dies and moulds

8/12/2019 dies and moulds

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Manufactur ing of D ies and Mold sTaylan Altan ( I ) , Blaine Li ll g, Y.C. Yen

Engineering Research Center for Net Shape ManufacturingDepartment 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.

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AbstractThe design and manufacturing of dies and molds represent a significant link in the entire production chainbecause nearly all mass produced discrete parts are formed using production processes that employ dies andmolds. Thus, the quality, cost and lead times of dies and molds affect the economics of producing a verylarge 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 andprocess design including process modeling, rapid prototyping, rapid tooling, optimized tool path generation forhigh speed cutting and hard machining, machinery and cutting tools, surface coating and repair as well as inEDM and ECM. This paper, prepared with input from many ClRP colleagues, attempts to review thesignificant advances and practical applications in this field.Keywords: Die, Mold, Manufacturing.

0 INTRODUCTIONThe authors would like to thank all of the colleagues whoresponded to the request for information in preparing thisreview paper, namely to Prof. Klocke -WZL Aachen, Prof.Tonshoff - IFW Hannover, Prof. Wertheim - Iscar, Ltd.(Israel), Profs. Kruth and Lauwers - Catholic UniversityLeuven, Prof. Rasch - NTNU Trondheim, Prof. Geiger andDr. Engel - LFT Erlangen, Prof. Weinert - ISF Dortmund,Dr. Leopold - GFE Chemnitz, Mr. Reznick - Extrude HoneCorp., Prof. Gunasekera - Ohio University, Prof. Bramley- University of Bath, Prof. Bueno - Fundacion Tekniker,Prof. Neugebauer and Dr. Lang - Fraunhofer IWUChemnitz. Thanks are also due to our co-workers and theERCINSM, as well as the co-workers of Prof. Klocke atWZL, and of Prof. Tonshoff at IFW, who assisted us incollecting the references and in the preparation of figures.Furthermore, we appreciate the response that we receivedfrom many of the participants of the 2001 Mold MakingConference.

1 BACKGROUNDProduction of industrial goods requires manufacturing ofdiscrete parts that are sub-assembled and assembled to aproduct ready for the customer. The manufacturing ofnearly all mass produced discrete parts require dies andmolds that are used in production processes such asforging, stamping, casting, and injection molding. Thus,the design and manufacturing of dies and molds representa very crucial aspect of the entire production chain. Thiscan be illustrated by the following observations:

Dies and molds, similar to machine tools, mayrepresent a small investment compared to the overallvalue of an entire production program. However, theyare crucial, as are machine tools, in determining leadtimes, quality and costs of discrete parts.Manufacturing and try-out of new dies and molds maybe critical in determining the feasibility and lead-time ofan entire production program. For example, inmanufacturing 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-outand robust process development using productionequipment. Considering that the OEMs require sampleparts, produced on production equipment (notprototypes), 6 to 9 months prior to start of production(SOP) of a new car model, the significance of moldmaking becomes obvious.The quality of the dies and molds directly affect thequality of the produced parts. Excellent examples aremolds used for injection molding lenses, or dies usedfor precision forging of automotive drive traincomponents.

1.1 Significance of the TechnologyThe observations listed above illustrate that die and moldmaking has a key position in manufacturing components invirtually all industries but especially in transportation,consumer electronics and consumer goods industries. Theeffectiveness of die making affects the entiremanufacturing cycle so that this technology must beconsidered to be a very essential link in the totalproduction chainDie and mold making covers a broad range of activities,including: a) manufacturing of new dies and fixtures, b)maintenance and modifications, and c) technicalassistance and prototype manufacturing for the customer,Figure 1 [ I ] . Process development and die try-out as wellas die maintenance are especially important because theytie up expensive production equipment and affect leadtimes. These activities must be scheduled and completedwithin very rigid deadlines. Such requirements makescheduling in a die shop an extremely challenging task.The automotive industry constantly tries to reduce thedevelopment time for new models which puts enormouspressure on die makers and requires new productionsystems [2].1.2 Variety of Dies and MoldsThe 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


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example, die-casting dies have more deep and thin ribcavities that cannot be easily machined than injectionmolding molds. As a result five times more plunge EDMmachines are used in the die casting industry than in theinjection molding industry. Another example is theextensive, nearly 50 % , use of wire EDM machines formaking blanking dies while only 5 % of these machinesare used to make extrusion dies.As seen in Figure 2, large deep drawing and stampingdies are made by machining cast iron or steel structureswhile dies for forging, die casting and plastics molding aremade from tool steel blocks, involving considerable roughmachining operations. Injection molding molds and diecasting dies allow the production of rather complex partswith undercuts andlor hollow geometries. Thus, thesetools usually have multiple motion slides and punches aswell as cooling channels that complicate themanufacturing process. Dies and molds are composed offunctional (cavity, core insert, punch) and supportcomponents (guide pins, holder, die plate). Often supportcomponents and a num ber of holes need only 2D or 2% Dmachining, but may require 50 to 60% of the totalmanufacturing time. This fact is often neglected but mustbe considered in effective planning of the machiningoperations.While metal cutting and EDM are the ma jor methods usedfor die and mold making, hobbing, micro machining andchemical etching methods are also used for manufacturingmolds for various applications.

Figure 1 Position of die and mold machining in productl ife cycle [I ] .

1.3 Economics of DielMold MakingAccording to a recent survey [4], major issues that face dieand mold makers are similar in all industrialized countries,namely:1. Declining prices and profit margins so that there is astrong 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 processdevelopment)

4. Lack and cost of skilled labor, which leads to the needto provide extensive training to employees and toutilize new technologies.5. Globalization that leads to increased foreigncompetition, especially from developing countrieswhere skill levels are increasing while salaries arecomparatively low.Priorities differ according to countries surveyed; forexample while North American and German mold makersare mainly concerned with foreign competition, Japanesecompanies concentrate on developing new markets. In allcountries, however, the acceptance of new technologiesis recognized to be one essential component that can leadto innovation and integration that are essential for growth[5]. New technologies are understood to include not onlymanufacturing techniques (high speed milling, hardmachining, automation, process modeling, etc.) but alsopre- and post-manufacturing, e.g. cost estimating andcontrol, documentation, training and operationsmanagement. Thus, two essential components forachieving a competitive position in dielmold industry are:a) capabilities of personnel, and b) utilization of optimizedand innovative production techniques 161.

Figure 2: Workpiece characteristics in die and moldmaking [3].Successful dielmold makers recommend that, for afinancially successful die making operation, it is necessaryto:1, Establish quantitative methods for cost estimating. Inthis industry cost estimates are often based on the

past experience and feel of the die maker andcomparison with similar dies. As a result the accuracy


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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 forworking hours, contracts, cost accounting).Establish a contractual basis so that items non-specified in the contract are only provided at extracharge [8]. In order to maintain deadlines, focus oncontract initialization and not on assembly of the mold,where considerable manpower is involved and it isdifficult to change a schedule.Provide services to the customer mainly in datamanagement at the start and during production butalso during process development with complex moldsthat may require considerable try-out time.

For successful die makers quality is a given. The time forwork in progress, or storage time, can be a significantfactor in low volume production, such as dielmold making.In this application it is estimated that 70% of the totalproduction time consists of storage time when no value isadded to the product. This situation can only be improvedby increasing machining capacity, machine utilization rate,orland improving the efficiency of the part handlingoperations [9]. To reduce the time for work in progress,many high technology die shops have separated tool pathgeneration from engineering and design of the dies. Whilethe latter is done in the engineering department, tool pathgeneration is done on the shop floor by the machineoperator.

