CN109522585B - Self-adaptive layering method for preventing feature migration of 3D printing model - Google Patents

Abstract

The invention discloses an adaptive layering method for preventing the feature deviation of 3D printing model, which includes establishing a three-dimensional model, performing grid processing on the three-dimensional model to generate an STL file, reading the STL file, and analyzing the data from the three-dimensional model. The data read in the STL file is subjected to a preprocessing operation to obtain preprocessed data, and the feature heights on the three-dimensional model after gridding processing are identified to obtain a set of height data.

Adjust the spacing between adjacent feature heights, that is, adjust the height data

Adjustment is performed, layered slicing is performed at the position of the feature height, adaptive layering is performed between adjacent feature heights, slice contour information is obtained, a print file is generated, and a 3D model is printed. The invention ensures that the layered plane passes through the position of the model features, thereby retaining the three-dimensional model features, can effectively prevent the feature offset and loss caused by the accumulation of materials layer by layer during the 3D printing model printing process, and reduce the step error.

Description

An Adaptive Layering Method to Prevent Feature Offset in 3D Printing Models

technical field

The invention belongs to the technical field of 3D printing, and in particular relates to an adaptive layering method for preventing the feature offset of 3D printing models.

Background technique

3D Printing (3D Printing) technology, also known as Rapid Prototyping (RP) or Additive Manufacturing (AM), is a technology that manufactures three-dimensional solid parts through CAD design and layer-by-layer accumulation of materials. The specific implementation steps are divided into: first, use CAD design or reverse engineering to obtain a 3D model, and convert it into a STL (Stereolithographic) format or other operable format file with a simple format and good versatility for processing; In the layer direction, planes with different heights are used to intersect the model to obtain a series of contour information; then according to the contour information, the printing path and other process parameters are determined, and the printer is controlled to stack and bond the materials layer by layer from low to high, and finally form a three-dimensional entity. The accumulation of materials layer by layer will cause obvious step error (or step effect, stair-stepping effect) and loss and offset of model features on the surface of the workpiece when printing inclined surfaces, which directly affects the surface quality of the workpiece and the determination of the layering position. And the number of layers will directly affect the surface accuracy and processing efficiency of the workpiece. However, whether the three-dimensional model of the workpiece is designed by CAD software or obtained by reverse engineering, the dividing direction and layer height must be selected first, and then sent to the printing equipment for processing after layering.

At present, the layering methods based on STL format mainly include equal thickness layering and adaptive layering methods. The equal-thickness layering method has a fixed layer thickness, is simple to implement, and has a fast processing speed. Although the step effect is obvious, it is widely used in the production of workpieces with large volumes and low precision requirements; although a smaller layer thickness can obtain higher Surface quality, but will increase the memory footprint, processing time and model printing time due to the increase in the number of layered layers, thereby reducing the processing efficiency. The slice thickness of the adaptive layering is not a constant, but is determined by the geometry of the model and the machining accuracy. The layer thickness will be automatically adjusted according to the surface complexity of the workpiece model to be processed, so that the surface is complex and severely inclined with large curvature. Models with smaller layer thicknesses have more layers, while models with simple surfaces have correspondingly fewer layers. The adaptive layering algorithm adopts the variable layer thickness, which reduces the step error to a certain extent and solves the contradiction between the machining accuracy and the efficiency during processing, but no matter how small the layer thickness is, the step effect cannot be completely eliminated, and it is impossible to Make sure that the layered plane passes through all model features, resulting in model features not being printed or the position of the features being offset, which cannot handle missing and offset features. Therefore, under the same surface quality, adaptive layering can obtain fewer layers than equal-thickness layering, so the processing efficiency is higher. However, most of the current adaptive layering algorithms mainly study the relationship between layer height and step error, and there are still few solutions to the problem of model feature offset and loss. Therefore, there is an urgent need to propose an improved adaptive layering method that effectively solves the problem of model feature offset, so as to further reduce the step effect and solve the problem of feature offset and loss.

SUMMARY OF THE INVENTION

In view of the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide an adaptive layering method for preventing the feature offset of 3D printing models, so as to solve the problems of step effect, feature offset and loss in the prior art .

In order to achieve the above object and other related objects, the present invention provides an adaptive layering method for preventing the feature offset of 3D printing model, and the adaptive layering method for preventing the feature offset of 3D printing model includes: establishing a three-dimensional model; The three-dimensional model is meshed to generate an STL file; the STL file is read, and a preprocessing operation is performed on the data read from the STL file to obtain preprocessed data; using the The preprocessed data is used to identify the feature heights on the gridded three-dimensional model to obtain a set of height data h; to adjust the spacing between adjacent feature heights, that is, to adjust the height data h; Perform layered slicing at the position of the described feature height, and use the tip height method to perform adaptive layering between adjacent feature heights to obtain slice outline information; generate a print file according to the slice outline information and G code format, and print the 3D model. .

In an embodiment of the present invention, the files stored in the three-dimensional model include STL files, AMF files, OBJ files, STEP files, 3MF files, IGES files, LEAF files, RPI files, and RP files.

