Ceramic Processing
Module CP- Ceramic Processing
Sample Lesson
Lesson CP-4: Forming Technology
TABLE OF CONTENTS
(Bookmarked for Convenience)
Introduction
In this lesson we will discuss the operations that form our ceramic
materials into a recognizable shape or product. You will note that we have divided the
subject into Traditional and Non-Traditional Forming Processes. This is a
somewhat arbitrary division, since some of the processes we have called non-traditional
have been in use for decades and are considered to be quite conventional to the industries
using them. On the other hand, we have put isostatic pressing in the traditional category,
even though it is a technique that has been widely used for only the past half century.
Compared to the "original" traditional processes of pressing, plastic forming,
or slip casting, which have been around for centuries or millennia, isostatic pressing is
a very new process.
Traditional Forming
Processes
We have grouped this section under three primary headings for the three
really traditional processes, Pressure Fabrication,
Plastic Forming, and Slip
Casting. However, under these headings are a number of sub-headings. Some
of these processes are a bit less traditional, but they have basic similarities and thus
fit fairly well. Under Pressure Fabrication we will discuss Conventional
Pressing and Isostatic Pressing.
Under Plastic Forming we will discuss Extrusion,
Jiggering, and Plastic
Pressing. Under Slip Casting, we will cover Conventional
Slip Casting, Pressure Casting,
and Tape Casting.
Pressure Fabrication
The most traditional process in this category is what has commonly been
called dry pressing. Dry pressing is not a
particularly accurate term, since most parts that are "dry pressed" are not
truly dry. They usually contain a few percent moisture, or a liquid other than water
in a few cases. The term pressure fabrication is more accurate, since this process is
characterized by the application of pressure to compact a ceramic powder or particulate
mass. When the pressure is applied basically uniaxially,
then we have the oldest traditional process. If pressure is applied from all directions,
or isostatically, then we have a newer
process called isostatic pressing.
Uniaxial pressure fabrication is a very common forming process. It is used
to form many tile and other flat shapes as well as simple shapes such as relatively short
cylinders. The cross-sections that can be formed are usually fairly simple geometrically,
although the pressed shapes can be machined into more complex geometries. The height is
usually limited relative to the lateral dimension or diameter. Pressing of wall tile is
one example of the process that has been highly automated. Very large hydraulic presses
are used to press a large number of tile shapes at each stroke.
Netzsch and Dorst Hydraulic Presses
Cammerzell 250-ton Trim Press
Laeis Bucher 1500-ton Hydraulic Tile
Presses
Laeis Bucher 3600-ton Hydraulic Refractory
Press with Comprehensive Numerical Control
The basic forming process is characterized by uniaxially pressing in a die
a powder material with a small amount of water or other liquid (1-7%). Normally there is a
binder material included in the composition to increase the green strength, or pressed
strength, of the part so it can be more easily handled for subsequent manufacturing
processes. This may be a natural material such as clay, or it may be an organic or other
inorganic binder of some sort. These were mentioned in Lesson CP-1, Raw Materials. The
binder phase contributes a small amount of plasticity, but this process is basically a
non-plastic forming process. Free-flowing pressing powder is delivered to a die cavity,
which consists of a rigid die cavity frame and moveable lower and upper die punches. The
amount of material delivered to the die cavity is controlled by the depth of the cavity,
which can be varied by moving the lower punch, and the bulk density of the pressing
powder. It is highly desirable that the material be delivered in such a way that the
cavity is filled fully and very uniformly. In a high speed hydraulic press, such as the
ones shown above for the tile industry, uniform die fill is not easy to achieve, and the
press manufacturers have expended a great deal of effort to develop good fill methods. Die
fill is greatly enhanced and made much simpler if the pressing powder is very
free-flowing. For products such as tile and technical ceramics, spray dried pressing
powder is ideal because of its flow properties and uniformity. In these industries, use of
spray dried powder has become nearly universal. It is also important that the particle
size distribution of both the original raw materials and the pressing powder (they will be
different for spray dried pressing powders) be consistent with time. The green density of
the pressed compact, as well as its uniformity, will depend on this. Pressed thickness
will also depend on the bulk properties of the powder because of their influence on die
fill and compaction. It is desirable to maximize the bulk density of the pressing powder
while still maintaining good flow. If the pressing powder has not been spray dried and has
less than desirable flow properties, as is likely with refractory brick bodies, for
example, control of die fill will be more difficult. Pressing speeds will likely have to
be lower to allow good die fill.
After the die cavity is filled, the powder is compacted in the cavity
between the upper and lower die punches. The powder as delivered to the cavity obviously
has much lower bulk density than the pressed compact, typically by a ratio of two to one
(pressed to powder density). The volume difference consists of air that must be removed
during the pressing step. This is not a trivial matter, especially in high speed presses.
If not given time and a mechanism for escape, air can be trapped and cause physical
defects, such as laminations. Pressing is usually done in steps, with the amount of
compaction and punch travel increasing with each step. On old mechanical friction presses,
which are still in some use, this pressing action was and is controlled by the press
operator. They usually became quite skilled at the "bumping" required to prevent
problems, but each operator performed the steps a bit differently. In modern hydraulic
presses, the pressing action can be programmed and controlled by computers. As mentioned
above, a simple die consists of the die block, upper punch, and lower punch. More complex
die sets might also contain secondary punches, core rods, pins, and relief on the punch
faces and/or in the die block. Die design, alignment, and maintenance are important to
successful pressing operations. The full pressing sequence can be quite complicated and is
usually completed in a surprisingly short time. Compaction may involve prepress, die table
movement, pauses in motion and dwell time for air release, variation in pressing speed,
etc. For example, the die table can be moved up after die filling, thus increasing the
cavity volume and causing the top surface of the powder to be lower than the cavity lip.
This prevents powder from being blown out of the cavity when the upper punch enters the
die. In the most modern presses, all such actions can be precisely programmed and
controlled. Some presses can measure and display the forces and movement of the top punch
and die frame as a function of time during pressing and display a curve of force vs.
movement on a monitor. This information can be used to identify sources of cracks, to
improve the pressing process, and optimize die design. It must also be noted that uniaxial
pressing does not provide uniform pressed density along the length of the pressed part.
