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NickelDevelopmentInstitute

Electroforming — a uniquemetal fabrication process

by Ron Parkinson

NiDI Technical Series Nº 10 084

The material presented inthis publication has beenprepared for the generalinformation of the readerand should not be used orrelied on for specific applica-tions without first securingcompetent advice.The Nickel DevelopmentInstitute, its members, staffand consultants do notrepresent or warrant itssuitability for any general orspecific use and assume noliability or responsibility ofany kind in connection withthe information herein.

Ron Parkinsonis a consultant to the

Nickel Development Institute



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Electroforming – a uniquemetal fabrication process

Electroforming plays an important role in our daily lives.We have contact with it many times each day and it greatlyenhances our lifestyle in a variety of ways. In addition, itis an extremely versatile process. For instance, it is usedto produce micro components for the medical and elec-tronics industries and huge components for the aircraftand aerospace industries. For many applications it hasbecome indispensable and yet outside the electroformingcommunity, little appears to be known about the processand its applications. Most metallurgists, engineers anddesigners are not well informed on the subject as it is

rarely, if ever, included in technical courses presented atcolleges or universities. Nevertheless it is a unique metalfabrication process and nickel is the dominant metal inthis industry.

It is beyond the scope of this publication to provide alldetails of the procedures used to produce electroforms butthe basic principles of the process and the reasons whynickel is so dominant are briefly explained. Applicationsare described to demonstrate the versatility of electroform-ing in order that new applications may become apparent tothose not already familiar with the process.

Ron Parkinson



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The Process

Electroforming has been described in different ways butASTM B 832-93 describes it simply and concisely as fol-lows: “Electroforming is the production or reproduction ofarticles by electrodeposition upon a mandrel or mould thatis subsequently separated from the deposit.” It is, therefore,a method of fabricating parts that are usually free standingonce separated from the mandrel and this definition will sat-isfy all purists in the technology. However, there are nowother procedures that are often classified as electroforming.For instance, a piece of the mandrel may be intentionallybonded to or encapsulated by the electrodeposit and there-fore become an integral part of the finished product. Inaddition, it has been used to describe the joining of parts byelectrodeposition when conventional welding or brazingtechniques are not feasible. This process has also been de-scribed as ‘electrojoining’ or ‘cold welding’ and in these casesit is necessary to accept a less rigid definition than that usedby ASTM. Electrofabrication is another term often used, es-pecially in the electronics industry.

Electrodeposition

Electroforming is an electrodeposition process, similar toelectroplating and electrorefining. Therefore, the processrequires two electrodes (an anode and a cathode) immersedin a conducting electrolyte containing metallic salts and asource of DC power. As current is passed between the twoelectrodes, metallic ions in solution e.g. Ni++, are convertedinto atoms on the cathode surface and these build up layerupon layer, micron upon micron to produce a continuousdeposit. It is possible, therefore, to visualize electroformingas the ‘growing of parts by electrodeposition’ and in fact theprocess has been described as ‘ionic fabrication’. Thisconcept of growing deposits explains why electroformingcan be used to make many thin products, such as metal foils,more economically than by normal metallurgical procedures.However, the process is certainly not limited to the deposition

of thin deposits. When the required deposit thickness hasbeen obtained, the electroform is separated from the mandrel.The process is shown schematically in Figure 1.

In nickel electroforming, the two most common elec-trolytes used are nickel sulphamate solutions and Wattssolutions. In the latter, nickel sulphate and nickel chlorideprovide the nickel units. It is the versatility of these elec-trolytes and the range of physical and mechanical propertiesof the deposits obtained from them that has made nickel sodominant. Properties such as hardness, ductility, strengthand internal stress can be varied significantly by changingelectrolyte composition and operating conditions, by theuse of organic additives or by codeposition of alloyingmetals such as cobalt. In addition, high deposition ratescan be obtained with these electrolytes and this is often animportant factor in electroforming.

The two other basic components for electroforming are theanode and the cathode. The anode is typically a suitable formof nickel metal that will dissolve efficiently under the anodicconditions used and thereby replenish the nickel being takenout of solution at the cathode. The cathode is the mandrel uponwhich nickel is deposited and mandrel quality and preparationare of the utmost importance. The success of electroforminghas resulted from the exceptional accuracy that can be obtainedin reproducing the shape and surface detail of the mandrel.

Mandrels

A wide range of materials is used for producing mandrels.The range includes many metals and alloys, plastics, glass,wood and fabrics. They can be classified, therefore, as con-ductors or non-conductors of electricity and so theprocedures required to prepare them for electroforming varygreatly. Within each of these two classifications, mandrelscan be expendable or permanent.

