AbstractMany shaping technologies are con-sidered for the production ofadvanced ceramics, like cold and hotisostatic pressing, uniaxial pressing,injection moulding, pressure casting,tape casting, etc. This paper presentsa comparison of properties ofadvanced ceramics (using alumina asmaterial for the comparison tests)formed by pressure casting, uniaxialhydraulic pressing and tape castingand gives some indication for properselection of the most suitable shapingtechnique regarding requirementsconcerning dimensions and shape of products, product quality,throughput capacity, and economicaspects. Adequate firing technologiesare also discussed. The results canhelp to design new plants as well asto optimize existing production facil-ities.

1 IntroductionFor manufacturing of advancedceramics or technical ceramics manydifferent shaping technologies can beused (in the context of this paper"advanced” or “technical” ceramicswill be referred to as ceramics pro-duced mainly from well defined rawmaterials, often synthetic powders,rather than “classical” ceramics main-ly made from natural raw materials).Such shaping technologies can bepressing (e.g. cold and hot isostaticpressing, uniaxial pressing), casting(pressureless casting, pressure cast-ing, tape casting), extrusion, injec-tion moulding, up to high sophisti-cated processes like rapid prototyp-ing or manufacturing, freeze gela-tion, printing technologies and manyothers [1 – 11]. Some of the technologies are wellintroduced and available also forlarge scale production, others areunder development or generally suit-able only for laboratory scale or pro-totype manufacturing. All of themhave specific advantages and restric-tions, making them more or less use-ful for a given task. When newadvanced ceramics products aredeveloped, the activities very often

are focused on material and recipeoptimization and thermal processes,whereas for shaping any availablelaboratory equipment is used, with-out searching for better alternativesand especially without respect to alater industrial scale production.TEAM by Sacmi, an alliance of sever-al companies belonging to the Sacmigroup (the world´s biggest supplierof machinery for ceramics produc-tion) and active in the field ofadvanced ceramics, offers a widerange of various shaping machineryand also provides outstanding R&Dfacilities. In order to provide a soundbackground for our customers tochoose the optimum forming tech-nology for new production lines,investment in production capacityincrease and other tasks, the capabil-ities of different shaping technologiesoffered by TEAM have been investi-gated, namely slip pressure casting,uniaxial hydraulic pressing and tapecasting.

Alfred KaiserLAEIS GmbHkaiser@laeis-gmbh.comRoel van LooAlpha CeramicsGmbH Josef KrausSama GmbH Andreas HajdukRiedhammer GmbH

cfi/Ber. DKG 86 (2009) No. 4 E 41

2 Experimental Details

2.1 Selection of ComparisonStandard Material / MaterialPreparationAs standard raw material for theseinvestigations, a reactive aluminapowder type CT 3000 SG (source:

Process Engineering

Comparison of Different ShapingTechnologies for AdvancedCeramics Production

Fig. 1Particle size distribu-tion of CT 3000 SGpowder and spraydried granules

slip for pressure

casting

spray dried powder

for pressing slip for tape casting

main organic

components

antifoaming agent;

deflocculant;

liquifier; wetting

agent; plasticizer

acrylate;

wax;

liquifier

acrylate based polymer

suspension;

liquefier;

content of

organics

(mass-%)

1 % 2,5 % 18,7 %

(including solvent)

solvent content

(mass-%)

30 %

(water)

0 %

(dry powder)

30 %

density (g/dm ) 2100 1200 1650

Tab. 1 Binder and additive systems for pressing and casting tests

Fig. 2Spray dried aluminaas used for pressingtests

For the tape casting slip a much high-er organic content was necessary.Due to this high organic content, aball mill was used for homogeniza-tion of the slip. Additionally to theacrylate based polymer which wasused as binder, also a high content ofliquefier had to be added to get agood castable slip. The final inorgan-ic solids content was about 51 %,with a slip density of about1650 g/dm³.

