Abstract: This paper introduces the composition, characteristics and motion control technology of a robot for rapid manufacturing of automotive body panel molds. The robot has five degrees of freedom and is directly driven by the three-dimensional CAD model data of the mold. It can automatically and efficiently complete two operations: metal spraying and electroplating without programming. The robot is used for the development of new car models and prototype production. It can quickly and cost-effectively manufacture automotive body panel spraying molds. Keywords: robot, automotive body panel, metal spraying, electroplating, rapid mold making, motion control Introduction The car body is composed of inner and outer body panels. Together with the chassis and engine, it constitutes the three major components of the car and is one of the main factors that determine the market competitiveness of automotive products. In order to quickly and continuously launch new models and improve the market competitiveness of automotive manufacturing enterprises, the development of new car models at home and abroad is mostly based on the redesign of the body shape on the basis of the unchanged "platform" (chassis, powertrain, etc.). The development of new models is, to some extent, the design and development of the body[1]. The manufacturing of prototype molds (also known as simple body panel molds) directly determines the cost and cycle of new model development. However, the mainstream manufacturing technologies for metal body panel molds at home and abroad, such as lost foam casting, large-scale precision CNC milling, and high-speed CNC milling, have long production cycles and high costs, making it difficult to meet the requirements of prototype vehicle trial production and small-batch production. Rapid mold making (RT) technology, which is based on RP technology, has the characteristics of flexibility, speed and low cost. Among them, metal spraying mold making method has great advantages in terms of cycle and cost for the manufacturing of large and medium-sized molds, and the working surface of the mold has good strength, hardness and wear resistance. However, since only low melting point metals can be used to manufacture sprayed molds in China, the surface hardness of the coating is relatively low, which seriously restricts the application range of metal spraying mold making method [2-5]. In view of this situation, the Advanced Manufacturing Technology Research Institute of Xi'an Jiaotong University proposed a rapid mold making technology for the integrated manufacturing of large and medium-sized automotive body panel molds by metal spraying/electroplated and developed the corresponding equipment. Metal spraying technology is used to spray a dense low melting point alloy coating with a certain thickness and strength on the surface of the master mold to form the required mold cavity. Electroplating technology is used to brush-plate a strengthening coating on the working surface of the mold. The integrated metal spraying/electroplating process combines the advantages of rapid deposition in metal spraying and the superior surface coating performance of electroplating. The metal spraying molding robot is one of the core technologies of this process. It has five degrees of freedom, and its motion control is directly driven by 3D CAD model data without programming. It can be used to automate and optimize both metal spraying and electroplating processes. This article mainly introduces the robot's composition, characteristics, and motion control technology. 1. Integrated Rapid Molding Process and Equipment for Metal Spraying and Electroplating The composite molding of arc metal spraying and electroplating is a composite molding technology that requires a physical model (or prototype) as a master mold. The master mold can be a rapid prototype or transitional model, a physical product, or a non-metallic model obtained through high-speed CNC machining. Arc metal spraying technology is used to spray a dense metal coating of a certain thickness and strength onto the surface of the master mold, thereby forming the required mold cavity. The coating material is a low-melting-point alloy. After filling with appropriate backing material and demolding, a reinforcing coating is applied to the working surface of the mold using electroplating technology, thus completing the rapid manufacturing of the mold. Arc metal spraying technology is a type of thermal spraying technology. Compared with other thermal spraying technologies such as ion spraying and flame spraying, it has advantages such as lower equipment investment, simpler process, higher production efficiency, energy saving, lower heat output to the substrate, and wider applicability. Due to the high bonding strength, dense structure, low porosity, high surface hardness, and high wear resistance of the coating obtained by arc spraying, it has received widespread attention in mold manufacturing in recent years. The rapid prototyping (RT) method of arc metal spraying based on RP technology is used in the manufacturing of large and medium-sized molds, which can significantly reduce costs and shorten the development cycle. Regarding the metal materials used for spraying, high-melting-point metals are prone to cracking and curling due to their large cooling shrinkage rate, making them difficult to adhere and form on the surface of the master mold. Low-melting-point metals such as Zn or Zn-Al pseudo-alloys have good mold-making effects and mature processes, but the coating hardness is relatively low, which seriously limits the applicability of sprayed molds. Electroplating is a surface strengthening technology that uses the principle of electrochemical deposition to rapidly deposit a coating of a specified thickness