improving mold sets for large-sized components prepared from aqueous slips. part 2. intensified...

SCIENTIFIC RESEARCH AND DEVELOPMENTS

UDC 669.762.2.001

IMPROVING MOLD SETS FOR LARGE-SIZED COMPONENTS

PREPARED FROM AQUEOUS SLIPS.

PART 2. INTENSIFIED TECHNOLOGY FOR PREFORMS SLIP-CAST

INTO POROUS MOLDS

E. I. Suzdal’tsev,1 D. V. Kharitonov,1 A. V. Dmitriev,1 and T. P. Kamenskaya1

Translated from Novye Ogneupory, No. 5, pp. 21 – 28, May, 2006.

Original article submitted January 10, 2006.

Basic methods for manufacture of large-sized complex-shaped ceramic components by casting from aqueous

slips — cryogenic, centrifugal, electrophoretic, etc. — are considered. Of these, the slip casting in porous

molds is advantageous in terms of cost and technological simplicity. To achieve further progress, attention

should be focused on the development of adequate molding equipment to minimize the rejection rate of fi-

nished product.

The shaping of ceramic preforms is a notoriously com-

plex technological process, and for this reason the techniques

and molding equipments normally used to prepare compo-

nents from aqueous slips need some comment. The shaping

of preforms from aqueous slips of inorganic materials has

been discussed in some detail in [1 – 4]. Major techniques

commonly employed for that purpose are: (i) casting into po-

rous molds, (ii) cryogenic, (iii) centrifugal, (iv), electropho-

retic shaping (v), shaping under pressure (vi) vacuum-as-

sisted shaping, and (vii) heat-assisted shaping.

The range of techniques suited for making large-sized

complex-shaped components such as aerial fairings is rather

limited. First, the size, profile, and performance parameters

of fairing preforms rule out the possibility of using cryogenic

shaping techniques. For example, viewed technologically,

one may imagine that the cryogenic method can be applied

if, during the molding, the slip is cooled through the inner or

outer shell of a metallic mold. However, as regards the qua-

lity of product, this method is clearly deficient since the po-

rosity of the molded preform (even if a highly concentrated

slip is used, with solid phase concentration CV > 0.74)

reaches 25 – 26%. Preparation of densely sintered high-

strength preforms from such a material requires very strict

control over a range of negative factors such as the crystalli-

zation of quartz glass, shrinkage and buckling of the pre-

forms during sintering, the use of high sintering tempera-

tures, and the difficulty of maintaining an optimum ratio of

crystalline phases in the glass ceramic of lithium aluminosili-

cate composition. A mold set for manufacture of the fairing

preforms from aqueous quartz glass slips by a cryogenic

method (using liquid nitrogen to cool the suspension on the

surface of a metallic core) was designed but, regrettably,

proved to be a modest success.

Sufficient attention has been given to the molding of pre-

forms from aqueous inorganic slips by centrifugal techniques

in the literature. However, most researchers agree that this

method is preferably used to shape cylindrical components

[1, 3]. Indeed, it is virtually impossible to provide an equal

centrifugal force to particles of different mass and staying

away from center of rotation at different distance (Fig. 1).

Furthermore, the centrifugal method for all its advantages

fails to provide high-density preforms uniform over the bulk,

of which data presented in Fig. 2 are illustrative .

In this context, the experience gained in the use of this

method at the Tekhnologiya Research and Production Enter-

prise is worthy of comment. Dating back some 30 years, a

Refractories and Industrial Ceramics Vol. 47, No. 3, 2006

158

1083-4877/06/4703-0158 © 2006 Springer Science+Business Media, Inc.

1Tekhnologiya Research and Production Enterprise, Obninsk, Ka-

luga Region, Russia.

unit “Agra” was designed for centrifugal molding of fairing

preforms shaped as a truncated cone with a base diameter of

350 mm and a length of 900 mm (Fig. 4). By the design, the

mold could be rotated about the proper axis simultaneously

with rotation around the angle of taper at a speed of

2 – 3 rev�min. It was clear, however, that in such a configu-

ration a high gain in material buildup rate was problematic,

whereas further increase in rotation speed would require the

creation of a prohibitively complicated structure, inefficient

from both economic and engineering standpoints.

Thus, the methods of centrifugal casting and cryogenic

shaping can find use in manufacturing components of ac-

ceptable size and shape, with relaxed requirements on the

uniformity distribution of physicomechanical properties over

the material bulk.

As regards the efficiency and product quality, the method

of electrophoretic molding from the aqueous slips of inor-

ganic materials may require more consideration [2, 5 – 7]. At

first sight, technologically this method has a wide potential

for molding components of different thickness. It has been

noted in [6] that quartz ceramic plates of thickness 30 mm

take 50 times less time to mold than those manufactured by

conventional casting into porous molds; for preforms 65 mm

thick, the savings in time improved by a factor of 300 or

higher.

However, in developing an electrophoretic molding tech-

nology for production of ceramic fairings we have met with a

major problem of deciding on an optimum diagram of the

process and selecting the material for electrodes and their de-

sign. Finally, based on the domestic and foreign expertise in

the field, an efficient molding equipment was proposed for

the electrophoretic molding of larger-sized complex-shaped

ceramic preforms (Fig. 5).

Improving Mold Sets for Large-Sized Components Prepared from Aqueous Slips 159

20

15

10

5

Wall

thic

kness,

mm

0 5 10 15 20 25 30

Molding time, min

2.3

1.5 0.75

Fig. 1. The preform buildup rate by centrifugal casting from me-

dium-disperse suspensions of quartz (– – –) and lithium aluminosili-

cate glass (——) at CV = 0.74 and different linear speed (numerals at

curves, m�sec).

20

15

10

5

Wall

thic

kness,

mm

10 15 20 25 30 35 40

Open porosity, %

2.3

1.5

0.75

2.3

1.5

0.75

Fig. 2. Open porosity distribution through the length in preforms

centrifugally cast from medium-disperse suspensions of quartz

(– – –) and lithium aluminosilicate glass (——) at CV = 0.74 and dif-

ferent linear speed (numerals at curves, m�sec).

10.0 m�

20 mm

15 mm

10 mm

5 mm

10.0 m�

10.0 m�

10.0 m�

Fig. 3. Photomicrographs of the material taken through the thick-

ness (5 – 20 mm) in a preform centrifugally cast from a lithium alu-

minosilicate glass slip at a linear speed of 1.5 m�sec (CV = 0.74).

Even a cursory glance at the diagram in Fig. 5 shows that

the manufacture of complex-profiled electrodes for the par-

ticular facility is an expensive and laborious operation. The

method proposed can be efficient only under the condition of

a large-scale and well-established production.

It was shown in [2, 8] that the molding of quartz cera-

mics can be accelerated substantially if the slip is fed into the

system under pressure. However, no unified approach to the

problem could as yet be reached. According to [2], the pre-

form buildup rate could be increased by a factor of 4 or

higher as the slip feed pressure was raised from atmospheric

to 0.42 MPa (Fig. 6). According to [8], the buildup rate

achieved was more modest amounting to a mere 70 – 80%.

One would hesitate to indicate with certainty the reason for

this discrepancy considering that in [2], unlike in [8], no es-

sential particulars on the precursor slip and operating condi-

tions were presented. Furthermore, it was clear in [8] that the

optimum slip feed pressure should be 0.3 MPa since any ex-

cess gave no useful effect. Still, the buildup rate curves in

both [2] and [8] disproved this conclusion. In both cases, the

buildup rate tended to increase with slip feed pressure (see

Fig. 6). The controversial evidence and the lack of an ade-

quate mechanism require further research.

The joint action exerted by the feed pressure and evacua-

tion in the plaster mold [created through the outer surface by

a VN-4M vacuum pump at 1732.9 Pa (13 mm Hg)] produced

little effect on the buildup rate acceleration (Fig. 7). Further-

more, increasing the slip feed pressure from 0.3 to 0.34 MPa

caused even a slight decrease in buildup rate (Fig. 7, curves 1

and 2 ) [8]. The insignificant effect of mold evacuation on the

buildup rate was also reported for large-sized components

shaped as a body of revolution [8]. In this case the molding

set (Fig. 8) was placed in a tightly sealed shell, and the eva-

cuation in the body of a plaster mold 30 – 40 mm thick was

created by applying vacuum of 1732.9 – 1999.5 MPa

160 E. I. Suzdal’tsev et al.

Preform

Slip

Fig. 4. Schematic diagram of a centrifugal molding technique for

nose fairings.

Inner

electrode

Outside

electrode

Preform

Feed source

Fig. 5. Schematic diagram of a molding equipment for electropho-

retic shaping of the fairing preforms.

20

16

12

8

4

0

Wall

thic

kness,m

m

1 2 3 4 5 6

Molding time, h

0.42

0.28

0.14

0.3

0.2

0.1

Atmospheric

Î

Î Î

Î

Î

Î

Î

Î

�

�

�

�

�

Î

Fig. 6. The buildup curves for preforms shaped from medium-dis-

perse quartz glass suspensions in plaster molds at different feed

pressures (numerals at curves, MPa) [2, 8].

