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230 Interceram 03–04/2011

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Polished Porcelain Stoneware Tiles

Ceramic Bricks Filling – Energy Saving

Including Special TILE & BRICK

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Trade Fairs & ConventionsPOWTECH 2010, Germany

CERA GLASS 2010, India

QUALICER 2010, Spain

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Ceramics ForumThe Glass Industry in the EU Today – a Survey

High-PerformanceCeramicsComposition Modifications on the Properties of Some Bioactive Glasses and Glass Ceramics

Titanium Nitride Coating of Cobalt Chromium Coronary Stents: a SEM-EDS Analysis

Ceramic Based Bio-Medical Implants

Preparation of Ca- / -Sialon Powders by Micro-wave Reaction Nitridation

Building MaterialsEffect of Bi2O3 on Cordie-rite Formation in Cordieri-te Based Bodies

TILE & BRICKThe Use of Residues in the Manufacture of Ceramic Tile Bodies

Hot-Pressed Gres Porcellanato Body

Effect of Calcite on the Brick Body Closing

Glossiness and Slipperi-ness of Polished Porcelain Stoneware Tiles

Effect of Diaspore Addition on Microwave-Assisted Sintering of Floor Tile

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1 IntroductionShaping of ceramics has a long tradition, starting from free hand forming several thousand years ago up to the most sophisti-cated modern technologies [1]. The most important technologies for “traditional” ce-ramics can be classified into casting technol-ogies, plastic forming technologies and pressing technologies [2]. These well proven and comparably economical technologies are also predominant in the manufacturing of advanced ceramics [3], at least when it comes to large scale production (in this pa-per “advanced” or “technical” ceramics will be referred to as ceramics produced mainly from well defined raw materials, often syn-thetic powders, rather than “traditional” or “classical” ceramics mainly made from nat-ural raw materials). In the course of new product developments in laboratories or in R&D centers, however, the focus usually is on material optimization and not on the shaping technology. Therefore, very often one of the available lab scale technologies is used without systematic investigation into better alternatives and especially without regard of a future industrial scale produc-tion. Sometimes, the available technologies are just small scale presses which do not provide all of the features of an up-to-date modern production size press and the result

of the investigation is the statement, that this pressing technology is not recommend-able for the new product. In other cases highly sophisticated shaping technologies are used which are still under development or generally suitable only for lab scale or prototype manufacturing. And again the result is the conclusion, that “simple” tech-nologies like uniaxial hydraulic pressing should not been considered.The aim of this paper is to show that mod-ern uniaxial hydraulic pressing technology can be used very effectively to produce ad-vanced ceramics with high quality and with comparably low capital expenditure. 2 Shaping technologies for advanced ceramicsMost of the well proven shaping technolo-gies for “traditional” ceramics are used also for the production of advanced ceramics. These are mainly uniaxial and isostatic pressing, slip casting and pressure casting as well as extrusion, often modified to meet the higher demands of technical ceramics [4–10]. Other also well established technol-ogies are not widely used in traditional ce-ramics production, but more dedicated to advanced ceramics, like hot isostatic press-ing, tape casting, injection moulding, thick film and thin film technologies [11–15], and further technologies are just being es-tablished or are under development, in-cluding the spark plasma sintering (SPS), also called FAST (field assisted sintering

technology), freeze gelation, so called “rap-id manufacturing” technologies (printing etc.) and many more [16–18].Technically they can be classified by various parameters, e.g. • body condition and moisture content

(dry, semi-dry, plastic, slip, …) • shaping temperature (cold, „warm“, hot) • pressure • atmosphere (air, inert, reducing, vacu-

um, ...) • content of organic additives

This classification, however, does not give an indication of the most suitable selection for a specific task, and in some cases it can be misleading. Table 1 (taken from [3]) e.g. shows a compilation of various organic ad-ditives used for the individual shaping processes. This table seems to indicate a low organic content for injection moulding and a high organic content for uniaxial pressing due to the number of different organic ad-ditives. If one looks at the quantity of or-ganics, however, it can be seen that the organic content of bodies for injection moulding typically is 1–2 orders of magni-tude higher than for uniaxial pressing (Table 2). Various papers compare data ob-tained with different shaping technologies [19–21], but generally the results require a very careful interpretation and must not be simply generalized. Table 2 gives also a synopsis of other charac-teristic features of selected shaping technol-ogies. Such a survey can help to restrict the

