Mouthpieces
One of the most important parts of the pot crucible, is the mouthpiece. The mouthpiece on all standard machines contains thirty holes (size 51), one hole for each pica of the slug, through which the metal flows into the mold cell to form a slug.
At the present time there are two styles of mouthpieces in general use: The wedge mouthpiece which has a gib or wedge to fasten it in the slot of the crucible to make a tight fit; and the screw mouthpiece which fastens on to the crucible by means of screws.
On all mouthpieces small cross vents are cut downward between each of the holes. There is also a vent which is cut from the first hole on the right-hand end of the mouthpiece. The cross vents allow the air to escape from the mold cell as the metal is forced in. These vents play a very important part in the casting of good slugs. If the vents are stopped up with dross or cold metal, the air can not escape from the mold cell. This causes a slug with air bubbles, making a light-weight slug. These air bubbles, when near the face of the type, allow the letters to be crushed in when put under pressure on the press. Also, the entire slug may be forced down, causing much delay in printing.
Care must be taken in cutting the vents in a mouthpiece so they do not extend very far above the top of the holes in the mouthpiece and that they are not too deep. They should be deep in the center and come to a very fine edge at the ends. All that is needed is to get the air out of the mold quickly. Ordinarily this can be accomplished by cleaning the vents with a sharp pointed scraper but be careful not to mar the mouthpiece. When the vents are opened properly there should be a sprue of metal below the vents on the back of the mold about ½ to ¾ of an inch long after the slug is cast.
A vent that is cut too deep will have too much sprue, causing an unnecessary amount of shavings on the floor and about the machine, and sometimes causing machine troubles.
It should seldom be necessary to drill out the holes in a mouthpiece. If the metal is properly cared for in remelting, and the mouthpiece is kept at the proper temperature, the metal will usually flow freely through the holes. However, if it is found necessary to drill out the holes, never use larger than a 1/16 inch or No. 51 drill, which will not make the holes larger than their original size. When using the drill, it should be immersed in oil after each hole is drilled, to prevent the drill from becoming too hot and breaking off in the hole. Enlarging the mouthpiece holes will work satisfactorily on large faces 8-point or above; but when the smaller faces are used the product will not have a clear-cut face.