The gating ratio for aluminum and magnesium castings is the relative cross-sectional area of the sprue, runner, and ingates. It has a direct effect on how fast the metal moves and how calmly it enters the mold cavity. For oxidation-sensitive aluminum and magnesium alloys, a stable entry into the cavity is usually more valuable than forcing the highest possible gate velocity. The ratios in this article are sand-casting starting references, not a substitute for a specific part review, filling simulation, or production trial.
Sprue, runner, and ingate area
Foundry engineers often write the gating ratio in the order:
sprue : runner : ingates
The ratio refers to the effective cross-sectional area of each part of the gating system. It does not describe the physical length of a runner or the number of gates by itself. A system with two ingates, for example, uses the combined effective ingate area when the ratio is calculated.
The ratio matters because it changes the velocity pattern through the system. It also changes whether the runner fills early, whether slag has a place to float, and how hard the metal stream hits the mold wall when it reaches the cavity.
There is no one ratio that works for every casting. Part size, wall thickness, alloy, mold type, gate location, feeding method, and the filling direction all change the answer. The practical starting point is to choose a system that gives the casting a quiet, controlled fill, then adjust it using trial results or simulation.
Constricted gating systems
A constricted system reduces in area from the sprue to the runner and then to the ingates:
Asprue > Arunner > Aingates
As the metal moves downstream, the available flow area gets smaller and velocity rises. The ingate can therefore deliver a fast stream into the cavity. A constricted system usually stays full from the beginning of filling, which can help slag float toward the top of the runner rather than travel directly into the mold cavity.
The downside is the gate. A fast stream entering a thin section or striking the mold wall can splash, entrain air, and create oxide inclusions. That risk is especially important for aluminum and magnesium alloys because their oxide films form easily and do not tolerate rough filling well.
For that reason, constricted systems are used less often when the main requirement is a calm fill of an aluminum or magnesium cavity. They are not automatically wrong; a compact casting with a well-controlled entry point may still use one. The mold layout has to support it.
Expanding and semi-expanding systems
An expanding system increases in area from sprue to runner to ingates:
Asprue < Arunner < Aingates
Older foundry references may call this an East–Wood system. Its behavior is the opposite of a constricted system. Metal moves more slowly through the runner and gates, so the flow entering the cavity is generally steadier. This is why expanding systems are commonly considered for medium and large aluminum or magnesium castings where oxidation control and stable filling matter.
The tradeoff appears early in the pour. The runner may not be completely full at the start, so its ability to trap slag is weaker during that first stage. A filter can be used where the process needs extra help with inclusions. The filter location and the actual mold layout still need to be checked; simply adding a filter does not repair a poor gate position.
A third option sits between the two forms. In a semi-expanding system, the runner is larger than both the sprue and the ingates, while the total ingate area remains larger than the sprue:
Asprue < Arunner > Aingates, with Aingates > Asprue
This arrangement can give a more stable flow than a constricted system while keeping better filling and slag-trapping behavior than a fully expanding system. It is often a practical fit for smaller, simpler aluminum or magnesium castings. For metal molds, it can also be useful when a filter is difficult to place in the runner.
Sand-casting ratio ranges for aluminum and magnesium
The ranges below are translated from the supplied foundry reference. They apply to commonly used sand-mold gating systems for aluminum and magnesium alloys. Each entry is written as sprue : runner : total ingates.
| Alloy family | Large casting | Medium casting | Small casting |
|---|---|---|---|
| Magnesium alloy | 1 : 3–5 : 3–8 | 1 : 2–4 : 3–6 | 1 : 2–3 : 1.5–4 |
| Aluminum alloy | 1 : 2–5 : 2–6 | 1 : 2–4 : 2–4 | 1 : 2–3 : 1.5–4 |
These are broad starting ranges. For expanding systems, the total ingate area may be roughly two to fourteen times the sprue area in aluminum and magnesium sand casting. That broad spread is a reminder that casting geometry controls the final decision.
The table does not account for cases where an ingate must also feed shrinkage. If a gate supplies a thick section that remains hot late in solidification, it may need more area than a simple filling calculation suggests. In that case, the gate is doing two jobs: delivering metal during filling and helping feed the casting afterward.
Do not enlarge every section of the system by default. Oversized runners and ingates can increase metal consumption, make flow distribution uneven, and add work when gates and risers are cut off. A larger ratio is only useful when it solves a real filling or feeding problem.
A frame-casting example
The source pages describe a magnesium-alloy frame casting with a sloping parting plane and several ingates located on an inclined surface. During the first trial, oxide inclusions appeared in a thin-wall area near the first ingate.
The problem was not simply that the ingate was close to the defect. Because the ingates were not on the same horizontal level, only the first ingate carried metal at the beginning of filling. At that moment, the effective system behaved like a constricted design: the sprue area was larger than the active ingate area. The metal hit the cavity at high velocity, splashed against the mold wall, and carried oxide film into the thin section.
The change was straightforward. Increasing the cross-sectional area of the first ingate gave the system an expanding character even at the beginning of the fill. The flow became steadier and the oxide-inclusion defect was removed in the trial casting.
That example is worth remembering because it shows why total gate area is not the only number that matters. When gates open at different times, the first active gate may control the first few seconds of cavity filling. Looking only at the final combined ingate area can hide that problem.
The runner connection also needs attention. A sharp connection between a sprue and a runner can create turbulence and air aspiration. With a single runner, the runner area should generally not exceed about three times the sprue area. With runners feeding both sides, the combined runner area should generally not exceed about five times the sprue area. If a larger runner is necessary, flare the sprue exit and blend the connection smoothly rather than joining two abrupt sections.
Thick sections, feeding, and gate size
Many aluminum and magnesium castings have a thick flange, boss, or raised pad that is a natural place to introduce metal. These areas can also be the last to solidify, so they need enough liquid metal to compensate for shrinkage.
Magnesium is sensitive to the flow path. Near an ingate, the metal can overheat a local area and create shrinkage or looseness if the section is not fed properly. Enlarging the ingate area—often by increasing gate thickness—can increase the expanding character of the system and leave more metal available for feeding. In some cases, the ingate area may reach ten to fourteen times the sprue area.
That does not mean every gate should be thick. If an ingate has no feeding role, making it too large adds metal, makes removal harder, and can upset distribution among several gates. Gate size should follow the casting’s filling sequence and solidification plan.
For a cover-like thin-wall casting, the important thing is often a short flow path. Multiple ingates around the perimeter can fill the cavity faster and more evenly than one gate at one end. A one-sided layout may leave the metal too cool by the time it reaches the far end, causing a misrun or incomplete fill.
Before a trial pour
Before cutting a pattern or core box, review the intended filling sequence with the foundry. The discussion should include the first active gate, the last area to fill, the thick sections that need feeding, and how the runner will handle oxide film or inclusions.
For aluminum and magnesium castings, it also helps to identify whether the part is more vulnerable to a fast impingement at the gate or to a long, cold flow path at the far end. Those are different problems and they rarely share the same fix.
Bring the full casting geometry, wall map, alloy, mold type, annual quantity, and quality requirements to the review. If the part needs pressure tightness, leak testing, X-ray, or machining through a critical wall, state that early. The gating system has to support the quality requirement, not merely fill the shape.
For a related review of where metal should enter the cavity, see casting gate location and ingate placement. A ratio can be reasonable on paper and still perform badly if the first gate points at the wrong wall or opens into the wrong section.
