Metal or alloy ingot structure not only have good internal quality, but also has good surface quality of the physical specifications. The quality of the surface of the ingot is, to a large extent, related to the internal quality of the coating applied to the inner wall of the ingot and the ingot itself. The sublimation (burning) of the coating leaves a very thin layer on the inner wall of the mold and has a certain intensity of coke black. This layer of coke black not only helps to form an ingot with good appearance, but also isolates the mold wall from the metal. Conducive to the release of the ingot.
When choosing a coating, the following factors should generally be considered:
(1) The coating should contain a certain amount of "volatile" material. The coating on the mold wall will sublimate as the metal is cast, producing volatile gases and solid residues (black or char). Too little volatiles will produce a large amount of solid residue. If there are too many volatile gases, bubbles will form on the surface of the ingot, causing the ingot to form a "pitting".
(2) The sublimation temperature of the coating should be consistent with the metal casting temperature. Coatings with a high sublimation temperature should not be used to cast metals with low temperatures.
(3) The paint should have the property of covering the mold wall, that is, the paint can adhere to the vertical wall of the mold. If the ability of the coating to adhere to the mold wall is poor, it may cause surface defects such as slag inclusions (carbon particles) or surface pores.
(4) The sublimation speed of the coating should be the same as the filling speed (casting speed) of the metal in the ingot mold. Faster casting speeds should be used when using sublimation coatings.
(5) The selected coating should be inexpensive and easy to buy.
According to the practice of some plants, when gold or silver ingot casting is performed, the choice of acetylene or oil (heavy oil or diesel oil) uniform flame on an inner wall of the mold a thin layer of tobacco smoked, with favorable results.
Since the cast silver ingot usually uses the combined vertical die casting method, in addition to the selection of a suitable coating, the quality of the casting operation has a great relationship with the quality of the ingot. When casting silver metal, the rate of rise of the liquid surface in the mold should be consistent with the sublimation speed of the coating (Figure 2). That is, the sublimation of the coating should be carried out simultaneously with the rise of the molten metal in the mold. At this time, all the residue generated during the sublimation process floats on the liquid surface, and gradually enters the upper part of the cap as the liquid level rises. After cutting the ingot, an ingot of good quality is obtained. If the sublimation of the coating is slower than the rate of rise of the liquid surface (Fig. 2), the gas produced by sublimation will enter the liquid metal and form a bubble or shell-like surface on the surface of the ingot. When the sublimation of the coating is faster than the rising speed of the liquid surface (Fig. 3), the metal will meet the residue when it is cast into the mold, causing severe slag inclusion defects. The vertical die casting method also requires the silver liquid to be vertically cast into the center of the mold. Otherwise, the silver liquid enters along the edge of the hat opening and flows down the inner wall of the mold. At this time, the metal flowing through the mold wall passes before the silver liquid is filled. The paint has been washed away by the silver liquid, so that the surface of the ingot has scouring marks, slag inclusions and pores, and even silver particles or delamination occurs.
Figure l Coating sublimation is consistent with liquid surface rising speed
Figure 2 Coating sublimation is slower than liquid surface rising speed
Figure 3 Coating sublimation is faster than liquid surface rising speed
The casting of the gold ingot, because of the open flat die, the operation is relatively simple, as long as the mold is placed on a horizontal surface, the crucible is perpendicular to the long axis of the mold, and the gold liquid is uniformly cast into the mold core. In order to protect the inner wall of the mold, the position of the gold liquid is continuously changed during casting to avoid eroding the center of the mold into a pit.
The coating applied to the mold wall should be thin and uniform, and the thickness of the coating at the corner of the mold wall should be consistent with that of the flat wall.
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