2.2 Shaping MachinesFor the comparison tests of slip pres-sure casting and hydraulic pressing,production scale machines have beenused which are available in our labsfor process development and opti-mization. The tests for tape castinghave been made to a much smallerextend on a pilot scale tape caster.They were made without detailedprocess optimization work, just toshow the preparation aspects of thinceramic plates, compared tohydraulic pressing technology. Slip pressure casting Slip pressure casting represents a suc-cessful combination of conventionalcasting and pressurised filtration,where the filter cake is formed to therespective article. This technologywas investigated already 100 yearsago, but could not be used in production lines due to the lack ofsuitable mould materials with suffi-cient strength and porosity, to pro-vide high numbers of casting cyclesat reasonable costs. Now fully porousresin moulds have been developedfeaturing the following characteris-tics:• Application of slip pressures up to

40 bar without additional reinforce-ment

• Minimum surface wear, thereforeexact and reproducible article con-tours

• Life span of up to several 10 000moulding cycles

• Homogeneous pore structureallowing a reliable demoulding andeffective backflushing

• Adjusted grade of flexibility servingfor safe sealing and preventing leak-age

• Simple production of workingmoulds at production site grantingthe highest production flexibility

At first, pressure casting was estab-lished in tableware production forshaping of non-rotation-symmetricarticles in two-part working moulds.Then sanitaryware production fol-lowed with the task to control theshaping of very large and quite com-plex articles, such as siphon-jet toiletbowls needing up to four-part work-ing moulds even with further insertedloose parts. Today also technicalceramic products are manufacturedin multi-partite as well as multi-cavitymoulds. Not only the shapingprocess, but also the downstreamprocesses like demoulding, automat-ic and highly precise fettling and fin-ishing, optimized drying and fullyautomatical transport and logisticsare at a very high level of sophistica-tion. For the slip pressure tests aSAMA PCM 100 was used (Fig. 3).Some general technical data of thismachine are given in Tab. 2. The slipwas stirred and pumped into a two-partite macro porous resin mould(material: SAMA-por Standard; d50 = 25 µm) with a central sprue,clamped to the closing unit. Themould diameter was 193 mm, with amould height (defining the thicknessof the green sample before drying) of8 mm. A feed pressure between 1and 5 bar was applied. After a fillingtime of about 15 s a slip pressure of15 to 40 bar (1,5 – 4 MPa) was builtup in about 20 s and the final pres-sure was maintained for 60 – 80 s.During the shaping process themould surface acts as a diffusion bar-rier, building the so-called “castingskin” as a spontaneous reaction whenthe slip is applied. This skin serves asself-filtration layer, on which the fur-ther filter cake forms up to the pre-determined body thickness. Afteropening of the mould and releasingthe plate from the mould by applyingcompressed air from the backside,

Fig. 3 SAMA PCM 100 slip pressure cast-ing machine

E 42 cfi/Ber. DKG 86 (2009) No. 4

Almatis, Frankfurt [12]) with 99,7 %Al2O3 was used. The CT 3000 wasprepared as spray dried powder in anindustrial scale spray dryer at ourtechnical center Alpha Ceramics inAachen. The particle size distributionof the CT 3000 SG powder and of thespray dried granules are shown in Fig.1 and 2. The slips for pressure castingand tape casting were prepared inlaboratory scale. A summary ofbinder and additive contents andproperties of the materials preparedfor the shaping tests is given inTab. 1.The binder system for the spray driedpressing powder with the main com-ponents acrylate and wax has beendeveloped for other applications andwas not optimized for this specialtask. The binder content was in therange of 2 mass%. The bulk densityof the spray dried powder was about1200 g/dm³. The slip for pressurecasting was prepared with a solidcontent of 70 mass% (density about2100 g/dm³). In order to get a goodhomogeneity and to avoid anyagglomerates, the slip was stirredwith a directed jet turbine mixer fortwo hours. A complex mixture oforganic additives had to be used toachieve a stable slip, but it was possi-ble to get a useful slip without anytemporary binder, therefore the totalcontent of organics was less than inthe pressing powder.

Process Engineering

Fig. 4 Green disc after slip pressure casting

closing pressure, max. 1000 kN slip filling pressure 1-5 bar casting pressure, max. 40 bar max. mould dimensions 900 x 900 mm cycle time (usually) 60 – 180* s

pressing force 15 000

1 500 kN

t max. useful die area 1 320 x 640 mm max. filling depth 120 mm theoretical no. of functional strokes 18 min