In an embodiment of the present invention, the identifying the feature height on the three-dimensional model to obtain a set of height data includes:

The basic features of the three-dimensional model include feature points, feature lines and feature surfaces;

Characteristic face, the triangular facet parallel to the layered plane is the characteristic face. When the Z coordinates of the three vertices of the triangular facet are the same, the triangular facet is determined to be the feature face, and the feature height is the triangular facet. The Z coordinate of one of the three vertices of the slice;

Characteristic line, two adjacent triangular patches have the same Z-coordinates of only two vertices in common, and the dihedral angle of these two triangular patches is greater than the threshold, and the connection between the two vertices is a characteristic edge, The feature height is the Z coordinate of a vertex on the feature edge, the dihedral angle is calculated from the normal vectors of two adjacent triangular faces, and the threshold is determined by formula (1):

为特征边阈值，β为STL文件中所有网格边对应的二面角的均值，λ＝log(Facetres)，Facetres为CAD模型中对生成的STL网格模型精度的控制参数；in,

is the feature edge threshold, β is the mean value of the dihedral angles corresponding to all mesh edges in the STL file, λ=log(Facetres), and Facetres is the control parameter in the CAD model for the accuracy of the generated STL mesh model;

Feature point, if the Z coordinate of a vertex P of one of the triangular patches is higher or lower than the Z coordinates of the other two vertices of all triangular patches with vertex P as the vertex, then vertex P is the feature point, and the feature height is the Z coordinate of vertex P.

In an embodiment of the present invention, adjusting the spacing between adjacent feature heights, that is, adjusting the height data h includes:

A set of height data h is preprocessed, and the height data with a height of 0 and the floor height data with the maximum height of the 3D model are inserted into the head and tail of the height data h respectively, and the floor height data with a height of 0, the 3D model are respectively inserted. The layer height data of the maximum height of the model are all boundary layer heights. Check whether the distance between the two boundary layer heights and their adjacent layer heights is less than the minimum layer thickness. If it is less than the minimum layer thickness, adjust the adjacent layer height to make the boundary layer height. The distance between the adjacent layer heights is equal to the minimum layer thickness, re-sort the height data h in ascending order, and detect whether there is a layer height between the adjusted layer height and the corresponding boundary layer height, and if there is a layer height, delete the existing layer height;

Given the maximum layer thickness h _max and the minimum layer thickness h _min of the 3D printer, the feature heights between the boundary layer heights are checked one by one using a traversal method to find the distance between two adjacent feature heights Whether there is a layer thickness smaller than the minimum layer thickness, if it is smaller than the minimum layer thickness, adjust the height of two adjacent features to obtain the feature height adjustment result. The method of adjusting the feature height is as follows:

The height difference between two adjacent feature heights is d;

Merge inward: If d≤h _min /2, shift both slice layers inward by d/2, that is, merge the two layers;

Outward expansion: if d>h _min /2, the two slice layers are shifted outward by |dh _min |/2;

According to the feature height adjustment result, and four adjacent layer heights, namely, two adjacent feature heights b and c to be adjusted, and two adjacent feature heights b and c to be adjusted, respectively The maximum height difference d _max of the two layer heights a and f, to determine whether the two adjacent feature heights b and c will be adjusted to the two adjacent feature heights b and c after the inward offset or outward offset height adjustment. Adjacent feature heights b and c have an impact on the two adjacent layer heights a and f. If there is an impact, the height adjustment is performed for different situations of inward merging, outward expansion and feature density. The height adjustment method is as follows :

Merge inwards for height adjustment;

Inwardly merge the two adjacent feature heights b and c in the middle, the height difference between the merged layer height e and its two adjacent layer heights is d ₁ and d ₂ respectively, at least one of them is still smaller than the minimum score. layer thickness h _min ;

1) If h _min ≤ d _max <2·h _min , then both d ₁ and d ₂ are less than h _min , delete the merged layer height e;

2) If d _max ≥ 2·h _min , only one of d ₁ and d ₂ is smaller than h _min , if it is d ₁ , adjust the combined layer height e so that d ₁ =h _min ; if it is d ₂ , Adjust the combined layer height e so that d ₂ =h _min ;

Expand outward for height adjustment;

The two adjacent feature heights b and c in the middle of the outward expansion, at least one of the layer height differences d ₁ and d ₂ between the two offset layer heights and the respective adjacent layer heights is still smaller than the minimum layer thickness _hmin ;

1) If d _max <3·h _min , then both d ₁ and d ₂ are less than h _min , the outward expansion is transformed into inward merging, and height adjustment is performed according to the inward merging;

2) If d _max ≥ 3·h _min , then only one of d ₁ and d ₂ is smaller than h _min , if it is d ₁ , adjust the height of the expanded layer and keep its distance at h _min , so that d ₁ = h _min ; if it is d ₂ , adjust the height of the layer after expansion, and keep the distance between them as h _min to make d ₂ =h _min ;

Feature dense for height adjustment;

For four consecutive adjacent high schools, the height difference d between the highest and the lowest layer heights is less than the minimum layer thickness h _min , delete the middle layer heights, and find the phases with the highest and lowest layer heights in the four high-level high schools respectively. For the height of adjacent layers, re-judge the heights of these four layers. If there are still problems of inward merging, outward expansion and feature density, use the corresponding height adjustment for processing; if there are no inward merging, outward expansion and feature density If there is a problem, end the height adjustment and search for the subsequent layer height.

In an embodiment of the present invention, adjusting the spacing between adjacent feature heights, that is, adjusting the height data h includes: the feature density is that in a set of height data h, there are three and more For the continuous feature heights above, the height difference between the highest story height and the lowest story height is less than the minimum layer thickness h _min .

In an embodiment of the present invention, layered slicing is performed at the position of the feature height, and adaptive layering is performed between adjacent feature heights using a tip height method, and obtaining slice outline information includes:

In the adjusted height data h, two adjacent heights are taken out in turn, and the model between the two adjacent heights is adaptively layered using the top height method. The top height method is the surface of the actual manufactured part and the surface of the CAD model. The maximum distance of , the triangular face of the STL mesh model is the hypotenuse of a right triangle, and the height on the hypotenuse is the top height d;

The right-angled side of a right-angled triangle is calculated as follows:

为三角面片的法向量，

为分层方向，垂直向上，θ为

和

的夹角，范围为0～π；where h is the layer thickness of the current layer,

is the normal vector of the triangular patch,

is the layering direction, vertically upward, and θ is

and

The included angle ranges from 0 to π;

Traverse all the triangular patches intersecting with the STL mesh model as a layered layer, and find the minimum layer thickness h _min , if the minimum layer thickness h _min is lower than or higher than the printing range of the 3D printer, the minimum layer thickness h min will be determined. The layer thickness h _min is changed to the minimum layer thickness or the maximum layer thickness; the minimum layer thickness h _min and the layer height l, that is, the height on the hypotenuse of the right triangle, are added to obtain the current layer layer plane height l′=h+ l, until a series of layer heights for adaptive layering of the STL grid model are obtained;

When adaptive layering is performed between two adjacent feature heights a and b, when d < 2 h _min , the distance is too small to perform adaptive layering, and the adaptive layering between a and b is ended. ; d represents the height difference between two adjacent feature heights; when d ≥ 2 h _min , perform adaptive layering between a and b, and calculate the distance between the current layered plane b when the layered plane height is determined If it is less than the minimum layer thickness h _min , adjust the layer height of this layer plane to make d′=h _min ; after adjustment, the distance between this layer plane and the previous adjacent layer plane is less than the minimum layer height layer thickness h _min , this layer plane is deleted.

As described above, an adaptive layering method for preventing the feature offset of 3D printing models of the present invention has the following beneficial effects:

The self-adaptive layering method for preventing the offset of 3D printing model features of the present invention ensures that the layered plane passes through the position of the model features, thereby retaining the three-dimensional model features, and can effectively prevent the layer-by-layer accumulation of materials during the printing process of the 3D printing model. Feature offset and loss, reducing step errors.

The invention takes into account the surface accuracy of the printed part and the printing efficiency, and under the premise of ensuring the printing accuracy, the number of layered layers is reduced as much as possible through the self-adaptive layering method of the tip height, thereby reducing the memory occupation and processing time, and improving the printing processing efficiency.

The invention provides a feature height identification method and a spacing adjustment method between the feature heights, so as to reduce the step effect of the workpiece as much as possible and improve the processing quality. The invention inputs the processed slice section profile information into the rapid prototyping processing equipment , forming a model processing trajectory, which is of great significance for further promoting the application of 3D printing technology in practice.

The invention is simple and efficient, has strong versatility and practicability, and has a wide range of applications.

Description of drawings

1 is a schematic flowchart of an adaptive layering method for preventing 3D printing model feature offset provided by an embodiment of the present application;

FIG. 2 is a three-dimensional model provided by an embodiment of the present application, wherein 1, 2, and 3 are corresponding features;

3 is a model feature provided by an embodiment of the present application, including feature surfaces, feature lines and feature points;

4 is a schematic diagram of inward merging of a feature height adjustment method provided by an embodiment of the present application;

5 is a schematic diagram of the outward expansion of the feature height adjustment method provided by the embodiment of the present application;

6 is a schematic diagram of an inward merging problem existing after feature height adjustment provided by an embodiment of the present application;

7 is a schematic diagram of the outward expansion problem that exists after the feature height adjustment provided by an embodiment of the present application;

8 is a schematic diagram of a feature density problem existing after feature height adjustment provided by an embodiment of the present application;

9 is a schematic diagram of a solution to the problem of inward merging existing after adjustment provided by an embodiment of the present application;

10 is a schematic diagram of a solution to the problem of outward expansion after adjustment provided by an embodiment of the present application;

FIG. 11 is a schematic diagram of a solution to a feature-intensive problem that exists after adjustment provided by an embodiment of the present application;

FIG. 12 is a schematic diagram of the top height provided by the embodiment of the present application;

FIG. 13 is a diagram of the layering result at the feature corresponding to 1 of the three-dimensional model in FIG. 2 by the method for iso-thickness layering provided by the embodiment of the present application;

FIG. 14 is a layered result diagram of the adaptive layering method provided by the embodiment of the application at the feature corresponding to 1 of the three-dimensional model in FIG. 2;

FIG. 15 is a diagram of the layering result at the feature corresponding to 1 of the three-dimensional model in FIG. 2 of the adaptive layering method for preventing the feature offset of the 3D printing model provided by the embodiment of the application;

FIG. 16 is a diagram of the layering result at the feature corresponding to 2 of the three-dimensional model in FIG. 2 by the method for iso-thickness layering provided by the embodiment of the application;

FIG. 17 is a layered result diagram of the adaptive layering method provided by the embodiment of the application at the feature corresponding to 2 of the three-dimensional model in FIG. 2;

FIG. 18 is a diagram of the layering result at the feature corresponding to 2 of the three-dimensional model in FIG. 2 by the adaptive layering method for preventing the feature offset of the 3D printing model provided by the embodiment of the application;

FIG. 19 is a diagram of the layering result at the feature corresponding to 3 of the three-dimensional model of FIG. 2 by the method for iso-thickness layering provided by the embodiment of the application;

FIG. 20 is a layered result diagram of the adaptive layering method provided by the embodiment of the application at the feature corresponding to 3 of the three-dimensional model in FIG. 2;

FIG. 21 is a diagram of the layering result at the feature corresponding to 3 of the three-dimensional model in FIG. 2 by the adaptive layering method for preventing the feature offset of the 3D printing model provided by the embodiment of the application.

Component label description

Steps S1～S6

Detailed ways

Embodiments of the present invention are described below through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following embodiments and features in the embodiments may be combined with each other under the condition of no conflict.

It should be noted that the drawings provided in the following embodiments are only used to illustrate the basic concept of the present invention in a schematic way, so the drawings only show the components related to the present invention rather than the number, shape and number of components in actual implementation. For dimension drawing, the type, quantity and proportion of each component can be changed at will in actual implementation, and the component layout may also be more complicated.

The present invention uses C++ language to realize the method of the present invention under the development environment of Visual Studio 2017. The maximum and minimum layer thicknesses are given according to the accuracy of the actual machine, and are set to 0.1mm and 0.3mm, and the top height is given according to the required accuracy, Set to 0.1mm. Combined with the top adaptive layering method, the flowchart of the improved self-adaptive layering algorithm in the present invention is shown in FIG. 1 . In order to further illustrate the superiority of the method, the layer number and feature offset obtained by comparing and analyzing the equal-thickness layering, the adaptive layering and the improved adaptive layering method of the present invention are explained, and the layering method proposed by the present invention is explained. It can be proved that through the height of the feature, it can effectively prevent the occurrence of model feature offset and loss, reduce the step error, and can also perform adaptive layering at places other than the height of the feature, so as to ensure the surface quality of the print at the same time. The number of layers is reduced, the processing efficiency is improved, and an adaptive layering method to prevent the feature offset of 3D printing models is provided for the practical application of 3D printing. The default layering direction of the present invention is the same as the positive direction of the Z-axis of the rectangular coordinate system where the vertexes of the triangular facets of the model are located. If the layering direction is different from the default direction of the present invention, the method proposed by the present invention can be used only by converting the three-dimensional coordinates of all triangular facet vertices of the model into a rectangular coordinate system with the layering direction as the positive direction of the Z axis.

According to the actual needs, the 3D model is generated by using 3D computer-aided design software, as shown in Figure 2, in which the circled 1, 2, and 3 are the characteristic positions corresponding to the 3D model, and the 3D model is meshed to generate a specific STL format file. data.

Read the STL file, adjust the Z coordinates of the vertices, sort the triangular facets, calculate the dihedral angle threshold of the feature line, and perform a series of preprocessing operations, including the following steps:

Read STL file

Read the model data according to the STL format, and use the hash table to remove redundant data. The vertices of the triangular patch are stored in the Point class, which contains the three-dimensional coordinates of the vertices. Triangular patches are stored in the Face class and contain information about vertices and adjacent triangular faces. In order to complete the judgment of all features in one traversal, add a judgment variable to the Point class and add three judgment variables to the Face class, which correspond to the three sides of the triangular face respectively. These judgment variables indicate whether the vertices or edges of the triangular face have been judged. . A STL model is represented by the Mesh class, which contains all vertex information (vector<Point>points) and triangular face information (vector<Face>faces).

Adjust vertex Z coordinate

Because the minimum value Z _min in the Z coordinates of all vertices on the three-dimensional model may not be 0, and the method of the present invention is used on the premise that the value of Z _min is 0, in order to ensure the correctness of the method, it is necessary to traverse all the vertices of the three-dimensional model, If Z _min is not 0, adjust it. The adjustment method is as follows: find the Z _min of the model, calculate the offset Δ=0-Z _min , and then add Δ to the Z coordinates of all the vertices of the model.

Triangular patch sorting

Because the top height method needs to traverse all the triangular planes that intersect with the previous layered plane, in order to reduce the time of searching for triangular faces, sort the faces in the Mesh, and sort the faces in ascending order according to the smallest Z coordinate value in the vertex of the triangular face. arrangement. At this point, you only need to find the corner mark a of the triangular face whose minimum Z coordinate value of the first vertex is less than or equal to the given height, and the maximum Z coordinate value is greater than or equal to the given height; meanwhile, find the minimum Z coordinate value of the first vertex greater than or equal to the given height. Given the angle label b of the triangular face of the height, the triangular face in the range of [a, b) is the triangular face to be found.

Calculate the dihedral angle threshold of the characteristic line

When calculating the threshold of the characteristic line, it is necessary to obtain the mean value β of the dihedral angles corresponding to all the mesh edges in the STL model. In order to calculate the β value, it is necessary to traverse the dihedral angles corresponding to the three sides of all triangular faces, and then obtain the sum of all the dihedral angles and divide by the total number of sides to obtain β. However, one edge will be shared by two triangular faces, so there will be repeated calculation. Therefore, when an edge is calculated, it is necessary to use the three judgment variables of the two related triangular faces, corresponding to this edge. The decision variable is set to true, so that when a certain side of a certain triangular face is calculated, if the decision variable corresponding to this side in the triangular face is true, the calculation of the dihedral angle of this side is skipped, thereby preventing repetitions. calculation situation. After calculating the value of β, in order not to affect the subsequent judgment of the feature edge, all judgment variables in all triangular faces are set to false.

Determine the features such as triangular faces, edges, and vertices on the model, identify the height of the feature, and obtain a set of ordered height data. Determining the feature height specifically includes the following steps:

(1) Complete the determination of all triangular faces, edges, vertices and other features in the model by traversing the faces in the Mesh once. Model features refer to the part of the 3D model that can characterize the shape of the model and has a non-smooth transition. The model shown in Figure 3 includes basic features including feature points, feature lines and feature surfaces. When a triangular face is traversed, the characteristic face, characteristic line and characteristic point are judged on the triangular face, edges and vertices in turn. If one of them is judged to be true, no subsequent judgment is made; if the obtained feature height is not In the feature height sequence h, it is stored in h. Among them, the specific methods for identifying feature surfaces, feature lines, and feature points are as follows:

Feature surface: The triangular surface parallel to the layered plane is the feature surface, as shown in Figure 3, 3 and 4. Then, when the Z coordinates of the three vertices of the triangular face are the same, it is determined that the triangular face is a characteristic face, and the characteristic height is the Z coordinate of a vertex.

