ТОР 5 статей:
Define the part of speech for the words in the right column. Guess their meanings and translate them.
Read Text A and translate it. Answer the following questions.
1) Are there various types of sand molds?
2) What types of sand molds do you know?
3) What type of sand molds is most commonly used?
4) Are green-sand molds economical?
5) What materials are mixed for producing green-sand?
6) For what metals is green-sand molding recommended?
Castings are produced into different types of molds. The various types of molds for sand castings are classified as green-sand, skin-dried, dry-sand and air-dried molds.
Green-sand molds are most commonly used. Most of small castings are made in green-sand molds, as green-sand molding is economical and simple.
Green-sand molds are formed by mixing silica, clay and a small amount of water. The permeability of the sand must be high so that the steam and gases formed by heat of metal can escape through the mold. The damp molding sand is rammed around the pattern. When the pattern is withdrawn, the mold is finished. The mold may be poured as soon as made or it may be held over a day or more, depending upon its size. Greensand causes less cracks when the casting cool to room temperature.
Green-sand molds are softer and weaker than all other types of molds. This characteristic is desirable for complicated castings which have high solid shrinkage.
Green-sand molding is recommended for cast iron and is applied to large-quantity production.
Since large molds may require much time for making and assembling, some other type of mold is usually used for larger castings.