Development of slip moulding methods

CERAMURGIA

INTERNATIONAL,

Vol. 3. n. 4. 1977

159

Development of Slip Moulding Methods INVITED REVIEW PAPER

A.G. DOBROVOLSKIY institut

Problems

Materiaiovedenia

A review of the factors affecting the bask properties of ceramic slips, i.e. viscosity, settling rate, tlxotropy and dilatancy, Is given. The mak technological features of slip castkg In porous forms, eiectrophoretk moulding and thermoplastic slip casting are described. It is shown that the slip casting process may be used not only for manufacturing complex shaped products but also for the production of special ceramics, such as ffbre relnforced materials, porous materials with oriented channel, cellular materials, multi layer ceramics. thin ffims, and others.

1 - INTRODUCTION Owing to its simplicity, slip moulding is widely applied in ceramics technology. The method enables complex, little and large sized, and hoiioy articles to be manufactured, and careful reproduction of the details of complex shapes and a uniform pore distribution in mouides articles. Practically any material (oxides, carbidmeos;,,bi;ides, nitrides, metals and other) may be slip Three main areas are presently of concern in the field of slip mouiding: 1 - development of theory 2- establishment of new forming methods and improvements of existing ones of materials with specific structures 3- development

2-

THEORY OF SLIPS

At present, the main problem of theory of slips consists in establishing quantitative relations defining the basic characteristics of the slips: fluidity, settling resistance, and limiting content of the solid phase. The basic factors influencing these characteristics are defined in coiloid chemistri theory ‘#I. However, most of the relations established in coiioid chemistry J-9 are erratic if used to calculate the viscosity of slips. Very often authors refer to the Michael equation ¶:

4=% where:

(

i+

’

1.25 Cv 1-

C./O.74

1

of the slip q - viscosity viscosity of the dispersing medium rbof the solid phase cv - volume fraction slip

111

in the

Eq. [I] is graphically represented by curve 6, Fig. 1. The Michael equation refers to ideal suspension of spherical particles with 0.74 as the limiting volume content of the solid phase. Nevertheless real slips are characterized by different limiting volumetric fillings of the solid phase. As shown in Fig. 1, the viscosity vs concentration curves of real slips differ from that computed with the Michael equation. The real value of the solid phase limiting volumetric

AN

fraction equation

USSR, Kiev,

in the ‘:

U.S.S.R.

slip

is accounted

2.5 Cv

f 1+

rl=r).

for

by the

Pivinsky

I2

where: CW, is the limiting volume fraction of the solid phase .in the slip. Equation [S] describes well the viscosity of quartz slips (curves 7 and 8 in Fig. I), but it cannot be used in other cases. For example, the estimated NbC (C&, = 0.66) curves (5) and (3) for a stabilized slip and a ZrBl (C,,i, = 0.52) slip, respectively, do not coincide with the actual curves (4) and (1) for these siilps (Fig. I) as welti ‘as equations [I] and [2] should not be used to estimate the viscosity of thermoplastic slips, since the experimental values considerably exceed the estimated ones. The difficulty of establishing viscosity - concentration relations, which can satisfactorily describe different slips, is due to the following factors: degree of stabilization, thickness and composition of the soivate iayers, thixotropy, particle shape, fraction of solid phase, etc. The increase in viscosity depends on the loss of stabilization and soivate layer thickening. This is shown in Fig. 1 by ‘the concentration curves for stabilized niobium carbide slips (curve 4) and non - stabilized niobium carbide slips (curve 21, and for zirconium boride slips (curve I) whose particles are surronded by a sodium carboxymethyi cellulose colloid soivate molecule layer. The sedimentation stability is defined by the particle settling rate which, for suspensions of low concentration, is determined by the Stokes formula: V = where:

d g pl p. g

-

g d’ (~1-~0)/18rl

131

particle diameter gravitational force particle density density of the slip viscosity of the slip

