model in step S3 is determined by linear fitting, and the linear regression equation of the strength of the sand mold in different directions and the printing parameters is obtained;

[0012] Step S5, based on the linear regression equation determined in step S4, the optimal printing parameter is determined, and the strength ratio of the sand mold in different directions is close to 1:1;

[0013] Step S6, according to the optimal printing parameter determined in step S5, the reliability of the linear regression equation is verified.

[0014] Further, in step S1, the sand mold sample is a column, the direction of the nozzle walking and spraying binder is defined as X direction, the direction perpendicular to the nozzle horizontally is defined as Y direction, and the direction of layer-by-layer powder laying and stacking is defined as Z direction; the X direction is selected to represent the radial strength, and the Z direction is selected to represent the axial strength.

[0015] Further, in step S1, the test block 1 is located at the top center of the sand mold, and the test block 1 plane is parallel to the axial direction; the test block 2 is located at the middle of the sand mold height direction, and the test block 2 plane is parallel to the radial direction of the sand mold; the tensile strength of the test block 1, i.e. the Z direction tensile strength, is represented by the symbol σ1, and the tensile strength of the test block 2, i.e. the X direction tensile strength, is represented by the symbol σ2.

[0016] Further, in step S1, the layer thickness is set to δ0 mm, and the liquid volume is set to V0 pL; in step S2, according to the ratio of the tensile strength of the test block 1 to the test block 2, the relationship between the X direction and the Z direction strength is established by data fitting, and formula (1) is obtained:

[0017] σ1=k0σ2 (1)

[0018] Wherein, k0 is the ratio of the Z direction tensile strength σ1 to the X direction tensile strength σ2 of the same sand mold sample.

[0019] Further, in step S3, the printing parameters include liquid volume and layer thickness, the tensile strength of the sand mold is taken as the dependent variable, and the liquid volume and layer thickness are taken as the independent variable, a mathematical model is established, and formula (2) is obtained:

[0020] σ∝αυ a δ b (2)

[0021] In formula (2), σ is the Z direction tensile strength or the X direction tensile strength, υ is the liquid volume, δ is the layer thickness, α is the correction coefficient, a is the index related to the liquid volume, and b is the index related to the layer thickness.

[0022] Further, step S4 includes the following steps:

[0023] Step S41: establish the relationship between the spray volume v and the X-direction tensile strength σ2, and obtain formula (3):

[0024] σ2=σ v +k v (v-V0) (3)

[0025] In formula (3), σ v is the tensile strength of the test block 2 when the spray volume is V0 pL, k v is the ratio between the difference in tensile strength and the difference in spray volume when the maximum spray volume and the minimum spray volume, and v is the spray volume.

[0026] Step S42: establish the relationship between the layer thickness δ and the Z-direction tensile strength σ1, and obtain formula (4):

[0027] σ1=σ hv -k h (δ-δ0) (4)

[0028] In formula (4), σ hv is the tensile strength of the test block 1 when the layer thickness is δ0 mm, k h is the ratio between the difference in tensile strength and the difference in layer thickness when the maximum layer thickness and the minimum layer thickness, and δ is the layer thickness.

[0029] Further, in step S41, L levels of the spray volume v and the corresponding experimental data of the X-direction tensile strength σ2 are introduced into a data simulation software, linear fitting is performed, and the slope k v of formula (3) is determined.

[0030] Further, in step S42, L levels of the layer thickness δ and the corresponding experimental data of the Z-direction tensile strength σ1 are introduced into a data simulation software, linear fitting is performed, and the slope k h of formula (4) is determined.

[0031] Further, in step S5, the best process parameters are determined through the spray volume and the layer thickness parameters adjustment, and the best process parameters satisfy:

[0032] σ2=σ v +k v (v-V0) = σ1=σ hv -k h (δ-δ0).

[0033] Further, in step S6, the sand printing is performed by using the best process parameters determined in step S5, and three eight-character test blocks are arranged along the X-direction, the Y-direction and the Z-direction respectively, and the reliability of the linear regression equation of step S4 is verified according to the tensile strength ratio of the three test blocks.

[0034] Compared with the prior art, the present application can achieve at least one of the following beneficial effects:

[0035] (1) The eight-shaped test block of the present application can be used as a representative of the mechanical properties of sand mold parts with complex shape, high dimensional and positional accuracy, and high surface quality requirements, and the eight-shaped test block can provide accurate parameters for the mechanical properties in all directions during 3D printing, which is beneficial to quickly and accurately determine the anisotropy of the mechanical properties of the sand mold.

[0036] (2) The present application considers the printing direction of the sand mold test block when detecting the strength of the 3D printed sand mold, which overcomes the problems of distortion of measurement results and casting defects caused by different printing directions which are not considered in traditional strength acceptance methods, and the determination result of the sand mold strength provided by the present application is more objective and scientific.

[0037] (3) According to the strength ratio of test blocks in different printing directions, the present application uses mathematical fitting method to find the rule of strength in different directions, so that the printing parameter adjustment and determination have clear directionality.

