Control inclusions with good gating design.

High quality castings require liquid metal free of metallic oxides and gases that form unwanted inclusions. Products of the melting process, inclusions are responsible for casting machining difficulties, surface pitting and reduced mechanical properties. Usually transferred as dross and slag from the furnace to the mold, most inclusions can be eliminated by properly-designed gating systems.

Dross is formed by a reaction of oxygen with liquid metal. Many alloys are sensitive to dross formation, such as aluminum that almost immediately forms a thin surface layer of dross behind a ladle skimmer. The film, actually solid aluminum oxide (|Al.sub.2~|O.sub.3~), is visible as a distinct color change from shiny silver to dull gray. Similar oxides form on other nonferrous alloys, such as magnesium, silicon bronze and aluminum bronze.

Slag is caused by a chemical reaction between metallic oxides and furnace, ladle or mold refractories. Typically, slags have a lower density than molten metal.

Alloy Sensitivity

In good gating design, inclusions can be floated out of molten metal before they enter the mold cavity. A gating system should prevent oxides from entering the mold cavity or the formation of new oxides within the system. Some metals are more sensitive to formation of oxides and understanding the sensitivity of an alloy is an important determinant in gating design. In general, there are three metal groups:

* low sensitivity--gray iron, malleable iron & red brass;

* medium sensitivity--ductile iron & steel;

* high sensitivity--aluminum & magnesium.

Gating Systems

Gating systems usually are classified as pressurized and nonpressurized, the difference being the location of the choke. A choke, given a fixed head pressure, controls the fill rate of the mold and contains the system's smallest total cross section. In a pressurized system, the choke is located at the ingates; in a nonpressurized system, it is at the bottom of the downsprue, as illustrated in Fig. 1.

The law of continuity states that the volume of liquid flowing through a full channel is the same at all points in the channel. Applied to a gating system, it follows that the quantity of metal flowing through a system remains constant as long as it is kept full. Changes in any gating component cross-sectional area will change only the speed or velocity of the metal flow. Velocity increases when cross-sectional area decreases; it decreases as the area increases.

This is a fundamental gating design rule; reducing velocity allows the system to float oxides and other inclusions out of the metal stream and attach to the molding sand. High velocity tends to keep the oxides entrained in the metal and could cause new oxides to form in sensitive alloys.

System Advantages

A nonpressurized gating system offers advantages for those alloys considered sensitive to dross and slag formation. The choke, located at the bottom of the downsprue, sets the flow rate, causing the metal to back up and quickly fill the sprue. This reduces the system's metal velocity and restricts oxides from traveling down the sprue. The runner cross section is increased to further reduce velocity and allow any inclusions that escaped the sprue filtration to float out of the stream and be trapped along the top of the runner. When the metal reaches the first ingate, all oxides and slags have been cleaned by the sprue and runner.

The cross-sectional area of ingates is typically 2-4 times greater than the sprue base. This enlargement makes the nonpressurized system very useful in reducing the reoxidation of sensitive alloys entering the mold.

The flow rate in a pressurized gating system is established at the ingates and works best for the less sensitive alloys. In this system, the sprue and runner have a greater total cross-sectional area compared to the sum of all its ingates. The ingates form the choke and act to restrict the metal flow. This allows the sprue and runner to quickly fill, reducing metal velocity and allowing dross and slag to float out of the metal stream. Unlike a nonpressurized system, the ingates, having the smallest total cross-sectional area, are the point of highest velocity and most turbulence.

This high flow velocity impacting a mold or core could precipitate erosion defects and cause a sensitive alloy to reoxidize upon entering the mold cavity. One of its benefits, seen in the finishing room, is the lower amount of labor required to remove the gating system and grind the smaller ingate contacts.