A stress-based 3D printing continuity path planning method

Abstract

The application aims to provide a stress-based 3D printing continuity path planning method. The application obtains a to-be-printed model, discretizes the to-be-printed model into a hexahedral grid, the side length of the grid is twice the printing path width, and the height is the pre-printed layer thickness, and the principal stress field is obtained by using finite element analysis. Since the path is generated on a two-dimensional quadrilateral grid, the principal stress direction field is projected onto the two-dimensional domain. For the stress value of the unit close to zero, the stress direction is arbitrary, a method of uniform principal stress direction field is proposed, the stress line is extracted from the principal stress direction field, a plurality of connection rules are adopted, the isolated points are eliminated, and the continuity is optimized. Finally, each stress line is offset to generate a path with a closed loop, and different paths are connected to generate a globally continuous path. In addition, the extrusion rate is adjusted adaptively according to the path direction to alleviate the overfilling problem caused by grid discretization. The model printed by the method has strong mechanical properties under the premise of guaranteeing the surface quality and the curved surface accuracy.

Description

Technical Field

[0001] This invention relates to the field of 3D printing, specifically a stress-based continuous path planning method. Background Technology

[0002] 3D printing, also known as additive manufacturing (AM), is a technology that uses adhesive materials to create objects layer by layer. In recent years, it has been widely applied in fields such as mechanical design, biomedical engineering, structural engineering, and education. The quality of products produced by additive manufacturing processes is affected by parameters such as layer thickness, molding temperature, printing materials, printing speed, and path planning. Among these, path planning has a significant impact on processing efficiency, surface finish, structural stiffness, and anisotropy.

[0003] Numerous path planning algorithms have been proposed to date. Some of these have been integrated into commercial software, such as Zigzag, Hilbert, Contour, and Fermat. Currently, the combination of the Contour and Zigzag algorithms has received widespread attention because it inherits both the surface accuracy of the Contour algorithm and the simplicity of the Zigzag algorithm. However, the aforementioned algorithms only focus on geometric accuracy and printing efficiency, without considering the mechanical properties of the manufactured object. When an object is subjected to external forces, the anisotropy of the path will have a certain impact on its mechanical properties. Experiments have shown that when the path direction is parallel to the principal stress direction, the tensile strength and compressive strength of the object can be increased by 176% and 21%, respectively. Therefore, considering the anisotropy of the path in path planning is necessary and important.

[0004] To address the above issues, this invention proposes a 3D printing stress path planning method with global continuity, which can improve the mechanical properties of the molded parts while ensuring printing speed and surface accuracy. Summary of the Invention

[0005] The main objective of this invention is to provide a stress-based continuous path planning method for 3D printing, in order to solve the problem of the influence of path anisotropy on the molded part in the prior art.

[0006] The technical solution adopted in this invention mainly includes the following steps:

[0007] Step 1: Discretize the model based on the pre-printed path width and layer thickness;

[0008] To generate custom discrete elements, a bounding box is first generated based on the model's dimensions, and then discretized into numerous hexahedral elements. The element's length and width are twice the predetermined path width (2d), and its height is the layer thickness (h). Next, the model is sliced ​​to obtain its contour within each slice. Finally, within each slice, the coordinates of the hexahedral element centers within that contour are obtained using a ray-drawing method.

[0009] Step 2: Stress smoothness optimization;

[0010] The model obtained in step 1, composed of numerous hexahedral elements, is imported into the finite element analysis software. Loads and boundary conditions are applied to the imported model to obtain stress data for each hexahedral element. Since the sign of stress represents direction, the stress direction of this part of the elements is complex and variable when the stress approaches zero. Next, a threshold η is set to capture this part of the elements, the stress directions of these elements are deleted, and the stress directions of the surrounding adjacent elements are assigned to these elements to ensure stress smoothness.

