photo-etching of tool steel - bohler uddeholm · impax supreme p20 analysis variation. ......

Photo-etching of tool steel

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ContentsThe photo-etching process ........................ 3

Advantages of textured surfaces .............. 3

Test programme ........................................ 4

Summary .................................................. 7

This information is based on our present state of knowledge andis intended to provide general notes on our products and theiruses. It should not therefore be construed as a warranty ofspecific properties of the products described or a warranty forfitness for a particular purpose.


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IntroductionA wide variety of moulded parts areproduced with a patterned or texturedsurface. Normally, the pattern is repro-duced on the moulding surfaces of thetool by the photo-etching process.

The photo-etchingprocessPublished information about thetechniques employed by the specialistphoto-etching companies is very limited.Essentially, however, the requiredpattern is transferred to the mouldingsurface by a photographic process. Thepattern is then etched to the requireddepth by the application of an appro-priate acid, under closely controlledcondition.

Photo-etching can be performedboth on complete tools or on specifiedareas of the tool only.

Photo-etching enables a wide varietyof different patterns to be produced invirtually any tool. The patterns may

Advantages oftextured surfacesA textured surface hides minor surfaceflaws which may occur in manufactureor during further treatment and fitting.Because of this, the rejection rate forthe finished products is lower. Moreover,photo-etching replaces the lengthy andexpensive finish polishing process.

The product is given an aestheticallyattractive surface finish. The surface iseasier to grip than a bright surface,which facilitates holding and handling.Irritating reflections are largely avoided.

A further advantage is that finger printsand similar marks do not show up asmuch as on a bright surface.

This brochure deals with photo-etching as a finishing process, with thepossibilities offered by it and with thefactors which must be taken intoaccount to ensure a satisfactory result.These factors were determined by a testprogramme carried out with thecooperation of a leading photo-etchingcompany.

resemble leather or wood graining, forexample, or be a straight forward linepattern with varying directions anddepths. Typical applications are for carinterior fittings, etc., and plastic casingsfor different kinds of machines andinstruments. In recent years, photo-etching has become an increasinglypopular and practical method forimparting attractive and appealingsurfaces to different products.

Textured mould and moulded part for automobile steering wheel.


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Test programmeTo ensure that the toolmaker and thetool-user gets the optimum results fromUddeholm tool steels that are photo-etched, Uddeholm Tooling has carriedout a series of tests. The test pro-gramme examined a number of influ-encing factors, including:• Photo-etching of different tool steel

grades, annealed and hardened• Flame-hardening, welding and EDM• Grain flow direction of the tool steel• Variations in steel analysis and

cleanliness• Material size

Several different tool steels werestudied, by etching plates measuring50 x 60 mm (2" x 2 1/4"). All surfaceswere ground with a 280-grain grindingwheel. In one set of tests, all the speci-mens were etched under identicalconditions in order to grade the ‘‘etch-ability’’ in terms of the amount of stockremoved from the different material.After this, etching conditions werevaried with the aim of producing opti-mum etching results.

The steels listed below have beenexamined in the first instance aslongitudinal specimens (in the rollingdirection of the material) in the soft-annealed state and also according tothe parameters shown in the chart.

Photograph of a patterned surface.

PHOTO-ETCHING OFDIFFERENT STEEL GRADES

Results

The etching results were assessedtaking into account the etching depth,pattern similarity, side-etching effectand surface appearance. The surfaceshave not only been visually appraisedbut also examined at a high magnifica-tion in order to detect and study anymicroscopic differences.

Soft-annealed Hardened to 55 HRC

Stavax ESR textured with special media.

Uddeholmgrade AISI Other parameters studied

RIGOR A2 Hardness: 60 HRCHigh retained austenite content.

CALMAX Hardness: 57 HRC

ORVAR SUPREME H13 Hardness: 52 HRCRough- and fine-spark-machined.

IMPAX SUPREME P20 Analysis variation.Flame-hardened to 54 HRC.Surface and centre of large dimension.Welded with IMPAX electrode.

