These conditions are obtained by using a coarse free sand and a binder. To give additional strength when necessary, iron wire or rods, or cast-iron core arbors are bedded in the core. These serve the same purpose in a core that the flask does in green-sand work.
The action of the binder enables the sand to retain its shape when the box is removed, and renders the core hard and strong when baked. In the mold the intense heat of the metal gradually burns out the organic matter or binder, leaving the core without bond. In this condition, the sand may readily be removed.
Too much binder tends to make the core sag out of shape before baking, and blow when metal strikes it; that is, give off more gas than the vents can carry away. With too little binder the sand does not bake hard, and cuts when the mold is poured.
The effectiveness of all binders, especially flour, depends upon their thorough mixing with the sand. The especial value of rosin and oil lies in the fact that by melting under the oven heat they form a more perfect bond with the sand.
Many intricate cores are now made with an oil mixture, without using rods or wires, which formerly were considered absolutely necessary for strength. Such cores must be well supported when green, must be thoroughly baked, and handled with much care until they are cold.
No universal mixture for core sand can be given, as sands vary so much in different localities. The mixtures, as shown on the following page, illustrate approximate proportions. In preparing core sand, the different ingredients should be measured out, thoroughly mixed, and sifted while dry. Temper the mixture a little damper than molding sand. Too much moisture makes the sand stick to the box. Not enough makes it hard to work and gives a crumbly surface if dried.
Blacking for light work should include one cup of molasses to a pail of water, into which is worked powdered charcoal until an even black coating is deposited upon the finger when dipped into the blacking and out again.
Small Cores (parts)
Large Cores (parts)
Sharp fire sand
Strong loamy sand
For heavy blacking there should be used about 2 parts charcoal and 1 graphite, mixed into thick clay wash.
In finishing small cores, they should be sprayed with weak molasses water while green, then well baked and removed from the oven. When cool enough to handle, they are dipped into the blacking; then put back in the oven until this facing has dried. For large cores the blacking is applied with a brush before baking.
All cores should be baked as soon as made, for air-drying causes the surface to crumble.
Cores must not be set in a mold while they are hot, or the mold will sweat, that is, beads of moisture will form on the inside faces. This would make the mold blow when poured.
A core should be rammed evenly and somewhat harder than a mold. Too hard ramming will make the sand stick in the box, besides giving trouble in casting. Too light ramming makes a weak core.
From the very nature of cores, the matter of venting them is very important and often calls for much ingenuity on the part of the core maker.
For simple straight work a good sized vent wire is run through before the box is removed. Half cores have their vents cut in each half before pasting together. Cinders are rammed in the center of large cores connecting through the prints, with the mold vents. For crooked cores, wax vents are rammed in the center - the wax-melts away into the sand when the cores are baked, leaving smooth even holes. This is illustrated in one of the following examples.
The examples here given serve to illustrate the principal methods used in making cores.
The simplest form of core is one which can be rammed up and baked as made by simply removing the box. Short bolt-hole cores, etc., are made in this way, as shown in Fig. 63.
Set the box on a flat bench top. Hold the two halves together by the clamp A. Ram the hole full of core sand by the use of a small rod. Slick off the top; run a good sized vent wire through the middle of the core. Remove the clamp. Set the box onto the core plate, rap the sides, and carefully draw them back from the core.
Larger cylindrical cores, up to about 1 1/2 inches diameter, are rammed in a complete box also, only they are rolled out on their sides, as shown in Fig, 64. This, however, tends to make a flat place on the side, from the weight of the sand supported on this narrow surface.
For this reason cylindrical cores of large diameter, and many symmetrical shapes, are made in half boxes. See Pattern-Making, Figs. 110, 208, 213, and 219. Such boxes are rammed from the open side. Wires are bedded when necessary about in the middle of the half core. The fingers and the handle of a trowel are often used to ram the sand, and with the blade of the trowel the sand is struck off and slicked to the level of the top of the box.
When baked, two half cores are held with their flat sides together, and any slight unevenness in the joint removed by a gentle rubbing motion. A vent channel is then scraped centrally on each half. Paste, made of flour and molasses water, is applied around the edges and the two halves pressed firmly together; care is taken to see that they register all around. The core should then be placed in the oven to dry out the paste. When pasting cores of 6-inch diameter and over, it is well to bind the halves at each end with a single wrap of small wire.
Fig. 64. Large Cylindrical Cores.