Characteristic line: The intersection line of two surfaces whose included angle is greater than the specified threshold and neither is a characteristic surface is the characteristic line, and this intersection line is perpendicular to the layering direction, as shown in Figure 3 1, 2, 5, and 6. Two adjacent triangular patches have the same Z coordinate only if they have two common vertices, and the dihedral angle of these two triangular patches is greater than a certain threshold, and the feature height is the Z coordinate of a vertex on the feature edge. The dihedral angle can be obtained by calculating the normal vectors of the two triangular faces. The formula for determining the threshold value of the characteristic edge is as follows:

为特征边阈值，β为STL文件中所有网格边对应的二面角的均值，λ＝log(Facetres)，Facetres为CAD模型中对生成的STL网格模型精度的控制参数；in,

is the feature edge threshold, β is the mean value of the dihedral angles corresponding to all mesh edges in the STL file, λ=log(Facetres), and Facetres is the control parameter in the CAD model for the accuracy of the generated STL mesh model;

Feature point: In the layering direction, the local highest or lowest point is the feature point, as shown in Figure 3 7. If the Z-coordinate of vertex P is higher or lower than the Z-coordinates of the other two vertices of all triangular faces with vertex P as the vertex, then vertex P is the feature point, and the feature height is the Z-coordinate of vertex P.

(2) After the feature judgment is completed, set the judgment variables in the three vertices Point of the triangular face to true; and set the three judgment variables in the triangular face to true; and then set the other three triangles related to the three sides respectively. The decision variable corresponding to the corresponding edge in the face is set to true. At the same time, in the judgment process, if the judgment variable in the vertex Point is true, or the judgment variable corresponding to an edge in the triangular face is true, the judgment and subsequent operations are not performed on this vertex or edge.

(3) Loop feature determination process until the end of traversal. Finally, sort the feature height sequence h in ascending order.

Adjust the spacing between adjacent feature heights, that is, adjust the feature height sequence h in step 3 to avoid feature offset and loss. The specific steps are as follows:

(1) First, preprocess the ordered feature height sequence h. 3D printers have maximum and minimum printable thicknesses, also called maximum layer thickness _hmax and minimum layer thickness _hmin . If the model has dense features in a certain height interval, in a set of ordered height data obtained after identifying and locating the features, it may appear that the height difference d between two adjacent heights is less than the minimum layer thickness h _min . Since the printing is stacked from low to high, the contour information on the higher layered plane cannot be printed, so these two heights need to be adjusted. In order to ensure that the height of the printed model remains unchanged, and the first and last layers can be printed, the ordered feature height sequence h needs to be preprocessed. The preprocessing method is as follows: firstly insert the layer height data with a height of 0 and the maximum height of the model at the head and tail of the sequence h (called these two layer heights as boundary layer heights). Then check whether the distance between the two boundary layer heights and their adjacent layer heights is smaller than the minimum layer thickness. If it is smaller, adjust the adjacent layer heights to make the distance equal to the minimum layer thickness. Then re-sort the sequence h in ascending order to detect whether there are layer heights between the adjusted layer height and the corresponding boundary layer height, and if so, delete these layer heights.

(2) After the preprocessing is completed, the ordered feature height sequence h is checked by traversing one by one to find out whether there are two adjacent heights with a spacing smaller than the minimum layer thickness, and the other two adjacent heights, and calculate The maximum height difference d _max of four adjacent storey heights (the two storey heights whose height difference is less than the minimum storey thickness, and the other adjacent storey heights of these two storey heights), and it is determined that it needs to be merged inward Or the outward expansion operation, adjust these two heights. The adjustment method is shown in Figure 4 and Figure 5, and the specific operation process is as follows:

Merge inward: If the height difference d≤h _min /2, shift both slice layers inward by d/2, that is, merge the two slices.

Outward expansion: If the height difference d > h _min /2, the two slice layers are shifted outward by |dh _min |/2.

(3) Then, according to the determined height adjustment result and the height difference d _max , determine whether the two adjacent feature heights will affect their respective adjacent heights after the inward merging or outward expansion height adjustment is performed. Among them, the two adjusted heights may still have other adjacent feature heights, and the adjusted heights may have an impact on the adjacent upper and lower feature heights. The specific performance of the impact is shown in Figure 6 and Figure 7 , as shown in Figure 8:

After inward merging, at least one of the layer height differences d ₁ and d ₂ between the combined layer height a and its two adjacent layer heights b and c is still smaller than the minimum layer thickness.

After the outward expansion, at least one of the layer height differences d ₁ and d ₂ between the two offset layer heights a and b and the respective adjacent layer heights c and d is still smaller than the minimum layer thickness.

The feature heights are too dense, and among three or four consecutive adjacent heights, the height difference d between the highest layer height and the lowest layer height is still smaller than the minimum layer thickness.

(4) Finally, according to the height adjustment result determined in the above step (2) and the judgment of the influence on the respective adjacent heights in the step (3), the corresponding adjustment is performed. If there is an impact, it will be adjusted for different situations of inward merging, outward expansion and too dense features. Take out the four adjacent layer heights with the above three problems (the two layer heights whose height difference is less than the minimum layer thickness, and the other adjacent layer heights of these two layer heights), as shown in Figure 9, Figure 10, Figure 11, and then adjust accordingly for different problems. Among them, the specific impact performance and corresponding treatment methods are as follows:

Inward merging problems and solutions:

a. If h _min ≤ d _max ＜2·h _min , after merging the middle two layers with heights b and c, both d ₁ and d ₂ are smaller than h _min . In order not to affect other feature heights, delete the merged layer directly high e;

b. If d _max ≥ 2·h _min , after merging the middle two layers of heights b and c, only one of d ₁ and d ₂ is less than h _min , assuming d ₁ , adjust the merged layer height e so that d ₁ =h _min .