A. Kaiser*, R. Lutz*

Uniaxial Hydraulic Pressing as Shaping Technology for Advanced Ceramic Products of Larger Size

This paper gives an overview about various shaping technologies and relat-ed selection criteria for the production of advanced ceramics. The uniaxial hydraulic pressing technology is ad-dressed in detail and recent develop-ments are discussed which enable new application possibilities of this technol-ogy, especially for the manufacturing of parts having larger dimensions. Sev-eral ex amples of oxide ceramics, non-oxide ceramics as well as carbon based products clearly show the broad range of utilization possibilities.

hydraulic pressing, shaping, advanced ceramics, high-perform-ance ceramics, vacuum pressing, large speci-menInterceram 60 (2011) [3–4]

The corresponding author, Dr. Alfred Kaiser, studied Chemistry at the University of Wuerzburg, Germany, and received his PhD in 1979. From 1979 to 1989 he was Project Manager, Depart-ment Manager and Deputy Director at the Fraunhofer Institute for Silicate Research in Wuerzburg, specializing in materials development (sol-gel process) and environmental aspects of glass technology. From 1989 to 1995 he was Manager Technol-ogy at the Test Center of Gustav Eirich, Hardheim, Germany,

where he focused on mixing and granulating technology. From 1995 to 2005 he worked as Manager Plant Engineering at Laeis GmbH in Trier, Germany. Since 2005 he has been Manager Business Development at the same company, now located in Wecker, Luxembourg. Dr. Kaiser is a member of the German Ceramic Society, the German Engineering Federation, and the American Ceramic Society. The author of numerous technical papers has served in various technical commit-tees. E-Mail: [email protected]

The auThor absTracT Keywords

* LAEIS GmbH, Am Scheerleck 7, L-6868 Wecker, Luxembourg


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number of suitable technologies, e.g. when high moisture contents are not acceptable or when high production capacities (i.e. short cycle times) are required. For practical pur-poses, however, the most important selec-tion criteria defining the optimum shaping process for advanced ceramics are • product geometry (maximum length and

width, product height and aspect ratio, complexity

• of product) • throughput capacity • special product requirements (density,

microstructure, contamination restric-tions, …)

• material properties (reactivity against wa-ter, sintering behavior, …)

• downstream production steps (possibility and costs of machining of the green and/or fired parts, necessity of near net shap-ing, …)

Further important aspects are the availabil-ity of a technology which has been proven already for comparable tasks, the necessary capital expenditure and the operating costs (CAPEX / OPEX) for technologies in ques-tion and last but not least sustainability and environmental impact.If these criteria are carefully evaluated, a comparative study as a basis for a decision

may show a result similar to that shown in Table 3 (comparison of hydraulic pressing, pressure slip casting and tape casting of alu-mina in various thicknesses).

3 Hydraulic pressingUniaxial pressing is one of the most used shaping technologies in practically all fields of ceramic production. The main advantag-es (especially with respect to advanced ce-ramics production) are: • possibility of „dry“ pressing: moisture

contents of press bodies are typically be-low 3 mass-% and very often even no moisture at all is applied; therefore no separate drying step before firing is neces-sary

• high green density, depending on the ma-terial properties and the applied pressure, similar to what can be reached with iso-static pressing

• good strength of the green pressed parts, allowing for an easy and safe handling even of large parts

• very low content of binder and other organic additives (typically less than 3 mass-%, for special requirements also shaping without any organic additives is possible, if the right pressing parameters are chosen); thus no separate debindering step is needed and no extensive emission control measures become necessary