2In manufacturing discrete parts using dies or molds, thepart design must be compatible with the process in orderto assure the production of high quality parts at low costwith short lead times. Thus, part and process designs arebest considered simultaneously, which is often not thecase in practice. This objective can only be achievedthrough good communication between the product andtool designer, who may be in different companies (OEMand supplier) andlor locations.

PART, DIE AND PR OCESS DESIGN

Or ig i na lEqu ipm ent F i s t -Ti e rManufac turers Supp l i e rs Subt i e rSupp l i e rsCADKEY

C A D D SCATIA I-DEAS

U n i g r a p h i c s l n t e g r a p hC A D D SI-DEAS CATIA

Pro lENGI N EERUnigra ph i cs

Figure 3: Proliferation of CAD systems inchain [lo].

ARIESA p p l i c o nANVILA u t o C A DPro lENGlNEERI-DEASPDGSHPl n t e g r a p hEUCLIDCATIAsupply

The use of different CAD systems by OEMs and suppliersfurther complicates communication within the supplychain. Figure 3, taken from [ lo ], shows the proliferation ofCAD systems in the top three tiers of the North Americanautomobile industry. Because die and mold making firms

tend to be third or fourth-tier suppliers, the interoperabilityproblem of reliably transferring CAD data between firms isparticularly acute in this industry.It is well known that the design actually represents only asmall portion, 5 to 15%, of the total production cost of apart. However, decisions made at the design stage have aprofound effect upon manufacturing and life cycle costs ofa product. In addition to satisfying the functionalrequirements, the part design must consider: a) theselected manufacturing process and its limitations, b)equipment and tooling requirements, c) processcapabilities such as size, geometry, tolerances, andproduction rate, and d) properties of the incoming materialunder processing conditions. Often the design requiresdevelopment of a new tooling andlor modification of anexisting process. In such cases dielmold development andtry-out can take as long or even longer that the timeneeded for die manufacturing.The assembly ready part geometry, usually in electronicform, must be used to develop the die or mold geometriesas well as to select the process parameters. Figure 4illustrates, using forging as an example, the flow ofinformation and activities in computer aided die andprocess design [ I l l . Processes such as stamping, hotand cold forging may require several operations startingwith the initial simple billet or sheet blank until the finishformed part is obtained. Thus, several die sets may beneeded. In processes, where the incoming material isshapeless, e.g. powder compaction, injection molding, diecasting, 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 processmodeling techniques to:1. Estimate material flow and die stresses.2. Establish optimum process parameters (machine and

ram speed, dielmold and material temperatures, timefor holding under pressure, etc.).

3. Design dielmold features, necessary to perform theprocess (flash and draft in forging, binder surface anddraw beads in stamping, gates and runners in injectionmolding and die casting).

4 . Finalize the product and die dimensions by predictingand eliminating defects while adjusting the processparameters for obtaining a robust process.

The application of process modeling, using 3D-FEM basedsoftware, is now considered routine in (i) permanent moldand die casting, (ii) injection, gas injection, compressionand blow molding, and (iii) sheet metal forming. In forging,while 2D simulation is widely practiced, 3D applicationsare being introduced by advanced technology companies.Research is being conducted on a reverse simulationapproach for designing forging preforms [12]. Examplesof FEM simulation results are seen in Figure 5 for forgingand Figure 6 for stamping [ I l l . Application of 2D-FEMsimulation in metal cutting is now being introduced bymany companies but it is still in the RBD stage. Mostprobably this application will be widely accepted during thenext two to four years and also expanded to full 3Dsimulations of the metal cutting operations. Before theycan be applied in the industrial environment, processsimulation must be further developed for (a) forming ofcomposite polymers (in mold lamination, compressionmolding of glass fiber reinforced polymeric composites)and b) sandwich sheet metal materials. Characterizationof composite materials and formulation of the deformationlaws represent considerable technical challenges and arestill in the development stage.


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3 PROTOTYPING AND RAPID TOOLING

3.1 Additive Manufacturing and Rapid Prototypingfor Die and Mold Production

The class of additive fabrication methods usually knownas rapid prototyping (RP) or solid freeform fabrication(SFF) processes have evolved considerably over the pastdecade. Although they were originally marketed as aids todesign visualization and prototyping, in recent years themost promising application of these technologies has beenin the area of rapid tooling for net shape processes. Anexcellent review was given as a ClRP keynote paper atthe 48th General Assembly [13].

All of the processes currently in use follow the same basicsequence of steps to construct a component. The processbegins with a CAD solid model of either a piece part ortool insert, which is typically transferred to the RP machinein STL format. This data structure reduces the solid modelto a set of triangular facets that define the surfaces of thepart. This STL file is then sliced by the machinecontroller software, turning what was originally a three-dimensional object into an ordered set of two-dimensionallayers. 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-freepart [ I ].

The RP processes differ in the particular method used toform the build material, as well as in the build materialitself. To date, the most common build materials are eithera liquid (stereolithography), an extruded solid (fused

Figure 4: Flow chart for product, die and process design deposition modeling) or a powder (selective lasersintering). The techniques used to shape the raw materialtypically use laser-activated chemical change(stereolithography), laser sintering (LENS, SLS), extrusion(FDM), or an adhesive binder (3D printing).3.2 Design and Visualization Too ls: New

DevelopmentsA major thrust in the RP market has been thedevelopment of low-cost 3D printers, which aredesigned for office use, and are intended solely asvisualization aids to part designers. All of these processesare intended as low-cost prototyping methods forproducing relatively fragile parts, allowing part designersto produce several iterations of a design quickly, and atlow cost. None of these processes are presently capableof producing parts able to withstand significant stresses.3.3 Rapid Production of ToolingRapid production of dies and molds using addit iveprocesses can reduce the time and cost of bringing newproducts to market by drastically cutting down on designiterations and prototyping cycles. The additive processes

(Example: Forging) [ I ] .

Figure 5: Simulation of forging an automotive crankshaftusing a 3D commercial FEM code [ I ] .


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have other advantages as well, such as the ability to buildconformal cooling lines around a mold cavity, and theability of some systems to tailor the material properties ofthe part as it is built.The concept of rapid tooling includes three distinctsegments: prototype tooling, bridge tooling, and toolingfor limited production runs. Prototype tooling is exactlywhat the name implies: a die or mold designed to test anew component design, a new material, or perhaps a newprocess. In this case the tool itself is not intended toproduce more than a few hundred parts, so tool life, cycletime, and part ejection are typically not design issues.Much slower cycle times and manual ejection are oftenemployed to simplify the tool design and save valuabletime and producing the prototype tool. Because the cost ofprototype tooling can be folded into the total tooling costand amortized over the entire product life of the finalproduct, cost is not a primary concern. On the other hand,product development constraints demand that the time toproduce the tool must be very short, typically only a fewdays or weeks.Bridge tooling is the name applied to dies and molds thatare designed to last for perhaps tens of thousands ofproduct cycles. These tools permit a new product to comeonto the market early, while the production tooling is stillbeing fabricated. While these tools do not require thedurability of production tools, and may not be optimized interms of the process parameters, they must be able towithstand several thousand cycles, while holdingproduction-level tolerances. Again, the cost of these toolscan be folded into the total tooling cost for the entireproduction run.Finally, the most demanding application is for tools forshort production runs. With the advent of leanmanufacturing and mass customization, the need toproduce tools that can produce quality parts in smallquantities, and do so cost effectively, has become a majorissue in many industries. Often it is unclear whether is itbetter to build a single die or mold to produce a limitednumber of parts, spread over several years, or is it betterto build cheaper, less durable tool, discard it at the end ofeach production run, and build a new tool when anotherproduction run is planned. This second point of view holdsthat the die is in the data: as methods for turning CADmodels into functioning tools become more sophisticatedand less time consuming, there is little to be gained frommaking a very expensive tool to produce a limited numberof parts. Better to discard a cheaper tool and buildanother, every time a new production run is needed. Asthis idea becomes more widely accepted, industries thatbuild components in small lots, such as the military andaerospace, may be more inclined to adopt net shapeprocesses.Rosochowski and Matuszak [I51 have proposed aclassification of rapid tooling processes, shown in Figure7, based on practical uses for the tooling, rather than onthe particular process used. They divide rapid toolingprocesses into three major groups: those that are used toproduce patterns for casting, those used to producepatterns for both soft and hard tooling (indirect tooling),and those that produce production-ready tools directly byRP methods (direct tooling). A good overview of currentindustrial uses for several of these techniques is given in

Patterns for Cast ingAlthough producing patterns for casting is not generallyregarded as rapid tooling, in fact, several RP processeshave been applied to pattern making for sand casting andinvestment casting, including FDM (Fused Deposition

[ I61.