[0038] (4) The present application reveals the influence of the two process parameters of liquid spray volume and layer thickness on the tensile strength of the sand mold in different directions by constructing a mathematical model, and through the tensile test of the eight-shaped test block, the linear regression equation of the constructed mathematical model can be obtained by using conventional data simulation software under the condition of fewer test times; further, through verification, it is judged that the linear regression equation obtained by data simulation has high fitting degree and reliability, thus it is proved that the determination method of 3D printing parameters established by the present application can efficiently determine the optimal process parameters of the 3D printing process, solve the problem that the printing parameters cannot be quickly and accurately determined to realize the isotropy of the sand mold strength in the prior art, and help to improve the controllability of the 3D printed sand mold strength and other properties, realize the uniformity and quality stability of the overall strength of the sand mold, and reduce the occurrence of sand mold fracture and other problems.

[0039] (5) The 3D printed sand mold parameter determination method in the present application can conveniently, quickly and reliably obtain the optimal process parameters, and the 3D printing experiment using the optimal process parameters obtained by the parameter determination method in the present application can obtain sand mold parts with good mechanical property isotropy.

[0040] (6) The present application has certain reference significance for the combination optimization problem of other similar process parameters (such as printing speed, resolution, etc.) in the 3D printing process, especially the related properties (gas evolution amount, air permeability, etc.) of the slender pipe sand mold 3D printing.

[0041] The above technical solutions in the present application can be combined with each other to realize more preferred combination solutions. Other features and advantages of the present application will be described in the subsequent description, and some advantages will become apparent from the description, or will be understood by implementing the present application. The purposes and other advantages of the present application can be realized and obtained through the description of the embodiments and the contents particularly pointed out in the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0042] The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiments of the application and together with the description serve to explain the principles of the application. In the drawings:

[0043] Figure 1 Structure schematic view of 3D printing sand mold of the embodiment of the present application;

[0044] Figure 2 In the present application, (a) is the front view of the eight-shaped test block of the embodiment of the present application, and (b) is the side view of the eight-shaped test block of the embodiment of the present application;

[0045] Figure 3 Statistical schematic view of the strength ratio of test block 1 and test block 2 of the embodiment of the present application.

[0046] Figure 4 Linear relationship schematic view of the embodiment of the present application established after regression analysis of the test of the influence of liquid injection volume on sand mold strength;

[0047] Figure 5 Linear relationship schematic view of the embodiment of the present application established after regression analysis of the test of the influence of layer thickness on sand mold strength;

[0048] Symbol explanation:

[0049] 1-test block 1; 2-sand mold; 3-test block 2. DETAILED DESCRIPTION

[0050] The preferred embodiments of the present application will be specifically described below in combination with the accompanying drawings, wherein the drawings constitute a part of the present application, and are used to explain the principles of the present application together with the embodiments of the present application, and are not used to limit the scope of the present application.

[0051] The principle of the 3D printing sand mold printing parameter determination method adopted by the present application is as follows: in the printing process, a layer of binder droplets is sprayed from the printing head every time a layer of silica sand is accumulated by layer-by-layer accumulation, the binder droplets enter the silica sand gap and solidify after dropping onto the surface of the silica sand, and finally the sand mold shape and size set by the program are formed. Due to the fluidity of the binder, the binder will be coated on the surface of the printed silica sand. In the single-layer printing state, the amount of binder coated around the printed silica sand is relatively uniform. With the printing process, under the action of the gravity of the sand mold, the thickness of the binder layer that has not completely solidified on the lower layer tends to be compressed and thinned due to the increase in the weight of the upper layer. In the case where the printing layer thickness and the printing head liquid volume remain unchanged, the binder layer thickness in the printing direction and the binder layer thickness perpendicular to the printing direction will be different due to the fixed liquid volume between layers, which is reflected in the sand mold as the difference in the binder content in the same thickness sand mold in different directions, and finally as the difference in the mechanical properties in different directions.

[0052] According to the characteristics of the different directions of the 3D printed sand mold, two eight-shaped test blocks (i.e., test block 1 and test block 2) are arranged along the radial and axial directions of the sand mold at the same time, and the tensile strength of the two directions of the test blocks is analyzed to determine the anisotropy of the sand mold. In order to realize the uniformity of the strength in different directions, first, a mathematical model of the relationship between the sand mold strength and the printing parameters is established; then, based on the single factor control variable method, the sand mold strength data under different printing parameters are obtained through experiments, and the linear regression equation of the radial and axial tensile strength of the sand mold and the printing parameters is obtained by linear fitting of the data points; further, based on the linear regression equation, the optimal printing parameters are determined to realize the tensile strength ratio of test block 1 and test block 2 close to 1:1, reduce the influence of anisotropy on the overall quality of the sand mold and the shrinkage of the casting, and realize the consistency of the shrinkage in different directions and the stability of the size in the solidification process of the casting.

[0053] Based on the principle, a 3D printing sand mold printing parameter determination method is proposed for the sand mold sample, which comprises the following steps:

[0054] Step S1, sample preparation detection: printing a sand mold sample, and printing two eight-shaped test blocks, i.e., test block 1 and test block 2.

[0055] The sand mold sample is printed, preferably a column; the printing direction is as follows Figure 1 ; the direction of the nozzle walking and spraying the binder is defined as the X direction, the direction perpendicular to the horizontal direction of the nozzle is defined as the Y direction, and the direction of layer-by-layer powder laying and stacking is defined as the Z direction; for the column-shaped sand mold, the X direction and the Y direction strength are basically the same, the X direction represents the radial strength, and the Z direction represents the axial strength; the specific shape of the sand mold can be selected as needed, preferably but not limited to a cylinder, a cube, etc.