1

2

3 4

20

16

12

8

4

0 1 2 3 4 5 6

�

�

�

�

�

�

Wall

thic

kness,

mm

Molding time, h

Fig. 7. Buildup curves for preforms cast from quartz glass suspen-

sion in plaster molds under different molding conditions [8]:

1 ) 0.3 MPa + vacuum; 2 ) 0.34 MPa; 3 ) 0.1 MPa + vacuum; 4 ) va-

cuum.

(13 – 15 mm Hg). Therefore based on the available evidence

we are of the opinion that any, even in-depth, efforts for im-

provement in this area using vacuum techniques promise lit-

tle (if any) success.

The temperature has been reported to produce a signifi-

cant effect on the preform buildup rate [1, 2, 9, 10]. The data

in [1, 10] revealed a relationship between the room tempera-

ture oscillation and the buildup rate of preforms from aque-

ous quartz and lithium silicate glass slips. In [12], the pre-

form buildup rate showed a 120% increase as the tempera-

ture of the quartz glass slip was raised from 20 to 40°C. Still,

systematic data on the effect of temperature on the buildup

rate from aqueous slips are sadly lacking in the literature.

Therefore further efforts in this direction may be of practical

interest.

Highly concentrated slips (CV = 0.76 – 0.78) of quartz

and lithium aluminosilicate glass of medium dispersity (with

22 – 25% particles of size � 5 �m and 4 – 9% of 63 –

500 �m) and plaster mold with a water-gypsum ratio of 1 : 1

were used in our study. For standardization of the experi-

ment, the molds and the slip, placed in tightly seal contain-

ers, were kept in a drying cabinet at 20 – 50°C. The slip was

poured in the molds and placed once again in the drying ca-

binet at a given temperature. The upper temperature limit

(50°C) was not exceeded to prevent the wear of mold mate-

rial because of dehydration. The preform buildup was made

on the vertical wall of the mold and controlled to a thickness

of 20 mm.

Rate curves for deposition from slips of quartz and li-

thium aluminosilicate glass are presented in Fig. 9. As is

seen, a common feature here is the increase in buildup rate

by a factor of greater than 2 as the molding temperature is

raised from 20 to 50°C. The porosity of molded preforms is

little affected by the raise in temperature (see Table 1). An

explanation to the observed increase in buildup rate should

be sought in the change in rheological properties of the aque-

ous slip with temperature. Data in [2, 3, 11] on aqueous slips

of various materials (amorphous and crystalline silica, zir-

con, alumina, mullite, kaolin etc.) and our results (see

Fig. 10) show a significant decrease in slip viscosity with

temperature — obviously because of the decrease in visco-

sity and density of the dispersion medium (water) and the de-

crease in thickness of the polymolecular water films on the

surface of solid phase particles [12] (Fig. 11). These factors

promote the percentage of kinetically free water in the slip,

which facilitates the movement of solid particles to the mold

wall and increase in the buildup rate with temperature. Fur-

thermore, the increase in molding temperature intensifies the

thermal agitation of solid particles, especially those of fine


Preform

Core

Porous mold

Vacuum

Fig. 8. Schematic diagram of a molding equipment for shaping pre-

forms in an evacuated mold.

20

20

16

16

12

12

8

8

4

4

2 4 6 8 10 12 14 16 18 20 22

40

30

20

20

0

4 6 8 10 12 14

50

50

40

30

20

à

b

��

��

�

�

�

�

�

�

�

�

�

��

�

�

��

��

��

�

�

�

�

�

�

�

�

Wall

thic

kness,

mm

Molding time, h

Fig. 9. Buildup curves for preforms cast from medium-dispersed

suspensions of quartz (a) and lithium aluminosilicate (b ) glasses in

plaster molds at different temperatures (numerals at curves, °C).

TABLE 1. Apparent Density and Open Porosity for Preforms Cast

from Suspensions of Quartz and Lithium Aluminosilicate Glasses at

Different Molding Temperature

Molding tem-

perature, °C

Lithium aluminosilicate glass Quartz glass

density, g�cm3 porosity, % density, g�cm3 porosity, %

20 2.10 14.0 1.95 12.7

30 2.09 14.4 1.93 12.7

40 2.07 15.0 1.93 12.7

50 2.07 15.0 1.92 13.2

fractions with their high percentage of 25%. The thermal agi-

tation of solid particles, in turn, provides conditions for the

shaping of equidense preforms, which is important for im-

proving the quality of finished products.

