Reference has been made in the first part of this article, under Divisions of Iron Molding, to the general difference between core work and green-sand work. This, and the section on Sands, the reader should review carefully.
Here, as in green-sand molding, the principal material used is a refractory sand. In molding sand, however, the alumina or clay forms a natural bond in the sand. To meet the necessary requirements of cores we must use a naturally free sand as a base, and give it bond by adding some form of organic matter as a binder, then bake the core.
The most common binders are the following four materials: Ordinary wheat flour is an almost universal material for use as a core binder. Every one is familiar with its action when moistened and baked. The hard vegetable gum rosin - a byproduct of the manufacture of turpentine - for use as a core binder, should be reduced to a powder. It melts under the heat of the oven, flows between the grains of sand, and upon cooling binds them firmly together. Linseed oil, made from flaxseed, acts in a way similar to rosin; a small proportion of oil together with some flour makes a very strong core. Glue, which is obtained from animal hoofs and from fish stock, is also used to some extent as a core binder. It should be dissolved in water before mixing with the sand.
A weak molasses water is used for tempering the sand for small cores; and on the larger work the same purpose is served by clay wash. There are many patent combinations of the above or similar materials put on the market as core compounds. There are two classes of these: dry compounds, and liquid compounds. The advantages claimed for them is that they are more economical - (1) because a smaller proportion of the compounds is sufficient to obtain the desired results; and (2) because a large proportion of the sand may be used over and over again.
Among other necessary core-room supplies are: annealed iron wire No. 6 to No. 16, and round bar iron in sizes 1/4-inch, 2-inch, 1/2-inch, 5/8-inch, and 3/4-inch, which are cut to length as needed, and are bedded in the core sand to strengthen the core, as will be demonstrated later.
A supply of clean cinders must be available also for venting larger cores. Small wax tapers make good vents for crooked cores. There is also a patented wax tent for sale on the market.
As before stated, charcoal with some graphite is the principal facing material used on cores. It is always applied in liquid form by dipping the core or by using a flat brush having extra long bristles.
The general tools of the core room are similar to those already mentioned. A piece of iron rod very often replaces the regular rammer on account of the small size of the opening into which sand must be packed.
The trowel is the most common slick, because most of the surfaces which require slicking are flat ones formed by striking off after packing the box. Except in the largest work, the entire face of the core is not slicked over, so a variety of small slicks is not needed.
A spraying can, shown in Fig. 57, is used for spraying molasses water over small cores. Fill the can two-thirds full and blow into the mouthpiece. Small cores are made up on a flat bench, the sand being in a small pile at the back. Larger boxes are rammed up on horses or on the floor, as is most convenient.
After being made up, cores are baked on core plates. The smaller plates are cast perfectly flat. Plates over 18 inches long are strengthened by ribs cast about 1 inch from the edge, as shown in Fig. 58; this keeps the plate from warping, and admits of its being picked up readily from a flat bench top or shelf.
Ovens are built with reference to the size of the cores to be baked. A good type of small oven is illustrated in Fig. 59. It can be run very economically with either coal or coke, and bakes cores up to 2 inches in diameter within half an hour. Each shelf is fastened to its own door, and, when open for receiving or removing cores, a door at the back of the shelf closes the opening. This prevents a waste of heat.
Fig. 60 shows the section through an oven suitable for the largest work, including dry-sand and loam molds. The fire box A is situated in one corner at the back; its whole top opens into the oven. At the floor level diagonally opposite is the flue B for conducting the waste heat to the stack C. The entire front of the oven may be opened by raising the sheet-steel door. Two tracks side by side accommodate cars upon which heavy work is run into the oven.
Fig. 58. Core Place.
Fig. 60. Core Oven for Large Work.
Fig. 61. Cast-Iron Car.
Fig. 61 shows a good form of cast-iron car. The wheels are designed on the roller principle to make it easier to start the car when heavily loaded.
For medium work smaller ovens of this type are used. Racks similar to the one shown in Fig. 62 may be bolted on the sides, arranged to hold the ends of the core plates; and the car may carry a line of double racks to increase the capacity of the oven.
As mentioned before, cores form those parts of a mold which are to be nearly or entirely surrounded by metal; in other words, such parts as would be in danger of breaking or require too much work to be constructed in green sand. The object, then, in making cores is to insure a better casting and to reduce costs.
Cores are held in position by means of core prints (see Pattern-Making). The main weight of the core is supported by these prints and through them all vent must be taken off and all sand removed in cleaning. Therefore, cores must be stronger than green sand, because, whether large or small, they must stand handling while being set and must not cut or break during pouring. They require greater porosity than green sand because their vent area is limited and their composition contains more gas forming material. Furthermore, cores must lose all their bond by the time the casting is cold, so that the sand may be easily removed no matter how small the available opening.
Fig. 62. Rack.