Silicone ~mouldsEPOSY ~moulds

Modeling), SLS (Selective Laser Sintering), and LOM(Layered Object Manufacturing). Patterns are typicallymade from wax, but complex patterns sintered frompolycarbonate by the SLS process have also been usedRecently, a new method has been developed forproducing large (1.5~0.75~0.753) sand molds and coresquickly. This technology uses an ink jet technology tospray binder onto layers of sand, followed by a reactantwhich is laid down selectively, resulting in a precise,strong, sand mold or core. This process is reportedly tentimes faster than Selective Laser Sintering, and can alsocreate wax models for investment casting as well [17].RP methods have also been used to form molds thatproduce foam patterns for the lost-foam casting process.A rather complex technique is used in the automotiveindustry to produce molds for polystyrene foam patternsby using LOM, RTV silicone, and a high temperaturealuminum filled epoxy. The combination of LOM and RTVsilicone produces a model that gives excellent surfacedetail of a complex part [18].

[I 51.

~ Spray metal ~ Metaltooling powder~ Electroform- ~ Ceramic

ed ooling powder

Rapid tooling

~ Cast metaltooling~ Keltooltooling

~ Microcasttools~ Laminatedtools

Figure 7: Classification scheme for rapid tooling [I51

lnd irect MethodsIn injection molding the RP process is most often used toproduce a model of the mold insert, with runner geometryattached. This mold insert is filled with silicone, which isallowed to cure. The silicone positive is then used to castan epoxy tool, which can typically withstand severalhund red injection-mo d ng cycles.Because epoxy molds are limited in the number of partsthey can deliver, there is much interest in industry infinding other techniques to cast tools around mastersderived from RP methods. A method was developed by acar manufacturer to spray molten tool steel onto ceramicmolds, which are produced from RP models. To date thismethod is used to produce relatively small (600 mm x 600mm) tool sets. However, work is in progress to scale upthe technique to produce sheet metal dies for auto bodypanels [17].Another related method is called Rapid Solidification. Itinvolves spraying a molten metal, in this case H I 3 toolsteel, against a ceramic master. The developers of themethod claim that the resulting tool steel shell showssuperior strength, hardness, and surface finish. The firsttool produced by this process is due to go online in July of2001 [19].


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Several other techniques use sc-called indirect methodsto produce tool sets in metals. These methods havealmost all been applied to injection molds, due to therelatively benign production environment. Ainsley andGong report on a method to slip cast stainless steel moldsusing RP masters of the mold [20]. Weaver et al. report ona method to produce the model of the tool set. Silicone isused as the intermediate material, and slurry of metalpowder and polymer is then cast around the silicone. Aftercuring, the tool is debound and sintered, which produces atool set with properties approaching hardened tool steel[21]. A similar process has been in use for several years. Ituses one of three proprietary metal composites, which iscast around a silicone master. Details on this techniqueare available in [22].Direct Method sThese processes all use RP technologies to make a die ormold directly from the CAD model, without using additionalpattern transfer techniques. The most common method fordirect tooling involves a green part that is created eitherby selective laser sintering (SLS) or three-dimensionalprinting (3DP) methods. SLS [23] deposits a layer ofpolymer coated metal powder, which is then selectivelysintered using a laser. With the 3D Printing process alayer of powder is first deposited, and then an adhesivebinder selectively applied. The major advantage of thepowder-based systems is that the powder that is not usedto form the part provides support for overhangingstructures. In this way, conformal cooling channels andundercuts can be created without need of additionalsupport material. An excellent overview of recent work invarious direct tooling methods is given in [24].In an overview of recent developments in direct toolfabrication using the SLS process, Klocke notes thatprecision on the order of hundredths of a millimeter is nowpossible, with attainable surface finishes (after some post-processing) of R= 15 pm or better. Rather complex partscan be made with these tools [25].Considerable work has gone into developing SelectiveLaser Sintering (SLS) as a process for building dies andmolds directly from the CAD model. Details are availablefor sheet forming dies [26], DTM rapid steel process [27]and the rapid mold process [28]. In addition RBD is inprogress for further improving the SLS process itself,specifically on powder deposition [29] and on the effects oflaser power and traverse speed upon microstructure andporosity of deposited surface [30].An overall practical review of rapid prototyping and rapidtooling is given in a recent publication [14].

4 CAVITY AND PUNCHlCORE MACHININGThe steps involved in manufacturing a typical mold forinjection molding is seen in Figure 8 [31]. The costcomponents of an example injection molded part are givenin Figure 9. This example illustrates that considerable costreduction potential exists in rough and finish machining ofdies and molds.4.1 Too l Path Generation and Optimi zationToday nearly all die and mold makers use High SpeedCutting or Machining (HSC or HSM) in cavity and punchmanufacturing. HSM requires not only specific machinetools (rigid, high spindle RPM, high feed rate, softwarewith look ahead capabilities, high acceleration anddeceleration) and cutting tools (ultra fine carbide withvarious and multiple coatings, optimized tool edgegeometry, high performance cutting tool materials, i.e.PCBN and ceramics) but also special CNC tool pathprogramming strategies. This is now being recognized not

only by research institutions but also by various CAMsystem vendors......

Design (plastic part geometry) by OEM or InjectionMolderProcess Simulation (mold design) by InjectionMolder or Mold MakerFirst Rough Machining of the Mold Steel Block, bySteel SupplierRough Machining of Cavity (milling, drilling) by MoldMakerSemi-Finish and Finish Machining (milling, EDM) byMold MakerPolishing and Assembly, by Mold MakerMold Try-out and Finish, by Mold MakerPre-Production Qualification, by Mold Maker,Injection Molder and OEM

Figure 8: Operations involved in making a typicalmold [31].

Figure 9: Cost Components (in %) in Manufacturing of anExample Automotive Part by Injection Molding(assuming 250,000 parts were produced in onesteel mold) [31].