The true settling rate of particles in highly concentrated slips differs from the calculated rate by two or three orders of magnitude, due to the influence of convection currents Iin the liiquid and by the random paths of the solid particles interacting with each other. The settling rate is also influenced by actual particle shapes, thixotropy and diiatancy of the slips. Some authors’ suggest using the Stokes formula to estimate the particle settling rate. The results, in this case, will be somewhat overstated (2 - 11 times), compared with real rates, but it allows a approxi,mate estimation of the particle settling rate. The difference between experimental particle settling rates and calculated ones depends upon the size differences between the particle fractions of the solid phase. The authors U showed that, for a stable slip, the maximum particle size should not exceed a diameter calculated by the formula: d,

=

6 Rt

K (PI -

c41 p.1

160

A.G. DOBROVOLSKlY

Belid phwe.

rol X

1 - Viscosity-concentration curves for real slips:

1 - ZrB2. FIGURE sodium carboxymethyl cellulose stabiliaed, C,,,i, = 0.59”‘; 2 - WC, non-stabilized, pH 5”; 4 - Nbc, NaOH stablIked. Cv,,‘, = 0.8% pH IO”; 7 - quartx. Cvcri, = 0.84’ and also the theoretical slips; 3, 5 and 8 calculated after the Michael equation improved by Pfvinsky ‘, rewctlvely for Cvcri, = 0.52, 0.99 and 0.84: 7 - calculated after the Michael equation 9.

where:

Pa tangential stress arising from gravity PI and pa - particle and suspensione densities, respectively; K - shape factor depending upon the stream curvatures which flow around the particles. The factor is 0.3 (for a particle size < 1 mm) and 0.6 for a particle size up to 40 mm).

This formula yields very important conclusions for the practice for preparing slips: Slip stability, other things being equal, may be increased by increasing Pt’ and lowering the (p’ - po) value. The decrease in (pl - a) may be reached by increasing the solid phase concentration in the slip or by strengthening its structure by means of an increase in thixo-

PH . Viscosity vs pH curves for real slips: 1 . MO”; 2 - W”: 3 . Si; 4. Cr; 5 - Cu”; 6 - Ni’$ 7 - Fe’$ 8 - stainless steel “; 9 - UO2”: 10 - Th02’*; 11 . AlZ03’9; 12. ZrO,m; 13 - 810,: 14 - SIC; 15 - NbC”; FIGURE 2

16 - MO&.

PH FIGURE 3 - Fundamental relations of the viscosity (I), of c potential (2) and of density of costing (3) vs pH of the sllp for basic (a) and acidic (b) materials.

tropy, for example. it should also be taken into account that the particles of the heavier materials should be finer. A low stability of aggregates, leading to a fioccuiation of the solid phase, may also increase the settling velocity. The aggregative stability of the solid phase is determined by a surface electric factor which causes the mutual repulsion of particles or their soivation. The electric factors occurs in the presence of electrolytes whose ions give rise to the electric double layer (Derjagin’s theory) of the surface of the particles of the slip. The aggregative stability improves with an increase of the absolute value, of the < - potential and with an increase of the thickness of the diffusion part of the electric double layer. To improve the aggreggative stability. it is necessary that the potential determining ions possess a specific affinity with the solid phase. The < - potential value increases with the decrease of the ,io.nic radius ‘and vaiency potential reaches its maximum absolute value at a given pH of the slip. The materials to be mouided may be subdivided into acicud ones (which ecidify the rispersing medium) and basic ones (which aikalize the dispersing medium). it was shown I3 that acidic materials stabilize better when NaOH is added to the slip, and the basic ones stabilize better with HCI additions. If better stabilization conforms to a minimum in the slip viscosity, then it may be illustrated by the results given in Fig. 2. The SIC, NbC, MO% slips (acidic materials) are better stabilized when NaOH is added “. The Si, Cr, SbN,, AirOs, ZrOn, BeO, UOI. ThOz slips [basic materials) show better fluidity when HCI is added. The overall dependency of slip viscosity (curve 1) and < - potential (curve 2) upon the pH is shown in Fig.1 3b for acidic materials, and in Fig. 3a for basic materials. The curves 3 in Fig. 3a and 3b show the density of the corresponding castings: the slips having a better aggregative stability result in denser castings. Thus, the main factor in the electrolytic stabilization of a slip is the electric one. Electrostatic forces are not always the unique factor responsible for slip stabilization. When electrostatic forces are not intense, aggregative stability of dispersion systems may be obtained by solvation of the solid phase. According to P.A. Rebinder’ the stabilization resulting from such a mechanism is determined by the formation of absorbed layers which have sufficient structure to control viscosity, elasticity, and mechanical strength. The high shearing resistance and high viscosity of the soivate layers impede their displacement from the surface during particle collisions, which prevent fioccuiation. Good wettability of the solid phase is needed for effec-

