Fiber shortening during injection molding of glass fiber-reinforced phenolic molding compounds: fiber length measurement method development and validation

Thermoset molding compounds based on glass fiber-reinforced phenolic resins are an attractive material for thermo-mechanically demanding applications. Usually, the average fiber length in commercially available molding compounds is significantly shorter than the critical fiber length. In order to investigate the potential for increasing the fiber length in the molded part, molding trials using a conventional short fiber and an experimental long fiber-reinforced compound are carried out and the fiber shortening during the process is analyzed. An improvement to the fiber length measurement is described, consisting of a repeatable and accurate down-sampling and fiber dispersion method. The nondestructive measurement of the fiber length distribution by means of X-ray computed tomography is evaluated regarding its suitability for molding compounds with a high glass fiber content. By using the introduced fiber length measurement method, a fiber shortening during the injection molding process from 370µm initial weighted average fiber length to 300µm is detected for conventional phenolic molding compounds. In contrast, the experimental long fiber phenolic compound is shortened from 11,800µm to 970µm. Analytical calculations indicate that the asymptotic fiber length, until which the fluid forces during processing are sufficiently high for breaking the fibers, is between the two measurements. This leads to the conclusion that the commercially available, state-of-the-art short fiber molding compounds leave a strong potential for improving the fiber length in the molded part unused.