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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 o f f 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 4 1 . (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 cheaper, less durable tool, discard it at the end of each production run, and build a new tool when another production run is planned. This second point of view holds that “the die is in the data”: as methods for turning CAD models into functioning tools become more sophisticated and less time consuming, there is little to be gained from making a very expensive tool to produce a limited number of parts. Better to discard a cheaper tool and build another, every time a new production run is needed. As this idea becomes more widely accepted, industries that build components in small lots, such as the military and aerospace, may be more inclined to adopt net shape processes. Rosochowski and Matuszak I51 have proposed a classification of rapid tooling processes, shown in Figure 7, based on practical uses for the tooling, rather than on the particular process used. They divide rapid tooling processes into three major groups: those that are used to produce patterns for casting, those used to produce patterns for both soft and hard tooling (“indirect” tooling), and those that produce production-ready tools directly by RP methods (“direct” tooling). A good overview of current industrial uses for several of these techniques is given in Patterns for Casting Although producing patterns for casting is not generally regarded as “rapid tooling”, in fact, several RP processes have been applied to pattern making for sand casting and investment casting, including FDM (Fused Deposition I 61. Silicone moulds EPOSY moulds Modeling), SLS (Selective Laser Sintering), and LOM (Layered Object Manufacturing). Patterns are typically made from wax, but complex patterns sintered from polycarbonate by the SLS process have also been used Recently, a new method has been developed for producing large (1.50.750.75 m3) sand molds and cores quickly. This technology uses an ink jet technology to spray binder onto layers of sand, followed by a reactant which is laid down selectively, resulting in a precise, strong, sand mold or core. This process is reportedly ten times faster than Selective Laser Sintering, and can also create wax models for investment casting as well 17. RP methods have also been used to form molds that produce foam patterns for the lost-foam casting process. A rather complex technique is used in the automotive industry to produce molds for polystyrene foam patterns by using LOM, RTV silicone, and a high temperature aluminum filled epoxy. The combination of LOM and RTV silicone produces a model that gives excellent surface detail of a complex part 18. I 51. Spray metal Metal tooling powder Electroform- Ceramic ed tooling powder I Rapid tooling I Cast metal tooling Keltool tooling Microcast tools Laminated tools Figure 7: Classification scheme for rapid tooling I51 lndirect Methods In injection molding the RP process is most often used to produce a model of the mold insert, with runner geometry attached. This mold insert is filled with silicone, which is allowed to cure. The silicone positive is then used to cast an epoxy tool, which can typically withstand several h u n d red injection- mo I d i ng cycles. Because epoxy molds are limited in the number of parts they can deliver, there is much interest in industry in finding other techniques to cast tools around masters derived from RP methods. A method was developed by a car manufacturer to spray molten tool steel onto ceramic molds, which are produced from RP models. To date this method is used to produce relatively small (600 mm x 600 mm) tool sets. However, work is in progress to scale up the technique to produce sheet metal dies for auto body panels 17. Another related method is called Rapid Solidification. It involves spraying a molten metal, in this case HI3 tool steel, against a ceramic master. The developers of the method claim that the resulting tool steel shell shows superior strength, hardness, and surface finish. The first tool produced by this process is due to go online in July of 2001 19. Several other techniques use sc-called “indirect” methods to produce tool sets in metals. These methods have almost all been applied to injection molds, due to the relatively benign production environment. Ainsley and Gong report on a method to slip cast stainless steel molds using RP masters of the mold 20. Weaver et al. report on a method to produce the model of the tool set. Silicone is used as the intermediate material, and slurry of metal powder and polymer is then cast around the silicone. After curing, the tool is debound and sintered, which produces a tool set with properties approaching hardened tool steel 21. A similar process has been in use for several years. It uses one of three proprietary metal composites, which is cast around a silicone master. Details on this technique are available in 22. Direct Methods These processes all use RP technologies to make a die or mold directly from the CAD model, without using additional pattern transfer techniques. The most common method for direct tooling involves a “green” part that is created either by selective laser sintering (SLS) or three-dimensional printing (3DP) methods. SLS 23 deposits a layer of polymer coated metal powder, which is then selectively sintered using a laser. With the 3D Printing process a layer of powder is first deposited, and then an adhesive binder selectively applied. The major advantage of the powder-based systems is that the powder that is not used to form the part provides support for overhanging structures. In this way, conformal cooling channels and undercuts can be created without need of additional support material. An excellent overview of recent work in various direct tooling methods is given in 24. In an overview of recent developments in direct tool fabrication using the SLS process, Klocke notes that precision on the order of hundredths of a millimeter is now possible, with attainable surface finishes (after some post- processing) of R , = 15 pm or better. Rather complex parts can be made with these tools 25. Considerable work has gone into developing Selective Laser Sintering (SLS) as a process for building dies and molds directly from the CAD model. Details are available for sheet forming dies 26, DTM rapid steel process 27 and the rapid mold process 28. In addition RBD is in progress for further improving the SLS process itself, specifically on powder deposition 29 and on the effects of laser power and traverse speed upon microstructure and porosity of deposited surface 30. An overall practical review of rapid prototyping and rapid tooling is given in a recent publication 14. 4 CAVITY AND PUNCHlCORE MACHINING The steps involved in manufacturing a typical mold for injection molding is seen in Figure 8 31. The cost components of an example injection molded part are given in Figure 9. This example illustrates that considerable cost reduction potential exists in rough and finish machining of dies and molds. 