DEVELOPMENT OF SLIP MOULDING METHODS

161

tive solvation. Thus, solvate layers are formed in such systems as watet=ci,ay, quartz, glass, paraffin - Incrgenlic materials and others. in such a case it is possible to make a slip without additives. if the solvate envelopes are not sufficienty strong, or the dispersed solid phase is poorly wetted by the liquid, slips are obtained by adding a surface - active agent. Nonpolar adsorbtives adsorb better on non-polar absorbents, and polar adsorbtives better on polar adsorbents. For polar substances the polarity rule after P.A. Rebinder’ gives the adsorption condition of a substance C at the interface of two other phases i.e. A and B: EA > EC > EB Or

Sk <

EC <

Ee

2 \ I a

- =-:_ = -_-

1

__:-

/3 /4 __

-rrrr-

6

6

c51

where: EA, EB, EC - are the dielectric constants of the phases A, B and C respectively. For example, in a water (E = 81) - toluene (E = 2.4) system, aniline (E = 7.3) is a surface - action agent. From the polarity rule it follows that a less soluble substance is adsorbed better, that is the greater the polarity difference between soluble substances, the better the adsorption. Slip viscosity also depends upon thixotropy and dilata,ncy, as explained by Freindlikh ” and Rebmder I. Thixotropy may arise in suspensions if the particle sizes are not larger then 5 Pm. It is shown u that dilatancy arises in coarser grained high concentrated suspensions. A reduction in particle size leeds to a decrease in dilatancy, and by overstepping a given limit of despersion dilatancy slip may tur,n .into thixotropy sllip. Thus, changes .in solid phase diepersion control the rheological ,prope,rbies of the slip.

3 - SLIP MOULDING

alumi.nous shps ” are obtained when the solid phase contains 45-65% of parti’cles < 1 urn, and 35.55% of particles equal to 1-3 ym. In most cases, the dispersion medium may be distilled water or aqueous solutions of colloidal substances, for

~

6

FIGURE 4 - Graphical representation of slip casting into porous moulds: a] vacuum casting: b) injection moulding; c) vacuum casting: 1 - mould; 2 -slip: 3 injection moulding; d) centrifugal casting: 4 -vacuum branch pipe; 5 - pressure branch pipe: 6 - forces acting on the slip.

METHODS

Based upon the mechanisms for forming a casting, slip moulding methods may be subd&ided as follows: slip casting in porous forms; electrophoretic slip moulding; thermoplastic slip casting. 3.1 - Slip casting in porous forms In the process the casting is formed due to capillarly forces of suction which cause the influx of the liquid phase to the form walls. The liquid is soaked up by the form when nearing the form wall, and the particles are packed up on the form wall making a gradually thickening layer. When the necessary thickness of the layer is reached, the process is ceased by removing the slip. Further development of this method required selection of a more suitable casting form material, optimization of the technological process, improvement of the quality of the casting, and mechanization of the process. The low cost of plaster moulds account for their wide use; however they exhibit some essential disadvantastrength, limited control of ges - e.g. low mechanical the casting rate, decrease in casting rate after repeated castings, and easy damage of the surface of the casting form. in addition, working with plaster mouids required a great number of casting forms. in order to circumvent these disadvantages, casting forms of other materials have been considered: paper moulsds, metallic perforated moulds w,ith inner paper interlayers ‘, and cerami.c moulds ‘. A ceremic mould is used for both casting ,and firing the article. Milling is performed by either wet and dry methods. Wet vibromilling is considered ‘. “. uI as the best method for achieving high stability slips. Dry milling is considered B, in fits turn, as more efficient (alumina mli~llingl, since it gives less thixotropic sld’ps and reduces shrinkage, moulding rate, warpage, and casting porosity. The optimal dispersity is selected from experi,ence. Stable