[0056] The two eight-shaped test blocks are printed at the same time when printing the sand mold, wherein the test block 2 is located in the middle of the sand mold in the height direction, the test block 2 plane is parallel to the radial direction of the sand mold, the test block 1 is located at the top of the sand mold and is over the center, and the test block 1 plane is parallel to the axial direction; wherein the tensile strength of the test block 1 is represented by a symbol σ1, and the tensile strength of the test block 2 is represented by a symbol σ2.

[0057] The printing is in a paving manner, the paving thickness is set as δ0 mm, and the liquid spraying volume is V0 pL; preferably, the liquid spraying volume is 100 pL-200 pL, the paving thickness is 0.1 mm-0.6 mm, the printing speed is 20 s / layer-40 s / layer, the sand size in the sand mold is 70 mesh-140 mesh, and the liquid spraying amount is 100%.

[0058] It can be understood that the eight-shaped test block in step S1 can be used as a representative of the mechanical properties in each direction of the sand mold part with complex shape, high size and position accuracy requirement and high surface quality requirement, and the eight-shaped test block can provide accurate parameters for the mechanical properties in each direction in the 3D printing process, which is beneficial to quickly and accurately determining the anisotropy of the sand mold mechanical properties; meanwhile, the printing direction of the sand mold test block is considered when detecting the strength of the 3D printing sand mold, which overcomes the defects that the traditional strength acceptance method does not consider the distortion of the measurement results and casting defects caused by different printing directions, and the determination result provided by the present application is more objective and scientific.

[0059] Step S2, determining the anisotropy of the sand mold according to the tensile strength ratio of the test block 1 and the test block 2;

[0060] After printing, the test blocks are placed for N hours to ensure that the binder is completely hardened; then the tensile strength values of the test block 1 and the test block 2 are measured, and the tensile strength ratio of the test block 1 and the test block 2 is calculated, wherein the tensile strength of the test block 1, i.e. the Z-direction tensile strength, is represented by a symbol σ1, the tensile strength of the test block 2, i.e. the X-direction tensile strength, is represented by a symbol σ2, the tensile strength ratio of the test block 1 and the test block 2 is the ratio of the Z-direction tensile strength σ1 and the X-direction tensile strength σ2, the experiment is repeated M times, and the ratio values obtained in M experiments are connected on the coordinate axis, in the data simulation software, the average value and the confidence interval of the tensile strength ratio are obtained through data fitting, the relationship between the X-direction and Z-direction strengths of the sand mold is obtained, and then the anisotropy of the sand mold strength is determined, which guides the direction of adjusting the printing parameters for isotropy in the next step. Specifically, the strength relationship between the X-direction and the Z-direction is established, and formula (1) is obtained:

[0061] σ1=k0σ2 (1)

[0062] Wherein, k0 is the ratio of the Z-direction tensile strength σ1 and the X-direction tensile strength σ2 of the same sand mold sample, when k0 > 1, the ratio k0 of the two can be close to 1 by adjusting the printing parameters to reduce σ1 or increase σ2; when 0 < k0 < 1, the ratio k0 of the two can be close to 1 by adjusting the printing parameters to increase σ1 or reduce σ2.

[0063] It should be noted that after printing, N hours are placed, and the value of N can be selected according to the type of binder and the size of the sample; in this embodiment, N = 24. Repeat the experiment M times, and the value of M can be set as needed, preferably, M ≥ 5; in this embodiment, M = 5.

[0064] Further, the confidence level of the confidence interval of the tensile strength ratio can be selected as needed; in this embodiment, the confidence level is 95%.

[0065] It can be understood that in step S2, according to the strength ratio of the test block in different printing directions, the mathematical fitting method is used to find the rule of the strength in different directions, so that the printing parameter adjustment and determination have clear directionality.

[0066] Step S3, establishing a mathematical model of the relationship between the sand mold strength and the printing parameters;

[0067] Generally speaking, the factors affecting the performance of 3D printed sand mold include layer thickness, spray volume, printing speed, etc. These factors jointly affect the performance of 3D printed sand mold, including sand mold strength, gas evolution, air permeability, etc. Among them, the spray volume and the layer thickness are significant factors affecting the sand mold strength, therefore, the tensile strength of the sand mold is taken as the dependent variable, and the spray volume and the layer thickness are taken as the independent variables, and a mathematical model is established, and formula (2) is obtained:

[0068] σ∝αv a δ b (2)

[0069] It should be noted that in formula (2), σ is the Z-direction tensile strength or the X-direction tensile strength, v is the spray volume, δ is the layer thickness, α is the correction coefficient, a is the index related to the spray volume, and b is the index related to the layer thickness.

[0070] Step S4, based on the single factor control variable method, the data of the sand mold strength under different printing parameters are obtained through experiments, the regression parameter value of the mathematical model in step S3 is determined through linear fitting, and a linear regression equation of the sand mold strength in different directions and the printing parameters is obtained.

[0071] Specifically, as can be seen from formula (2), the influencing factors of Z tensile strength or X tensile strength σ mainly include two variables of spray volume v and layer thickness δ. For a multi-factor (multi-variable) problem, the method of controlling factors (variables) is adopted to change the multi-factor problem into a plurality of single-factor problems, and only one factor is changed to study the influence of the factor on the strength of the test block, which is studied respectively, and finally the problem is solved comprehensively, that is, the controlled variable method is adopted to study the relationship between the X tensile strength (σ2), the Z tensile strength (σ1) and the single factor;

[0072] Specifically, the method comprises the following steps:

[0073] Step S41: It can be understood that with the increase of the spray volume, the penetration ability of the liquid droplets is improved, the diffusion in the X direction and the Y direction is improved, the effective binder between the sand particles of the same layer is increased, and the X tensile strength σ2 is significantly improved; therefore, it is determined that the spray volume is a significant factor affecting the X tensile strength.

