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Manufacturing of Dies and Molds Taylan Altan (I), Blaine Lillg, Y.C. Yen Engineering Research Center for Net Shape Manufacturing Department of Industrial, Welding, and Systems Engineering The Ohio State University, Columbus, Ohio, U.S.A. Submitted by Taylan Altan (I), Columbus, Ohio, U.S.A. 1 2 Abstract The design and manufacturing of dies and molds represent a significant link in the entire production chain because nearly all mass produced discrete parts are formed using production processes that employ dies and molds. Thus, the quality, cost and lead times of dies and molds affect the economics of producing a very large number of components, subassemblies and assemblies, especially in the automotive industry. Therefore, die and mold makers are forced to develop and implement the latest technology in: part and process design including process modeling, rapid prototyping, rapid tooling, optimized tool path generation for high speed cutting and hard machining, machinery and cutting tools, surface coating and repair as well as in EDM and ECM. This paper, prepared with input from many ClRP colleagues, attempts to review the significant advances and practical applications in this field. Keywords: Die, Mold, Manufacturing. 0 INTRODUCTION The authors would like to thank all of the colleagues who responded to the request for information in preparing this review paper, namely to Prof. Klocke - WZL Aachen, Prof. Tonshoff - IFW Hannover, Prof. Wertheim - Iscar, Ltd. (Israel), Profs. Kruth and Lauwers - Catholic University Leuven, Prof. Rasch - NTNU Trondheim, Prof. Geiger and Dr. Engel - LFT Erlangen, Prof. Weinert - ISF Dortmund, Dr. Leopold - GFE Chemnitz, Mr. Reznick - Extrude Hone Corp., Prof. Gunasekera - Ohio University, Prof. Bramley - University of Bath, Prof. Bueno - Fundacion Tekniker, Prof. Neugebauer and Dr. Lang - Fraunhofer IWU Chemnitz. Thanks are also due to our co-workers and the ERCINSM, as well as the co-workers of Prof. Klocke at WZL, and of Prof. Tonshoff at IFW, who assisted us in collecting the references and in the preparation of figures. Furthermore, we appreciate the response that we received from many of the participants of the 2001 Mold Making Conference. 1 BACKGROUND Production of industrial goods requires manufacturing of discrete parts that are sub-assembled and assembled to a product ready for the customer. The manufacturing of nearly all mass produced discrete parts require dies and molds that are used in production processes such as forging, stamping, casting, and injection molding. Thus, the design and manufacturing of dies and molds represent a very crucial aspect of the entire production chain. This can be illustrated by the following observations: Dies and molds, similar to machine tools, may represent a small investment compared to the overall value of an entire production program. However, they are crucial, as are machine tools, in determining lead times, quality and costs of discrete parts. Manufacturing and try-out of new dies and molds may be critical in determining the feasibility and lead-time of an entire production program. For example, in manufacturing automotive interior components by in- mold lamination complex molds that are used may cost up to $0.5 million and require 6 to 9 months for try-out and robust process development using production equipment. Considering that the OEMs require sample parts, produced on production equipment (not prototypes), 6 to 9 months prior to start of production (SOP) of a new car model, the significance of mold making becomes obvious. The quality of the dies and molds directly affect the quality of the produced parts. Excellent examples are molds used for injection molding lenses, or dies used for precision forging of automotive drive train components. 1 1.1 Significance of the Technology The observations listed above illustrate that die and mold making has a key position in manufacturing components in virtually all industries but especially in transportation, consumer electronics and consumer goods industries. The effectiveness of die making affects the entire manufacturing cycle so that this technology must be considered to be a very essential link in the total production chain . Die and mold making covers a broad range of activities, including: a) manufacturing of new dies and fixtures, b) maintenance and modifications, and c) technical assistance and prototype manufacturing for the customer, Figure 1 I. Process development and die try-out as well as die maintenance are especially important because they tie up expensive production equipment and affect lead times. These activities must be scheduled and completed within very rigid deadlines. Such requirements make scheduling in a die shop an extremely challenging task. The automotive industry constantly tries to reduce the development time for new models which puts enormous pressure on die makers and requires new production systems 2. 