The composition and mixing of loam has already been described. Loam cores made over a perforated barrel or pipe have also been alluded to.
Where the core is long and can best be supported at the ends only, loam cores are frequently used. They are also used where the core would be very large and heavy if made of dry sand. The advantages possessed by loam cores are that they are lighter, more cheaply made and offer better facilities for the escape of the gases than do the solid cores of dry sand. The method of making loam cores is very simple. A piece of pipe about three inches smaller in diameter than the outside diameter of the core is selected to form the center. This pipe is perforated by a large number of holes. If the pipe is more than three or four inches in diameter, centers or trunnions are placed in the ends to serve as bearings. This pipe is then arranged to revolve freely as shown in Fig. 71. A crank handle is attached to the pipe by which it is turned. The pipe is first wound with hay rope having a diameter of from ¾ to 1 inch. Where only a small amount of hay rope is used it is either bought ready-made or is twisted by hand. In foundries where large quantities are used the rope is made in machines built especially for the purpose. The rope should be made of long wisps and tightly twisted. It is laid on the pipe by fastening with a wire at one end and winding tightly to the other end where it is fastened in a similar manner. After the rope has been wound on and secured in position the loam is applied. This is done by means of an adjustable board A Fig. 71. The beveled edge of this board is set so that its distance from the center of the pipe is exactly equal to the radius of the core that is to be formed. The loam is then shoveled upon the board A and the pipe turned in the same direction as when the rope was being wound. The first coat of loam used should contain a considerable proportion of clay. This will make it tough and adhesive. The first coat must be well rubbed in and worked into all of the uneven places and cavities. If this is not done the casting will be rough and lumpy. This work is done by hand as the barrel is being slowly turned. Too much care cannot be taken in this portion of the work. After the rope has been covered and the loam pleased into it as firmly as possible, take a small iron bar,5/8 or ½ inch in diameter and press against the joints between the ropes, turning the barrel and working from end to end. When this is done, work in more loam and then press the whole down firmly with a block of wood while the pipe is being slowly turned. The second coat of loam which should be of such thickness as to leave about 3/8 inch for the finishing coat is then put on. The second coat should be made very stiff. Unless this is done it will bag and make an uneven core. Sometimes it is advisable to so adjust the sweep board A that a smooth surface is given to the second coat. This facilitates the laying of the finishing coat. The barrel is usually placed in the oven and baked before the finishing coat is applied.
While it is still hot it should be removed from the oven and rubbed down. This is done by setting it in its bearings and pressing a brick against it, while it is being turned. This breaks the skin, roughens the surface and removes the smoke black.
The finishing coat should be put through a sieve of eight meshes to the inch and thoroughly mixed. Put enough on the sweep board A to finish the core. Before making an application of the loam, brush off the prepared surface and moisten slightly. When this has been done, let the barrel be turned slowly, and apply the loam with the hands. After one revolution make the loam thinner. The more rapidly the core is finished the better will be the results. Two revolutions of the barrel is better than a greater number. When the core has been built to size, move the loam back from the edge of the board A and then withdraw the board while the barrel is still in motion.
Rough places and lumps should be smoothed with a block of wood and not with a trowel. If the core is of large diameter, it should be turned until the loam has set in order to avoid a tendency to bag. The thickness of the loam to be put over the rope depends upon the thickness of the casting to be made around it. Castings '2 inches thick will need about 1 inch of loam. In such cases the pipe should be 4 inches smaller in diameter than the finished core. As soon as the surface of the mould has become hard enough to absorb the blacking it should be applied. The best condition of mould or core for blacking is when it is just damp enough to soak up the material, and be ready for sleeking in five or six minutes thereafter. In sleeking a loam mould, press on gently with the tools. Too great a pressure closes the pores and tends to start the blacking from the surface of the mould. The less the sleeking the better. Rough places or marks left by sleeking can be smoothed by going over the surface with thin blacking-applied with a camel's hair brush. After blacking, the core is again dried in the oven, when it will be ready for use.
The object of using hay rope about the pipe is two-fold: it serves as a convenient binder for the loam and by burning or disintegrating under the influence of heat, the pipe is loosened and easily withdrawn from the casting. If the loam is too thin, the rope will be entirely burned away. When this occurs, the loam is crushed in and the iron, in contracting, is apt to so pinch the pipe that it can only be removed with difficulty.
Loam Moulds may be made without any patterns, with partial patterns and with complete patterns. The first is only used where the casting is of the simplest form. The second is the common method of making loam moulds. The third js seldom used, since a complete pattern makes it possible to mould in green or dry sand, which is cheaper than the use of loam. Loam moulds are usually built of bricks over which a coating of loam is spread. The loam, that is spread on brick work, is of the same character as that already described for loam cores. It is applied and treated in practically the same manner. Loam moulds are usually employed where the casting is large; where only one of a kind is to be made, and where it would be expensive to provide a pattern. They are also used in emergencies where the casting can be made in loam more quickly than a pattern for dry or green sand moulding could be put together. The most frequent application of loam moulding is to be found in the making of large cylinders for steam engines.