ТОР 5 статей:
Read and translate Text B. Answer the following questions.
1) In what molds are sand castings produced?
2) How many castings may be produced from one sand mold?
3) What it used for making the cavity of the sand mold?
4) What material gives the strength to the molding sand?
5) What does the porosity of the sand depend upon?
6) What sand must be used for producing the casting with smooth surface?
Most castings are produced in sand molds, since this method of production is relatively simple, inexpensive and is not limited to any type of metal or sizes and shapes of castings. Of course, sand molds are used only once, and each casting requires a new mold.
In the production of sand castings the important functions are making the pattern, melting and alloying the metal, pouring the alloy into the mold and cleaning the castings.
Sand molds are made of refractory molding sand. It is a mixture of sand and clay. The clay helps the sand to stick together, and the strength of the molding sand depends upon the clay content in it. Large proportion of clay makes the sand less porous, however. And the sand porosity is very important in medium and heavy work.
The molding sand is to be mixed with water in certain proportion. Too much water should not be poured into the molding sand. The quality of castings often depends upon the amount of water the sand contains.
It is necessary to make a casting with smooth surface; fine sand is to be used. The finer is the sand; the smoother will be the casting. Coarse sand is usually used for making large iron and steel castings.
26. Divide Text B into 2 logical parts. Which of the following titles is the most suitable one for the second part?
1) Making the sand molds.
2) The production of sand castings.
3) Molding sand.
27. Are these sentences true or false?
1) Sand casting is the most economical method in foundry.
2) This method is used only for producing the casting of simple shape.
3) The porosity of the molding sand depends upon the proportion of clay in it.
4) The molding sand for medium and heavy castings (works) must have high clay content.
5) Molding sand must be refractory.
6) The coarse sand makes the surface of the casting less smooth.
7) The more is the clay content in the molding sand; the more is its porosity.