Copper alloy die castings (Brass Parts Die Castings) have the highest mechanical properties and corrosion resistance of all die casting parts. Because of high metal temperatures and shorter die casting mold life, die casting brass has not been readily accepted as a good die castings alloy. Of the diecast brass alloys, the yellow brass die casting alloys are the most commonly die cast in this country and overseas. Advantages of brass parts die castings, as with other die casting alloys, include good surface finish, precise dimensional control, and high production rates.
In the die casting of brass parts, induction brass melting usually is preferred. This form of melting will produce less melt loss through dross formation, faster melting, better control of brass composition and temperature, and less gas absorption by the metal. Speed of the brass melting also is important, to reduce vaporization of volatile alloying elements—particularly zinc.
Fluxes are not always required in melting diecasting brass alloys. However, since dirt is unavoidable present, some fluxes may be advisable to assure cleanliness of the molten brass metals. A typical flux consist of three pounds of anhydrous borax and one pound of silica per hundred pound of brass metal.
Whether melting for brass die casting is performed under oxidizing or reducing conditions (induction melting is oxidizing), it is preferable to keep gas absorption at a minimum, because the absorbed gas must be removed prior to pouring. When die casting brass melting is done in a neutral atmosphere, a protective cover (usually, charcoal) must be used to prevent further absorption of oxygen and hydrogen. Dissolved hydrogen is removed from the molten metal with and inert gas (usually, nitrogen).
Cold chamber die casting machines used for die casting other die casting alloys are suitable for brass alloys. Diecast Ram pressures range from three thousand to ten thousand psi. Speed of the die casting plunger may vary from ten to two hundred and fifty psi. Regulation of speed, pressure, gating, venting, and die temperatures are essentially the same… and influence the quality of the die castings the same as with aluminum die castings and zinc die castings.
Tungsten hot work tool steels (H-20, H-21 and H-22) heat treated to Rockwell C38 to C45 are more commonly used for die casting molds and cores used for die casting brass. Moving parts not exposed to molten brass metal can be made of H-13 or 6145 heat treated to Rockwell C40 to C50.
The surface finish on die casting mold cavities must be satin or matte for purposes of surface tension. For die casting thin-wall parts, a die casting mold life of 40,000 to 50,000 shots can be expected, in yellow brass die cast parts. For alloys requiring higher metal temperatures, die casting mold life can fall to 10,000 die casting parts.
In die casting brass, the die cast mold life usually is given higher consideration than surface finish of the casting. If temperatures are too low, scrap parts are produced. If the die temperature is too high, blistering will occur. Normal diecasting mold and core temperatures are between 600-1300F. Oil or water usually is circulated through the die to control die temperatures. Die lubricant is used sparingly in die casting brass, since lubricant has the tendency to burn into the brass part casting and result in defective die cast brass parts. Generally, oil and/or graphite is used on die casting plunger tips or a spot where release of the diecasting is a problem.
Normally, scaling of the dies used in diecasting brass is common and the die has to be removed from the die casting machine (perhaps every eight hours) and polished. Repeated cleaning will cause dimensional changes… and where dimensions are critical, machining of the brass die casting may become necessary.
The most important factors in the die cast production of brass die castings are shape, weight, quantity, and likely replacement orders, for providing and informed quotation. Die casting defects, mis-runs excessive porosity, and flow lines are among the pitfalls. A proper balance of brass metal temperatures, diecast die temperature, venting and gating must be achieved to solve or eliminate the defect problem.
Because of the poor diecast mold life experienced in die casting brass parts or other copper alloys—in addition to the cost of the alloys, themselves—the copper alloy castings cannot be as price competitive as aluminum, zinc, or magnesium die castings. The copper-alloy die casters, however, provide a process where quantities of superior diecastings are produced that are not otherwise obtainable by other processes.
Kinetic Die Casting Company