Outward expansion problems and solutions:

a. If d _max <3·h _min , the heights b and c, d ₁ and d ₂ of the two layers after the outward expansion are all smaller than h _min . Merge, at this time, the problem of outward expansion is transformed into the problem of inward merger, and it is dealt with according to the processing method of inward merger problem;

b. If d _max ≥ 3·h _min , after expanding the two storey heights b and c outward, only one of d ₁ and d ₂ is smaller than h _min , assuming d ₁ , adjust the expanded storey height b, While keeping their spacing h _min , let d ₁ =h _min .

Feature height is too dense problem and processing method:

The feature heights are too dense, and among three or four consecutive adjacent heights, the height difference d between the highest layer height and the lowest layer height is still smaller than the minimum layer thickness. d _max <h _min , in order not to affect other feature heights, delete the middle layer height directly, then search for the adjacent layer heights of the highest and lowest layer heights of the original four-layer high school respectively, and re-judge the four layer heights , if the above three problems still exist, use the corresponding solution; if not, end the adjustment and continue to search for the subsequent layer heights.

After layering and slicing at the feature height, adaptive layering is performed between every two adjacent feature heights using the top height method to obtain slice contour information. Among them, the specific description of the top height method is as follows:

The feature height refers to a series of numerical values obtained after steps S3 and S4, and these numerical values represent the height from the bottom of the model, that is, the height of the model feature relative to the bottom of the model. It is a fixed value, not an interval.

Layering or slicing refers to using an infinite plane (called a layered plane) to intersect the model at a certain position of the model to obtain the required outline information of the model, which is simply equivalent to cutting with a knife Model, will get a very flat cross section, the profile of the cross section is the information I want. This plane is perpendicular to the layering direction, but generally the default layering direction is vertical upward, so this plane is generally horizontal. If the layering direction is not vertical, the model can be rotated to make the layering method vertical, and the method of the present invention can also be used.

Layering at the feature height is to intersect the model with a horizontal plane whose vertical distance from the horizontal plane at the bottom of the model is the feature height to obtain the contour information here.

为三角面片的法向量，

为分层方向，θ为

和

的夹角，范围为0～π，d即为顶尖高度。对应层高计算方法如下公式所示。遍历与分层面BC相交的所有三角面片找到最小的h，若h低于或高于可打印范围，则将h改为最小层厚或最大层厚。之后将h与分层面BC的高度l相加得到当前层分层平面的高度l′＝h+l。循环此步骤便得到对模型自适应分层的一系列层高。In order to take into account the surface accuracy and processing efficiency of the workpiece, and the model between the feature heights does not need to consider the feature problem, this part of the model directly adopts the top height method for adaptive layering. The top height refers to the maximum distance between the surface of the actual manufactured part and the surface of the CAD model, as shown in Figure 12. BC is the triangular patch of the STL model, AB and AC are the surface of the printed part, AC is also the previous layer plane, h is the layer thickness of the current layer,

is the normal vector of the triangular patch,

is the layering direction, and θ is

and

The included angle ranges from 0 to π, and d is the height of the tip. The corresponding layer height calculation method is shown in the following formula. Traverse all the triangular patches that intersect with the layered layer BC to find the smallest h. If h is lower or higher than the printable range, change h to the minimum layer thickness or the maximum layer thickness. Then, add h to the height l of the layered plane BC to obtain the height l′=h+1 of the layered plane of the current layer. Looping through this step yields a series of layer heights that adaptively layer the model.

Note that when performing adaptive layering between two adjacent feature heights a and b (set b higher than a), pay attention to the spacing between a and b.

(1) When d < 2 h _min , the spacing is too small to perform layering, and the adaptive layering between a and b is ended;

(2) When d ≥ 2 h _min , adaptive stratification can be performed between a and b, but each time the height of the stratified plane is determined, the distance d' from the current stratified plane to b needs to be calculated, if it is less than the minimum Layer thickness, adjust the layer height of this layered plane so that d′= _hmin ; if the distance between this layered plane and the previous adjacent layered plane after adjustment is less than the minimum layered thickness, delete this layered plane .

Generate the corresponding print file according to the slice format, and print the 3D model. Select 5 models and use the equal thickness layering, the adaptive layering method of the top height method and the improved adaptive layering method of the present invention for layering respectively. The given maximum and minimum layer thicknesses are 0.1mm and 0.3mm respectively, The height is set to 0.1mm. In order to minimize the model feature offset caused by the equal thickness layer, the thickness of the equal thickness layer is 0.1mm. The results shown in Table 1 are obtained. Table 1 shows the three layering methods. Stratification results.