[0074] The relationship between the spray volume and the X tensile strength σ2 is further determined by using the controlled variable method. First, the layer thickness is kept unchanged at δ0, the spray volume is adjusted by the liquid droplet detection system, different sizes of liquid droplet pulse parameters are embedded in the printing system of the 3D printer, and the test eight-shaped test block (i.e. test block 2) is arranged in the X direction and printed into a test block; after the test block 2 is printed into a test block, the tensile strength of the test block 2 is measured by using the sand mold strength testing machine; for example, the spray volume (unit: pL) can be selected as L levels, that is, …V -2 , V -1 , V0, V1, V2…; preferably, L≥5, the difference of the L levels is the same, which is f; preferably, the value range of the spray volume v is 100 pL-200 pL; in this embodiment, V0=150 pL, f=10 pL, L=5, that is, the five levels are 130 pL, 140 pL, 150 pL, 160 pL and 170 pL.

[0075] The L levels of the spray volume (v) and the experimental data of the corresponding X tensile strength (σ2) are imported into the data simulation software, the spray volume (v) is taken as the X axis, the X tensile strength (σ2) is taken as the Y axis, the “XY scatter plot” is selected, and the complete button is clicked to obtain the original form of the scatter plot;

[0076] Further, the scatter plot is observed to determine whether the point column distribution has a linear trend; from the experimental results, the data has a linear distribution characteristic, a linear regression analysis method is adopted to establish the relationship between the spray volume (v) and the X tensile strength (σ2), and formula (3) is obtained:

[0077] σ2=σ v +kv (v-V0) (3)

[0078] wherein σ v is the tensile strength of the test block 2 when the spray volume is V0 pL, k v is the ratio between the difference in tensile strength and the difference in spray volume when the maximum spray volume and the minimum spray volume, and v is the spray volume.

[0079] Further, in the data simulation software, the "regression" option is selected, and the input areas of X and Y values, confidence level and other parameters are set respectively, the "linear fitting graph" option is selected, the regression result is obtained, and the "slope k v " is read; since step S2 is based on V0 pL when determining the anisotropy of the sand mold, in order to ensure the stability of the data, the tensile strength at V0 pL is selected as the calibration reference value, and the mathematical model of the simple linear relationship between the spray volume (v) and the tensile strength in X direction (σ2) is determined according to the "slope k v ", that is, the linear regression equation (3) is obtained.

[0080] It can be understood that, under the condition that the spray volume is unchanged, as the layer thickness increases, the distance between layers increases, the effective binder between layers decreases, and the tensile strength σ1 in Z direction decreases significantly. Therefore, the layer thickness is set as a significant factor affecting the tensile strength σ1 in Z direction.

[0081] The relationship between the layer thickness δ and the tensile strength σ1 in Z direction is further determined by using the control variable method. First, the spray volume V0 is determined to be unchanged, and the parameters of different layer thicknesses are respectively embedded in the printing system of the 3D printer, and the test eight-shaped test block (i.e. test block 1) is arranged in the Z direction and printed into a shape. After the test block 1 is printed into a shape, the tensile strength of the test block 1 is measured by using the sand mold strength testing machine; exemplarily, the layer thickness δ (unit: mm) can be selected as L levels, that is, … δ -1 , δ0, δ1, δ2, δ3…; preferably, L≥5, the difference values of the L levels are the same, and are g; preferably, the value range of the layer thickness δ is 0.1 mm-0.6 mm; in this embodiment, δ0=0.25 mm, g=0.1 mm, and L=5, that is, the five levels are 0.15 mm, 0.25 mm, 0.35 mm, 0.45 mm and 0.55 mm respectively.

[0082] The L levels of the layer thickness (δ) and the experimental data of the corresponding tensile strength in Z direction (σ1) are imported into the data simulation software, the layer thickness (δ) is taken as the X axis, the tensile strength in Z direction (σ1) is taken as the Y axis, the "XY scatter plot" is selected, and the complete button is clicked to obtain the original form of the scatter plot;

[0083] Further, observing the scatter plot, it is determined whether the point column distribution has a linear trend; from the experimental results, the data has the characteristics of linear distribution, a linear regression analysis method is used to establish the relationship between the layer thickness (δ) and the Z-direction tensile strength (σ1), and formula (4) is obtained:

[0084] σ1=σ hv -k h (δ-δ0) (4)

[0085] In the formula, σ hv is the tensile strength of the test block 1 when the printing thickness is δ0mm, k h is the ratio between the difference in tensile strength and the difference in layer thickness when the maximum layer thickness and the minimum layer thickness, and δ is the layer thickness.

[0086] Further, in the data simulation software, the “regression” option is selected, the input areas of X and Y values, the confidence level and other parameters are set, the “linear fitting graph” option is selected, the regression result is obtained, and the “slope k h ” is read out. Since step S2 is to determine the anisotropy of the sand mold, printing is performed based on δ0mm, in order to ensure the accuracy of the data, the strength value when δ0mm is selected as the calibration basic value, and the mathematical model of the simple linear relationship between the layer thickness (δ) and the Z-direction tensile strength (σ1) is determined according to the “slope k h ”, that is, the linear regression equation (4) is obtained.

