I. Introduction The CAD 3D modeling of the product is an important part of the whole product design process. In order to improve the 3D design efficiency and quality of the product, the user must master the principle of 3D CAD modeling and fully understand the modeling method in the CAD software. Making full use of modern advanced CAD technology can not only help designers to complete their design ideas, reduce labor intensity, improve efficiency and precision, improve visual effects, but also lay the foundation for subsequent analysis, mold design, NC processing and so on. UnigraphicsNX provides technology based on parameterization, feature-based solid modeling, inter-component correlation design, and expert design systems. Its core technology is the use of non-uniform rational B-spline (NURBS) as the basis of surface modeling, fusion wireframe model, surface modeling, solid modeling as one, parameterized and characterized hybrid modeling system. The system is based on a unified and relational database, and provides complete correlation in engineering to make the CAD/CAM/CAE data linkage and linkage. UGNX absorbs the advantages of various modeling techniques and is the first to introduce the most advanced composite modeling method, integrating parametric solid modeling, advanced free-form surface modeling and wireframe modeling. Composite modeling technology represents the trend of CAD development, that is, easy to use, free and efficient, and is the most optimized modeling method in the current CAD industry. It allows engineers to work freely and quickly without having to be bound by a single modeling approach at work. It has both flexible and convenient parametric modeling and unique advantages in handling complex products. UGNX is oriented to the actual engineering needs and gradually developed into a CAD/CAM/CAE/PDM integrated system. The system also has a variety of expert design modules such as sheet metal design, injection mold design, stamping die and progressive die. Second, UGNX / CAD cover product design 1. Design example of cover piece based on SMC molding The SMC-formed cover is a fully composite structure consisting mainly of skin and ribs. The ribs in the skin serve as a skeleton to increase the strength and rigidity of the cover, as shown in Figure 1. The composite cover must meet certain weight and stiffness requirements, such as deformation and torsional stiffness under the action of gravity, surface bubbles and thermal deformation of the high temperature paint. Through the SMC forming process and structural research of the cover, the forming method adopts block molding and finally bonding molding, which is composed of three parts: outer skin, inner reinforcing rib and front face. Due to the complicated structure of the cover, when the forming scheme is designed, the skin and the rib are separately molded, and then bonded into one body, and local reinforcement is performed according to the strength and rigidity requirements. Its structural design has the following characteristics: (1) Internal and external structural shape of the product The outer shape of the product should be simple, and the inner cavity is the main profile of the product. To ensure easy release and molding, it should be designed as a rounded structure. Avoiding excessive depth of the cavity is difficult to mold. All of the joint features of the three components of the cover are made of potentially large rounded corners, making them easy to form. (2) Demoulding slope For thermosetting plastics, the draft angle is generally not less than 1 在 on the surface parallel to the pressing direction. For brittle hard materials, the draft angle should be designed to be larger. For products with higher precision requirements, a smaller drafting angle should be adopted. For products with complicated shapes and large shrinkage, thick-walled fabrics that are not easily demolded should adopt a large draft. Due to the large size of the cover, the draft design is generally designed to be more than 3 inches, and the local area is between 1.5 ゚ and 8 ゚. (3) Wall thickness Due to the good fluidity of SMC, it is possible to form various thin-walled and thick-thickness products. The thickest section should be kept below 13mm. For thicknesses exceeding 25mm, a special initiation system can be used. When designing SMC products, avoid it. If the size is too thin, the large-area size should be larger than 1mm thick. Because of the unevenness of the wall thickness, deformation is easy. Therefore, in the variable thickness area, a smooth transition should be adopted. The thickness of the SMC molded cover product is generally about 3mm. (4) Reinforcement and flange structure For the thin-walled structure of the cover, in order to increase the rigidity and prevent the deformation of the product, the rib and the flange structure are designed in many places, the sides of the rib are both the draft angle of more than 3 和 and the fillet radius of 3 mm or more. The reinforcement structure is shown in Figure 2 below. For the working conditions after the overall bonding, the thin-plate aluminum alloy beam is reinforced in a local area, and the SMC and the aluminum alloy beam are connected by bonding and screws. Structural strength stiffness optimization is mainly enhanced by the use of box-shaped members, rectangular ribs and flanges. 2. UGNX based cover design skills The cover is relatively complex and requires a reasonable decomposition of its structure to determine the main features of the product structure. In the modeling, the characteristic curve is established according to the main structure of the product, and a reasonable "blank" is established by stretching, rotating, scanning, etc., and then the detailed design of the appearance is performed, and the appearance shape of the product is mostly smooth and smooth. composition. To ensure that the constructed surface is smooth and meets certain accuracy requirements, it is necessary to master certain surface modeling techniques. Due to the complexity of the design of the cover product itself, the requirements of the designer are relatively high. The following are some of the experiences that the author has summarized when completing the product design: (1) The combination of overall post-local, parametric and non-parametric In engineering, it is often difficult to express a complex product shape with a continuous, complete surface. The surface thus constructed tends to be unsmooth and produces large deformations. At this time, according to the surface modeling method provided by the software, combined with the shape of the product, it is divided into a plurality of regions to construct a plurality of curved surfaces, and then stitched or linked with the transition surface. For the needs of the product and the goal of optimizing the design, the mixed use of parameterization and free-form surface modeling provided by UGNX should be used reasonably during the modeling. For example, the parameterized wireframe model is used in the layout design of the ribs. In the optimization analysis and surface reconstruction design, the design quality and efficiency of the product are improved; for the local features such as the R angle of the SMC molding, the result after the finite element analysis can be finally determined. (2) Skilled use of basic modeling functions In the design of SMC-formed cover products using UGNX, there are many basic curved surface operations, such as the commonly used Ruled Surface, Extruded Surface, Swept Surface, and Zoom Surface. Enlarge Surface), Offset Surface, Blend Surface, Trim Surface, Sew Surface, Extend Surface, Boundary Surface. Pay attention to the use characteristics of these basic and derived surfaces, such as the stretch function for surface design with oblique draft angle. Also for basic curves such as lines, arcs, Trim curves, Arc lengths, Offset Curves, Project Curves, Intersection Curves, and Curve Connections (Join Curve) and other generations and their editing and modification should be flexible and proficient. At the same time, we should distinguish between the use of Sketch and Curve and the flexible use of coordinate systems. (3) use of shortcut keys, layers and colors Since the workload of the cover product design, mold design and numerical control programming is relatively large, the use of shortcut keys helps to reduce labor intensity. Reasonable planning layers and color representations of geometric objects help to improve design efficiency and information expression. Taking full advantage of the Array and Transform functions of graphic objects can greatly improve the design efficiency and quality of products. (4) From the perspective of the mold The ultimate goal of three-dimensional modeling of products is to manufacture. Most of the product parts are produced by the mold. Therefore, in the three-dimensional modeling, it is necessary to consider from the perspective of the mold. After determining the direction of the product, the surface should be inspected for mold release and whether there is a reverse buckle phenomenon (ie, the mode is dialed). The angle is a negative angle. If an undercut phenomenon is found, the control line of the surface should be modified to reconstruct the surface. This is often overlooked, but it is very important. It is especially important in the design of SMC molded parts. Due to the requirements of SMC molding process, the fillet is generally not less than R3, and the draft angle is not less than 1 ゚. Next page