The selection of a conducting or a non-conducting man-drel determines the procedures required to prepare it forelectroforming. Conducting mandrels are usually puremetals or alloys to which the electrodeposit can bond suf-

1) Mandrel Preparation 2) Electroforming 3) Separation

Figure 1 Schematic of an electroforming operation.

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May 96/0.0

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ficiently well to prevent premature separation but will al-low easy removal of the finished electroform. Stainless steelis frequently used because the natural surface oxide enablesthese requirements to be met. Other materials may requirethe application of a thin parting film such as an oxide orchromate to facilitate removal of the electroform. This isnot required if the mandrel is expendable i.e. if it is removedby melting or chemical dissolution, for instance.

Non-conductors have the disadvantage of having to bemade conductive to allow the electrodeposition process tobegin. This is done by applying a thin metallic film, usu-ally silver or nickel. A typical procedure is to spray thenon-conductive surface with solutions from a dual nozzlespray gun, which react “in situ” to produce a thin silverfilm by chemical reduction. Electroless nickel or sputteredmetallic films can also be used.

The choice of a permanent or an expendable mandreldepends largely on the particular electroform being pro-duced. It becomes a question of design and the number ofelectroforms required. If no re-entrant shapes or angles areinvolved, it is possible to use permanent, rigid mandrelsthat can be separated from the electroform and re-used manytimes. This is the preferred procedure in high volume pro-duction operations. However, if re-entrant shapes or anglesare involved, mandrel materials must be used that can beremoved by melting or chemical dissolution e.g. in elec-troforming nickel bellows and waveguides.

The various types of mandrel materials each have theirown advantages and disadvantages. The success of an elec-troforming operation is largely dependent on making a goodchoice of mandrel type, material and pretreatment and theimportance of this will become obvious from the range ofapplications described.

Applications

Screen ProductsThe production of mesh or screen products is a major partof the nickel electroforming industry. Applications includelarge, industrial centrifuge screens used in sugar produc-tion, small centrifuge screens used in kitchen appliances,razor foils, filters and precision sieve screens. However,the largest application is in the printing industry in whichflat and cylindrical, rotary screens are used for printing avariety of products including textiles, wallpaper and car-pets. The production of rotary printing screens is the largest,single application of nickel electroforming.

Screen patterns are usually produced using photoresistmaterials. These are applied to metallic mandrels either as adry film or as an emulsion, which is subsequently dried andhardened. A high contrast film or mask on which the requiredpattern has been produced with great precision is then placedover the photoresist. The precision and edge quality can begreatly enhanced by preparing the original artwork on a muchlarger scale than required and reducing it to final sizephotographically on the film. On exposure to an appropriatelight source, usually ultra violet (UV) light, the exposed areasof the photoresist are cured and become stable to solutionsused for developing the pattern. Unexposed areas are dissolvedduring the development stage and so using this technique,generally referred to as the ‘direct method’, a mandrel forelectroforming a simple screen product would consist of ametal substrate with a pattern of discrete islands of photoresiston the surface, corresponding to the holes in the screen. Suchmandrels have a life expectancy of only one to five cyclesafter which the pattern must be produced again.

The preparation of more permanent mandrels that cantypically be used for up to two hundred cycles is more com-plex. This is the ‘indirect method’ and also makes use ofphotoresists. The coating of the mandrel and the exposureto a suitable light source is similar to the procedure used inthe direct method, except that the film is reversed i.e., apositive rather than a negative is used. After development,therefore, the mandrel surface is masked off with photore-sist except for the hole area in the pattern. The exposedareas of metal, often copper or brass, are then etched toproduce a pit pattern corresponding to the holes in the finalscreen. The photoresist is then removed and the pitted sur-face is coated with a resin with good adhesive properties,typically a filled epoxy. The resin is ‘worked’ over the sur-face to ensure all pits are filled and is then allowed to cure.When fully cured, the resin is machined or sanded in somesuitable way depending on the shape of the part so that theonly resin remaining is in the etched areas and the remain-ing metal is exposed. The mandrel surface is now ready tobe prepared for electroforming. The ‘direct’ and ‘indirect’methods of producing screen patterns for electroformingare shown in Figure 2.

Direct Method Indirect Method

Separation

Electroforming

Development

UV Exposure

Separation

Electroforming

Development

UV Exposure

Etching

Filling

Figure 2 Direct and indirect methods of producing screenpatterns using photoresists.



