In injection molding, the linear expansion coefficient of resin is greater than that of metal, resulting in a molded product that is smaller than the size of the mold cavity, a phenomenon known as "mold shrinkage. It is known that the magnitude of mold shrinkage differs between the direction of resin flow in the mold and the direction orthogonal to it, indicating anisotropy in mold shrinkage. This is thought to be due to two major factors: the effect of solidification layer growth and stiffness that occurs within the molded product during the solidification process in the mold, and the anisotropy of the molecules themselves, since the resin is made of polymers. Furthermore, experimental results show that the mold shrinkage anisotropy is not the same within a molded product, but varies depending on the location within the molded product.
Fig. 1. a): initial shape in the mould, b): matrix for deformation.
When molding shrinkage anisotropy is distributed within a molded product, the molded product is deformed. Figure 1 shows schematically how a flat-plate molded product would be shaped depending on the presence of the phenomena of molding shrinkage distribution and molding shrinkage anisotropy, given a mold for forming flat plates. If the rectangular flat plate shown in Figure 1a) is formed from three square sections, anisotropic molding shrinkage means that the square sections are deformed into rectangles. The molding shrinkage is expressed as the change in area of each rectangular portion corresponding to a square. The fact that there is a distribution of anisotropy means that the orientation of the rectangles within the molded product can be modeled as changing from place to place. Here, if there is a distribution in the anisotropy of molding shrinkage within the molded product, the straight portion of the outer circumference of the molded product is considered to be no longer straight.
Since the model of this single plate only has a curved periphery, it is impossible to represent the warp deformation of the molded product in Figure 1.
Figure 2: Deformation Pattern of a Product Consisting of Two Plates Laminated Together
with a Distribution of Shrinkage Anisotropy
Figure 2 shows a schematic diagram of a product consisting of two plates with anisotropic molding shrinkage distribution in the product. The two plates need to share a single ridge line. For clarity, we have drawn a rectangle that shows the deformation of the parts that make up the molded product, as shown in Figure 1. By thinking in this way, the molded product will have a warped shape, which is often referred to as a "saddle-shaped warp.
The distribution of anisotropic mold shrinkage within a molded product varies depending on the molding conditions, which means that the shape of the warped deformation of the molded product will vary depending on the molding conditions.
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