As gas prices continue to soar, the demand for lighter automobiles to improve fuel economy will increase, as will the trend of using plastics in substitution of metal components among automotive manufacturers. However, producing lighter plastic parts that exhibit the same strength and durability characteristics as their metal counterparts is not an easy task. Many vehicle makers have adopted long fiber materials as a solution to this problem.
It has been proven in many scientific studies that the inclusion of fiber reinforcements results in greatly improved mechanical properties of composite materials. Reported benefits of long fiber products include better fracture toughness, impact resistance, and creep performance. In other words, added fibers lend strength and durability to plastic parts.
However, there are still challenging issues to be addressed when manufacturing composite parts. The injection molding process is the most common processing technique used to manufacture fiber-reinforced thermoplastics, but fibers break during the process. For example, a poorly controlled plastication process may result in a 50% reduction in length, i.e., from 10 to 5 mm on average. With the hardest-hit factor being the impact strength of the product, the quality and strength of fiber-reinforced plastic parts could then be compromised.
For industries like automotive that use long fiber applications quite often, it has become apparent that understanding and predicting fiber length reduction in the screw section is vital for part design. This is a crucial factor to be taken into consideration when manufacturing fiber-reinforced parts because the fiber length has such a strong effect on the mechanical properties, strength, stiffness, and impact resistance. The length reduction phenomenon in the injection phase will most certainly decrease the reinforcement efficiency of fibers. Screw-induced attrition in fiber length is more unpredictable than mold filling. However, computer-aided engineering (CAE) technology now makes it possible to visualize fiber breakage behavior, making valuable information available to automotive part designers for the first time.
Consider this actual case study on a center-gated disk. The initial fiber length for the center-gate disk mold filling system is 13 mm. CAE technology enables the evaluation of the critical problem of long fiber length attrition (shown in Fig. 1).
Because of processing from the screw, flowing through the gate and entering to the part, two length units are usually used: number-average length (Ln) and weight-average length (Lw). During both screw and filling stages, users can obtain the fiber length distributions (shown in Fig. 2 and Fig. 3, respectively).
A good quantitative agreement between the experiment data and the prediction by CAE software is demonstrated below (shown in Fig. 4). This is extremely helpful for fiber-reinforced thermoplastic industrial fabrication.