Chopping is an efficient way to produce short carbon fiber (CF). Generally, there are two types of fixing constraints available in the chopping process: rigid-fixing and flexible-fixing. Simplified experiments were performed using glass and rubber as the fixing constraints in cutting a single polyacrylonitrile-based CF to reveal the influence of the fixing constraints in CF chopping. The cutting forces and the bending angles with different fixing constraints were analyzed. Furthermore, the failure surface of the CF was observed. Due to an additional bending effect in the flexible-fixing cutting, the failure surface of the CF was rough, and the cutting-off force was approximately 5% of the force in rigid-fixing cutting. Therefore, flexible-fixing cutting is a suitable way to decrease the cutting-off force in CF chopping. Moreover, it was concluded that the fiber fracture in rigid-fixing cutting is caused by compression, whereas in flexible-fixing cutting, it results from bending. We hope our work is beneficial to the design of the chopping procedure for short CF.
Short carbon fiber is an effective and essential reinforced material in composite domains. Generally, it is produced by chopping technology, among which pressing roller and grooved wheel are two popular chopping techniques. According to the working principle of these two techniques, the pressing roller can be simplified as a flexible-support cutting in off-axis transverse cutting, meanwhile the grooved wheel can be represented by a non-support cutting. To reveal the fracture difference of carbon fiber between the pressing roller and the grooved wheel technique, comparative investigation of failure behavior between a single carbon fiber (also simplified as a filament) with flexible support and with non-support was performed in off-axis transverse cutting using a custom-designed fixture. Both the cutting force and the fracture surface of the filament were comparatively analyzed in cutting process. It was found that the failure of single carbon fiber was caused by tensile effect in a non-support cutting, whereas that in a flexible-support cutting was caused by bending effect. The cutting-off force and cutting-off depth of filament cut by use flexible support are significantly lower than that by use non-support under low pre-tension. It is interesting to find out that the failure spot of carbon fiber filament in non-support cutting under low pre-tension tends to occur randomly. Meanwhile, in a flexible-support cutting, the break point of carbon fiber filament is determined by the tip of the blade. After a comprehensive comparison, the flexible-support cutting is a prior selection to produce short carbon fibers compared with the non-support cutting.
Short carbon fiber (short-CF) has been extensively used as enforcement in composites or conductors in fibrous network materials, owing to its good subsequent processing compatibility with routine manufacturing technologies. Conventionally, short-CFs were made by cutting continuous CF-tows from thousands of meters into several millimeters length scale using a radial chopping technology, mainly through sizing, drying and chopping processes in sequence. In this work, four kinds of CF-tows with different physical properties were chopped to study the effect of a sizing agent on the production of short-CFs, including product morphology and yield ratio. All experiments were conducted on polyacrylonitrile-based CFs sized by polyurethane (PU)-acetone solution. Results show that the uniformity (an important index of short-CF appearance) of short-CFs is closely related to the unit sizing weight. The yield ratio is affected by the relative sizing amount of PU coated on CF-tows. In addition, a raw CF-tow with little fiber hairiness is beneficial to increase the yield ratio of short-CFs.
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