White White Wares and Heavy Clay wares
Slip Casting and Pressure Slip Casting
Slip casting is a method for powder-based shaping of ceramic components that has been used for a long time in the traditional ceramic industry for the manufacture of tableware and sanitary ware. Slip Slip casting is occasionally also used in the manufacture manufacture of o f advanced (technical) ceramics. Slip casting is a filtration process, in which a powder suspension usually a water-based suspension is poured into a plaster mould, which by its porosity creates capillary forces and removes liquid from the suspension (slip). When the liquid (filtrate) is sucked into the plaster mould, the powder particles are forced towards the mould walls and a consolidated layer (filter cake) is gradually built up. When a desirable layer thickness has been obtained, the casting process is stopped either by having the excess slip removed, or by letting the casting fronts approach each other in the center of the piece to form a solid body. After a certain period of drying the shaped piece can be released from the mould for further drying drying and firing firing (sintering).
In slip casting, water is removed from the powder powder suspension by the water suction of the plaster mould and a consolidated layer consisting of packed particles builds up. When a desirable thickness has been reached the excess slip is removed (drain casting), or the casting proceeds until the casting fronts approach each other and a solid body has been obtained (solid casting). casting).
The advantages of slip casting as a forming method are mainly that complex geometries can be shaped, and good material homogeneity is generally achieved. Furthermore, the mould material is cheap. The disadvantages are that a large-scale production requires many moulds and large areas, coupled with the fact that the plaster moulds have a limited durability, as plaster of Paris erodes/corrodes in water processing. To get around these problems a method called pre pr essur e sl sl i p casting or pres pr ess su r e casti casti n g has been developed. Instead of plaster moulds, moulds of polymeric
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White Wares and Heavy Clay wares materials are used, and these have a porosity consisting of larger pores that do not give the same capillary forces but require an externally applied pressure to drive the filtration process. However, as much higher pressure (<40 bar or 4.0 MPa) is applied, this gives much faster casting cycles than in slip casting where the capillary forces correspond to a pressure of 1–2 bar (0.1–0.2 MPa). Furthermore, the high pressure gives such dry, cast pieces that de-molding can be done immediately and a new casting cycle can be started. The polymeric materials used have much better durability than plaster and, therefore, it is possible to achieve shaped products with better dimensional tolerances. The Swedish Ceramic Institute (SCI) possesses a pressure-casting machine, a small production unit, suitable for casting pieces up to about 1 dm3. In pre-studies for large-scale pressure casting, filter pressing can be used and SCI has designed a filter press adapted for use in a universal testing machine, where the plunger movement and load (pressure) can be accurately controlled.
Casting
Richard Burkett - SDSU 1993
Slip is a mixture of finely ground stone and clay with water added to make a creamy consistency. In casting method following steps are to be followed carefully. ?
Plaster molds for casting should be dried sufficiently. The mould should not be dried above 130° F or so otherwise it gets deteriorate. The mould for casting should be clean and free of all oil, mold release, or grease.
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If the mold is completely dry (no dampness whatsoever) it should be slightly dampened before slip casting. If the plaster is not slightly damp, the slip will cast with varying density (harder on the outside where the dry plaster quickly draws the water out/softer toward the inside) and cracking will be more likely to occur. Air bubbles and porous casts are another possibility when casting was done in a dry mold. A damp mold will also make dusting more effective (see step 4).
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Mould must be dusted before use. Talc is fine for low-fire casting slip. Silica is better for porcelain casting slip, as the talc will melt on the surface during firing. Too much dust and the fine details in the mold will be lost, and the casting may come loose prematurely and
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White Wares and Heavy Clay wares collapse. This dusting allows the casting to release from the mold earlier and more easily than it wo uld otherwise and is not always necessary with simple molds.
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The mould should be fastened together firmly and securely with large rubber bands, pieces of rubber inner tube, or heavy string tied, twisted, and wedged snugly about the mold. Be sure the mold is quite securely fastened, as the slip pressure inside even moderately sized molds is surprisingly great.