• low dimensional tolerances and high con-ture sharpness, especially compared to isostatic pressing

• smooth and even surface quality and also the possibility to press structures directly into top and bottom surfaces; thus, to-gether with the low dimensional toleranc-

Table 1 • Qualitative compilation of organic additives used for various shaping tech-nologies (from [3])

Liqu

ifye

r

Tem

pora

ry b

inde

r

Pres

sing

aid

Lubr

ican

t

Plas

tify

er

Ther

mop

last

ic b

inde

r

Feed

stoc

k

Sepa

rati

ng a

gent

Ant

ifo

am a

gent

Filt

rati

on a

id

Tape casting x x x x x

Slip casting x x

Pressure casting x x x

Hot casting x

Injection moulding x

Extrusion x x x

Hot extrusion x x

Uniaxial pressing x x x x x

Hot pressing x x

Cold isostatic pressing (CIP) x x x x

Hot isostatic pressing (HIP)

Table 2 • Characteristic features of selected shaping technologies

Tota

l org

anic

con

tent

Typi

cal m

oist

ure

Typi

cal p

ress

ure

Typi

cal t

empe

ratu

re

Typi

cal c

ycle

tim

e

/ mass-% / mass-% / MPa / °C / s

Tape casting 5 … >50 30 0 RT –

Slip casting 1 30 0 RT 100

Pressure casting 1 ... 3 30 0,15 … 4 RT 10 … 100

Injection moulding 10 … 30 0 50 120 … 200 25 … 60

Extrusion 10 10 5 … 20 RT –

Uniaxial pressing 1 … 3 0 … 5 50 … 250 RT … 150 2 … 20

Hot pressing <1 … >5 0 10 … 20 600 … 2400 *

Cold isostatic pressing (CIP, dry bag) 1 … 3 0 … 5 400 RT 10 … 120

Cold isostatic pressing (CIP, wet bag) 1 … 3 0 … 5 400 RT 500

Hot isostatic pressing (HIP) 1 0 200 500 … 2200 *

* cycle time defined by sintering process, not by shaping method



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es a subsequent adjusting/machining can be reduced to a minimum

• flexible and versatile control system, to-gether with a high degree of automation; many parameters can be adjusted to opti-mize the process and the product proper-ties; storage of parameter settings provide a good reproducibility

• high throughput capacities, due to short cycle times of only a few seconds

• good economic efficiencyOn the other hand, sometimes also sup-posed limitation factors for uniaxial hy-draulic pressing of advanced ceramics are mentioned like: • suitable only for products with „simple“

geometries and/or small dimensions

• big variation of green density; especially top to bottom for products with larger height (up to >10 % variation reported) [2, 22]

• limited dimensional accuracy (thickness variations up to 3 % reported) [22]

However, due to recent developments in pressing technology these perceived disad-vantages can be considered something of the past. As shown in the next chapter, uniaxial hydraulic pressing allows for the shaping of very large products and also of fairly complex shapes. As has been shown earlier, a very even vertical density distribu-tion can be achieved also with real “high” products (i.e. large dimension in pressing direction) [5] and thickness variations can

be reduced to a minimum [20]. Such im-provements could be realized e.g. by adap-tion of mould filling technologies and espe-cially with the introduction of advanced hy-draulic systems and new electric control concepts including a closed loop control concept as shown in Fig. 1 [23]. With an up-to-date hydraulic press control, many parameters can be easily adjusted in a wide range, like: mould filling box movement, punch entry speed, speed ratio upper punch/mould frame, pressure increase curve, pressure holding time, de-aeration strokes or vacuum pressing regime, ejection speed and ejection under reduced load and many others. Another big step forward was the introduc-tion of vacuum pressing technology [10]. Though its principle was well known long time ago, it was only rarely applied in pro-duction plants. It is, however, one of the key factors which allow for defect-free large-sized specimens to be pressed from fine powders with low or even zero binder con-tent. Fig. 2 illustrates the effect of pressing with and without application of vacuum for large scale alumina blocks.Besides the presses for small product ge-ometries that have been known for a long time, today uniaxial hydraulic presses with pressing forces up to >50,000 kN are availa-ble for advanced ceramic production. De-pending on the required specific compac-tion force, they allow for products with a surface area of up to approx. 0.8 m² and more to be pressed. Various types of presses can be chosen, which are distinguished mainly by their maximum filling depth: modified tile press-es with a standard filling depth of 60 mm or with an enhanced filling depth of 120 mm (Fig. 3) for more or less flat products and presses with a filling depth of up to 600 mm or 800 mm for special shapes with larger height, including cylinders, tubes and others (Fig. 4). The latter ones are working accord-ing to the well known HPF principle which means that the pressing occurs by moving down the upper die whereas the lower die is fixed (Fig. 5). As the upper die descends, the mould frame moves down simultaneously at a defined speed which can be selected via

