Integration Concept of Injection, Forming and Foaming: A Practical Approach to Manufacture Hybrid Structures

Motivated by the concept of the integrative production systems, the hybrid process of polymer injection molding and sheet metal forming, known as polymer injection forming (PIF), has been introduced to manufacture sheet metal-polymer components in a single operation. Despite the wide potential application of this technology, its implementation in actual industrial production has been hindered due to several challenges; a thick layer of polymer where there is deep deformation, non-uniform deformation due to pressure loss and the opposite phenomena of shrinkage and springback. To mitigate these practical issues, the novel idea of integrating supercritical fluid (Sc.F.) technology with the PIF process is introduced in this work. As the proposed technology is a manufacturing innovation, with no available information in the literature correlating to this concept, two sets of experiments are designed to investigate the feasibility of this integration. In the first set, the effect of blank material and shot volume as design variables were investigated over a range of Sc.F. weight percentage. To improve the cell morphology in experiments with the low-strength sheet material, several other processing scenarios are explored in the second set of experiments. The results of this study clearly demonstrate the capabilities of this concept manufacturing process in terms of initiating the foaming process within the simultaneous injection/forming process, ensuring weight reduction (of up to 16%) and complete elimination of issues related to shrinkage

Comparing faceted and smoothed tool surface descriptions in sheet metal forming simulation

This study deals with different tool surface description methods used in the finite element analysis of sheet metal forming processes. The description of arbitrarily-shaped tool surfaces using the traditional linear finite elements is compared with two distinct smooth surface description approaches: (i) Bézier patches obtained from the ComputerAided Design model and (ii) smoothing the finite element mesh using Nagata patches. The contact search algorithm is presented for each approach, exploiting its special features in order to ensure an accurate and efficient contact detection. The influence of the tool modelling accuracy on the numerical results is analysed using two sheet forming examples, the unconstrained cylindrical bending and the reverse deep drawing of a cylindrical cup. Smoothing the contact surfaces with Nagata patches allows creating more accurate tool models, both in terms of shape and normal vectors, when compared with the conventional linear finite element mesh. The computational efficiency is evaluated in this study through the total number of increments and the required CPU time. The mesh refinement in the faceted description approach is not effective in terms of computational efficiency due to large discontinuities in the normal vector field across facets, even when adopting fine meshes.The authors gratefully acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) via the projects PTDC/EME-TME/118420/2010 and PEst-C/EME/ UI0285/2013 and by FEDER funds through the program COMPETE – Programa Operacional Factores de Competitividade, under the project CENTRO-07-0224-FEDER-002001 (MT4MOBI). The first author is also grateful to the FCT for the PhD grant SFRH/BD/69140/2010.info:eu-repo/semantics/publishedVersio

An improved relationship between Vickers hardness and yield stress for cold formed materials and its experimental verification

Cold formed products are increasingly serving as high duty machine parts. Designers and users need to know their properties as accurate as possible. One such product property is the new yield stress, which can be approximated by the final flow stress of the workpiece material during forming. Vickers hardness measurements provide an easy and inexpensive method for evaluating the new local yield stress in cold formed workpieces. In this study, an improved relationship for the conversion of Vickers hardness values to yield stress is proposed. The agreement between theoretical and experimental results is better than 4 % error

EQUIVALENT STRAIN AND STRESS HISTORY IN TORSION TESTS

Bulk and plane torsion tests are being used in industry to determine the flow curve of metals up to very high plastic strains. Despite their wide usage the experimental results obtained in torsion tests are still not being evaluated uniformly. Differences arise basically due to the conversion of the measured angle of twist into the corresponding equivalent strain. This paper shows that the often used equivalent strain expression by Nadai and Davis as well as Eichinger is invalid, since the so-called redundant shear during torsion deformation has been ignored. Also, the histories of different stress measures such as the convected and rotated stress tensors are computed to discuss the reasons for differences between the flow curves obtained from torsion and simple tension tests

State-of-the-art of simulation of sheet metal forming

The paper starts with a brief historical overview to the attempts of numerical simulation of sheet metal forming. Comparison between bulk and sheet metal forming processes from the simulation point of view is given. Basic requirements of the applier to the simulation tools are summarized. Various possible methodologies are briefly discussed. Special emphasis is given to the static explicit and dynamic implicit finite element procedures. Also different element types are overviewed. Available important commercial finite element packages are given. The current state of the application of the simulation tools is discussed. Some typical industrial applications are reviewed to demonstrate the current abilities of analysis. Finally, an attempt to prognosticate some future developments is made. (C) 2000 Elsevier Science S.A. All rights reserved

Fully automatic simulation of bulk metal forming processes

The paper presents a fully automatic finite element procedure for the simulation of industrial bulk metal forming processes. The procedure is based on a rigid (-quasi-viscous) plastic material law. Thermomechanical analyses are performed by loose or full coupling of the heat balance and the equilibrium equations. To ensure a real automatic simulation, the numerical procedure is made robust by using Euler time integration, a Newton-Raphson solver which is coupled with a direct iteration solver, a linear contact algorithm and, finally, a completely automatic hexahedral mesh generation scheme. Severe deformations are detected automatically and are handled by mesh smoothing and/or by remeshing. The all-hexahedral-mesh generation is based on a modified octree scheme. The method is implemented in the ANSI-C programming language. Effectiveness of the method is demonstrated by an industrial application

Improved relationship between Vickers hardness and yield stress for cold formed materials

Cold-formed metal products are increasingly serving as high duty machine parts. Designers and users need to know their properties as accurately as possible. One such product property is the new yield strength, which can be approximated by the final flow stress of the workpiece material during forming. Vickers hardness measurements provide an easy and inexpensive method of evaluating the new local yield stress in cold-formed workpieces. The well-known available models given in literature to convert the measured hardness number into the corresponding yield stress have an error of up to 25%. This is basically due to the facts that cold formed material experiences large plastic strains in the main forming stage, the hardening behaviour is anisotropic and, moreover, the material properties are inhomogeneous especially at the workpiece surface. The purpose of this study is to improve the accuracy of the well-known available correlation models between Vickers hardness measurements and yield stress. This is achieved by utilizing finite element simulations of the indentation process. The models currently incorporate only the isotropic strain-hardening behaviour of the work material. The new suggested model decreases the theoretical conversion error to less than 10%

Shape optimization with the biological growth method: A parameter study

Studies the effect of parameters controlling the biological growth method by applying it to the classical optimization problem of a plate with a central hole under biaxial stress state. It has been found that the optimization character of the method depends strongly on the so-called reference stress. Depending on the magnitude of this parameter either a local or global optimum is approached. A global optimum corresponds to the minimum possible v. Mises stress along the hole boundary (and hence in the plate), whereas a local optimum presents the modified shape of the hole yielding an uniform stress distribution whose magnitude is larger than the minimum possible value and which is equal to the specified reference stress. The magnification factor applied to the iterative displacement results influences the optimization speed. Too large factors lead to divergence of the solution. Furthermore, it has been found that the dimension of the optimization domain has a critical effect on the optimization result