1.2 Variety of Dies and Molds The four major processes that utilize dies and molds a) require different technologies for design and production, and b) utilize different terminologies, Figure 2 3. For example, die-casting dies have more deep and thin rib cavities that cannot be easily machined than injection molding molds. As a result five times more plunge EDM machines are used in the die casting industry than in the injection molding industry. Another example is the extensive, nearly 50 %, use of wire EDM machines for making blanking dies while only 5 % of these machines are used to make extrusion dies. As seen in Figure 2, large deep drawing and stamping dies are made by machining cast iron or steel structures while dies for forging, die casting and plastics molding are made from tool steel blocks, involving considerable rough machining operations. Injection molding molds and die casting dies allow the production of rather complex parts with undercuts andlor hollow geometries. Thus, these tools usually have multiple motion slides and punches as well as cooling channels that complicate the manufacturing process. Dies and molds are composed of functional (cavity, core insert, punch) and support components (guide pins, holder, die plate). Often support components and a number of holes need only 2D or 2% D machining, but may require 50 to 60% of the total manufacturing time. This fact is often neglected but must be considered in effective planning of the machining operations. While metal cutting and EDM are the major methods used for die and mold making, hobbing, micro machining and chemical etching methods are also used for manufacturing molds for various applications. Figure 1 : Position of die and mold machining in product life cycle I. 1.3 Economics of DielMold Making According to a recent survey 4, major issues that face die and mold makers are similar in all industrialized countries, namely: 1. Declining prices and profit margins so that there is a strong need to control and reduce costs. 2. Demands for building dieslmolds in far less time (nearly 50 % less) than before. 3. Need for extended customer service (data handling, advice, prototype parts, assistance in process development) . 4. Lack and cost of skilled labor, which leads to the need to provide extensive training to employees and to utilize “new technologies“. 5. Globalization that leads to increased foreign competition, especially from developing countries where skill levels are increasing while salaries are comparatively low. Priorities differ according to countries surveyed; for example while North American and German mold makers are mainly concerned with foreign competition, Japanese companies concentrate on developing new markets. In all countries, however, the acceptance of “new technologies” is recognized to be one essential component that can lead to innovation and integration that are essential for growth 5. New technologies are understood to include not only manufacturing techniques (high speed milling, hard machining, automation, process modeling, etc.) but also pre- and post-manufacturing, e.g. cost estimating and control, documentation, training and operations management. Thus, two essential components for achieving a competitive position in dielmold industry are: a) capabilities of personnel, and b) utilization of optimized and innovative production techniques 161. Figure 2: Workpiece characteristics in die and mold making 3. Successful dielmold makers recommend that, for a financially successful die making operation, it is necessary to: 1, Establish quantitative methods for cost estimating. In this industry cost estimates are often based on the “past experience” and “feel” of the die maker and comparison with “similar” dies. As a result the accuracy of the estimations, that may determine profit or loss, may be in the range off 20 % 7. Determine the entire process chain for die making, from inquiry until delivery to the customer. Identify all cost parameters and quantify cost factors, eventually by reviewing past history (data collection for working hours, contracts, cost accounting). Establish a contractual basis so that items non- specified in the contract are only provided at extra charge 8. In order to maintain deadlines, focus on contract initialization and not on assembly of the mold, where considerable manpower is involved and it is difficult to change a schedule. Provide services to the customer mainly in data management at the start and during production but also during process development with complex molds that may require considerable try-out time. For successful die makers quality is a given. The time for work in progress, or storage time, can be a significant factor in low volume production, such as dielmold making. In this application it is estimated that 70% of the “total production time” consists of storage time when no value is added to the product. This situation can only be improved by increasing machining capacity, machine utilization rate, orland improving the efficiency of the part handling operations 9. To reduce the time for work in progress, many high technology die shops have separated tool path generation from engineering and design of the dies. While the latter is done in the engineering department, tool path generation is done on the shop floor by the machine operator. 