Table 3 • Pre-selection table for shaping technologies (excerpt from [20])

Slip pressure

casting Hydraulic pressing

Tapecasting

Prod

uct

prop

erti

es green density + ++ –

density distribution ++ ++ +/–

dimensional accuracy ++ ++ –

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

surface quality +/– ++ –

Proc

ess

char

acte

rist

ics

reproducibility ++ ++ ++

material requirements slip powderslip

(water/solvent)

binder content low low high

drying necessary yes noyes

(+debindering)

large specimen limitedyes

(3 dimensions)yes

(2 dimensions)

product / wall thickness limitedflexible

(~0,5–>50 mm)flexible

(<0,1–3 mm)

geometry complexity ++ + –

Gen

eral process flexibility high high lower

invest costs medium high low

production capacity medium high medium

1 Fig. 1 • Closed loop

control; controller

design:

1) nominal value,

2) motion controller,

3) proportional valve,

4) cylinder,

5) pressure transducer,

6) measuring

rod/actual value

2

Fig. 2 • High alumina plates, pressed with vacuum

(right) and without vacuum (left)


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the press control panel relative to the plung-er speed (so called “active mould”). The speed of the mould can be changed several times at pre-defined plunger positions or af-ter reaching pre-selected pressures, thus providing a precisely controlled densifica-tion regime.Additional filling and fill compensation measures can be applied for complex shapes and vacuum systems are available for all types of presses. If necessary, also further measures can be taken to avoid contamina-tion, e.g. selection of special materials for all components coming in contact with the press body and even by enclosure of the whole press and running the process under inert atmosphere.

4 ApplicationsThe presses mentioned above are currently being used for the production of very differ-ent types of technical ceramics. Some of them are described here in order to high-light the possibilities which are offered now-adays by state-of-the-art uniaxial hydraulic presses.

4.1 Ceramic armourBallistic protection plates for body protec-tion as well as for vehicle protection is one of the present top themes and steadily in-creasing production capacities are planned and realized. Whilst vehicle protection plates and tiles of different formats and thicknesses are still mainly made of high alumina, the majority of the body protec-tion armour plates tends to be made of sili-con carbide (SiC), due to the lower weight

and/or better performance. Flat plates or tiles are pressed and sintered in the tradi-tional way. For the multi-curved body pro-tection plates different technologies have been established: they are either flat pressed and sintered on curved supports for bend-ing, or they are directly pressed into the curved shape. In the latter case, extraordi-nary care and attention has to be paid to a proper filling of the mould cavity. Fig. 6 shows a multi-curved SiC body protection plate as pressed, with a good green strength for handling before firing.Ceramic armour can be pressed in uniaxial hydraulic presses for subsequent pressure-less sintering. Sometimes, however, uniaxial pressing is used also as a pre-densification step for a hot pressing process to follow, in order to make better use of the capacity of the hot press.

