MAT E 322
INTRODUCTION TO CERAMIC PROCESSING
EXPERIMENT # 6
SLIP CASTING OF A PORCELAIN BODY
(revised by Xinhang Shen, 2015)
Introduction
The process of slip casting involves preparation of the mold, usually made of
gypsum (sometimes polymer based molds are also used), preparation and optimization of
ceramic slip, and the casting process itself. The most critical part of the slip casting
operation is the formulation of the slip constituent and optimization of its casting
characteristics which will have a profound effect on the quality of the fired/finished product.
This experiment deals preparation of a porcelain slip with optimum solids contents, and
casting a ceramic crucible using a previously prepared mold. We will also characterize the
slip cast parts for green density.
Background
Slip casting is a technique for the mass-production of pottery, especially for
shapes not easily made on a wheel. A liquid clay slip is poured into plaster molds and
allowed to form a wet stiff ceramic layer inside the cavity of the mold. This layer is
called cast. The slip can be formulated to mature at a variety of temperatures. In a solid cast
mold, ceramic objects such as handles and platters are surrounded by plaster (industry uses
the term plaster interchangeably with gypsum) on all sides with a reservoir for slip, and are
removed when the solid piece is held within. In a pour mold, once the plaster has absorbed
most of the liquid from the outside layer of clay the remaining slip is poured off for later
use, and the item is left to dry. Finally the finished item is removed from the mold,
"fettled" (trimmed neatly), and allowed to air-dry. This produces a greenware piece, which is
then generally bisque fired, to harden it before the application of glaze and the final glaze
firing.
Ideally, the behavior of the casting slurry should not be sensitive to slight variations
in chemical composition or particle size distribution. The slurry should also have a
reasonable storage life. Occasionally, aging may be necessary to obtain a time- stable
slurry. Control parameters are the proportions and dispersion of the components in the
slurry, flow of slurry during mold filling, casting rate, the density and the yield strength of
the cast, flow rheology on draining, shrinkage and release of the cast from the mold, the
strength and the mechanical toughness of the product after removing from the mold, and the
trimming and the surface finishing.
Casting slurries are typically formulated to be shear thinning and have an apparent
viscosity below 2000 cP (2000 mPa·s = 2 Pa·s) at a mold-filling shear rate of 1 - 0.1 s-1.
This viscosity behavior is convenient for mixing and pumping, and for flow when the
mold is filled. The density and the apparent viscosity of the slurry should be high enough
to minimize particle settling during casting. For particles larger than a few microns, the
stress required to suspend a particle is:
2
τ > a ( Dp − DSlurry ) g
(1)
3
Here, a is the radius of the particles suspended in the slurry, Dp is the density of
the particle and Dslurry is the density of the slurry. The stress is given by t.
Permeable molds for casting are commonly gypsum with 40-50% porosity and an
effective pore size of 1-5 µm. The casting time may range from a few minutes for a thin cast
from a porcelain slurry to about 1 hr for a thick porcelain cast. The consolidated layers
of particles formed on the mold surface occur by the process of filtration. For uniaxial
filtration, the cast thickness L as a function of casting time is given by:
𝐿=
!!∆!
!!!
𝑡+
!
!!
!!
!
− !
!!
!!
!
(2)
here, L is thickness cast, J is the volume of the cast or the volume of the liquid removed, Rc
is the resistance to liquid transport in the cast, Rm’ is the resistance to the liquid transport in
the mold, ΔP is the apparent mold suction and η is the viscosity of the liquid transported.
In deriving the above result, it is assumed parameters other than time on the right
'
hand side of the equation stay constant during casting. Commonly the Rm / R
c ratio is
small, and can be ignored. The simplified equation is then given by:
𝐿=
!!∆!
!!!
𝑡
(3)
The above equation indicates the nature of parameter adjustments to increase the casting
rate. J is higher when the difference in the liquid concentration between the slurry and the
cast is smaller. A higher solids loading in the slurry will increase J.
