Some molten metal alloys generate more slag/dross than others and are more prone to contain small, round-shaped nonmetallic inclusions trapped in the casting. Unless a specific application is exceedingly critical, a few small, rounded inclusions will not affect casting structure significantly. In most commercial applications, nonmetallic inclusions are only a problem if they are encountered during machining or appear in a functional as-cast cosmetic surface.

The best defense against nonmetallic inclusions is to inhibit their formation through good melting, ladling, pouring and gating practices. Ceramic filters, which can be used with alloys that have good fluid life, have advanced the metalcaster’s ability to eliminate nonmetallics. Vacuum melting and pouring are applied in extremely dross-prone alloys, like titanium.

4. Pouring Temperature
Even though molds must withstand extremely high temperatures of liquid metals, interestingly, there are not many choices of materials with refractory characteristics. When pouring temperature approaches a mold material refractory limit, the heat transfer patterns of the casting geometry become important.

Sand and ceramic materials with refractory limits of 3,000-3,300F (1,650-1,820C) are the most common mold materials. Metal molds, such as those used in diecasting and permanent molding, also have temperature limitations. Except for special thin designs, all alloys that have pouring temperatures above 2,150F (1,180C) are beyond the refractory capability of metal molds.

It’s also important to recognize the difference between heat and temperature; temperature is the measure of heat concentration. Lower temperature alloys also can pose problems if heat is too concentrated in a small area—better geometry choices allow heat to disperse into the mold.