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In recent years an indenting method has largely replacedthe etch and fill method, for producing the huge mandrels usedfor rotary printing screens. The method requires the cylindricalmandrel to be rotated on a lathe, while a special tool producesan indented pattern corresponding to the millions of holes inthe finished screen. At this stage, the indented mandrel is similarto the previously described etched mandrel and can be pre-pared for electroforming in the same way, i.e. coated with anon-conductive resin to fill the indentations and surfacefinished to display the metal substrate and hole pattern.

Typical rotary printing screens for textiles are 3 - 6 m (10- 20 ft) long with a diameter of approximately 20 cm (8 in).For carpets, they may be up to 2 m (7 ft) diameter. The mostfrequently used textile printing screens have a uniform meshsize, usually in the range 40 to 200 mesh. Typically hole sizeis 125µm and with 60 holes/linear cm, each screen containsmillions of holes, all of which are produced with remarkableprecision by electroforming. This type of screen is called a‘lacquer’ screen. Mandrels are rotated rapidly in a horizontalposition during electroforming. Depending on tank designthey can be totally or partially submerged in the electrolyteand a 100µm thick screen is produced in about one hour.One essential requirement of the nickel deposit is that it mustbe deposited in a state of low compressive stress. This reduces

the risk of the nickel lifting from the mandrel in the earlystages of deposition but more importantly provides a methodof separating the finished electroform. The required level ofstress is obtained by controlled addition of a stress reducersuch as saccharin to the electrolyte.

The removal of a thin, fine mesh screen from an untapered,cylindrical mandrel that may be 6 m (20 ft) long appears tobe a major problem, yet it is quite simple. After removalfrom the electroforming tank and rinsing, the cylinder isrotated along its axis and pressure is applied to the surfaceby a pad. The pad traverses the full length of the cylinderand the pressure causes release of the compressive stressand sufficient expansion of the nickel deposit to allow it tobe easily withdrawn from the mandrel. It is possible that atleast two hundred screens can be produced from a mandrelbefore any maintenance is required.

For printing, a screen is required for each colour in thepattern and so the more colours and more complex the pat-tern, the more screens are required for printing. The screensare installed in the printing machine and rotate in precisesynchronization with each other as the material to be printedpasses beneath and in contact with them. Each colour, there-fore, requires its own pattern on the screen, which consistsof an open mesh area through which ink can flow and asealed area which prevents flow. Typical lacquer screensare shown in Figure 3. The ink is forced through the openarea of the screen from the inside as it rotates and contactsthe material to be printed. As the name implies, the patternon lacquer screens is produced by lacquer applied to theelectroformed screens. The entire screen is coated with aphotoresist lacquer and the pattern produced by standardphotoresist techniques. Alternatively, the patterns can beproduced by a computer controlled laser engraving tech-nique, in which the lacquer is burned off from the requiredopen areas using carbon dioxide lasers. A series of electro-formed lacquer screens in use on a textile printing machineis shown in Figure 4.

Figure 3 Typical lacquer screens. Figure 4 Textile printing machine using electroformednickel screens.



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thickness of 3.75µm. Electronic designers and assemblers haverealised significant improvements in using electroformedstencils and these have translated into less rework, higher yieldsin assembly and increased product quality.

MouldsElectroformed moulds of all sizes are produced for manyapplications and their production now represents a largepart of the industry. Electroforming is the best way of re-producing surface detail and in some cases it is the onlyway that the necessary precision and accuracy can be pro-duced economically. Perhaps the best example of this arethe moulds or ‘stampers’ used for injection moulding ofpolycarbonate resins in the production of compact discs(CDs) or other optical read-out discs. But the applicationsof electroforming are not limited to small moulds with sub-micron replication requirements and in fact, the size of themould that can be produced is usually limited only by thesize of the electroforming tank. For instance, one-piecemoulds for entire aircraft wing sections have been producedby electroforming.

Nickel moulds are used in compression, injection, lay-up, resin transfer moulding (RTM), rotational, slush,vacuum and blow moulding of plastics for instance. Theyhave also been used for moulds in the glass, rubber andpowder metallurgy industries, for die-casting of low melt-ing point alloys and many other applications. They canprovide high strength in high pressure applications such ascompression and injection moulding and excellent corro-sion resistance when this is a factor.

Electroforming is not the answer to all mould makingproblems. However, it should be considered by designerswhen there is a requirement for:

• Extreme accuracy of surface detail;• Multiple impression dies in a single mould;• More than one mould;• Good wear and corrosion resistance;• A complex die cavity.