[0087] Step S5, based on the linear regression equation determined in step S4, the optimal printing parameters are determined to realize that the strength ratio of the sand mold in different directions is close to 1:1.

[0088] Specifically, the following steps are included:

[0089] Step S51: According to formula (1) and formula (3), the liquid volume is adjusted to change the resin content between the same layers of sand particles under the condition that the layer thickness is unchanged, so as to achieve the purpose of adjusting the tensile strength of test block 2; through the above parameter adjustment, the tensile strength ratio of test block 1 to test block 2 is close to 1:1, that is, σ2=σ v +k v (v-V0)=σ1, so as to achieve the purpose of keeping the shrinkage consistent during the solidification process;

[0090] Step S52: According to formula (1) and formula (4), the layer thickness is adjusted under the condition that the liquid volume is unchanged, that is, the effective binder volume between the same layers is unchanged, so as to achieve the purpose of adjusting the tensile strength of test block 1, that is, σ1=σ hv -k h (δ-δ0)=σ2, so as to achieve the purpose of keeping the shrinkage consistent during the solidification process;

[0091] Step S53: According to steps S51 and S52, the optimal printing parameter values (layer thickness, liquid volume) that make the tensile strength ratio of the Z direction and the X direction close to 1:1 are determined, and the optimal process parameters satisfy:

[0092] σ2=σ v +k v (υ-V0)=σ1=σ hv -k h (δ-δ0)。

[0093] Step S6: According to the optimal printing parameters determined in step S5, the reliability of the linear regression equation is verified.

[0094] Specifically, the optimal layer thickness and liquid volume determined in step S5 are used as the parameters of the sand mold 3D printing to perform sand mold printing to obtain a 3D printed sand mold, and three eight-shaped test blocks are arranged along the X direction, the Y direction and the Z direction, respectively, that is, test block 1 is arranged and printed in the Z direction, test block 2 is arranged and printed in the X direction, and test block 3 is arranged and printed in the Y direction. Test block 2 and test block 3 are located in the middle of the sand mold height direction, the planes of test block 2 and test block 3 are parallel to the radial direction of the sand mold, test block 1 is located at the center of the top of the sand mold, and the plane of test block 1 is parallel to the axial direction. After test block 1, test block 2 and test block 3 are printed and formed, the tensile strength of test block 1, test block 2 and test block 3 is measured by using a sand mold strength testing machine. According to the tensile strength ratio of the three test blocks, the reliability of the linear regression equation in step S4 is verified.

[0095] According to the test results, it is found that the ratio of the tensile strength in the X direction, the tensile strength in the Y direction and the tensile strength in the Z direction is close to 1:1:1, which indicates that the fitting degree of the established linear regression equation is good and the reliability is high. The optimal process parameters obtained by using the parameter determination method in the present application are used for 3D printing experiments, and a sand mold part with good mechanical property consistency in all directions can be obtained.

[0096] It should be noted that the data simulation software in steps S2 and S4 can be selected as needed, including but not limited to Excel, Origin, Matlab, etc. In the present embodiment, the data simulation software is Excel.

[0097] The 3D printing method in steps S1 to S6 can be selected as needed, including but not limited to fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering / melting (SLS / SLM), three-dimensional printing process (3DP), etc. In the present embodiment, the 3D printing method is three-dimensional printing process (3DP).

[0098] It can be understood that step S3 reveals the influence of the liquid injection volume and the layer thickness on the tensile strength of the sand mold in different directions by constructing a mathematical model. In step S4, the linear regression equation of the mathematical model can be obtained by using a conventional data simulation software in a case that the number of test times is small, through the tensile test of the eight-shaped test block. Based on the linear regression equation, the optimal printing parameter is determined through step S5. Further, in step S6, it is known that the linear regression equation has high fitting degree and reliability through the verification, so that it is illustrated that the 3D printing sand mold printing parameter determination method established by the present application can efficiently determine the optimal process parameter of the 3D printing process, solves the problem that the printing parameter cannot be quickly and accurately determined to realize the isotropy of the sand mold strength in the prior art, and is helpful to improve the controllability of the 3D printing sand mold strength and other parameters, realize the uniformity of the overall strength and the quality stability of the sand mold, and reduce the problems such as sand mold fracture.

[0099] The technical solutions of the present application are further described in detail in combination with specific embodiments.

[0100] Embodiment 1

[0101] Step S1: A cylindrical sand mold is printed by using a three-dimensional printing process (3DP) technology, the diameter of the sand mold is 100 mm, the height is 200 mm, the binder is furan resin, and the specification of the sand in the sand mold is 100 mesh; the printing direction is as Figure 1 , the direction of the nozzle walking and spraying the binder is defined as the X direction, the direction perpendicular to the nozzle horizontally is defined as the Y direction, and the direction of layer-by-layer powder laying and stacking is defined as the Z direction; for the cylindrical sand mold, the X direction and the Y direction strength are basically the same, the X direction is selected to represent the radial strength, and the Z direction represents the axial strength.

