Rapid Tooling for Sand Casting : MoldMaking Technology
Gas and steam generated during casting exit through the permeable sand or via risers , [note 1] which are added either in the pattern itself, or as separate pieces. 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—flat on one side—the lower portion of the box, closed at the bottom, will be filled with a molding sand.
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 casting possible without waiting for the sand to dry. Sand may also be bonded by chemical binders, such as furane resins or amine-hardened resins.
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Additive manufacturing can be used in the sand mold preparation, so that instead of the sand mold being formed via packing sand around a pattern, it is 3D-printed. This can reduce lead times for casting by obviating patternmaking. To control the solidification structure of the metal, it is possible to place metal plates, chills , 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.
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In controlling the way a casting freezes, it is possible to prevent internal voids or porosity inside castings. To produce cavities within the casting—such as for liquid cooling in engine blocks and cylinder heads —negative 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.
With a completed mold at the appropriate moisture content, the box containing the sand mold is then positioned for filling with molten metal—typically iron , steel , bronze , brass , aluminium , magnesium alloys, or various pot metal alloys, which often include lead , tin , and zinc. After being filled 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.
In the case of metals that are significantly heavier than the casting sand, such as iron or lead, 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. After 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—in the case of steel or iron, by quenching in water or oil. The casting may be further strengthened by surface compression treatment—like shot peening —that adds resistance to tensile cracking and smooths the rough surface. And when high precision is required, various machining operations such as milling or boring are made to finish critical areas of the casting. Examples of this would include the boring of cylinders and milling of the deck on a cast engine block.
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.
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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. In general, we can distinguish between two methods of sand casting; the first one using green sand and the second being the air set method. These castings are made using sand molds formed from "wet" sand which contains water and organic bonding compounds, typically referred to as clay.
Green sand is not green in color, but "green" in the sense that it is used in a wet state akin to green wood. Contrary to what the name suggests , "green sand" is not a type of sand on its own that is, not greensand in the geologic sense , but is rather a mixture of:. There are many recipes for the proportion of clay, but they all strike different balances between moldability, surface finish, and ability of the hot molten metal to degas. Green sand casting for non-ferrous metals does not use coal additives, since the CO created does not prevent oxidation.
Green sand for aluminum typically uses olivine sand a mixture of the minerals forsterite and fayalite , which is made by crushing dunite rock. The choice of sand has a lot to do with the temperature at which the metal is poured. At the temperatures that copper and iron are poured, the clay gets inactivated by the heat, in that the montmorillonite is converted to illite , which is a non-expanding clay. Most foundries do not have the very expensive equipment to remove the burned out clay and substitute new clay, so instead, those that pour iron typically work with silica sand that is inexpensive compared to the other sands.
As the clay is burned out, newly mixed sand is added and some of the old sand is discarded or recycled into other uses. Silica is the least desirable of the sands, since metamorphic grains of silica sand have a tendency to explode to form sub-micron sized particles when thermally shocked during pouring of the molds.
These particles enter the air of the work area and can lead to silicosis in the workers. Iron foundries spend a considerable effort on aggressive dust collection to capture this fine silica. Often, combustible additives such as wood flour are added to create spaces for the grains to expand without deforming the mold. Olivine , chromite , etc. Since they are not metamorphic minerals , they do not have the polycrystals found in silica , and subsequently do not form hazardous sub-micron sized particles.
The air set method uses dry sand bonded with materials other than clay, using a fast curing adhesive. The latter may also be referred to as no bake mold casting. When these are used, they are collectively called "air set" sand castings to distinguish them from "green sand" castings.
Two types of molding sand are natural bonded bank sand and synthetic lake sand ; the latter is generally preferred due to its more consistent composition. With both methods, the sand mixture is packed around a pattern , forming a mold cavity. If necessary, a temporary plug is placed in the sand and touching the pattern in order to later form a channel into which the casting fluid can be poured. Air-set molds are often formed with the help of a casting flask having a top and bottom part, termed the cope and drag.
The sand mixture is tamped down as it is added around the pattern, and the final mold assembly is sometimes vibrated to compact the sand and fill any unwanted voids in the mold. Then the pattern is removed along with the channel plug, leaving the mold cavity.
The casting liquid typically molten metal is then poured into the mold cavity. After the metal has solidified and cooled, the casting is separated from the sand mold. There is typically no mold release agent, and the mold is generally destroyed in the removal process. The accuracy of the casting is limited by the type of sand and the molding process. Sand castings made from coarse green sand impart a rough texture to the surface, and this makes them easy to identify.europeschool.com.ua/profiles/ledetem/guky-pof-las-palmas.php
Rapid Tooling Guidelines For Sand Casting
Castings made from fine green sand can shine as cast but are limited by the depth to width ratio of pockets in the pattern. Air-set molds can produce castings with smoother surfaces than coarse green sand but this method is primarily chosen when deep narrow pockets in the pattern are necessary, due to the expense of the plastic used in the process. Air-set castings can typically be easily identified by the burnt color on the surface. The castings are typically shot blasted to remove that burnt color.
Surfaces can also be later ground and polished, for example when making a large bell. After molding, the casting is covered with a residue of oxides, silicates and other compounds. This residue can be removed by various means, such as grinding, or shot blasting.
During casting, some of the components of the sand mixture are lost in the thermal casting process.