-1 vacuum system min. residual pressure

approx. 20 mbar

* depending on the wall thickness and the filtration behaviour of the slip

Tab. 2 SAMA PCM 100 slip pressure casting machine; technicaldata

Tab. 3 LAEIS ALPHA 1500 hydraulic press; technical data

the plate was removed manually (Fig.4). The residual moisture of the sam-ples was about 16,4 %.Uniaxial hydraulic pressingUniaxial hydraulic presses of thetypes ALPHA and OMEGA are gener-ally used for the production of ceram-ic tiles, slabs or plates, characterizedby large pressing area and limitedheight. Applications include techni-cal ceramic products like sputteringtargets, ceramic armour plates, wearprotection plates, substrates for elec-tronic devices and many more. Suchpresses are available with pressingforces in the range from 600 t up to4200 t and a useful pressing area upto 1600 x 1000 mm². Generally theyhave a maximum filling height of 60mm, but some of them are availablewith a filling height of up to 120 mm.Products of the same shape with dif-ferent thicknesses can be made in thesame mould, simply by changing thefilling height. With active mould ele-ments also more complex shapes canbe produced. The hydraulic pressing testsdescribed here were done using aLAEIS press ALPHA 1500-120 (Fig. 5).Some general technical data of thismachine are given in Tab. 2. Thepowder was dry pressed to rounddiscs in a single cavity steel mould.The mould diameter was the samethan for the slip pressure casting tests(193 mm). No specific measureswere taken with respects to surfacetreatment (polishing etc.) of the dies.Discs with various thicknesses (nomi-nal green thickness of 10 mm, 5 mmand 2,5 mm) were pressed using thesame mould with various fillingheights, with specific pressures of 50,100 and 150 MPa. Such pressures aretypical for uniaxial hydraulic pressingof technical ceramics, but for special

applications specific pressures up toapprox. 400 MPa have also beenapplied. The mould was volumetri-cally filled by hand. The press was equipped with a vacu-um system, which allows the mouldcavity to be evacuated before thedensification starts. Pressing undervacuum provides many advantages,e.g. better densification, no forma-tion of layers caused by entrappedair, and shorter pressing cycles, sincemultiple de-aeration strokes can beavoided [13]. Vacuum pressing tech-nology is especially recommendedfor press bodies with a high densifi-cation factor, for extremly fine pressbodies (not granulated, highly dis-persed powders) or for productgeometries with large volume or withhigh aspect ratios. In the tests report-ed here, the mould was evacuated toa residual atmospheric pressure< 50 mbar (min. 90 % vacuum). After pressing, the discs were ejectedand manually removed from themould. Even very thin plates (greenthickness down to about 1 mm)showed a good strength and couldbe handled without damage (Fig. 6).The total pressing cycle time includ-ing evacuation was about 20 s.Tape castingCompared to other shaping methodsapplied in ceramic production, tapecasting is a relatively young shapingmethod, first introduced successfullyfor the production of dielectricceramic tapes. Today tape casting isvery common for the preparation ofthin ceramic tapes, e.g. for ceramicsubstrates in electronic applications,piezo actuators, micro filters, compo-

Fig. 5 LAEIS ALPHA 1500 hydraulic press

cfi/Ber. DKG 86 (2009) No. 4 E 43

nents for fuel cell stacks and manyothers. The technology is based on avery simple principle: a plane surface(polished steel belt or polymer carriertape) is driven with constant speedunder the doctor blade, which isexactly adjustable with regard toheight. Thus the ceramic slip is dis-tributed in a homogeneous coat lay-er. Precondition is a homogeneouslyprepared slip, without bubbles andfree-flowing, which will dry to a use-able, faultless tape. Two kinds of slipcan be used:• Slips with organic solvent offer a

numerous variety of combinationsof additives (solvent, binder, plasti-fier, liquefier, wetting agent, etc.) toreach a maximum density and asufficient stability of the slip. How-ever, due to regulations regardingtoxicity, explosion risk and environ-mental pollution, additional mea-sures have to be taken which can bequite cost-intensive.

Process Engineering Fig. 6Green discs afterhydraulic pressing

Fig. 7SAMA FGT 250 tapecasting machine

Fig. 8a, b Green tape after casting

useful casting width 250 mm casting track length 3 200 mm casting speed 20 - 200 mm/min casting height 0,05 - 4 mm drying system radiation (IR panel)

Tab. 4SAMA FGT 250 tapecasting machine;technical data

dure, and the casting speed has to beadjusted accordingly. Tape castersallow to adjust the width of the tapevery easily. According to the workingprinciple, the side areas of the tapedo not show the same propertiesthan the main tape area and need tobe cut off. A high organic content isnecessary, which allows a very flexi-ble tape to be produced, but alsorequires a separate debinderingprocess before firing of the tapes. The final shaping of the product isdone after drying by cutting, milling,boring, punching and/or lamination,allowing a broad range of differentproducts to be manufactured. A bigadvantage is that such differentgeometries are achieved by almoststandardized machines. For the tapecasting tests the SAMA tape castingunit FGT 250 (Fig. 7) was utilised. Thetechnical data of this laboratoryequipment are summarized in Tab. 4.The base of the casting station is aprecisely machined and polishedhard-stone plate, on which the cast-ing unit including doctor blade isfixed. The homogeneously preparedand de-aired slip was added througha slot gate onto the casting surface(polyester foil). The requestedamount was adjusted by the gateopening. Subsequently the slip wasspread uniformly onto the carrier foilby means of the exact height adjust-ment of the doctor blade. During thetrials the absolute wet tape thicknesswas varied from 0,08 to 3,0 mm.After casting, the ceramic tape waspassed through the drying zone,which was equipped with an infraredpanel in order to accelerate the dry-ing process. In case the casting speedwas too high or the tape too thick,the casting process was stopped.After completion of the dryingprocess the ceramic tape was sepa-rated from its carrier foil. The driedgreen tape showed a good flexibility

