# Thermoforming Simulation of Woven Carbon Fiber Fabric/Polyurethane Composite Materials
## 1 Introduction  
Composite materials are popular candidates for weight saving in structure components that are applied in aerospace, automobile, sports, and other industries because they have high stiffness/weight and strength/weight ratios. Currently, thermoset composite materials are dominant in these applications, considering their good mechanical properties. Even though thermoplastic composite materials have no competitive stiffness and strength, they have very good toughness and recyclability, a fast forming cycle, and an excellent shelf life. Based on these advantages, thermoplastic composite materials have started to become a better choice in some situations. To circumvent the high viscosity and cost, thermoplastic composite materials may be used in a semi-finished form, such as sheets, for further manufacturing processes. Therefore, forming processes like thermoforming, hydroforming, vacuum forming, rubber pad forming, and diaphragm forming are possible choices [1, 2]. The main considerations for the selection of forming processes are the cost of the mold and the quality of the final part. Matched-die press forming, otherwise called thermforming, is a suitable selection for thermoplastic composites because it can be easily accessed, automated, and used to make components with complicated shapes. In the thermoforming process, the thermoplastic blank is heated to a temperature between the glass transition temperature and the melting temperature such that it is soft. Then, the blank is pressed with molds having complicated shapes, and the solidification iscomplete after the cooling process is finished. Generally, thermoforming is a simple, fast, and high-deformation process, and it can be automated. Therefore, it draws widespread interest in thermoplastic composites. For instance, [PERSON] et al. [3] thermoformed a woven glass fabric/polyamide 6 thermoplastic blank into a three-dimensional hat profile at around 270 \({}^{\circ}\)C. Then, it was bonded with a closing plate into a crash box for crashworthiness investigation. [PERSON] et al. [4] developed a fully automated forming process to construct the same glass fiber-reinforced thermoplastic composite sheet at 260 \({}^{\circ}\)C into a down-scaled battery tray for a plug-in-hybrid automotive vehicle. Using two subsequent forming steps and local heating of the structure, [PERSON] et al. [5] could manufacture a thermoplastic composite shaft with an integrated flange from a semi-finished tube at a temperature range that is higher than the melting point. Because thermoforming is a temperature-related process, to obtain a product with good quality, the process parameters, especially those related to temperature, need to be carefully selected. [PERSON] et al. [6] considered the effect of cooling rates on the mechanical properties of the thermoformed glass fiber-reinforced polypropylene composites. Judging from the final dimensions of the products, [PERSON] et al. [7] tried to find the optimal molding temperature for forming the carbon fiber/polyphenylene sulfide sheets into V-shaped parts. To prevent the wrinkling of the glass fiber-reinforced polypropylene composite laminates, the effect of the stacking sequence at the drawing stage [8] was studied. For the products having complicated double curvatures, the various thermoplastics and fiber orientations of thermoplastic composite blanks and the fixed method of mold were experimentally proven to be very important [9]. In addition, forming rate and blank holding pressure are also important parameters. Therefore, thermoforming is a very complicated, nonlinear, and multivariable process. To find the optimal parameters for the thermoforming process of thermoplastic composite materials, finite element simulation is a better choice instead of theoretical modeling. The current methods of simulating this thermoforming process can be roughly classified into the continuous method, discrete method, and semi-discrete method.