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POLISHING OF MOULD STEELDescription of abrasivesIt is important that the abrasivefulfills requirements with respect to: hardness sharpness thermal resistance chemical stabilityToday, the following five main groupsof synthetic abrasives are used, fulfilling the above requirements togreater or lesser extents.1. Diamond designation SD2. Aluminium oxide designation A (SG)3. Silicon carbide designation C4. Boron carbide designation B4C5. Cubic boron nitride designation BAbrasives have different applicationareas, depending on their particularcharacteristics, as shown partially intable 2 oroncarbideCBNHARDNESSKNOOPTHERMALSTABILITYIN AIR C F7 00065012002 100200036302 500120021902 9004 70027001300489025503. Silicon carbide (SiC)Has a needle like blocky structure.Used for rougher surface finishes.4. Boron carbide (B4C)Is hard and has a blocky crystal structure. Fast material removal generatingmoderate surface finish.5. Cubic Boron Nitride (CBN)Is produced basically in the same wayas synthetic diamond and is usedwhen grinding hard materials like HSSand hardened high carbide toolsteels.RecommendationsMaterial removal in hardened steelsis more consistent and repeatablewhen diamond products are used.Precision hand tools incorporatinglinear movement of the workingtools, grinding files and polishingstones, give a less troublesomepreparation process. A good practice is to work perpendicular tothe grooves in all preparation stepsand to verify with optical examination that all scratches from theprevious step have been completelyremoved. Note, that heavy coldworked material beneath the surface needs to be removed for aperfect end result.Table 2.1. DiamondThe hardest material known, has asharp and angular structure. Fastmaterial removal and the best possible planarity in combination withexcellent surface finishes.Distinguish between mono andpolycrystalline diamonds. Monocrystalline are best for lapping, sincethey are round and have many cuttingedges. Natural gives better cuts whilesynthetic are harder, a mix is the bestsince it last longer.2. Aluminium oxide (Al2O3)Is relatively hard and has a sharpangular structure. It is often usedduring the last polishing step since itgives excellent and highly glossy surface finishes. Is relatively inexpensive.GuidelinesNo general recipe exists for all typesof steels, but the experience andability to adjust the polishing technique to every single mould and tominor variations in the surface is ofcrucial importance for the end result.As a general guideline the procedurefor high gloss polishing shown belowcan be adopted i.e.; starting from a ground surfacewhere the roughness Ra/Rz shouldbe maximum 0.5/5 µm use stones/grinding papers for thefirst steps, stepwise grinding to1200 Mesh spend more time on the coarsesteps before changing to the finersteps polishing with diamond compoundfrom 15 µm down to 1 µm grain,use as short time as possible always be careful when using softcarriers (felt, brushes, cloths) asthere is a risk of “orange peel”formation on the polished surfaceA reflective surface starts to appearat Ra/Rz approaching 0.1/1 µm, andthe final surface roughness Ra/Rzshould be less than 0.005/0.04 µm fora high gloss polished surface.FINE GRINDINGFine grinding should smooth thesurface before the diamond polishingstage commences. Working tools andcompound media are built up arounddifferent kinds of abrasives whichconsists of small and hard particleswith sharp edges and irregularshapes.PRACTICAL HINTS FOR GRINDINGIt should be emphasized that thegrinding operation forms the basis fora rapid and successful polishing job.In grinding the marks left by therough machining operation are removed and a clean and geometricallycorrect surface is obtained. The practical hints mentioned below apply toboth mechanical grinding and manualstoning. To avoid adding heat and stress intothe surface, do not use too muchpressure and use plenty of coolant. Use only clean and free-cuttinggrinding tools with soft stones forhard surfaces. It is very important that the workpiece and the hands of the polisherare carefully cleaned between eachchange of grain size. This is done toprevent coarse particles and dustfrom being carried over to the nextgrinding step.7

