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The main objective is to analyze the mechanical features of electroformed nickel shells, studying different aspects related to theirmetallographic structure, hardness, internal stresses and possible failures, by relating these features to the parameters of production of the shellswith an electroforming equipment. Finally a core was tested in an injection mold. 2006 Elsevier B.V. All rights reserved.Keywords: Electroplating; Electroforming; Microstructure; Nickel1. IntroductionOne of the most important challenges with which modernindustry comes across is to offer the consumer better productswith outstanding variety and time variability (new designs). Forthis reason, modern industry must be more and more compet-itive and it has to produce with acceptable costs. There is nodoubt that combining the time variable and the quality vari-able is not easy because they frequently condition one another;the technological advances in the productive systems are goingto permit that combination to be more efficient and feasiblein a way that, for example, if it is observed the evolution ofthe systems and techniques of plastics injection, we arrive atthe conclusion that, in fact, it takes less and less time to puta new product on the market and with higher levels of qual-ity. The manufacturing technology of rapid tooling is, in thisfield, one of those technological advances that makes possiblethe improvements in the processes of designing and manufac-turing injected parts. Rapid tooling techniques are basicallycomposed of a collection of procedures that are going to allowus to obtain a mold of plastic parts, in small or medium series,in a short period of time and with acceptable accuracy levels.Their application is not only included in the field of makingE-mail address: mmonzondim.ulpgc.es (M.D. Monzon).plastic injected pieces 13, however, it is true that it is wherethey have developed more and where they find the highestoutput.This paper is included within a wider research line where itattempts to study, define, analyze, test and propose, at an indus-trial level, the possibility of creating cores for injection moldsstarting from obtaining electroformed nickel shells, taking asan initial model a prototype made in a FDM rapid prototypingequipment.It also would have to say beforehand that the electro-forming technique is not something new because its appli-cations in the industry are countless 3, but this researchwork has tried to investigate to what extent and under whichparameters the use of this technique in the production ofrapid molds is technically feasible. All made in an accu-rate and systematized way of use and proposing a workingmethod.2. Manufacturing process of an injection moldThe core is formed by a thin nickel shell that is obtainedthrough the electroforming process, and that is filled with anepoxic resin with metallic charge during the integration in thecore plate 4 This mold (Fig. 1) permits the direct manufactur-ing by injection of a type a multiple use specimen, as they are0924-0136/$ see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2006.04.003274M.D. Monzon et al. / Journal of Materials Processing Technology 176 (2006) 273277Fig. 1. Manufactured injection mold with electroformed core.defined by the UNE-EN ISO 3167 standard. The purpose of thisspecimen is to determine the mechanical properties of a collec-tion of materials representative industry, injected in these toolsanditscoMParisonwiththepropertiesobtainedbyconventionaltools.Thestagestoobtainacore4,accordingtothemethodologyresearched in this work, are the following:(a) Design in CAD system of the desired object.(b) Model manufacturing in a rapid prototyping equipment(FDM system). The material used will be an ABS plastic.(c) Manufacturing of a nickel electroformed shell starting fromthe previous model that has been coated with a conductivepaint beforehand (it must have electrical conductivity).(d) Removal of the shell from the model.(e) Production of the core by filling the back of the shell withepoxy resin resistant to high temperatures and with therefrigerating ducts made with copper tubes.Theinjectionmoldhadtwocavities,oneofthemwastheelectro-formed core and the other was directly machined in the movingplaten.Thus,itwasobtained,withthesametoolandinthesameprocess conditions, to inject simultaneously two specimens incavities manufactured with different technologies.3. Obtaining an electroformed shell: the equipmentElectrodeposition 5,6 is an electrochemical process inwhich a chemical change has its origin within an electrolytewhen passing an electric current through it. The electrolyticbath is formed by metal salts with two submerged electrodes,an anode (nickel) and a cathode (model), through which it ismade to pass an intensity coming from a DC current. Whenthe current flows through the circuit, the metal ions present inthe solution are transformed into atoms that are settled on thecathode creating a more or less uniform deposit layer.The plating bath used in this work is formed by nickelsulfamate 7,8 at a concentration of 400ml/l, nickel chloride(10g/l), boric acid (50g/l), Allbrite SLA (30cc/l) and Allbrite703 (2cc/l). The selection of this composition is mainly dueto the type of application we intend, that is to say, injectionmolds, even when the injection is made with fibreglass. Nickelsulfamate allows us to obtain an acceptable level of internalstresses in the shell (the tests gave results, for different processconditions, not superior to 50MPa and for optimum