STAVAX ESR 420 Hardness: 300 HB 55 HRCWelded with STAVAX electrode.

ELMAX Hardness: 58 HRC

Annealed material

Depending on the type of etchingmethod used, a special etching mediamay be needed when etching steelswith good corrosion resistance. This isvalid for Stavax ESR, Stavax Supreme,

Corrax and Elmax. However, owing toits alloy content also Orvar Supremeand Calmax gives weaker etching thanother grades when the standard mediais used and in view of this the specialmedia is recommended.


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Flame-hardened material

The influence of flame-hardening onthe etching of Impax Supreme was alsostudied and here there is a decideddifference between the locally hard-ened zone and the hardened and tem-pered basic material. In the flame-hardened zone, a faint streakinesssimilar to that in hardened specimens isdiscernible. In addition, there is a differ-ence in etching depth between flame-hardened and hardened and temperedmaterial.

The other steels examined showgood results upon visual examinationafter having been etched by the stand-ard process. When the surfaces areexamined under the microscope (9 xmagnification), some minor differencescan be observed.

The observed differences normallyhave no practical significance. Theynevertheless show that if a tool withinserts which are to be etched with thesame pattern is being made it is advis-able for material from the same bar orblock to be used in all parts in order toget a pattern of identical and uniformappearance on the moulding. (See‘‘Grain flow direction of the tool steel’’,page 6.)

Hardened material

All grades were examined in the fullyhardened condition. Here, too, the fourgrades Orvar Supreme, Calmax, StavaxESR and Elmax differ from the others inrespect of etchability.

When the surfaces are studied underthe microscope, some tendency tostreakiness is discernible in some of thehardened specimens. The streaks areparallel to the direction of rolling, andthe phenomenon is an expression of thenormal rolling direction which appearsin alloyed tool steels. The streakiness,however, is of such modest proportionthat it lacks significance when usingtool steels with normal degrees of seg-regation, but at the same time it dem-onstrates the importance of selecting asteel that is as homogeneous and uni-formly worked as possible.

The presence of a high content ofretained austenite in a hardened tool isnormally a disadvantage. Etchability,however, is not affected even by a rela-tively high content of retained austeniteaccording to a test performed on Rigor.

Nitrided material

When a tool or insert is to be nitrided,this must be done after photo-etching.

Photo-etching on a rough-sparked surface.

Basic material. Flame-hardened zone.

the response to etching will varybetween the weld and the base metaland this will result in a witness mark onthe plastic component. Welds in ImpaxSupreme, Stavax ESR, Stavax Supremeand Calmax with Impax Weld, StavaxWeld or Calmax Weld (or TIG-Weld) orCorrax TIG-Weld will normally not bediscernible after photo-etching.

More information on welding is givenin the brochure ‘‘Welding of Tool Steel’’.

Areas which have been weldedshould always be clearly indicated to thephoto-etching company.

Electrical discharge machining(EDM)

If EDM is not carried out in the rightway, some defects may remain in thesurface of the material. The influence ofspark-erosion on photo-etchability hastherefore been studied. Specimens withboth a rough-sparked and a fine-sparkedsurface were tempered at 250°C (480°F).Photo-etching on a rough-sparkedsurface gives a very poor result. Evenafter a careful fine-sparking operation,it may be difficult to get an acceptableresult.

Flame-hardening, therefore, shouldbe carried out after photo-etching,wherever possible.

Welding

In certain circumstances it may benecessary to weld a tool, for instancefor repair purposes. Welding alwaysseverely affects the uniform structure ofthe parent material.

The weld metal and the base steelmust be similar in composition if awelded surface of a plastic mould is tobe textured via photo-etching. If not,


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Tempering does not give an appreci-able improvement. If doubts are enter-tained as to how the spark-machininghas been carried out the materialshould always be ground or polished toremove any residual traces of the spark-ing. Special test kits are available forchecking removal of residual effectsafter spark-erosion.