POLISHING OF MOULD STEEL When changing to the next finergrain size, it is recommended thatthe grinding direction be changed to45 . Cross-grinding is very simple,but extremely effective. It increasesstock removal and it makes it easierto detect scratches from the previous steps and improve the dimensional accuracy. Figure 5 A–C. Select the sequence of movementsso that all surface segments areprocessed for an equally longperiod. With a rotating grindingdisc there will be a risk that therewill be less stock removal on theedge than in the centre of thesurface. Figure 5 D.StandardA45 PRACTICAL HINTS FOR POLISHINGAbove all, cleanliness in every step ofthe polishing operation is of suchimportance that it cannot be overemphasized. Each polishing tool should be usedfor only one paste grade and keptin dust proof containers. Paste should be applied to thepolishing tool in manual polishing,while in machine polishing the pasteshould be applied to the workpiece. Polishing pressure should beadjusted to the hardness of thepolishing tool and the grade ofpaste. For the finest grain sizes, thepressure should only be the weightof the polishing tool. Work with hard carriers for asmany steps as possible and workfor as short a period as possiblewith soft carriers. Polishing should start in the corners, edges and fillets but be carefulwith sharp corners and edges sothey are not rounded off.BMillingTurningEDM’ing Finish polishing step should, if possible, be carried out in the releasedirectional of the moulded part. With softer carrier the abrasive isable to penetrate deeper into thecarrier. This will result in that thesurface will be finer for the samesize of abrasive. See Figure 6 below.SoftFeltMediumHardWoodSteelHardened steelFigure 6. The hardness of the carrieraffects the exposure of the diamond grainsand the removal rate.TYPICAL POLISHING SEQUENCESThe choice of grinding and polishingsequences are determined by theexperience of the operator and theequipment he/she has at his/herdisposal. The properties of the toolmaterial can also influence thesequence.RoughgrindingRough Grainnumber90 5080C120180Irregular overlappingFine 220FinegrindingRough GrainD320Fig. 5. Grinding directions.FEPAPolishing withdiamond pasteRough Micronsize45 µm25800151200Fig. 7. This figureshows example of howthe polishing sequencecan be selected.89631FineFine

POLISHING OF MOULD STEELEXAMPLE OF DIFFERENTPOLISHING STRATEGIES AT HIGHGLOSS POLISHINGAll polishers have their own procedures for high gloss polishing. Thedata, in Tables 3–5, reflects that differ-ent manual polishing strategies canbe adopted to reach the same finalsurface finish by using rigorous andwell proven working procedures.The achieved surface finish is lowerthan Ra 0.01 µmSTEPTECHNIQUETYPE OF TOOLLUBRICATION1Hand-held unitStone320Dielectric oil2Hand-held unitStone400Dielectric oil3Hand-held unitStone600Dielectric oil4Hand-held unitPaper400Dry5Hand-held unitPaper600Dry6Hand-held unitPaper800Dielectric oil7Hand-held unit (linear)Brass 5 x 5 mmDP 9 µmDielectric oil8Hand-held unit (linear)Wood 5 x 5 mmDP 9 µmDielectric oil9Hand-held unit (linear)Wood 5 x 5 mmDP 6 µmPolishing oil10Hand-held unit (rotational)Hard felt 10 mmDP 3 µmPolishing oil11Hand-held unitPiece ofcotton woolDP 1 µmPolishing oilThe tables 3 and 4 showexamples of specific stepby-step information regarding high gloss polishing ofUddeholm Stavax ESR andUddeholm Unimax.Table 3.STEPTECHNIQUETYPE OF TOOLLUBRICATION1Ground2Hand-held unitSiC paperK320Dry3Hand-held unitSiC paperK800Dry4Hand-held unitSiC paperK1500Dry5Hand-held unitAcrylD fluid 6 µmPolishing oil6Hand-held unitAcrylD fluid 3 µmPolishing oil7Hand-held unitCottonD fluid 3 µmPolishing oilTable 4.STEPTECHNIQUETYPE OF TOOLLUBRICATION1Reciprocating machine 9500 RpmAmplitude movment 0.2 mmBrass carrierPlastic carrierDP W 15 µmPolishing oil2Reciprocating machine 9500 RpmAmplitude movment 0.2 mmBrass carrierPlastic carrierDP W 10 µmPolishing oil3Reciprocating machine 9000 RpmAmplitude movment 0.2 mmBrass carrierPlastic carrierDP W 5 µmPolishing oil4Reciprocating machine 7500 RpmAmplitude movment 0.2 mmBrass carrierPlastic carrierDP W 3 µmPolishing oil5Turning toolsWool blanketsDP W 1 µmPolishing oilObserve carefully, duringthe polishing steps, if anydeep marks are visible inthe polished surface. If thisproblem occur it is neededto immediately reduce thepressure, put on polishingoil or if more diamondpaste needs to be added.Table 5.9