example, sodium or ammonium aiginates ‘%38or sodium carboxymethyl cellulose I”. Colloid additions are introduced to improve stability. Ethyl alcohol and other organic liquid media are used when water reacts with the solid phase. Slips are obtained by mixing the powder with the liquid ‘phase, or by mixing as the milling is ,performed. The last variant *is consid.ered to be more &rational ‘, U because a better wetting of the surface of the sollid ,phase occurs. To ‘inorease the slip density, a slip limiting saturation method or a granular filler limiting saturation methodU are used. In these methods, after the slip is properly mixed at a reduced viscosity, the initial powder or the granular filler is added with further mixing of the slip. According to the di(mensions and thickness of the wails of the products pouring, casting-on, and rod-using procedures are applied. The volumetric mass of casting is almost independent of the density of fine-grained shps 12,j’. If coarse-dispersed additions are used the density of casting increases simultaneously with th,e rise of the shp de,nsity. The porosity of casting may be reduced by two methods. in one method the coefficient of packing of the semi-product is expressed by the following formula: K, =

K, +

Kr (I - K.)

[‘31

where K, - coefficient of packing of the mass sampled from the finegrained casting slip, Kt - coefficient of packing of the grainy filler. According to the other method, the concentrated finely dispersed slip is saturated by the dense coarse - grained fractions of 0.5 - I mm. The volumetric filling by the granular filler should not exceed a value of 0.35 - 0.45, because a greater coarse - fraction content gives rise to cavities and heterogeneities. These values correspond to granular filler packing coefficients for dry castings which are 0.45 - 0.50 and values of casting porosity as low as 7 - 8% for quartz moulding, and 28 - 30% for China - clay moulding I. The rate of mass build up depends upon filtration of the cake, which, in turn, is determined by the solid phase dispersity, solid phase stability and casting po-

162

A.G. OOBROVOLSKIY

ros.ity. The rate of mass build up deoreases sign.ificantly when the casting porosity is decreased: it may be reduced by 50 times by decreasing the porosity from 20% to 10%” In factories, slip ca$ting is performed with stands and automatic moulding machines 338 w. To speed the casting process and to improve the quaIbty of casting, heating of th,e slip, vacuum-form:ing, a,nd vibration treatments are used. Results from these slip casting methods are summarized in Reference 35. The best effects are obtained by combining several methods. By vacuum-forming the casting densities are improved and the vibration treatment makes it possible to use thixotropic slips M and mould elongated articles. In order to raise productivity, vacuum casting, pressure casting, simultaneous vacuum and pressure casting arvd a true centrifugal casting method were developed “e‘*. The scheme of these processes is shown in Fig. 4. In these cases, external forces are applied to the slip which make it possible to double the casting rate and to improve the casting densities at the same time. Slip casting Iinto porous fonms ena’bled them to obtain articles of different shapes and ‘mass; for example, arbicles of hard alloys M, thermocouple envelopes 39, ce,ramic meltilng pots 19, quartz refr#ige.rators 1o, and others. 3.2 - Electrophoretic

slip moulding

This method involves applying an electrical field to the slip which causes the slip particles to be transferred to the oppositly charged electrode form and settle upon it boulding up the particle layer on the form. The ,electrode form shape conforms with the outer configuration of the article. Configurations possible by the electrophoretic method are limited by electrotransfer possibilities. In Reference 41, it is stated that optimal moulding conditions for dense blanks are restricted by the voltage Interval (2 - 5 V/cm for a porcelain mass). When the voltage values are too high, a porous layer is formed. However, in moulding blanks of dense quartz slips”, the casting densities depend little on the voltage over a wide interval (6,6 - 33,3 V/cm). Therefore the problem of specifying the influence of the applied voltage on the electrophoretic moulding process has not yet been satisfactorily solved. Slip temperature and casting rate increase with an increase in current density, but the current density must not be raised above the limit at which the slips begin to release oxygen due to water electrolysis. In other studies u8‘3 it is stated that the rate of electrophoretic moulding does not depend on the density of the manufactured article and increases with an increase in slip concentration. This rate may exceed by 3 to 100 times the rate of moulding into porous forms. The efficiency of this method increases when denser castin,g are Irequired. Melt,ing ,pots”, ,plates 30’mm thick and 1000 mm ‘long”. and othe’r products have been made by this method. 3.3 - Thermoplastic

slip casting

(Hot casting)