4.1 Tool Path Generation and Optimization Today nearly all die and mold makers use High Speed Cutting or Machining (HSC or HSM) in cavity and punch manufacturing. HSM requires not only specific machine tools (rigid, high spindle RPM, high feed rate, software with look ahead capabilities, high acceleration and deceleration) and cutting tools (ultra fine carbide with various and multiple coatings, optimized tool edge geometry, high performance cutting tool materials, i.e. PCBN and ceramics) but also special CNC tool path programming strategies. This is now being recognized not only by research institutions but also by various CAM system vendors. . . . . . Design (plastic part geometry) by OEM or Injection Molder Process Simulation (mold design) by Injection Molder or Mold Maker First Rough Machining of the Mold Steel Block, by Steel Supplier Rough Machining of Cavity (milling, drilling) by Mold Maker Semi-Finish and Finish Machining (milling, EDM) by Mold Maker Polishing and Assembly, by Mold Maker Mold Try-out and Finish, by Mold Maker Pre- Prod uction Qualification, by Mold Maker, Injection Molder and OEM Figure 8: Operations involved in making a typical mold 31. Figure 9: Cost Components (in %) in Manufacturing of an Example Automotive Part by Injection Molding (assuming 250,000 parts were produced in one steel mold) 31. Constant Chip Load and Cutting Speed Early studies on this topic concentrated on the optimization of the tool paths for 3D milling of sculptured surfaces with ball end mills. The basic approach was to maintain the cutting speed and the chip load approximately constant by controlling the spindle speed and the feed rate. Computer codes were developed, based on this principle and allowed the reduction of milling time 20 to 30%, depending upon die geometry while increasing cutter tool life 32, 331. Recently, process simulation and feed rate adoption to maintain constant chip load have been developed and applied to 5-axis CNC machining of sculptured surfaces using torus as well as ball end mill cutters 34, 351. Especially in rough milling operations feed rate adoption helps to avoid unacceptably excessive tool deflection and deviation from the theoretical design surface. Simulation of five-axis milling allows the operator to examine the influence of different milling strategies and to improve the process reliability while reducing the machining time. The optimization of NC programs for five-axis milling of dies and molds has also been suggested by 36. In this approach multiple process models are used for technological optimization of NC programs. A software module has been developed which extends the functionality of a commercial CAD/CAM system. NC tool path generation for five-axis machining has also been optimized by a team of researchers 37. This “extended CAM system” for multi-axis milling integrates tool path generation, axes transformation and NC simulation. The system performs an immediate verification of each generated cutter location and in case a collision occurs (e.g. between machine and part), it takes the appropriate action by applying a collision avoidance algorithm. Thus, the system facilitates the use of five-axis machining and allows the variation of the tool inclination during tool path generation in order to achieve the best combination of scallop height, workpiece accuracy, surface roughness and machining cost 38. The effect of cutting speed and lead angle on tool wear in ball end milling has been also investigated in a recent study 39 where the strategies for optimizing the CNC programs are also reviewed. It is shown that by applying appropriate machining strategies in hard milling, it is possible to achieve cost savings of up to 30 percent. Some of the cutter path optimization strategies are now being implemented into controllers of HSC machine tools 40. In milling with a ball nose end mill, a constant spindle speed can produce a variety of surface speeds depending upon the contact point of the tool on the workpiece. The effective tool radius (RI or Rz) changes with the angle of contact, (affecting the cutting speed at constant RPM), as seen in Figure 10. A feature built in the controller of a CNC machine can calculate the contact radius of the ball based on the angle of the tool path and overrides the spindle RPM to maintain a constant cutting speed during 3D milling. Thus, the spindle speed is a servo-controlled parameter, just like the X, Y, and 2 motions. In addition, the feed rate is also modified to maintain a constant chip load, similar to the R 8 D studies conducted earlier in various laboratories. This controller feature allows the program to specify a desired cutting speed and let the control work this surface speed target by continuously calculating the position of the tool. R2= Rxcosl5” U R 2 Figure 10: Schematic illustration of the cutting speed variation in ball end milling with contact angle 401. A so called “in corner cutting” capability is said to be in the latest stage of development and will allow a machining center to mill out a “sharp” corner that can only be produced today by EDM. As seen in Figure 11, as the spindle rotates, the position, i.e. the axis location, of the spindle is continuously changed in coordination with machine motion in X and Y. A special triangle-shaped tool is necessary for performing this cutting operation. Start + + - I . I I + - - End V i , LSharp Corner Tool Figure 11 : Schematic illustration for the rotating sequence of a “Corner Cutting” cutter (a special triangular tool is used and the position of the centerline is continuously changed during tool rotation) 40. Tool Path Strategies In a recent study, dies for forging and stamping and molds for injection molding have been manufactured using HSM with the application of a “circular strategy” in pocket machining, as seen in Figure 12 41. Here, the tool is moved alternately from one side (AB) of the machined slot to the other side (CD) as indicated with the path of the spindle axis. Thus, the tool contact time with the workpiece is reduced and the tool is allowed to cool. Carbide tools with TiAlN coating, cylindrical bull end for rough cutting (6, 8 and 10 mm diameter I 4 teeth) and ball end for finishing (2, 4, and 10 mm I 2 and 4 teeth), were used to machine cast iron (GG 25) and three different tool steels (38 to 48 HRC) using a high speed milling machine (24,000 RPM spindle). The results of this study agrees with the well known machining strategy in HSM where the heat generated by the cutting process is discarded with the chip and the tool is cooled because of limited engagement with the workpiece during a single rotation. Figure 12: Tool path trajectory generated for the “circular strategy” 41. A good review of hard milling strategies is available 42. The authors point out that in HSM of hardened die and mold steels with carbide tools, for a given cutter radius, R, the arc of contact, a, is governed by the radial width of cut, a,. Thus, a increases with increasing a, and the end mills cutting edges remain longer in the cut which leads to heat build up and a decrease in tool life. Therefore, there are recommended values of a, depending upon cutter diameter and the workpiece hardness. The so called HSM peel strategy is used for milling a slot into the workpiece material. As seen in Figure 13, by maintaining a limited radial width of cut (a,), the arc of contact between the tool and the workpiece is reduced 42. In conjunction with proper feeds and speeds (recommended for tool diameter, workpiece hardness and slot dimensions), the end mill can remove a large amount of material without generating excessive heat that increases tool wear. In Figure 13 the a , is exaggerated for illustrative purposes. This figure shows a view towards the plane of the slot directed from the machine spindle. The HSM helix strategy, Figure 14, is widely used for vertical milling into the material, i.e. to begin rough cavity machining from a block of die steel using experience- based values for tool diameter and axial depthlrevolution for a given hole diameter and material hardness. In ramping, i.e. in milling a slot form into the material when entry must be from the top of the part, certain values are recommended for ramping angles and axial depth of cut, ap, depending upon material hardness. Strategies have been developed for modifying the CNC programs so that tool deflection in milling sculptured surfaces can be compensated for. Thus, the geometric definition of complex surfaces can be maintained within close tolerances. As an example, a software module has been developed and integrated with the multi-axis machining programs 43. This software predicts the cutting forces, estimates the tool deflection and performs the necessary corrections on the CNC program. Thus, this approach is particularly useful in milling with thin and long tools. Figure 13: The “peel strategy” to reduce the arc of contact by maintaining a limited radial width of cut a , 42. 