(Fig. 8) and could be processed easi-ly (cutting and firing) for further char-acterization.

2.3 Thermal TreatmentAfter shaping, the firing process isanother key technology which deter-mines the quality of the final prod-uct. For industrial high temperatureapplications in the ceramic industrycontinuous kilns are used in thedesign as tunnel kilns with cars con-veyance, as roller hearth type or aspusher type. Intermittent high tem-perature kilns are used in the ceram-ic industry whenever high flexibilityis required. Depending on the necessary temper-ature profile, the required through-put capacity and other factors, Ried-hammer offers various solutions ofboth continuous and intermittenthigh temperature kilns for heat treat-ment of advanced ceramics, e.g.:• Tunnel kiln: gas heated kilns with

car conveyance with continuousoperation for high throughputcapacities, with different cross sec-tion dimensions depending on thecapacity requirement. Kiln lengthbetween 24 and 60 m, maximumtemperature up to 1800 °C.

• Roller kiln: gas and/or electricallyheated kilns, designed for continu-ous operation for low, medium andhigh throughput capacities. Widthup to 1,5 m, depending on maxi-mum temperature even wider,length from 6 m to 60 m, depend-ing on product requirements. Max-imum temperature up to 1600 °C.

• Top-hat-kiln: batch operation fordifferent firing conditions due todifferent product requirementsconcerning temperature profile,atmosphere and cycle time. Electri-cally heated with gas-tight designfor technical ceramic applications.Useful volumes from 0,13 m³ up to

Fig. 9 High temperature shuttle kiln type RIEDHAMMER WMG8/10U/160-TNV for operating temperature up to 1600 °C (photo:courtesy Helmut Kreutz GmbH, Haiger, Germany)

E 44 cfi/Ber. DKG 86 (2009) No. 4

• Aqueous systems provide a consid-erably lower range of possibilities toselect and combine the differentadditives and therefore cause amuch higher effort in development.Also the drying period is longer andrequires a bigger variety in adjust-ing the drying conditions, but theeffort regarding environmental andsafety measures is considerably less.

Very thin ceramic tapes (green thick-nesses as low as 0,05 mm, up toabout 4 – 5 mm as a maximum) witha very large surface area can be pro-duced. The higher the thickness, themore difficult is the drying proce-

Process Engineering

Fig. 10 leftRIEDHAMMER kilnHTK 150 with sam-ple plates stack

Fig. 11 rightFiring curve

max. operating temperature 1 800 °C useful volume 0,5 x 0,5 x 0,6 m no. of burners 4 fuel natural gas firing principle burners working with preheated

combustion air and diffusion air control temperature and atmosphere flue gas evacuation forced evacuation with kiln

room pressure regulation by

induced draught blower heat exchanger pre-heated combustion

air up to 450 °C

Tab. 5 RIEDHAMMER hightemperature cham-ber kiln HTK 150;technical data

1,74 m³. Maximum temperature1600 °C.• Shuttle kiln: simpler design as top-

hat kiln, but with the possibility ofgas and electric heating. Useful vol-umes from 0,5 m³ up to 30 m³.Maximum temperature up to 1800°C. Fig. 9 shows a typical intermit-tent high temperature shuttle kilnused in the production of advancedalumina products.

