Material anisotropy and warpage of nylon 66 composites

Abstract

AbstractIt has been found that the warpage mode of an injection molded disk depends on the type of reinforcement of the material. To clarify the reason for the finding, correlations among the aspect ratio (λ) of reinforcements, anisotropy in linear thermal expansion coefficient (α), and warpage have been studied for an injection‐molded disk. The λ is defined as the ratio of the length to the radius. The warpage of the disk is defined as the difference between the maximum and the minimum of the deformation at the free state, which is observed immediately after the disk is released from the mold. The study has been conducted with respect to the PA66 composites compounded with the reinforcement of λ = 1 and λ = 4. Rα≡α11/α22 has been defined to represent the anisotropy of α, where α11 is the α in the flow direction, and α22 is that in the transverse to the flow direction. The flow is defined as the flow of the melted polymer in the injection molding. It has been experimentally found that Rα = 1.0 for unreinforced PA66, Rα < 1.0 for PA66 compounded with λ = 4 reinforcement, and Rα > 1.0 for PA66 compounded with λ = 1 reinforcement. The warpage mode in the Rα < 1.0 composite resembles a coffee cup, while the warpage mode in the Rα > 1.0 composite resembles a saddle. The results indicate that Rα determines the warpage mode of the disk. To prove that Rα is the adequate condition for determining the warpage mode, finite element analysis was conducted for the material of Rα < 1.0 and for the material of Rα > 1.0. It has also been found that the amount of warpage in the case of Rα 1.0 is approximately 10 fold that in the case of Rα = 1.0. On the other hand, the warpage of a disk molded from the blend of Rα < 1.0 composite and Rα > 1.0 composite decreases to the warpage level of the disk of Rα = 1.0. It is deduced that Rα is the important parameter for controlling the warpage since Rα < 1.0 and Rα > 1.0 represent the two counteracting material properties in warpage.