Abstract:

Objective Formation of hollow special-shaped carbon fiber composites usually needs the support of mandrels. The methods currently used to prepare mandrels however involve complex process and long production period, but require complex equipment to complete the demolding after the formation of carbon fiber composites, which can easily cause damage to the final products. Consequently, the problems on mandrel manufacturing and demolding after the formation of carbon fiber composite are solved.
Method An aqueous solution of H₂O₂ was adopted to oxidize quartz sand, which was then coated with aluminum chloride and PVP-K30 using wet coating method. Since aluminum chloride in the shell an catalyze the curing of a modified adhesive, a mandrel was prepared by point-bonding the coated sand in a mold using the modified adhesive. In order to prepare the mandrel using 3D printing method, the 3D printability of the modified adhesive and coated sand was also investigated using a 3D printing inkjet system and a 3D printer. Two specially designed coatings were applied to the surface of the mandrel to improve the smoothness and prevent the penetration and adhesion of resin happening during the formation of carbon fiber composite materials. A hollow special-shaped carbon fiber composite was then prepared as an example with such a mandrel via vacuum bagging method. Thanks to the water-soluble shell of the coated sand, the mandrel was water-collapsible, so that the formed composite could be easily demolded in the presence of water.
Results Compared with quartz sand, the hardness and tensile strength of the mandrel prepared from the coated sand were significantly improved. The hardness increased from 44.3 to 54.5 HD, and the tensile strength increased from 1.85 to 2.44 MPa. Moreover, the mandrel could quickly collapse when exposed to water, facilitaing the formation and demolding of carbon fiber composite materials. After the mandrel was modified with the special coatings, its roughness decreased from 25.8 to 3.5 μm. The coatings simultaneously blocked the penetration and adhesion of resin during the formation of carbon fiber composite materials, and the inner surface roughness of the molded carbon fiber composite materials was as low as 0.9 μm. The viscosity and surface tension of the modified adhesive were 10.8 mPa·s and 36.8 mN/m, respectively, allowing it to be sprayed continuously and stably in the nozzle of 3D printer. The reasonable particle size and distribution and excellent flowability made the coated sand spread smoothly in the sand spreading system of 3D printer, and the AlCl₃ in the shell of the coated sand catalyzed the curing of the modified adhesive, so that the coated sand could be bonded into a special-shaped sand mold.
Conclusion Oxidation modification and coating of aluminum chloride and PVP-K30 could repair the surface defects of quartz sand without affecting its flowability and particle size and distribution, which could lay the structural foundation for improving the strength of mandrel, and the water-soluble shell formed by aluminum chloride and PVP-K30 could make the mandrel water collapsible. High strength and hardness as well as good water collapsibility enabled the formation of hollow special-shaped carbon fiber composite materials with the mandrel as a supporter to be feasible. The suitable viscosity and surface tension of the modified adhesive as well as the excellent flowability and uniform particle size and distribution of the coated sand made the formation of 3D printed mandrel possible, which could provide complicated mandrels for the formation of carbon fiber composite materials.

Key words: coated sand, point bonding, sand mandrel, mandrel, quartz sand, carbon fiber composite material, 3D printing