Natural molding sands are used for brass work. They are usually finer than sands used in iron work, because brass parts are generally small and often have fine detail which must be brought out very sharply in the mold. For this reason, also, the sands should have more alumina or bond than iron sands. This increase of bond is possible because the metals entering the mold are not as hot as iron would be, and therefore do not require as much vent, but they have a greater tendency to cut the mold.
Burnt sand, powdered charcoal, and partainol are all good parting materials; the last two are best on small work, as they make a cleaner joint. Since they make a good facing for the mold, they are not blown off of the patterns.
Fig. 175. Spill Trough.
The brass molder uses practically the same kind of tools, such as shovels, sieves, rammers, and molder's tools generally, as already have been described.
Snap flasks may be used, but the pins, hinges, and catches must be kept in careful adjustment so that the parts of the mold shall register perfectly. The same is true of the larger bo* flasks for floor work.
The most typical brass flasks are of cast iron with accurately fitting round steel pins, as seen in Fig. 174 at A. They have holes on the joint at one end of the flask so that the mold may be set upright when pouring. This gives a decided additional pouring pressure with a minimum thickness of sand over the castings.
Great care is taken in the brass shop to save all the shot and spilled metal possible. To this end, when the molds are to be poured on end, they are leaned against a cast-iron spill trough such as shown in Fig. 175. There should be a 1-inch layer of sand over the bottom of this tray. The crucible is held over it when pouring the molds, thus making it possible to conveniently catch, any metal that is spilled.
For thin work the face of the molds are skin-dried to drive off the moisture before the metal enters the mold. Drying stoves, similar to that shown in Fig. 176, are used for this purpose. When the mold is finished, the two halves are carefully sprayed with a weak molasses water, and the flask is set on end on the wide platform with the face of the mold next to the stove. When sufficiently dry, the mold is closed and poured at once.
Brass work deals, as a rule, with smaller quantities in every way than does iron work. The patterns are generally smaller, and the brass molder takes particular pride in making all his joints so neat that hardly a fin shows on his castings. The matter of catching the shot metal has been mentioned. Up to the time of the introduction of the oil-melting furnace, it was customary to heat a pot of metal for each molder. These heats were comparatively small, so that the molder would make up possibly 6 or 8 molds, then draw his pot and pour them, running in this way several heats in a day. Using the furnace, several heats are run each day, but a much larger quantity of metal is melted at each heat, so that the work of several molders is poured with exactly the same metal.
A mold for brass should be rammed about the same as for iron. On name plates and thin work, after the initial facing of sifted sand has been properly tucked with the fingers, the flask is filled heaping full of sand. Then by the aid of a rope hanging down from the ceiling, the molder springs up on top of the flask and packs the mold with his feet, the weight of his body giving the right degree of firmness to the sand. Stove-plate molders often pack their flasks in the same way.
The main differences between making up molds for brass and those for iron are due to the three following causes: brass melts at a lower heat; it does not run as fluid as iron; it has about double the shrinkage of iron. For these reasons the sand may be somewhat less porous and still vent sufficiently, if risers are placed to allow for the escape of the air. On bench work the vent wire is not used. The runners for brass should be larger than for iron, and the gates, instead of being broad and shallow, should be more semicircular in section. Pouring molds on end gives the pressure necessary to force a more sluggish metal to take a sharp impression, and the heavy runners shown in the following examples serve to feed the casting as it shrinks. Forms of skimming gates, as explained in an earlier section, are used to good advantage when the work is of a very particular nature.
Cores for brass work are made up as previously described. To give a smoother surface on the small cores, about 1/3 molding sand is often mixed in with the beach sand of the stock mixture.