Ceramic forming techniques
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Ceramic forming techniques are ways of forming ceramic shapes. This can be used to make everyday tableware from teapots, to engineering ceramics such as computer parts. Methods for forming ceramic powders into complex shapes are desirable in many areas of technology. For example, such methods are required for producing advanced, high-temperature structural parts such as heat engine components, recuperators and the like from ceramic powders. Typical parts produced with this production operation include impellers made from stainless steel, bronze, complex cutting tools, plastic mold tooling, and others. Typical materials used are: wood, metal, water, plaster, epoxy and STLs, silica, and zirconia. This production operation is well known for providing tools with dimensional stability, surface quality, density and uniformity. For instance, on the slip casting process the cast part is of high concentration of pure ceramic powder with little additive, this improves uniformity. But also, the plaster of Paris mold draws water from the poured slip to compact and form the casting at the mold surface. This forms a dense cast.
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[edit] Forming Techniques
There are many forming techniques to make ceramics, but one example is slipcasting. This is where slip, liquid clay, is poured into a plaster of Paris mold. The water in the slip is drawn 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.
[edit] Slip Casting
The original mold for a slip cast, as well as the pieces themselves in many individual works of ceramics, can be 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 un-centered 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.
Slip-casting methods provide superior surface quality, density and uniformity in casting high-purity ceramic materials over other ceramic casting techniques, such as hydraulic casting, since the cast part is a higher concentration of pure ceramic powder with little additives. Ceramic powder is compacted in the slip casting process and sintered or bonded together at high furnace temperatures. A slip is a crowded suspension of fine ceramic powder in a liquid such as water or alcohol with small amounts of secondary materials such as dispersants, surfactants and binders. Early slip casting techniques employed a plaster-of-Paris block or flask mold. The plaster-of-Paris mold draws water from the poured slip to compact and form the casting at the mold surface. This forms a dense cast form removing deleterious air gaps and minimizing shrinkage in the final sintering process.
[edit] Ceramic Shell Casting
Ceramic shell casting techniques using silica, zirconia and other refractory materials are currently used by the metal parts industry for `net casting,` forming precision shell molds for molten metal casting. The technique involves a successive wet dipping and dry powder coating or stucco to build up the mold shell layer. The shell casting method in general is known for dimensional stability and is used in many net-casting processes for aerospace and other industries in molten metal casting. Automated facilities use multiple wax patterns on trees, large slurry mixers and fluidic powder beds for automated dipping.
[edit] Other Techniques
There are also several traditional techniques of handbuilding, such as pinching, soft slab, hard slab, and coil construction.
Other techniques involve threading animal or artificial wool fibre through paperclay slip, to build up layers of material. The result can be wrapped over forms or cut, dried and later joined with liquid and soft paperclay.
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.)
[edit] Technical Ceramics
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 iso-statically 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 paraffin wax.
There are also many more techniques, such as gel casting and injection molding that are often used to create engineering ceramics.
