This type of forming is done with press machines, which work on the principle of applying high pressure to force the clay into a mould. Press moulding of soft clay is mainly used for forming roofing tiles with complicated interlocking shapes, but there are drawbacks with soft clay pressing: clay easily sticks to steel dies, the soft products are difficult to handle, high drying shrinkage causes warping and cracking during drying. Dies for soft clay pressing are often made from plaster provided with air holes so release is done by forcing compressed air through the mould.
Semidry forming processes were developed to reduce some of these problems associated with plastic clay forming.
Semidry powder pressing
Semidry powder means clay with 7 to 12 % water content that is prepared in powder form. The clay looks like powder, but when some of it is squeezed in the hand, it sticks together and holds its form. Because of this characteristic, semidry forming is limited to products that can be made under pressure in a mould or die, such as tiles, simple electrical articles (fuse holders, low tension insulators) and kiln shelves.
Leather-hard means clay with about 15 % water content. It is prepared to approximately the final shape when plastic, then allowed to dry until leather-hard before pressing.
Semidry forming either requires beating by hand, or the use of a press machine with a metal die.
This uses clay with only 2 to 7 % water content. Because this requires great pressure, the machinery is too expensive for small producers.
The major single user of semidry pressing is the wall and floor tile industry. Tile making is an attractive industry, as use is growing rapidly all over the world, and the market is increasing for exports from developing countries. Tiles at first glance seem a simple product to make, since they are just flat squares of clay. However, the big problem is keeping them flat and square through the various stages of drying and firing, and keeping them the same size in firing (unless the temperature is perfectly even, there will be differences in firing shrinkage).
In order to reduce these problems as much as possible, there is clearly an advantage in semidry pressing, which at least prevents most drying shrinkage problems.
large tile industries:
However, as mentioned in the introduction, the small producer cannot compete with highly-automated, large-scale industries. The state of the art for large-scale wall tile production is based on using local red-firing clays, automatically dry pressing them under tremendous pressure, and then fast-firing them (under 45 minutes) in roller hearth kilns. Very accurate temperature control produces accurately-sized tiles, with a very small rejection rate. These production lines are often computerized, and all the handling of tiles is done on automated conveyor belts - even grading and selecting and packaging of the finished products.
This leaves the small tile producer with the possibility of
other market niches, such as hand-decorated tile (relief, engobe, underglaze,
overglaze) and especially unglazed floor and wall tile made from local
red-firing clay. These tiles can be produced with relatively inexpensive
machinery, and periodic kilns can be used for firing with profitable results.
Semidry pressing is then feasible (either powder or leather-hard system).
Marketing can be aimed either at the local construction industry, or at the
specialty export sector.
Nowadays in developing countries, there is a rapidly growing middle and upper class that is interested in adding status to their homes. This means that there is a growing market for items like tiles for the outside of houses, either glazed or unglazed, facing bricks (which are tiles made to imitate good quality bricks), floor tiles for interior and exterior, and even glazed roofing tiles. All of these are made by semidry pressing.
Because the work flow for semidry pressing is very different from plastic clay, and different clay preparation machinery is used, it usually is not combined in the same factory as plastic clay forming, or else a separate section of the factory is set up exclusively for this process.
For making simple electrical goods like fuse holders, where a nonhollow form is required, semidry pressing is also the method of choice. Hollow electrical goods are generally slip-cast. A successful fuse holder factory can make use of cottage industry, as described in the introduction. By only producing clay body and firing finished goods, this type of industry reduces labor problems and factory space, by simply purchasing the unfired fuse holders.
5.2.2. HAND-OPERATED SCREW PRESS
5.2.3. FRICTION PRESS
5.2.4. TOGGLE PRESS
5.2.5. ECCENTRIC PRESS
5.2.6. SIMPLE LEVER PRESS
5.2.7. HYDRAULIC PRESSES
Because press machines are needed for most of these products,
this section will describe various types that are available.
All press machines depend on a mould, which, because of the heavy pressure used, is made of steel. This type of mould is called a die.
Inexpensive dies are made from mild steel, which is easy to machine but does not have a long life ( 1-2 months for dies that are used daily). They are suitable for very simple tiles that do not have details like curved edges, and for small pressings like fuse holders.