conditionsaround 2MPa). Nevertheless, such level of internal pressure isalso a consequence of using as an additive Allbrite SLA, whichis a stress reducer constituted by derivatives of toluenesulfon-amide and by formaldehyde in aqueous solution. Such additivealso favours the increase of the resistance of the shell whenpermitting a smaller grain. Allbrite 703 is an aqueous solutionof biodegradable surface-acting agents that has been utilizedto reduce the risk of pitting. Nickel chloride, in spite of beingharmfulfortheinternalstresses,isaddedtoenhancetheconduc-tivity of the solution and to favour the uniformity in the metallicdistribution in the cathode. The boric acid acts as a pH buffer.The equipment used to manufacture the nickel shells testedhas been as follows: Polypropylene tank: 600mm400mm500mm in size. Three teflon resistors, each one with 800W. Mechanical stirring system of the cathode. System for recirculation and filtration of the bath formed bya pump and a polypropylene filter. Chargingrectifier.Maximumintensityincontinuous50Aandcontinuous current voltage between 0 and 16V. Titanium basket with nickel anodes (Inco S-Rounds Elec-trolytic Nickel) with a purity of 99%. Gases aspiration system.Oncethebathhasbeendefined,theoperativeparametersthathave been altered for testing different conditions of the processhave been the current density (between 1 and 22A/dm2), thetemperature (between 35 and 55C) and the pH, partially mod-ifying the bath composition.4. Obtained hardnessOne of the most interesting conclusions obtained during thetests has been that the level of hardness of the different electro-formed shells has remained at rather high and stable values. InFig. 2, it can be observed the way in which for current densityvalues between 2.5 and 22A/dm2, the hardness values rangefrom 540 and 580HV, at pH 40.2 and with a temperatureof 45C. If the pH of the bath is reduced at 3.5 and the tem-perature is 55C those values are above 520HV and below560HV. This feature makes the tested bath different from otherconventional ones composed by nickel sulfamate, allowing tooperate with a wider range of values; nevertheless, such opera-Fig. 2. Hardness variation with current density. pH 40.2, T=45C.M.D. Monzon et al. / Journal of Materials Processing Technology 176 (2006) 273277275tivitywillbelimiteddependingonotherfactors,suchasinternalstress because its variability may condition the work at cer-tain values of pH, current density or temperature. On the otherhand, the hardness of a conventional sulfamate bath is between200250HV, much lower than the one obtained in the tests. Itis necessary to take into account that, for an injection mold,the hardness is acceptable starting from 300HV. Among themost usual materials for injection molds it is possible to findsteel for improvement (290HV), steel for integral hardening(520595HV), casehardened steel (760800HV), etc., in sucha way that it can be observed that the hardness levels of thenickel shells would be within the mediumhigh range of thematerials for injection molds. The objection to the low ductilityof the shell is compensated in such a way with the epoxy resinfilling that would follow it because this is the one responsiblefor holding inwardly the pressure charges of the processes ofplastics injection; this is the reason why it is necessary for theshell to have a thickness as homogeneous as possible (above aminimum value) and with absence of important failures such aspitting.5. Metallographic structureInordertoanalyzethemetallographicstructure,thevaluesofcurrentdensityandtemperatureweremainlymodified.Thesam-ples were analyzed in frontal section and in transversal section(perpendicular to the deposition). For achieving a convenientpreparation, they were conveniently encapsulated in resin, pol-ished and etched in different stages with a mixture of aceticacid and nitric acid. The etches are carried out at intervals of15, 25, 40 and 50s, after being polished again, in order to beobserved afterwards in a metallographic microscope OlympusPME3-ADL 3.3/10.Before going on to comment the photographs shown in thisarticle,itisnecessarytosaythatthemodelsusedtomanufacturetheshellsweremadeinaFDMrapidprototypingmachinewherethe molten plastic material (ABS), that later solidifies, is settledlayer by layer. In each layer, the extruder die leaves a threadapproximately 0.15mm in diameter which is compacted hori-zontal and vertically with the thread settled inmediately after.Thus, in the surface it can be observed thin lines that indicatethe roads followed by the head of the machine. These lines aregoingtoactasareferencetoindicatethereproducibilitylevelofthe nickel settled. The reproducibility of the model is going tobe a fundamental element to evaluate a basic aspect of injectionmolds: the surface texture.The tested series are indicated in Table 1.Table 1Tested seriesSeriespHTemperature (C)Current density (A/dm2)14.2 0.2552.2223.9 0.2455.5634.0 0.24510.0044.0 0.24522.22Fig. 3. Series 1 (150), etch 1.Fig. 4. Series 2 (300), etch 2.Fig. 3 illustrates the surface of a sample of the series after thefirst etch. It shows the roads originated by the FDM machine,that is to say that there is a good reproducibility. It cannot bestill noticed the rounded grain structure. In Fig. 4, series 2, aftera second etch, it can be observed a line of the road in a wayless clear than in the previous case. In Fig. 5, series 3 and 2etch it begins to appear the rounded grain structure although itis very difficult to check the roads at this time. Besides, the mostdarkened areas indicate the presence