Table 2

From the layering results of the five models in Table 1, it can be seen that the number of layers obtained by the adaptive layering and the method of the present invention is less than that obtained by the equal-thickness layering. A large number of previous studies have proved that the adaptive layering method can effectively improve the processing efficiency, and the improved adaptive method of the present invention has a similar layer number to the traditional adaptive method, indicating that the present invention adopts the adaptive layering method between the two feature heights. The number of layers can be effectively reduced, and the processing efficiency can be improved to a certain extent on the premise of ensuring the surface accuracy. In addition, when the number of feature heights is small, the results of the method of the present invention are similar to the results of adaptive layering, such as the layering results of models 1-3 in Table 1; and as the number of feature heights increases, the number of layers obtained by the method of the present invention The number of layers obtained by self-adaptive layering is less, which further improves the processing efficiency. This is because in order to ensure that the outline at the feature height can be printed, the method of the present invention adopts the adaptive layering of the top height method in the part between the feature heights, which may cause the layering adjacent to and lower than the plane where the feature height is located. The spacing of the planes is close to twice the minimum layer thickness, so there is one less layered plane than adaptive layering. When the number of feature heights increases, the number of occurrences of this situation will increase, so the number of layers obtained by the method of the present invention will be significantly less than the number of layers obtained by adaptive layering. The reduction of the number of layers can save memory space and processing time for subsequent software processing and design processing, and improve processing efficiency. The height adjustment method of the invention ensures that the layered position passes through the feature height, can effectively prevent feature offset and loss, and improve processing quality.

In addition, taking the 3D model in Figure 2 as an example, three features are selected for comparison. The detailed results are shown in Figure 13 to Figure 21, and the feature offsets are shown in Table 2. Table 2 shows the results of the three hierarchical methods. Offset, the minimum and maximum values of the Z coordinate given in the experiment are 0mm and 54mm respectively. Three layering methods are respectively: equal thickness layering, self-adaptive layering of the top height method and the improved self-adaptive layering method of the present invention, and the obtained contour information is stored in the SLC format file, and then the three files are respectively combined with. The STL file of the original model is imported into the Materialise Magics software together, and the coordinate system of the model is completely coincident with the coordinate system of the contour, and the offset of the model features is obtained by measuring.

Table 2

From the feature offset details of features 1-3 shown in FIGS. 13 to 21 , it can be clearly seen that the method of the present invention does not generate offset, and since the layered plane passes through the feature height position, there is no feature loss problem. The results in Table 2 show that the feature offsets processed by the method of the present invention are all 0mm, and there are different degrees of offset for the equal thickness layer and the adaptive layer, which further shows that the method is better than other methods, and proves that the improved method of the present invention The adaptive layering method can effectively prevent the offset and loss of model features and reduce the step error.

To sum up, the self-adaptive layering method for preventing 3D printing model feature offset of the present invention ensures that the layered plane passes through the location of the model features, thereby retaining the 3D model features, and can effectively prevent the 3D printing model from being caused by materials during the printing process. The feature offset and loss caused by layer-by-layer accumulation reduce the step error.

The above-mentioned embodiments merely illustrate the principles and effects of the present invention, but are not intended to limit the present invention. Anyone skilled in the art can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical idea disclosed in the present invention should still be covered by the claims of the present invention.

Claims (5)

1. An adaptive layering method for preventing feature migration of a 3D printing model, which is characterized by comprising the following steps:

establishing a three-dimensional model, and carrying out gridding processing on the three-dimensional model to generate an STL file;

reading the STL file, and carrying out preprocessing operation on the data read from the STL file to obtain preprocessed data;

identifying the characteristic height on the three-dimensional model after gridding treatment by using the preprocessed data to obtain a group of height data h;

adjusting the distance between adjacent characteristic heights, namely adjusting the height data h;

carrying out layered slicing at the positions of the feature heights, and carrying out self-adaptive layering between adjacent feature heights by adopting a tip height method to obtain slice profile information;

generating a printing file according to the slice outline information and the G code format, and printing a 3D model;

wherein the identifying the feature height on the three-dimensional model to obtain a set of height data comprises:

the basic features of the three-dimensional model comprise feature points, feature lines and feature surfaces;

the characteristic surface is a triangular surface patch parallel to the layering plane, and when the Z coordinates of three vertexes of the triangular surface patch are the same, the triangular surface patch is judged to be the characteristic surface, and the characteristic height is the Z coordinate of one vertex of the three vertexes of the triangular surface patch;

the characteristic line, two adjacent triangular patches have and only have the same Z coordinate of their two common vertexes, and the dihedral angle of these two triangular patches is greater than the threshold value, the line between two vertexes is the characteristic edge, the characteristic height is the Z coordinate of a certain vertex on the characteristic edge, the dihedral angle is calculated and obtained through the normal vector of two adjacent triangular patches, the threshold value is confirmed through the formula (1):

wherein,

the method comprises the steps that a characteristic edge threshold value is adopted, beta is the mean value of dihedral angles corresponding to all grid edges in an STL file, lambda = log (Facetres) is adopted, and the Facetres is a control parameter for the accuracy of a generated STL grid model in a CAD model;

and if the Z coordinate of one vertex P of one of the triangular patches is higher or lower than the Z coordinates of the other two vertexes of all the triangular patches taking the vertex P as the vertex, the vertex P is taken as the characteristic point, and the characteristic height is taken as the Z coordinate of the vertex P.

2. The adaptive hierarchical method for preventing feature shift of 3D printing model according to claim 1, characterized in that: the files stored in the three-dimensional model comprise an STL file, an AMF file, an OBJ file, a STEP file, a 3MF file, an IGES file, a LEAF file, an RPI file and an RP file.