Density will be highest at the top and bottom faces and lowest in the middle of the part.
This effect is greater if the part is thick relative to the lateral dimension. This
difference is caused by die and interparticle friction and is inherent in uniaxial
pressing. Density variation during pressure fabrication has received a great deal of
study, since it can dramatically affect product properties and quality. The photos below
show density gradients in six pressed green pieces, showing both the lower density in the
center and wall friction effects.
Density Gradients in Pressed Green Parts
Density gradients in pressed ware carry over to the fired piece, as is
illustrated in the photos below.
Development of Central Fired Defect from
A) Green B) Fired 5 min. and C) Fired 20 min.
After all pressing actions have been completed, the part is ejected from
the cavity by the lower punch. During ejection from the die, friction with the die walls
and part expansion as cavity pressure is release can cause mechanical problems in the
part. Removal of the pressed part from the press is almost always done by an automated
handling system on tile presses but may sometimes be done by hand on brick presses.
There are two basic types of pressing operation that are available: pressing-to-size and pressing-to-density.
In pressing-to-size, the motion of the punches is controlled so that the pressed thickness
is consistent from part to part. If the die fill and powder properties are not consistent,
the pressed green density will vary. This can lead to subsequent variation in shrinkage
during firing and in fired properties. In pressing-to-density, the pressing pressure is
limited to the pressure required to achieve a given pressed density. If the die fill and
powder properties are not consistent, the pressed thickness will vary while firing
characteristics will be more consistent.
We will discuss another pressure fabrication (dry
pressing) manufacturing process for forming continuous tapes, called roll compaction, in this
section and duplicate it in the section on tape casting below. The process is based on
feeding a prepared powder (usually spray dried) between two large diameter steel rolls.
The powder normally contains the ceramic raw material(s), an organic binder, and a small
amount of water. The powder is pressed into a tape by the rolls. The key to the process is
control of the powder feed to the "nip" point of the rolls. Typical tape widths
are 200-300 mm, and thicknesses range from 0.1 to 3 mm. Capital investment tends to be
high, and maintenance of the roll surfaces can be a problem. However, there is very little
drying necessary. This process is an alternative to the more commonly used tape casting
process for producing thin, continuous tapes for the electronics industries.
Isostatic pressing applies
pressure from all three directions rather than only uniaxially. This results in more
uniform density and greater compaction. The basic process uses a chamber filled with a
liquid which is applied under hydraulic pressure to all surfaces of the part being formed.
The part being pressed is enclosed in a shaped membrane or "bag", which serves
as the mold for forming the part and isolates the powder from the hydraulic liquid.
Loomis Carousel Dry-Bag Isostatic Press
Wet Bag Isostatic Pressing
Dry Bag Isostatic
Pressing
The figures above show a drawing of a carousel dry
bag isostatic press and then schematics of the processes of wet bag and dry bag isostatic
pressing. In wet bag pressing, the powder to be compacted is loaded into a bag mold while it is outside
the press (A above left). The sealed bag (B above left) is placed inside the pressure
chamber (C above left), hydraulic pressure is applied (D above left), and compaction
completed (E above left). The bag and compressed part are then removed from the chamber
and the part removed form the bag. This obviously involves many steps and considerable
labor. Dry bag pressing is similar in nature except that the bag is integrated with the pressure
chamber. The powder (A above right) is loaded into the tooling dry bag (shown empty in B
above right and filled in C above right). Pressure is then applied (D above right) and the
part is compacted. Finally, the compacted part is removed for the chamber (E above right).
This method simplifies and speeds the process.
Isostatic pressing enhances uniformity of density in
pressed parts, but it does not produce absolute uniformity. The density at the very center
of a pressed part will still be lower that the density at surfaces. Early isostatic
pressing operations were slow and labor intensive, since many were hand-operated wet bag
systems. New automated dry bag pressing systems have allowed isostatic pressing to be used
on high volume products like dinner plates. Automated isostatic pressing has replaced
plastic forming methods like jiggering in producing plates in many manufacturing
operations. There have been problems with density differences on the foot of the plate,
where the part thickness is greater, but in general the production rates and lower costs
have justified the change. Even some deep shapes like cups are being pressed
isostatically.
Plastic Forming
Plastic forming traditionally relies on a plastic component, very often a
clay, made plastic by the addition of sufficient water. There are cases where a non-clay
plasticizer,such as polyvinyl alcohol, methocellulose, or other material, is used instead.
In most clay-based plastic forming processes, water content will be in the 15-25% range.
In a few cases, a liquid other than water is used.
Extrusion is
probably the most common of the plastic forming processes in use. Large tonnages of brick
and other structural clay products are produced by extrusion, and it is used as an initial
forming step in many whiteware manufacturing facilities. In this case, it might be
followed by jiggering, another plastic forming process which will be discussed below.
Large and small diameter hollow clay sewer pipe is extruded, with a bell-shaped end formed
at one end. Blanks for large electrical post insulators can be extruded, dried, and then
machined in the green or "leather-hard" state, thus forming the typical flutes
on the outer surface. These will normally be a porcelain composition and contain ball
and/or kaolin clays to give plasticity. Small diameter tubes can be extruded from
porcelain, mullite, or alumina compositions for use as thermocouple protection tubes.
These can contain one, two or even four holes down their length. In some cases,
particularly for high alumina compositions, organic plasticizers may be required.