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Make sure that the casting slip is well blunged (mixed), free of lumps, and of the proper consistency. If not sieve the slip before casting.
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Pour the casting slip into the mold quickly, filling the mold to the very top. As the slip level drops in the mold, keep refilling the mold with casting slip. The actual time of casting will depend on the mold, slip thickness, and how thick of a casting is desired. How long the casting takes dependant on how thick the ware is required. The shorter the time the thinner and more delicate the body. During casting the thickness of the cast can be observed by pouring a small amount of slip out of the mold and cutting through the casting with a needle tool.
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Drain the excess casting slip from the mold (Incase of Hollow Casting). You may have to use a knife to open the sprue if it has cast shut. After most of the slip has drained out of the mold, prop the mold so that the sprue is down, and all remaining liquid casting slip continues to drip out. A good method is to place two sticks across the top of a bucket and prop the mold on the sticks to that it drains into the bucket. Allow the mold to drain upside down like this for at least 15-20 minutes. Avoid shaking the mold! This may loosen the casting from the plaster and cause it to collapse.
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Allow the mold to dry until it can be disassembled without tearing or collapsing the casting. This drying time will depend on the wetness of the mold and the thickness of the casting. Typical time from pouring to removal from mold is 1-4 hours, occasionally it takes overnight
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White Wares and Heavy Clay wares
Preparation of Slip In slip casting the first step to success is quality slip preparation. Casting efficiency of slip mainly depends on the following properties. For casting the slip should have a specific gravity of 1.75-1.8 and it should be fluid enough to pour easily. That’s all there is to it. No matter what is the water:clay:electrolyte recipe. For making a good quality of slip following factors are to be
considered. a. Composition. b. Grain Size. c. Flocculation & Deflocculation. d.
Viscosity of Suspension.
e. Density / Specific Gravity of Slip. Composition: As far as slip-forming characteristics are concerned, the body composition
can be divided into two categories, Clay containing bodies and non-clay bodies. Clay in suspension exhibit some colloidal properties, being fine grained and these colloidal properties are used to keep the particles of the body in suspension. And also adjust its Rheological properties. Different clays show different suspensibility. For e.g. China clay/ Kaolin, due to its coarse grains and open structure formation allow the water to move towards the mould surface producing rapid casting and quick hardening of the cast. But we cant get much green strength. Whereas if we use plastic clay/ ball clay we can get high green strength and fired strength due to their fine grain size. But too high an amount of ball clay lengthens the time of casting by forming a denser and less permeable skin on the mould surface. It is therefore customary to use a mixture of different types of clay so that variation in any one of them will not affect the casting slip. Grain Size and colloidal content: The processing of ceramic materials and their
composition are all aimed at obtaining finer grain size of particles so as to bring out their surface properties to the maximum advantage during forming stages, like plasticity, suspensiblity and sensitivity towards absorbed ions as well as to increase their reactivity during heating. Too fine grinding, say to colloidal size, however not only economical and impractical but also causes low casting rate of suspension, low packing density, high drying & firing shrinkage and other problems. For casting slips the grain size or rather its size distribution should be such
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White Wares and Heavy Clay wares that, it should be free from coarse particles. Otherwise they settle out in the suspension more quickly than the rest and should not be fine enough to retard the rate of casting. Flocculation & Deflocculation: It was known that surface of solids absorb ions present in
water when immersed in it. This is said to be due to broken bonds at the surface and to some extent due to the instauration effect by the lattice substitution. The individual particle in the colloidal suspension carries a charge even though the suspension whole remains neutral. Being charged particles they tend to attract other charged particles, usually ions in order to neutralize the charge. If the surface charge is positive (which is very rare) said to be hydroxyl ions are absorbed from the water to form a bound inner layer. This leads to an overall negative charge, which is balanced by an outer layer of cat ions. Often clay particles are negatively charged and absorb cat-ions directly. The size and charge of cat-ions determine the behavior of the suspension. Small sizes pack closer to the surface. Similarly cat-ions of large charge like Ca2+, Mg2+ and Al3+ are required in lesser numbers and so can pack into a smaller space to get closer to their surface. There are two forces acting on the particles in suspension, they are repulsion due to negative charges on the particles and the gravitational attraction due to the mass. Among these first one leads to Deflocculation and a stable state where as latter one leads to flocculation. So small ions with large charge like Ca2+, Mg2+ and Al3+ act as flocculant and ions with small charge like Na+ and K+ act as deflocculant. For simple reason that the former ions after charge neutralization allow the particles to come closer for gravity to act, where as later one does not. Before going on, we should consider deflocculants. Although the chemistry of how they work is quite complex, for now just think of a deflocculant as a source of ions that charge clay particles to repel each other electrostatically and thus produce a slurry that is thinner than it would otherwise be. Soda Ash & Sodium Silicate : Sodium silicate is a liquid deflocculant that has been the
standard for many years. Powdered soda ash is a ‘slip softener’ and produces protective colloids to deal with the anti-deflocculant effects of organic materials in the clays. These two materials work well together. Merits of Deflocculants in Slip
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Lower minimum viscosity can be achieved and maintained longer.