2 In manufacturing discrete parts using dies or molds, the part design must be compatible with the process in order to assure the production of high quality parts at low cost with short lead times. Thus, part and process designs are best considered simultaneously, which is often not the case in practice. This objective can only be achieved through good communication between the product and tool designer, who may be in different companies (OEM and supplier) andlor locations. PART, DIE AND PROCESS DESIGN Original Equipment Fi rst-Ti er Manufacturers Suppliers Subtier Suppliers I CADKEY CADDS CATIA I-DEAS Unigraphics lntegraph CADDS I-DEAS CATIA ProlEN GI N E E R Unigra phi cs Figure 3: Proliferation of CAD systems in chain lo. ARIES Applicon ANVIL AutoCAD ProlENGlNEER I- D EAS PDGS HP lntegraph EUCLID CATIA supply The use of different CAD systems by OEMs and suppliers further complicates communication within the supply chain. Figure 3, taken from lo, shows the proliferation of CAD systems in the top three tiers of the North American automobile industry. Because die and mold making firms tend to be third or fourth-tier suppliers, the “interoperability problem” of reliably transferring CAD data between firms is particularly acute in this industry. It is well known that the design actually represents only a small portion, 5 to 15%, of the total production cost of a part. However, decisions made at the design stage have a profound effect upon manufacturing and life cycle costs of a product. In addition to satisfying the functional requirements, the part design must consider: a) the selected manufacturing process and its limitations, b) equipment and tooling requirements, c) process capabilities such as size, geometry, tolerances, and production rate, and d) properties of the incoming material under processing conditions. Often the design requires development of a new tooling andlor modification of an existing process. In such cases dielmold development and try-out can take as long or even longer that the time needed for die manufacturing. The assembly ready part geometry, usually in electronic form, must be used to develop the die or mold geometries as well as to select the process parameters. Figure 4 illustrates, using forging as an example, the flow of information and activities in computer aided die and process design Ill. Processes such as stamping, hot and cold forging may require several operations starting with the initial simple billet or sheet blank until the finish formed part is obtained. Thus, several die sets may be needed. In processes, where the incoming material is shapeless, e.g. powder compaction, injection molding, die casting, a single set of dies or molds have to be designed. Die design is essentially an experience-based activity. However, it is enhanced significantly by utilizing process modeling techniques to: 1. Estimate material flow and die stresses. 2. Establish optimum process parameters (machine and ram speed, dielmold and material temperatures, time for holding under pressure, etc.). 3. Design dielmold features, necessary to perform the process (flash and draft in forging, binder surface and draw beads in stamping, gates and runners in injection molding and die casting). 4. Finalize the product and die dimensions by predicting and eliminating defects while adjusting the process parameters for obtaining a robust process. The application of process modeling, using 3D-FEM based software, is now considered routine in (i) permanent mold and die casting, (ii) injection, gas injection, compression and blow molding, and (iii) sheet metal forming. In forging, while 2D simulation is widely practiced, 3D applications are being introduced by advanced technology companies. Research is being conducted on a “reverse simulation approach” for designing forging preforms 12. Examples of FEM simulation results are seen in Figure 5 for forging and Figure 6 for stamping Ill. Application of 2D-FEM simulation in metal cutting is now being introduced by many companies but it is still in the RBD stage. Most probably this application will be widely accepted during the next two to four years and also expanded to full 3D simulations of the metal cutting operations. Before they can be applied in the industrial environment, process simulation must be further developed for (a) forming of composite polymers (in mold lamination, compression molding of glass fiber reinforced polymeric composites) and b) sandwich sheet metal materials. Characterization of composite materials and formulation of the deformation laws represent considerable technical challenges and are still in the development stage. 