on selected areas of a conductive workpiece surface. Its key characteristics include: the anode contacts the workpiece surface through a wrapping material (composed of cotton or chemical fibers and a sheath); the anode and the selected local surface of the workpiece move relative to each other at a certain speed; and the use of high current density and precise control over the coating thickness. By preparing an electroplated layer on the surface of parts, the surface hardness, strength, wear resistance, corrosion resistance, and high-temperature oxidation resistance of the parts can be greatly improved. While electroplating can create high-hardness mold surfaces, its deposition rate is much lower than that of arc metal spraying, making it difficult to meet rapid deposition requirements. Furthermore, the safe thickness for electroplating is generally less than 1 mm. Therefore, the integrated rapid molding process combining arc metal spraying and electroplating utilizes the advantages of both processes, achieving complementarity in coating thickness, coating performance, and deposition efficiency. The main parameters of arc spraying include: spraying voltage, current, air pressure, spraying distance, and spray gun movement speed; the parameters of electroplating include: plating current, relative movement speed of the plating brush, and flow rate of the plating solution. Both processes involve both motion and non-motion parameters, requiring experimental optimization. The movement patterns of the plating brush and spray gun on the mold surface are basically the same, and their axes must be perpendicular to the surface where the working point is located. Currently, arc spraying and electroplating are mostly done manually. On the one hand, when the mold surface area is too large, the labor intensity of arc spraying and electroplating is too high, and the working environment is harsh, making manual operation difficult. On the other hand, it is not conducive to optimizing process parameters and cannot guarantee the quality of mold manufacturing. Therefore, automating the arc spraying and electroplating processes is crucial for the manufacturing of large and medium-sized molds. The level of automation in the manufacturing process directly affects the mold manufacturing quality and cycle time, and will also directly affect the promotion and application of this technology. Figure 1 shows the integrated arc metal spraying and electroplating mold manufacturing equipment, developed by the Advanced Manufacturing Technology Research Institute of Xi'an Jiaotong University. This equipment is specifically designed for manufacturing automotive body panel molds and consists of a computer, motion and process parameter controllers, arc spraying equipment, electroplating equipment, and a five-degree-of-freedom robot. After receiving the 3D CAD design data of the body panel mold, the computer on the equipment processes the data and, without programming, directly drives the robot to move along an optimized process trajectory, automatically completing both electroplating and arc spraying operations. It can also control the process parameters for both processes. Under computer control, the robot ensures that the axes of the plating pen and spray gun are always perpendicular to the surface where the working point is located. The conversion between the plating pen and the spray gun is achieved through an adapter on the device. The maximum size of the automotive body panel mold that this device can manufacture is 3500mm × 2000mm × 500mm. Figure 1: Composition of the integrated arc metal spraying and electroplating mold manufacturing equipment . 2. Composition of the metal spraying robot system. 2.1 Performance requirements and structure of the metal spraying robot's mechanical body are shown in Figure 2. The diagram shows three degrees of freedom (X, Y, and Z), with the remaining two rotational degrees of freedom unlabeled, for a total of five degrees of freedom. The displacement parameters for each degree of freedom are as follows: ① X direction, range of motion 3500mm; ② Y direction, range of motion 2000mm; ③ Z direction, range of motion 500mm; ④ Oscillating motion, rotation around the Y-axis, with the positive X direction as the initial direction, the spray gun's maximum oscillation range is ±90°; ⑤ Rotational motion, rotation around the Z-axis, with the positive X direction as the initial direction, the spray gun's maximum rotation range is ±90°. Motion in the X and Y directions is driven by AC servo motors, while the other three directions are driven by stepper motors. 2.2 Hardware Composition of the Robot Motion Control System The robot control system is a typical multi-axis real-time motion control system. Traditional robot control systems employ a closed architecture consisting of a dedicated computer and multiple microcontrollers with multiple control loops. This type of controller has technical bottlenecks in high-speed, high-precision, and multi-axis synchronous motion control. Furthermore, it suffers from high manufacturing and usage costs, long development cycles, difficulty in upgrading and replacing systems, and the inability to add new system functions. Therefore, we ultimately chose the domestically produced MCT8000F4 motion control card, which features an open architecture. A key characteristic of this robot control system is its open architecture combining a general-purpose personal computer and a DSP with multiple control loops, along with its network control capabilities. The MCT8000F4 motion control card provides 16-bit PIO, 32-bit DI and DO, 6 ADC channels, and 4 DAC channels, enabling simultaneous control of 4 servo motors and 4 stepper motors. The I/O interface board also provides opto-isolated inputs, conveniently used for limit switches, system zeroing, and interrupt inputs for other signals. The robot control system based on the MCT8000F4 motion control card is shown in Figure 3. This system uses an industrial computer with a PC bus as the hardware platform to handle non-real-time tasks in robot control, while real-time tasks are handled by the MCT8000F4 motion control card. Figure 3. Hardware Composition of Robot Motion Control System 3. Metal Spraying Robot Motion Control Technology 3.1 Holographic Data Based on STL Model Layering Processing The robot can automatically complete both spraying and electroplating operations. Regardless of the shape of the master mold, there is no need to write motion control programs for specific master mold prototypes; it is entirely driven by 3D CAD model data. Its motion control data is obtained through layering processing of the mold's STL model, as shown in Figure 4. We know that the layer contour data obtained after layering the STL model in a traditional RP system is a series of three-dimensional straight line segments connected end-to-end, without including the normal vector of each straight line segment. Furthermore, the RP process does not require the normal vector of each straight line segment. However, metal spraying and electroplating processes require the spray gun and plating brush to always be perpendicular to the surface where the working point is located, necessitating knowledge of the normal vector of each straight line segment. Therefore, the SLF format file in Figure 4 contains the normal vector. We call the layer contour data containing the normal vector obtained after layering the standard STL model "holographic data." Figure 4. Obtaining Robot Motion Control Data Based on STL Model Layered Processing. Our defined SLF format file has two representation methods: ASCII and binary. The following uses the ASCII format file as an example to illustrate the definition of holographic data. In an ASCII format SLF file, there are 10 keywords, separated from other items by spaces. They are: part, layersum, layerthk, layerbeg, layerend, layerpsn, nodesum, node, vector, and endpart. The structure of an ASCII-formatted SLF file is as follows: part [PartName] (Part name) layersum Layer_Sum (Total number of layers) layerthk Layer_thickness (Layer thickness) layerbeg layerpsn z (Layer z-coordinate) nodesum node_sum (Total number of nodes in a layer) node x0, y0 (Coordinates of the 0th node of the layer outline) … … node x node_sum-1, y node_sum-1 (Coordinates x, y of the -1th node of the layer outline) vector nx0, ny0, nz0 (Normal vector data of the 0th segment of the layer outline) … … vector nx node_sum -1, ny node_sum -1, nz node_sum –1 (Normal vector data of the -1th segment of the layer outline) layerend … … endpart [PartName] (Part name) 3.2 The software provided by the MCT8000F4 motion controller used for robot motion control includes: the BIOS function library of the motion control card (allowing users to directly operate all I/O of the MCT8000), the basic motion function library (1-3D motion interpolation and motion control functions), the extended motion function library (robot kinematic model and intelligent PLC code translator, etc.), and the Internet online controller (based on TCP/IP point-to-point secure communication protocol). Utilizing the software system and graphical development platform provided by the MCT8000F4, the development cycle of the robot control system can be significantly shortened. The control flow for robot motion control and non-motion parameters such as process parameters is shown in Figure 5. Figure 5: Control Flowchart of Integrated Arc Metal Spraying and Electroplating Rapid Molding Equipment. The holographic data used for motion control in Figure 5 is obtained by layering the mold CAD model after repositioning it in the robot coordinate system. Mold CAD model repositioning refers to: creating a physical prototype from the mold data model; loading the physical prototype onto the robot's work platform; measuring the position of the prototype in the robot's coordinate system; and feeding this position data back to the layered processing software to determine the placement of the mold's 3D CAD data model in the coordinate system, ensuring that the physical prototype and the data model have the same position and orientation. This allows the layered holographic data to be directly used for robot motion control. 4. Conclusion The metal spraying robot is a core piece of equipment in the rapid manufacturing system for automotive body panel molds. It can automatically and efficiently complete both metal spraying and electroplating processes, providing a convenient and fast way to manufacture large and medium-sized spraying molds and offering an experimental platform for optimizing process parameters. The robot's control system, composed of an industrial control computer and a domestically produced DSP processor-based motion controller, not only achieves robot motion control but also easily controls non-motion quantities such as process parameters. Using holographic data obtained from layered processing of the STL model to control the robot's motion allows for convenient acquisition of holographic data regardless of the model's simplicity, without the need for programming specific mold models. Furthermore, by controlling the generation of the STL model, minute changes in the mold surface can be ignored, which is beneficial for the motion control of the robot. References [1] Zhou Yongtai. Market forecast for the mold industry during the "15th Five-Year Plan" period [J]. Mold Industry, 2000, (1): 3-6. [2] Margaret BVS. Rapid tooling-another better idea from Ford [J]. Forming & Fabricating, 2002, 9 (5): 29-34. [3] Takeo Nakagawa. 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