Many other screen products are produced on mandrelsprepared using photoresist techniques. For instance, elec-tric razor foils are electroformed on flat stainless steel,nickel or brass mandrels that produce up to one thousandfoils per cycle. The pattern ensures that the foils are at-tached to each other so that the whole sheet can be removedtogether and individual foils can then be separated in muchthe same way as postage stamps. The same technique isused for segments of permanent coffee filters and othersmall screen products. At a typical current density of 8A/dm2, the required thickness of 50µm for razor foils is ob-tained in about thirty minutes.

The precision and reproducibility attainable by nickelelectroforming cannot be matched by other screen produc-tion methods. For instance, nickel sieves are produced withhole sizes as small as 3µm and a thickness of 25µm. Inaddition to the advantages of reproducibility, electroformedsieves have extremely sharp edge definition without burrsand unlike woven screens they present no opportunity formaterial entrapment. A comparison of electroformed andwoven fine screens is shown in Figure 5.

Electroformed nickel screen products also find use astemplates and masks in critical situations where it is essen-tial to obtain uniform application of coating materials. Inthis area, nickel electroforming has found a growing appli-cation in the electronics industry as a method of producingscreens or stencils for fine pitch surface mount technology.The application is an excellent example of the many waysin which electroforming screen products can solve designproblems, especially when precision and consistency aremajor factors.

The trend in the electronics industry has been very stronglytowards more complex devices and smaller circuits. This meansdesigners have greatly reduced the area available for leads assmaller pin connections and tighter spacing are required.Devices with over a hundred leads with sizes as small as 175µmand separated by only 325µm are not uncommon. The preciseapplication of solder paste to these lead areas is a key factor indetermining the extent to which thesize of these devices can bereduced. It is imperative that theamount of solder at each locationor pad is the same and that it isinsufficient to cause shorting be-tween leads. Electroformed nickelmasks have solved this challengeand demonstrated their superiorityover competitive methods such aschemical etching, electropolishingand laser cutting. Typically, stencilshave thicknesses up to 300µm butelectroforming places no lowerlimit on thickness and screens with400 lines/cm can be produced at a

Figure 5 A comparison of finescreens, electroformed(above) and woven (right).



A typical procedure for producing a mandrel for an elec-troformed mould begins with a model of the finishedproduct. The electroformed mould can only be as good asthe model or pattern from which it is produced. In somecases the original model is made from wood and surfacedetail is produced on it. For instance, if a rotational mouldfor producing vinyl skins with a leather texture and ap-pearance is required, a selected piece of leather will bebonded to the model. Similarly if a particular wood grainis required as in the production of hardboard, the modelsurface is a carefully selected piece of wood with the re-quired grain.

The original model is rarely used to produce the elec-troform. Instead, the surfaces are sealed if porous, and aplastic mould is made to replicate in reverse the shape andsurface texture of the model. The mould would typicallybe a fibre-glass reinforced thermoset plastic or an RTV sili-cone rubber. From this part, another similar plastic castingis made, which should now be a perfect replica of the origi-nal model. This will be used as the mandrel on which theelectroformed moulds will be made.

This is the traditional method of producing mandrelsfor mould production but there are others. For instance,

when hundreds of moulds of simple shape are required,permanent stainless steel mandrels could possibly be ma-chined. Or in the production of CDs, where extremely finesurface detail must be perfectly replicated, very specialprocedures are required, as described later.

Moulds are usually electroformed in nickel sulphamatesolutions at low current densities, especially large, com-plex moulds. The usual objectives are to obtain zero or lowtensile stress in the deposit and to optimize current distri-bution and therefore thickness uniformity. It is importantthat low stress, typically less than 35MN/m2, be obtainedto ensure that the nickel deposit does not separate from themandrel in the early stages. Separation not only destroysthe electroform but can create huge costs in wasted timeand materials. For instance, if large moulds are involvedthat must be built up to a thickness of 1.5 cm (0.6 in), it ispossible that the loss in time could be as much as four weeksand the metal loss as much as two tonnes.

Electroformed nickel moulds are used extensively in manydifferent industries. The automotive and aircraft/aerospaceindustries not only create a high demand for electroformedmoulds, but their requirements often include some of thelargest moulds produced anywhere. For instance, they havebeen used for producing radomes, huge jet engine housings,wing and body fairings and the truly impressive mould forproducing the upper and lower graphite/epoxy wing skinsfor the Harrier jump-jet. These 8.5 m (28 ft) long moulds,shown in Figure 6, were electroformed to a thickness ofapproximately 5 mm (0.2 in) and enabled the productionrate of the composite skins to be increased by 30% over thatobtained with machined steel moulds. This application is anexcellent example of large scale electroforming that engineersand designers should consider when reviewing the process.