4.2 High Speed and Hard Machining - Overview In a conventional die making operation, the cavity is usually machined to about 0.3 mm oversize dimensions. The die is then hardened, which may cause some distortion, and then EDMed to final dimensions. The trend in dielmold manufacturing today is towards hard machining, both in roughing and finishing, and in replacing EDM whenever possible. Thus, the number of necessary machine set-ups is reduced and the throughput is increased. High speed machining of hardened dielmold steels makes this trend feasible and economical. Figure 14: The HSM helix strategy for vertical milling into the workpiece material 42. The main objective of high speed machining of hardened dies is to reduce benching by improving the surface finish and distortion. Thus, quality is increased and costs are reduced. High speed machining of hardened dies (40 to 62 HRC) has within an approximate range, the following requirements and characteristics: Feed rates; 15 rnfmin or higher when appropriate pressured air or coolant mist is provided, usually through the spindle. Spindle RPM; 10,000 to 50,000 depending upon tool diameter. Surface cutting speeds: 300 rnfmin to 1000 rnfmin, depending upon the hardness of the dielmold steel and the chip load. High speed control with high-speed data and look forward capability to avoid data starvation. Look forward capability tracks surface geometry, allowing the machine to accelerate and decelerate effectively for maintaining the prescribed surface contour. High acceleration and deceleration capabilities of the machine tool in the range of 0.8 to 1.2 rnfs. 4.3 Machining of Dies for Sheet Metal Stamping Large dies for sheet metal forming are manufactured by rough and finish machining cast iron or steel die structures. In machining of cast iron, coated carbides, CBN, SIN and coated carbides are the most commonly used tools. Recently, the performance of various CBN grades and coated carbides were investigated 44. Milling experiments were performed on a four-axis high- speed horizontal milling center. Tool geometry and cutter specifications are given in Figure 15. Using TiN-coated carbide tools instead of uncoated carbide tools increases productivity in terms of cutting speed by 25% while tool like increases by more than 500%. In addition, TiAIN- coated inserts ran at least three times as long as TIN- or TiCN-coated inserts at any cutting speed, Figure 16. However, PCBN inserts outperformed the coated carbide inserts. Tests were aborted after A = 1.6 m2 of surface area was machined and tool wear on PCBN 2 was measured at V B , , = 60 pm and on PCBN 0 at V B , , = 85 pm. Abrasion and thermal fatigue were identified as the main wear mechanisms. Higher CBN content and higher hardness exhibited favorable wear resistance. 4.4 Machining of Dies and Molds from a Tool Steel Block In general molds for injection molding, blow molding and compression molding, as well as dies for die casting, forging and tube hydroforming, are manufactured from a block of tool steel (in exceptional cases molds are also made from aluminum or copper alloys). In these applications a large amount of material must be removed by rough machining. Consequently, machinability of tool steel, together with many different aspects of hard and HSC milling, is an important issue, especially for rough machining. In a recent study, the machinability of three mold steels with different sulfur contents but of the same hardness (HB 300) was investigated by varying the cutting conditions 31. It was found that a) the main parameters affecting the milling process (using round carbide insert in a 40 mm torus tool) were cutting velocity, feed per tooth and radial and axial depths of cut, b) the addition of sulfur to the mold steel brings an increase of about 50% in tool life, and c) when machining at high feed rates an increase of 75% in tool life could be obtained. Tool Life Criterion I ToolType I Single insert indexable ball end mill I VB= 0.150 mm I Tool Diameter I D= 25.4 mm I I No. Cutting Edges I z= 1 I I Tool Geometry 1 y= -go, a= 16 I I Tilt Angle 1 pm=30 I I Lead Angle 1 Br= 0 I Fundamentals of high speed and hard machining are still partially understood. Research is being conducted in various laboratories to understand the basic physical phenomena that determines chip formation, the effect of cutting speed and temperatures. In machining hardened steels, with increasing cutting speed the chip formation changes from continuous to serrated chip form. Thus, the cutting forces may be reduced and tool life may be increased provided the temperatures at the tool edge can be maintained within allowable limits 45. Evaluation of the cutting power in function of cutting speed (100 to 3000 mlmin) showed that there seems to be a material specific cutting speed where the cutting power reaches its maximum value. Beyond this value, with increasing speed the cutting force remains approximately constant and the power increases linearly with the cutting speed 46. Figure 16: Summary of all tool life experiments in pearlitic cast iron (symbols explained in Figure 15) 44. An excellent example of HSC milling of hardened dies is discussed in a recent study 47. The investigation was focused on dies for forging turbine and compressor blades (die material 1.2343 heat treated to 53 HRC with an ultimate strength of 1600 Nlmm) that had surface dimensions of 185 x 125 mm and 600 x 330 mm. The results of this investigation illustrates many aspects of HSC milling faced in industrial practice, namely: Die and mold makers are provided with a large amount of information on test results on tool life. However, this information is difficult to apply to the specific conditions without knowing the exact test conditions, i.e. feed, speed, infeed, angle of inclination, etc. To succeed in the application of HSC milling, teamwork (cooperation between 3D design engineers, NC mill operators) and documentation (in addition to program documentation this includes milling strategies, milling machine and operation parameters and test results) are essential. It is important to invest more engineering time (calculations, programming) at the start of manufacturing a new die to determine the best machining conditions. Thus, it is possible not to remain conservative and use low feed rates resulting in long machining times. In rough machining it is better to use relatively smaller tool diameters and increased feed rates. Thus, more uniform machining allowance is left, which facilitates finish milling. As a result the time necessary for roughing may increase but the time needed for finishing will decrease so that the total milling time may be reduced. Rough machining pre-hardened die blocks requires more time than machining in the soft state. However, when rough and finish machining in hardened