[0011] Step 3: Continuous stress path;

[0012] Stress lines are generated using stress, and these lines are composed of numerous discrete element coordinates. Paths with closed loops are generated by offsetting the stress lines to both sides. The offset distance varies depending on the direction of the stress line: (d / 2) when the stress line is horizontal or vertical, and (√2d / 4) when the stress line is inclined. Finally, a depth-first search algorithm is used to connect each adjacent path and merge them into a single path.

[0013] Step 4: Molding technology for different path widths;

[0014] When the path direction is ±45°, if the path width is (d), this will result in overfilling, affecting the surface finish of the molded part. Therefore, the path in this direction is optimized by adjusting the extrusion ratio.

[0015] The beneficial effects of this invention are as follows:

[0016] In the planar dimension, this invention employs quadrilateral discretization, which effectively improves the stress concentration areas of existing stress paths. Furthermore, a global continuity algorithm is introduced; from a certain layer perspective, this algorithm can complete all paths in one stroke, eliminating nozzle jumps and idle strokes, significantly improving printing efficiency. Simultaneously, the absence of stringing caused by idle strokes improves the surface accuracy of the model. Attached Figure Description

[0017] Figure 1 This is a flowchart of the present invention.

[0018] Figure 2 This is a schematic diagram of the bounding box discreteness.

[0019] Figure 3 This is a schematic diagram comparing the stress direction before and after smoothing.

[0020] Figure 4 Schematic diagrams before and after stress line optimization.

[0021] Figure 5 Custom bias diagram.

[0022] Figure 6 Global continuity optimization. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] like Figure 1 As shown, the method for generating a continuous stress path planning function mainly includes the following steps:

[0025] Step 1: Discretize the model based on the pre-printed path width and layer thickness;

[0026] The 3D model in STL format is discretized, and the AABB (axis-based) bounding box of the 3D model relative to the world coordinate system is calculated. Then, according to the required resolution, the bounding box is divided into a set of hexahedral voxels of the same size. For example... Figure 2 As shown, the voxels of the entire bounding box are obtained. The higher the resolution, the smaller the voxels. The length and width of the voxels are set to twice the path width (2d), and the height is the layer thickness (h). In this example, the size of the hexahedral unit of the voxel is 1.12*1.12*0.2 (unit: mm).

[0027] The model is sliced ​​using traditional methods based on layer thickness to obtain the boundary contour of each layer. All voxel blocks within the bounding box are traversed, and ray casting is used to obtain the voxel units inside the model contour. The model discretization is now complete, and the result is as follows: Figure 1 As shown in (a).

[0028] Step 2: Stress Extraction and Optimization;

[0029] The discretized hexahedral elements are imported into finite element analysis software. Loads and boundary conditions are applied to them, and finite element analysis is performed to obtain the principal stresses [α] along the X and Y axes of each discrete element. x ,α y ] and the shear stress τ of the plane xy Next, the maximum principal stress and direction of the maximum principal stress for each discrete element are calculated using the stress formula for the element body. The formula is as follows:

[0030]

[0031]

[0032] After obtaining the stress data, the sign of the maximum principal stress represents its direction. When positive, the stress is tensile; when negative, it is compressive. When the stress value of an element is very small, that element exhibits a lack of directionality. Therefore, a threshold η is set to filter out these elements, their stress directions are deleted, and the stress directions of their neighboring elements are assigned to them to ensure stress smoothness. Figure 3 This is a schematic diagram comparing the stress direction before and after optimization.

[0033] Step 3 has a stress path with global continuity;

[0034] To extract stress field lines based on the stress field and its corresponding direction field, starting with the discrete element with the highest stress, the eight adjacent elements surrounding that element are divided into four regions, with each region consisting of two diagonally opposite elements. Based on the stress direction of that element, the adjacent elements of a region are designated as candidate elements for that element. The next element to be connected is then selected from these two candidate elements. If the element has fewer than eight adjacent elements (e.g., ...), the process continues. Figure 2 If the corner elements and boundary elements shown do not point to any region where adjacent elements exist, then no candidate element is assigned.

