7. Use your translation of 6 in describing the process of «Investment Casting» as a summary.

8. Translate at sight

В прецизионном литье применяется литниковая система, которая изготовляется из того же материала, что и восковая мо­дель. Металлическая подмодель имеет как раз ту форму, которую должна получить окончательная отливка. Литник прикрепляется к модели в любой желаемой точке.

Монтированная модель сначала облицовывается тонким слоем огнеупорного цемента, а затем сушится несколько часов.

Закалённая форма нагревается до такой температуры и по­мещается в такое положение, что воск плавится и полностью ис­чезает, оставляя полости точно такой же формы, как и у восковой модели и литников. Форму прокаливают в печи в течение 24 ча­сов. Заливают её ещё горячей.

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Unit 10 DIE CASTING

The use of die casting is growing faster than that of any other type of casting. This is because die castings offer the user small-size non-ferrous castings in large quantities as an exceptionally profitable combination of:

• low cost;

• excellent precision;

• excellent surface finish;

• maximum design flexibility.

In fact, for parts, which are adaptable, the casting represents the ultimate in economy. Here are the more important limits of applicability.

1. Metals. Aluminium, zinc, magnesium, copper-base alloys and other metals with relatively low melting temperature {1600 °F or less).

2. Size. Less than an ounce to 50 pounds. The size range is being extended steadily.

3. Quantity. Since dies cost several thousand dollars each, the annual quantity required must be high-normally at least 10.000 pieces.

In the die-casting process molten metal is forced into a steel die under high pressure. After a short cooling period, the casting is ejected, the excess metal is trimmed off, and the casting is ready for use. The casting process is usually done in high- speed automatic machines op­erating on predetermined cycles. The process is rather simple. The moulding operation is performed simply by opening and closing the die. The pouring operation is performed automatically by the action of the machine. After casting, the only operation required is the trimming of excess metal. This can be frequently done by forcing the part through a trimming die. The die is produced from two steel blocks. A skilled die maker machines into one side of the die an impression of one half of the casting. The impression of the reverse half is machined into the opposite half of the die.

This must be done with great accuracy so that casting dimen­sions can be held to close tolerances.Die life varies from up to a mil­lion parts with zinc-base alloys, to about 100.000 parts with aluminium *nd magnesium alloys, and from 10.000 to 50.000 parts with low-

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melting brasses. In designing parts for die-casting sections must be kept thin to permit solidification within the cycle of operation. Ribs may be added for necessary strengthening. The part must also be adapted to the removal of necessary metal cores prior to the ejection of the piece from the die. Details of die design, including parting line lo­cation arrangement of parts in multiple cavity moulds, gating, vents, cores, etc., should be worked out by a competent die-caster.

Exercises