There are certain principles underlying iron molding which hold good in all classes of founding, and a practical understanding of these principles is necessary for good work in any line. Aside from the fact that generally a mold is wanted which takes the least possible time to put up, three things aimed at in green-sand work are: (1) a sound casting, which is free from internal imperfections, such as blow holes, porous spots, shrinkage cracks, etc.; (2) a clean casting, which is free from dirt, such as slag, sand, etc.; and (3) a smooth casting, having a uniform surface free from scabs, buckles, cold-shuts, or swells.
The natural sands best adapted to obtain these results have already been dealt with. The methods of adding new sands vary with different classes of work. For light work the entire heap should be kept in good condition by adding a little new sand every day, for the light castings do not burn out the sand to a great extent.
On heavier work of 50 pounds and upward, the proportion of sand next the pattern is so small compared with that used simply to fill the flask, that it does not pay to keep the entire heap strong enough for actual facing. The heap should be freshened occasionally with a cheap molding sand, but for that portion of the mold which forms the joint surface, and especially that which comes in contact with the metal, a facing sand should be used.
The range of new sand in facing mixtures on a 10-part basis, with sea coal in addition, is as tabulated herewith;
Fig. 13. Illustrating Method of Clamping.
(Basis of 10 Parts).
Sea Coal (additional part)
3 - 6
6 - 2
1 - 2
These proportions, and the thickness of the layer of facing sand, vary with the weight of metal in the casting. Too much new sand tends to choke the vent and to cause sand to cake; too little new sand renders facing liable to cut or scab. Too much sea coal makes sand brittle and more difficult to work, and also gives off too much gas which is liable to cause blowholes in casting. Not enough sea coal allows the sand to cake, making cleaning difficult.
To prepare foundry sand for making a mold, it must be tempered and cut through. This is now usually done by laborers. To temper the sand, throw water over the heap in the form of a sheet by giving a peculiar backward swing to the pail as the water leaves it. Then cut the pile through, a shovelful at a time, letting the air through the sand and breaking up the lumps. This moistens the clay in the sand, making it adhesive and puts the pile in the best condition for working.
To test the temper, give one squeeze to a handful of sand. An excess of water will at once be detected by the soggy feeling of the sand. Now hold the egg-shaped lump between thumb and finger of each hand and break it in the middle. The edges of the break should remain firm and not crumble. Too much moisture will make excess of steam in the mold, causing blowholes. Not enough moisture renders sand weak and apt to wash or cut.
The sand next to the joint and over the pattern should be sifted. The thickness of this layer of sifted sand varies from about 3/4 inch for light work to 2 inches on very heavy work. The fineness of the sieve used depends upon the class of work. No. 16 or 12 would be used for small name plates, stove plate, etc., while No. 8 or 6 is good for general machinery work. On floor work, from 4 to 6 inches of sand back of the facing should be riddled through a No. 4 sieve to ensure more even ramming and venting.
The object of ramming is to make the sand hang into the flask and to support the walla of the mold against the flow and pressure of the metal. The knack of ramming just right only comes with continued practice and comparison of results. Hard ramming closes up the vent, causing blowholes. Iron will not "lay" into a hard surface. Soft ramming leaves a weak mold surface, and the flow of the metal as it enters the mold washes or cuts the sand, leaving a scab on one part of the casting and sand holes on another. A mold rammed too soft tends to swell under the pressure of the liquid metal, making the casting larger than the pattern or leaving an unsightly lump on the casting. The bottom parts of a mold, being under greater casting pressure, must be rammed somewhat harder than the upper portions. The joint also should be packed firmly, as it is exposed to more handling than any other part.
Crossbars are put in the cope to make it possible to lift the sand with the cope without excessively hard ramming. As an additional support for the cope sand on large work there are used gaggers, which are L-shaped pieces of iron made from wrought or cast iron of from 5/16 inch to 1/2-inch square section.
The force of sand pressing against the long leg of the gagger holds it in place and the short leg supports the sand about it. Therefore the gagger will hold best when the long leg is placed tight against the crossbar and is plumb. The long leg of the gagger should not project above the level of the cope, as there is much danger of striking it and breaking in the mold after the flask is closed. In Fig. 19 are shown the right and wrong ways of setting gaggers.