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fractions; however, this cannot be done in practice since it would then
be impossible to maintain a satisfactorily large particle-size ratio. For
ternary mixes, the optimum proportions amount approximately to 50%
coarse, 40% fine and 10% medium fraction; the reduction of porosity as a
result of the introduction of the third fraction is relatively small.
The given principles are exploited particularly in the manufacture of
refractory materials; however, they hold generally and are utilized in
fine ceramics by using mixes with a broad particle-size distribution.
If the particle-size distribution and particle shape remain the same,
a change in absolute particle size does not affect porosity but
influences the pore size substantially and thus also the permeability of
the body for gases and liquids (cf. Chapter IV, Section 7.2).
1.2 Preparation of mixes for forming
The most general method is wet mixing in a ball mill or in a blunger
(paddle stirrer). In the first case, simultaneous grinding is performed.
If the raw' materials have been supplied in ground form, then the
operation has the purpose of breaking up clusters and mixing the material
thoroughly. The slurry then passes through a screen and a magnetic
separator in order to remove lumps and iron which may have entered the
mixture during transport and grinding. The mixing (blending) is most
efficient in aqueous suspension, but in some instances is also carried
out in the dry state, so that water need not be eliminated before dry
granules FIG. 154. Spray dryer.
Plastic mass is traditionally prepared by partial removal of water
from the slurry in filter presses and by subsequent kneading in augers,
usually equipped with a vacuum chamber for removing air from the body.
For pressing, the water contcni has to be reduced further by partial
drying and the material is granulated into particles about 1 mm in
diameter which readily fill the die shapes by flow and ensure uniform
compression. Granulation is carried out by breaking lip of the dry mass
and then sieving.
The above operations have recently been rcplaccd by spray-di ycrs. in
is removed from the suspension droplets thus producing granules with a
particle size of the order of tenths of mm (cf. Fig. J54). The suspension
is sprayed into the top of a heated chamber, the droplets are dried in
the fluid state and the granulated product settles in the bottom from
where it is collected for further processing. The water content in the
granules and the granule size can be controlled.
The granules have to be sufficiently strong to withstand charging into
the die, but they have to be deformed under the forming pressure to
produce a dense pressing. The strength can be controlled using temporary
binders, e.g. paraffin emulsions, polyvinyl alcohol, methylcellulose,
The method described above is particularly advantageous for the
preparation of dry press mixes; however, it is also used in the
preparation of plastic mass where the granules have to be remoistened.
The main advantages are the continuous process and the savings provided
by elimination of the laborious filter-press operation w'hich is also
demanding in terms of required floor space. The recycling of waste heat
for drying is also favourable in this case.
The procedures described above are used chiefly in fine ceramics. For
plastic forming of coarse ceramics, it is common to use wet pan mixers or
pug mills in which the wet mix is kneaded with rotating knives.
Subsequent degassing in a vacuum chamber and extrusion are usual.
1.3. Rhsological propsrties of plastic masses
In ceramics, the term plastic mass or paste denotes an easily workable
mixture which keeps its shape after forming. Up to a certain stress, such
a mixture behaves as a solid and exhibits approximately elastic
behaviour. It is irreversibly deformed only beyond a critical stress
called the yield point. Beyond the yield point its behaviour can be
characterized by the respective deformation rate from which the so-called
plastic viscosity can be determined. The rhcological behaviour involved
is essentially of the
FIG. 155. Types of Theological behaviour: (a) Newtonian liquid; (,b)
anomalous (pseudoplastic) liquid; (c) Bingham body; (d) real plastic
body; (e) thixotropic body, (/) dilatant body. The viscosity is given by
the tangent of the indicated angle.
Bingham type, even though substantial deviations from the ideal Bingham
body may occur.
Rheological behaviour expressed by the dependence of the rate of
deformation on stress is illustrated for various types of materials in
Fig. 155. A Bingham body is described by the relationship
where ò is shear stress, r0 is the yield point, f?pl is plastic viscosity
and de/d/ is the rate of relative deformation (s-1). With real materials