Characterizing ceramic extrusion
properties
Introduction
The rheology of a ceramic body is
critical in the ‘plastic making’ of
tableware. The structure of the
ceramic needs to be right to reduce
processing problems such as a large
pressure build up, cracking on the
roller, sticking to the roller head or sag
after moulding.
The Rosand capillary rheometer is
able to simulate the range of shear
rates (typically quite low i.e. 10 to
1000s-1) that would be seen in
extrusion and injection moulding (as
for kiln furniture, etc). The RH2000 is
usually used by ceramics
manufacturers, which has a maximum
load of 20kN. High forces are
sometimes measured, but they are
usually due to friction between the
barrel and the piston.
The test is designed to measure the
viscosity of the body during the
extrusion process. The results (shown
opposite) can be used to compare
with that of ideal body systems, or to
optimize the roller head settings.
Interpretation
The results show that Ceramic A has
a lower viscosity than B and will
probably extrude more easily. It is
more shear thinning than B, which
indicates that it may be more suitable
for plastic making.
The Bagley correction (Cogswell
model) uses a long capillary die and a
zero length orifice die. This correction
accounts for extensional properties
seen at the die entrance, and allows
both shear and extensional properties
to be plotted separately. The shear
viscosity can indicate how a sample
1
might flow into moulds or in pumping
conditions. The extensional properties
can have a large influence on how
well the ceramic responds to the
process. By correlating both the shear
and extensional properties to ‘real’
situations, formulations can be
evaluated without the need for
extrusion trials.
Measurement Conditions
Conclusion
Pre-test
The Rosand Capillary Rheometer can
be used in conjunction with the lab
and pilot scale extruders to formulate
high quality ceramic paste
compounds.
Stage 1: 2 Mpa Pleft, 1Mpa Pright /
50mm.min-1 hold for 4 minutes,
Rosand application note
MRK596-01
Samples: Fibre Filled Ceramic
Pastes
Set-up
Dies: 16 mm x 1 mm x 90º & 0.25 mm
x 1mm x 90º
Transducers: 10 KPSI, 1.5 KPSI
Stage 2: 2 Mpa Pleft, 1Mpa Pright /
50mm.min-1 hold for 5 minutes.
Table of shears
-1
Range: 2 - 5,000 s , 8 steps / up / log
Temperature: 90ºC or extrusion
temperature (usually in the range
ambient to 90°C, or up to 200°C for
injection moulding)
Equilibrium:
By volume: 60 samples/ml, 0.1 mins
max interval, no filter
Pressure deviation: 0.5%, Window: 6
samples, Average: 4 samples, max
samples: 100, trip: 90%.
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2
Rosand application note
MRK596-01