The gating system in sand casting is the set of channels molded into the sand that carries molten metal from the ladle into the mold cavity: the pouring basin, sprue, sprue well, runner and ingates, often with a ceramic filter. Its job is to fill the cavity completely and in time without carrying air, slag or loose sand into the casting. British foundries call it the running system.
The foundry designs the gating and mounts it permanently on the pattern. The buyer supplies the drawing’s critical surfaces and the allowable defect levels, which tell the foundry where gates may sit and how clean the metal has to arrive.
What a Gating System Has to Do
A gating system has to get clean, calm metal into every part of the cavity before it freezes, while leaving as little metal as possible in the channels. That breaks down into five jobs:
- Fill in time: metal that arrives too slowly freezes before the cavity is full; metal that arrives too fast erodes the sand.
- Keep the flow calm: turbulent metal folds air and its own oxidized surface into the melt, and those films end up as inclusions and porosity.
- Hold back slag and dross: slag floats on iron and steel, so the basin, runner and filter are laid out to catch it before the ingates.
- Feed the risers hot metal: metal is usually routed so the riser holds the hottest metal and the casting solidifies toward it.
- Waste little metal: everything left in the channels is cut off and remelted, so the system is kept as small as the other jobs allow.
Components of a Gating System
In the order the metal meets them, a gating system is made of the pouring basin, sprue, sprue well, runner with its extension, and ingates, often with a filter in the runner. The riser is usually counted separately as a feeding reservoir that makes up for shrinkage, though some textbooks list it as a gating element because metal often flows from the runner through the riser into the casting.
Pouring Basin
The pouring basin receives the stream from the ladle and steadies it, so the sprue fills without drawing in air.
Sprue
The sprue is the vertical channel that takes metal from the basin down to the parting line. It narrows toward the bottom because a falling stream narrows as it speeds up; a sprue shaped like the stream stays full and does not draw air in along its walls.
Sprue Well
The sprue well is an enlarged pocket at the bottom of the sprue that takes the impact of the falling metal and turns it into the runner.
Runner and Runner Extension
The runner carries metal horizontally from the sprue well and distributes it among the ingates. A runner of enough length gives floating slag time to rise and stay in the channel before it reaches the cavity.
The runner extension continues past the last ingate as a dead end. The first metal down the runner, the coldest and dirtiest, runs into it and stays there.
Ingates
Ingates are the short channels where the runner joins the cavity, and they set the direction and speed at which metal enters the casting. Several ingates let a large casting fill from more than one side at high volume and low velocity.
Ingates commonly enter the heaviest section, or the metal is routed through the riser first. After shakeout the ingates are cut off and the stubs ground flush, so their location matters on a machined or sealing surface.
Filters
A ceramic filter usually sits in the runner near the sprue well. It catches slag and non-metallic inclusions picked up in the ladle and basin, and it slows and smooths the flow downstream of it.
The filter’s position has to be designed together with the sprue well: a filter placed where the metal arrives as a jet can throw fresh jets downstream and form new oxides.
Types of Gating Systems
Gating systems are classified by where the choke sits, which makes a system pressurized or unpressurized, and by where the metal enters the cavity: top, bottom, parting line or in steps. The choke is the smallest cross-section in the system.
Pressurized and Unpressurized Systems
In a pressurized system the choke is at the ingates. The total cross-section shrinks from sprue to runner to ingates, so back-pressure keeps every channel full of metal.
In an unpressurized system the choke is at or near the sprue base and the area grows after it. The metal slows down, but the runner and ingates may run only partly full. The gating ratio, the cross-sectional area of the sprue to the runner to the total of the ingates, expresses the difference; foundry course material gives the orders of magnitude shown in the chart.
A pressurized system runs full, so it draws in little air, and its smaller channels leave more of the poured metal in the casting. Metal leaves its ingates at high velocity, though, with turbulence at corners and erosion of the mold. An unpressurized system delivers slower metal and erodes the mold less, but its partly empty channels draw in air and its larger runners lower yield.
Practitioners often prefer unpressurized systems for smoother flow. In pouring trials on a cast iron exhaust manifold, however, a pressurized system with larger runners that held gate velocity to 1.0 m/s cut the defect rate tenfold.
Top, Bottom, Parting-Line and Step Gating
- Top gating: metal drops into the cavity from above. The hottest metal ends up at the top, which favors solidification toward the risers, but the falling stream erodes the sand and is highly turbulent.
- Bottom gating: ingates in the drag fill the cavity from below, and the metal rises steadily up the walls with little turbulence or erosion and a good surface. The cost is that the coolest metal ends up at the top, working against solidification toward the risers, and an ingate that freezes early can choke off the flow before the mold is full.
- Parting-line gating: ingates are cut along the parting line between cope and drag, the easiest place to mold them. The cavity below the parting line fills as if top-gated and the part above as if bottom-gated, so it falls between the two.
- Step gating: a vertical runner feeds ingates at several heights on a tall casting, so each higher ingate takes over as the metal level reaches it. It keeps the fall into the cavity short and brings hot metal up through a casting too tall for bottom gating alone.