According to customer´s request sev-eral options are available like: • Swinging, sliding or lifting doors as

closure of the chamber and shuttlekilns

• Flue gases extraction via the kiln carsoles and the kiln rear wall and con-duction from there via heatexchanger into the open air.Depending on kiln size, flue gasesare exhausted by means of a fluegas channel underneath the kiln

• Thermal post combustion for allabove mentioned kiln types

• Selective debindering measures aslow-O2-technology, depending onbinder content and binder compo-sition [14, 15]

For firing of the tests samples in thisinvestigation, a high temperaturechamber kiln RIEDHAMMER HTK150 (Fig. 10) was used, one of thesmallest standardized kilns of thedelivery program with automatictemperature and atmosphere con-trol. This type of kiln is normally usedfor high temperature sinteringprocesses, for testing the shrinkage ofrefractory materials and for testingkiln furniture. A summary of technicaldata of this kiln is given in Tab. 5. Thebasic temperature and diffusion air

curve used for the tests is shown infig. [11]. The samples were pre-driedat 120 °C to constant weight. Then aheating-up gradient of 1 K/min hasbeen applied up to the maximumtemperature of 1600 °C. Between200 and 400 °C the heating-up gra-dient was reduced to 0,65 K/min foran optimal debindering. Diffusion airwas used to control the debinderingprocess by cooling down the flametemperature. While heating up afterdebindering the diffusion air wasreduced to a minimum to save fueland to control the oxygen content inthe atmosphere. The soaking time at1600 °C was 4 h. During the coolingphase again diffusion air was insertedto control the temperature. Theresulting total firing cycle time was66 h. It has to be mentioned that thiscycle time was not optimized, butwas kept constant in order to getcomparable results for the differenttest samples.Some samples have been fired also inan electrically heated chamber kiln,using the same firing curve as in thegas fired kiln. The results were quitesimilar, but the temperature distribu-tion seemed to be less homogeneousbecause of the lack of convection.

2.4 Characterization of theSamplesFor comparison of green sampleproperties, all samples were dried at120 °C to constant weight. Theywere visually inspected and the thick-ness was measured at different spotsat the outer, middle and central areaof the plates (Fig. 12). After firing, the

Tab. 6Green plate thick-ness and density

cfi/Ber. DKG 86 (2009) No. 4 E 45

thickness of the samples was mea-sured accordingly. Then the sampleswere divided in smaller parts and thedensity of the fragments was deter-mined according to the Archimedes´principle. Typically, about 2 frag-ments from the center area, 4 frag-ments from the middle area and 6fragments from the outer area weremeasured for each sample plate. Forsome samples also the green densityand the density distribution was con-trolled. The green tape cast sampleswere only visually inspected, thick-ness and density were measured onlyat fragments of the fired tape. Onlyfragments from the inner strip of thetape were investigated, side stripswere neglected.

3 Results and DiscussionIn the following chapter, some prop-erties of green and fired samples, pre-pared with the different shapingtechnologies as described above, willbe compared and discussed. The aimof this work was not to optimize spe-cial properties of the products for aspecific application, but to show gen-eral advantages and/or restrictions ofthe shaping processes.

Process Engineering

green plate thickness green density

shaping

technique

specific

pressure

average min max standard

deviation average

standard

deviation

MPa mm mm mm mm g/cm g/cm

pressing 50 8,55 8,33 8,87 0,15 2,24 0,009

pressing 100 8,40 8,20 8,68 0,10 2,28 0,007

pressing 150 8,30 8,16 8,40 0,07 2,32 0,008

casting 1,5 8,08 7,99 8,20 n.d. 2,15 n.d.

casting 2 8,06 7,80 8,30 0,08 2,16 0,016

casting 2,5 8,08 8,30 8,11 n.d. 2,18 n.d.

casting 3 8,11 8,06 8,15 n.d. 2,19 n.d.

Fig. 12Sketch of characteri-zation areas of castand pressed plates-blue = outer area-purple = middlearea yellow = cen-tral area

increasing pressure leads to a reduc-tion in pressed sample thickness.Both variations result in higher greendensity with increasing pressure. Thethickness variation of the green plateswere quite low, from plate to plate(when shaped under identical condi-tions) as well as at different spots ofone plate. In case of pressed plates,the thickness variations were mainlycaused by the manual filling of themould, the variations of the castplates mainly came from the centralsprue. Both systems still can be opti-mized depending on the geometry ofthe specimen to be produced. Thegreen density of the pressed plateswas somewhat higher than that ofcast plates, due to the higher residualporosity after drying out of the water.As to be expected, the green densitycould be increased with increasingspecific pressure. Interestingly, dou-bling of the pressure led to anincrease of 0,04 g/cm³ in green den-sity in both systems. The green den-sity variations of the plates againshowed very satisfying results. Withstandard deviations of less than 0,01g/cm³ (pressed plates) and 0,016g/cm³ (cast plates) both technologiescan be compared easily with other"advanced" shaping technologies.The somewhat higher density fluctu-ation of the cast plates again aremainly caused by the differencebetween the central sprue area and