In hot casting, slips are melted and shaped with a shaper (metal mould), allowed to solidify during cooling, and finally are separated from the shaper. These casting show no pores owing to the presence of a thermoplastic bond between the particles. The most widely bonding phase is paraffin ‘I8“-“. Tellows, pitches, resins and plastics are also used”, Q. @. The most effective surface active ,additives to paraffi,ns are bees wax and oleiniP4’. Someti~m~es‘in order to lreise the yield hmit thermoreactive substances are int*rod,uced into sl,ips ‘I. The densest slips are obtained when the componcqts (additives and powdered material) are mixed and milled together*. It is recommended that

mixing be made in vibrating and rotating hot mixing mills. An increase of the density of thermoplastic casting slips and castings can be achieved either when the pores of the carcass formed by the coarse-gralned powder, having a discontinuous granulometric composition, are impergnated by a highly concentrated casting slip or when the casting slip contains the grainy filler, as described before ‘. Even by using these packing methods It is not possible to obtain the casting densities that may be obtained by plaster-mould casting or electrophoretic moulding. It may be explained by the fact that for thermoplastic castings the packing coefficients will be the same as for the slips whereas the packing coefficients for electrophoretic casti,ngs and the blanks made by slip casting into porous forms are higher than for slips. To obtain defect free castings, slips should be vacuumtreated. The optimal time for vacuum treatment in a rotating vacuum mixer is 45-60 min.‘6. Thermoplastic slip casting uses metal moulds and casting quality depends upon mould filling rates, slip temperature and shape, mould construction, and slip characteristics ‘I, O. Smazhevskaja” showed that, when the slip movement is laminar, casting densities increase with the increase in mould filling rate. However, when turbulence occurs casting densities decrease with the increase of casting rate. The reduction in casting density is caused by a non-uniform distribution of the soft phase, which, concentrates on the periphery of castings at low pressures (1 atm). Uniform casting density is improved when the casting pressure is more than 5 atm. A higher slip temperature is used in manufacturing thin-walled bianks dr articles having shapes with inter-

FIGURE 5 - Graphlcal representation of slip solidiffcation in the moulds con&bring differwt conffguretions of castings, cooling dlrection, sizes and disposition of pouring getes: a, c. d. ecastings of non-porous forms b, f -testings of porous forms.

nal structures, because slip viscosity and solidification duration are determined by the slip temperature. The densest castings are obtained when slips, heated up to 80 to 100% are moulded into moulds at 0 to 20°C ‘I. It is reasonable to increase the temperature of moulds in cases when the constructing of the moulds contains elements hindering a shrinkage of the casting. The quality of the products depends upon the construction of the moulds. It may be seen from Flg. 5 that voids (denoted by a fat line] are formed in those parts of the castings where the crystallization of the casting slip occurs lastly and where a compensation of shrinkage by supplying additional amounts of casting slip through holes is difficult. Therefore a pouring gate is provided in the srde of tie mould with the greatest cross section. If necessary, two or three pouring gates are provided, and the slip moves along the pins #. An increase in viscosity, heat conduction or solidiflca-

