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.
To Remove a Mouthpiece
When it is necessary to remove a mouthpiece for cleaning out the throat of the crucible, mark a line on the face of the crucible to align with the first hole on the right end of the mouthpiece. When replacing the mouthpiece the line will be your guide. By so adjusting there will be a full hole on each end of the slug when casting the different lengths, with the exception of half-pica measures. A hole on the adjustable or left end being partly covered by the liner in the mold would cause no trouble. If part of the first hole should be covered by the constant liner, the first letter on the right end of the slug would be blurred, or would not cast sharply, because the metal cools quickly on the ends of the mold, and a full, free flow is necessary.
The mouthpiece should always be removed when the metal is hot. If removed when cold there would be danger of breaking the pot crucible.
When removing the wedge style mouthpiece, place the vise in second position, lift out the mold slide, place a block of wood between the right side of the pot jacket and the slideway, drive the mouthpiece toward the keyboard, using a piece of brass as a drift. The above operation is necessary to loosen the wedge. The instant the mouthpiece moves, the wedge will become loose and can be lifted out.