the peripheral areas of the discs. Thedensity variations of the pressedplates can be reduced further byincreasing the homogeneity of theplate thickness with improved fillingtechnology.Thickness and density variationsafter firingFor the investigation of fired sampleproperties, slip pressure cast plateshave been used which where pre-pared with slip pressures of 2,0 , 3,0,3,5 and 4,0 MPa and plates pressedto nominal green thicknesses of 2,5,5,0 and 10,0 mm, all with a specificpressure of 90 MPa. For samples fromthe tape casting experiments, ran-dom fragments of the tape has beenused with various thickness. Thethickness of the plates has been mea-sured at different spots from the out-er, middle and centre area, repre-sented by different colours accordingto Fig. 12. The thickness variation offired plates obtained with slip pres-sure casting (left) and with hydraulicpressing (right) are shown in Fig. 13.The pressure cast plates have a sys-tematically increasing thickness fromthe outer region towards the centerwith differences in the rangebetween 0,05 to 0,08 mm. Theabsolute values increase with increas-ing pressure up to 3,5 MPa, indicat-ing that with increasing pressuremore material has been pressed intothe mould as could be expected. The

Fig. 13Thickness distribu-tion of fired testplates left: slip pressurecasting with slippressures of 2 / 3 /3,5 / 4 MParight: hydraulicpressing (90 MPa)to green thicknessof 2,5 / 5 / 10 mm(colour code seeFig. 12)

Fig. 14Density distributionof fired test platesleft: slip pressurecasting with slippressures of 2 / 3 /3,5 / 4 MParight: hydraulicpressing (90 MPa)to green thicknessof 2,5 / 5 / 10 mm(colour code seeFig. 12)

E 46 cfi/Ber. DKG 86 (2009) No. 4

Thickness and density variations ofgreen pressed and pressure castplates With both shaping technologies,hydraulic pressing and pressure cast-ing, good samples with a high greenstrength could be prepared. Table 6gives a survey about some character-istic data of the green samples. Eachline for pressed samples in the tablerepresents a collective of up to 15identical samples with up to 9 mea-suring spots each. For cast samplesthe line with a slip pressure of 2 MParepresents the same number of sam-ples, whereas for the other pressuresonly 2 samples were available, whichis not enough for a statistical evalua-tion. Therefore, in these cases nostandard deviation values could becalculated. The pressure cast sampleswere characterized after drying,whereas the pressed samples weretaken just as they came out of thepress. With the hydraulic press, sam-ples of various thicknesses between< 1 mm and > 20 mm were prepared(Fig. 6), but in this comparison onlyplates of about 8 mm green thicknesshave been included, prepared inboth machines with different pres-sures.The average thickness of the greencast plates shows practically no varia-tion with different slip pressures, dueto the fixed mould geometry. Withhydraulic pressing, however, an

Process Engineering

drop in thickness from 3,5 to 4,0 MPacan not be explained at the moment,but shows that an increase in pres-sure does not automatically give bet-ter results.The thickness variations of thepressed plates are in the same rangethan the pressure cast plates. Theabsolute values are in the range of 2mm, 4 mm and 8 mm, respectively,but the right picture in Fig. 13 hasbeen recalculated to the same scaleas in the left part for the pressure castsamples. The pressed plates, howev-er, do not show a systematic effect inany direction, the variations are dueto fluctuations caused by the manualfilling of the mould. The linear firingshrinkage, both in diameter and inthickness, was about 17 % for allplates.Fig. 14 shows the density variationsof the same set of plates. It can beseen, that all pressed plates and thepressure cast plates prepared with2,0 and 3,0 MPa slip pressure havevery similar and consistent densitiesin the range of 3,92 – 3,93 g/cm³,which is very close to the maximumfired density of 3,96 g/cm³ as speci-fied by the supplier. The density ofthe slip pressure cast plates madewith a pressure of 3,5 and 4,0 MPahave significantly lower densities andespecially the plates made with 3,5MPa also have bigger variations fromthe outer to the center area. Thisresult confirms again, that at least theconditions which have been appliedin these tests do not lead to betterresults with increasing pressure. Alsothe pressed plates show no advan-tage of higher pressure after firing.However, the green strength is high-er with increasing pressure (for bothtechnologies) and allows an easierhandling of the products before fir-ing.During the tape casting tests, tapeswith various casting thickness fromabout 0,05 mm up to about 3 mmhave been produced. From the cen-ter area of the thinner tapes smallersegments have been taken and thick-ness and density of such segmentsafter drying and firing have beendetermined. In fig. 15 are the densi-ties sorted with increasing thicknessbetween 0,05 and 0,4 mm, each barrepresenting an average of severalsamples of the same thickness.Despite the fact, that density mea-surement of such thin samples ismore difficult and the error of themeasured values might be largerthan for thicker plates, it is very clearthat the density variations are muchhigher than for the plates and also