Constant Chip Load and Cutt ing SpeedEarly studies on this topic concentrated on theoptimization of the tool paths for 3D milling of sculpturedsurfaces with ball end mills. The basic approach was tomaintain the cutting speed and the chip loadapproximately constant by controlling the spindle speedand the feed rate. Computer codes were developed,based on this principle and allowed the reduction of millingtime 20 to 30%, depending upon die geometry whileincreasing cutter tool life [32, 331. Recently, processsimulation and feed rate adoption to maintain constantchip load have been developed and applied to 5-axis CNCmachining of sculptured surfaces using torus as well asball end mill cutters [34, 351. Especially in rough millingoperations feed rate adoption helps to avoid unacceptablyexcessive tool deflection and deviation from the theoreticaldesign surface. Simulation of five-axis milling allows theoperator to examine the influence of different millingstrategies and to improve the process reliability whilereducing the machining time.The optimization of NC programs for five-axis milling ofdies and molds has also been suggested by [36]. In thisapproach multiple process models are used fortechnological optimization of NC programs. A softwaremodule has been developed which extends thefunctionality of a commercial CAD/CAM system. NC toolpath generation for five-axis machining has also been


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optimized by a team of researchers [37]. This extendedCAM system for multi-axis milling integrates tool pathgeneration, axes transformation and NC simulation. Thesystem performs an immediate verification of eachgenerated cutter location and in case a collision occurs(e.g. between machine and part), it takes the appropriateaction by applying a collision avoidance algorithm. Thus,the system facilitates the use of five-axis machining andallows the variation of the tool inclination during tool pathgeneration in order to achieve the best combination ofscallop height, workpiece accuracy, surface roughnessand machining cost [38].The effect of cutting speed and lead angle on tool wear inball end milling has been also investigated in a recentstudy [39] where the strategies for optimizing the CNCprograms are also reviewed. It is shown that by applyingappropriate machining strategies in hard milling, it ispossible to achieve cost savings of up to 30 percent.Some of the cutter path optimization strategies are nowbeing implemented into controllers of HSC machine tools[40]. In milling with a ball nose end mill, a constant spindlespeed can produce a variety of surface speeds dependingupon the contact point of the tool on the workpiece. Theeffective tool radius (RI or Rz) changes with the angle ofcontact, (affecting the cutting speed at constant RPM), asseen in Figure 10. A feature built in the controller of a CNCmachine can calculate the contact radius of the ball basedon the angle of the tool path and overrides the spindleRPM to maintain a constant cutting speed during 3Dmilling. Thus, the spindle speed is a servo-controlledparameter, just like the X Y, and motions. In addition,the feed rate is also modified to maintain a constant chipload, similar to the R D studies conducted earlier invarious laboratories. This controller feature allows theprogram to specify a desired cutting speed and let thecontrol work this surface speed target by continuouslycalculating the position of the tool.

R2= Rxcosl5

UR2Figure 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 thelatest stage of development and will allow a machiningcenter to mill out a sharp corner that can only be

produced today by EDM. As seen in Figure 11, as thespindle rotates, the position, i.e. the axis location, of thespindle is continuously changed in coordination withmachine motion in X and Y. A special triangle-shaped toolis necessary for performing this cutting operation.

Start -II - - Endi

L S h a r p C or ner To olFigure 11 Schematic illustration for the rotating sequence

of a Corner Cutting cutter (a special triangulartool is used and the position of the centerline iscontinuously changed during tool rotation) [40].

Tool ath StrategiesIn a recent study, dies for forging and stamping and moldsfor injection molding have been manufactured using HSMwith the application of a circular strategy in pocketmachining, as seen in Figure 12 [41]. Here, the tool ismoved alternately from one side (AB) of the machined slotto the other side (CD) as indicated with the path of thespindle axis. Thus, the tool contact time with theworkpiece is reduced and the tool is allowed to cool.Carbide tools with TiAlN coating, cylindrical bull end forrough cutting (6, 8 and 10 mm diameter I 4 eeth) and ballend for finishing (2, 4, and 10 mm I 2 and 4 teeth), wereused to machine cast iron (GG 25) and three different toolsteels (38 to 48 HRC) using a high speed milling machine(24,000 RPM spindle). The results of this study agreeswith the well known machining strategy in HSM where theheat generated by the cutting process is discarded withthe chip and the tool is cooled because of limitedengagement with the workpiece during a single rotation.

Figure 12: Tool path trajectory generated for the circularstrategy [41].


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inserts. Tests were aborted after A = 1.6 m2 of surfacearea was machined and tool wear on PCBN 2 wasmeasured at VB = 60 pm and on PCBN 0 at VB = 85pm. Abrasion and thermal fatigue were identified as themain wear mechanisms. Higher CBN content and higherhardness exhibited favorable wear resistance.4.4 Machining o f Dies and Molds from a Too l SteelBlockIn general molds for injection molding, blow molding andcompression molding, as well as dies for die casting,forging and tube hydroforming, are manufactured from ablock of tool steel (in exceptional cases molds are alsomade from aluminum or copper alloys). In theseapplications a large amount of material must be removedby rough machining. Consequently, machinability of toolsteel, together with many different aspects of hard andHSC milling, is an important issue, especially for roughmachining. In a recent study, the machinability of threemold steels with different sulfur contents but of the samehardness (HB 300) was investigated by varying the cuttingconditions [31]. It was found that a) the main parametersaffecting the milling process (using round carbide insert ina 40 mm torus tool) were cutting velocity, feed per toothand radial and axial depths of cut, b) the addition of sulfurto the mold steel brings an increase of about 50% in toollife, and c) when machining at high feed rates an increaseof 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 II No. Cutting Edges I z= 1 II Tool Geometry y= -go,a 16" II Tilt Angle pm= 30 II Lead Angle B r 0 I

Fundamentals of high speed and hard machining are stillpartially understood. Research is being conducted invarious laboratories to understand the basic physicalphenomena that determines chip formation, the effect ofcutting speed and temperatures. In machining hardenedsteels, with increasing cutting speed the chip formationchanges from continuous to serrated chip form. Thus, thecutting forces may be reduced and tool life may beincreased provided the temperatures at the tool edge canbe maintained within allowable limits [45]. Evaluation ofthe cutting power in function of cutting speed (100 to 3000mlmin) showed that there seems to be a material specificcutting speed where the cutting power reaches itsmaximum value. Beyond this value, with increasing speed

the cutting force remains approximately constant and thepower increases linearly with the cutting speed [46].

Figure 16: Summary of all tool life experiments in pearliticcast iron (symbols explained in Figure 15) [44].

An excellent example of HSC milling of hardened dies isdiscussed in a recent study [47]. The investigation wasfocused on dies for forging turbine and compressor blades(die material 1.2343 heat treated to 53 HRC with anultimate strength of 1600 Nlmm') that had surfacedimensions of 185 x 125 mm and 600 x 330 mm. Theresults of this investigation illustrates many aspects ofHSC milling faced in industrial practice, namely:

Die and mold makers are provided with a large amountof information on test results on tool life. However, thisinformation is difficult to apply to the specific conditionswithout 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 toprogram documentation this includes milling strategies,milling machine and operation parameters and testresults) are essential.It is important to invest more engineering time(calculations, programming) at the start ofmanufacturing a new die to determine the "best"machining conditions. Thus, it is possible not to remainconservative and use low feed rates resulting in longmachining times.In rough machining it is better to use relatively smallertool diameters and increased feed rates. Thus, moreuniform machining allowance is left, which facilitatesfinish milling. As a result the time necessary forroughing may increase but the time needed forfinishing will decrease so that the total milling time maybe reduced.Rough machining pre-hardened die blocks requiresmore time than machining in the soft state. However,when rough and finish machining in hardened state, anew tool set up is eliminated.While the decision for hard machining has to be madefor each application individually, in the present caseHSC milling in hardened state could be doneeconomically 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 advancedtechnology forging shops in North America and in Japan.


