Good sprue design eliminates gas, oxide entrainment.

The basic objective of good gating design centers on hydraulics, the study of liquids in motion. All gating system components must support this objective as closely as mechanics and costs allow. Ignoring the basics would result in severe metal flow difficulties involving air and gas entrainment at abrupt changes in stream velocity and direction, the formation of vortexing and excessive turbulence due to splashing of the metal stream.

Designing for Consistency

Keeping the system full at all times is a prime gating design consideration because of the two distinctly different types of metal flow within it. One, transient flow, is turbulent due to the metal stream's tendency to splash and tumble over itself, delivering a constantly changing volume. The fill time with transient flow also is incalculable and thus undesirable.

The second type is a steady state flow, characterized by a smooth laminar flow and constant volume. Because the flow rate is constant, the sizes of sprues, runners and ingates can be calculated to deliver a known quantity of liquid. Reducing stream turbulence also eliminates the entrainment of undesirable oxides and gases. A full gating system is key to obtaining and sustaining this flow.

Pouring Basin Design

The velocity energy generated by a falling stream is important in designing a gating system that will deliver a desired quantity of metal to a mold cavity. The further a liquid metal falls, the faster its stream velocity. Many times this height is determined by the pouring distance from the lip of the ladle to the pouring cup. When the ladle is full of liquid metal, this height tends to be greater than when the same ladle approaches empty.

Therefore, the velocity of the metal entering the gating system is constantly changing from mold to mold, resulting in constantly varying fill times for the same casting. This could result in defective castings because of too rapid and too slow fill times.

The design of a proper pouring basin eliminates this variable. It takes the art out of pouring by creating a constant height regardless of who is manning the ladle and results in actual pour times that resemble calculated pour times.

The use of an offset rectangular pouring basin, is recommended to slow the initial metal velocity and develop a constant height from the basin to the parting line. Thus, regardless of the amount of liquid remaining in the ladle, the velocity of the metal stream at the parting line is constant.

The best shape for a pouring basin is rectangular, this helps to eliminate the formation of a vortexing action down the sprue, which results in the entrainment of oxides and gases. It is important that a pouring basin have sufficient size to allow the pour stream to keep both the basin and the gating system full. A large basin also aids the pourers to hit the sprue without wasting excessive amounts of metal to the cope surface of the mold.

Downsprue Design

The downsprue is the vertical passage from the pouring basin to the runner system and its design incorporates the physics of a free-falling fluid stream. The farther the fall, the greater the velocity. The law of continuity states that the faster a liquid travels, the smaller its cross-sectional area. The natural shape of a free-falling stream always forms a tapered shape. The best shape for a sprue is tapered with the small end down. A rectangular taper is better for reducing the formation vortex down the sprue.

When comparing a straight sprue to a tapered sprue, the metal stream in the straight sprue will always tend toward a natural taper, creating air pockets between the metal stream and the sprue wall. Because of the molding sand permeability, the metal stream can develop an area of low pressure between the metal stream and mold wall that invites the creation of undesirable oxides and the entrainment of gases.

Many molding operations and machines are unable to use a tapered sprue and often require the use of reverse taper sprues. This is the least desirable sprue design, but the use of a choke core at the bottom of the reverse tapered downsprue or the reduction of runner cross section next to the sprue base can partially overcome this disadvantage.

For those wanting to learn more about gate and riser design, CMI will offer three courses (May, July and September '92). For more information call CMI at 800/537-4237.