A multi-part molding box (known as a casting flask, the top and bottom halves of which are known respectively as the cope and drag) is prepared to receive the pattern. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object¡ªflat on one side¡ªthe lower portion of the box, closed at the bottom, will be filled with prepared casting sand or green sand¡ªa slightly moist mixture of sand and clay. The sand is packed in through a vibratory process called ramming and, in this case, periodically screeded level. The surface of the sand may then be stabilized with a sizing compound. The pattern is placed on the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed on the box and it is turned and unlatched, so that the halves of the mold may be parted and the pattern with its sprue and vent patterns removed. Additional sizing may be added and any defects introduced by the removal of the pattern are corrected. The box is closed again. This forms a "green" mold which must be dried to receive the hot metal. If the mold is not sufficiently dried a steam explosion can occur that can throw molten metal about. In some cases, the sand may be oiled instead of moistened, which makes possible casting without waiting for the sand to dry. Sand may also be bonded by chemical binders, such as furane resins or amine-hardened resins.
Chills
To control the solidification and metallurgical structure of the metal, it is possible to place metal plates¡ªchills¡ª in the mold. The associated rapid local cooling will form a finer-grained structure and may form a somewhat harder metal at these locations. In ferrous castings the effect is similar to quenching metals in forge work. The inner diameter of an engine cylinder is made hard by a chilling core. In other metals chills may be used to promote directional solidification of the casting. In controlling the way a casting freezes it is possible to prevent internal voids or porosity inside castings.
Cores
To produce cavities within the casting¡ªsuch as for liquid cooling in engine blocks and cylinder heads¡ªnegative forms are used to produce cores. Usually sand-molded, cores are inserted into the casting box after removal of the pattern. Whenever possible, designs are made that avoid the use of cores, due to the additional set-up time and thus greater cost.
Two sets of castings (bronze and aluminium) from the above sand mold
Two sets of castings (bronze and aluminium) from the above sand mold
With a completed mold at the appropriate moisture content, the box containing the sand mold is then positioned for filling with molten metal¡ªtypically iron, steel, bronze, brass, aluminium die casitng, magnesium alloys, or various pot metal alloys, which often include lead, tin, and zinc. After filling with liquid metal the box is set aside until the metal is sufficiently cool to be strong. The sand is then removed revealing a rough casting that, in the case of iron or steel, may still be glowing red. When casting with metals like iron or lead, which are significantly heavier than the casting sand, the casting flask is often covered with a heavy plate to prevent a problem known as floating the mold. Floating the mold occurs when the pressure of the metal pushes the sand above the mold cavity out of shape, causing the casting to fail.
Left:- Corebox, with resulting (wire reinforced) cores directly below. Right:- Pattern (used with the core) and the resulting casting below (the wires are from the remains of the core)
Left:- Corebox, with resulting (wire reinforced) cores directly below. Right:- Pattern (used with the core) and the resulting casting below (the wires are from the remains of the core)
After die casting, the cores are broken up by rods or shot and removed from the casting. The metal from the sprue and risers is cut from the rough casting. Various heat treatments may be applied to relieve stresses from the initial cooling and to add hardness¡ªin the case of steel or iron, by quenching in water or oil. The casting may be further strengthened by surface compression treatment¡ªlike shot peening¡ªthat adds resistance to tensile cracking and smooths the rough surface.oil painting,Oil Painting Reproduction
Design requirements
The part to be made and its pattern must be designed to accommodate each stage of the process, as it must be possible to remove the pattern without disturbing the molding sand and to have proper locations to receive and position the cores. A slight taper, known as draft, must be used on surfaces perpendicular to the parting line, in order to be able to remove the pattern from the mold. This requirement also applies to cores, as they must be removed from the core box in which they are formed. The sprue and risers must be arranged to allow a proper flow of metal and gasses within the mold in order to avoid an incomplete casting. Should a piece of core or mold become dislodged it may be embedded in the final casting, forming a sand pit, which may render the casting unusable. Gas pockets can cause internal voids. These may be immediately visible or may only be revealed after extensive machining has been performed. For critical applications, or where the cost of wasted effort is a factor, non-destructive testing methods may be applied before further work is performed.
Essential improvements of the foundry technology
An increasing demand for castings in the growing car and machine building industry during and after the World War One and Two, stimulated new inventions in mechanization and later automation of the sand casting process technology. The bottleneck for a faster casting production was not only the molding speed, but also slow molding sand preparation in the slow sand mixers, slow core manufacturing processes and slow metal melting rate in the cupola furnaces. In 1912 sand slinger was invented by the American Company Birdsley & Piper. In 1912 the first sand mixer with individually mounted revolving plows was marketed by Simpson Company. In 1915 first experiments started with bentonite clay in stead of simple fireclay as bonding additive to the molding sand. This increased tremendously the green and dry strength of the moulds. In 1918 the first fully automated foundry fabricating hand grenades for the U.S. Army. In 1930 first high-frequency coreless electric furnace was installed in the U.S. In 1924 Henry Ford sets record producing 1 million cars, consuming one-third of the total casting production in the U.S. In 1943 ductile iron was invented by adding magnesium to the widely used grey iron. In 1940 thermal sand reclamation was applied for molding and core sands. In 1952 D-process was developed for making shell molds with fine, pre-coated sand. In 1953 hotbox core sand process was invented by thermally curing cores. In 1954 a new core binder - water glass hardened with CO2 from the ambient air, was applied.
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