The making of a core is matter which requires fully as much attention as the mold itself, and one is apt to think that as long as a core is made so that it will hold together and is well vented, that this is all that is necessary. Such an idea is wrong, says the "Brass World." A core for one purpose requires a different treatment from that of another, and to produce a core which will answer certain requirements, needs a careful understanding of the properties of the various binders.
The ideal core would be one which does not give off any gas at all when the hot metal strikes it. In addition to this, the core must be yielding so that the metal will not crack when it shrinks around it. Such a core material is unknown, and it is believed that the nearest approach to it is a core made of sand held together with glue. The virtue of the glue lies in its ability to bind the core together and, as far as known, glue is the best adhesive in existence. For a given weight, glue will hold more sand together than any other known substance.
This means that a smaller amount may be used for mixing with the sand than any other material, and with the accompanying virtue of giving off less gas when the hot metal strikes the core. It will, no doubt, be apparent that the less binder that is used in making a sand core, the less the amount of gas which will be given off. This is why glue is so useful in making the cores. So small an amount is used that very little gas is given off.
The method of using the glue for the manufacture of cores is to boil it up with enough water to dissolve it, and then add cold water until a very thin glue water is had. This is used to sprinkle the core sand mixture. The cores are then made in the usual manner and dried. If the cores are too hard, then less glue must be used in the water. The amount to be used can only be told by experience.
The advantage of glue is very apparent in the manufacture of small cores as the cost is not an item as it is in the use of a binder for large cores. For small cores, where there is little opportunity to vent them properly and little opening in casting for the escape of gas, glue will be found particularly serviceable. By its use it is possible to make a core which gives off practically no gas at all. This, of course, is on the assumption that a core can be used which is very soft. if trouble is experienced with cores of a small nature, core maker would do well to glue. Many know of its value but imagine that the cost is so much that it cannot be considered. As very much less can be used than in the case of flour, the cost is not so much as one might naturally believe.
Glue is very valuable in the case of large cores which are of such shape that they are apt to break in handling. By putting sufficient glue in the water which is used to wet the core sand, a core may be made which will stand a large amount of rough usage.