Die Casting

Die casting has a long-standing problem: when a casting has uneven wall thickness, the thick sections cool slowly. When the metal solidifies and shrinks, there isn't enough molten metal to feed those thick sections, leaving shrinkage cavities or porosity behind. These defects hide inside the casting, only detectable by X-ray inspection, and in sealed components they cause gas or oil leaks. Local squeeze is designed specifically to tackle this problem.

The principle isn't complicated: after the molten metal finishes filling the cavity, but before it fully solidifies, a squeeze pin delivers a targeted "second strike" at the thick-wall location. At this point the metal is in a semi-solid state—the surface has already formed a hard shell, while the interior is still mushy. The squeeze pin breaks through the surface layer and presses the not-yet-solidified metal toward the shrinkage cavity, forcing feed compensation.

Timing is critical. Start too early, and the metal is still fully liquid—the pin just stirs the liquid around without achieving effective feeding. Start too late, and the dendrite network has already formed, gaining strength, so the pin can't push through and the liquid can't flow. Research shows that initiating the squeeze about 3 seconds after filling completes works best, when the dendrite network is still relatively weak; under pressure, the dendrites break apart and the liquid between them is forced out to flow and feed the shrinkage. Japanese researchers quantified the timing as "0.4 to 1.4 seconds after filling" and recommended a pin advance speed of 1.2 to 2.63 mm/s, so the pin can keep pace with the moving solidification front.

The results are striking. In the die casting production of a heavy-duty transmission rear cover, the maximum wall thickness was 30 mm while the average was only 7 mm. This extreme disparity caused severe shrinkage cavities, and the leakage scrap rate once reached 30%. After adopting local squeeze, the air-tightness test scrap rate dropped to about 3%. Another study on ADC12 alloy showed that local squeeze significantly improved the fatigue strength of castings—the smaller and sparser the defects, the higher the fatigue strength.

But more pressure isn't always better. Beyond a certain point, the trend of shrinkage reduction clearly slows, while excessive pressure transmits to the mold, causing cracks and reducing die life. A CAE simulation indicated that the optimal local squeeze pressure for a certain casting was around 125 MPa; beyond that, returns diminish while mold risk rises.

Simply put, local squeeze means that while the metal hasn't yet fully "frozen hard," you deliver a precise extra squeeze at the thick-wall location—strangling shrinkage cavities before they can form.