In the automotive industry, electroformed moulds areused for producing structural and decorative components.For instance, truck cabs, body panels and protective sidestrip moulding are all made in electroformed nickel moulds.A large nickel mould for producing reinforced plastic bodycomponents for the automotive industry is shown inFigure 7. Interior applications are mostly for nickelrotational moulds, which are used for producing vinyl skinsfor dashboards, door panels etc. One of the very attractivefeatures of these moulds is that the surface texture of themandrel is perfectly replicated in reverse on the electro-form. This enables any surface texture, such as leather, tobe produced on the vinyl skin.

Electroformed moulds also enable many home productsto be produced economically. For example, they are usedto produce plastic bathtubs, sinks and shower enclosures.A major home application is in producing moulds for pan-elling and exterior siding. Hardboard used for siding isactual wood that has been processed to greatly expand thefibres and then compression moulded under high pressure.The result is an extremely strong, durable product with 50%

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Figure 6 Electroformed nickel mould for wing skins forHarrier Jump-jet.

Figure 7 Nickel mould for automotive body component.



higher density than that of the original wood. By installingelectroformed moulds on the platens of the press, their sur-face texture is produced in the hardboard. Therefore, fromany selected natural wood surface, the texture and graincan be reproduced by electroforming and transferred to thehardboard during compression. Moulds may be as large as7.5 x 2.5 m (25 x 8 ft) and the nickel thickness is typically6.3 to 12.7 mm (0.2 to 0.5 in). A typical mould still at-tached to the mandrel is shown in Figure 8.

Applications in recreational products include moulds fora variety of watercraft such as canoes, paddle boats, andsurfboards. Many injection moulds for contemporary golfclub grips, where texture and feel are so important, are alsoelectroformed nickel. These tubular moulds are producedon expendable plastic or aluminum mandrels. Althoughmost applications for electroformed nickel moulds are inthe plastics industry, nickel/cobalt moulds have been usedvery successfully in the glass and zinc diecasting industries.

Two of the best examples of the ability to perfectlyreplicate surface detail in moulds are found in productswhich we encounter every day, i.e. optical read-out discs,such as CDs, and security holograms on credit cards.Neither would be available to us without electroforming.

The moulds or ‘stampers’ used for producing audiorecords have been made by electroforming since about1930. Over many years great process improvements oc-curred that provided the high quality sound available fromLPs. However, the development of compact discs requiredtremendous additional refinements to be made that haveresulted in the exceptional sound quality that we now ex-perience. The need for refinement is evident by comparingLP and CD surfaces.

An LP surface has a continuous spiral groove, typically50µm deep and 50µm wide with a spacing of up to 100µmbetween the grooves. CD surfaces have a spiral track of smallpits that are typically 0.4µm wide and 0.12µm deep, withabout a 1.6µm spacing between the pits. These sub-micron

pits are shown in Figure 9 and there are approximately 30billion of them on a CD, all of which must be perfectlyreproduced. Refinements for CD stamper production includethe need for clean-room conditions, sealed electroformingtanks and greater control of the nickel deposit properties suchas thickness and stress. In addition, the starting surface forthe two types of records are very different. For LPs theoriginal sound is transcribed onto a nitrocellulose lacqueron an aluminum disc and for CDs it is transcribed by laseronto a photoresist film on a polished, optically flat glass disc.For both, it is then necessary to reproduce these surfacesthrough three electroforming stages to obtain the stampersfor compression moulding of LPs or injection moulding ofCDs. The original transcribed surface is replicated in reversein the first electroforming stage. This produces one ‘master’and this surface now becomes the mandrel for the secondstage. From each “master”, approximately ten electroformsare produced. These are referred to as ‘mothers’ and similarly,several ‘stampers’ are electroformed from each ‘mother’. Inthis way, a sufficient number of identical stampers is producedto operate the many moulding machines required to meetproduction requirements. In CD production the electroform-ing stages are fully programmed to control nickel propertiessuch as stress and thickness. High deposition rates areobtained at current densities of up to 30A/dm2 so thatstampers with a thickness of 295±5µm are electroformed inabout 45 minutes. The reliability of electroforming inreproducing such great detail through three stages of repli-cation has enabled the CD industry to grow to the point wheremany plants around the world routinely produce over 100million CDs per year.

Electroforming has also played a key role in the emer-gence of holography into every day use. Thethree-dimensional holographic imaging on credit cards isproduced by embossing aluminized plastic film with anelectroformed nickel ‘stamper’ or embossing tool. Thesetools are obtained by three electroforming stages similar to

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Figure 8 Large nickel mould for compression moulding ofhardboard panels.