Better dies are made from hardened steel, and are correspondingly expensive. They make up for their cost by having a long working life, and can be machined to have sophisticated rounded edges.
A set of dies consists of a top die, also called a plunger, and a bottom die. The bottom die is shaped like a shallow box. It has a metal frame of mild steel which makes up the sides, and in good dies, this frame is removable from its support, which is designed to fasten to the base of the press. The frame wears down after many thousands of pressings, and periodically must be replaced. The floor of the die can slide up and down in the frame, and is provided with a lifting device that fits into it from the bottom. After pressing a tile, the lifter is operated, and the tile is raised up to the level of the frame for easy removal.
The top die is fitted to the screw in such a way that it does not rotate. Correct alignment of top and bottom dies is EXTREMELY important, as an expensive die can be damaged beyond repair very easily. If the press is operated without any clay in the die, this will also damage the die.
In some tile dies the frame is lifted for releasing the pressed tile instead of the bottom plate. This arrangement is used on the friction press in Fig. 5.2.3-A.
The size of tile that can be powder-pressed depends on the amount of pressure applied. One source¦ gives the following data:
2-7 % water
9-12 % water
15-20 % water
Successful pressing also depends on the type of clay body used, and needs to be tried out.
Standard friction presses are rated at 40 to 80 tonnes total pressure. This equals 80000 kg force applied to the die. Because the pressure of the die is distributed across the total area to be pressed, the kg per square centimeter will be less with larger dies. For example, a tile die measuring 15 cm x 15 cm has an area of 225 cm². Then:
40000kg/225 cm² = 178 kg per cm²
This may not be sufficient pressure to produce a strong 15 x 15 cm tile.
However, a tile die measuring 10 x 10 cm has an area of 100 cm². Then:
40000kg/100cm² = 400 kg per cm²
This would make a compact and strong tile on the same press.
For smaller tiles, dies capable of pressing up to 4 tiles at the same time can be made.
Dies for fuse holders and other small shapes sometimes are fairly complex, and are made from mild steel by machining. They often have removable pins for producing necessary holes in the pressing. These pins are removed after each pressing, and replaced for the next.
Dies are also used for pressing thimbles small devices for separating tiles during firing.
Large dies are used for pressing saggers, firebricks, and items like flowerpots.
For semi-automatic production, presses are equipped with rotating heads that have up to 6 bottom dies. As one die is pressed, the next one is being filled, and the previously-pressed tile is removed. This increases production quantities greatly. This type of press is often used for roofing tiles.
For making kiln shelves (setter slabs), powder clay is used in a
metal frame, which is made from reinforced angle iron. It is fitted with a sheet
metal bottom plate. This is not used in a press machine, because the size
requires more pressure than ordinary presses can supply. Instead, the clay is
beaten into the frame by hand (see kiln shelves, below).
5.2.2. HAND-OPERATED SCREW PRESS
This machine uses a long screw to supply pressure to a metal die. The screw turns in a brass bearing, and the quality of this bearing determines the usefulness of the machine. The machine is available in a variety of sizes, and depends on a heavy hand-turned flywheel to supply the necessary pressure.
It is quite a heavy machine, and must be fixed to a foundation to keep it from shifting in use. This type of press can be used for any product, with either powder or leatherhard clay. Small screw presses are used for tiles, and have a two- or three-man team: one man manages the clay and die, and the other two turn the flywheel. The largest sizes press saggers and firebricks, and may have as many as 3 workers turning the flywheel, while two workers manage the clay and heavy die.
Daily production figures quoted from Europe (8-hour day) can be
up to 2000 tiles, or one every 14 seconds. This is with a very experienced and
motivated work crew. An inexpert crew might manage half that amount. It also
assumes that there are no delays, such as lack of clay, machinery breakdowns,
5.2.3. FRICTION PRESS
This is simply a motorized version of the screw press, which has two movable friction wheels which can be moved horizontally. These wheels are driven by a motor through a belt system, and in one position move the press down, and in the opposite mode move it up. They are usually made of cast iron or mild steel, and the flywheel is provided with a leather or fiber surface (the same as used for flat belt drives) which provides the necessary friction.