In this experiment, we will use the optimum slip additives we established in the
Rheology of Ceramic Slips experiment to prepare the slip. Determine the optimum
solids content and cast a ceramic crucible for future firing experiment.
Experimental Procedure
A. Solids Content:
1. Weigh 750 g of solid with compositions of 35% SiO2, (US Silica) 25% Feldspar (S.D.
cluster), 25% Ball Clay C and 15% Kaolin (Edgar Plastic).
2. Weigh each component in plastic containers and carefully mix the powders.
3. Measure 400 mL of deionized water and 8 mL of 2 . 5 w t % Darvan CN into a wide
mouth plastic jar and add the powder mixture SLOWLY WITH MIXING (carefully mix
for approximately 15 minutes).
4. This 600 mL slip should make 45 vol% solids. Measure its pH.
5. Divide the slip (45 vol% solid content) into two equal parts.
One part will be used for casting a crucible. The other part will be used for casting pellets.
Crucible
1. If the pH is less than 7, add NaOH solution dropwise to bring it to 7.
2. Wipe the crucible mold with wet sponge and pour the first portion of the slip slowly into
the gypsum mold cavity and wait for approximately 4-5 minutes. Visually check the
thickness of the cast wall.
3. When the wall thickness is judged to be 2-3 mm thick, pour out the remaining slip.
4. Let the cast air dry until the end of the lab session, then remove the cast crucible.
Mark your crucible and place it in drying oven at 110˚C for at least overnight.
1.
2.
3.
4.
5.
6.
7.
8.
9.
Pellets
Use the acrylic molds with ½ inch holes. Wipe the gypsum block with wet sponge and
place the mold on the gypsum block.
If the pH is not between 4 and 5, add NaOH solution or HCl solution dropwise to bring it
to between 4 and 5.
Measure t h e v i s c o s i t y o f t h e s l i p u s i n g a p p r o p r i a t e s p i n d l e a n d s p e e d
(you should have gained experience from the previous lab to choose
a p p r o p r i a t e s p i n d l e a n d s p e e d ) . T r a n s f e r 15 mL slip from the second portion
of the slip to a syringe to pour the slip in the first row of mold cavities. Add slip to the
cavity carefully, so that it doesn’t spill over. Pour around 1 mL initially into each cavity,
and when the mold has absorbed some of the slip, pour some more to fill the cavity.
Do not pour back the remaining slip (from the 15 mL taken) into the beaker. Rather,
you may keep the remaining slip inside the disposable syringe for adding to the pellet
molds to keep the slip level at the top of the mold.
Add 1 mL of HCl solution to the remaining slip and increase the pH to between 5 and 6,
and repeat the previous step (step 3) to m e a s u r e t h e v i s c o s i t y a n d cast a
second row of pellets.
Repeat step 5 by adding 1 mL HCl solution and bring the pH to between 6 to 7, and fill
molds in the 3rd row.
Repeat step 5 by adding 1 mL HCl solution and bring the pH to between 7 to 8, and fill
molds in the 3rd row.
Leave the slips to dry, carefully remove the pellets from the die, mark each set of pellets
and place in a drying oven for at least overnight.
Take the pellets out of the oven, let them cool to room temperature, and measure
the diameter and thickness of each pellet with a caliper after grinding the top layer
carefully to obtain smooth and uniform cylindrical shape, weigh them for density
calculations.
Report
The group report should be written carefully following the information provided for report
contents and format at the beginning of the class. It should specifically include the
following information:
• Slip preparation, calculations (in table format) solids contents
• Plot of viscosity vs. solids content at constant shear rate, additives, time,
• Casting behavior of crucible (qualitatively)
• Green density of pellets you produced as a function of solids content
• Discussion of value of the experiment, what was achieved, what difficulties have
been experienced, what could be done to improve the value of the experience.
• Appropriate literature citation.