In an extruder, also sometimes called a pug mill, the plastic ceramic body
is forced through a die to produce a shape. The raw materials and appropriate amounts of
water will be mixed in a plastic mixer, perhaps in a high intensity mixer. The plastic mix
will be fed into the upper hopper of the extruder, where it feeds directly into the feed
chamber. Most large extruders are auger fed and are thus continuous. Virtually all
extruders will have an de-airing device to remove any air brought in with the starting
material. De-airing is accomplished by providing a vacuum chamber between the material
entry camber, which will have its own auger feeder, and the actual extrusion chamber. This
is critical so that air is not trapped in the material during extrusion, which would cause
voids or delaminations in the extruded material. Smaller extruders can be batch-type,
ram-fed designs. For hollow products, there will be an internal die to form the central
hole or holes. This will be supported by three or more "spiders" attached to the
wall of the extruder. The material must flow around the spiders and re-knit at the exit
end. Proper die design and material control becomes critical in such cases. Velocity
differences can occur in the material as it flows through the extrusion chamber and exits
through the die. Part of this is caused by friction between the material and the chamber
walls. These velocity gradients can cause laminations and other defects in the final
product or can cause the material to "curl" or deviate from a straight line
after leaving the die. Materials with high plasticity are more prone to such problems. The
inclusion of non-plastic components, such as grog, and help alleviate such problems. Grog
is pre-reacted (pre-fired) material of the same composition as the material being
extruded. In sewer pipe or brick, this is often crushed defective or waste, fired product.
In clay-based compositions, the clay platelets will tend to align parallel to the axis of
extrusion, and this can cause differential shrinkage rates during subsequent drying.
Proper die design and
balance are important to successful extrusion. Dies are generally designed to provide a
gradual taper form the extrusion chamber diameter to the final die outlet diameter. It is
common for one extruder to handle a variety of die diameters. The die should have a final
straight section or land of sufficient
length to assure adequate compaction pressure on the mix. If this is not done correctly, a
defect called feathering, which consists of
surface tears in the material, can occur.
Eirich Extruder
Loomis Ram Tilt Extrusion Press
Lancaster Autobrik Machine
Another plastic forming process is the stiff mud process. This is
an ancient method that originally was used to make mud brick in wood molds. The process is
still used; the author has seen the process in use to make building brick in Mexico. The
"Autobrik" machine shown above is a mechanized version of the stiff mud process
to make what appear to be rustic, hand-made brick.
Jiggering is a process commonly used in the whiteware industry to form products
like plates, platters, cups, bowls, suspension insulators, and similar shapes. In this
process, a piece of plastic material, normally prepared by extrusion, is placed on a mold
and formed into a pancake bat by pressing against a flat, rotating plate. The bat is then
slapped onto a plaster mold in the shape of the piece to be formed. This might be the
eating surface of a dinner plate, or the cup-receiving face of a saucer. This mold is also
rotated and the bat is forced onto the mold using a shaped cutting tool or jigger head.
The mold may contain decorative surface relief. After forming, the piece is dried on the
mold and then removed.. Cups can be formed in a similar manner in a deep mold with a
rotating metal jigger head. Jiggering was originally done manually, but as is true of most
operations today, the process is now highly automated. One disadvantage is the large
number of molds required for production. They require drying between operations, and since
the part must also be dried on or in the mold until it can be handled, molds spend a great
deal of time essentially out of service. Isostatic pressing has replaced jiggering form
many shapes since it is faster and does not require much water removal from the formed
part (however, since spray dried powder is needed, there is a large energy penalty for
water removal at the body preparation stage).
RAM pressing is another plastic forming technique, typically used to form large
platters, particularly oval ones that cannot be jiggered easily, and large floor tile.
Modern RAM pressing uses a hydraulic press with two porous or permeable dies to form the
finished shape. These have typically been made of gypsum cement (plaster), although harder
and stronger compositions of several types have been developed to increase die life. An
associate of the author developed a glass-bonded alumina die composition for abrasive
floor tile pressing that very dramatically increased die life and improved consistency of
pattern relief on the product. RAM pressing was introduced in the mid-1940's. The original
presses were fully manually controlled, but newer presses have sophisticated PLC
controllers to give automated and consistent process control and improved worker safety.
Presses range from 30 to 150 ton capacity.
During RAM pressing, a bat of plastic material is placed on the lower mold and is pressed to shape
by the upper mold. To accomplish removal of the part, air is applied behind the lower
mold. The air passes through the mold material and gently blows the part free. The part
stays with the upper mold and then, when the upper mold is at its raised position, is
released onto a tray or holder by application of air pressure behind and through the upper
mold.
RAM dies are typically reinforced by metal and have
an air purge coil incorporated in their design. Early systems used water for press
cooling, but new systems use recirculated cooling oil. Dies can be quite large, up to 2 x
2.5 feet.
Slip Casting
Conventional slip casting
utilizes a stable suspension of the ceramic body, normally in water. This suspension, or
casting slip, is poured into a plaster mold. Water is absorbed by the mold at its surface,
resulting in the formation of a layer of material that is low enough in water content that
it will be self-supporting when the mold is removed. After a layer of proper thickness has
formed, the remaining slip is poured or drained from the mold. The part is normally
allowed to remain in the mold for additional water removal, by mold absorption and
evaporation, until the part has become physically strong enough to support itself. The
mold is taken apart to release the part, which is then dried.
Control of the casting slip is critical in this process. We will spend a
bit of time discussing slip rheology.
Discussion of the control aspects will occur in Lesson CPC-5, Control of Forming
Operations. Basically, one wants to have a fluid slip, which will easily fill the mold and
not trap air bubbles, but one also wants to maximize slip bulk density. This minimizes the
amount of water that must be removed in the casting process, thus maximizing the density
of the cast piece and speeding the process. These two mutually exclusive properties can be
attained by controlling slip rheology, or flow, accomplished by the use of proper deflocculants. We will discuss three basic types
of fluid flow, and one sub-set, of importance to casting operations. Many liquids are Newtonian or ideal
liquids. Water is an example. When a shear stress is applied to an ideal liquid, its flow
is proportional to the applied stress. Doubling the applied stress will double the flow
rate. A Newtonian liquid begins to flow as soon as a stress is applied. This is shown in
figure a) below. Bingham flow is similar in
nature, but flow does not start until a "threshold" stress is reached. This is
not desirable for good mold filling in casting. Ketchup is a common example. Tipping the
bottle does not produce enough applied stress to cause much flow, perhaps not even enough
to reach the threshold level. Modest shaking probably will not produce much. Violent
shaking will usually result in large deposits of ketchup on your food, or you. The flow
response is shown in figure b) below.
a) Newtonian or Ideal Fluid Flow
b) Bingham Flow
The second major flow category is thixotropic
flow. In this case, shown in c) below, the rate of flow increases
dramatically at higher applied stresses, and there generally is a threshold stress to
initiate flow. Many lake beds and clay-containing landfills are thixotropic. They will
support buildings and other structures until an earthquake occurs, producing large stress
levels. Then they turn to a liquid and can no longer support anything. Thixotropic flow is
very undesirable in slip casting, since the slip will not flow easily into the mold under
the stress of gravity alone. Thixotropic casting is unrelated to slip casting and is used
for certain castable refractory materials. These castables are designed to be highly
thixotropic, and it is an interesting example of the use of thixotropy. The mix looks like
a wet powder, but when it is vibrated, it turns into a very fluid liquid. As soon as the
vibrational stress is stopped, the material becomes quite solid again very quickly.
Thixotropic casting is not done into plaster molds and does not rely on the absorption of
water from the material to build up a layer. The third flow category is dilatant flow. This is virtually the opposite of
thixotropic flow. At low stresses there is normal flow, but when high stress is applied,
there is unexpected resistance to flow. Wet beach sand is an example. You can run your
fingers or toes through it easily at low speed, but when you run on wet sand, it feels
like rock. The resistance results from a high stress-induced breakdown of the water
separating and lubricating the sand grains. At high stress the grains are in direct
physical contact. Dilatant flow is an interesting thing to know about, but it is not a
factor in slip casting.
c) Thixotropic Flow
d) Dilatant Flow
There are a number of casting slip
properties that are desirable to allow an optimum process. These
properties include: 1) low viscosity (high
flow rates) to allow all parts of the mold to be easily filled and to prevent trapping of
air bubbles; 2) high specific gravity to
shorten casting time, increase green density, lower drying shrinkage, and lower the amount
of water that must be processed; 3) a deflocculated slip,
which allows 1) and 2) above to occur in the same slip; 4) good
casting rate; 5) easy mold release;
6) good drainage from the mold at the end of
the cast; 7) adequate green strength in the
cast layer to allow easier handling; 8) low drying shrinkage;
and 9) Newtonian flow.
Newtonian flow is desirable
in a casting slip because it will allow the slip to flow
well under all applied stress conditions, will aid in filling all mold cavities, will help prevent bubbles from being trapped, will help prevent sudden slumping caused by thixotropy
during handling and transport of the cast piece, and will aid
in achieving uniform casting rates. Casting slips, especially when the
specific gravity is high and the body contains clay minerals, tend to be thixotropic. The
particles tend to aggregate, a process called flocculation. To counter this, a variety of
chemicals are used to deflocculate the slip, or break up these aggregates. The basic
process is to add like charges to the particle surfaces so they repel each other. This
decreases viscosity and changes the flow characteristics toward or to Newtonian flow. A
number of deflocculation agents are in use, including sodium silicate (water glass),
sodium carbonate, sodium salts of phosphoric acid, penta-sodium tripolyphosphate (STP,
given by the formula Na5P3O10), sodium
carboxymethylcellulose (Na-CMC), poly(vinyl alcohol) (PVA), polyvinylpyrrolidone (PVP),
and others. By way of a simplified explanation, deflocculants containing the Na+
ion act to cause surface replacement of Ca2+ ions on the particle surfaces.
Since not enough Na+ ions can find room on the surface, the particles are given
a net negative charge and repel each other. A drawback of using deflocculants containing
Na+ ions is that the composition then contains a powerful fluxing agent, which
may be detrimental to firing or final electrical properties. The organic deflocculants
leave no "residue" like this behind.
It is much easier to produce a stable casting slip for compositions
containing clay minerals. They act as suspension agents and binders. They aid in achieving
desirable flow characteristics. Producing a workable casting slip for bodies devoid of
plastic materials like clay is more difficult. Alumina presents a good case in point. It
is a dense material and is thus difficult to keep in suspension. A high alumina ceramic
composition will have no clay, and if the alumina content must be very high, it will
consist of nearly pure alumina. Materials like Na-CMC act as combined binders and
deflocculants. The curve below shows the effect of additions of Na-CMC to a high specific
gravity alumina suspension. As is typical for deflocculation curves, the addition of
Na-CMC dramatically decreases viscosity (round dots), with a minimum at 0.35 wt% to 0.65
wt%. Above this level, viscosity again rises. The thixotropy index (square dots) reaches
essentially zero (no thixotropic behavior) at about 0.65 wt%.
Flow Properties for Alumina Casting Slip
Vs Na-CMC Addition
As can be seen in the graph below for this composition, the green density
reaches a maximum and the shrinkage reaches a minimum at this optimum level of Na-CMC
addition. This type of data, achieved by careful experimentation, is invaluable in
developing optimized casting slips and casting processes.
Cast Properties for Slip Cast Alumina Vs
Na-CMC Addition
Pressure slip casting
is basically the same process but with pressure applied to the slip in the mold. This
helps force water from the slip through the part and into the mold. The effect of applied
slip pressure is shown in the curve below. The dashed line to the right gives the cast
thickness Vs casting time with no additional applied pressure. One atmosphere pressure = 1
bar = 105 Pa. As can be seen, increasing the applied slip pressure
dramatically cuts the time required to develop wall thickness.
Effect of Casting Pressure on Cast
Thickness Vs Casting Time
Pressure casting requires special molds and stronger mold materials, which
can withstand the applied pressure. Plaster molds cannot withstand the pressures used, and
special plastic materials have been developed. These materials must have high porosity,
high mechanical strength, and good elasticity. The latter property is required to allow
mechanical pressure to be used to provide a very tight seal around the periphery of the
molds without mold cracking. A typical medium pressure casting machine and molds are shown
below.
Typical Netzsch Medium Casting Machine
Mold Line
Views of Molds in Medium Pressure Casting
Machine
Pressure casting is being applied in the sanitaryware industry and has
produced a number of advantages. Casting times are significantly cut. Parts can be easily
demolded. Molds require no drying between casting cycles and thus can be returned to
service immediately; an air purging system is used to dewater molds. Mold life is much
longer than conventional plaster molds, and fewer defects occur because of mold wear.
Product quality is more consistent and the cast part has less moisture to remove. This
eases drying requirements and cuts drying defects and losses. Parts with variable
thickness are easier to mold. One person can operate two or three casting machines,
including fettling of parts, and the operation can be run two or three shifts per day. The
net result is greater throughput, lower labor costs, and lower overall production costs.
Centrifugal slip casting
is another means of increasing pressure at the casting face. In this case, a cylindrical
plaster mold is rotated about its axis, and the centrifugal pressure aids in the casting
process. The pressures are lower than in the process described above, and the process has
been used to produce parts with multiple layers of different compositions. The process is
limited to relatively simple cylindrical geometries and is shown schematically below.
Schematic of Centrifugal Slip Casting of
Small Gear Shape
Tape casting is a
second cousin to the casting processes described above. The process is used to produce
thin layers of a flexible tape containing the ceramic composition. It is a common process
for substrates for electronics, such as semiconductor substrates, and to form layers for
multilayer capacitors. It is very difficult, if not impossible, to form very thin ceramic
parts by traditional methods such as dry pressing or extrusion. This is especially true if
holes for pins or feedthroughs are needed. The tape casting process has been developed and
refined over the past fifty or so years to meet this need.
The basic tape casting process
consists of producing a thin, continuous layer of a ceramic slip on a non-porous substrate
and removing the liquid phase to give a thin layer of the ceramic composition. The liquid
used to suspend the ceramic materials has typically been an organic solvent and not water.
A polymer binder is also typically used to give the tape needed strength. Since the
process does not rely on absorption of the liquid carrier by a porous material like
plaster of Paris, the liquid carrier must be evaporated. Because the layers are very thin,
no bubbles or other disruptions can be tolerated. Use of solvents greatly enhances the
ease of producing a defect-free, thin, continuous, and handleable tape. Recently, there
have been environmental and health-related pressures to use water as the liquid medium,
and considerable progress has been made in this direction. Solvents, however, are still
common. Thin layers are easily produced by the "doctor blade" technique. This is
a flat knife blade that is held slightly above the substrate onto which the material is to
be deposited. A reservoir of the ceramic slip is placed behind the blade so that material
can flow out through the gap between the blade bottom and the substrate. Either the doctor
blade assembly is moved over the substrate, which might be a glass plate in this case, or
a flexible substrate is drawn continuously under the assembly. In production environments,
the latter arrangement is universal. Typically, a long Mylar sheet is pulled under the
doctor blade so that a layer of ceramic slip is placed on it, the solvent carrier is
evaporated and collected for reuse, and the Mylar sheet/ceramic tape is rolled up for
storage before it is used in subsequent manufacturing steps. With appropriate binders to
give the tape adequate strength, the tape can easily be cut and punched for a variety of
part geometries. Poly(vinyl butyral) is a commonly used binder. Current tape casting
machines can produce tape thicknesses from 12mm up to
3+mm. Tapes as thin as 5mm have been made. Metal conductor
paths can be silk screened onto the tape. Layers of tape can then be assembled into
multilayer parts, such as capacitors or electronic packages, and fired. Common products
include alumina substrates, barium titanate-based capacitors, as well as related
compositions, and polymer-based membranes for lithium-ion batteries. Current developments
include the use of finer particle powders for thinner layers, including use of nanosized
particles.
Double Doctor Blade Assembly for Precision
Tape Casting
Typical Small Tape Casting Machine
 |
| Large Commercial
Tape Casting Unit |
We are going to duplicate the discussion of an
additional manufacturing process for forming continuous tapes, called roll compaction, in this
section even though it is a pressure fabrication (dry pressing) process. The process is
based on feeding a prepared powder (usually spray dried) between two large diameter steel
rolls. The powder normally contains the ceramic raw material(s), an organic binder, and a
small amount of water. The powder is pressed into a tape by the rolls. The key to the
process is control of the powder feed to the "nip" point of the rolls. Typical
tape widths are 200-300 mm, and thicknesses range from 0.1 to 3 mm. Capital investment
tends to be high, and maintenance of the roll surfaces can be a problem. However, there is
very little drying necessary.
Gelcasting is a
process that could be categorized several ways. It is a casting process, so one might put
it here in the slip casting area as a similar process. But it is a new process, somewhat
related to sol gel processing, but still quite different. It even has some similarities to
injection molding. It involves casting a part from a mix of ceramic powders and an organic
gel. Since it is a new, non-traditional process, we have arbitrarily elected to discuss it
below in the "non-traditional" forming processes. If you would like to study it
now, click here to link to the gelcasting discussion.
Non-Traditional
Forming Processes
As we did above in the Traditional Processes section, in this section on
Non-Traditional Processes we will have a number of sub-groupings. These will include the
processes of Hot Pressing (and isostatic hot pressing), Vapor
Deposition, Injection Molding,
Gel Casting, and Sol
Gel Processing. Some of these have been around for a while, but they are
relatively specialized and are much less commonly used in the ceramic industry. We have
also included a section on Miscellaneous Processes
to allow discussion of processes that do not fit anywhere else.
Hot Pressing
Hot pressing is analogous to
conventional pressure fabrication except that pressing occurs at high temperature.
Basically, the process combines pressing with firing. In many cases, this is the only way
some ceramic compositions can be produced at full or theoretical density. Generally, only
simple shapes can be made. The process is limited by the die materials available, since
they must contain the ceramic part under high pressure at high temperatures. Graphite dies
have traditionally been used, but graphite is a weak material and must be protected from
oxidation.
Elatec Hot Press
Hot isostatic pressing, or HIP, is basically isostatic pressing at elevated
temperatures. Pressure is applied from all sides as the part is held at elevated
temperatures. Thus, this process combines isostatic forming and firing into one step. We
might also call HIPing "pressurized sintering". The process often allows
densification to occur at lower temperatures than would be the case without the applied
pressure, or it allows attainment of theoretical density in materials where this would be
difficult or impossible otherwise. Containment of the part as pressure is applied requires
special materials and ingenious designs.
