Preventing Porosity and Shrinkage in Zinc Alloy Die Castings: Key Controls

In B2B zinc alloy die casting, "Gas Porosity" and "Shrinkage Porosity" are the two most critical defects faced by engineers. Gas porosity spoils surface finishes (causing electroplating blisters), while shrinkage voids weaken mechanical integrity, raising fracture risks.

Leveraging decades of mold tooling and casting floor experience, Dinad Industry shares how scientific runner design and process controls solve these issues at the root.

1. Porosity vs. Shrinkage: Causes & Diagnostics

To solve these quality issues, we must first correctly identify the defect type:

• Gas Porosity: Typically appears as smooth, circular, or elliptical cavities with shiny inner walls. This happens when high-speed metal flow traps air in the cavity or vaporizes die release agents, failing to vent in time.
• Shrinkage Porosity: Shows irregular, spongy, or crystalline cavities with rough, dark inner walls. This happens because zinc shrinks by ~1.1% upon solidifying. Thick areas that lack cooling and metal feeding develop voids as the core cools last.

2. Preventing Gas Porosity: Venting and Runner Layout

The core logic of reducing gas porosity is: "Guide molten metal in an orderly, wave-like flow to push air efficiently into overflows and vents."

• Optimized Runner and Gate Design: Gate placement dictates metal trajectory. Molten metal should enter as a continuous "liquid wall" to sweep air out, rather than dividing into chaotic streams that seal vents prematurely.
• Venting Grooves & Overflows: Overflow wells must be positioned at the last places to fill and where cold flow gathers. Venting slots should be machined at 0.05mm - 0.10mm deep for zinc. This allows air to escape while freezing the zinc, preventing excessive flash.

3. Eliminating Shrinkage: Cooling and Coring

Eliminating shrinkage requires achieving "directional solidification" or "uniform cooling":

• Apply "Coring" (Wall Uniformity): Redesign thick features to include hollowed-out centers (cores). Keeping walls uniform prevents localized hot spots.
• Targeted Mold Cooling Lines: Run dedicated water lines near thermal centers to speed cooling in thick sections, minimizing temperature gradients.
• Fast Intensification Phase: Adjust the die-casting machine's fast-injection profile to trigger high pressure immediately after filling (typically under 50ms). This squeezes liquid metal through the gate to feed the shrinking core.

4. Mold Flow Simulation (Computational Fluid Dynamics)

Historically, gating design relied on trial and error. At Dinad, we believe that "thorough upstream engineering planning prevents endless downstream mold modifications." Before cutting steel, we run CFD simulations: