Bjorn’s Corner: Aircraft Structures Part 19. Liquid Composite Molding

Resin infusion methods for composite manufacturing are categorized into three main processes: Resin Transfer Molding (RTM), Vacuum-Assisted RTM (VARTM), and Compression Resin Transfer Molding (CRTM). Resin Transfer Molding requires a closed mold, allowing heated resin to be injected under pressure into a dry fiber preform, which is then cured in a heated mold. This method offers precise control over geometry, making it suitable for Out of Autoclave production of limited-size parts. VARTM uses a one-sided female mold with a vacuum bag to consolidate the preform before resin infusion, followed by curing in a hot oven. CRTM involves placing a dry fiber preform between a dual-sided tool, where resin is injected and then mechanically compressed to rapidly drive the resin through the fibers, leading to fast cycle times but requiring complex tooling. The article provides historical context on CRTM's application in Boeing interior sidewall manufacturing and Freightliner semi-truck hood production, detailing process evolution from manual compression molding methods.

The transition between mold closure methods—closed RTM versus partial/forced closure in CRTM—reveals a fundamental trade-off between geometric control and manufacturing speed. Closed molds prioritize geometric precision at the expense of initial cost, fitting niche applications like OoA production. Conversely, CRTM prioritizes high-volume throughput via mechanical force, which necessitates highly complex tooling, suggesting that rapid manufacturing often imposes significant capital expenditure demands. The historical trajectory shown by Boeing's move from manual compression molding to automated processes highlights a long-standing industry tension between bespoke quality control and mass production efficiency. This dynamic suggests that the optimization of composite processing is less about finding a single superior method and more about selecting the process dictated by the specific constraints of geometry, volume requirements, and economic feasibility at any given stage of product development. The evolution toward CRTM reflects an ongoing search for processes that balance physical forces with fluid dynamics to achieve minimal cycle times. What assumptions about tooling cost versus time savings drive the current industrial adoption? How does the historical precedent influence modern expectations for process innovation?