1. In the bottom pouring all slag remains on the top of the metal.

2. All the ladles should be preheated to about 1500 °F.

3. It is desirable to ensure the dryness of the ladle so that hydro­gen and oxygen will become dissolved in the molten metal.

4. In the teapot pouring metal leaves the ladle through the top side and emerges through die «teapot» spout.

5. Read the following text and find the answers to the ques­tion below

1. Why must the mould be strong enough?

2. When does the necessity of making a mould appear?

3. What types of pouring are mentioned in the text?

Having been melted, the metal is tapped from the furnace into a pouring ladle. Then the molten metal is poured from the ladle into the mould that must be strong enough so that it does not melt or change shape when full of molten metal. The necessity of making a mould ap­pears just after the casting procedure has been selected.

Ladles are known to be of three common types. But all of them should be preheated to about 1500°F before use to avoid the loss of temperature between the furnace and the mould.

Metal may be poured over the lip of the ladle (lip pouring), may leave the ladle through the bottom side and emerge through the «tea­pot» spout (teapot pouring), or as in the bottom pour metal leaves the ladle through the nozzle in the bottom. Slag being held back, clean metal enters the mould.

6. Use your translation of 5 in describing the process of «Metal Handling» as a summary.

7. Translate at sight

Известно, что расплавленный металл подается из печи в ли­тейный ковш, миксер. Желательно, чтобы ковш был сухим, так чтобы водород и кислород из паров воды не растворялись в рас­плавленном металле.

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Ковши бывают различных видов. Большие - обычно состоят из металлического кожуха, облицованного огнеупорами, способ­ными противостоять температуре 3100 °F. Меньшие ковши также могут быть облицованы огнеупорными материалами или могут иметь сборную футеровку и форму тиглей, сделанных или из свя­занного графита или карбида кремния.

В зависимости от вида разгрузки расплавленного металла шлак может задерживаться внутренней клюшкой, скребком для скачивания шлака или поглощаться литниковой чашей для круп­ных отливок, или удерживаться на поверхности металла, в то вре­мя как чистый металл входит в форму.

Unit8

Shell moulding

Shell mouding is a semiprecision method for producing castings repetitively in high volume. It provides the user with the following advantages.

• Closer tolerances than obtainable in sand moulding.

• Improved surface smoothness and surface appearance.

• Greater design latitude in the use of sharp corners, small holes, intricate contours, or thin wall sections.

• More consistent quality is obtainable.

These advantages are all related to the improved precision of the process as compared with green sand moulding. The gains in precision are made because stable phenolic resits are used in the moulding mate­rials instead of the relatively unstable water, binders, and core oils, which are used in sand moulding and sand cores. Also, the mould is rigid before removing the pattern.

The Process

A phenolic resin is mixed with fine silica sand. This mixture must be dry or in the presence of alcohol. No water is included, a ma­chined metal pattern is used usually of gray iron, aluminium, or brass.

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The pattern is heated from 450 to 55-0 °F. Then the sand resin mixture is dumped or blown on the surface of the pattern. Upon contact with the hot pattern, the resin melts, hardens, and bonds the sand grains strongly together. Heat of the pattern is sufficient to melt the resin for a depth of 1/8 inch or more, and a shell is, thus, formed having the shape of the pattern. The uncured mould is then moved into an oven where it is cured for 15 to 120 seconds. Aftei curing, the shell is rigid and can be stripped from the pattern by means of ejector pins, and readily stopped and handled.

The two halves of the mould are then assembled with the aid of alignment bosses. They may be joined together by pasting and curing, and by clips, or clamps, or tapes. Handling shell moulds is carried out in machines designed for such purpose.

In general, the process is quite adaptable to automation for low cost. The completed shell moulds may be stored for several days or even weeks before pouring. For the pouring process they may be placed in a box and supported by filling the box with steel shot of gravel. In place of shot or clamps framework supporting the shells at critical points may be employed. Moulds may be poured in an upright position horizontally, or tilted depending on the design of the casting and layout of the pattern.

After the casting cools, it is removed from the shell, usually on a shakeout machine, and the burned shell sand is discarded.

Shell moulding is ordinarily used only for castings, which are run in reasonably high volume, most shell-moulded castings are small in size, although, a few are produced in sizes up to several hundred pounds.

In most cases shell-moulded c-astings will cost more than green-sand castings. Hence, most applications are found in those cases where one or more machining operations can be eliminated, or where a grind­ing operation can be substituted for a machine cut. And because the process is so readily adaptable to a high degree of mechanization, it is likely that the economy of shell moulding versus sand moulding will improve rapidly for small parts, which are made in relatively high volume.

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Exercises