However, for all its advantages, this technology for

molding large-sized components has not yet gained accep-

tance in practice for a number of reasons. First, even a simple

technique to warm up the mold containing slip (Fig. 12a) re-

quires the use of a voluminous drying cabinet and a signifi-

cant energy consumption, which is economically unprofit-

able. Second, the high wall thickness of the plaster mold (up

to 40 mm) and its low heat conductivity require that the tem-

perature of the outer surface of the mold be sufficiently high,

which is inconvenient operationally. Third, preheating the

mold in a drying cabinet to 40 – 50°C and itd subsequent fill-

ing is little effective for the reason that the mold, because of

the high coefficient of linear thermal expansion of the plas-

ter, may easily fail.

Therefore to achieve technological and engineering im-

provements in the field, further efforts are needed. Essential

points to be primarily considered are the choice of an effi-

cient heat carrier, direction of the heat flow and its distribu-

tion over the molding set, slip heating regime prior to shap-

ing, adequate mixing of components, etc.

A solution to the problem is the molding set shown in

Fig. 12b. Its major components are a plaster mold and a core

heatable core. The core is a hollow shell made of a heat-con-

ducting material; inside it, a distributing branch pipe is

mounted which is connected to a heat-carrier supply system.

Using such a facility, the mold and the slip contained in it can

be readily heated to 50°C, without the risk of overheat and

impairing damage to plaster molds. However, the shortcom-

ings of this molding set should not be overlooked: the core

was complex in design, especially for preforms of larger size.

Another grave shortcoming of the heated-core molding

technology is that at molding temperatures above 50°C, the


1.0

0.8

0.6

0.4

0.2

0

� �, Pà sec

30 60 90 120 150 0 20 40 60 80 100

20

70

40

20

7040

a b

P, Pà P, Pà

Fig. 10. Viscosity � plotted a function of the shear stress P for sus-

pensions of quartz (a) and lithium aluminosilicate (b ) glasses at dif-

ferent temperatures (numerals at curves, °C).

0.9

0.7

0.5

0.3

0.995

0.990

0.985

0.980

0.975

0.970

�, g ñm� 3�, nm

9

8

7

6

5

4

3

2

1

00 20 40 60 80

t, °C

1

2

3

� �, Pà sec

Fig. 11. Viscosity � (1 ), density � (2 ), and polymolecular water

film thickness � (3 ) plotted as a function of temperature t.

HeatingHeat carrier

Heated core

Porous mold Porous mold

Preform Preform

Core

Heating coil

à b c

Fig. 12. Schematic diagrams for shaping ceramic preforms using a drying cabinet for heating the plaster mold and slip (a), using a

heated core (b ), and using a heating coil incorporated in the plaster mold’s body (c).

slip particles stick on the heated core, which causes

delamination in the preform material and the occurrence of

pores and voids in it. This can be prevented by conveying

heat to the slip material in the plaster mold on the mold side.

However, heating the mold from the outside is little efficient,

and an alternative variant was proposed where the plaster

mold was heated by means of a heating coil embedded in the

mold’s body (Fig. 12c). Such a mold was used to shape

large-sized preforms (of layer thickness 20 mm, length

1100 mm, and base diameter 400 mm) from a lithium alumi-

nosilicate glass slip. The molding temperature was main-

tained within 40 – 45°C, which allowed one to reduce the

molding time from 15 to 8 h.

Preforms molded by the newly-developed technology

were not inferior in density and porosity to those prepared by

the conventional slip casting technology. Despite the encour-

aging results, further efforts are required along the way of

improving and optimizing the technology and design of

molding equipment.

As was shown above, the material of the mold, the di-

mensions of the molding set, the molding temperature, and

the excess slip feed pressure affect the preform buildup rate.

Other important factors involved are the material of solid

phase of the slip and its performance characteristics.

As is seen in Figs. 13 and 14, the wall buildup rate tends

to decrease monotonically with layer thickness deposited on

the surface of the mold. This behavior is typically observed

in both quartz and lithium aluminosilicate glass slips, and a

variety of factors can be involved in it, such as the cumula-

tive ability of the mold material, density and thickness of the

near-surface layer, and conditions for the filtration of free

water through the growing layer of deposited solid phase.

The higher electrokinetic potential of the lithium alumino-

silicate glass slip (130 – 150 mV) versus that of quartz glass

(30 – 50 mV) and the denser near-surface layer of the former

(of porosity 8 – 9% versus 10 – 11%) are the reasons for

more lower buildup rate of preforms cast from lithium alumi-

nosilicate glass slips.