The HIP process was developed on the basis of using a containerized powder
or preform shape. Glass or metal containers can be used. If a presintered part is used, a
container may not be required. The part to be HIPed is placed in a furnace chamber, which
is pressurized with a gas, usually argon. Nitrogen is commonly used when HIPing nitrides,
such as silicon nitride, to prevent loss of nitrogen from the composition. The part is
heated under pressure through a prescribed temperature/pressure/time cycle. HIP furnaces
can range in size from 2" to 60" in diameter. Furnaces of 10" to 20"
diameter are becoming more common for ceramic applications. Pressures can range to 3,000
bar (about 45,000 psi). Firing temperatures generally range from 500oC to 2200oC.
Fully automated and very sophisticated control systems have been built into HIP furnaces.
Complex shapes can be produced with near-net shape capability. This offers an advantage
over hot pressing, in which only simple geometries can be handled and in which expensive
machining is required for complex shapes. HIP processing can very significantly cut the
final cost of complex parts when compared to the hot pressing/machining route. Rapid
cooling processes have been developed to shorten the cycle, thus increasing throughput and
decreasing processing costs. The HIP process is now a mature, if expensive, manufacturing
process for the ceramic industry. A wide variety of special application parts are
economically feasible using the HIP process.
Vapor Deposition
Vapor deposition is a "forming" process in which the raw
materials are gases. It is not really a new process; the author did his Ph.D. thesis on
chemical vapor deposition of pyrolytic boron nitride over thirty years ago! Chemical vapor deposition (CVD) and the related chemical vapor infiltration (CVI) process are
receiving a lot of attention in the production of composite materials and semiconductor
materials, to name two areas of current activity.
The process has a number of forms, including physical vapor deposition and
the two processes mentioned above. We will be concentrating on CVD and CVI. A wide variety
of oxide and non-oxide ceramic materials can be made by the process, including graphite or
carbon, diamond, SiO2, BN, SiC, tungsten silicide, Si3N4,
TiN, TiC, Ti(C,N), ZrN, Zr(C,N), and others. A gas or gases are reacted in a vacuum
furnace to produce a coating on a substrate or to infiltrate a preform, such as a
continuous fiber preform to make a composite material. A variety of gases are used as
reactants, including methane, CH4; carbon tetrafluoride, CF4; boron
trichloride, BCl3; diborane, B2H6; ammonia, NH3;
dichlorosilane, H2SiCl2; silicon tetrafluoride, SiCl4;
and silane, SiH4. There are a number of reactions that can be used to produce
the desired ceramic material:
BCl3 + NH3 = BN + 3 HCl
CH4 + SiCl4 = SiC + 4 HCl
Silane and oxygen will yield SiO2 and ammonia and
dichlorosilane will yield Si3N4.
Materials deposit as fine grain, dense layers on the substrate or in the
preform. It is possible to create virtually void-free infiltrates by the CVI method.
Perhaps the most widely studied composites are carbon/carbon composites, in which carbon
fibers are imbedded in a carbon matrix. Such composites can be produced by CVD/CVI or by
repeatedly infiltrating a carbon fiber preform with pitch or phenolic and carbonizing.
Anisotropic materials such as graphite and hexagonal BN are deposited by CVD as very
anisotropic layers, since the hexagonal sheet structure is laid down parallel to the
deposition surface. For example, the thermal conductivity of the layer will be several
orders of magnitude higher along the surface compared to through the surface. Generally,
deposition rates are not high, and the resulting materials are relatively expensive, but
for special applications these processes are one of the few methods available to achieve
desired materials.
Injection Molding
Injection molding is basically a plastic molding process essentially
borrowed from the plastics (as in polymer plastics) industry. For the ceramic industry, we
should more properly call it powder injection molding,
or PIM, because we are injecting a mixture
of a plastic material and ceramic powders into a mold to form a shape. The process has
been in use for over three decades. In the originally developed process, a mixture of
ceramic powder and a polymer/wax binder system was heated to soften the polymer/wax and
injected into a steel mold to form a complex shape. Removal of the organic portions to
produce a ceramic preform that could then be sintered was a complex, difficult, and
time-consuming process. In recent years new molding systems have been developed to
overcome many of the earlier difficulties. With these systems, the advantages of PIM in
producing complex parts at economical costs using the automated equipment developed for
the plastic molding industry can be more fully realized.
Most of the binder systems
previously (and still) used for PIM have been thermoplastic. That is, these materials
become soft and plastic upon heating to 100-220oC but return to a rigid or
semirigid state at room temperature. Binders can be classified into three types: wax- or oil-based, water-based,
or solid polymer solutions. The binder
system normally represents 15-50 volume percent of the PIM system, with 45-50% being more
typical. The binder system must provide the means for giving the plasticity needed for the
injection molding process and also the shape retention needed after that process. It must
allow for easy binder system removal (debinding)with
no distortion of the part, other than shrinkage, before final firing. Wax/oil-based binder systems
normally contain three or four components. The backbone polymer, often polypropylene,
provides or controls viscosity, green strength after molding, and debinding behavior.
Waxes or oils are fillers and provide better flow and are removed early in the debinding
process. Surface agents improve the interaction between the polymer and wax/oil. Sometimes
additional plasticizers are added to modify flow properties. Water-based
binder systems utilize gels, water soluble polymers, or glasses. Molding
pressures at often lower. A system developed by AlliedSignal uses a water-based gel that
requires heating to only 85oC and can be injected at 200-500 psi. This allows
use of low cost epoxy molds for short-run or prototype production. A gelling agent in the
mix causes the part to gel to a silicone rubber consistency. These parts are easy to
handle and are not prone to chipping or cracking. The final part contains 18-19% water,
which is removed much the same way water is removed from a slip cast or extruded
clay-based ceramic system. Thick parts can be handled, and there normally is no debinding
process after drying, since only about 3% of the gelling agent is left at that stage. This
is burned out early in the sintering schedule. Solid-polymer-solution
binder systems have been developed that minimize shape distortion during
debinding. These often are based on the use of polystyrene. The binder component is
removed using a solvent after molding, leaving the backbone polystyrene structure to
support the shape.
A variety of batch, semicontinuous, and continuous mixers are used to mix
the ceramic powder with the binder systems. Normally, the binder system has to be heated
to allow uniform incorporation of the powder and to provide plasticity for mixing.