[0035] Starting with the element with the highest stress, select the element with the highest stress from its candidate elements and connect them. After connection, the connected element is called the current element. Starting with the current element, if there is only one candidate element, connect directly. If there are two candidate elements, prioritize connecting those with the larger angle to the stress line. If there are no candidate elements, stop connecting and start connecting from a new element. Following this connection rule, after traversing all discrete elements, multiple stress lines will be generated.

[0036] Each element is assigned a weight: a degree of 1 for an endpoint of a stress line, 2 for an intermediate point, and 0 for an element not included within a stress line. To optimize the continuity of the stress line, all elements are traversed; if an element has a degree of 1 and its surrounding elements have degrees of 1 or 0, it is merged or connected. Figure 4 The diagram shows the before and after optimization.

[0037] The optimized stress lines are offset. When the stress lines are vertical or horizontal, the offset distance is (d / 2). When the stress lines are obliquely connected, the offset distance is (√2d / 4). (See reference.) Figure 5After the paralysis is complete, the loop is sealed to create a line segment with a closed loop, which can be called a path. If there are still elements not included in the stress line, a square path with side length d is generated around the center point of that element. After generating the paths, an adjacency matrix is ​​generated based on whether each pair of paths is adjacent. Then, a depth-first traversal algorithm is used to sort all paths according to the adjacency matrix, and a connecting line is generated between two paths. This connecting line prioritizes endpoints to reduce the number of turns in the final path. Finally, all paths are merged according to the connecting lines to achieve a globally continuous path. Figure 6 As shown.

[0038] Step 4: Print and shape;

[0039] Due to the limitations of planar quadrilateral voxels, overfilling can occur when printing paths at an angle. Therefore, vertical, horizontal, and angled paths are divided into two categories: the first category has a path width of √2d, and the second category has a path width of d. Different extrusion rates are used to achieve different path widths. In this example, the path widths are 0.56 and 0.4 (unit: mm). Figure 1 c is the optimized printout. Figure 1 d represents the molded part.

Claims

1. A stress-based method for continuous path planning in 3D printing, comprising the following steps: Step 1: Discretize the model based on the pre-printed path width and layer thickness; First, a bounding box is generated based on the dimensions of the model. Then, the bounding box is discretized into numerous hexahedral elements, where... The length and width of the unit are twice the predetermined path width d, and the height is the layer thickness h. Next, the model is sliced ​​to obtain the contour of the model in each slice. Finally, within each slice, the center coordinates of the hexahedral unit within the contour are obtained using the ray-drawing method. Step 2: Optimize stress smoothness; The model obtained in step 1, consisting of numerous hexahedral elements, is imported into the finite element analysis software. Loads and boundary conditions are applied to the imported model to obtain stress data for each hexahedral element. Next, a stress threshold η is set to obtain the elements in that part. Stress directions of elements in that part that are less than η are deleted, and the stress directions of the surrounding adjacent elements are assigned to these elements to ensure stress smoothness. Step 3: Generate a continuous stress path; Stress lines are generated using stress, and these stress lines are composed of numerous discrete unit coordinates. Paths with closed loops are generated by offsetting the stress lines to both sides. The offset distance varies depending on the direction of the stress line. When the stress line is horizontal or vertical, the offset distance is d / 2, and when the stress line is inclined, the offset distance is √2d / 4. Finally, a depth-first traversal algorithm is used to connect each adjacent path and merge them into a single path.

2. The stress-based 3D printing continuity path planning method according to claim 1, characterized in that: During breakpoint optimization, each discrete element is assigned a degree, with endpoints assigned a degree of 1, intermediate elements assigned a degree of 2, and elements not included in the stress line assigned a degree of 0. Finally, the two stress lines are merged based on the degree.

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