4.2 Carbon based productsManufacturing of various carbon based products can also be a challenging task. Sev-eral presses of various types have been used to produce such different items like bipolar plates for low temperature (PEM) fuel cells (Fig. 7) or large sized carbon blocks made from carbon black and graphite as interme-diate products for carbon brushes etc. In this case it is very important to guarantee a very even density distribution throughout the whole volume of the block, since the small final products must have identical properties regardless whether they are cut from the center or from the outer area of such a block. HPF type presses are used for the production of these carbon blocks.

Another very interesting application is the manufacturing of carbon-based filters for molten metal purification. Fig. 8 demon-strates the very delicate design of such fil-ters, which have been pressed directly into the shape as shown in the picture, including the fine pore structure. Up to now, this proc-ess has been realized only in pilot scale.Also carbon fibre reinforced carbon materi-als have been pressed under elevated tem-perature (approx. 150 °C) in order to pro-duce pre-forms for ceramic brake discs. Fig. 9 shows the very homogeneous struc-ture of a disc with a diameter of 300 mm and a thickness of approx. 30 mm.

4.3 Sputtering targetsSputtering targets are used in PVD coating (PVD = physical vapor deposition) as a source for the coating material, which through the bombardment with high energy particles transfers into the gas phase and is then deposited in extremely thin layers on different surfaces. By this method, coatings with special optical, electrical or other char-acteristics are being achieved for example on large plasma screens, displays of laptops, mobile phones or architectural glass. For this application a wide range of different presses have been supplied to various cus-tomers (see Table 4). Materials to be used are indium tin oxide (ITO), alumina doped zinc oxide (AZO) and others. Again, the uniaxial pressing is used either as the only shaping method for a subsequent pressure-less sintering or as pre-densification for a second compaction step in an isostatic press. The advantage there is reduction of scrap to

3

Fig. 3 • LAEIS press Alpha 1500, max. filling depth

120 mm

4

Fig. 4 • LAEIS press HPF 2500, max. filling depth

600 mm

Fig. 5 • HPF principle (“active mould”)

5



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a minimum (especially ITO is a very expen-sive material) and to increase the through-put capacity of the isostatic press.

4.4 Other applicationsFurther applications to mention are shaping of stacking aids with very complex geome-try, with especially large differences in thick-ness which makes it difficult to achieve ho-mogeneous densification in all parts of the specimen (Fig. 10); substrates for electronic applications (PTC) with a thickness down to the range of about 1 mm and an area of, for example, 300 x 300 mm²; silicon nitride (Si3N4) plates for hobs, structured supports for firing of electronic devices and many more. The range of interesting applications is expanding continuously.

5 ConclusionsThough uniaxial hydraulic pressing is an “old” technology, it can be considered to be a very modern shaping technology for the production of advanced ceramics. Due to recent developments and process optimiza-tions, it is also available and suitable for the production of really large specimens, e.g. of sputtering targets with more than half a square meter of pressing area. On the other hand, it provides for high production capac-ities at affordable prices, especially when compared to highly sophisticated alterna-tives, like isostatic pressing. Therefore it is always worthwhile to take this technology

into account when a new product becomes marketable and an appropriate production process needs to be developed.

References [1] Reh, H.: Die Geschichte der keramischen Formgebung.

cfi/Ber. DKG 84 (2007) [10] D29–D32 [2] Heinrich, J.G.: Einführung in die Grundlagen der kera-

mischen Formgebung.cfi (2005) (CD) [3] Kollenberg, W. (ed.): Technische Keramik: Grundlagen-

Werkstoffe-Verfahrenstechnik, 2nd edition. Vulkan-Ver-lag, Essen (2009). ISBN 978-3-8027-2927-7

[4] Schulle, W.: Die Preßformgebung in der Keramik. Ker-am. Z. 44 (1992) [11] 754–759

[5] Kaiser, A.: Hydraulic pressing of advanced ceramics. cfi/Ber. DKG 84 (2007) [6] E27–E32

[6] Brook, R.J., Cahn, R.W., Haasen, P., Kramer, E.J.: Processing of ceramics part I and part II. In: Materials science and technology, Volume 17A + 17B. VCH Verlag Weinheim (1996). ISBN-10: 3-527-29356-6

