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AbstractThis paper shows how a selected forming process (coldextrusion) can be evaluated by using the part informationavailable from the CAD-system, e.g. volume, geometryand material. In particular, the energy needed to form thepart as well as the amount of zinc-phosphate coatingnecessary for the forming process are estimated based onthis information.It is shown how the part information is retrieved fromthe CAD-system, how the cold extrusion process ismodeled in an object-oriented manner and how the LCIdata is obtained from this process model. Furthermore,the interface to an evaluation system is discussed.42495
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1 Introduction and GoalsThe need for environmentally sound products is forvarious reasons rising constantly in today’s society. A trueenvironmental soundness presents a challenge to theproduct developer. The complete product life-cycle mustbe assessed since the sum of all environmental impactsover the product life should be minimized. A singularview at one life-cycle phase does not guarantee anoptimum. For example, aluminum parts are light andrequire little energy for movement; however, aluminumproduction is very energy-demanding.The product developer must be supported in theproduct assessment, since the influences of the differentlife-cycle phases on each other are complex. Currently, a research project at Darmstadt Universityof Technology titled „Development of EnvironmentallySound Products – Methods, Tools and Instruments“,which is funded by the German Research Foundation(DFG), is engaged in creating a product developmentenvironment which will deliver this support. The projectwill enable the product developer to perform a holisticLCA of a product that yet exists virtually within theCAD-system only.The product design environment is centered around theCAD-system. Together with information about the life-cycle phases of the virtual product, all relevant LCI datacan be computed. An evaluation system processes the datain order to conduct the LCA.The evaluation system considers environmental,economic and technological aspects of the product inorder to assist the engineer in finding an overall optimum.Only the environmental aspects of cold extrusion, whichare essential for the conduction of an LCA, are discussedhere.One of the production processes implemented in theproduct design environment is cold extrusion. For thecalculation of LCI data for this process, the difference inshape between semi-finished product and product must beknown. The product developer only defines the geometryof the finished part. Since it is created in a formingprocess, the semi-finished product which must be useddoes not have larger overall dimensions than the finalproduct.Several aspects of the inclusion of the cold extrusionprocess in the product design environment require closerattention. The determination of the optimal way torepresent the cold extrusion process within the productdesign environment in order to obtain LCI data is ofmajor importance. Also, a method must be found todetermine the product’s geometry exactly with the datareadily available from the CAD system and without usinga feature-based approach.
2 Cold extrusionCold extrusion is a mass-production forming processusing cylinder-shaped semi-finished products. The mostcommon procedures are direct extrusion and indirectextrusion, also known as cup extrusion. Typical shapes produced in one-stage processes are shafts with twodifferent diameters linked by a truncated cone (directextrusion) and cups (indirect extrusion) as shown inFigure 1. One of the most widely produced parts are bolts,which are made in a multi-stage process.d0d1αd2ld0l1ll1ll2l1Figure 1: Standard extrusion parts. Directextrusion (top) and indirect extrusion(bottom)Cold extrusion is conducted at room temperature. Dueto the forming process, however, the workpiece heats upand may reach temperatures around 200° C.The cold extrusion production process consists ofseveral steps (Figure 2) and requires careful preparationof the semi-finished product involved. First, theworkpiece must be produced. This is usually done byshearing from wire or rod. A heat treatment (annealing)to improve the material properties for the forming processmight follow. In the next step the surface must be cleaned,smoothed and coated with a high-performance lubricant,typically zinc phosphate. Following the actual formingprocess, the workpiece is freed from the remaininglubricant.SurfacetreatmentCleaningShearingHeat treatmentColdext r us ionFigure 2: Cold extrusion process3 Obtaining LCI dataIn order to obtain LCI data, the cold extrusion processmust be modeled in such a way that material flows as wellas energy consumptions can be estimated based on thefinal product’s properties.3.1 Need for a true process modelFor the product yet only virtually existent in theproduct design environment, the LCI data has to bedetermined. While the product developer is working,geometry data is automatically generated by the CAD-system.A very efficient way to process data electronically isthe application of matrix calculus. The shape and materialof the product can be described in a joint vector. The sameholds true for the production process parameters as wellas the LCI data which results from the cold extrusionprocess of this particular product. In order to obtain theLCI vector from the geometry vector, a transformationmatrix (or a set thereof if intermediate steps such as theproduction process itself should be described) is needed.This, however, would result in a fixed relation betweenthe dimensions of the shape vector and the LCI vector, ineffect saying that any cold extrusion part which has ashape vector of the same size as another part generates theidentical number and kinds of inventory data. Obviously,this defies reality and a different method of describing therelationship between the product properties and the LCIdata is needed.Vector functions prove to be an elegant description ofthe relationship between LCI vector and shape vector. As finite-element-simulation exist [1]. Due to the complexityof these systems as well as the time they require, theirapplication is unacceptable in this context.An easier method which yields results with acceptableaccuracy is the elementary theory of plasticity. Based onthe part geometry and the material's flow curve, it ispossible to calculate the strain energy for the formingprocess. However, certain geometry data is needed. Thestraight retrieval of this data would be difficult andrequire a feature-based approach.The strain energy depends mainly on the deformedvolume and on the natural strain, which is defined as thenatural logarithm of the quotient of the starting crosssection to the end cross section:ϕ= lnAA01(2)For indirect extrusion there is no simple definition ofthe natural strain [2]. The production of a cup throughindirect extrusion can be viewed as a double upsetting [3],which requires the definition of two natural strains.4.2 Calculation from the CAD-dataThe calculation of volume, surface, maximum diameterand overall length of a product are standard functions ofCAD systems. The technological aspects of theinformation retrieval are briefly discussed in chapter 6.The surface is a function of volume, maximum diameterand overall length under the condition that the part hasbeen deformed and is not a cylinder anymore.For direct extrusion, the surface is also a function ofthe die's angle which forms the truncated cone betweenthe slimmer cylinder and the not deformed cylinder.Common angles are α=30°…60°.