state, a new tool set up is eliminated. While the decision for hard machining has to be made for each application individually, in the present case HSC milling in hardened state could be done economically for both rough and finish machining, using full carbide ball end mills and inserts, Figure 17. This experience is very similar to that of many advanced technology forging shops in North America and in Japan. To increase wear resistance and die life, forging dies are often surface welded with high temperature alloys. In the past, the finishing of such surface welded dies, new or after repair, was done by EDM. Recently, specifically designed cutters with PCBN inserts were used for high speed machining of such welded surfaces 48, 391. Studies with different cutter materials demonstrated that specific PCBN grades (with appropriate tool edge preparation and machining strategies) offer long life and cost effective machining of hard welded surfaces. The machining of injection molding molds and die casting dies present particular challenges because these applications have many thin and deep cavities to be machined. In the past the strategy in manufacturing such molds consisted in rough machining, hardening and finishing by EDM. To save set-up time and decrease the total manufacturing time it is desired to rough and finish machine in one set-up and in hardened state. Thus, it is necessary to develop techniques for milling deep cavities using long solid carbide milling cutters and with appropriate cutting conditions. A study, conducted for machining hardened dies (1400 to 1500 Nlmm strength) for casting, illustrated many of the difficulties and remedies related to milling with long and thin cutters. A software module was developed for estimating the cutter conditions for selected cutter geometries 49. This study and others indicate that, with appropriate process selection of parameters, EDM can be replaced in many applications WI. Figure 17: Development and reduction of machining times for one forging die 46. 4.5 Machine Tools and Cutting Tools Machine Tools It is well known that the milling machines for high speed and hard machining must be stiff and have high acceleration and deceleration capabilities. This is especially important in machining of small dies and molds, where it is rare to have large, relatively flat surfaces to cut. Thus, the tool must continuously accelerate and decelerate to machine the specified contour. For example, the response of a typical high-speed milling machine (Makino AS), used in die and mold manufacturing, was measured when machining a specific sculptured surface. At a programmed feed rate of 20 rnfmin the machine needed almost 70 mm to reach the desired velocity. At moderate feed rates, say at 5 mlmin, it needed about 4 mm to reach the target feed rate 50. While accelerating and decelerating the chip load can not be maintained and is reduced since, in most machines, the spindle continues to rotate at the same speed which may lead to “rubbing” action on the tool, increasing tool wear. In machining a selected sculptured surface, it was shown that increasing the nominal feed rate might not result in a proportional reduction in machining time, since the actual feed rate is determined by machine dynamics. This is illustrated in Figure 18, where the doubling of the nominal feed rate has different effects on machining time, depending on the range considered. Of course, this result is valid only for the specific machined geometry but illustrates the importance of machine inertia on total machining time. Similar studies indicate the importance of machine inertia and die geometry in reducing machining times 45. Figure 18: Actual and calculated (using the software OPTIMILL) machining times for a selected sculptured surface 49. The use of linear motor drives in large milling machines, used for manufacturing automotive stamping dies, is already well known. The trend appears to be in increased application of linear drives in milling machines. Machines that provide the tool motion using parallel kinematics and Hexapod concepts are being evaluated in various laboratories. Even though these machines appear to be suitable for die and mold manufacturing, there is no proved industrial application of such machines in industrial die manufacturing practice. Recently an innovative machine tool concept has been developed for quick repair of large stamping dies 39, 511. This machining unit can be brought to the press without taking the dies out for repair, thus providing “on-site machining” capability. To provide multi-axis capability with a rigid frame, this machine is built with a hybrid-parallel and serial-kinematical structure, Figure 19. The prototype of this machine has five-axis capability and is being a depth of cut “ a ; and step over distance “a,“, is usually evaluated under practical die repair conditions, Figure 20. considerably lower than the selected cutting velocity, based on spindle RPM and cutter diameter alone. In hard milling, the dimensions of the tool edge radius (about 30 pm for roughing and 10 pm for finishing) and that of the chip thickness (about 25 to 50 pm for roughing and 10 to 12 pm for finishing) are very small. Therefore, it is necessary to estimate and work with actual cutting speeds, which are estimated to be about 1.8 times the nominal cutting speed, in order to determine a reliable process window, Figure 21. Thus, based on empirical data the finish slot machining of a hardened tool steel, typical cutting speed vc= 300 rdmin, can be increased up to vc= 500 rdmin. In another example, by increasing the chip load from an estimated h , = 1 pm and by doubling the cutting speed, it was possible to quadruple the productivity 2 1 . wfactor 1 8 1 6 1 4 1 2 1 0 8 0 0 2 0 4 0 6 0 8 - 1 I &Id Figure 19: Concept of the transportable machining unit 39 Figure 21 : Geometric conditions and the ratio of “numerical versus cutting speed, v,-Factor, in function of ste p-over distance” 52. Figure 20: Transportable milling machine used for die repair 51 Tool Geometry In addition to the characteristics of the machine tool, the factor that affects the success of high-speed milling is the selection of tool material, coating and geometry, in accordance with cutting conditions. For example, the tool edge radius for hard machining is in the range of 5 to 30 pm. In rough machining a forging die with a 8 mm diameter ball end mill at feed per tooth of fz= 0.087 mm, the mean chip thickness “ h , ” is 5.7 pm 51. In gradually increasing the feed rate and the feed per tooth to 0.12 mm and the chip load to 8.2 pm, it was possible to improve productivity approximately 40%. This was achieved because the effective cutting velocity, based on The application of high speed hard milling for finishing requires that roughing must a) be conducted with a large metal removal rate and b) results in a machined surface that is very near the finish geometry and leaves a relatively uniform machining allowance for finishing (0.05 to 0.5 mm). Thus, for effective application of HSM alternative insert geometries are considered. Tools with round or octagonal inserts give, with comparable depths of cut, significantly better sculptured surface contours than cylindrical end mills that are in turn most effective for pocketing and slotting 53. The round inserts offer maximum strength and represent an excellent solution for different milling conditions 54. With these inserts the chip thickness varies with the depth of cut and feed rate must be increased to achieve appropriate chip thickness and increased metal removal rate. Die and mold machining requires a wide selection of