CompositesWorld
Issue link: https://cw.epubxp.com/i/517026
JUNE 2015 32 CompositesWorld WORK IN PROGRESS channels (inter-tow spaces) for voids to reach the resin fow front." Looking ahead, says Advani, more work needs to be done to understand what variables enhance bubble mobility. Large bubbles move faster than small bubbles, he says, and resin viscosity and surface tension play obvious and important roles. Experiments will further evaluate the roles of tow size, channel size, fabric archi- tecture and rate of vacuum pull in minimizing OOA void content. "One thing we know," says Advani, "is that if we can get the bubble to the front of the fow front, it's much easier to move it." Ultimately, says Cender, he'd like to develop impregnation data for a variety of fber forms that prepreg suppliers can use to optimize the products they provide to OOA fabricators. In any case, he says he wants to bring a greater level of understanding and sophistication to OOA molding: "We want to dismiss the misconception that if you apply vacuum to your laminate, that the bubbles will always reach air pathways and be vented from the system. Bubbles can still get stuck if the impregnation is too high, no matter how long the vacuum dwell is held." Still images and video of bubble development and movement have proved valuable in understanding what conditions facilitate bubble migration, and what conditions hinder it (Figs. 2 & 3). "We experimentally observe bubbles moving two to four times faster than the resin fow — in the channels between fber tows, specifcally, the undulations of the fabric," Cender reports. "Tis makes it possible for bubbles to reach the fow front and be vented from the laminate. We also make note of conditions under which the bubbles will become stuck in the laminate. Bubbles often do not break [free] once the resin flls these inter-tow channels. If a bubble can penetrate the fber tow, it always moves in the longitu- dinal direction — with the fbers. Bubbles never penetrate fber tows in the transverse direction — across the fbers." Although Cender's work on bubble mobility is ongoing, he says results, thus far, indicate that an OOA semipreg should have an initial resin fraction of not more than 30%. If initial impregnation is more than 30%, says the paper, it "will take longer to de-gas the laminate and does not allow enough space for resin fow inside Read this article online | short.compositesworld.com/OOAvoids Bubble behavior videos taken by Advani's team are available on the CW YouTube page | short.compositesworld.com/Bubbles Jef Sloan is editor-in-chief of CompositesWorld, and has been engaged in plastics- and composites-industry journalism for 22 years. jef@compositesworld.com Walton Process Technologies, Inc. Best Customer Service in The Industry www.autoclaves.com Mansfield TX 682-518-9002 Service/Repair Retrofit/Relocate Autoclaves Bond Presses Batch Process Controls Ovens Parts