Friction presses also come in a variety of sizes. They normally are rated according to tonnes, which means the amount of tonnes/force they can supply. A common size for up to 15 by 15 cm tiles is 80 tonnes. This size of press commonly uses a 3-5 HP 3-phase motor. Larger presses are available. Friction presses do not necessarily apply more pressure than hand screw presses. The advantage is that they do it with less manual labor.
When selecting any screw press, it is important to be sure that it has sufficient stroke length. This is the distance the top die travels from top to bottom of each stroke. For pressing saggers, there needs to be enough distance to remove the sagger easily.
Typical production figures for friction presses with inexperienced workers are about 125 presses per hour (double stroke).
Friction presses are often equipped with double bottom dies. In
this system, the dies slide back and forth, so that one is being pressed while
the other is emptied and refilled. This greatly increases production
5.2.4. TOGGLE PRESS
The toggle press is used for light pressing, such as fuse
holders and other very small items. It works on a rack and pinion gear
5.2.5. ECCENTRIC PRESS
The eccentric press is also commonly used for tile production,
but only with leatherhard clay. It works on the principle of an eccentric shaft
which rotates and activates a piston on which the die is mounted. This supplies
a very high mechanical advantage, so high pressure can be supplied with a
relatively small motor. However, it has the drawback of a short stroke length.
For leather-hard tiles, it is an efficient and low-cost machine. It cannot be
used for powder pressing, because it is too difficult to fill the die.
5.2.6. SIMPLE LEVER PRESS
There are two types of lever press which can be used for pressing soft clay, as for tiles with relief designs. These are used for low production, custom-made tiles.
One is simply a long lever arm attached to a fulcrum, with a plaster mould or a metal die connected. A slab of clay is placed on the table, the mould is pressed into it, and excess clay is then trimmed from the sides.
The other type uses a mechanical principle which is the same as an automobile jack, except that it is operated by a lever arm rather than a screw. Mechanical advantage comes at the top of the stroke, where rather high pressure is obtained. This is also used with plastic clay.
soil block press
Another type of lever press is commonly used for pressing soil
blocks for house construction. There are many variations on this, with the
original known as the Cinva-Ram. This type of press could be used
for firebrick pressing, and even for tiles, but those who have tried it say that
it is slow and produces uneven work.
5.2.7. HYDRAULIC PRESSES
Hydraulic presses use hydraulic cylinders to provide pressure. They have the advantage of producing very high pressure, but also are very expensive, and unless specially equipped, are slower than friction presses.
hydraulic ram press
A specialized type of hydraulic press is known as the ram press. It uses plastic clay and special moulds made of hydrostone, that are fitted with air tubes to release the pressing. This was a patented process until recently, and only used in the U.S.
This process is used for all types of tiles floor, wall, and roof. The general work flow is shown in Fig. 5.3-A.
leather-hard clay requirements
Clay for leather-hard pressing varies according to the type of product. As usual, large tiles (such as roof tiles) should have a higher percentage of nonplastic materials like sand or grog added. The clay can be prepared by hand, or for larger production by machines (see chapter 4.1).
As seen in the work flow, the first step is preparing blanks. A blank is simply a slab of clay, which can be made in a number of ways. Frequently, slabs are cut from a clay block using the sticks and cutting wire method. However, since the tile blanks should all be a uniform size, frequently a wooden or metal mould is used.
The mould is sprinkled with a dry separator, which usually is dry clay powder kept in an open-weave cloth bag (such as a jute bag). This is quickly done by shaking the bag in the mould. Then a piece of clay is pressed into the mould, and the excess is cut from the top with a cutting wire. The mould is inverted onto a ware board, so that the blank neatly falls out.
The water release method is also used, where the blank is formed in a metal frame with water used as a separator.
The leather-hard tiles, after pressing, are set on ware boards to dry further. In some cases the edges of the tiles are made smooth immediately after pressing, but this may also be done just before firing. This process is called fettling.
During drying the tiles may need to be turned once or twice to avoid warping. When they are distinctly leather-hard, they are checked for warping. Warping can be corrected by placing the tile in a single bottom die, and gently beating it. They then are dried completely.