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To increase wear resistance and die life, forging dies areoften surface welded with high temperature alloys. In thepast, the finishing of such surface welded dies, new orafter repair, was done by EDM. Recently, specificallydesigned cutters with PCBN inserts were used for highspeed machining of such welded surfaces [48, 91.Studies with different cutter materials demonstrated thatspecific PCBN grades (with appropriate tool edgepreparation and machining strategies) offer long life andcost effective machining of hard welded surfaces.The machining of injection molding molds and die castingdies present particular challenges because theseapplications have many thin and deep cavities to bemachined. In the past the strategy in manufacturing suchmolds consisted in rough machining, hardening andfinishing by EDM. To save set-up time and decrease thetotal manufacturing time it is desired to rough and finishmachine in one set-up and in hardened state. Thus, it isnecessary to develop techniques for milling deep cavitiesusing long solid carbide milling cutters and withappropriate cutting conditions. A study, conducted formachining hardened dies (1400 o 1500 Nlmm strength)for casting, illustrated many of the difficulties and remediesrelated to milling with long and thin cutters. A softwaremodule was developed for estimating the cutter conditionsfor selected cutter geometries [49]. his study and othersindicate that, with appropriate process selection ofparameters, EDM can be replaced in many applicationsWI

Figure 17:Development and reduction of machining timesfor one forging die [46].

4.5 Machine Too ls and Cutting Too lsMachine ToolsIt is well known that the milling machines for high speedand hard machining must be stiff and have highacceleration and deceleration capabilities. This isespecially 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 anddecelerate to machine the specified contour. Forexample, the response of a typical high-speed millingmachine (Makino A S ) , used in die and moldmanufacturing, was measured when machining a specificsculptured surface. At a programmed feed rate of 20rnfmin the machine needed almost 70 mm to reach thedesired velocity. At moderate feed rates, say at 5 mlmin, itneeded about 4 mm to reach the target feed rate [50].While accelerating and decelerating the chip load can notbe maintained and is reduced since, in most machines,

the spindle continues to rotate at the same speed whichmay lead to rubbing action on the tool, increasing toolwear. In machining a selected sculptured surface, it wasshown that increasing the nominal feed rate might notresult in a proportional reduction in machining time, sincethe actual feed rate is determined by machine dynamics.This is illustrated in Figure 18,where the doubling of thenominal feed rate has different effects on machining time,depending on the range considered. Of course, this resultis valid only for the specific machined geometry butillustrates the importance of machine inertia on totalmachining time. Similar studies indicate the importance ofmachine inertia and die geometry in reducing machiningtimes [45].

Figure 18: Actual and calculated (using the softwareOPTIMILL) machining times for a selectedsculptured surface [49].

The use of linear motor drives in large milling machines,used for manufacturing automotive stamping dies, isalready well known. The trend appears to be in increasedapplication of linear drives in milling machines. Machinesthat provide the tool motion using parallel kinematics andHexapod concepts are being evaluated in variouslaboratories. Even though these machines appear to besuitable for die and mold manufacturing, there is noproved industrial application of such machines in industrialdie manufacturing practice.Recently an innovative machine tool concept has beendeveloped for quick repair of large stamping dies [39, 11.This machining unit can be brought to the press withouttaking the dies out for repair, thus providing on-sitemachining capability. To provide multi-axis capability witha rigid frame, this machine is built with a hybrid-paralleland serial-kinematical structure, Figure 19. The prototype


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of this machine has five-axis capability and is being a depth of cut a and step over distance a,, is usuallyevaluated under practical die repair conditions, Figure 20. considerably lower than the selected cutting velocity,

based on spindle RPM and cutter diameter alone. In hardmilling, the dimensions of the tool edge radius (about 30pm for roughing and 10 pm for finishing) and that of thechip thickness (about 25 to 50 pm for roughing and 10 to12 pm for finishing) are very small. Therefore, it isnecessary to estimate and work with actual cuttingspeeds, which are estimated to be about 1.8 times thenominal cutting speed, in order to determine a reliableprocess window, Figure 21. Thus, based on empirical datathe finish slot machining of a hardened tool steel, typicalcutting speed vc= 300 rdmin, can be increased up to vc=500 rdmin. In another example, by increasing the chipload from an estimated h= 1 pm and by doubling thecutting speed, it was possible to quadruple the productivity~ 2 1 .

w f a c t o r1 81 6

1 4

1 2

10 8

0 0 2 0 4 0 6 0 8 - 1 IId

Figure 19: Concept of the transportable machining unit [39]

Figure 21 Geometric conditions and the ratio of numericalversus cutting speed, v,-Factor, in function ofste p-over distance [52].

Figure 20: Transportable milling machine used for dierepair [51]

Tool eometryIn addition to the characteristics of the machine tool, thefactor that affects the success of high-speed milling is theselection of tool material, coating and geometry, inaccordance with cutting conditions.For example, the tool edge radius for hard machining is inthe range of 5 to 30 pm. In rough machining a forging diewith 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]. Ingradually increasing the feed rate and the feed per tooth to0.12 mm and the chip load to 8.2 pm, it was possible toimprove productivity approximately 40%. This wasachieved because the effective cutting velocity, based on

The application of high speed hard milling for finishingrequires that roughing must a) be conducted with a largemetal removal rate and b) results in a machined surfacethat is very near the finish geometry and leaves arelatively uniform machining allowance for finishing (0.05to 0.5 mm). Thus, for effective application of HSMalternative insert geometries are considered. Tools withround or octagonal inserts give, with comparable depths ofcut, significantly better sculptured surface contours thancylindrical end mills that are in turn most effective forpocketing and slotting [53]. The round inserts offermaximum strength and represent an excellent solution fordifferent milling conditions [54]. With these inserts the chipthickness varies with the depth of cut and feed rate mustbe increased to achieve appropriate chip thickness andincreased metal removal rate.Die and mold machining requires a wide selection ofcutting tools and inserts for facing of large and smallareas, machining deep walls and shoulders, as well asmachining of long, both deep and narrow slots [55]. Thus,


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tools are necessary for plunging or ramp down operations, carbide with 8 to 12% cobalt and TiAlN coating is bestmilling, as well as drilling including deep drilling, reaming, suited. It is also interesting to note that the qualitytapping and chamfering. differences between tools offered by various suppliers hasThe main geometries for face-, shoulder-, and slot milling been reduced [9].include flat positive inserts, inserts with ground andmolded chip formers and helical, non-flat inserts withmodified rake and clearance faces, which provideimproved performance. Tool producers offer end mills withinterchangeable heads for various milling and profilingoperations. The helical cutting edge concept, developedrecently, is used with solid cutters as well as with inserts,Figure 22. This geometry and the helical curve cuttingedge as well as curved rake and clearance faces result inconstant rake and clearances on the tool during milling. Inaddition, each cutting edge penetrates gradually into theworkpiece with a gradual increase in the cutting force.Square, multi purpose helical inserts are available for 90degrees shouldering, facing and slotting [55].

Figure 23: Groove type chipformer for improved toollife [55].

Figure 22: The helical cutting edge concept [55].

Octagon inserts, Figure 23, offer cutting edges for moreconomical facing, shouldering, slotting, recessing anchamfering applications. Some of these inserts aravailable with a series of depressions to reduce thecontact area between the chip and the insert rake face toa) reduce heat flow to the insert, b) reduce friction, and c)improve tool life. Figure 23 illustrates schematically theheat flow into and from the tool. Heat generated by frictionand deformation flows from the curled chip into the tooland is illustrated by horizontal arrows. The depressions inthe tool reduce the amount of toollworkpiece contact andthe heat flow into the tool. Furthermore, as indicatedschematically by inclined arrows, convective heat lossfrom the tool is also increased. It is claimed that, thepositive rake angle, in combination with these depressionsalso reduces cutting forces and improves chip flow. Asseen in Figure 24, octagon and round inserts can be usedfor a variety of milling applications.Normally up to about 8 mm diameters, ball nose end millsare made of submicron substrate for improved toughnessand PVD coating (TiCN, TiAIN) for hardness and highwear resistance. For larger diameter end mills, inserts andincreasingly used. For high speed milling at higher cuttingspeeds, normally solid endmills are recommended. Insome cases, however, screw clamped or blade typeinserts can also be used.Tool Materials and CoatingsFor tools ranging from 12 mm to 35 mm diameter, carbideinsert tools are shown to be effective. TiCN coatings aresufficient for machining die steels less than 42 HRC whileTiAlN coatings are used for 42 HRC and over. For toolswith a diameter of 12 mm and under, sub-micron solid

Figure 24: The multifunction milling option with octagonaland round inserts [55].