How a Gating System Is Sized
A foundry sizes a gating system from the pouring time outward, because the choke calculation needs the pouring time and every other channel is scaled from the choke:
- Pouring time: set from the casting’s weight and, for gray iron, its section thickness.
- Choke area: the smallest cross-section that delivers that weight in that time.
- Runner and ingates: scaled from the choke through the gating ratio.
- Ingate velocity: checked against the limit for the alloy, with ingates enlarged or added if it is too high.
Before the pattern is made, the sized layout is checked by mold-filling and solidification simulation.
Pouring Time
Pouring time is set from empirical formulas built on the casting’s weight. For steel the formula uses weight alone:
t = K × √W
For gray iron the formula adds the average section thickness T in inches, and the root changes at 1,000 lb:
t = K × (0.95 + T / 0.853) × √W, for castings under 1,000 lb
t = K × (0.95 + T / 0.853) × ∛W, for castings over 1,000 lb
In each, t is the pouring time in seconds, W the casting weight in pounds and K a fluidity factor. K is the foundry’s own input for its metal and pouring temperature.
Choke Area
The choke area is calculated as the cross-section that passes the casting’s weight of metal in the chosen pouring time, under the head of metal driving the flow:
A = W / (c × d × t × √(2gH))
Here A is the choke area, W the casting weight, d the density of the liquid metal, t the pouring time, g gravitational acceleration and H the effective head of metal. The nozzle coefficient c accounts for friction losses in the channels.
The effective head depends on where the metal enters: with bottom gating it falls as the cavity fills. Like K, the coefficient and the head are inputs the foundry sets for its own layout.
Metal Velocity at the Ingate
Iron should enter the cavity at no more than about 1.0 m/s (40 in./s), the typical maximum used in gray iron gating design.
A lower critical velocity of about 0.5 m/s, above which the metal surface breaks up and folds oxide films into the melt, was measured on aluminum. One simulation paper points out that it equals a free fall of only about 12.5 mm, so a gravity-poured sand mold almost never stays under it.
For steel the guidance is qualitative: fill at high volume and low velocity. The foundry controls velocity through the total ingate area: more or larger ingates deliver the same flow more slowly.
Gating Gray Iron, Ductile Iron and Steel
Gray iron, ductile iron and steel use the same channels but are gated differently because each forms a different defect when its stream meets air, and so needs its flow kept calm to a different degree.
Gray Iron
Gray iron is gated to keep turbulence down, because in gray iron a rough fill affects the graphite as well as leaving inclusions. A published study of ASTM A48 test bars poured both ways found more and finer Type A graphite flakes in the bottom-filled bars. Its filling simulation also found far more air trapped in the top-filled mold. Bottom filling cost some yield: 43 % against 50 % for the top-filled mold.
Top gating is still used on gray iron where the casting tolerates the turbulence.
Ductile Iron
Ductile iron is gated above all to limit dross, which is mainly a problem in large castings such as heavy ductile housings and valve bodies. The gating keeps air out of the stream, because every bit of air folded in during filling adds to the dross, and a filter after the sprue well catches part of the dross the ladle carries.
Carbon and Stainless Steel
Carbon steel is gated to fill fast through a full system with bottom ingates, because steel freezes quickly and its main inclusion defect, reoxidation, comes from air entrained during the pour. Simulations of a 344 lb low-alloy steel casting found these reduce entrained air:
- Steady entry: a stream centered over the pouring cup, or an offset pouring basin.
- Low head height: less height for the metal to fall into the basin.
- High flow rate: a shorter fill entrains less air.
- Pressurized gating: ingate area kept small enough that the system stays full for most of the fill.
- Bottom ingates: a top ingate entrained more than five times the air of a bottom one.
Bottom gating needs a long sprue and special molding such as ceramic tile runners, so steel castings are often gated at the parting line instead.
In steel, most inclusions come from the ladle and pouring cup rather than from the channels, so a good gating system also works as a trap. In 36 experimental steel castings, an offset basin, tapered sprue, streamlined runner and tapered bottom ingate gave 2.5 times less inclusion area than a cup, oversized sprue and runner and a high side ingate.
Stainless steel is gated on the same principles.
Defects Caused by Poor Gating
The casting defects that trace back to the gating system are the ones formed while the metal flows into the mold:
- Oxide films and reoxidation inclusions: high ingate velocity breaks the metal surface and folds its oxide into the melt.
- Slag and dross inclusions: slag the basin, runner and filter fail to hold back, and in ductile iron, dross formed by entrained air.
- Gas porosity from entrapped air: air drawn in by a partly empty sprue or runner, or by metal falling inside the cavity.
- Sand inclusions and erosion: a fast or falling stream erodes the mold walls and carries the loose sand into the casting.
- Misruns and cold shuts: a fill that is too slow, or flow paths too long, so the metal freezes before the cavity is full.
- Shrinkage: slow, uneven or long-path flow delivers cool metal to the risers and upsets feeding.