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CHAPTER III

PREPARING THE MATERIAL FOR AND HARDENING MOLDS

THE worker often demands a mold that will make a number of casts; this where there is no very marked under-cutting is easily possible to accomplish by baking or hardening the mold, so that it is durable and will make a number of casts or pieces of work before it outlives its usefulness. Where the design is very intricate and with deep under-cutting this method is impossible, as it will be found impracticable to remove the cores from the deep hollows; but in the majority of designs it works very well, and with all the success of a sectional plaster mold.

The chief requisite for a baked mold is a material that, when subjected to moderate heat, will harden or "bake" until it is not easily penetrated by anything it comes in con

tact with, as well as being strong enough to stand a moderate amount of handling without danger of breaking.

This can be obtained by the use of common molasses mixed with your molding sand; the molasses makes a very strong adhesive, and when subjected to a moderate heat hardens nicely, and makes a core or mold that will secure a number of casts in concrete without injury to the mold. The quantity to use must be determined by the sand you are working with; the best guide is to mix it slowly with the sand, adding until the sand is of a putty-like consistency or so it can be modeled with the fingers and will retain its shape. It can then be packed into the flask, over the pattern, first covering the pattern with dry sand sprinkled over same so the molasses and sand mixture will not stick to the pattern, when you wish to remove same.

Another excellent mixture is the use of sand and a thin paste made of flour and water; this bakes quite hard and is excellent for a hardened mold, as well as having the advantage of being cheap in cost. It is used in the same manner

as the molasses, mixing with the sand until the necessary quality of adhesiveness is secured, then pack into mold over pattern.

There are many other similar compounds that may be used, as sand and sour beer or sand and thin glue-water; anything that will harden under heat until it is not easily penetrated, or broken.

The flask for a hardened or baked mold must be such that it can be removed before putting in the oven, as the wood would take fire from even the moderate heat required to bake or harden the mold. This is secured by making the flask or mold form in a box form, but hinged at three of the corners with the fourth corner to fasten with a hook and eyelet; this enables the hook to be unfastened and the wood flask folded back from the sand mixture, inside same, thus removing it before placing in the oven.

To prevent the sand mixture from falling down or breaking, as well as cracking or checking from the heat, there must be some uninflammable material to enclose the mold; this need not be strong and can be made of a strip of tin,

sheet iron or any metal riveted together so it will just sit or go inside the flask; the sand mixture is packed inside this and when the wood part of flask is removed the metal casing remains, to hold the mold in shape while it is being hardened by the heat of oven.

The same as the wood flask, a substitute for the cover board, or board on which the mold rests, must be secured; otherwise the heat of oven would burn that also. This may be secured in any heavy metal sheet or a flat grate from the oven of a stove or any similar flat sheet of metal which can be supplied by the scrap heap at your local foundry.

The oven can be anything to generate the heat to harden or bake the material, of which the mold is composed; as these are not usually large an old cook-stove for small work will be ideal; the mold is placed on the flat sheet of metal and then the wood flask unfastened and removed, by folding back carefully from the work; it is then ready for the oven as the pattern has been removed before the wood flask, it is then a simple matter to place in the oven

and generate heat for from four to six hours, even less is often ample, as the only requirement is to harden the material, and when that is accomplished the mold is ready for removal. It is often advisable where a mold is valuable to again encase it with the wood flask, after baking, so that any rough usage to which it might be subjected would not be so apt to injure it.

This material is valuable in the event of cores that are to be a separate part of mold, these can be baked hard and so stand rough handling without danger of breakage.

Where the worker has at his command the regular iron flasks, as used for this work in the metal foundry, he can work more rapidly, as they will not require removal before placing in the oven, which does not generate heat enough to have any effect upon the metal, as that is not required.

It is not general in practice, but I have found it to be of value to protect the molding surface of such molds before using, as when a number of casts were required from the same mold the

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