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A NEW SOLVER FOR THE FORMING PHASE FROM THE THERMOFORMING PROCESS

The thermoforming process allows manufacturing products on an extensive range of length scales, from millimeters to meters, and with a wide range of industrial products including packaging (the most relevant), advertisement, housings, automotive and aeronautical industries, among several others. It is designated as a secondary process in the plastics industry because it uses plastics in the form of sheet or film. It consists, sequentially, of the following phases: heating, forming, cooling, demoulding, and trimming, all affecting the quality of the manufactured part. The process starts by heating the sheet (a thermoplastic semi-finished product) until the forming temperature is reached, then the forming starts by applying a pressure gradient on both sides of the sheet, which forces it to conform to a mould with the desired shape. While getting in touch with the mould, which is at a lower temperature, the sheet cools down, until reaching a temperature where the new shape will be sustained for the demoulding phase, where the reshaped sheet is removed from the mould. The final product is obtained after trimming the excess material. There are some thermoforming methods, and some are distinguished by the approach used in the forming phase, the one known as vacuum forming is a process in which the air located in between the heated sheet and the mould is evacuated, and then, since the other side of the sheet is subjected to the atmospheric temperature, the pressure gradient forces the sheet towards the mould walls. There are a number of advantages that makes this process widely used, such as the low cost associated to the process, and the flexibility of the materials used, since almost all thermoplastics that can be manufactured into a sheet form. However, are a number of difficulties hard to tackle, such as assuring a uniform thickness distribution in the final product. The large number of degrees of freedom for this process and its inherent non-linearity, difficult the proper definition of the process parameters. This research group is developing a simulation framework for the thermoforming process, using the OpenFOAM computational library, which is going to cover all the relevant phases of the process. This work presents a proposal to support the design activities to mitigate some pointed limitations and disadvantages, where the simulation framework can be used to predict possible problems before building any manufacturing device used for the process. The developed methodology will be illustrated with some case studies, comparing the initial and the final thickness for the sheet and for its distribution in the final part.