Typically, the mixed material is pelletized and cooled for storage before being used in
the PIM process. Early injection molding machines used a piston/barrel arrangement to
force the material into the mold. This was a batch type system, although reloading was
relatively fast. Newer, higher capacity machines use continuous auger feed systems.
Schematic of A) Screw-type and B)
Piston-type Injection Molding Feeders
Examples of PIM Parts
Gelcasting
Gelcasting is similar in some respects to slip casting. Both processes
convert a slip or slurry into a rigid cast part, almost always of intricate shape. Slip
casting is virtually always an aqueous process, and gelcasting can be either aqueous or
nonaqueous in nature. Slip casting produces parts that are somewhat fragile and cannot
easily be machined in the green state. Gelcasting uses a high solids content of ceramic
powders in an organic gel. The solids loading is higher than is the case in slip casting.
Gelcast parts are strong and can be machined. Slip casting is a much more prolonged
process than gelcasting, and molds for slip casting must be carefully dried between casts
and are easily damaged. Gelcasting is perhaps a second cousin to sol gel processing,
discussed below. However, the sol gel process involves low loadings of ceramic precursors
in an inorganic gel, and it is much more difficult to produce large monolithic parts.
Gelcasting can produce intricate parts similar in nature to powder injection molding,
discussed above. However, gelcasting processes are less likely to have molding defects,
will produce parts with greater green strength, do not involve complex and long debinding
steps, which can lead to part distortion or defects, and can produce thick-section parts,
which is a problem with injection molding. Gelcasting is its own unique process. Compared
to PIM, gelcasting separates the mold filling process from the setting operation, although
both occur in the mold in both processes. In most PIM systems, the part must be cooled via
the mold in order to achieve rigidity. In gelcasting, gelation occurs passively in the
mold as a separate chemical process within the cast part. The mold is basically a passive
container at this stage. When compared to PIM, gelcasting does have some similarity to
some of the new water-based, gel-type PIM processes discussed above.
One almost needs to be an organic chemist to fully appreciate or
understand gelcasting. Ceramic powders are suspended in a monomer solution, which is
polymerized in the mold to form a cross-linked, rigid, polymer/solvent gel. The mold is
not porous, as is necessary in traditional slip casting. The system typically contains
only 10-20 weight percent polymer, and the solvent portion is removed by a rather
conventional drying step after the part is removed from the mold. We cannot cover all the
possible gel systems that have been used for gelcasting. We will give only a few examples
to indicate their nature and complexity. A nonaqueous
gelcasting system for casting alumina ceramics might contain 50-55 vol%
alumina, with the balance being the dispersing solution. This solution will have about 10%
dispersant, such as Rohm & Haas Triton X-100 or N-100 in DuPont dibasic ester (DBE) or
ICI Americas Solsperse 2000 in dibutyl phthalate (DBP), and 90% gelcasting premix. The
gelcasting premix might include 20 vol% monomers, such as trifunctional trimethylolpropane
triacrylate (TMPTA) and difunctional 1,6 hexanediol diacrylate (HDODA), both from Hoechst
Celanese, 1 vol% dibenzoyl peroxide initiator, and 79% solvent, either DBE or DBP. Aqueous gelcasting systems are more closely
related to traditional ceramic processing, have easier drying processes, better fluidity
during casting, and fewer environmental problems. An example is the acrylamide system.
This system is based on the use of the monomers monofunctional acrylamide (AM) and
difunctional N,N'-methylenbis-acrylamide (MBAM), with the solvent being water. The premix
into which the ceramic powder is dispersed might include 5-19 vol% of a mixture of
AM and MBAM, with the balance being water. The initiator might be ammonium persulfate, (NH4)2S2O8.
Obviously, gelcasting generally involves very complex chemical systems and specialty
chemicals. However, most of these are readily available and are not prohibitively costly.
Sol Gel Processing
Sol gel processing is a relatively new process that has received attention
over the past decade. It is, for all practical purposes, more of an organic/inorganic
chemical process than it is a "ceramic" process, at least as one would
traditionally define it. The process can be used to produce very special ceramic raw
materials, ceramic coatings, composite materials, fibers, or monolithic ceramic parts.
Before we can discuss sol-gel processing, we need to define a few terms. A
sol is a stable dispersion of very fine
particles in a liquid phase in which the particles remain suspended indefinitely by
Brownian motion. If the liquid phase is water, a sol is possible with particles under
about 1µm. When there is a weak liquid/particle interaction, the sol is classified as lyophobic. When the interaction is strong, the sol
is classified as lyophilic. A gel is a solid composed of liquid and solid
phases, with these phases both highly dispersed and with an internal network structure. In
sol-gel processing for ceramics, we need to convert a sol containing ceramic precursors
into a gel. Generally, only lyophilic sols can be converted to gels. The gel can then be
heat-processed to form a variety of ceramic compositions.
Sol-gel processing can involve either aqueous-based solutions of a metal
salt or alcohol-based solutions of a metal alkoxide. Gelation of an aqueous-based sol can
be accomplished by either removal of water (dehydration gelation) or increase of pH
(alkaline gelation). In alcohol-based, metal alkoxide processes, a lot of the effort for
ceramics has involved preparation of glasses from tetraethyl or tetramethyl orthosilicate.
However, a wide variety of ceramic compositions can be prepared via the sol-gel process,
including powders of ZnO, TiO2, Al2TiO5, and Al2ZrO5,
thin films of Ta2O5, and layers of lead zirconate titanate.
Monolithic, amorphous gels of zirconia can be prepared by the hydrolysis of
tetrapropylzirconate solution in cyclohexane. Non-oxide materials, such as silicon
carbide, have been prepared by the sol-gel route. If this starts to sound a bit foreign to
your concept of ceramics, you are not entirely alone in that perception!
