Void Formation During Liquid Composite Molding Under Varied Pressure with Mechanical Properties Validation

Composite materials have revolutionized products and how they are manufactured. Most large-scale aerospace companies utilize a carbon fiber pre-impregnated with a resin in conjunction with an autoclave to minimize voids, but it is very expensive and requires a great amount of space. The less expensive alternative to prepreg-autoclave processes is known as Liquid Composite Molding (LCM), in which the resin is brought into a mold via overpressure on the resin supply or by a vacuum in the mold. But this process is more prone to voids due to bubble formation. Voids are a critical issue as common failure modes such as interlaminar shear strength of the composite are highly sensitive to it This study examines LCM void formation in-situ during manufacturing, via two main sets of infusion experiments. The first set of experiments were pulsed infusions, in which the pressure was adjusted to set values at set increments of flow front filling distance. The second set of experiments was conducted by using a constant flow rate machine, which moved a piston a set increment over a set period of time. Both pulsed pressure and constant flow front velocity experiments were utilized to identify if pulsed pressure can result in a lower void content than constant pressure and if there was any correlation between void formations on the top-most surface and the interlaminar shear strength of the composite. With the experiments executed, there was no significant difference between pulsed pressure infusions and constant pressure infusions, and the best correlating factor to predict shear strength was the modified capillary number, which is derived directly from the flow front velocity. Ultimately, the modified capillary number is the best predictor of top-most void content and interlaminar shear strength.