[0102] When the sand mold is printed, two eight-shaped test blocks are printed at the same time for strength measurement, wherein the test block 2 is located in the middle of the height direction of the sand mold, the plane of the test block 2 is parallel to the radial direction of the sand mold, the test block 1 is located at the top of the sand mold and passes through the center, and the plane of the test block 1 is parallel to the axial direction; wherein the tensile strength of the test block 1, i.e. the Z direction tensile strength, is represented by the symbol σ1, and the tensile strength of the test block 2, i.e. the X direction tensile strength, is represented by the symbol σ2; the size of the eight-shaped test block is: 66 mm in length direction, 36 mm in width direction, 23 mm in thickness direction, 41 mm in gauge length, and 23 mm in width at the narrowest part in the middle.

[0103] The printing adopts a flat laying mode, the layer thickness is 0.25 mm, the liquid injection volume is 150 pL, the liquid injection amount is 100%, and the printing speed is 30 s / layer (second / layer).

[0104] It should be noted that for the volume of the liquid, the preferred range is 100 pL-200 pL, which can avoid the situation that the resin droplet volume is too small to cause insufficient strength of the sand mold or the resin droplet volume is too large to cause deformation between sand particles due to sliding; in the embodiment, the middle value 150 pL of the preferred range is selected as the basis to determine the anisotropy of the sand mold.

[0105] It should be noted that for the layer thickness, due to the minimum printing layer thickness limit and the minimum strength limit of the 3D printer, preferably, the value of δ is in the range of 0.1 mm-0.6 mm; it can be understood that too large layer thickness will result in reduced tensile strength and cannot meet the minimum strength requirement of the casting. In the embodiment, according to the ±0.5 mm casting tolerance requirement, the maximum printing layer thickness is selected to realize the casting size tolerance requirement, i.e., δ = 0.25 mm, which is used as the basis to determine the anisotropy of the sand mold.

[0106] Step S2: Measure the tensile strength values of the test block 1 and the test block 2 after 24 hours of printing, the strength test machine type is YQY-II type, and the test national standard is GB / T2684-2009 Casting Sand Machine Mixture Test Method; calculate the ratio of the tensile strength of the test block 1 and the test block 2, which is shown in Table 1 below, and the ratio is connected on the coordinate axis as shown in Figure 3 According to the fact that the strength of the test block 1 is greater than the strength of the test block 2, it is determined that the resin amount between the sand particles in the Z direction of the sand mold is greater than the resin amount between the sand particles in the same layer in the parallel direction;

[0107] Table 1 Tensile strength and ratio of test block 1 and test block 2

[0108]

[0109] According to Figure 3 The ratio of the tensile strength of the test block 1 and the test block 2 is basically between 1.44±0.18, wherein the average value of the ratio of the tensile strength is 1.44, the confidence interval is ±0.18, the confidence level is 95%, and the relationship between the test block 1 and the test block 2 is established, and formula (1) is obtained:

[0110] σ1=1.44σ2 (1)

[0111] As can be seen from formula (1), the Z-direction tensile strength σ1 of the sand mold is greater than the X-direction tensile strength σ2, in order to realize the ratio of 1:1, the best value of the printing parameter in the 3D printing process can be determined through the following steps S3-S5.

[0112] Step S3: Establish a mathematical model of the relationship between the sand mold strength and the printing parameter;

[0113] The relationship (2) between the tensile strength of the sand mold and the printing parameters (i.e., the spray volume, the layer thickness) in the 3D printing process is established as follows, wherein σ is the tensile strength in the Z direction or the tensile strength in the X direction, υ is the spray volume, δ is the layer thickness, α is a correction coefficient, a is an index related to the spray volume, and b is an index related to the layer thickness;

[0114] σ∝αυ a δ b (2)

[0115] Step S4: Based on the single-factor control variable method, the data of the tensile strength of the sand mold under different printing parameters are obtained through experiments, the regression parameter values of the mathematical model in step 3 are determined through linear fitting, and the linear regression equation of the strength of the sand mold in different directions and the printing parameters is obtained.

[0116] Step S41: The control variable method is adopted, the layer thickness is controlled to be constant, according to the ±0.5mm casting tolerance requirement, the maximum printing layer thickness is selected to realize the casting size tolerance requirement, that is, δ = 0.25mm, the spray volume is changed, the influence of the spray volume on the tensile strength σ2 of the sand mold in the X direction is verified, the printing parameters and the test results are shown in Table 2, and the data in Table 2 are connected on the coordinate axes as shown in Figure 4

[0117] Table 2 Determination results of the tensile strength σ2 in the X direction after changing the spray volume

[0118] Spray volume v / pL 130 140 150 160 170 X-direction tensile strength σ2 / Mpa 0.74 0.91 1.24 1.49 1.70

[0119] According to Figure 4 The tensile strength σ2 of the sand mold in the X direction increases with the increase of the spray volume, it is determined that the influence factor of the spray volume on the tensile strength σ2 in the X direction is positive, that is, it is determined that a is positive, and the simple linear relationship between the spray volume and the tensile strength σ2 in the X direction is established through the data in Figure 4

[0120] σ2=σ v +k v (v-150) (3)

[0121] In the formula, σ v is the tensile strength (i.e., the tensile strength in the X direction) of the test block 2 when the spray volume is 150pL, k v is the ratio between the difference of the tensile strength in the X direction and the difference of the spray volume when the maximum spray volume and the minimum spray volume, and υ is the spray volume.