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They produce a slip with a faster casting rate at minimum viscosity.
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White Wares and Heavy Clay wares ?
They encourage a higher green strength, allowing removal from the mold at higher water content.
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They afford better absorption and penetrat ion into plaster.
Viscosity of the Suspension: Maintaining stringent control on slip viscosity is crucial. The
best measure of Deflocculation-flocculation of a suspension is the viscosity. Viscosity refers to the mobility of the slip; its “thickness” or “runniness”. A slip that has high viscosity is thick like syrup and one that has low viscosity is fluid. A deflocculant is used to lower the viscosity of a slip and make it more fluid. This lower
Viscosity
viscosity, in turn, makes it possible to add more dry material, which again raises the viscosity. It was known that by adding electrolyte the viscosity of
pH y t i s o c s i V
slip could be decreased. In the same time pH value of slip gradually increases. The viscosity decreases for a particular level
Electrolyte
and it increases suddenly after certain addition of deflocculant.
Specific Gravity This is the weight per unit of volume of the slip. Specific gravity is
defined as the comparison of a liquid’s weight with the weight of an equal volume of water. ( In metric it is even simpler: water weighs one gram per cc (ccs and milliliters are the same). If a liquid weighs 1.8 grams per cc, then it has a specific gravity of 1.8; it is 1.8 times heavier than water.) A slip with a specific gravity that is too high is said to be “heavy”. If the viscosity is too high, it is said to be “thick”. The more water in a slip, the lower its specific gravity will be. The more solids, the higher it will be. As mentioned, slip with too much water will soak the molds more quickly, give slow casts, and result in excessive shrinkage that cracks the ware. For drain casting the specific gravity of slip is maintained in between 1.65 to 1.85 and the viscosity in the range of 1 to 1.5poise. And for solid cast slip specific gravity is in the range of 1.75 to 1.95 and viscosity from 5 to 50 Poise. Normally a specific gravity of 1.8 can be taken as a commercial average. Such a slip is made in to a pour able slip by judicious use of electrolytes. The slip will then contain only 25-26% of water.
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White Wares and Heavy Clay wares Preparation of Casting Slip
In general filter pressed cakes from wet process are blunged in blunger with calculated amount of water to form a slip. While making the casting slip, most common method of preparation is to achieve the specific gravity first using the needed deflocculant, then fine-tune the viscosity using a little more of the same deflocculant. Specific gravity of 1.75 to 1.8 is maintained with water content of about 35-45%.
Empty Mould
Mould Filled with slip
Excess Slip is poured Off after attaining sufficient Thickness
Ware releases from mould on drying
Final Shape Hollow Shape
Hollow Casting
Empty Mould
Mould Filled with slip
Ware releases from mould on drying
Final Shape Solid Shape
Solid Casting
Blank Mould
Mould filled with slip
Ware released from mould
Final Shape
Core Casting
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