All sands are formed by the breaking up of rocks due to the action of natural forces, such as frost, wind, rain, and the action of water.
Fragments of rocks on the mountain sides, broken off by action of frost are washed into mountain streams by rainfall. Here they grind against each other and pieces thus chipped off are carried by the rush of the current down into the rivers. Tumbled along by the rapid current of the upper river, the sand will finally be deposited where the stream flows more gently through the low land stretches below the hills. Here the slight agitation tends to cause the finer sand and the clay to settle lower and lower down in the bed. Thus we find beds that have been formed in ages past; possibly with a top soil formed over them, so long have they been deposited. But on removing this top soil we find gravel or coarse sand on top; this merges into finer sand and this again finally into a bed of clay.
Sand (per cont)
(Tore Sand (percent)
Light (per cont)
Medium (per cent)
Light (per cent)
Alumina (clay). .. Al2O3
Iron Oxide....... Fe2O.2
(b) Lime carbonate.. CaCO3
• • • •
Combined water... . H2O
Degree of fineness......
Rocks, however, are very complex in their composition, and sands contain most of the elements of the rocks of which they are fragments. For this reason molding sands in different parts of the United States vary considerably.
A good molding sand first of all, should be refractory, that is, capable of withstanding the heat of molten metal. It should be porous to allow the escape of gases from the mold. It should have a certain amount of clay to give it bond or strength, and should have an even grain. All of these properties will vary according to the class of work for which the sand is used.
The two important chemical elements in such sands are silica, which is the heat-resisting element, and alumina, or clay, which gives the bond. Other elements which are found in the molding sands are oxide of iron, oxide of lime, lime carbonate, soda potash, combined water, etc. The analyses shown in Table I, made by W. G. Scott, give an idea of the proportions of these elements in the different foundry sands.
Silica alone is a fire-resisting element, but it has no bond. These other elements help in forming the bond. But under heat, silica combines and fuses with them, forming silicates. These silicates melt at a much lower temperature than does free silica. Therefore with sands carrying much limestone in their make up, or with those containing much oxide of iron, soda potash, etc., the molten iron will burn in more, making it more difficult to clean the casings.
The limestone combinations also go to pieces under heat, tending to make the sand crumble, which may result in dirty castings.
The proportions given in Table I must not be considered as absolutely fixed, for no two samples of sand, even from the same bed, will analyze exactly alike. The table is instructive, however, because it indicates the reasons why the different sands are especially adapted to the use to which they are put in practice.
Such sand is used in the daubing mixture for repairing inside of cupola and ladles, and should be in the highest degree refractory, and should contain as little matter as possible that would tend to make it fuse or melt.
This sand is used for castings such as stove plate, etc., which may have very finely carved detail on their surfaces, but are thin. The sand should be very fine to bring out this detail; it must be strong, i.e., high in clay, so that the mold will retain every detail as the metal rushes in. On the other hand, the work will cool so quickly that after the initial escape of the air and steam there will be very little gas to come off through the sand.
Sand of this grade is used in bench work and light floor work, for making machinery castings having from 1/2- to 2-inch sections. These will have less fine detail, so the sand may be coarser than in the previous case. The bond should still be fairly strong to preserve the shape of the mold, but the tendency of the large proportion of clay to choke the vent will be offset by the larger size of the grain. This vent must be provided for because the metal will remain hot in the mold for a longer time and will cause gases to form during the whole of its cooling period.
This grade of sand is used for the largest iron castings. Here the sand must be high in silica and the grain coarse, because the heat of the molten metal must be resisted by the sand, and gases must be carried off through the sand for a very long time after pouring. The amount of bond or clay must be small or it will cause the sand to cake and choke these gases. The detail is generally so large that the lack of bond is compensated for by the use of gaggers, nails, etc. The coarse grain is rendered smooth on the mold surface by careful slicking.
Core sand, often almost entirely surrounded by metal, must be quite refractory but have very little clay bond. This bond would make the sand cake, choking the vent, and render it difficult of removal from a cavity when cleaning the casting. Compared with medium molding sand, it shows higher in silica, although having less than half the proportion of alumina.
Sands having practically no clay in them are called free sands. Of these there are two kinds in use: river sands, and beach sands.
The grains of river sand retain the sharp fractured appearance of chipped rock, and these little sharp grains help much in making a strong core because the sharp angular grains interlock one with another. River sand is used on the larger core work.
Beach sand is considerably used in coast sections because it is relatively inexpensive, but its grains are all rounded smooth by the incessant action of the waves. It will pack together only as will so many minute marbles. For this reason it is used only for small cores.