the absolute value of the density ismuch lower, in average well below3,85 g/cm³. There is also no system-atic change of the density withincreasing thickness.Influence of firing technologyThe firing parameters have also beenmodified during these tests, but theirinfluence is not to be discussed herein detail. Of course heating rates andmaximum firing temperature are veryimportant. Plates with the samedimensions as described above couldbe fired to a density of 3,85 - 3,88g/cm³ only, when heated to a maxi-mum temperature of 1550 °C. Alsoattention needs to be paid to the typeof kiln and the heating system. In anelectrically heated kiln e.g. lesshomogeneous density distributionswere achieved due to a less homoge-neous temperature in the kiln volume(not enough convection).General aspects of product propertiesBoth slip pressure casting andhydraulic pressing can be operated at

Tab. 7Pre-selection tablefor shaping tech-nologies

cfi/Ber. DKG 86 (2009) No. 4 E 47

very low binder contents with sur-prisingly high green strength, even ofrather thin plates with a high aspectratio. The moisture content of thepressure cast plates causes a linearshrinkage in the diameter of theplates during the drying process,decreasing with increasing slip pres-sure (about 1,3 % at 1,5 MPa andabout 0,9 % at 3 MPa). In case ofhydraulic pressing, there is a slightspringback effect after ejection,which leads to an increase of theplate diameter in the range of 0,1 to0,3 %, increasing with increasingthickness of the plate. Tape castingrequires a much higher organic con-tent in the slip, leading to much high-er shrinkage and to lower final densi-ties. The smoothness of the surface aswell as the sharpness of the edges areclearly better at the pressed plates,due to the porosity of the pressurecasting mould. The pressed platesshowed a surface roughness Ra of <0,6 µm after firing without anymachining which can still be

Process Engineering slip pressure casting hydraulic pressing tape casting

green density + ++ --

fired density ++ ++ +-

density distribution ++ ++ +-

dimensional accuracy ++ ++ -

conture sharpness +/- ++ -

green strength ++ ++ (rigid) ++ (flexible)

surface quality +/- ++ + p

rod

uct

pro

per

ties

reproducibility ++ ++ ++

material requirements slip powder slip (water /

solvent)

binder content low low high

drying necessary yes no yes

(+debindering)

yes yes large specimen limited

(3 dimensions) (2 dimensions)

flexible flexible

product / wall thickness limited (~ 0,5 - > 50 mm) (< 0,1 - 3 mm)

geometry complexity ++ + -

process scrap neglectible neglectible yes

pro

cess

ch

ara

cter

isti

cs

process flexibility high high lower

invest costs medium high low

mould costs medium high none

production capacity medium high medium

range of available machines lower higher lower gen

era

l

useful for advanced ceramics

production +++ +++ +++

• Hydraulic pressing and slip pressurecasting both are mainly used for ful-ly dense product geometries likeplates, blocks, cylinders etc.Hydraulic pressing allows also shap-ing of hollow cylinders with simplegeometry, but no undercut is possi-ble. In contrary, pressure castingcan realize much more complexdesigns like curved hollow tubesand pipes

• Pressure casting is more restrictedin product thickness and wall thick-ness of hollow articles (typically 3 -15 mm). Each variation in thicknessrequires a separate mould.Hydraulic pressing allows the varia-tion of product thickness in a muchwider range (< 1 mm up to 50 mmor more) without changing themould (only adjustment of fillingheight necessary)

• Tape casting is a flexible technolo-gy for production of thin tapes, butis limited simple shapes which haveto be tailored in additional steps

• Pressed products can be fireddirectly, while slip pressure cast arti-cles need previous drying and casttapes mostly require an additionaldebindering step

• Firing to full density is easier forpressed and pressure cast productsthan for tapes

Depending on local conditions andwhich product requirements areparamount, the right technology canbe chosen. The synopsis given intable 7 can help to identify the opti-mum technology for a given task.Additionally, tests which originalmaterial can be done in our technol-ogy centers, where different tech-nologies are available and can beinvestigated even up to a certain pro-duction scale.

