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Photocuring rapid prototyping is recognized as the most accurate rapid prototyping method. Its essence is to meet the requirements of the user model. No matter what structure, it can be discrete into a set of two-dimensional thin layers, scan the solidification adhesion layer by layer, and finally form a physical prototype. It has the advantages of high manufacturing efficiency and material utilization rate close to 100%, and can form particularly complicated and fine parts. The accuracy of the formed part is inevitably related to a variety of factors.
Today, we analyze the factors affecting the accuracy of molded parts from the pre-printing, printing and post-processing processes, and hope to help you.
Before printing, we usually need to create a 3D model and process the previous data. In this process, errors affecting the accuracy of the molded part are mainly reflected in the conversion and output of the STL format and slicing.
Since the 3D printing device can only accept the external contour information of the model, the 3D printing device can be recognized only after discretizing the designed 3D graphics into a series of two-dimensional slices. STL is a model file obtained by discrete triangulation of CAD 3D models, which is formed by using many small triangles to approximate the free surface of the CAD 3D model. The number of small triangles directly affects the approximation accuracy. In other words, the higher the precision, the more small triangles are used. However, the number of small triangles cannot be increased indefinitely, because this causes a sharp increase in the amount of data in the STL file, which increases the difficulty of computer data processing, so the accuracy cannot be mentioned too high. When the model is completely planar, there will be no approximation error; but for the surface, no matter how high the accuracy is, it is impossible to completely fit the original image, so this approximation error is inevitable, which is bound to be Affects the precision of molded parts.
When the 3D model is layered, the information between the two layers is inevitably lost, resulting in prototype shape and size errors. The smaller the layer thickness, the smaller the error. However, the thickness of the layer is affected by accuracy, production time, and cost, rather than being as small as possible. Therefore, a reasonable choice of layer thickness helps to reduce or eliminate errors. When the dimension in the tangential direction can be eliminated by the thickness of the layer thickness, there is no error.
In the printing process, there are many factors that affect the accuracy of the molded parts. The main reasons are equipment errors, resin shrinkage deformation errors, and process parameter selection errors.
The most primitive factor affecting the accuracy of molded parts is the equipment error. This error is mainly caused by the molding machine. It can be controlled from the design and hardware system. The manufacturing equipment can be adjusted before leaving the factory to improve the precision of the printed parts and the equipment hardware. System reliability. The equipment error mainly manifests in the motion error in the X, Y, and Z directions and the positioning error of the laser beam or the scanning head.
Worktable Z-direction motion error
The workbench is a support plate, and the final part processing is completed by the up and down movement, and the up and down movement of the support type is realized by the screw. Therefore, the motion error of the table directly affects the layer thickness accuracy of the molded part, and finally leads to the dimensional error in the Z direction. The shape, large roughness and positional error of the formed part are mainly caused by the linearity error of the table. Therefore, the light curing equipment owned by Zhongrui Technology adopts the marble integrated Z-axis lifting platform, which minimizes the straightness error of the table in the vertical plane and obtains high printing precision.
Scanning mirror deflection
In the scanning process of the scanning mirror system, there are inherent geometric distortions, and the system itself has linear and nonlinear errors, which will affect the laser scanning quality of the vibrating mirror scanning system during the photocuring process.
Changes in material morphology directly affect the accuracy of the molded part. During the molding process, the resin produces linear shrinkage and volume shrinkage from liquid to solid. Linear shrinkage creates complex interlaminar stresses when layers are stacked, resulting in warpage of parts, resulting in reduced precision. The mechanism is quite complex, related to material composition, photosensitivity, and polymerization rate. Volume shrinkage has a certain effect on the warpage of the molded part, but there is no direct quantitative relationship. Warpage mainly comes from linear shrinkage. However, the quantitative relationship between warpage and linear contraction is not constant. Therefore, in order to improve the accuracy, the shrinkage of the material should be reduced. In addition to the low resin shrinkage, it is also possible to reduce shrinkage by improving the formulation of the material.
Influence of spot diameter
For a photocuring machine using ultraviolet light as a light source, the spot diameter is much larger than the laser spot diameter and cannot be regarded as a spot. At a certain scanning speed, the actual cure line width is equal to the actual spot diameter. If no compensation is made, the outer contour of the solid part of the molded part is larger than the spot radius, resulting in a positive deviation of the physical size of the part, especially the fillet at the corner. As shown, it directly affects the size of the molded part.
Scanning speed
The scanning speed refers to the linear velocity of the ultraviolet scanning two-dimensional laminate. The size of the resin is related to the depth of cure of the photosensitive resin. The lower the scanning speed, the more energy the resin absorbs, the deeper the cure depth, and the higher the degree of cure. Especially in the vicinity of the edge of the molded part, the scanning speed is low, and the curing phenomenon often occurs at the edge due to the change of the scanning direction and the holding time. However, the scanning speed cannot be blindly pursued low, and the scanning speed that is too low will cause the curing speed of each layer of the resin to be too fast, affecting the connection of the parts of each layer, and causing the warpage of the parts. When the scanning speed is high, the resin cannot absorb sufficient energy to cure, or the curing depth is insufficient, so that the adhesion between adjacent layers of the workpiece becomes small, and the interlayer bonding is unreliable and slipped.
Influence of scan spacing
The scanning pitch refers to the distance between adjacent scanning lines when the ultraviolet beam scans a two-dimensional area. In the scanning curing process, the plane is composed of a plurality of curing lines, and adjacent lines are embedded in each other. The size of the scan spacing determines the degree of embedding between adjacent cure lines in the same layer and the number of scan lines. When the scanning pitch is large, the number of scanning lines is small, and the embedding degree of adjacent curing lines is also small, and a sawtooth effect occurs, which seriously affects the surface quality of the molded part. When the scanning pitch is larger than the curing line width, there will be a gap between the lines filled with the liquid resin, which will destroy the entire molding portion. When the scanning pitch is small, the number of scans required for the same curing area increases, the number of curing lines is large, and the energy is too high. The shrinkage deformation of the later scanning lines may affect the cured parts, which may easily cause shrinkage, warpage or even cracking of the molded parts. . Therefore, the choice of scan pitch should take into account manufacturing accuracy, strength and molding efficiency requirements.
After the photocuring member is formed, the molded part needs to be disassembled and post-treated, including removal of the bracket, secondary curing of the incompletely solidified part, polishing, grinding, and surface treatment. These processes will have an impact on the accuracy of the molded parts. E.g:
After the bracket is removed, the shape and size of the formed part may change, ruining the existing accuracy. Therefore, a reasonable support structure should be selected in the support design, which can play a supporting role and can be easily disassembled, so that the support can be minimized within the allowable range.
Due to changes in temperature, environment, etc., parts may continue to deform after forming, causing errors. Due to the forming process or structural process of the part itself, residual stress is generated in the molded part, and some or all of the aging effect disappears, which may cause errors.
Problems such as small processing steps, small defects, and inaccurate size due to layered manufacturing may not meet the needs of users. Therefore, it is necessary to perform grinding, repair, polishing, and shot peening, and any processing problem affects the accuracy of the molded part.