Analysis of non-uniform expansion behavior of injected EVA

Abstract

Currently the sports footwear market is very competitive, in which it is sought to produce products that provide high performance, using technology in the production processes as well as in the materials used. One of the main materials used in the composition of soles and midsole of these products is EVA (Ethylene-Vinyl Acetate), which brings several benefits such as cost reduction, comfort, lightness and softness. The injection molding process is the most efficient for obtaining EVA soles, however, EVA undergoes large expansions that may vary according to its formulation and geometry of the desired model. The objective of this work is to analyze the non-uniform deformation of EVA, used in the injection molding processes, as well as one method used to correct such distortions that occur in the expansion of this material. For this, a bibliographic review was carried out to analyze the chemical composition of the EVA, whose composition will determine the characteristics of expansion and softness. The work also addresses the steps of the injection process by analyzing the temperatures and pressure employed and an analysis of how to predict and correct the distortion in EVA expansion. It is proposed to use a section of a 3D model, where it is determined where compensations should be applied and using 3D modeling software. To analyze the generation of smaller bubbles in less thick regions of an EVA sole, a scanning electron microscope (SEM) analysis was performed. In addition, the measurements of the analyzed model were compared with the virtual model through a 3D scan. Hardness tests on the Asker C scale and injection simulations were also performed to analyze the cooling time of the part after leaving the mold, volumetric contraction that occurs in the extraction of the mold part, and the thicknesses of the walls of the product. The results indicate that the comparison of the models was within the accepted tolerance, which allows a variation in the expansion of up to 1mm in the length of the evaluated points. From the analyzes made in the scanning electron microscope and in the hardness test, it was possible to verify that in regions of smaller thickness and smaller volume, the EVA blend cannot have its total expansion, i.e., micro bubbles of gases generated in the expansion are smaller in these regions when compared to regions of greater volume and thickness. Consequently, these regions become denser and less soft. Simulations indicated that in regions where bubbles do not expand, there is also an early cooling of the material when compared to regions with higher volume, proving that the cooling time is directly linked to the EVA expansion rate.