cutting tools and inserts for facing of large and small areas, machining deep walls and shoulders, as well as machining of long, both deep and narrow slots 55. Thus, tools are necessary for plunging or ramp down operations, carbide with 8 to 12% cobalt and TiAlN coating is best milling, as well as drilling including deep drilling, reaming, suited. It is also interesting to note that the quality tapping and chamfering. differences between tools offered by various suppliers has The main geometries for face-, shoulder-, and slot milling been reduced 9. include flat positive inserts, inserts with ground and molded chip formers and helical, non-flat inserts with modified rake and clearance faces, which provide improved performance. Tool producers offer end mills with interchangeable heads for various milling and profiling operations. The helical cutting edge concept, developed recently, is used with solid cutters as well as with inserts, Figure 22. This geometry and the helical curve cutting edge as well as curved rake and clearance faces result in constant rake and clearances on the tool during milling. In addition, each cutting edge penetrates gradually into the workpiece with a gradual increase in the cutting force. Square, multi purpose helical inserts are available for 90 degrees shouldering, facing and slotting 55. Figure 23: Groove type chipformer for improved tool life 55. Figure 22: The helical cutting edge concept 55. Octagon inserts, Figure 23, offer cutting edges for mor economical facing, shouldering, slotting, recessing an chamfering applications. Some of these inserts ar available with a series of depressions to reduce the contact area between the chip and the insert rake face to a) reduce heat flow to the insert, b) reduce friction, and c) improve tool life. Figure 23 illustrates schematically the heat flow into and from the tool. Heat generated by friction and deformation flows from the curled chip into the tool and is illustrated by horizontal arrows. The depressions in the tool reduce the amount of toollworkpiece contact and the heat flow into the tool. Furthermore, as indicated schematically by inclined arrows, convective heat loss from the tool is also increased. It is claimed that, the positive rake angle, in combination with these depressions also reduces cutting forces and improves chip flow. As seen in Figure 24, octagon and round inserts can be used for a variety of milling applications. Normally up to about 8 mm diameters, ball nose end mills are made of submicron substrate for improved toughness and PVD coating (TiCN, TiAIN) for hardness and high wear resistance. For larger diameter end mills, inserts and increasingly used. For high speed milling at higher cutting speeds, normally solid endmills are recommended. In some cases, however, screw clamped or blade type inserts can also be used. Tool Materials and Coatings For tools ranging from 12 mm to 35 mm diameter, carbide insert tools are shown to be effective. TiCN coatings are sufficient for machining die steels less than 42 HRC while TiAlN coatings are used for 42 HRC and over. For tools with a diameter of 12 mm and under, sub-micron solid Figure 24: The multifunction milling option with octagonal and round inserts 55. Similar results were obtained in other investigations. It was found that in using TiAlN coating, tool life increases and then decreases with increasing TilAl ratio. The best combination of TilAI was found to be 0.35 (Ti) to 0.65 (Al). This coating (Ti 0.35 Al 0.65 N) gave better tool life than TiCN in machining 57 HRC die steel under similar high speed milling conditions, Figure 25 52. The application of CBN in hard turning is well known. The process has the potential to reduce manufacturing costs by eliminating or reducing grinding and by eliminating the use of lubricants. To achieve the potential of PCBN, all aspects of hard turning must be considered, including machine tool, work holding, tool compensation, insert material grade and edge quality, and stability 56. These issues as well as achievable surface integrity and form accuracy are reviewed in a recent publication 57. Tool Holders In high speed milling with high spindle RPM, tool holder balance is very critical to avoid premature tool failure and to obtain good surface finish. Run out is recommended to be less than 5 pm. Experience indicates that for each 10 pm runout, in general, tool life is reduced about 50 percent. Thus, shrink fit holders are the best and easy to balance while hydraulic chucks are acceptable at moderate RPMs 58. In addition, it is necessary that inserts and tool holders have better dimensional tolerances. The length of the tool affects the dynamic behavior of the tool, especially when cutter shank length to diameter ratio exceeds three to one. As a result, tool life decreases with increasing tool shank length. Clearly, the tool shank should be made as short as possible. The insert shape also influences vibration by altering the entry angle of the cutting edge to work. Round inserts are most prone to vibration while the inserts with 45 lead angles show the least tendency to vibration 54. Figure 26: Predicted temperature distribution from FEM simulation - Max. tool temperature = 605 C, Max workpiece temperature= 601 C 61 Figure 25: Tool wear in rough milling 57 HRC die steel with TiCN (left) and TiAlN (right) coatings 52. Modeling of Machining Operations A recent ClRP publication reviewed and summarized a myriad of modeling techniques to predict forces, temperatures and chip formation for turning, milling, and other metal cutting operations 59. Among all available models, the FEM based process modeling of metal cutting appears to offer great potential for predicting the parameters of the machining processes. This methodology is still in development stage and it is practical today for only two-dimensional chip flow analysis, it has great potential for estimating temperatures, chip flow, tool wear, residual stresses and microstructural variations during machining. As an example, recent studies applied the FEM analysis to 2D machining to predict a) continuous and serrated chip formation 60, 611, b) the effect of edge preparation including sharp, honed and T-land edges, c) temperatures in the workpiece, chip and tool under various cutting conditions, d) estimate tool wear with uncoated carbide tools, and e) the cutting forces. Figure 26 shows the temperature distribution calculated for specific orthogonal cutting conditions. (Workpiece: P20 (30 HRC), tool: uncoated carbide WC, vc = 150 mlmin, f = 0.105 mmlrev, rake angle = O, relief angle = 6, edge radius = 0.020 mm) Figure 27 illustrates the effect of tool edge hone radius upon the tool stresses. (Workpiece: HI3 (46 HRC), tool: uncoated carbide WC, vc = 200 rdmin, f = 0.25 mrdrev, rake = -5, relief = 5) The results are compared with experimental data and demonstrated that FEA of metal cutting can certainly predict process parameters in simple cutting conditions. Furthermore, preliminary 3D process simulations, although requiring consid era ble computational resources, indicate that FEM will soon become a very powerful process simulation tool for selecting optimum cutting conditions, tool materials and coatings, and tool wear for complicated milling operations that are encountered in die and mold making. (a) Hone radius = 0.01 rnrn Max. eff. stress = 3950 MPa (b) Hone radius = 0.05 rnrn Max. eff. stress = 2700 MPa (c) Hone radius = 0.1 rnrn Max. eff. stress = 2000 MPa (d) Hone radius = 0.2 rnrn Max. eff. stress = 3800 MPa Figure 27: Effect of edge preparation on tool effective stress in a honed edge tool, as estimated by FEM simulations 61. 