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CASTING MOLD. GENERAL

Founding is one of the most ancient methods of metal shaping.

Foundry practice includes such basic production processes as melting of metal, manufacture of molds, pouring of the metal into molds, solidification, shakeout and fettling of the castings.

The principle of casting consists in the following. The metal of the required chemical composition and quality is poured into a casting mold whose cavity conforms to the shape of the desired casting. As metal cools and solidifies in the mold, a casting results. During its crystallization and cooling in the casting mold, the metal acquires certain mechanical properties and service characteristics. The cooled and solidified casting is removed from the mold, cleaned, and subjected to further treatment if necessary. There are a great variety of casting processes which can produce castings of any shape, size, and mass, for example, from several grams to a few hundred tons.

Castings are made from irons and steels and from nonferrous alloys such as copper, aluminum, magnesium, zinc, and other types. The castings from metals and alloys find wide use as parts of machines and instruments turned out by the machine-building and instrument-making industries. What makes the foundry practice most popular is the possibility of producing cast parts of complex shapes with minimum machining allowances and good mechanical and service properties. Mechanized and automatized casting processes help decrease the cost of castings.

A casting is produced when molten metal is poured into a casting mold and left to cool and solidify. Consider the sequence of steps involved in molding a casting of simple shape, for example, an iron buses 1 (Fig. 1). First, it is necessary to make a wood pattern 3 using the part drawing.

The pattern is a form bedded down in sand to make an imprint in the mold conformable to the shape of the desired casting. Since the melt that fills the mold contracts as it cools and solidifies, the pattern maker must take into account the shrinkage of metal. He must also consider a finishing allowance for subsequent machining of certain portions of the casting. So, to produce a casting of the required shape, the pattern should be slightly larger in all dimensions than the finished piece to allow for metal shrinkage and the stock left for machining. Patterns are made of wood, metal, gypsum, plastics, and other materials.

The pattern shown in the figure is a split piece that consists of two halves mutually centered and locked in place with tenons and mortises.

The core 9, which is part of the mold, serves to form the central hole of bushing 1.The core is prepared from core sand compacted in a core box 2 into the shape of the resultant cavity. The shape is then baked dry to give it strength. In the assembled mold the core is held in its correct position by impressions —known as core prints (core print seats) — formed in the mold by suitably shaped extensions (core prints) 4 on the original pattern 3. The core is thus made longer than the casting cavity by the size of prints.



The mold for the bushing is made up of two mold halves, the upper 6, called the cope, and the lower 14, known as the drag. They are prepared from molding sand rammed in metallic box-like frames, Called flasks, or boxes. A molding box which holds the top half or cope of the mold is called the cope, and that which holds the bottom half or drag of the mold is termed the drag.

 

Fig. 1. Sequence of steps in molding an iron bushing

Production of the mold for the bushing.To form the desired imprint in the mold drag, one half-pattern 3 is placed on a molding board 10' along with a flask drag 7 (drag box) as shown in Fig. 1a. The surface of the half-pattern and board is now dusted with dry facing sand or sprinkled with a parting liquid (kerosene), and the flask drag is filled up with ordinary molding sand, which is rammed to consolidate it. Any excess of the molding sand is cut off level with the up-per edges of the drag and then the drag is turned over and placed on the molding board 10. The upper half of the pattern is now fitted on its lower half and the flask cope is placed on top of the drag (Fig. lb). Next, facing sand is sprinkled over the surface of the pattern, the sprue and riser pins are set up, and then molding sand is riddled into the flask cope and rammed over the entire surface.

The cope half of the flask is now lifted off and set aside, the pattern is rapped and withdrawn, the core is put in position, and then the mold is closed by placing the cope on top of the drag (Fig. lc), location being effected by pins 5 and holes 11 in the lugs of the flask. The molten metal filling the cavity exerts pressure on the mold walls and tends to lift the cope, with the result that a gap may form at the parting plane, through which the melt can run out from the mold. To preclude leakage of the metal, the cope is fastened to the drag with clamps 13 (Fig. 1d) or sometimes weighted.

In pouring molten metal into the mold, the metal enters the mold cavity 8 through a system of channels, called gating. Fig. 1 shows the gating system that consists of a sprue 12, slag trap 15, and ingate 16 to admit the molten metal to the mold cavity. The gating system also includes a flowoff 17 (Fig. le) whose principal function is to allow the air and gases evolved during pouring to escape from the mold and reveal the level of metal in the mold.

After the metal has solidified and cooled, it is necessary to shake out the casting and thus destroy the mold, knock out the core, cut off the gating, and clean the casting of the molding sand.

The described casting mold is dispensable. Such molds are made of molding sands, the basic constituent of which is quartz sand. Clay is used as a bonding substance which imparts strength to the sands. Since the strength of such molding sands is relatively low, while the pressure of molten metal on the mold walls is rather high, sand molds must be made thick-walled. The use of binders which give high strength to the molding sand enables the production of shell-type (thin-walled) molds. This also effects a drastic cut in the consumption of Nand and, besides, ensures closer dimensional tolerances and better surface finish of castings owing to the specific properties of the molding sand.

Thick-walled sand molds allow for molding intricately shaped castings of a few grams to tens of tons in mass from various alloys in piece, batch, or quantity production. The production process is relatively simple, makes use of cheap materials, provides for adequate accuracy and tolerable surface roughness of castings, and is adaptable to mechanization and automatization.

Permanent metal molds find wide uses in the foundry practice. These molds are adaptable to the production of tens and thousands of castings from steel, cast iron, and nonferrous alloys. The permanent mold process can produce castings of intricate shape, which may range up to several tons in mass. Generally, this process is practical for making parts of small or medium mass (a few tens of kilograms) from light nonferrous alloys.

The castings produced in permanent molds have smooth surface and increased accuracy of dimensions. Permanent mold casting excludes the use of molding sands, improves the conditions of labor, and is adaptable to mechanized and automated processes. A rather high cost of metal molds, however, limits their uses only to large-scale and quantity production.

 

Task 5. Match the following:

1. dispensable 2. iron 3. wood 4. desired 5. finishing 6. molding 7. gating 8. shell 9. slag 10. intricately 11. batch 12. surface a) roughness b) system c) trap d) bushing e) pattern f) mold g) casting h) production i) board j) allowance k) walled l) shaped

Date: 2016-04-22; view: 898


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