POLISHING OF MOULD STEELPolishing problemscan be solvedThe predominant problem in polishing is so-called “overpolishing”. Thisterminology is used when a polishedsurface gets worse the longer youpolish it. There are basically twophenomena which can appear when asurface is overpolished: “orange peel”and “pitting” (pin holes). These problems often occur when changingfrom hard to a soft tool (felt/brush).A material at higher hardnesscan better withstand a high polishingpressure compared with prehardened steel. Subsequently materialwith low hardness will become“over-polished” more easily.Orange peelThe appearance of an irregular, roughsurface, which is normally referred toas “orange peel”, might depend ondifferent causes. The most commonis polishing with high pressure andprolonged time during the last polishing steps. A material at high hardnessis less sensitive to problems with“orange peel” compared to prehardened or soft annealed material. If a polished surface shows signsof an appearance like “orange peel”;stop polishing! There is no idea toincrease the polishing pressure andcontinue to polish. Such a course ofaction will only result in a worseset of problems. Following steps are recommendedto restore the surface. Remove thedefective surface layer by regrindingit, by using the last grinding stepprior to polishing. Use a lowerpressure and shorter time duringthe polishing steps than what wasused when the problems occurred.PittingThe very small pits (pin holes) whichcan occur in a polished surface generally result from non-metallic inclusions or hard carbides which havebeen torn out from the surfaceduring the polishing process. Pittingcan also be caused by hard particlesembedded in a softer matrix. Duringpolishing the matrix will be removedat a more rapid rate than the hardparticles. Polishing will gradually“undermine” the hard particles untilthey are torn out of the material byfurther polishing. The problem ismost often encountered when polishing with diamond paste grain size lessthan 10 µm and soft polishing tools(felt, brush).If pitting occurs the followingmeasures should be taken: regrind the surface carefully usingthe last grinding step prior topolishing use a hard coarse tool and repeatthe polishing process0.5 mm“Pitting”0.5 mm“Orange peel”10When using grain size 10 µm andsmaller: the softest polishing tools shouldbe avoided the polishing process should becarried out for the shortest possible time and under the lowestpossible pressure

POLISHING OF MOULD STEELMeasuringsurface roughnessand qualityPolished mould surfaces are traditionally estimated by the naked eyeand/or measured by mechanicalprofilers for surface roughness, commonly described with the Ra, Rz andRt values.However, these methods are bothsubjective and uncertain compared tomore advanced surface- and subsurface measurement devices capableof measuring to fractions of nanometres. The use of 3D-instrumentation with higher resolution providesmore accurate surface measurementsof moulds with complex geometrieswhich in turn means that quantitativesurface quality controls can be performed.SURFACE ROUGHNESS ACC. TO DIN/ISO 1302ROUGHNESSROUGHNESSRa, µmRmax, µmN1N2N3N4N5N6N7N8N9N 10N 11N 120.0250.050.10.20.40.81.63.26.312.52550SURFACE ROUGHNESS ACC. TO SPIACHIEVED AFTER GRINDING/POLISHING �1010–2020–40 60 125 250 5003 µm Diamond Paste6 µm Diamond Paste15 µm Diamond Paste600 Grit Paper400 Grit Paper320 Grit Paper600 Grit Stone400 Grit Stone320 Grit Stone600 Stone Prior to Dry Blast Glass Beads #11400 Stone Prior to Dry Blast #240 Alminium oxide320 Stone Prior to Dry Blast #24 Aluminium oxideA-1A-2A-3B-1B-2B-3C-1C-2C-3D-1D-2D-3Table 6. Approximate comparison between requested surface roughness measured by mechanical prof ilers andinternational standards.Surface assessment byroughness parametersThe benefit to measure surfaces isboth the possibility to study them inthe micro- and nano-scale, and a wayto quantitatively evaluate them. But,there is a huge amount of available2D- and 3D parameters (abbreviatedR- and S-parameters, respectively), sohow do you know which to use?2D parameters, usually obtained bya mechanical profiler, can be used toquantify the surface quality in alimited extent. The most frequentused in practical work with moulds isthe Ra-value describing the averageheight of the measured surface.However, it is a rather poor description of the mould surface sincesmaller defects and certain textureswill be “averaged out” and/or undetected. See figure 8.Ra – the arithmetical mean deviation ofthe prof ile is the mean value of theabsolute value of the profile departurey within the reference length l.Source: The f igure to the left is fromT.R. Thomas book “Rough surfaces”2nd edition.0100200300400500600 µm050100150200250300350400450µmThe A- & B-profiles illustrate one ofthe major disadvantages of the 2Dprofilometry; A – a surface with pores,and B – a “defect free” surface, i.e. theresults are strongly dependent on theprofile location.Ra 2.4 µmIllustration of different surfacetopographies with equal Ra-value;Ra 2.5 µm i.e. the Ra-value itself is not enough tofully describe the surface structureSource: Illustration from T.R. Thomasbook “Rough surfaces” 2nd edition.Ra 2.4 µmFig. 8.11