[0122] As can be seen from Table 2 and Figure 3 , the spray volume and the tensile strength σ2 in the X direction are in a simple linear relationship, linear fitting is performed through the data simulation software, and the slope k v ​​= 0.024 Mpa / pL, it can be known that the linear regression equation between the tensile strength of X direction and the liquid volume is: σ2=1.24+0.024(v-150), wherein v is in pL; since the volume of resin droplets and the relationship between the sand particles affect, in order to avoid the sand mold strength not enough caused by too small resin droplet volume or the deformation between sand particles caused by too large resin droplet volume, the value range of v is 100 pL-200 pL; since the printing is based on the liquid volume V0 of 150 pL in step S2 when determining the anisotropy of the sand mold, in order to ensure the stability of the data, the tensile strength of 150 pL is selected as the calibration reference value.

[0123] It can be understood that, with the increase of the liquid volume, the penetration ability of the droplets is improved, the diffusion of X direction and Y direction is improved, the effective binder between the sand particles of the same layer is increased, and the tensile strength σ2 of X direction is increased.

[0124] Step S42: the control variable method is used to control the liquid volume unchanged, that is, v=150 pL, the layer thickness is changed, the influence of the layer thickness on the tensile strength σ1 of the sand mold in Z direction is verified, the printing parameters and the test results are shown in Table 3, and the data in Table 3 is connected on the coordinate axis as shown in Figure 5 .

[0125] Table 3: Z-direction tensile strength σ1 measurement results after changing the layer thickness

[0126] Lay-up thickness δ / mm 0.15 0.25 0.35 0.45 0.55 Z-direction tensile strength σ1 / Mpa 2.23 1.96 1.60 0.75 0.48

[0127] According to Figure 5 , the tensile strength σ1 of the sand mold in Z direction decreases with the increase of the layer thickness, it is determined that the influence factor of the layer thickness on the tensile strength σ1 of the sand mold in Z direction is negative, that is, it is determined that b is negative, and the simple linear relationship between the layer thickness and the tensile strength σ1 of the printed sand mold in Z direction is established through the data in Table 3 and Figure 5 , and formula (4) is obtained:

[0128] σ1=σ hv -k h (δ-0.25) (4)

[0129] In the formula, σ hv is the tensile strength of test block 1 when the layer thickness is 0.25 mm (that is, the tensile strength in Z direction), k h is the ratio between the difference of the tensile strength of test block 1 and the difference of the layer thickness when the maximum layer thickness and the minimum layer thickness, and δ is the layer thickness.

[0130] It can be known from Table 3 and Figure 5 that, under the condition that the liquid volume remains unchanged, the tensile strength and the layer thickness are in a linear relationship, and the linear fitting can be obtained through the data simulation software, and the slope k h= -4.4 Mpa / mm, it can be known that the linear regression equation between the tensile strength in Z direction and the layer thickness is σ1=1.96-4.4(δ-0.25), wherein δ is in mm; due to the minimum printing layer thickness limit and the minimum strength limit of the 3D printer, the value range of δ is 0.1 mm-0.6 mm; since step S2 is based on the maximum layer thickness 0.25 mm for ensuring that the size tolerance of the casting meets ±0.5 mm in determining the anisotropy of the sand mold, in order to ensure the accuracy of the data, the tensile strength value when 0.25 mm is selected as the calibration basic value.

[0131] It can be understood that, under the condition that the spray volume is unchanged, with the increase of the layer thickness, the distance between layers increases, the effective binder between layers decreases, and the tensile strength σ1 in Z direction decreases accordingly.

[0132] Step S51: according to formula (1) and formula (3), the spray volume is adjusted to change the resin content between the same layer sand particles under the condition that the layer thickness is unchanged, so as to achieve the purpose of adjusting the tensile strength (i.e. the tensile strength in X direction) of the test block 2; through the above parameter adjustment, the tensile strength ratio of the test block 1 to the test block 2 is close to 1:1, i.e. σ2=1.24+0.024(v-150)=σ1.

[0133] Step S52: according to formula (1) and formula (4), the layer thickness is adjusted to change the tensile strength between the accumulated layers under the condition that the spray volume is unchanged, i.e. the effective binder volume between the same layers is unchanged, so as to achieve the purpose of adjusting the tensile strength (i.e. the tensile strength in Z direction) of the test block 1, and realize that the tensile strength ratio of the test block 1 to the test block 2 is close to 1:1, i.e. σ1=1.96-4.4(δ-0.25)=σ2.

[0134] Step S53: according to steps S51 and S52, the optimal printing parameters are determined, which make the tensile strength ratio of Z direction to X direction close to 1:1, and the optimal process parameters satisfy σ2=1.24+0.024(v-150)=σ1=1.96-4.4(δ-0.25).

[0135] It should be noted that there are multiple groups of optimal printing parameters that satisfy the above linear relationship equation σ2=1.24+0.024(v-150)=σ1=1.96-4.4(δ-0.25); exemplarily, the optimal printing parameters include but are not limited to,

[0136] The first group: the layer thickness is 0.4 mm, and the spray volume is 150 pL;

[0137] The second group: the layer thickness is 0.3 mm, and the spray volume is 170 pL;

[0138] The third group: the layer thickness is 0.5mm, and the spray volume is 140pL.