5 SummaryThe results of the reported investiga-tions confirm, that slip pressure cast-ing, uniaxial hydraulic pressing andtape casting are generally very suit-able technologies for shaping ofadvanced ceramics. All of them areavailable and well proven for indus-trial scale production. Each of thesetechnologies provides special advan-tages and restrictions, e.g. regardingpossible product geometries,throughput capacities, materialpreparation etc. When such limita-tions are taken into consideration,they allow in combination with ade-quate firing technologies the manu-facturing of advanced ceramic prod-ucts fulfilling utmost quality require-ments.

TEAM by Sacmi offers a uniqueopportunity to evaluate alternativesolutions for any given task in techni-cal centers equipped with productionscale machinery. Thus, optimal solu-tions for an economical productionare provided in direct cooperationwith the customers, taking varioustechnologies into consideration.

AcknowledgementsThe authors want to thank AlmatisGmbH for supply of the alumina rawmaterial, and the technical staff in theTEAM labs for the experimental andanalytical work.

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lagen der keramischen Formgebung; cfi (2005) (CD)

[2] Schulle, W.: Die Preßformgebung in derKeramik; Keram. Z. 44 (1992) [11], 754-759

[3] Hofer, B.: Heißisostatisches Pressen (HIP);Handbuch der Keramik; Gruppe I D 3.4;Beilage zu: Keram. Z. 38 (1986) [3]

[4] Hewson, G.B.: Today`s HIPs: bigger,faster, cost-efficient; Ceramic Industry,May 2007, 8-9

[5] Yasuda, K.; Kitamura, T.; Matsuo, Y.; Shio-ta, T.: Densification process of powdercompaction by cyclic CIP; ProceedingsMS&T Innovative Processing and Synthe-sis of Ceramics, Glass and Composites(2006), 495-506

[6] Kaiser, A.: Hydraulic pressing of advancedceramics; cfi/Ber.DKG 84 (2007) [6] E27-E32

[7] Kremer, R.; Lutz, R.: Quality improvementof shaped refractories by modern press-ing technology; Preprints 49. Internat.Colloquium on Refractories 2006, pp.223–226

[8] Jahn, P.; Mussler, B.; Rabenstein, M.; Rieß,W.; Eckardt, C.; Lehmann, J.; Krenkel, W.:Druckschlickerguss technischer Keramik;cfi/Ber.DKG 82 (2005), 307-311

[9] Twiname, E.R.; Mistler, R.E.: Tape casting:theory and practice; The AmericanCeramic Society (2000), ISBN 978-1-57498-029-5

[10] Cremer, I.: Metal injection moulding ismature; cfi/Ber.DKG 83 (2006),[10] E22-23

[11] Soltmann U.; Böttcher H.; Koch D.;Grathwohl G.: Freeze gelation: a newoption for the production of biologicalceramic composites (biocers); MaterialsLetters 57 (2003), 2861-2865

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[13] Kaiser, A.; Kremer, R.: Fast acting vacuumdevice – guaranteed quality for pressedrefractories; Interceram RefractoriesManual (2003) 28–33

[14] Becker, F.H.; Hajduk, A.: Innovative heattreatment of ceramics by using low O2technology; Interceram 58 (2009), 11ff

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Fig. 15Density distributionof fired tape frag-ments

E 48 cfi/Ber. DKG 86 (2009) No. 4

improved by polishing of the moulddies, if required. With tape casting, asimilar surface quality can beobtained, as long as the air isremoved completely from the slip.

4 Selection Factors forShaping TechnologiesThe results of these investigationsshow, that all three technologies arevery useful for shaping of advancedceramics. However, each of them hasspecial advantages and restrictions,which have to be considered whenthe most adequate shaping technol-ogy for a planned production has tobe selected. Thereby, requirementsregarding product quality as well asvarious technological aspects andalso economic factors have to be con-sidered, e.g.:• Both hydraulic pressing and slip

pressure casting systems can pro-duce near net shape articles, butdue to the mould materialhydraulic pressing can provide bet-ter surface quality and sharpness ofedges

• Investment costs for machines andmoulds are higher for hydraulicpressing technology than for slippressure casting, but also the pro-duction capacity (e.g. in terms ofm² production per hour) is muchhigher

• Hydraulic pressing requires drymaterials with a suitable flowabilityfor an even mould filling, preferablyspray dried powder; for pressurecasting and tape casting a slip hasto be prepared

• Two-part moulds with one closingdirection are standard for pressurecasting machines, but also 3-, 4- or5-part moulds for 2 or 3 closingdirections are possible. Hydraulicpresses generally have only oneclosing direction, but also dividedmoulds can be used for articles withmore complex shapes

Process Engineering