14

12

10

8

6

4

2

0

15 10 5 0 15 10 5 0

0 2 4 6 8 10 12 14 16 18 20 0 5 10 15 20 25 30 35 40

à b

Wall

thic

kness,

mm

Molding time, h Molding time, h

Fig. 13. Buildup curves for preforms cast from aqueous slips of quartz (a) and lithium aluminosili-

cate (b ) glass of different fineness (numerals at curves indicate the residue retained on a No. 0063 mesh

sieve, %).

14

12

10

8

6

4

2

0

0 2 4 6 8 10 0 5 10 15 20

à b

0.8 0.76 0.74 0.74 0.72 0.70

Wall

thic

kness,

mm

Molding time, h Molding time, h

Fig. 14. Buildup curves for preforms cast from aqueous slips of quartz (a) and lithium aluminosilicate

(b ) glass of different solid-phase concentrations (numerals indicated at curves).

The buildup rate of preforms cast from aqueous slips into

porous molds is strongly related to grain size distribution

(Fig. 13) and solid phase concentration (Fig. 14). The data in

Figs. 13 and 14 imply that the buildup rate is controlled by a

number of technological parameters associated with the type

of mold, slip, molding conditions, and preform size and may

vary within a wide range. In practice, a trade-off is usually

sought between specified parameters. As shown in [1, 10],

such an approach is quite practicable in the production of

preforms from quartz and lithium aluminosilicate glass ce-

ramics with good product quality. Further efforts in this area

are needed aimed at the optimization of technology, and re-

duction of materials consumption and cost of the finished

product.

To briefly summarize, all the technologies for molding

large-sized complex-shaped ceramic components from aque-

ous slips that have been considered above can give high-

quality products; still, slip casting in porous molds is more

appealing in terms of cost and technological simplicity. To

achieve further progress, attention should be focused on the

development of adequate molding equipment to minimize

the rejection rate of finished product.

REFERENCES

1. A. G. Romashin, E. I. Suzdal’tsev, and M. Yu. Rusin, “Theoreti-

cal and practical aspects in the manufacture of large-sized com-

plex-shaped components from quartz ceramics,” Novye Ogne-

upory, No. 9, 34 – 40 (2004); ibid., No. 11, 20 – 27 (2004).

2. Yu. E. Pivinskii and A. G. Romashin, Quartz Ceramics [in Rus-

sian], Metallurgiya, Moscow (1974).

3. Yu. E. Pivinskii, Theoretical Aspects in the Technology of Ce-

ramics and Refractories [in Russian], St. Petersburg, Stroiizdat

(2003).

4. E. I. Suzdal’tsev, High-Refractory Radio Transparent Glass Ce-

ramic Materials and Their Use in the Production of Aircraft

Fairings, Author’s Abstract of Candidate’s Thesis [in Russian],

Moscow (2002).

5. A. S. Berkman, “Electrophoretic intensification in the casting of

ceramic components,” Tr. Keram. Inst., No. 20, 6 – 12 (1948).

6. V. F. Tsarev, “The effect of molding parameters on the proper-

ties of quartz ceramics,” Steklo Keram., No. 11, 21 – 23 (1979).

7. D. V. Kharitonov, Development of an Efficient Technology for

Production of Large-Sized Complex-Shaped Ceramic Compo-

nents Based on Aqueous Suspensions of Lithium Aluminosili-

cate Glass, Author’s Abstract of Candidate’s Thesis [in Rus-

sian], Moscow (2005).

8. V. F. Tsarev, L. S. Koneva, and N. V. Solomin, “Vacuum and

pressure as factors affecting the preform buildup rate and den-

sity of quartz glass ceramics,” in: Synthesis, Technology and

Testing Methods in the Production of Refractory Inorganic Ma-

terials, Collection of Research Papers, Issue IV [in Russian],

ONTI, NITS, Moscow (1975), pp. 42 – 46.

9. A. G. Dobrovol’skii, Slip Casting [in Russian], Metallurgiya,

Moscow (1974).

10. E. I. Suzdal’tsev, “Statistical analysis in the production of com-

ponents from glass ceramics of lithium aluminosilicate compo-

sition,” Ogneup. Tekh. Keram., No. 3, 12 – 18 (2003).

11. Yu. E. Pivinskii, Ceramic and Refractory Materials [in Rus-

sian], St. Petersburg, Stroiizdat (2003).

12. R. Aiado, The Chemistry of Silica [Russian translation], Mir,

Moscow (1982).


improving mold sets for large-sized components prepared from aqueous slips. part 2. intensified...

Documents