4.6 Application of Lasers in Die and Mold Making Laser surface treatment is already in use in a number of applications in industry, to enhance the wear characteristics of dies and molds 62. In this technique, a limited area of the die surface is melted using high-energy laser radiation. A path is created by the feed motion of the workpiece in relation to the laser beam. Several tracks are laid alongside one another. A range of different materials, selected for the specific applications involved, can be fed in powder form to harden and coat a specific portion of the die, as seen in Figure 28. The materials suitable for alloying applications are carbides such as WCICo, WCICr, TIC and VC. Cobalt and nickel alloys are generally used in coating applications. The alloying depths that can be achieved range from 0.3 to 0.8 mm, permitting laser processed dies to be finish machined. Laser surface treatment technique has been applied to the repair, restoration, and reconfiguration of dies that require minor surface modifications using the Direct Metal Deposition process. An industrial laser and powdered material are used to create and reconfigure fully densed parts or layers directly from a CAD file 63. Application of lasers for die maintenance and improvement includes not only die surface repair and modification but also localized laser beam hardening, surface coating and laser beam alloying and dispersing 391. Laser technology is also applied to manufacturing of small dies. A 100 W YAG last cutting machine provides a 0.1 mm diameter laser beam that can be precisely controlled to machine cavities in a wide range of materials, including ceramics 64. Using special software, the laser pulses are controlled to vaporize the material in layers of about 5 pm, resulting in surface finishes in the order of 1 to 2 pm. The material removal rate is 13,000 mm3/min. The working envelope is about 406 mm x 305 mm x 559 mm (x, y, z). Maximum machining depth is 10 mm. Another model (15 kW and 12,000 RPM spindle) combines conventional milling for rough machining with laser milling for finishing. Laser treated zone I- Figure 28: Laser surface treatment 62. 5 Traditional manufacturing processes (Electro Discharge Machining-EDM, Laser Beam Material Removal (LBM), High Speed Cutting and Hard Machining (HSCIHSM) greatly affect the surface integrity of manufactured die or mold, namely: 1. macro- and micro-surface quality, accuracy, and roughness 2. sub-surface microstructure and composition 3. sub-surface residual stresses, and 4. surface and sub-surface microhardness. These issues have been discussed in an excellent review prepared for ClRP 65. DIE AND MOLD SURFACES AND DIE LIFE The conditions (surface topography, heat transfer, friction) at the dielmaterial interface affect not only the surface and appearance of the finished product but they also influence the process conditions, especially in metal forming, i.e. forging and stamping. Thus surface finish and coating of the dies are very critical in improving lubrication and metal flow as well as die life. A recent study investigated the influence of excimer laser treatment of cast iron and ceramic surfaces, used as sheet forming dies, upon the tribological behavior 66. The study illustrated that microtextures produced by this technique, improves the lubrication conditions on ceramic surfaces but not on metals. However, the geometry (size and depth) of the textures strongly influences the results. In cold forging die life and reliability are two very important factors influencing process economics. The use of carbide inserts, punches and coatings in the industry are standard procedures for reducing tool wear in cold forging. Using process modeling techniques for estimating metal flow and tool stresses it is today possible to design cold forging tooling so that stress concentrations are eliminated and the fatigue life of dies and punches is extended. Application of FEM modeling to production cold forging of bevel gears and constant velocity joint (CVJ) components have been described in earlier publications 67. Recently, a so-called SVL (strength versus load) concept was developed to combine the proved methods of numerical process simulation with statistical analysis to predict tool life 68. The concept has been applied and has been shown to give reasonable results. It was also shown that the accuracy of tool life prediction depends on the availability and quality of input parameters, necessary for the implementation of the SVL methods. In hot forging die life is mainly affected by abrasive wear that leads to a) bad surface finish and b) out of tolerance dimensions of forged parts. Thus, understanding and controlling of die wear in hot forging is extremely critical in determining the technical and economic feasibility of warm and hot precision forging processes. Extensive investigation, conducted on this topic, helped to gain considerable insight on this complex phenomena to tool wear in hot and warm forging 69. Die wear can be reduced and die life can be improved by surface treatment techniques. Classical methods such as flame hardening, nitriding, boriding, and surface welding of die surfaces with high temperature alloys are well known Surface layers generated by laser processing, as discussed in section 4.7, also produce a wear resistant surface that is metallurgical bound to the sub-surface. Other applications of laser surface treatment include distortion free laser assisted hardening, surface alloying, and generation of functional layers using laser coating. These techniques offer the potential for a) integrating surface treatment into the production chain of dies and molds, thus reducing the total production time 70, and b) performing rapid repair of dies and molds, which is very critical in terms of maintaining the production and keeping the processing equipment running 39. Die finishing and polishing is labor intensive and time consuming. High-speed milling operations reduce the need for die polishing to some extent in certain applications, mainly in forging and stamping dies. Injection molding molds and cold forging and extrusion dies, however, must still be polished to obtain desirable metal flow and specified surface finish on molded and forged products. An excellent process for automatic polishing of dies is the so-called abrasive flow process 71 that is being used in the die and mold shops around the world. 481. The application of high-speed milling allows obtaining good surface finishes within an acceptable machining time. However, there are still applications where hand finishing of the dies is necessary. In such applications three-axis or five-axis CNC grinding of the die surfaces is still more cost effective than hand finishing, Figure 29. The results of a study on automated grinding of dies are available and summarize the contact conditions in 3 axis grinding, the magnitude of grinding forces, the quality of the ground surfaces and the application of belt grinding to large dies with relatively large surface curvatures 72, 731. Figure 29: NC grinding of sculptured surfaces 72. 6 NON-TRADITIONAL MACHINING OF DIES AND MOLDS While high speed machining of hardened tool steel continues to attract much interest, EDM remains an indispensable process in the die and mold making industry. EDM and other non-contact processes such as ECM and hybrid processes continue to develop, both in terms of machining efficiency and their ability to produce precise die geometries in difficult to machine metals and geometries. In the past decade, increased control of the EDM process has provided a higher level of machining precision, along with decreased damage to the workpiece and reduced machining times. At the same time, EDM processes have become more tightly integrated in the total die and mold making process, leading to increased use of both wire and sinker EDM in a “lights out” mode 74. 