Figure 9 Submicron pits on CD surface produce highquality sound. Perfect replication is dependent onnickel electroforming.



those used for CD production and the pattern consists of aseries of very fine lines, as shown in Figure 10, varying inwidth and depth with both dimensions less than 1µm. Elec-troforming is the only economically acceptable method ofproducing embossing tools for security holograms.

Electroformed nickel bellows also provide an excellentexample of the precision and quality afforded by this pro-cess. Electroformed bellows are made in a range of sizesand have a variety of applications, e.g. for metallic her-metic seals, volume compensators, pressure andtemperature sensors and flexible connectors. Some verysmall bellows are shown in Figure 11. One of the majorapplications is in flexible electromagnetic interference(EMI) shielding in aircraft and satellites. Quality is suchthat extremely small, electroformed bellows with a wallthickness of only 25µm can be joined to other componentsby various welding techniques to produce gas-tight as-semblies with leak rates of less than 1x10-9ml/sec. Thefact that the physical properties of electroformed nickelcan be controlled by the deposition conditions is againmost advantageous. Aluminum is usually used as anexpendable mandrel as it can easily be machined to theconvoluted configurations and sizes required for thisapplication and then dissolved out of the electroform insodium hydroxide.

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Figure 10 Electroformed nickel embossing tools forholograms have a pattern of submicron grooves.

Aircraft/Aerospace ApplicationsThere are many impressive applications for electroform-ing in these industries other than the production of moulds.The process is widely used to produce nickel or nickel al-loy components directly. These include lightweight,precision parts such as waveguides and antennae, erosionshields for helicopter and aircraft engine blades, bellowsand heat shields for aerospace and missile applications, fuelliners and manifolds for propeller driven and jet poweredaircraft and regeneratively cooled thrust chambers for rocketengines.

Electroforming is the only practical way to produce thetwists and transitions that are characteristic of manywaveguides, while maintaining the internal dimensionalprecision and surface finish. For complex waveguides, ex-pendable mandrels such as aluminum are frequently used.Aluminum can be prepared with a 3 to 5 microinch finishthat is often required to be reproduced on the inside ofwaveguides. If the waveguide shape is simple enough toallow for a mandrel to be withdrawn from the electroform,a permanent stainless steel mandrel might be used.

Nickel electroforming has long been used as a methodof producing erosion shields for components such as he-licopter blades. By electroforming, it is possible toreproduce the precise airfoil contour and to control currentdistribution so that the thickest deposit is at the leadingedge where erosion problems are most serious. In this typeof application, it has generally been shown to beadvantageous to deposit the nickel in a state of compres-sive stress as this has a positive effect on fatigue resistanceby inhibiting crack propagation.

Figure 11 Precision bellows of all sizes can be produced bynickel electroforming.

Designers in the aerospace industry have workedclosely with electroformers to resolve some difficultfabrication problems. An impressive example of this is inthe use of electroforming to create internal coolingpassages in hardware of complex shape. The techniquehas been successfully used in Europe and North Americafor producing regeneratively-cooled thrust chambers andnozzle extensions for advanced rocket engines in whichthe weight of the chamber per unit of power produced isof critical importance. In these assemblies, it is normalfor the fuel to flow through channels in the chamber wallto lower the operating temperature of the liner for longerthermal fatigue life, while the fuel itself is preheated,before being injected into the combustion section. Allmaterial properties are critical, particularly high and lowtemperature strength and heat conductivity. Electroform-



Figure 14 A machinedelectroformedthrust chamberfor the Arianelauncher.

ing with nickel has provided satisfactory properties andlightweight. Typical applications have been in the thirdstage of the Ariane space launcher and the main engine ofthe NASA Space Shuttle.

To provide a brief description of the process does notdo justice to the technology and creativity involved inthis development but the general procedures are asfollows. The inner part of the chamber, or liner, has beenmade from stainless steel, nickel, copper or speciallydeveloped alloys and is normally produced by spinningor machining. Alternatively, electroformed nickel orcopper have been used. Cooling channels are cut alongalmost the entire length by electrical dischargemachining or milling to a depth that leaves only about0.9 mm (0.04 in) of stock between the bottom of thechannel and the inner surface of the chamber. Thecomponent, therefore, has little strength prior to thenickel electroforming stage. The channels are then filledwith wax, which is preferably loaded with filler to reduceshrinkage and provide good sanding properties. Prior to

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electroforming with nickel, the wax must be madeconductive by burnishing with silver and the metal ribsmust be activated to obtain the best possible bonding ofthe nickel. Nickel sulphamate electrolyte is used withvery low chloride content and deposit thickness may beas great as 1.2 cm (0.5 in). The electroformed nickelshell not only closes out the passages and provides therequired strength but when necessary coolant feed ports,for instance, can be grown into it. The sequence ofoperations is shown schematically in Figure 12 and themachined channels in the liner can be seen in Figure 13.