[0139] Step S6: using the optimal process parameters determined in step S5, for example, the layer thickness is 0.4mm, and the spray volume is 150pL, as the parameters of the sand mold 3D printing, the sand mold is printed to obtain a 3D printed sand mold;

[0140] The tensile strength in each direction of the obtained 3D printed sand mold is tested, and the results are as follows:

[0141]

[0142] According to the test results, the ratio of the tensile strength in X direction, the tensile strength in Y direction and the tensile strength in Z direction is close to 1:1:1, and the sand mold part with good isotropy is obtained by using the optimal printing parameters determined by the method.

[0143] The above is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this. Any changes or replacements within the technical range disclosed by the present application can be easily thought by those skilled in the art, and should be covered within the protection scope of the present application.

Claims

1. A method of determining printing parameters for 3D printing of sand molds, characterized in that, The method comprises the following steps: Step S1, detecting sample preparation: printing a sand mold sample, and printing two eight-shaped test blocks, namely test block 1 and test block 2; Step S2, determining the anisotropy of the sand mold according to the tensile strength ratio of test block 1 to test block 2; Step S3, establishing a mathematical model of the relationship between the sand mold strength and the printing parameters; Step S4, obtaining the data of the sand mold strength under different printing parameters through experiments based on the single-factor control variable method, determining the regression parameter value of the mathematical model in step S3 through linear fitting, and obtaining the linear regression equation of the sand mold strength in different directions and the printing parameters; Step S5, determining the optimal printing parameters based on the linear regression equation determined in step S4, so that the sand mold strength ratio in different directions is close to 1:1; Step S6, verifying the reliability of the linear regression equation according to the optimal printing parameters determined in step S5.

2. The method of claim 1, wherein, In step S1, the sand mold sample is a column, the direction of the nozzle walking and spraying the binder is defined as the X direction, the direction perpendicular to the nozzle horizontally is defined as the Y direction, and the direction of layer-by-layer powder laying and stacking is defined as the Z direction; the X direction is selected to represent the radial strength, and the Z direction is selected to represent the axial strength.

3. The method of claim 2, wherein, In step S1, the test block 1 is located at the top center of the sand mold, and the plane of the test block 1 is parallel to the axial direction; the test block 2 is located at the middle of the height direction of the sand mold, and the plane of the test block 2 is parallel to the radial direction of the sand mold; the tensile strength of the test block 1, that is, the Z-direction tensile strength, is represented by the symbol σ1, and the tensile strength of the test block 2, that is, the X-direction tensile strength, is represented by the symbol σ2.

4. The method of claim 3, wherein, In step S1, the layer thickness is set as δ0 mm, and the spray volume is set as V0 pL; in step S2, the strength relationship between the X direction and the Z direction is established according to the tensile strength ratio of the test block 1 to the test block 2 through data fitting, and formula (1) is obtained: σ1=k0σ2 (1) Wherein, k0 is the ratio of the Z-direction tensile strength σ1 to the X-direction tensile strength σ2 of the same sand mold sample.

5. The method of claim 4, wherein, In step S3, the printing parameters include the spray volume and the layer thickness, the sand mold tensile strength is taken as the dependent variable, and the spray volume and the layer thickness are taken as the independent variables, a mathematical model is established, and formula (2) is obtained: σ ∝ αv a δ b (2) In formula (2), σ is the Z-direction tensile strength or the X-direction tensile strength, υ is the spray volume, δ is the layer thickness, α is the correction coefficient, a is the index related to the spray volume, and b is the index related to the layer thickness.

6. The method of claim 5, wherein, The step S4 comprises the following steps: Step S41: establishing the relationship between the spray volume υ and the X-direction tensile strength σ2, and obtaining formula (3): σ2= σ v + k v (υ - V0) (3) In formula (3), σ v is the tensile strength of the test piece 2 at a spray liquid volume of V0pL, k v is the ratio between the difference in tensile strength and the difference in spray liquid volume at the maximum spray liquid volume and the minimum spray liquid volume, and υ is the spray liquid volume. Step S42: establishing the relationship between the layer thickness δ and the Z-direction tensile strength σ1, and obtaining formula (4): σ1= σ hv -k h (δ-δ0) (4) In formula (4), σ hv is the tensile strength of test piece 1 when the ply thickness is δ0mm, k h is the ratio between the difference in tensile strength and the difference in ply thickness when the maximum ply thickness and the minimum ply thickness, δ is the ply thickness.

7. The method of determining 3D printing sand mold printing parameters according to claim 6, characterized in that, In step S41, L levels of the spray volume v are introduced into the data simulation software with the corresponding experimental data of the X-direction tensile strength σ2, linear fitting is performed to determine the slope k of formula (3) v .

8. The method of claim 7, wherein, In step S42, the L number of ply thicknesses δ and the corresponding experimental data of Z-direction tensile strength σ1 are imported into the data simulation software, linear fitting is performed, and the slope k of formula (4) is determined h .

9. The method of claim 8, wherein, In step S5, the optimal process parameters are determined through the adjustment of the spray volume and the layer thickness parameters, and the optimal process parameters satisfy: σ2= σ v + k v (υ - V0) = σ1= σ hv - k h (δ - δ0).

10. The method of claim 9, wherein, In step S6, the sand mold is printed by using the optimal process parameters determined in step S5, three eight-shaped test blocks are arranged along the X direction, the Y direction and the Z direction respectively, and the reliability of the linear regression equation of step S4 is verified according to the tensile strength ratio of the three test blocks.

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