3 PROTOTYPING AND RAPID TOOLING 3.1 Additive Manufacturing and Rapid Prototyping for Die and Mold Production The class of additive fabrication methods usually known as “rapid prototyping” (RP) or “solid freeform fabrication” (SFF) processes have evolved considerably over the past decade. Although they were originally marketed as aids to design visualization and prototyping, in recent years the most promising application of these technologies has been in the area of rapid tooling for net shape processes. An excellent review was given as a ClRP keynote paper at the 48th General Assembly 13. All of the processes currently in use follow the same basic sequence of steps to construct a component. The process begins with a CAD solid model of either a piece part or tool insert, which is typically transferred to the RP machine in STL format. This data structure reduces the solid model to a set of triangular facets that define the surfaces of the part. This STL file is then “sliced” by the machine controller software, turning what was originally a three- dimensional object into an ordered set of two-dimensional layers. The part is then reconstructed, one layer at a time 41. (b) BHF= 30 tons (wrinkles are eliminated) Figure 6: Example of FEM simulation in stamping. By optimizing the Blank Holder Force (BHF) control, it is possible to form a wrinkle-free part I I. The RP processes differ in the particular method used to form the build material, as well as in the build material itself. To date, the most common build materials are either a liquid (stereolithography), an extruded solid (fused Figure 4: Flow chart for product, die and process design deposition modeling) or a powder (selective laser sintering). The techniques used to shape the raw material typically use laser-activated chemical change (stereolithography), laser sintering (LENS, SLS), extrusion (FDM), or an adhesive binder (3D printing). 3.2 Design and Visualization Tools: New Developments A major thrust in the RP market has been the development of low-cost “3D printers”, which are designed for office use, and are intended solely as visualization aids to part designers. All of these processes are intended as low-cost prototyping methods for producing relatively fragile parts, allowing part designers to produce several iterations of a design quickly, and at low cost. None of these processes are presently capable of producing parts able to withstand significant stresses. 3.3 Rapid Production of Tooling Rapid production of dies and molds using additive processes can reduce the time and cost of bringing new products to market by drastically cutting down on design iterations and prototyping cycles. The additive processes (Example: Forging) I I. Figure 5: Simulation of forging an automotive crankshaft using a 3D commercial FEM code I I. have other advantages as well, such as the ability to build conformal cooling lines around a mold cavity, and the ability of some systems to tailor the material properties of the part as it is built. The concept of “rapid tooling” includes three distinct segments: prototype tooling, “bridge” tooling, and tooling for limited production runs. Prototype tooling is exactly what the name implies: a die or mold designed to test a new component design, a new material, or perhaps a new process. In this case the tool itself is not intended to produce more than a few hundred parts, so tool life, cycle time, and part ejection are typically not design issues. Much slower cycle times and manual ejection are often employed to simplify the tool design and save valuable time and producing the prototype tool. Because the cost of prototype tooling can be folded into the total tooling cost and amortized over the entire product life of the final product, cost is not a primary concern. On the other hand, product development constraints demand that the time to produce the tool must be very short, typically only a few days or weeks. “Bridge tooling” is the name applied to dies and molds that are designed to last for perhaps tens of thousands of product cycles. These tools permit a new product to come onto the market early, while the production tooling is still being fabricated. While these tools do not require the durability of production tools, and may not be optimized in terms of the process parameters, they must be able to withstand several thousand cycles, while holding production-level tolerances. Again, the cost of these tools can be folded into the total tooling cost for the entire production run. Finally, the most demanding application is for tools for short production runs. With the advent of lean manufacturing and mass customization, the need to produce tools that can produce quality parts in small quantities, and do so cost effectively, has become a major issue in many industries. Often it is unclear whether is it better to build a single die or mold to produce a limited number of parts, spread over several years, or is it better to build cheape