After electroforming, the exterior part must bemachined to drawing requirements and the wax removedby melting and flushing with hot solvent. Since electro-deposition provides a method of obtaining metallurgicalbonding at near ambient temperatures, unlike thermalfusion techniques such as welding or brazing, the depositproperties remain excellent. Induced bond failure may beexpected to occur in the weaker of the joined metals.Typical room temperature and cryogenic properties ofnickel deposits for this application are as follows.

20°C -196°CUltimate Strength (MN/m2) 180 1000Yield Strength (MN/m2) 510 615Elongation (% in 5cm) 12 21

A machined electroformed thrust chamber for the Arianelauncher is shown in Figure 14.

Other ApplicationsThere are many otherinteresting applications fornickel in electroforming. Forinstance, there are obviousadvantages in producingnickel foil by electroformingrather than by standardmetallurgical processing andthe thinner the foil thegreater the advantage. Foil isproduced on titanium cylin-drical mandrels, rotatingwith their axes in a horizontalposition, while partiallysubmerged in a nickel sulph-amate electrolyte. The foil ispeeled from the mandrel asit exits the electrolyte anddeposition restarts at the re-entry point. Deposition iscontinuous, therefore, andfoil thickness is controlled bythe diameter and rotational

Figure 12 Stages in the electroforming of internalpassages.

Figure 13 Channelsmachined inthe metal linerof a rocketthrustchamber, forthe NASASpace Suttle.



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speed of the mandrel and the current density. The electro-forming of nickel foil is shown in Figure 15. Nickel foilis produced at typical thicknesses of 6 to 200µm andcurrent densities as high as 45A/dm2, corresponding todeposition rates of up to 0.5 mm (0.02 in) per hour.Applications for nickel foil include solar energyabsorbers, high temperature gaskets, pressure controlmembranes and fire resistant barriers.

refrigeration equipment and for electromagnetic shielding butthe major application is now in rechargeable battery systems,such as nickel-cadmium and nickel-metal hydride batteries.The nickel foam electrode is an alternative to the sinteredelectrode and foam is now produced in continuous lengths atseveral plants around the world.

Basically, production is very simple. The mandrel materialis polyurethane foam with controlled porosity, onto whichnickel is electroformed and the organic material is subsequentlyburned out. However, practically, the process is more complex.Practical and economic considerations demand that the foambe produced on a continuous basis, which means that rolls offoam mandrel material are unwound and pass through theprocessing tanks under automatically controlled conditions.Initially, the expendable foam mandrel must be uniformlycoated with an electrically conductive layer and both graphiteand electroless nickel have been used. Nickel sulphamate elec-trolytes are normally used under standard conditions to producelow stress. Current density is maintained at low levels initially,typically 2-5A/dm2, but may be increased to levels as high as10A/dm2 as the thickness of the nickel deposit increases.Following the electroforming stage, pyrolysis is used to removethe polyurethane foam. Obtaining maximum porosity, whilemaintaining adequate strength and ductility in the nickel foamis a major requirement of this innovative application for nickelelectroforming. The skeletal structure and high porosity ofthe foam is shown in Figure 16.

The LIGA ProcessRecently much work has been done in developing a pro-cess for electroforming micro components, much smallerthan could be imagined before this era of miniaturization.This is the LIGA process originating in Germany with thename being derived from the three production stages -lithographie (lithography), galvanoformung (electroform-ing), and abformung (moulding). The process was originally

Figure 15 Electroforming of nickel foil.

Another example of the use of rotating mandrels is inthe production of seamless nickel photoreceptor beltsfor high speed copying machines. This application hasnow been phased out due to changes in technology buttestament to the success of this application is the factthat millions of belts have been produced over the lasttwenty years, typically at a rate of about 600,000 peryear. In this process, permanent chromium platedaluminum mandrels were used, rotating on a vertical axisand at current densities of 30A/dm2, the 125µm thickbelts were produced in only eighteen minutes. Inremoving the electroforms from the mandrels, advantagewas taken of the difference in thermal expansion betweennickel and aluminum.

The intricate designs produced by master engravers for banknotes are perfectly reproduced through several electroform-ing stages before the final nickel printing plate is installed onthe press. The nickel, deposited at quite low current densities,is often hardened by the use of organic additives to the nickelsulphamate electrolytes. Most currency around the world isproduced with nickel printing plates. In the U.S., for instanceabout four thousand plates are electroformed each year forprinting over six billion notes.