of pitting by inadequateconditions of process and bath composition.Fig. 5. Series 3 (300), etch 2.276M.D. Monzon et al. / Journal of Materials Processing Technology 176 (2006) 273277Fig. 6. Plane transversal of series 2 (600), etch 2.Thisbehaviorindicatesthat,workingatalowcurrentdensityand a high temperature, shells with a good reproducibility of themodel and with a small grain size are obtained, that is, adequatefor the required application.If the analysis is carried out in a plane transversal to thedeposition, it can be tested in all the samples and for all the con-ditionsthatthegrowthstructureofthedepositislaminar(Fig.6),what is very satisfactory to obtain a high mechanical resistancealthough at the expense of a low ductibility. This quality is due,above all, to the presence of the additives used because a nickelsulfamate bath without additives normally creates a fibrous andnon-laminar structure 9. The modification until a nearly nullvalue of the wetting agent gave as a result that the laminar struc-ture was maintained in any case, that matter demonstrated thatthedeterminantforsuchstructurewasthestressreducer(AllbriteSLA).Ontheotherhand,itwasalsotestedthatthelaminarstruc-ture varies according to the thickness of the layer in terms of thecurrent density.6. Internal stressesOne of the main characteristic that a shell should have for itsapplicationlikeaninsertistohavealowlevelofinternalstresses.Different tests at different bath temperatures and current densi-ties were done and a measure system rested on cathode flexuraltensiometer method was used. A steel testing control was usedwith a side fixed and the other free (160mm length, 12.7mmwidthandthickness0.3mm).Becausethemetallicdepositionisonly in one side the testing control has a mechanical strain (ten-sile or compressive stress) that allows to calculate the internalstresses. Stoney model 10 was applied and was supposed thatnickelsubstratumthicknessisenoughsmall(3?m)toinfluence,in an elastic point of view, to the strained steel part. In all thetested cases the most value of internal stress was under 50MPafor extreme conditions and 2MPa for optimal conditions, anacceptable value for the required application. The conclusionis that the electrolitic bath allows to work at different condi-tions and parameters without a significant variation of internalstresses.Fig. 7. Analysis by photoelasticity of injected specimens.7. Test of the injection moldTests have been carried out with various representative ther-moplastic materials such as PP, PA, HDPE and PC, and ithas been analysed the properties of the injected parts such asdimensions, weight, resistance, rigidity and ductility. Mechani-calpropertiesweretestedbytensiledestructivetestsandanalysisbyphotoelasticity.About500injectionswerecarriedoutonthiscore, remaining under conditions of withstanding many more.In general terms, important differences were not noticedbetween the behavior of the specimens obtained in the core andthe ones from the machined cavity, for the set of the analysedmaterials. However in the analysis by photoelasticiy (Fig. 7)it was noticed a different tensional state between both typesof specimens, basically due to differences in the heat transfer-enceandrigidityoftherespectivemoldcavities.Thisdifferenceexplainstheductilityvariationsmoreoutstandinginthepartiallycrystalline materials such as HDPE and PA 6.For the case of HDPE in all the analysed tested tubes it wasnoticed a lower ductility in the specimens obtained in the nickelcore, quantified about 30%. In the case of PA 6 this value wasaround 50%.8. ConclusionsAfter consecutive tests and in different conditions it has beencheckedthatthenickelsulfamatebath,withtheutilizedadditiveshas allowed to obtain nickel shells with some mechanical prop-erties acceptable for the required application, injection molds,that is to say, good reproducibility, high level of hardness andgood mechanical resistance in terms of the resultant laminarstructure. The mechanical deficiencies of the nickel shell willbe partially replaced by the epoxy resin that finishes shaping thecore for the injection mold, allowing to inject medium series ofplastic parts with acceptable quality levels.References1 A.E.W. Rennie, C.E. Bocking, G.R. Bennet, Electroforming of rapid pro-totyping mandrels for electro discharge machining electrodes, J. Mater.Process. Technol. 110 (2001) 186196.2 P.K.D.V. Yarlagadda, I.P. Ilyas, P. Chrstodoulou, Development of rapidtooling for sheet metal drawing using nickel electroforming and stereolithography processes, J. Mater. Process. Technol. 111 (2001) 286294.M.D. Monzon et al. / Journal of Materials Processing Technology 176 (2006) 2732772773 J. Hart, A. Watson, Electroforming: A largely unrecognised but expand-ing vital industry, Interfinish 96, 14 World Congress, Birmingham, UK,1996.4 M. Monz on, et al., Aplicaci on del electroconformado en la fabricaci onr apida de moldes de inyecci on, Revista de Pl asticos Modernos. 84 (2002)557.5 L.F. Hamilton, et al., C alculos de Qu mica Anal tica, McGraw Hill,1989.6 E. Julve, Electrodeposici on de metales, 2000 (E.J.S.).7 A. Watson, Nickel Sulphamate Solutions, Nickel Development Institute,1989.8 A. Watson, Additions to Sulphamate Nickel Solutions, Nickel Develop-ment Institute, 1989.9 J. Dini, Electrodeposition Materials Science of Coating and Substrates,Noyes Publications, 1993.10 J.W. Judy, Magnetic microactuators with polysilicon flexures, MastersReport, Department of EECS, University of California, Berkeley, 1994.www.bsac.eecs.berkeley.edu/arhive/masters/jjudy/chapter3.pdf (cap?. 3).