Ceramic forming techniques are ways of forming ceramic shapes.
This can be used to make everyday tableware, such as a teapot,
to engineering ceramics such as computer parts.
There are many forming techniques to make ceramics, but one
used for making mass quantities of commercial tableware is
slip casting. This is where slip, liquid clay, is poured into
a plaster of Paris mold. The water in the slip is sucked out
of the slip, leaving an inside layer of solid clay. When this
is thick enough, the excess slip can be removed from the mold.
When dry, the solid clay can then also be removed. The slip
used in slip casting is often liquified with a substance that
reduces the need for additional water to soften the slip;
this prevents excessive shrinkage which occurs when a piece
containing a lot of water dries.
The original mold for a slip cast, as well as the pieces
themselves in many individual works of ceramics, are generally
thrown on a potter's wheel. The advantage of the wheel in
forming ceramic vessels is that its rotation allows symmetrical
adjustments to the piece, resulting in a uniform and balanced
pot. Throwing, as forming ceramics on a wheel is called, consists
of three or four steps. First, the clay must be centered on
the wheel. (The pot will likely be ruined if this step is
completed improperly or if the piece is allowed to become
uncentered at any point in the process.) Second, the center
must be opened. Third, the clay forming the walls of the pot
must be squeezed gently in order to force the clay upwards,
causing the pot to become taller. Fourth (this step is omitted
entirely in the creation of simple objects such as cylinders
and bowls) the pot must be coaxed into the desired shape by
carefully pushing in the appropriate direction. A finished
pot is cut off the wheel with a wire tool. "Feet"
may be trimmed into the bottoms of some pieces; this is accomplished
by allowing the thrown pot to dry to leather hard and then
centering it upside down on the wheel, then carving into the
middle of the base of the piece with a trimming tool.
There are also several techniques of handbuilding, such as
pinching, soft slab, hard slab, and coil construction.
When forming very thin sheets of ceramic material, "tape
casting" is commonly used. This involves pouring the
slip (which contains a polymer "binder" to give
it strength) onto a moving carrier belt, and then passing
it under a stationary "doctor blade" to adjust the
thickness. The moving slip is then air dried, and the "tape"
thus formed is peeled off the carrier belt, cut into rectangular
shapes, and processed further. As many as 100 tape layers,
alternating with conductive metal powder layers, can be stacked
up. These are then sintered ("fired") to remove
the polymer and thus make "multilayer" capacitors,
sensors, etc. According to D. W. Richerson of the American
Ceramic Society, more than a billion of such capacitors are
manufactured every day. (About 100 are in a typical cellular
telephone, and about a thousand in a typical automobile.)
Details of the binder systems and defect-free forming techniques
are described in "Organic Additives And Ceramic Processing"
by D. J. Shanefield (Kluwer Acad. Publ, 1996).
When forming technical ceramic materials from dry powders
prepared for processing, the method of forming into the shape
required depends upon the method of material preparation and
size and shape of the part to be formed. Materials prepared
for dry powder forming are most commonly formed by "dry"
pressing in mechanical or hydraulic powder compacting presses
selected for the necessary force and powder fill depth. Dry
powder is automatically discharged into the non-flexible steel
or tungsten carbide insert in the die and punches then compact
the powder to the shape of the die. If the part is to be large
and unable to have pressure transmit suitably for a uniform
pressed density then isostatic pressing may be used. When
isostatically pressed the powder takes the shape of a flexible
membrane acting as the mold, forming the shape and size of
the pressed powder. Isostatic presses can be either high speed,
high output type of automatic presses for such parts as ceramic
insulators for spark plugs or sand blast nozzles, or slower
operating "wet bag" presses that are much more manual
in operation but suitable particularly for large machinable
blanks or blanks that will be cut or otherwise formed in secondary
operations to the final shape.
If technical ceramic parts are needed where the length to
diameter ratio is very large, extrusion may be used. There
are two types of ceramic extruders one being piston type with
hydraulic force pushing a ram that in turn is pushing the
ceramic through the loaded material cylinder to and through
the die which forms the extrudate. The second type of extruder
is a screw, or auger, type where a screw turns forcing the
material to and through the die which again shapes the part.
In both types of extrusion the raw material must be plasticized
to allow and induce the flow of the material in the process.
Complex technical ceramic parts are commonly formed using
either the injection molding process or a process known as
"hot wax molding" both of which rely upon heat sensitive
plasticizers to allow flow of the material into a die which
forms the part. The part is then quickly cooled for removal
from the die. The injection molding process is much like injection
molding of plastic materials using various polymers for plasticizing
while the hot wax molding process largely uses parafin wax.
There are also many more techniques, such as gel casting
and injection molding that are often used to create engineering
ceramics.
