Investment casting

The investment process makes it possible to produce a casting of a wide range of shapes and contours in small-size parts. In fact, a large per cent of the products in industry are the parts which would be virtually impossible to produce by any other process.

Investment casting, once more generally known as the lost-wax process, is an old casting method. The basic investment process was developed in ancient Egypt for producing jewellry. It has been used for years in making dental inlays. Since World War II its industrial uses have grown rapidly.

A metal master pattern is used^to form out of wax or synthetic resin what may be called a negative pattern, that is, a shape having exactly the form desired in the finished casting. The gating system of the same material as the wax pattern is put together separately and at­tached to the pattern at any desired points. The assembled pattern 's then coated by dipping of spraying with a thin layer of refractory cement.

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After drying for several hours, the coated pattern is placed in a flask, and the mould is completed by filling the flask with a refractory mixture.

This again must be dried for several hours. The hardened mould is heated to a temperature sufficient to melt the wax and placed in such a position that the wax will drain completely leaving cavities of exactly the shape of the wax pattern and gates. If resin is used instead of wax a higher temperature is required. The mould is fired in a furnace for ap­proximately 24 hours.

After firing, the moulds are poured while still hot. Moulds may be poured statically or centrifugal ly. Many investment moulds are poured from a special furnace, to which the mould is clamped and then inverted to allow the metal from the furnace into the mould. An inter­esting modification of the investment process utilizes frozen mercury in place of wax. Parts formed of mercury and cooled to 90 °F below zero can be manipulated briefly at ordinarily temperature, but must be heated to 1350 °F to get rid of the organic binder used in the investment.

The process seems to be elaborate and expensive, but it has proved to be advantageous in a large number of cases. Although it is more costly than other casting processes, the investment process offers low-cost solutions to problems where the part is small, the metal is hard to work or to machine, and intricate contours and excellent sur­face finish are needed.

Exercises

1. Answer the following questions

1. What parts may be produced by the investment process?

2. Where was the basic investment process developed and for what purpose?

3. What pattern is used in investment casting?

4. Is the gating system put together separately? Where can it be attached?

5. How is the mould completed (hardened)?

1. What is the pattern coated with?

2. Where and when is the coated pattern placed?

6. What temperature is required for heating the hardened mould?

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7. How long is the mould fired in the furnace?

8. In what way may the moulds be poured?

9. Can mercury be used instead of wax?

10. Why must the finished mould free from mercury at room temperature be heated to 1850 °F?

10. Is the investment process elaborate and rather expensive?

10. Does it have any advantages compared with other casting processes, although, it is more costly than others?

10. In what cases is the investment casting utilized?

2. Match the words on the left with their definitions on the right

1) cavity a) a casting process by which it is

possible to obtain castings of high dimensional accuracy and good surface quality

2) to coat b) a rigid frame designed to hold and

support the sand in a sand mould

3) flask c) an auxiliary metal pattern used for

the production of a metal pattern proper

4. gate d) empty space within a solid body

5. investment casting e) to face foundry moulds

6. master pattern f) a channel in a mould through

which molten metal is poured into the mould cavity

3. Give the definition to a negative pattern in English.

3. Translate the passage of the text in writing beginning with «An interesting modification of the investment process...» to the end of the text.

5. Find out which sentences do not correspond to the con­ tents of the text

1. Investment casting is an old method.

2. If resin is used instead of wax a lower temperature is required.

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3. After drying for an hour, the coated pattern is placed in a flask.

4. After firing, the moulds are poured while still hot.

6. Read the following text and give the title to it

Investment casting is such a casting process by which it is possi­ble to obtain castings of high dimensional accuracy and good surface quality.

Although, this process seems to be elaborate and expensive, a large per cent of the products in industry are the parts, which would be virtually impossible to produce by any other process. Besides, invest­ment casting has a lot of advantages over other casting processes. It appears to be an old casting method developed in ancient Egypt for producing jewelry.

In investment casting a great number of shapes and contours in small-size parts may be cast. In this casting process wax is usually used. If synthetic resin is used in place of wax a higher temperature is needed. An interesting modification of the investment process utilizes frozen mercury instead of wax.