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

[8] Janney, M.A.: Plastic forming of ceramics: Extrusion and injection moulding. In: R.A.Terpstra, P.P.A.C. Pex,

A.H. de Vries: Ceramic Processing, Chapman & Hall London (1995)

[9] Kremer, R., Lutz, R.: Quality improvement of shaped refractories by modern pressing technology. Preprints 49. Internat. Colloquium on Refractories, Aachen (2006) 223–226

[10] Kaiser, A., Kremer, R.: Fast acting vacuum device: guar-anteed quality for pressed refractories. Interceram – Refractories Manual (2003) 28–33

[11] Hofer, B.: Heißisostatisches Pressen (HIP). In: Hand-buch der Keramik; Gruppe I D3.4. Beilage zu: Keram. Z. 38 (1986) [3]

[12] Hewson, G.B.: Today’s HIPs: bigger, faster, cost-effi-cient. Ceramic Industry, (May 2007) 8–9

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

[14] Mutsuddy, B.C., Ford, R.G.: Ceramic Injection Molding. Chapman & Hall, London (1995)

[15] Teng, W.D., Edirisinghe, M.J.: Development of continu-ous direct ink jet printing of ceramics. Brit. Ceram. Trans. 97 [4] (1998) 169–173

[16] Herrmann, M., Raethel, J., Schulz, I.: Spark plasma sintering/field assisted sintering of ceramic materials. Interceram 58 (2009) [2–3] 109–114

[17] Soltmann, U., Böttcher, H., Koch, D., Grathwohl, G.: Freeze gelation: a new option for the production of biological ceramic composites (biocers). Materials Let-ters 57 (2003) 2861–2865

[18] Günster, J., Engler, S., Heinrich, J.G.: Forming of com-plex shaped ceramic products via layer-wise slurry deposition (LSD). Bull. Eur. Ceram. Soc. 1 (2003) 25–28

[19] Krell, A., Klimke, J.: Effect of the homogeneity of parti-cle coordination on solid state sintering of transparent alumina. J. Am. Ceram. Soc. 89 (2006) [6] 1985–1992

[20] Kaiser, A., van Loo, R., Kraus, J., Hajduk, A.: Compari-son of different shaping technologies for advanced ce-ramics production. cfi/Ber. DKG 86 (2009) [4] E41–E48

[21] Yasuda, K., Kitamura, T., Matsuo, Y., Shiota, T.: Densi-fication process of powder compaction by cyclic CIP. Proceedings MS&T Innovative Processing and Synthesis of Ceramics, Glass and Composites (2006) 495–506

[22] Reckziegel, A., Willmann, G.: Konstruieren mit Keramik; Herstellung und Maßhaltigkeit ohne Nachbearbeitung. Sprech saal 118 (1985) 332–338

[23] Lutz, R.: Use of closed loop controls in hydraulic press forming of ceramic products to obtain highest dimen-sional accuracy. Proceedings of the 47th International Colloquium on Refractories, Aachen (2004) 222–224. ISBN-13: 3-514-00712-8

Received: 25.05.2011

Table 4 • Presses used in the production of sputtering targets

Press type Pressing force Typical target

sizeTypical

thicknessMax. applicable

pressure

/ t / kN / cm2 / mm / kN/cm2

Omega 2100

2100

21000

3600 8 5,8

1000 12 21,0

Omega 3000 3000 30000 3400 14 8,8

Alpha 1500/120 1500 15000 1400 35 10,7

Alpha 4200

4200

42000

4000 8–15 10,5

6800 8–15 6,1

PH 6500

6500

65000

6500 8–15 10,0

7200 8–15 9,0

8500 8–15 7,6

6

Fig. 6 • Body protection armour plate (as pressed)

7

Fig. 7 • PEM bipolar plate (as pressed)

8

Fig. 8 • Carbon filter (size approx. 100 x 100 mm²)

9

Fig. 9 • Brake disc pre-form

Fig. 10 • Complex shaped stacking aids

10

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