Areas which have been spark-erodedshould be clearly indicated to thephoto-etching company.

The photograph showsstreakiness in photo-etchingof a pre-hardened mould steelwith high sulphur content.

VARIATIONS IN STEELANALYSIS AND CLEANLINESS

There are always minor differences inthe analysis of every steel to occur fromone heat to another. In this context,two extremes in the analysis of ImpaxSupreme were examined, but no differ-ences in the results of the etching wereobservable. Normal variations in analy-sis of Uddeholm Tooling tool steels thushave no influence on photo-etchability.

The cleanliness of the steel, andespecially its sulphur content, can affectthe appearance of photo-etched pat-terns. Impax Supreme pre-hardenedmould steel is particularly suitable forphoto-etching for two reasons: it has avery clean microstructure, being sub-jected to a vacuum degassing processduring manufacture; it also has a verylow sulphur content (max. 0,010%).

Grain flow directionof the tool steel

Calmax has been examined on both thelengthwise and crosswise direction inthe soft-annealed state. No appreciabledifference between the specimens wasobserved. For fine patterns, however,experience shows that some differencecan occur. Where it is important thatphoto-etched patterns on differentmould parts match exactly, e.g. whenusing inserts, the following procedure isstrongly recommended:

1. Make all parts to be textured fromthe same bar or block of steel

2. Make sure that all surfaces to betextured have the longitudinal grainflow in the same direction

MATERIAL SIZE

When manufacturing materials in heavysections differences in the microstruc-ture of the material can be observedbetween the surface and the centre.In order to study the influence of thesedifferences on the photo-etchability ofImpax Supreme in the size 500 mm(20") dia., specimens from the surfaceand centre were photo-etched.

Surface. Centre.Impax Supreme Ø 500 mm (20").

No differencebetween the twospecimens wasobservable.

A wood-grain textureon a moulded handle fora saucepan.

There are, however, similar types ofsteels with far higher sulphur contents(0,08%), which can give rise to streaki-ness in photo-etching, as evident fromhe following photograph.


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SummarySeveral different grades of UddeholmTooling tool steels have been tested forphoto-etchability. The results of theetching tests and other experiencegained can be summarized as follows:

• All of the grades examined can bephoto-etched with satisfactoryresults. There are certain microscopicdifferences, but these normally haveno practical significance whatsoever.

• Orvar Supreme, Calmax, Stavax ESR,Stavax Supreme and Elmax shouldbe etched by a special process.

• If nitriding is to be carried out it mustbe done after photo-etching.

• Flame-hardening prior to photo-etching should be avoided, since thepattern will be etched differently inthe flame-hardened zone and inhardened and tempered basematerial.

• A welded tool can in certain circum-stances be photo-etched, but this isconditional upon using the samematerial in the weld as in the parentmaterial.

• Spark-machined surfaces should beground or polished in order to be onthe safe side. A poor etching resultwill be obtained on surfaces marredby residual traces of spark-machin-ing.

• Areas of tools which have beenflame-hardened, welded or spark-eroded should always be clearlyindicated to the photo-etching com-pany.

Part of an automobile steeringwheel produced from a photo-etchedImpax Supreme mould.

Photo-textured body for Polaroid instant camera.Mould material: Stavax ESR.

• If several parts are included in a tooland are to be photo-etched withexactly the same pattern, the samegrade of material and the same grainflow direction should be chosen forall the parts.

• Normal variations in analysis for thesame grade of steel have no adverseinfluence. Steels with a clean micro-structure and low sulphur contentgive the most accurate and consist-ent pattern reproduction.

• Different sizes of starting material ofone and the same grade do notusually show any differences.

• Initial machining operations shouldbe followed by stress-relieving priorto finish-machining.

• Coarser abrasives than 220 grainmust not be used on surfaces whichare to be photo-etched.

photo-etching of tool steel - bohler uddeholm · impax supreme p20 analysis variation. ......

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