Casting Fillet Radius Design: Small Changes That Reduce Defects

The cheapest time to fix casting defects is before the molten metal is poured. Most drawings that arrive at a casting factory already carry defects inside them — and the fillet radius is where a lot of them hide. A sharp internal corner, or a radius too small for the wall thickness, is a classic source of casting defects in both investment casting and aluminum casting. It is a small detail. It costs nothing to change in the CAD model. But left alone, it comes back later as shrinkage porosity, hot tearing, or a casting that fails leak testing after machining. This article covers why fillet radius matters, how to size it, and what to put on the drawing so the part passes inspection the first time.

What a sharp internal corner does to solidification

When a casting cools, it solidifies from the outside in. The mold pulls heat away from the surface of the part. Now look at an internal corner — the concave angle where two walls meet. What actually happens in that corner? The mold material inside it is surrounded by metal on two sides. It absorbs more heat than anywhere else in the mold. It gets hotter. And it stays hot longer.

The result is a hot spot. The last liquid in that region solidifies slowly, and as it shrinks it pulls a void into place. That void is shrinkage porosity. In steel castings — including 304 and 316 stainless, and carbon and alloy steels — the same corner often develops hot tears as the casting contracts. The outer sections solidify and shrink first. The semisolid core cannot follow. So the sharp corner tears.

There is a second problem: stress. A sharp internal corner concentrates stress. When the part is later pressurized or cycled, cracks start exactly there. That matters for valve bodies and pump parts, which are pressure-containing by nature. A radius spreads the stress over a larger area. A knife edge concentrates it into one line.

Finally, in aluminum casting — whether low pressure or gravity — sharp corners affect mold filling. They pinch the flow of liquid metal and create turbulence. Turbulence traps gas. Trapped gas becomes porosity. A generous radius lets the metal flow smoothly into every cavity.

Sizing the fillet: rules that work

The starting point used in casting design is simple: the internal fillet radius should be at least as large as the adjacent wall thickness. A 5 mm wall gets a 5 mm inside radius. Where three or more sections meet — a boss joining a wall and a rib, for example — use the thickest section as the basis and go larger, usually one and a half times it.

External radii are less demanding. The convex outer corners do not create the same hot spot, since the mold there is surrounded by metal on one side only. A small radius or chamfer is usually enough to prevent edge tearing and handling damage. But do not mix the two up. On many drawings the only note is “break all sharp edges.” That tells the foundry nothing. You get whatever the operator feels like that day — inconsistent geometry and plenty of arguments at inspection.

Wall thickness transitions deserve the same attention. A step from a thick section straight into a thin section creates the same problem as a sharp corner: uneven cooling and a stress concentration. Blend the change with a radius or taper over a length of at least three times the thickness difference.

Material behavior varies, but the principle holds. Steels contract as they solidify and are prone to hot tearing at sharp corners. Aluminum alloy has high solidification shrinkage, so hot spots turn into porosity. In both cases, a generous radius is the cheapest insurance you can design into a part.

Where bad fillets show up in inspection

A fillet problem never stays hidden for long. It shows up in almost every inspection method we use in our quality lab.

X-ray inspection is the most direct. Our XTH320 X-ray unit is used for exactly this class of internal defects — shrinkage porosity and gas porosity. Put a part with a sharp internal corner in front of it, and the porosity cluster sits right at that corner. The same defect that X-ray catches in a valve body also shows up as a leak: the porosity network reaches the surface, and pressure testing fails.

Coordinate measuring machines show the geometry side of the problem. We run COORD3 and ZEISS SPECTRUM units for dimensional inspection. A tight radius is hard to cut and hard to measure. If the drawing says R2 and the actual corner measures R1.2, and nobody agreed on a tolerance, the CMM report becomes a dispute instead of an answer.

Metallurgical checks tell the rest of the story. Our inverted metallurgical microscope reveals micro-porosity and micro-shrinkage at the corner — the early stage of what becomes a leak or a fatigue failure. And impact testing, including at low temperature, is very sensitive to notches. A sharp corner is a notch. It fails early and it fails fast.

The point for a buyer is timing. All of these checks happen after tooling, casting, and machining. A defect found then means rework or scrap. That is the expensive way to learn that the radius was too small.

Fix it in the drawing, not in the die

Here is where a buyer actually controls the cost. Changing a fillet radius in CAD is free. Changing it after tooling is not.

In investment casting, the radius is cut into the wax injection die. Reworking a hardened tool pocket is a precision job, and in some cases the die must simply be remade. In low-pressure and gravity aluminum casting, the radius lives in the permanent mold — same problem. If the defect does not surface until first article, the whole sequence stops while the tool is reworked.

The cheaper path is the engineering review at the quoting stage. Our process starts with your drawings and requirements, then engineering review and quote, then casting, machining, and delivery. That review is the moment for design-for-manufacturing feedback: “this internal corner needs a larger radius,” “this step will hot tear,” “this edge will trap gas.” A serious supplier flags these things before you commit money to tooling.

That is also why First Article Inspection exists. The first casting out of the tool is measured against the drawing. If the radius on the drawing is wrong — too small, or missing a tolerance — the part is both correct and defective at the same time, and the FAI passes a bad design into production.

There is a machining side to this as well. Sharp internal corners force small tools, slow feeds, and sometimes EDM work. A slightly larger radius keeps the machining operation fast, and machining cost is part of every casting price. When the foundry does its own CNC machining — Waking runs 400+ CNC machines in-house — it can tell you exactly what a tight corner costs you.

Drawing notes that prevent arguments

Five things to put on your drawing so fillet geometry is clear before the quote, not after the rejection:

1. Dimension internal fillet radii explicitly. Put a real value on every internal corner, or define an uncontrolled-radii note that states a minimum and says which corners it covers.
2. Give radii a tolerance like any other feature. Radii are measured with CMMs when there is a dispute. Decide the tolerance before casting, not after.
3. Separate external from internal radii. They do different jobs. Grouping them into one “small radius” note is how defects and arguments both start.
4. Check the corner against machining and surface treatment. If the part is machined after casting — the normal flow in a one-stop shop like ours — the cutting tool has to reach that radius. If the part gets passivation, anodizing, plating, or coating, a sharp corner will have thin or missing coverage. A radius lets the treatment apply evenly.
5. Walk the thick-to-thin transitions. Go through the section changes along the part. Wherever a heavy wall meets a light wall, there should be a taper or a large radius, not a step.

None of this is exotic. It is standard casting design, but it is surprising how often the drawing that arrives for quoting still carries a sharp corner labeled “as drawn.” The foundry then has two choices: flag it and wait, or pour it and hope. A real casting factory flags it. Waking has been one of those factories since 2002, and the engineering review exists precisely to catch these small drawing details before they become casting defects.

A fillet radius is a small circle on a big drawing. In casting, small circles decide whether a part is sound or porous, whether it leaks or holds pressure. Spend five extra minutes on radii at the design stage and you save months of trouble at inspection. That is the cheapest defect prevention a buyer will ever get.