MACHINING PROCESSES FOR PRECISION MOLDS

E. Brinksmeier, O. Riemer, J. Osmer, S. Twardy, K. RickensLaboratory for Precision Machining LFM

University of Bremen, Germany

INTRODUCTIONThe continuing drive towards miniaturisation in anumber of industries including semiconductor,optoelectronic and medical is stimulated byrequirements which include enhanced functio-nality, performance and reliability, increasedenergy efficiency, a cleaner environment,improved healthcare, together with reducedcosts etc. [1, 2]. Also, new products for fast-growing industries like IT and electronics areincreasingly implemented by using ever smallerand more complex modules. Because of thecontinuing demand for miniaturisation, complexsurface figures with and without precisemicrostructures are becoming more and moreimportant in technical applications. The highvolume production of these parts requiresreliable replication techniques by the usage oflong term durable molds. Here, the challengelies in the development of sufficiently preciseand accurate, deterministic manufacturingprocesses for these molds.There is a large number of machining processesavailable for making precise mold inserts for e.g.out of metal-based, machinable materials likestainless steel or advanced, difficult-to-cutmaterials like semiconductors or ceramics. Theapplicable processes differ in the spectrum ofmachinable geometry, removal rate, achievableform accuracy, surface roughness and sub-surface integrity. Therefore, hard milling anddiamond machining are state-of-the-artmachining technologies with high potentials andvarious opportunities for further processimprovements that make these technologiesmore effective and flexible. Otherwise, precisiongrinding and polishing are limited in contrast toe.g. diamond machining, but harder materialslike ceramics are only machinable by thesebounded or loose abrasive precision processes.

HARD MILLINGThe micro-cutting of steel with tungsten carbidetools can meet many of the demands ofminiaturised components. In contrast to macros-copic milling a homogeneous material behaviourcannot be assumed. Varying grain size and

dislocations in the material are annoying theprocess stability. A prerequisite for a stablemachining process is a homogeneous, hardworkpiece material with no internal stresses.These tools are commercially available down toa diameter of 50 µm and are still produced bytool grinding.Nevertheless, this technology is restricted bymaterial, hardness and aspect ratio. Theachievable maximum surface roughness isbelow 3 µm Sz. But the most important processparameter is the cutting velocity. Depending onthe hardness of the material, higher cuttingvelocities lead to better surface qualities,whereby surface roughness of 200 nm Sq andbelow can be achieved (cf. Fig. 1).For wear reduction special coatings of TiAlN areapplied. The actual coating thickness is reducedunder one micrometer, so the increasingrounding of the cutting edge is not effective.

FIGURE 1: Hard milled mold of hardened coldworking steel (60 HRC) for mirco rotaryswagging of thin wires

DIAMOND MACHININGThe largest spectrum of geometries ismachinable by diamond turning and millingprocesses. The processes are adaptable tocontinuous and structured surfaces, whereby themachining of structured surfaces can be done bycircumferential or ball end milling. For machiningstructures like prisms or spherical lens arrays,diamond tools with high form and angularaccuracy are necessary. Moreover, the toolshave to be in an accurate position within the toolholder and in relative position within thecoordinate system of the machine tool. To

prevent geometrical deviations when machiningmicro structures, the diamond tool has to bepositioned in the machine tool with respect to itscoordinate system.

A novel development for the diamond machiningof steel molds is the nitriding of the workpiecematerial. Due to the catastrophic tool wear whendiamond turning steel, the spectrum ofmachinable materials is so far limited. Thus, theamorphous electroless nickel coating is the onlydiamond machinable hard material (approx.550V). This is state of the art for making moldinserts for injection molding, but this materialsuffers form its low hardness and lowtemperature stability above approx. 200°C, e.g.for hot pressing of glass. Mold materials withsufficient properties are up to now machinableby grinding and polishing only, therefore,structured surfaces or cavities with small radiiare almost not machinable. However, twoalternative processes seem to beat thislimitation. The ultrasonic assisted diamondturning, introduced by Moriwaki [3] and revisitedby Brinksmeier [4], and the thermo-chemicalmodification of the mold material by a newnitriding process [5].

FIGURE 2: (a) Cross section and dominantphases of steel after thermo-chemical treatment;(b) Depth profile of micro hardness for twoworkpiece material after thermo-chemicalmodification; (c) diamond turned aspherical hardsteel molds turned with one diamond tool

The basic idea of the new method is to avoidchemical reactions between the carbon of the

diamond tool and the iron of the workpiece byestablishing a chemical bond between the ironand another chemical element. This was doneby a nitriding process, where after the surfacezone of the workpiece material (tempering steel42CrMo4) consist of a layered structure (cf. Fig.2a). The compound layer is composed of twophases, an ε-Fe2-3N-phase and a γ’-Fe4-N-phase, where all iron atoms are bonded tonitrogen atoms.Fig. 2b shows, that the thickness of thecompound layer of two used workpieces, a soft,stress relieved steel (340HV) and a hardenedsteel (730HV), is approx. 15 µm in both cases.Within the compound layer the hardnessincreases with increasing depth from2000 N/mm2 to 6000 N/mm2. Therefore, thereshould be no difference in cutting soft orhardened steel from the chemical point of view.During the development of the nitriding processdiamond face turning experiments wereperformed at sintered high alloy tool steel(dw = 22 mm) with a Vickers hardness of approx.1200HV. Here, surface roughness of better than10 nm Sa was obtained (cf. Fig. 2c).