In the past few years, the production of nickel foam byelectroforming has been of great interest and demand hasgrown enormously. It has been widely used for many yearsfor filters, as a carrier for catalysts, for oil-mist separators inFigure 16 The skeletal structure of electroformed nickel foam.



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developed for electroforming small moulds for plastic partsbut is now also used for the direct production of micrometallic components. The great interest in the LIGA processhas developed from the recognition that electroforms canbe made with very high aspect ratios (height to width ra-tios) and with vertical walls. For instance, parts with a widthof only several microns can be electroformed to a height of3 mm (0.1 in) while maintaining parallel, vertical wallsand great precision.

Basically the process for producing mandrels is similarto the photoresist method described for screen productionbut materials and exposure conditions are markedly differ-ent. PMMA (polymethylmethacrylate) replaces thephotoresist, X-rays replace the UV light and a thin goldmask replaces the photographic film. Typically the processbegins with the application of a layer of PMMA to ametallic substrate. Layer thickness is dependent on theheight requirements of the finished part and the energy ofthe synchroton X-ray radiation source. The mask typicallyconsists of a silicon or beryllium wafer on which the pat-tern is produced in gold, 30-35µm thick. The wafer materialsmust be invisible to X-rays, while the gold acts as anabsorber to block them. After exposure the PMMA is de-veloped and the irradiated areas are dissolved away, downto the metallic substrate. Electrodeposition can then beginon the exposed areas of the substrate to produce a part withparallel, vertical walls, defined by the PMMA. The maxi-mum thickness, or height, of the electroform is thereforedetermined by the thickness of the PMMA.

This is a very simplistic view of the LIGA process andmany technical challenges have to be met in each applica-tion. For instance, the development of say, a 25µm featurethrough a PMMA thickness of 3 mm (0.1 in) and the sub-

sequent deposition of nickel in such a narrow slot presentenormous solution transfer problems. Nevertheless, thechallenges are being met and many micro components havebeen produced for a wide range of industries. For instance,applications are to be found in microfiltration systems,micro optics, ultrasonic sensors, fuel injection and in jetprinter nozzles, micro actuators and micro motors. Spe-cific examples include precision gear wheels smaller thanthe eye of a needle and a micro turbine/cutting tool de-signed to pass along diseased arteries to removeaccumulated plaque. Progress in this technology has beenpromoted by the enormous interest in miniaturization andthe number of applications is expected to increase. A se-lection of small components produced by LIGA technologyis shown in Figure 17.

Figure 17 Gears and spacers for micro-magnetic motorapplications.

Summary

This general review of electroforming has been preparedto introduce the subject to those who are not familiar withthe capabilities of the process. It was not the intention todescribe the details of the process such as mandrel selec-tion, design and preparation or electrolyte composition andoperating conditions or separation techniques for remov-ing the electroform but rather to bring forth the importanceof these parameters with reference to many successful elec-troforming applications.

Electroforming is a unique metal fabrication process thatis indispensable in producing many items with which wehave daily contact. It is also sufficiently versatile that it isused successfully to produce huge components for the aero-space and automotive industries and yet is able to perfectlyreplicate surfaces with submicron size features. It is used

to produce many types of screen products and moulds ofall sizes. It is the only practical method of producing manycomplex forms such as waveguides and bellows and therange of applications continuous to expand.

Electroforming is an electrodeposition process in whichnickel is the material of choice for most applications.Nickel can be deposited with a wide range of mechanicalproperties by controlling the deposition conditions, by theuse of electrolyte additives or by alloying. Nickelelectrolytes have the advantages of being easy to controland they are capable of fast deposition rates. Nickel andits alloys will continue to dominate this industry and byincreasing the awareness of the electroforming processamongst designers, engineers and metallurgists, continuedgrowth should be assured.

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The NickelDevelopmentInstitute isan internationalnonprofitorganizationserving the needs ofpeople interestedin the application ofnickel andnickel-containingmaterials.

Members of NiDICompanhia Níquel TocantinsEmpresa de Desenvolvimento de Recursos Minerals “CODEMIN” S.A.Falconbridge LimitedInco LimitedMorro do Níquel S.A.Nippon Yakin Kogyo Co., Ltd.Outokumpu OyP.T. International Nickel IndonesiaPacific Metals Co., Ltd.QNI LimitedSherritt International Corporation Inc.Shimura Kako Company, Ltd.Sumitomo Metal Mining Co., Ltd.Tokyo Nickel Company Ltd.WMC Limited

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DateOct 98/5.0

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