3. The adaptive layering method for preventing feature shift of a 3D printing model according to claim 1, wherein adjusting the distance between adjacent feature heights, namely adjusting the height data h, comprises:

preprocessing a group of height data h, respectively inserting layer height data with the height of 0 and layer height data with the maximum height of a three-dimensional model into the head and the tail of the height data h, respectively detecting whether the distance between the two boundary layer heights and the adjacent layer height is less than the minimum layer thickness, if so, adjusting the adjacent layer height to enable the distance between the boundary layer height and the adjacent layer height to be equal to the minimum layer thickness, sequencing the height data h in an ascending order again, detecting whether the layer height exists between the adjusted layer height and the corresponding boundary layer height, and if so, deleting the existing layer height;

given the maximum layer thickness h of the 3D printer _max And a minimum delamination thickness h _min Checking the feature heights between the boundary layer heights by adopting a one-by-one traversal method, searching whether the distance between two adjacent feature heights is smaller than the minimum layer thickness, if so, adjusting the two adjacent feature heights to obtain a feature height adjusting result, wherein the feature height adjusting method specifically comprises the following steps:

the height difference between two adjacent feature heights is d;

inward merging: if d is less than or equal to h _min 2, shifting the two slicing layers inwards by d/2, namely combining the two layers;

outward expansion: if d > h _min And/2, respectively shifting the two sliced layers outwards _min |/2；

According to the characteristic height adjusting result, four adjacent layer heights, namely two adjacent characteristic heights b and c for adjustment and the maximum height difference d of two layer heights a and f respectively adjacent to the two adjacent characteristic heights b and c for adjustment _max Judging whether two adjacent feature heights b and c have influence on two layer heights a and f adjacent to the two adjacent feature heights b and c after the height adjustment of inward deviation or outward deviation is carried out, if so, carrying out height adjustment aiming at different situations of inward merging, outward expansion and feature density, wherein the height adjustment method comprises the following steps:

inward combination for height adjustment;

inwardly merging two adjacent characteristic heights b and c in the middle, wherein the layer height difference between the merged layer height e and the two adjacent layer heights is d ₁ And d ₂ In (e), at least one remains less than the minimum delamination thickness h _min ；

1) If h _min ≤d _max ＜2·h _min Then d is ₁ And d ₂ Are all less than h _min If yes, deleting the merged layer height e;

2) If d _max ≥2·h _min Then d is a ₁ And d ₂ In is only one less than h _min If is d ₁ Adjusting the combined layer height e so that d ₁ ＝h _min (ii) a If it is d ₂ Adjust the combined layer height e so that d ₂ ＝h _min ；

Expanding outwards to adjust the height;

two adjacent characteristic heights b and c in the middle are expanded outwards, and the layer height difference d between the two layer heights after the displacement and the layer height of the adjacent layer ₁ And d ₂ In (e), at least one remains less than the minimum delamination thickness h _min ；

1) If d _max ＜3·h _min Then d is a ₁ And d ₂ Are all less than h _min Outwardly expanding the rotorChanging into inward combination, and performing height adjustment processing according to the inward combination;

2) If d _max ≥3·h _min Then d is ₁ And d ₂ In is only one less than h _min If it is d ₁ Adjusting the height of the expanded layer to keep the distance between the layers as h _min While d is being caused ₁ ＝h _min (ii) a If is d ₂ Adjusting the height of the expanded layer to keep the distance between the layers as h _min At the same time, make d ₂ ＝h _min ；

Carrying out height adjustment on the dense features;

the height difference d between the highest layer height and the lowest layer height of four continuously adjacent layer heights is less than the minimum layered thickness h _min Deleting the middle layer height, respectively searching the adjacent layer heights of the highest layer height and the lowest layer height in the four layer heights, judging the four layer heights again, and if the problems of inward combination, outward expansion and dense features still exist, using corresponding height adjustment to process; and if the problems of inward combination, outward expansion and dense features do not exist, finishing the height adjustment and searching the subsequent layer height.

4. The adaptive layering method for preventing feature shift of a 3D printing model according to claim 3, wherein the adjusting the distance between adjacent feature heights, namely adjusting the height data h, comprises: the feature density is that in a group of height data h, three or more continuous feature heights exist, and the height difference between the highest layer height and the lowest layer height is smaller than the minimum layering thickness h _min 。

5. The adaptive layering method for preventing feature shift of a 3D printing model according to claim 3 or 4, wherein layered slicing is performed at the feature heights, adaptive layering is performed between adjacent feature heights by using a tip height method, and obtaining slice profile information comprises:

sequentially taking out two adjacent heights from the adjusted height data h', and carrying out self-adaptive layering on a model between the two adjacent heights by adopting a centre height method, wherein the centre height method is the maximum distance between the surface of the actually manufactured part and the surface of the CAD model, a triangular surface patch of the STL grid model is the inclined edge of a right-angled triangle, and the height on the inclined edge is the centre height d;

the calculation method of the vertical upward right-angle side of the right-angle triangle is as follows:

wherein h is the layering thickness of the current layer,

is the normal vector of the triangular patch,

in the layered direction, vertically upwards, theta is

And

the included angle of (3) is in the range of 0-pi;

traversing all the triangular patches of the STL mesh model as hierarchical planes, and finding out the minimum hierarchical thickness h _min If the minimum lamination thickness h _min Below or above the printing range of the 3D printer, the minimum lamination thickness h will be _min A minimum layer thickness or a maximum layer thickness instead; will minimum lamination thickness h _min Adding the height l of the layering plane, namely the height on the hypotenuse of the right triangle, to obtain the height l' = h + l of the current layering plane until a series of layer heights for self-adaptive layering of the STL mesh model are obtained;

when adaptive layering is performed between two adjacent feature heights a and b, when d is less than 2 · h _min When the distance is too small, the self-adaptive layering cannot be carried out, and the self-adaptive layering between the a and the b is finished; d represents the height difference between two adjacent feature heights; when d is more than or equal to 2. H _min When the height of the layering plane is determined, the distance d 'from the current layering plane to the b is calculated, and if the distance d' is smaller than the minimum layering thickness h _min Then, the layer height of the layered plane is adjusted so that d' = h _min (ii) a The distance between the layered plane and the adjacent layered plane is less than the minimum layered thickness h _min Then the hierarchical plane is deleted.

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