Die Casting

When a die casting has defects, there is ultimately only one reason: the molten metal completes filling and solidification in an extremely short time, and this process itself is full of contradictions.

Filling must be fast, or the metal cools before it fills the cavity. But fill too fast, and air gets trapped. Solidification must be uniform, or thick sections haven't hardened while thin sections have already shrunk. But uneven wall thickness is the norm in structural design. To vent gas, you need gaps; leave gaps, and the molten metal may flash out.

Every defect is the result of these contradictions "fighting it out" under specific process conditions.

Cold Shut: Two Streams of Metal That Can't "Shake Hands"

Molten metal usually flows through the cavity along multiple paths and converges at some location. If the metal has cooled too much by the time it reaches the convergence point, the surfaces of the two streams have already formed oxide films and solidified shells. When they meet, they cannot fuse, leaving a "seam" at the junction—this is a cold shut. It hides inside the casting like a crack and is one of the most feared defects in structural parts. Too low a pouring temperature or an unreasonable ingate design that makes the flow path too long will both cause the metal to "go cold" before it converges.

Gas Porosity: Air That Didn't Have Time to Escape

When molten metal rushes into the cavity at high speed, it entraps air. If the venting system can't expel this gas in time, the gas gets "frozen" in place by the solidifying metal, forming gas porosity. Thin-walled parts and deep cavity areas are especially prone to this, because the air there is inherently hard to vent. Matching pressure and speed is critical: too little pressure, and the metal can't be pushed forward; too much speed, and air entrapment worsens. Vacuum die casting is effective precisely because it removes the air from the cavity in advance, so there is no gas left to entrap when the metal enters.

Shrinkage Cavity: Thick Sections That "Starve"

When a casting has uneven wall thickness, the thick sections cool slowly and solidify last. As they solidify and shrink, they need liquid metal to compensate for the volume reduction. But if the surrounding metal has already solidified, no liquid can flow in, and a void is left inside the thick section—this is a shrinkage cavity. Shrinkage cavities have irregular shapes and rough walls, and concentrate at hot spots. The local squeeze process targets exactly this problem: while the thick section hasn't fully solidified, a squeeze pin "feeds" metal in to compensate for shrinkage.

Misrun: Metal That "Didn't Make It"

A misrun means certain areas of the cavity were never filled by molten metal at all. It occurs mostly at corners, deep recesses, thin walls, and cylindrical hole walls—locations with high flow resistance. Causes may include insufficient pouring temperature, inadequate pressure, too low a cavity temperature, or poor venting that lets gas block the metal. Misrun castings are usually scrapped outright, because the shape is already incomplete.

Cracks: Stress That "Can't Be Held Back"

During solidification and cooling, a die casting develops shrinkage stress. If mold temperature is uneven, ejection timing is wrong, or the casting geometry itself has stress concentrations, the stress will exceed the material's strength and cause cracking. Some cracks appear at ejection; others only surface during subsequent machining or in service. Uneven mold temperature causes different sections of the casting to shrink out of sync, generating thermal stress—a major cause of cracks.

In the end, every parameter in die casting—temperature, pressure, speed, timing—is about finding a balance point among these contradictions. Defects aren't caused by "carelessness"; they are the result of that balance being broken. To understand the causes of defects is, at heart, to understand the inherent tensions of the die casting process itself.