FIGURE 5.3-A Work flow for leather-hard pressing.(A; B-1)
A) Plastic clay is taken from store.The clay is prepared as
described in chapter 4.1.
B-1) Blanks are formed in a mould.Excess clay is cut off by a wire.
FIGURE 5.3-A Work flow for leather-hard pressing.(B-2)
B-2) Blanks can also be extruded and then cut.
FIGURE 5.3-A Work flow for leather-hard pressing.(C; D)
C) Blanks are placed on ware boards and left to dry until
D) The blanks are taken to the press machine for pressing.
FIGURE 5.3-A Work flow for leather-hard pressing.(E; F)
E) After pressing the tiles are placed on boards and then
finished by hand.
F) Drying completely.
FIGURE 5.3-A Work flow for leather-hard pressing.(G)
G) Kiln firing is started with a long smoking pericol.
5.4.1. SEMIDRY POWDER PRESSING WORK FLOW
5.4.2. HINTS FOR SUCCESSFUL PRESSING
5.4.3. SAFETY SAFETY SAFETY !
5.4.4. PRESSING PROBLEMS AND SOLUTIONS
As mentioned before, the advantage of pressing with semidry
powder is that the pressed product is almost dry, and has almost no shrinkage,
and thus minimum warping. Besides the actual pressing, the clay body needs to be
specially prepared in the form of granules containing 6 to 10% water. Various
methods for doing this are described below. The work flow for semidry powder
5.4.1. SEMIDRY POWDER PRESSING WORK FLOW
FIGURE 5.4.1-A Work flow of semidry pressing.(A; B)
A) Granules of clay with right moisture content are taken from
store to press machine.
B) Measured amount of body is filled in the die, leveled off and pressed twice.
FIGURE 5.4.1-A Work flow of semidry pressing.(C)
C) The pressed tiles are placed on boards.
FIGURE 5.4.1-A Work flow of semidry pressing.(D)
D) Tile edges are fettled with a damp sponge or by maving the tile back and forth on a coarse screen
FIGURE 5.4.1-A Work flow of semidry pressing.(E; F)
E) The tiles are dried completely.
F) Firing after a long soaking period.
There are three main processes for preparing clay granules, with the following work flows:
- DRY PROCESS
This is the simplest of the processes in terms of equipment required and number of steps taken.
Many variations in this work flow are possible. The optimum process depends on the nature of the clay, the force of the press and the size and shape of the tile. It may be possible just to grind the clay, adjust its moisture content and then press it. Granulation can also be done in a pin mill without a screen. The pan mill and the final screening to remove the fines may be left out if the press body does not tend to cause lamination. Granules can also be produced in a pan mill with perforated bottom. The mixing can be done in the pan mill if this is operated in batches instead of continuously.
FIGURE 5.4.1-B Work flow of dry process granulation (A)
A) Clay from storage.
FIGURE 5.4.1-B Work flow of dry process granulation (B; C)
B) Clay is dried to less than 10% moisture content.
C) Clay is powdered in a hammer mill or pin mill.
FIGURE 5.4.1-B Work flow of dry process granulation (D; E)
D) Powdered clay is screened. This may not be necessary.
E) Clay is mixed with possible other materials in a drum mixer or with a shovel.
FIGURE 5.4.1-B Work flow of dry process granulation (F; G)
F) Clay body with addition of water is mixed in a pan mill
G) Screening through 10 mesh and 40-60 mesh. Material coarser than 10 mesh and finer than 40-60 mesh is returned to the pan mill.
FIGURE 5.4.1-B Work flow of dry process granulation(H)
H) Granules with a moisture content slightly higher than needed for pressing are left to mature for several days.
FIGURE 5.4.1-B Work flow of dry process granulation(I)
I) Moisture content is checked before pressing. Granules are taken to press machines
- WET PROCESS
This starts with the standard wet process of preparing clay (see chapter 4.1.2). Following that process the plastic clay is here used as the starting point of the granulation process.
Although complicated and requiring expensive equipment (filter press, blunger, agitator, diaphragm pump) this prepares reliable body that works with any suitable body composition.