Similar results were obtained in other investigations. It wasfound that in using TiAlN coating, tool life increases andthen decreases with increasing TilAl ratio. The bestcombination 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 thanTiCN in machining 57 HRC die steel under similar highspeed milling conditions, Figure 25 [52].The application of CBN in hard turning is well known. Theprocess has the potential to reduce manufacturing costsby eliminating or reducing grinding and by eliminating theuse of lubricants. To achieve the potential of PCBN, allaspects of hard turning must be considered, includingmachine tool, work holding, tool compensation, insertmaterial grade and edge quality, and stability [56]. Theseissues as well as achievable surface integrity and formaccuracy are reviewed in a recent publication [57].Tool HoldersIn high speed milling with high spindle RPM, tool holderbalance is very critical to avoid premature tool failure andto obtain good surface finish. Run out is recommended tobe less than 5 pm. Experience indicates that for each 10pm runout, in general, tool life is reduced about 50percent. Thus, shrink fit holders are the best and easy tobalance while hydraulic chucks are acceptable atmoderate RPM's [58]. In addition, it is necessary that


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inserts and tool holders have better dimensionaltolerances. The length of the tool affects the dynamicbehavior of the tool, especially when cutter shank length todiameter ratio exceeds three to one. As a result, tool lifedecreases with increasing tool shank length. Clearly, thetool shank should be made as short as possible. Theinsert shape also influences vibration by altering the entryangle of the cutting edge to work. Round inserts are mostprone to vibration while the inserts with 45 lead anglesshow the least tendency to vibration [54].

Figure 26: Predicted temperature distribution from FEMsimulation - Max. tool temperature = 605 "C,Max workpiece temperature= 601 "C [61]

Figure 25: Tool wear in rough milling 57 HRC die steelwith TiCN (left) and TiAlN (right) coatings [52].

Modeling of Machining OperationsA recent ClRP publication reviewed and summarized amyriad of modeling techniques to predict forces,temperatures and chip formation for turning, milling, andother metal cutting operations [59]. Among all availablemodels, the FEM based process modeling of metal cuttingappears to offer great potential for predicting theparameters of the machining processes. This methodologyis still in development stage and it is practical today foronly two-dimensional chip flow analysis, it has greatpotential for estimating temperatures, chip flow, tool wear,residual stresses and microstructural variations duringmachining.As an example, recent studies applied the FEM analysis to2D machining to predict a) continuous and serrated chipformation [60, 611, b) the effect of edge preparationincluding sharp, honed and T-land edges, c) temperaturesin the workpiece, chip and tool under various cuttingconditions, d) estimate tool wear with uncoated carbidetools, and e) the cutting forces. Figure 26 shows thetemperature distribution calculated for specific orthogonalcutting conditions. (Workpiece: P20 (30 HRC), tool:uncoated carbide WC, vc = 150 mlmin, f = 0.105 mmlrev,rake angle = O , relief angle = 6 , edge radius = 0.020mm) Figure 27 illustrates the effect of tool edge honeradius upon the tool stresses. (Workpiece: HI3 (46 HRC),tool: uncoated carbide WC, vc = 200 rdmin, f = 0.25mrdrev, rake = -5 ,elief = 5 ) The results are comparedwith experimental data and demonstrated that FEA ofmetal cutting can certainly predict process parameters insimple cutting conditions. Furthermore, preliminary 3Dprocess simulations, although requiring consid era blecomputational resources, indicate that FEM will soonbecome a very powerful process simulation tool forselecting optimum cutting conditions, tool materials andcoatings, and tool wear for complicated milling operationsthat are encountered in die and mold making.

a) Hone radius = 0.01 rnrnMax. eff. stress = 3950 MPa b ) Hone radius = 0.05 rnrnMax. eff. stress = 2700 MPa

c ) Hone radius = 0.1 rnrnMax. ef f . st ress = 2000 MPa

d) Hone radius = 0.2 rnrnMax. eff. stress = 3800 MPa

Figure 27: Effect of edge preparation on tool effectivestress in a honed edge tool, as estimated byFEM simulations [61].

4.6 Application o f Lasers in Die and Mold MakingLaser surface treatment is already in use in a number ofapplications in industry, to enhance the wearcharacteristics of dies and molds [62]. In this technique, alimited area of the die surface is melted using high-energylaser radiation. A "path" is created by the feed motion ofthe workpiece in relation to the laser beam. Several tracksare laid alongside one another. A range of differentmaterials, selected for the specific applications involved,can be fed in powder form to harden and coat a specificportion of the die, as seen in Figure 28. The materials


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suitable for alloying applications are carbides such asWCICo, WCICr, TIC and VC. Cobalt and nickel alloys aregenerally used in coating applications. The alloyingdepths 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 therepair, restoration, and reconfiguration of dies that requireminor surface modifications using the Direct MetalDeposition process. An industrial laser and powderedmaterial are used to create and reconfigure fully densedparts or layers directly from a CAD file [63].Application of lasers for die maintenance andimprovement includes not only die surface repair andmodification but also localized laser beam hardening,surface coating and laser beam alloying and dispersing[391.Laser technology is also applied to manufacturing of smalldies. A 100 W YAG last cutting machine provides a 0.1mm diameter laser beam that can be precisely controlledto machine cavities in a wide range of materials, includingceramics [64]. Using special software, the laser pulsesare controlled to vaporize the material in layers of about 5pm, resulting in surface finishes in the order of 1 to 2 pm.The material removal rate is 13,000 mm3/min. Theworking envelope is about 406 mm x 305 mm x 559 mm(x, y, z). Maximum machining depth is 10 mm. Anothermodel (15 kW and 12,000 RPM spindle) combinesconventional milling for rough machining with laser millingfor finishing.

Laser treated zone

IFigure 28: Laser surface treatment [62].

5Traditional manufacturing processes (Electro DischargeMachining-EDM, Laser Beam Material Removal (LBM),High Speed Cutting and Hard Machining (HSCIHSM)greatly affect the surface integrity of manufactured die ormold, namely:1. macro- and micro-surface quality, accuracy, and