163

DEVELOPMENT OF SLIP MOULDING METHODS

tion rate deteriorates moulding characteristics and castings quality; vice versa, go.od fluidity, low heat conduction and a low solidification rate improves the casting process “, 16. In References ‘I, u-6, the following thermo,plastic slip moulding methods are described: injection moulding. centrifugal casting, chill casting, swaging. hot drawing, welding, and others. The most widely used method is injection moulding. The most prolonged stage in the casting process is removal of the bonding phase. In Reference 50 it is shown that the adsorbent material does not significantly influence bond removal process, but pore size does. The binder is removed either in fillings or without fillings. IPlreferred #is the second process Ibecause lit i,s simpler, does not lead to contamination of the components, rooms and air. When the binder is removed without fillings the critical temperature of the casting has to be increased. In order to do that either polystyrene ‘I and thermally active agents ” are added to the casting or the casting is treated by water or steam at the melting point of the casting slip “, 16.The removal of the binder without filling is done on porous supports either by heating the whole volume of the casting or by a downallows an ward-directed heating &. The last procedure increase of the rate of the process and application of a single heat treatment. Wh,en moulding machines are used, efficiency of hot casting may ‘be compared with the efficiency of automatic presses. It ‘is a progressive process for manufacturing articles of va,rious materials such as ceramic spark plugs s2, multichannel ,insu,lat.ing pipes, coils M, art’icles hav,ing open-worked srehef surface *3, electronic ceramic substrates ” and circuit boards &. As a rule, the compon,ents manufactured by the hot casting method have little ,mass. Such a method is optimal in lmouId.ing smah components because large products with thimck walls impede defectless bond removal from the castings. In choosing between various moulding methods, it is necessary to take into account: 11 All slip moulding methods make it possible to obtain complex shaped articles. In moulding bottle-like and large-sized products, slip casting into porous moulds or electrophoretic moulding is recommended. Complex-shaped, small and miniature products are usually manufactured by using thermoplastic slip moulding. Using the electrophoretic moulding method it is possible to obtain only products within a restricted range of configurations. 21 Use af electrophoretic moulding or casting into porous moulds make it possible to obtain less porous castings, for example a porosity of 7 - 8% ‘, “. The porosity of thermopl’astkz b,lanks after the bond removal is more than 17 - 20% 3,‘! ‘5, 16. 3) The least efficient method is slip casting into porous moulds. Electrophoretic moulding efficiency exceeds by 3 - 100 times the efficiency of slip casting into method is therporous moulds ‘* ‘*. The most efficient moplastic slip moulding. But, in estimating the efficiency of thermoplastic slip moulding, one must consider the duration of thermoplastic bond removal which increases significantly with the article thickness and overall dimensions. -. 3.4 - Slip moukling as a method for manufacturing articles with special characteristics Due to its distinctive mechanisms, slip moulding may be used not only for manufacturing of products with complex configurations but also for the production of materials with special structures. In Reference 54, it was shown that the slip casting into plaster moulds makes it possible to obtain materials with a uniform distribution of fibers throughout the vo-

Lime of the ceramic body. The fiber reinforoi’ng was made by lintroduction of either lnichrome wire or by stainless steel fibre. The authors 36developed a method for manufacturing products with directed holes, 0,75-5 millimeter in diameter, using water based ceramic slips. It the new procedure the slip (is introduced into moulds with h,emp cords disposed beforehand anld vacuum treatment and vibration ,processes used. After obta’ining the desired ‘mass, the casting reinforced with he’mp cords
4 - SUMMARY

AND CONCLUSIONS

Further development of the slip moulding method involves both improving the theory of the process and achieving a better understanding of the phenomena occurring in practice. Qualitative descriptions of the slip behaviour and moulding processes have already been given: however a quantitative determination of the functional relations among various casting parameters and other factors that affect slip behaviour have not yet been attained. Development of the moulding practice is concerned with the search for new methods and the improvement of existing ones. A general improvement of efficiency, by using new dispersing media and surface active agents, the application of new approaches in manufacturing products with more specific and unusual structures are required.

REFERENCES 1. IS. GALINKER and P.I. MEDVEDEV. Fizichenskava I Kolloidnava . Khimiya, Vysshaya Shkola, Moskva, 1972. 2. S.S. VOYUTSKIY. Kurs Kolloidnov Khimiv. Khimiva, Moskva. 1964. a. YU. E. PIVINSKIY and A.G. ROMASHKIN. Kvartsevaya Keramika i Metallurgiya. Moskva. 1974. 4. E.R. HERRMANN and J.B. CUTTER, Trans. Brit. &ram. Sot. 4 119621 207. 5. V. VAND. J. Phys. Coil. Chem. 52 (1952) 300. 6. M. MOONY. J. Coil. Sci. 12 (19571 243. 7. J.R. RUTGERS, Rheol. Acta 3 (1962) 305. a. R.F. DEACON and S.F. MISKIN. Trans. Brit. Ceram. Sot. 9 (1964) 473. 9. Ceramic Fabrication Processes, W.D. Kingery (Ed.), J. Wiley 8 Sons, N.Y. Li958). 10. A.G. DOBROVOLSKIY. G.G. DOBROVOLSKIY, T.A. LUDVINSKAYA