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Porosity & Shrinkage Defect Root Cause Analysis & Mold-side Countermeasures

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  • Release time: 2026-09-29

Porosity & Shrinkage Defect Root Cause Analysis & Mold-side Countermeasures

Porosity and shrinkage are the most common internal defects of aluminum castings, accounting for 58% of total rejection in automotive aluminum casting. Many defects originate from mold design rather than melting process. This article classifies pore types and introduces mold optimization solutions from Xinfeng mould case library.

Gas porosity: caused by trapped air, mold release agent decomposition gas or hydrogen. Mold-side causes: insufficient vent area, blocked vents, residual moisture on cavity surface. Countermeasure: increase vent slot area, shorten vent cleaning cycle, fully dry mold after spraying release agent.

Shrinkage porosity: formed during solidification volume contraction at hot spots. Mold-side causes: improper riser position, insufficient riser volume, unbalanced cooling. Countermeasure: move riser to hot spot, add exothermic sleeve, add local cooling channels to realize sequential solidification.

Oxide inclusion defects: turbulent filling or air entrainment. Mold-side causes: unreasonable gating design, high filling speed. Countermeasure: adopt bottom laminar gating, reduce melt flow velocity, add slag trap at runner.

Blowholes from release agent: thick coating or incomplete drying. Mold-side control: limit release agent thickness 0.2–0.3 mm, ensure full drying before casting.

Hot spot detection: solidification simulation at mold design phase, verify hot spot location and feeding distance. Maximum feeding distance for A356 under LPDC condition is 32 mm.

For CPC mold, argon leakage will introduce air and oxide pores. Check sealing flatness and gasket integrity every batch.

Local thick boss without feeding channel is high risk for shrinkage. Mold design to add feeding channel or reduce boss wall thickness by structural optimization.

X-ray defect mapping can record defect location, then modify mold cooling and riser layout in targeted way, reducing shrinkage rejection by up to 40%.

FAQ

Q1: What is the difference between gas porosity and shrinkage porosity? A1: Gas porosity comes from trapped gas; shrinkage porosity forms due to volume contraction at hot spots. Q2: What is the maximum feeding distance for A356 alloy LPDC casting? A2: Maximum effective feeding distance of A356 under LPDC process is 32 mm. Q3: How to reduce oxide inclusion from mold design perspective? A3: Adopt bottom laminar filling gating, lower flow speed and add runner slag trap.

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