roughness2. sub-surface microstructure and composition3. sub-surface residual stresses, and4. surface and sub-surface microhardness.These issues have been discussed in an excellent reviewprepared 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 andappearance of the finished product but they also influencethe process conditions, especially in metal forming, i.e.forging and stamping. Thus surface finish and coating ofthe dies are very critical in improving lubrication and metalflow as well as die life. A recent study investigated theinfluence of excimer laser treatment of cast iron andceramic surfaces, used as sheet forming dies, upon thetribological behavior [66]. The study illustrated thatmicrotextures produced by this technique, improves thelubrication conditions on ceramic surfaces but not onmetals. However, the geometry (size and depth) of thetextures strongly influences the results.In cold forging die life and reliability are two very importantfactors influencing process economics. The use ofcarbide inserts, punches and coatings in the industry arestandard procedures for reducing tool wear in cold forging.Using process modeling techniques for estimating metalflow and tool stresses it is today possible to design coldforging tooling so that stress concentrations are eliminatedand the fatigue life of dies and punches is extended.Application of FEM modeling to production cold forging ofbevel gears and constant velocity joint (CVJ) componentshave been described in earlier publications [67]. Recently,a so-called SVL (strength versus load) concept wasdeveloped to combine the proved methods of numericalprocess simulation with statistical analysis to predict toollife [68]. The concept has been applied and has beenshown to give reasonable results. It was also shown thatthe accuracy of tool life prediction depends on theavailability and quality of input parameters, necessary forthe implementation of the SVL methods.In hot forging die life is mainly affected by abrasive wearthat leads to a) bad surface finish and b) out of tolerancedimensions of forged parts. Thus, understanding andcontrolling of die wear in hot forging is extremely critical indetermining the technical and economic feasibility of warmand hot precision forging processes. Extensiveinvestigation, conducted on this topic, helped to gainconsiderable insight on this complex phenomena to toolwear in hot and warm forging [69].Die wear can be reduced and die life can be improved bysurface treatment techniques. Classical methods such asflame hardening, nitriding, boriding, and surface welding ofdie surfaces with high temperature alloys are well knownSurface layers generated by laser processing, asdiscussed in section 4.7, also produce a wear resistantsurface that is metallurgical bound to the sub-surface.Other applications of laser surface treatment includedistortion free laser assisted hardening, surface alloying,and generation of functional layers using laser coating.These techniques offer the potential for a) integratingsurface treatment into the production chain of dies andmolds, thus reducing the total production time [70], and b)performing rapid repair of dies and molds, which is verycritical in terms of maintaining the production and keepingthe processing equipment running [39].Die finishing and polishing is labor intensive and timeconsuming. High-speed milling operations reduce theneed for die polishing to some extent in certainapplications, mainly in forging and stamping dies.Injection molding molds and cold forging and extrusiondies, however, must still be polished to obtain desirablemetal flow and specified surface finish on molded andforged products. An excellent process for automaticpolishing of dies is the so-called "abrasive flow" process[71] that is being used in the die and mold shops aroundthe world.

[481.


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The application of high-speed milling allows obtaininggood surface finishes within an acceptable machiningtime. However, there are still applications where handfinishing of the dies is necessary. In such applicationsthree-axis or five-axis CNC grinding of the die surfaces isstill more cost effective than hand finishing, Figure 29.The results of a study on automated grinding of dies areavailable and summarize the contact conditions in 3 axisgrinding, the magnitude of grinding forces, the quality ofthe ground surfaces and the application of belt grinding tolarge dies with relatively large surface curvatures [72, 731.

Figure 29: NC grinding of sculptured surfaces [72].6 NON TRADITIONAL MACHINING O F DIES ANDMOLDSWhile high speed machining of hardened tool steelcontinues to attract much interest, EDM remains anindispensable process in the die and mold makingindustry. EDM and other non-contact processes such asECM and hybrid processes continue to develop, both interms of machining efficiency and their ability to produceprecise die geometries in difficult to machine metals andgeometries.In the past decade, increased control of the EDM processhas provided a higher level of machining precision, alongwith decreased damage to the workpiece and reducedmachining times. At the same time, EDM processes havebecome more tightly integrated in the total die and moldmaking process, leading to increased use of both wire andsinker EDM in a lights out mode [74].6.1 Advances in EDMWhile EDM will never be able to compete with metalcutting in terms of removal rate, recent advances inmachine and control technology have greatly increasedthe cutting speed of both wire and sinker EDM. WEDMcutting speeds have increased over 800% in the past 20years, while SEDM cutting speeds have also increasedsignificantly [75]. At the same time, EDM machinemanufacturers have reduced the severity of post-EDMsurface damage, with recast layers as thin as 1 or 2 pm.The use of fuzzy logic and other advanced control logichas been an industry standard for several years. Processcontrollers have also benefited from advances in computerspeed, as well. It is now possible to buy EDM machineswith control loops operating on the order of a few

microseconds, which can be several orders of magnitudefaster than a typical discharge.The mechanical design of the machines has alsoimproved considerably in the past decade. Theintroduction of linear motors to drive the axes in bothSEDM and WEDM machines has led to much improvedmachine response to process instability. [75, 761 Whencoupled with ultrafast controllers operating on 2 microseccontrol loops and glass scales to directly measure themotion of the machines axes, linear motors enable EDMmachines to react to process instabilities quickly, leadingto increased process speed and decreased risk of damageto the workpiece.While the role of machine and controller dynamics inmaintaining a stable process is crucial, other researchershave looked into altering the characteristics of the EDMdielectric to achieve the same result. [77] An interestingmethod for achieving greater control of the erosionprocess involves mixing additives such as silicone,chrome, or graphite to the EDM dielectric in SEDM, toenhance process stability. [76, 651 A major machine toolsupplier markets this technology under the acronymDDM, which stands for diffuse discharge machining.They claim that use of additives can result in a finersurface finish, while maintaining metal removal rates. [78]Other promising areas of research include EDM ofceramics [79] the use of EDM for texturing mold surfaces[80], and the development of several hybrid EDMlECMmachines, which will be covered more fully in the nextsection.6.2 ECM and EDMlECM HybridsAlthough it has never achieved the importance of EDM,electrochemical machining (ECM) continues to be anattractive non-traditional machining method for a varietyof applications. An excellent overview of recentdevelopments in this field is given in [81]. In die and moldmanufacturing, ECM has been limited by the difficulty ofpredicting the exact shape of a tool to machine a specificcavity to a high degree of precision. Rajurkar and hiscolleagues have applied orbital tool motion to conventionalECM to improve machining accuracy, as well as usingpulsed current with passivating electrolytes. [82]Recently, several efforts at combining EDM and ECM in asingle machine have appeared. [83, 84, 851 Thisapproach clearly removes the problem of preciselymaintaining relative positioning while moving the tool andworkpiece, but adds the problem of keeping the EDMdielectric and ECM electrolyte from mixing. DeSilva andMcGeough, on the other hand, have looked not only athybrid EDMlECM processes, but also at what they termelectro-erosion4issolut ion machining, in which a pulsedpower source creates discharges in an aqueouselectrolyte, followed by a pulsed ECM cycle. [86]6.3 Micromachin ing with EDM and ECMInterest in creating engineered structures at nanometerand micrometer scales continues to grow very rapidly.While nanometer scale engineering is still largely thedomain of silicon-based photolithography, there is alsoconsiderable interest in being able to economicallyproduce components from plastics, metals, and ceramicsat dimensional scales less than one millimeter, but greaterthan 1 pm. [87] While micrc-injection-molding hasperhaps attracted the most attention in this domain [88],other researchers are at work adapting net shapeprocesses such as die casting and metal forming tomicroscale applications [89]. A complete review of thistopic is available in [go, 911.


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Both EDM and ECM processes also possessdisadvantages for micromachining as well. EDM toolswear quickly during the machining process, leading tosome uncertainty regarding the exact shape of the toolduring metal removal. It is also not always possible tocontrol the erosion zone precisely, leading to overcut andtapering of high aspect ratio cavities and holes. Inaddition, forces due to steam pressure within thedischarge bubble, as well as electrostatic andelectromagnetic forces can cause workpiece distortionwhen machining very small features.The problem in ECM also revolves around control of theerosion zone. In pulsed ECM this is a function of the pulseparameters, the shape of the tool, the width of themachining gap, and the type of electrolyte employed.Nevertheless, both processes are the subjects of muchinterest as researchers seek to find better ways tomachine dies and molds at dimensions between 1 and1000 pm.Because tool wear in micro-EDM can be minimized, butnever completely eliminated, the ability to predict thedegree of tool wear with a high level of certainty is clearlya major issue. Rajurkar and Yu have developed what theycall the uniform wear method, and applied it successfullyto the machining of features on the order of 100 pm. [92,931The use of wire EDM for machining structures at sub-millimeter dimensional scales showed that furtheradvances in this area will clearly require the use of smallerdiameter wire than is currently practical, with a necessaryimprovement in wire guide and wire transport systems,and pulse generators capable of higher frequency andshorter duration pulses. [94]At this early stage relatively little attention has been paidto adapting pulsed ECM technology to the task ofmicromachining, even though this process seems topossess inherent advantages at this scale. ECM creates astress-free, undamaged surface, and has the majoradvantage of zero tool wear. On the other hand, the abilityto confine the erosion to a narrow zone appears to be themajor stumbling block to wider use for high precisionapplications. DeSilva and his colleagues have recentlypublished two very interesting papers on their efforts toimprove the precision of pulsed ECM by developing anempirical model of the process based on the characteristicrelationships of the process parameters [95]. They havebeen able to achieve accuracies greater than 5 pm, withsurface finishes in the range of 0.03 pm R