Clay body is first prepared as in the wet process, but instead of being pumped into a filter press, it is pumped into a spray drying tower, where it is sprayed into a stream of hot air that dries it to the correct moisture content, as well as producing more or less spherical granules. It then goes directly to the press. This is the state of the art system which is appropriate only for relatively large-scale industries, due to cost and sophistication of the machinery.
FIGURE 5.4.1-C Work flow of wet process granulation (A; B)
A) Clay cakes are taken from filter press to drying area.
B) The clay is air-dried to a water content slightly higher than needed for pressing.
FIGURE 5.4.1-C Work flow of wet process granulation (C)
C) The clay with a water content of 10 - 15% is granulated in a pan mill or pin mill.
FIGURE 5.4.1-C Work flow of wet process granulation (D)
D) Granules are matured for a few days until the moisture content is correct.
FIGURE 5.4.1-C Work flow of wet process granulation (E)
E) Moisture content is checked before taking the granules to
5.4.2. MINTS FOR SUCCESSFUL PRESSING
There are some aspects of the process which are worth discussing in detail, as they are problems that have come up in our experience and that are not discussed in available books.
The main purpose of granulating is to get the clay particles into coarse condition, so that they consist of small clumps or balls of clay body larger than 40 to 60 mesh. This is important to prevent lamination problems (discussed in pressing, below).
Granulating is not necessary for all clay bodies. Generally, bodies which are fairly coarse - containing coarse fireclay and grog (ranging approximately up to 30 mesh) - can be dry-blended and then mixed with the correct amount of water. With such easy-to-press bodies, tbe water is usually estimated by volume, and is sprinkled evenly into the dry body while stirring and turning it with a shovel. It is correct when body squeezed in the hand holds its shape.
A simple form of granules can be produced by adding all water to the grog which is then mixed with the dry clay body. The clay will stick to the grog and form small granules and at the same time the water will be distributed evenly.
The body is often mixed with kerosene oil about 1-2% (sometimes up to 5% is necessary) as well as water, which prevents sticking to the die. This type of body may also press successfully without maturing, although it is always better to mature clay before forming it. Both green and fired strength of the pressed tile increase considerably if the body is allowed to mature for some days.
Bodies that contain high amounts of fine clay (ball clay, clay high in montmorillonite), or slippery materials such as some kinds of talc, cause greater problems for powder pressing, and require granulation. For tiles, this is corrected in two ways: by adding grog to the body, and by adding wetting agents (soap) to the water.
pressing work flow
Pressing itself has a fairly complicated work flow:
- Inspection: Checking that die is clean. Clay tends to stick to the die, and needs to be removed with a stiff brush (fiber or brass wire type) periodically.
- lubrication of die: The die needs to be lubricated periodically with light oil to prevent clay from sticking. Frequency of lubrication and type of oil that is successful depend on the clay body. Commonly used oils are kerosene, diesel oil, waste crankcase oil (either plain or thinned with kerosene), coconut oil, palm oil thinned with kerosene, etc. This can be applied with a piece of cloth.
- filling the die: In order to get the same thickness of tile every time, the die must be filled the same amount. This is done by adjusting the bottom plate, so that when filled to the top and levered, the tile is the correct thickness. It is easy to make a filling device, which is simply a metal or wooden frame that contains clay powder. Sliding the frame over the bottom die automatically fills the die, and sliding it back on the press table scrapes the clay level.
- pressing stroke: With coarse bodies, it is possible to press only one time. However, with most bodies, two pressings are required for each tile. The first press stroke is done rather slowly - its purpose is to allow air to come out of the clay powder. Then, a second press stroke is done rapidly, which fully compresses the clay.
- lifting the bottom die: This is done as the top die is raised to its highest position.
- removing the tile: The tile is lifted carefully from the die,
and placed on a ware board. If the bottom die is plain, without relief pattern
or rounded edges, the filling frame can be used to push the tiles to the other
side, where it is picked up by an assistant or by a conveyor belt.
5.4.3. SAFETY SAFETY SAFETY!
ALL PRESS MACHINES ARE DANGEROUS! It is very easy to lose a finger or even a hand. Workers need to be instructed in correct use of the machine. The main point is that BOTH HANDS MUST BE USED TO PULL THE PRESS LEVER, so that there is no chance of having a hand under the die. It is much safer for the same person who fills the die to also pull the press lever. If two persons are working with the press (as is common with presses having two sliding dies), there needs to be a system of signaling between the workers.