6.1 Advances in EDM While EDM will never be able to compete with metal cutting in terms of removal rate, recent advances in machine and control technology have greatly increased the cutting speed of both wire and sinker EDM. WEDM cutting speeds have increased over 800% in the past 20 years, while SEDM cutting speeds have also increased significantly 75. At the same time, EDM machine manufacturers have reduced the severity of post-EDM surface damage, with recast layers as thin as 1 or 2 pm. The use of fuzzy logic and other advanced control logic has been an industry standard for several years. Process controllers have also benefited from advances in computer speed, as well. It is now possible to buy EDM machines with control loops operating on the order of a few microseconds, which can be several orders of magnitude faster than a typical discharge. The mechanical design of the machines has also improved considerably in the past decade. The introduction of linear motors to drive the axes in both SEDM and WEDM machines has led to much improved machine response to process instability. 75, 761 When coupled with ultrafast controllers operating on 2 microsec control loops and glass scales to directly measure the motion of the machines axes, linear motors enable EDM machines to react to process instabilities quickly, leading to increased process speed and decreased risk of damage to the workpiece. While the role of machine and controller dynamics in maintaining a stable process is crucial, other researchers have looked into altering the characteristics of the EDM dielectric to achieve the same result. 77 An interesting method for achieving greater control of the erosion process involves mixing additives such as silicone, chrome, or graphite to the EDM dielectric in SEDM, to enhance process stability. 76, 651 A major machine tool supplier markets this technology under the acronym “DDM”, which stands for “diffuse discharge machining”. They claim that use of additives can result in a finer surface finish, while maintaining metal removal rates. 78 Other promising areas of research include EDM of ceramics 79 the use of EDM for texturing mold surfaces 80, and the development of several hybrid EDMlECM machines, which will be covered more fully in the next section. 6.2 ECM and EDMlECM Hybrids Although it has never achieved the importance of EDM, electrochemical machining (ECM) continues to be an attractive “non-traditional” machining method for a variety of applications. An excellent overview of recent developments in this field is given in 81. In die and mold manufacturing, ECM has been limited by the difficulty of predicting the exact shape of a tool to machine a specific cavity to a high degree of precision. Rajurkar and his colleagues have applied orbital tool motion to conventional ECM to improve machining accuracy, as well as using pulsed current with passivating electrolytes. 82 Recently, several efforts at combining EDM and ECM in a single machine have appeared. 83, 84, 851 This approach clearly removes the problem of precisely maintaining relative positioning while moving the tool and workpiece, but adds the problem of keeping the EDM dielectric and ECM electrolyte from mixing. DeSilva and McGeough, on the other hand, have looked not only at hybrid EDMlECM processes, but also at what they term “electro-erosion4issolution machining”, in which a pulsed power source creates discharges in an aqueous electrolyte, followed by a pulsed ECM cycle. 86 6.3 Micromachining with EDM and ECM Interest in creating engineered structures at nanometer and micrometer scales continues to grow very rapidly. While nanometer scale engineering is still largely the domain of silicon-based photolithography, there is also considerable interest in being able to economically produce components from plastics, metals, and ceramics at dimensional scales less than one millimeter, but greater than 1 pm. 87 While micrc-injection-molding has perhaps attracted the most attention in this domain 88, other researchers are at work adapting net shape processes such as die casting and metal forming to microscale applications 89. A complete review of this topic is available in go, 911. Both EDM and ECM processes also possess disadvantages for micromachining as well. EDM tools wear quickly during the machining process, leading to some uncertainty regarding the exact shape of the tool during metal removal. It is also not always possible to control the erosion zone precisely, leading to overcut and tapering of high aspect ratio cavities and holes. In addition, forces due to steam pressure within the discharge bubble, as well as electrostatic and electromagnetic forces can cause workpiece distortion when machining very small features. The problem in ECM also revolves around control of the erosion zone. In pulsed ECM this is a function of the pulse parameters, the shape of the tool, the width of the machining gap, and the type of electrolyte employed. Nevertheless, both processes are the subjects of much interest as researchers seek to find better ways to machine dies and molds at dimensions between 1 and 1000 pm. Because tool wear in micro-EDM can be minimized, but never completely eliminated, the ability to predict the degree of tool wear with a high level of certainty is clearly a major issue. Rajurkar and Yu have developed what they call the “uniform wear method”, and applied it successfully to the machining of features on the order of 100 pm. 92, 931 The use of wire EDM for machining structures at sub- millimeter dimensional scales showed that further advances in this area will clearly require the use of smaller diameter wire than is currently practical, with a necessary improvement in wire guide and wire transport systems, and pulse generators capable of higher frequency and shorter duration pulses. 94 At this early stage relatively little attention has been paid to adapting pulsed ECM technology to the task of micromachining, even though this process seems to possess inherent advantages at this scale. ECM creates a stress-free, undamaged surface, and has the major advantage of zero tool wear. On the other hand, the ability to confine the erosion to a narrow zone appears to be the major stumbling block to wider use for high precision applications. DeSilva and his colleagues have recently published two very interesting papers on their efforts to improve the precision of pulsed ECM by developing an empirical model of the process based on the characteristic relationships of the process parameters 95. They have been able to achieve accuracies greater than 5 pm, with surface finishes in the range of 0.03 pm R , . 7 SUMMARY AND FUTURE DEVELOPMENTS Die and molds manufacturing will continue to represent a very significant aspect of production technology. The present developments and future trends can be summarized as follows: Dies and molds must be manufactured with even shorter lead times to offer flexibility and rapid introduction of goods to market. Thus, the role of process modeling, especially in making complex dies or molds, becomes very important for reducing time allocated for process development and try out. High speed machining is well established while hard machining is being rapidly accepted. Optimized tool path generation, to maintain constant chip load in machining complex sculptured surfaces, is offered by research centers as well as some software suppliers. The wider use of these technologies will allow die makers to become even more competitive. 