7 SUMMARY AND FUTURE DEVELOPMENTSDie and molds manufacturing will continue to represent avery significant aspect of production technology. Thepresent developments and future trends can besummarized as follows:

Dies and molds must be manufactured with evenshorter lead times to offer flexibility and rapidintroduction of goods to market. Thus, the role ofprocess modeling, especially in making complex diesor molds, becomes very important for reducing timeallocated for process development and try out.High speed machining is well established while hardmachining is being rapidly accepted. Optimized toolpath generation, to maintain constant chip load inmachining complex sculptured surfaces, is offered byresearch centers as well as some software suppliers.The wider use of these technologies will allow diemakers to become even more competitive.

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he trend for unattended machining is very strong,nainly in industrially developed high wage countries.his mode of manufacturing requires robustrocesses, advanced tool path generation, and bestlossible use of machine tools and cutters.he cutting tool industry continues to develop newutter geometries and coatings for obtaining betterurface finish and long tool life. Obviously this trendAll continue. However, these new developmentsequire that the users, i.e. die shops, keep training theirlersonnel and keep up to date about newlevelopments in the industry. The continuouslevelopment of cutting tools is now being assisted byising FEM based simulation of the cutting process.Vhile these techniques are still at their infancy, there is10doubt that process simulation in machining will beccepted by the industry, as it is the case with processnodeling of stamping, injection molding and forging.: is desired to machine the dies and molds in oneingle set up. Thus, deep cavities usually machined byDM are often manufactured by milling with long andi in cutters. While this trend will continue, still thereAll be many applications where EDM is still the onlyost effective method of manufacture.he machine tool and software suppliers offer, overall,ather good products for die manufacturing. Theutting tools, including geometry, substrate materialInd coating, need continuous improvement in order toJrt her improve the machining conditions.

[EFERENCESKlocke, F., Klotz, M., Knodt, S., Altmueller, S. 1999,The Process Chain in Die and Mold Manufacturing,(in German) presented at the EDM TechnicalConference, Aachen, Nov. 4-5.Neugebauer, R., Stoll, A,, Schneeweiss, M., 2000,New Production Systems for Die Making (inGerman), ZWF, Vol. 95, p. 612.Klocke, F. and Knodt, S., 1999, Hard-Fast-Dry:Advances in Machining of Dies and Molds, (inGerman), 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 DieManufacturing with Future Aachen, Sept. 27-28.Casellas, A,, 2000, The Future of Die Manufacturingin Europe, Conference on Die Manufacturing withFuture Aachen, Sept. 27-28.Friedrich, G., 2000, Estimation of Die Costs, (inGerman) presented at the Conference on DieManufacturing with Future Aachen, Sept. 27-28.Bogenschutz, U., 2000, New Technologies - Limitsof Feasibility in Tool and Die Industry, (in German),Conference on Die Manufacturing with FutureAachen, Sept. 27-28.Hock, S., Wenserski, J., 2000, EffectiveManufacturing Processes in DielMold Making, (inGerman) presented at the Conference on 30Experiences in Die and Mold M aking Dresden, MayBrunnermeier, S.B., et al ., 1999, lnteroperability CostAnalysis of the U.S. Automotive Supply Chain,Center for Economics Research, RTI, ResearchTriangle Park, N.C.

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[I31 Kruth, J.P., 1998, Progress in Additive Manufacturingand Rapid Prototyping, ClRP Annals, Vol. 47, No. 2,p. 525.

[I41 Anonymous, 2001, Rapid ManufacturingTechnologies, Advanced Materials and Processes,May, p. 32.

[I51 Rosochowski, A,, A. Matuszak, 2000, Rapid Tooling:the State of the Art, J. of Materials ProcessingTechnology, Vol. 106, p. 191-198.

[I61 Noken, S., 2000, Rapid Prototyping and RapidTooling 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

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[81] Rajurkar, K.P., 1999, New Developments in Electro-Chemical Machining, Annals of the CIRP, Vol. 48/2,[82] Rajurkar, K.P., 1999, Improvement ofElectrochemical Machining Accuracy by UsingOrbital Electrode Movement, Annals of the CIRP,[83] Karden, A,, 1999, Faster to the Final Shape - TheEDMlECM Process Chain (in German), EDMTechnical Conference, Aachen, Nov. 4-5.[84] Klocke, F., Sparrer, M., 1998, ElectrochemicalFinishing - The Faster Way to Finished Dies andMolds, Int. J. for Manufacturing Science andProduction, Vol. 1, No. 4, p. 247-256.[85] Klocke, F., Karden, A,, 1999, Material Characteristicsafter Cavity Sinking by EDM, Production

Engineering, Vol. 6/2, p. 35-38.[86] De Silva, A,, McGeough, J. , 1998, HybridElectrodischarge - Electrochemical Process forRoughing and Finishing Dies and Moulds, Proc. ofISEM XII, Aachen, VDI Berichte 1405. VDI-VerlagDusseldorf, p. 397-406.[87] Van Brussel, H. , Piers, J., Reynaerts, D.,Delchambre, A,, Reinhart, G., Roth, N., Weck, M.,Zussman, E., 2000, Assembly of Microsystems,Annals of the CIRP, Vol. 49/2, p. 451-472.[88] Kemmann, O., Schaumburg, C., Weber, L., 1999,Micro molding behavior of engineering plastics,Symposium on Design Test and Microfabrication ofMEMS and MOEM S Paris, p. 464-471.[89] Hirt, G., Rattay, B., 2001, Coining of Sheet Metal toProduce Thin Plates with Micro Channel Structures,Proceedings of 9th International Conference onSheet Metal Leuven, April, p. 119-126.[go] Masuzawa, T., Tonshoff, H.K., 1997, ThreeDimensional Micromachining by Machine Tools,Annals of the CIRP, Vol. 46/2, p. 621-628.[91] Masuzawa, T., 2000, State of the Art ofMicromachining, Annals of the CIRP, Vol. 49/2, p.473-488.[92] Yu, Z .Y . , Masuzawa, T., Fujino, M., 1998, Micro-EDM for Three-Dimensional Cavities - Developmentof the Uniform Wear Method, Annals of the CIRP,[93] Rajurkar, K.P.,Yu, Z . Y . ,2000, 3D Micro-EDM UsingCAD/CAM, Annals of the CIRP, Vol. 49/1, p. 127-130.[94] Uhlmann, E., St. Appel, Doll, U., 1998, WEDM ofMicrostructured Component Parts, Proc. of ISEM XII,Aachen, VDI Berichte 1405. VDI-Verlag Dusseldorf,[95] De Silva, A.K.M., Altena, H.S.J., McGeough, J. A,,2000, Precision ECM by Process CharacteristicModeling, Annals of the CIRP, Vol. 49/1, p. 151-156.

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Vol. 48/1, p. 139-142.

Vol. 47/1, p. 169-172.

p. 369-374.

Documents

dies and moulds