POLISHING OF MOULD STEELMeasurement devices and analysis techniques available toquantify engineered surface topographiesMECHANICAL PROFILER (STYLUS)Typical outputparameters are theRa (arithmetic meanvalue of a profile),the Rz (mean peakto valley height), and the Rmax (orRt, the maximum peak to valleyheight). Notice: most often R-valuesare filtered per default (connected toactual measurement length and cutoff).LtLsVBVtPointsPick-upSample D ground 15 rep Unimax 01µm1.75 mm2.5 µm350 µm0.50 mm/s3500PHT 350P000 007 PCD: R(LC GS 0.3 mm)2.00-2.01.3 mm0.3 mm/divRa 0.083 µmRz 0.78 µmSCATTEROMETER (GLOSSMETER)The surface is illuminated and the reflected/scatteredlight is detected.Simple glossmetersmeasure reflections in defined angles,whereas scatterometers include thetotal reflection.Scattering data need to be correlated to roughness data by verifications with other measurementdevices. Typical output is e.g. anaverage rms-values, the ratio ofthe diffuse reflection or the reflection of light at a defined angle.Intensity db500-5040-4020-2000-2020-4040Y scatter angle (degree)X scatter angle (degree)INTERFEROMETERadvantages in laboratories due toits sensitivity to vibration, but newinstruments are coming90that can be used for in-linemeasurements. Typical outputare 3D maps and areal surfaceparameters (e.g. Sa and St whichcorrespond to the Ra and Rt respectively). Also other parameterfamilies are available, e.g. areal,volume and functional parameters.nm020406080µm10060120140Height deviationsare detected byutilising interferencepatterns formed/arised when tworeflected lightbeams, one from the sample and onefrom a reference surface, interact.Features down to 1 µm in spatialresolution and sub-nm in height canbe detected. The technique is 02520151050nm180CONFOCAL MICROSCOPE12405060708µm 0 90100160307020504µm 0 30201001401012080than optical quality. Typical outputare 3D maps and areal surface 190parameters (e.g. Sa and St0 10which correspond to the Ra andRt respectively). Also other parameter families are available, e.g. areal,volume and functional parameters.1301401201008060402090Builds up 3D mapsbased on stacks ofimages recorded atdifferent heights,excluding pointsthat are out of focus. The techniqueis preferential for surfaces rougher1601500

POLISHING OF MOULD STEELSEM/EDSA focused electronbeam raster-scansthe surface; theenergetic electronsinteract with theatoms in the sample within a few nmto several µm of the surface, i.e.scattering events take place (primaryelectrons loose energy and/orechange direction). The emittedelectrons are “collected” by differentdetectors. The EDS, a type of X-rayspectrometer, allows elementalanalysis. Typical output are thetopographical contrast (based on SE),chemical contrast (based on BSE) andphase composition (based on X-ray).ATOMIC FORCE MICROSCOPEµmSimply described asa tiny profiler/stylusoperating withextremely smallprobe tips barelytouching the surface resulting in 3Dresolutions close to atomic level.Typical output are 3D maps and arealsurface parameters.3530252015105005101520253035 µmSELECTION OF MEASUREMENT DEVICES AND GENERAL SCOPESEM/EDSATOMICFORCEMICROSCOPEGLOSSMETERxy: 10-6z: 10-7xy: 10-9z: 10-9xy: 10-10z: entUnlimitedDevicedependent(often up to2–10 kg)Devicedependent(often up to2–10 s forcleanlinessderminationRisk of surfacedamage, fragilestylus/pick-upSensitive tovibrationsLarge depth offocus, problemswith artefactsSTYLUSINTERFEROMETERCONFOCALxy: 10-7z: 10-6xy: 10-6 – 10-4z: 10-9xy: 10-6z: 10-10µm-mmµm-cmHeight infoNo2D/3DComponentanalysisResolution (m)MeasurementareaUsabilityMeasurementtimeSize ofworkpieceOtherWork in vacuum, Noise sensitive,needs solid andfragileconductingstylus/pick-upsamples, ability toimage undercutsOnly averageroughness dataTable 7. The figures shown should only be considered as guidelines.13

POLISHING OF MOULD STEELDDEHOLM/ TRYCKER120305.1000ART ANDE FEC T CHFO RD HIN TS; KARLSTADI KNAPPENINGSS POLISH ESHIGH GLOSU RFACOF ST EE L2013101The most common defects foundwhen polishing are described in theUddeholm brochure “Defect Chartand Hints for High Gloss Polishing ofSteel Surfaces”.UDDEHOLM DEFECT CHARTScattered (pin) holes dispersedover the majority of the surface.0.5mm Shorten the polishing time (use enoughbut short steps) Use lower pressure Use harder carriers/tools – combinationdiamond paste and lubricants is important Avoid unidirectional movements duringpreparation of the surfaces Dry the workpiece and store properly toavoid corrosion attacks on the surface If the pitting defects only appears in alocal area on the surfaces it probably dueto impurities in the material0.5mmHINTS Avoid unidirectional movements Use higher rotational speed if manualpolishing0.5mm Choose a cleaner steel i.e. ESR steel grade Use softer carriers/tools (without lint) Use lower pressure Napless polishing cloths reduce the riskfor pull-outs Use a fluoride-free polishing clo

High gloss polishing To ols for plastic moulding do require a high surface finish especially when extreme transparency and/or high gloss are aimed for. In such cases it is of utmost importance to choose a proper tool material and establish a suitable surface preparation tech-nique. To achiev