It is generally recommended that you should cast at 100 °F above the flow point of the metal to allow it to be thoroughly molten. Caution should always be taken to avoid overheating. As a rule, the mold should be 800 °F to 1000 °F below the melting point of the metal at the time of casting. The flask will cool approximately 100 °F per minute after removal from the oven. Have everything ready and check your equipment before you start to melt to be sure it will operate properly. Melt the metal thoroughly and cast immediately. Overheating is often the result of malfunctioning equipment.
Porosity – Causes
There are two types of porosity: gas porosity and shrinkage porosity. Gas porosity is usually caused either by overheating the metal or exposing the molten metal to air. It can also be caused by sulfur dioxide from recycled metal contaminated with investment. Shrinkage porosity is by far the most common porosity. It occurs when hot molten metal is not allowed to feed the casting as it cools and shrinks.
Sprue size – Make sure the sprue is at least as heavy as the heaviest section of the casting. Metal shrinks as it solidifies, if additional molten metal is not allowed to fill in, a void will result. If the sprue is too thin, it will solidify before the casting cutting off the supply of molten metal to the still molten inner sections of the casting. A common mistake is to have a large sprue that is tapered down as it attaches to the casting piece. This restricting acts like a nozzle and the molten is sprayed into the mold. The sprue should be uniform in thickness up to the point where it attaches to the piece. At that point it should be flared to allow a smooth flow of metal into the casting. Metal flow into the piece should be as straight as possible.
Always attach the sprue to the heaviest section of the casting. This allows the heavier section to be fed after the thinner section has solidified. Multiple sprues are sometimes necessary if the casting has more than one heavy section.
Placement of Casting on the Tree
Always place the heaviest pieces close to the button and the lightest pieces at the other end where there is more pressure to promote better filling. In addition, the casting should not be placed too close to one another on the tree. When pieces are too close, localized heating of the investment results in poor heat extraction, i.e. “hot spots.”
It is very important to protect the metal from oxidizing and absorbing unwanted gases. Protecting the metal from contacting the air can be achieved in several ways: with a gas flame, a protective inert gas such as Argon, Nitrogen, or by using flux.
Heating the Metal
The proper temperature is a critical factor in obtaining good castings. When the metal is too cold, it freezes in the mold before completely filling it. When the metal is too hot, shrinkage porosity occurs in the heavier sections or immediately adjacent to them. Overheating the metal causes base metals (primarily zinc) to be “burnt off” or evaporated. As the base metals are lost, the percentage of gold (karat) increases. Each time the alloy is reused some of the special deoxidizers are lost. To minimize losses, keep a close watch on the metal temperature and always use at least 50% to 75% new alloy. We strongly discourage adding pure zinc or alloy to compensate for losses.
The casting temperatures given in the table are suggested starting temperatures. These temperatures result in good casting for most people. You may have to increase or decrease these slightly. (Usually not more that 50 °F)
Depending upon the size of the pieces being cast and how much fine detail they contain, the temperature will have to be adjusted. Large plain pieces require a lower flask temperature of 800 °F to 950 °F while pieces with fine detail or filigree require higher flask temperatures of 900 °F to 1150 °F. Another rule of thumb is when flasks are used in a vacuum/vacuum-assist casting machine; they should be 75 °F to 100 °F hotter than when casting the same pieces in a centrifugal machine.