Metal Pot Plunger
The metal is forced by the plunger from the well, through the throat of the crucible, into the mold cell, and up against the line of matrices aligned in front of the mold.
When the machine is in normal position, the plunger should be high enough in the well to permit the metal to run into the well through the holes in the sides. If the plunger does not set above the holes, the metal can not flow into the well and throat, so when the line is cast the slug will appear porous or spongy and of light weight.
If a plunger is worn, it will permit the metal to escape around the sides of the plunger when the cast is made, giving a poor slug. To remedy this trouble it is sometimes necessary to put in a new plunger a trifle oversize (.005), and fit it to the well.
A dirty plunger may bind and cause a splash of metal to be ejected before the pot locks up tight and, chilling on the back of the mold, prevent a lockup, so the metal will flow out over the back of the disk when the plunger acts. A dirty plunger will often cause a slug to have the appearance of cold metal. Keep the plunger and the well of the pot clean.
Any kind of a wire brush may be used to clean a plunger, but the Ewald cleaning box is recommended, because it keeps all the dust confined in the box. There are several kinds of brushes and scrapers manufactured for use in cleaning the well. Use whatever method desired, but be sure it keeps the well clean.
If using a rotary well brush to clean the walls of the well, be sure to turn it always to the right, otherwise the bristles may break off and stay in the well.
If a plunger should stick in the well very tightly, raise the temperature a little, apply a wrench to the plunger rod and twist it carefully. Do not use too much force or the rod may be broken. If a plunger sticks so that it can not be removed by the above method, dip enough metal out of the crucible to expose the well; squirt some oil between the plunger and the inside wall of the well, or drop a piece of tallow in the well; let it stand for a few minutes, after which the plunger can usually be loosened with the wrench as described. Do not pry up on the plunger for there is danger of breaking the rod.
When the plunger forces the metal through the throat of the crucible, there will be nothing to retard the flow of metal against the face of the matrices if the throat of the crucible is clean. However, if the throat should be stopped up with dross or dirt, retarding the flow of metal, the face of the slug would be glassy, and have the appearance of cold metal.