Warpage Mechanism due to Fiber Orientation during Injection Molding

Abstract

A simple method that has been adopted in recent years for increasing the strength and modulus of injection-molded plastic products is to mix in short glass fibers when the products are molded. Because the mechanical properties and dimensional accuracy of fiber-reinforced products strongly depend on the fiber orientation and fiber content, it has become increasingly important to clarify experimentally the fiber orientation distribution in molded products. The fiber orientation can sometimes cause molding defects, typified by warpage, because it induces anisotropic mechanical properties or anisotropic shrinkage rates in molded products. There can also be times when the injection gates can not be ideally positioned due to the cooling pipe arrangement for the mold or in the case of products with corners, such as box-shaped items or ones with reinforcement ribs. In such cases, the local fiber orientation can produce a nonuniform fiber orientation distribution in various parts of injection-molded products or along their thickness, which is thought to be conducive to warping.In this research, a ribbed flat plate was used to make clear how the presence or absence of fibers and the injection gate positions affected warpage behavior (direction and amount), and the mechanism of warping was examined on the basis of observation results. Warpage predictions were also made using an evaluation method that was independently devised from the observation results.These efforts made the following points clear. The amount of warpage increased depending on the fiber strength and, when a side gate was used, the peak deformation shifted toward that gate. The increase in warpage due to the fiber strength is attributed to a difference in shrinkage rates stemming from differences in the fiber orientation between the plate and rib portions of the molded specimen. The shift in peak deformation toward the side gate is ascribed to the influence of the gate position on the state of the fiber orientation. In addition, it was found that the evaluation method proposed here was capable of accurately assessing the contribution of fibers to inhibiting shrinkage in the direction of warping, and the results obtained with this method explained well the Warpage mechanism originating in the fiber orientation.