1 n 1 P P 1 C S n rl P d d V n a n Ii E tl U S n C 1 r, a fl C F he trend for unattended machining is very strong, nainly in industrially developed high wage countries. his mode of manufacturing requires robust rocesses, advanced tool path generation, and best lossible use of machine tools and cutters. he cutting tool industry continues to develop new utter geometries and coatings for obtaining better urface finish and long tool life. Obviously this trend All continue. However, these new developments equire that the users, i.e. die shops, keep training their lersonnel and keep up to date about new levelopments in the industry. The continuous levelopment of cutting tools is now being assisted by ising FEM based simulation of the cutting process. Vhile these techniques are still at their infancy, there is 10 doubt that process simulation in machining will be ccepted by the industry, as it is the case with process nodeling of stamping, injection molding and forging. : is desired to machine the dies and molds in one ingle set up. Thus, deep cavities usually machined by !DM are often manufactured by milling with long and iin cutters. While this trend will continue, still there All be many applications where EDM is still the only ost effective method of manufacture. he machine tool and software suppliers offer, overall, ather good products for die manufacturing. The utting tools, including geometry, substrate material Ind coating, need continuous improvement in order to J rt her improve the machining conditions. EFERENCES Klocke, F., Klotz, M., Knodt, S., Altmueller, S. 1999, The Process Chain in Die and Mold Manufacturing, (in German) presented at the EDM Technical Conference, Aachen, Nov. 4-5. Neugebauer, R., Stoll, A, Schneeweiss, M., 2000, New Production Systems for Die Making (in German), ZWF, Vol. 95, p. 612. Klocke, F. and Knodt, S., 1999, Hard-Fast-Dry: Advances in Machining of Dies and Molds, (in German), EDM Technical Conference, Aachen, November 4-5. Christman, A, 2001, Moldmakers Catch 22, Moldmaking Technology, March, p. 19. Gnass, C., 2000, Globalization in Die Manufacturing - The Jump to USA, Conference on Die Manufacturing with Future, Aachen, Sept. 27-28. Casellas, A, 2000, The Future of Die Manufacturing in Europe, Conference on Die Manufacturing with Future, Aachen, Sept. 27-28. Friedrich, G., 2000, Estimation of Die Costs, (in German) presented at the Conference on Die Manufacturing with Future, Aachen, Sept. 27-28. Bogenschutz, U., 2000, New Technologies - Limits of Feasibility in Tool and Die Industry, (in German), Conference on Die Manufacturing with Future, Aachen, Sept. 27-28. Hock, S., Wenserski, J., 2000, Effective Manufacturing Processes in DielMold Making, (in German) presented at the Conference on 30 Experiences in Die and Mold Making, Dresden, May Brunnermeier, S.B., et al., 1999, lnteroperability Cost Analysis of the U.S. Automotive Supply Chain, Center for Economics Research, RTI, Research Triangle Park, N.C. 25-26. I l l Altan, T. et al, 1999, Simulation of Metal Forming Processes - Applications and Future Trends, Proceedings of the 6th ICTP, Vol. 1, Nuremberg, Sept. 19-24, p. 23. I21 Chang, C.C., Bramley, A.N., 2000, Forging Preform Design Using a Reverse Simulation Approach with the Upper Bound Finite Element Procedure, Proceedings Inst. Mechanical Engineers, Part C, Journal of Mechanical Engineering Science, Vol. 214 (CI), p. 127. I31 Kruth, J.P., 1998, Progress in Additive Manufacturing and Rapid Prototyping, ClRP Annals, Vol. 47, No. 2, p. 525. I41 Anonymous, 2001, Rapid Manufacturing Technologies, Advanced Materials and Processes, May, p. 32. I51 Rosochowski, A, A. Matuszak, 2000, Rapid Tooling: the State of the Art, J. of Materials Processing Technology, Vol. 106, p. 191-198. I61 Noken, S., 2000, Rapid Prototyping and Rapid Tooling in Product Development, (in German) Conference on Die Making for the Future, Aachen, Sept. 27-28. I71 Kochan, A, 2000, Rapid Prototyping Gains Speed, Volume and Precision, Assembly Automation, Vol. I81 Gervasi, V.R., F.Z. Shaikh, 2000, Indirect Rapid Molds for Prototype Lost-Foam Pattern Production, Proceedings of the 2000 Solid Freeform Fabrication Symposium, Austin, Texas, p. 506-51 3. I91 McHugh, K., J. Knirsch, 2001, Producing Production Level Tooling in Prototype Timing, Time Compression Technologies, March, p. 23-24. 20 Ainsley, C., H.Q. Gong, 1999, Costs and performance of injection molding tools produced using slip casting, Rapid Prototyping Journal, Vol. 5, No. 1, p. 35-44. 21 Weaver, T.J. et al., 2000, Time Compression Rapid Steel Tooling for an Ever Changing World, Materials and Design, Vol. 21, p. 409-415. 22 Zelinski, P., 2001, The Rapid Tooling Alternative, Modern Machine Shop Online, (http:/www. M) Article 109705, January. 23 Barlow, J.W., Beaman, J.J., Balasubramanian, B., 1996, A rapid mould-making system: material properties and design considerations, Rapid Prototyping Journal, Vol. 2, No. 3, p. 4-15. 24 Karapatis, N.P., van Griethuysen, J-P.S., Glardon, R., 1998, Direct rapid tooling: a review of current research, Rapid Prototyping Journal, Vol. 4, No. 2, p. 25 Klocke, F., Wagner, C., 2000, New Developments in the Technology of Selective Laser Sintering, Proceedings of the 2nd International Conference on Machining and Measuring of Sculptured Surfaces, Kra kow. 26 Hui, D.Z., Kai, C.C., Sen, C.Y., Gek, L.-L. K., Tiak, L.S., 2000, Advanced Sheet Metal Manufacturing using Rapid Tooling, Proceedings of the 2000 Solid Freeform Fabrication Symposium, Austin, Texas p. 27 Dalgarno, K.W., Stewart, T.D., Childs, T.H.C., 2000, Production Tooling for Polymer Components via the DRM Rapidsteel Process, Proceedings of the 2000 Solid Freeform Fabrication Symposium, Austin, Texas, p. 125-132. 20, NO. 4, p. 295-299. 77-89. 522-536. 28 Killander, L.A., 1996, Rapid Mould: epoxy-infiltrated, laser-sintered inserts, Rapid Prototyping Journal, Vol. 2, No. 1, p. 34-40, 29 Van der Shueren, B., Kruth, J.P., 1995, Powder Deposition in selective metal powder sintering, Rapid Prototyping Journal, Vol. 1, No. 3, p. 23-31. 30 Kobryn, P.A., Moore, E.H., Semiatin, S. L., 2000, The Effect of Laser Power and Traverse Speed on Microstructure, Porosity, and Build Height in Laser Deposited Ti-6AI-4V, Scripta Materialia, Vol. 43, p 31 LeCalvez et al., 2001, Rough Milling Characterization and Optimization of Steels for Plastic Injection Molds, presented at the ClRP Workshop on Machining of Dies and Molds, Paris, January 24. 32 Bergs, T. et al., 1999, Tool Path Optimization for Finish Milling of Die and Mold Surfaces - Software Development, Transactions of NAMRI-SME, May 24, p. 81. 33 Yazar et al., 1994, Feed Rate Optimization based on Cutting Force Calculations in 3-Axis Milling of Dies and Molds with Sculptured Surfaces, International Journal of Machine Tools and Manufacturing, Vol. 34 Weinert, K. et al., 2000, Increasing the HSC-milling Process Reliability by Simulation-based Feed Rate Adaptation, Department of Machining Technology, Univ. of Dortmund (in print). 35 Weinert, K., Stautner, M., 2001, An Efficient Discrete Simulation, Department of Machining Technology, Univ. of Dortmund (in print). 36 Tonshoff, H.K., Trampler, J., 1997, Optimizing NC Programs for 5-Axis Milling by Combining Process Models, presented at the International Conference and Exhibition on Design and Production of Dies and Molds, Istanbul, May 20-22. 37 Lauwers, B. et al., 2000, Efficient NC-Programming of Multi-Axes Milling Machines through the Integration of
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