Many countries now have safety regulations for workers using dangerous machinery. It is common for press machines to be equipped with two switches, which must be held down in order for the press to move -one is on the press lever, and the other is located some distance away so that both hands are out of the way before the machine will operate. This is difficult to do with a friction press, but it is highly recommended that the manufacturer be required to equip his machinery with such a safety device.
Semidry bodies produce a lot of dust during body preparation and
during pressing and fettling. Inhalation of clay dust over a long period of time
will damage the lungs. Therefore, workers should be equipped with dust masks and
good ventilation is needed in work areas.
5.4.4. PRESSING PROBLEMS AND SOLUTIONS
The main problem with powder pressing is lamination in tiles, which means that the tile will split apart in horizontal layers. These layers are caused mainly by sideways slipping of the clay under pressure, which causes clay particles to line up in the same direction and not to get good adhesion. It is made worse when air cannot escape quickly enough from the body, which also causes separation of clay layers. Another factor in lamination is suction caused when the die is lifted: if the die does not release easily from the clay, it will actually tear the tile apart.
Lamination can be difficult to cure. Possible solutions that can be tried are:
- Changing the clay body, which means reducing the amount of slippery substances such as talc, or changing the type of talc from plate-type particles to rough particles (there are many varieties of talc), or using talc with a rougher particle size.
- Adding coarser clays and/or grog, which gives the clay more tooth and helps prevent it from slipping.
- Adding organic binders, such as starch, sugar, CMC gum.
- Adding wetting agents to the water, such as liquid soap, or sodium lauryl sulfate (which is the common wetting agent in liquid soap).
- Maturing the clay for a longer period.
- Changing from dry clay processing to the leather-hard method.
- Prepressing the clay body into bricks or blocks (with a slightly higher water content), then drying them to the necessary water percentage, and breaking and granulating them.
- increasing the granule size.
Cracking of tiles normally only shows after firing. Its main cause is lamination. Other causes: the green strength is too low to withstand stresses from handling and drying. The cure is to increase: thickness of the tile, moisture content of press body, the proportion of plastic clay, the pressing force or maturing time of body. The pressure can be increased by reducing the size of the tile, by increasing the speed of the flywheel or by pressing and extra time.
- Uneven thickness is corrected by ensuring that the same amount of clay is used for each tile, by keeping moisture content constant, and by applying the same pressure to each tile.
- Crumbling corners are caused by too low moisture content, by attempting to press too thin tiles, by lack of pressure or by uneven filling of the die.
- Top center section separating from the rest of the tile is a
form of lamination. The pressure in the center is always less than at the edges,
where the clay powder is better compacted when the material bounces back from
the die frame. The cure is to strengthen bonding by increasing moisture content
and if that is not sufficient by granulating the press body.
Kiln shelves (sometimes called setter slabs) for low and medium temperatures can be made from a simple body of 50% fireclay and 50% grog (with the dust fraction removed). This is prepared by blending by hand with about 10% water- as described above, a small amount squeezed in the hand will stick together and hold its shape. This is aged for at least 24 hours, and then is filled into the metal frame described in dies, above. The clay is beaten with a wooden mallet, taking care to beat it well into the corners. More clay is added, and again beaten until the frame is filled level and the clay is as compressed as possible. Then the frame is turned over, and the opposite side is also beaten thoroughly. The shelf is removed from the frame, and set aside for drying.
Other refractory products for kiln construction can be made in a similar way. Fig. 5.4.5-A shows a die and moulding table for production of large refractory firebricks. A similar system could be used for kiln shelf production.
Sufficient beating is very important, in order to closely pack all the clay particles. This makes a shelf that will not easily bend in firing or crack during handling. Standard press machines do not provide enough pressure for the large surface area of kiln shelves (an 80-tonne press used for a 30 x 30 cm shelf can supply only 88 kg per cm2). For more information on making refractories see The Self-Reliant Potter: Refractories and Kilns also in this series.