Sol-gel processing, even though it appears to be very
"non-traditional", offers a number of advantages and/or unique opportunities for
the ceramic industry. Chemical homogeneity on the molecular or atomic scale can be
attained. Because of the very small particle sizes produced, there is an enormous amount
of surface area involved. This can lead to much lower sintering temperatures than would
otherwise be the case. Glasses can be made in compositional regions where phase separation
would occur if conventional melting techniques were used. Another advantage relates to
very high chemical purity and lack of contamination. The process can be used to produce
reactive powders, fibers, and coatings; to infiltrate preforms (such as continuous fiber
preforms) to produce composite materials; and to produce monolithic materials. In the
latter case, and in some cases with coatings, the large volume change that often occurs
during heat processing of the gel can create problems and barriers.
It is, of course, beyond the scope of this course to delve deeply into
sol-gel processing. It is not, at least as yet, a mainstream manufacturing process.
However, sol-gel processing and a number of related chemical processing techniques will
undoubtedly have a growing role in specialty applications. Will dinnerplates ever be made
this way? That's very unlikely! But in the electronics, optical, communications,
structural, and similar ceramic arenas, sol-gel processing offers some intriquing
potential and opportunities.
Miscellaneous Processes
There are a number of processes that are unique and/or seldom used. Some
of these processes will become more common in the future. We will not be able to cover all
of them, and we will not be able to cover any in great depth. However, it is important to
at least introduce them.
The refractories industry has long produced fused
cast refractory shapes, primarily for the glass industry. This is
basically a metallurgical process. The ceramic composition, such as alumina or
alumina-zirconia-silica (AZS), is actually melted in an electric arc furnace at very, very
high temperatures and cast by pouring into a mold. During solidification, large
interlocking crystals grow in the piece, and a large volume decrease occurs. Shrinkage
voids or "pipes" occur at the center of the cast part. Proper design of the
casting feed system will help eliminate these defects from the actual finished piece.
Fused cast blocks are very resistant to glass attack and are thus used as the working
lining in glass melting furnaces. They are also used where metallurgical slag attack is a
problem.
There have been a number of techniques developed to allow fabrication of
prototype or one-of-a-kind ceramic parts without the need for complex molds, dies, or
other tooling. Fusion Deposition of Ceramics (FDC)
makes use of injection molding ceramic compositions in filament form as a feedstock. This
is melted, extruded, and placed via CAD computer control onto the prototype. This allows
the construction of an intricate shape in a "freeform" environment. The process
is shown schematically below:
Schematic of Fusion Deposition of Ceramics
Material Feeding and Deposition Head for
FDC Process
Unfired Ceramic Parts Produced by Fusion
Deposition of Ceramics
Another similar technique is called Solid
Freeform Fabrication (SFF). It also relies on CAD techniques to convert a
part drawing into directions for building a freeform part. The part is electronically
sectioned into horizontal slices for part fabrication. For ceramic parts, several
techniques are being developed. One deposition technique, which is conceptually similar to
FDC, discussed above, uses chemical gelation of ceramic slurry layers. This is shown
schematically below:
Schematic of Freeform Fabrication of
Ceramic Parts Using Slurry Gelation
Using this technique, high strength alumina parts were fabricated using an
aqueous, low-viscosity, very fine grain (0.35 µm average particle size) magnesia-doped
alumina slurry with dissolved ammonium alginate. A dispersant was also used, as was a
small amount of sodium citrate to prevent premature gelation. Individual layers of the
slurry were deposited, leveled, and then gelled using calcium chloride salt solution
airbrushed over the surface. This is obviously not a production forming process, but for
prototypes it is useful. Another process uses ceramic tape similar to that used for tape
casting. Layers of tape of the appropriate shape for each layer are laminated and pressed
into a green preform. Different compositions can be used in each layer if a composite
structure is desired. The preform can receive further machining and then be fired. The
process, called Computer-Aided Manufacturing of Laminated Engineering Materials (CAM-LEM)
is shown schematically below.
Schematic of the CAM-LEM Process
Pentronix, Inc.: http://www.ptx.com/
Unique/Pereny Tape Casting Machines: http://www.hed.com/procast.html
Tape Casting Warehouse: http://www.drblade.com/TCW.htm
EPSI Isostatic Pressing: http://www.epsi-highpressure.com/isostatic/pressing.htm
Pressure Technology, Inc. Hot isostatic Pressing: http://www.pressuretechnology.com/
Argonne National Laboratory Hot Isostatic Pressing: http://anl.gov/LabDB/Current/Ext/H498-text.003.html
Fullman-Kinetics Vapor Deposition: http://www.fullman.com/semiconductors/cvd.html
Metal-organic CVD: http://bittburg.ethz.ch/LSST/mocvd/default.html
Contents of Chemical Vapor Deposition 6/96: http://www.wiley-vch.de/contents/jc_2112/199606.html
Oak Ridge National Laboratory "Injection Molding of a Ceramic
Turbocharger Blank": http://www.cped.ornl.gov/cped_ce/text/jep3.html
Elatec Bindervac Binder Removal Furnace: http://www.bindervac.com/main.html
Fraunhofer Institut Injection Molding of Aluminum Nitride Ceramics: http://www.ikts.fhg.de/publications/lenk/7.html
Oak Ridge National Laboratory Gelcasting: http://www.ornl.gov/MC-CPG/gelcasting.html,
http://www.ornl.gov/orcmt/capabilities/dtin386.html
and http://www.ornl.gov/ORNLReview/rev28-4/text/gelcast.htm
CAM-LEM Inc.: http://www.camlem.com/ceramics.html
Digital Fire, "Understanding Slip Casting Bodies": http://digitalfire.com/magic/slip.htm
Tape Casting, Cranfield University, http://www.cranfield.ac.uk/sims/materials/processing/tcintro.htm.
Ceramic Substrates for the Electronics Industry, CeramTec, http://www.ceramtec.com/Services/Laser_Facilities/Ceramtec_NAEAs/ceramsubstrates.htm.
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Student Exercise for
Lesson CP-4
For the composition(s)/product(s) selected for this module, discuss the
following:
a) What forming operations are used?
b) Describe the production equipment used for each process, including
make, age, production rate, condition, strong and weak points, degree of automation, and
safety.
c. What improvements in existing processes could be made? Could newer or
different processes, like isostatic pressing or gelcasting, replace any existing
processes? Discuss these issues.
