Carbon fiber, as a reinforcing phase in fiber-reinforced composite materials, has a wide range of application scenarios [
1,
2,
3,
4]. Despite exceptional mechanical properties exhibited by carbon fiber composites, the matrix penetration effect in the carbon fiber is not ideal during the preparation of composite materials, resulting in easy production of gaps and defects between individual filaments [
5,
6]. At the same time, the diameter of tow fibers is also prone to fiber buckling and the dislocation of the layering angle during the molding process, resulting in mechanical properties that cannot meet design requirements. Carbon fiber composite structures made of thin layers of carbon fiber prepreg tow have excellent static and dynamic mechanical properties [
7,
8,
9,
10]. However, the difficulty and the cost of preparing thin-layer carbon fibers have become a difficult problem restricting the development of high-performance carbon fiber composites [
11,
12,
13]. Therefore, the development of a widening technology for thin lamination of carbon fiber can effectively solve the above problems, improve the performance of the material and promote the large-scale application of thin-layer carbon fiber prepregs.
To ensure the integrity of the mechanical properties of fiber bundles, the automatic stretching technology is used to reduce the thickness of carbon fiber bundles and increase their width, to obtain a thin layer of carbon fiber. The commonly used widening techniques mainly include the mechanical widening method, the ultrasonic widening method, the electrostatic widening method, and the air flow widening method [
14]. Studied on the use of ultrasonic widening to expand carbon fibers have been conducted by relying on the effect of ultrasonic vibration to expand carbon fibers. Ma’s group [
15] from Jiangnan University invented a widening method that combines ultrasonic and chemical solutions. In the above research, the ultrasonic widening method is inseparable from the chemical solution, and the chemical solution needs to be prepared according to the sizing agent on the surface of carbon fiber, which has relatively high cost and thus a low economic benefit. There are also studies using the method of electrostatic widening to charge carbon fibers, relying on the method of mutual repulsion of the same charges to thin carbon fibers. Ren et al. [
16] used the electrostatic widening method to carry out the secondary widening of fiber bundles. The results showed that the high-voltage electrostatic field widening is conducive to the dispersion of fiber bundles and at the same time reduces the abrasion and fracture of fiber bundles during the widening process. However, the electrostatic method needs to remove static electricity from the fiber, and the fiber is prone to problems such as yarn. Some studies have used the method of air flow [
17] for widening, relying on the effect of air flow on the carbon fiber, and the fiber bundle becomes curved. The hot air flow melts the paste on the surface of the carbon fiber, weakening the bonding effect between the fiber monofilaments. Due to different air flow rates at different locations, the carbon fiber exerts a transverse force, and the tow becomes thinner. Niu used a two-stage airflow widening device [
18] to stretch the carbon fiber tow. Huang’s group [
19] used a four-stage air flow stretcher to expand the CF tow step by step, and the fiber bundle width gradually increases with the increase of the stretcher. The air flow widening method can ensure no damage to CF while widening, but it needs to accurately control the size of the air flow and design a suitable air flow stretcher. In the mechanical widening method, a heating roller is used to melt the paste on the surface of the carbon fiber to reduce the bonding effect between the monofilament fibers, and the friction between the fiber and the widening roller or the widening rod is used to expand the CF filament bundle. Scholars such as Ma’s group invented a method to regulate the heating of CF filaments by regulating the contact area of a CF filament bundle on a heating roller and at the same time to expand the CF filament bundle under the action of the yarn guide roller rolling. Karl Meyer (China) Co., LTD [
20] uses the conductivity of CF to provide heat by electrifying a CF bundle, thereby removing the sizing agent from the CF surface. Park et al. [
21] successfully achieved the expansion of 12 K carbon fiber from 7 mm to 20 mm by employing a self-designed tow spreading device under optimized process conditions. The mechanical widening method has the characteristics of simple device structure and low cost, but if the friction is too large, it is easy to damage the CF tow and reduce the mechanical properties of the prepreg prepared later. Compared with the other four widening methods, the mechanical widening method has the advantages of simple structure, low cost, and easy improvement later, so this study chooses the mechanical widening method.
In this study, the relationship between the broadening width of carbon fiber and the process parameters was studied by using the mechanical broadening method. The impacts of four parameters, namely the initial tension, the quantity of straight rods, the quantity of convex rods, and the winding speed, on the width of broadening were investigated through the establishment of an automated broadening platform. Experimental results were analyzed using an innovative regression model, and the broadening effect was assessed through SEM imaging and mechanical testing. Furthermore, a comprehensive investigation of the factors influencing width and defects in the broadening process was conducted, leading to the determination of optimal parameters for achieving +effective broadening. This study provides valuable insights for enhancing the carbon fiber broadening process, improving the properties of carbon fiber composites and facilitating their widespread industrial application.