Another challenge in mold making is the directdiamond machining of micro cavities. Thespectrum of machinable structures by diamondturning and milling is inherently limited by thetool geometry and its intrinsic kinematic.Whereas the machining of retro-reflective triplestructures is state of the art in diamond cuttingthe machining of a more efficient retro-reflectivecube corner array cannot be machined becauseof the need to generate a 3-fold pyramidalcavity. A machining technology that offers theability to machine sharply ended discontinuousstructures in a range between 50 to 500 µm willbe a key technology for new applications. Withthis technology available new optical functionscan be implemented in complex opticalcomponents leading to a new degree of freedomin optical design.For meeting this challenge a novel cuttingprocess, diamond micro chiseling (DMC), wasbeing developed [6]. This process enables thegeneration of discontinuous structures like cubecorner arrays with hexagonal aperture (similar tocat’s eye of bicycles, cf. Fig. 3). Here, eachmicro structure has to be cut separately andfrom several directions (depending on thecomplexity) and in multiple layers (depending onthe aspect ratio). Hence, an ultraprecisionmachine tool with three linear and two rotationalaxes is required, which has to be adjusted with a

tolerance below 0.1 µm (resp. 0.1°) in alldegrees of freedom. The applied tool is similarto a V-shaped diamond tool and the cuttingprocedure for a 4-sided pyramidal cavity willtake at least 4 individual cuts.

FIGURE 3: (a) Tool motion and cutting process(1-4) for micro cutting of a four sided pyramidalmicro cavity; (b) SEM images of four sidedpyramidal cube corner array manufactured bydiamond micro chiseling in OFHC-copper

PRECISION GRINDINGPrecision grinding technologies have to bedeveloped in order to get optical or near opticalsurface quality when machining ceramic moldmaterials for mass replication of opticalelements. For e.g. cemented carbide compo-sites are widely used in industrial and researchapplications as die material in glass mouldingprocesses due to their high hardness and lowcoefficients of thermal expansion, which ensuresto maintain the mold shape at the requiredpressing temperatures. Otherwise, due to thehigh hardness and brittle behaviour of cementedcarbide materials precise grinding processeswith fine-grained diamond abrasive tools areessential [7].Conventional cemented carbides are charac-terized by a cobalt proportional weight of 5% to20% and a Vickers hardness of HV10 < 2200. Incontrast, suitable die materials are nano-crystalline binderless tungsten carbides with agrain size below 200 nm, having less than 0.3%cobalt and a hardness of HV10 > 2800. Thebinderless type provides the chemical andabrasive wear-resistance of the mould materialagainst the melted glass.

Fig. 4a shows tungsten carbide molds withground surfaces in pre-machined and precisionground quality out of an optimized precisiongrinding process with a pin-type, fine-grained,resin-bonded diamond grinding wheel. For theprecision grinding process the followingparameters have been used: cutting velocityvc = 2 m/s, depth of cut dc = 0.2 µm, tangentialinfeed velocity vft = 0.1 mm/min and 5%-emulsion as cooling fluid. White-light interfero-metry and 3D-profilometry of the precisionground surfaces lead to a resulting surfaceroughness below 4 nm Sa and a figure accuracybelow 150 nm PV (cf. Fig. 4b).

FIGURE 4: (a) Precision grinding of a tungstencarbide mold insert with a pin-type diamondgrinding wheel; (b) different ground mold insertsafter conventional pre-machining (left), precisionrough (centre) and fine grinding (right)

POLISHINGIn some cases the surface quality of precisionground molds with structured elements superimposed onto envelope surfaces like plane,spherical, aspheric as well as free formedsurfaces is not sufficient for optical applications.Therefore, deterministic polishing processes toimprove surface roughness of structuredsurfaces are necessary. However, sincepolishing of aspheres, free-forms and structuresis a point polishing process, the challenge of thepolishing process is to maintain the formaccuracy out of the precision grinding processduring the polishing process. Severalapproaches for point polishing of micro andoptical components are known [8].

Besides the continuous optics, structured opticswith V-shaped or cylindrical groves withstructure size less than 1 mm are increasinglyrequired. These kinds of structures cannot bepolished using state of the art polishing tech-nology. A new structure polishing technology tomeeting these demands has been introduced in[9]. By using conical pin type tools or conicalpolishing pads cylindrical and V-shaped groveswith structure size less than 1 mm could bepolished after grinding (cf. Fig 5). The goal is, todecrease surface roughness without loss of formaccuracy.Polishing of steel or ceramics is still a processthat bases on the experience of the operator ofthe polishing machine tool. Here, detailedknowledge of the material removal process isnecessary for finding optimal processparameters. Moreover, the demands on the formaccuracy of polished steel and ceramic moldsare almost below one micrometer withaspherical or free-form shapes. Therefore, thenecessary step to a deterministic polishingprocess needs a fundamental knowledge aboutmechanical and chemical interaction betweenworkpiece and tool during the polishing process.

FIGURE 5: Structure polishing process forpolishing a cylindrical groove using a plasticpolishing pad (right); White light interferometricimages of cylindrical grooves in tungsten carbideafter grinding and additional polishing (left).

SUMMARYThe mass production of high precision moldswith aspheric, structured or free-form shape isstill a great challenge, whereas, the number ofoptics, its complexity and its quality neededincreases year by year. Innovative fast, preciseand deterministic machining and measuringtechnologies are required to meet this challengeand the integration of measuring techniques isrequired. The described machining processesfor the manufacturing of mold inserts forinjection and glass molding processes, hard

milling and diamond machining as well asprecision grinding and polishing, representspossible manufacturing technologies to fulfill theabove mentioned requirements.

ACKNOWLEDGEMENTThe works are funded by the German ResearchFoundation (DFG) at the University of Bremenwithin the Transregional Collaborative ResearchCenter SFB/TR4 "Process Chains for theReplication of Complex Optical Elements", theCollaborative Research Centre 747 “Micro ColdForming” and the Federal Ministry of Educationand Research (BMBF) within the researchproject “Fast hot pressing of glass for highquality micro optical components - FastMold”

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