Porosity is one of the most frequent and costly defects in high pressure die casting. It can lead to leakage, weak mechanical performance, poor machining results, and coating failures. Understanding the root causes of die casting porosity—and fixing them systematically—is the key to stable mass production.
In this article, we’ll break down what air pores are, why they form, and proven methods to reduce or eliminate them.
What Is Die Casting Porosity?
Die casting porosity (air pores / gas pores) appears as round or flattened bubble-like voids inside the casting. Typical features include:
- Shape: circular or oval cavities
- Size: usually about 1–20 mm in diameter
- Interior: smooth surfaces often covered with an oxide film
- Location: commonly scattered beneath machining surfaces
Porosity may be visible after machining, X-ray inspection, pressure testing, or even after painting/plating due to blistering.
Why Porosity Happens: Main Root Causes
Porosity is rarely caused by one single factor. Your article summarizes several high-frequency causes:
1. Excess Impurities in Raw Material
Oxides, hydrides, oil residues, and other inclusions in virgin or recycled alloys can release gas during melting, leaving pores in the final part.
2. Gas Dissolution in the Melt (Hydrogen in Aluminum)
During aluminum melting, moisture in air can react with molten aluminum and generate hydrogen. If degassing/refining is insufficient, hydrogen remains in solution and forms pores during solidification.
3. Unstable or Incorrect Process Parameters
Poor parameter settings increase turbulence and air entrapment, such as:
- Metal ladling too fast
- Plunger speed too high during early filling
- Flow becoming chaotic and trapping air
4. Lubricant / Coating Volatilization
Release agents or coatings that are sprayed unevenly, not dried properly, or contain unsuitable ingredients can vaporize at high temperature, generating gas that becomes trapped in the cavity.
5. Wrong High-Speed Switching Point
If the slow-to-fast shot changeover is not set correctly:
- Too early: melt jets in, gas can’t escape in time
- Too late: melt loses temperature, flow weakens, exhaust becomes poor
Both increase porosity risk.
6. Improper Pouring / Melt Temperature
- Too high: melt dissolves more gas and becomes harder to degas
- Too low: poor fluidity traps gas during filling
A stable temperature window is crucial.
7. Poor Venting or Exhaust Blockage
If vent design is weak or vents are blocked early by melt, cavity gas cannot escape, leaving pores.
8. Shrinkage-Related Gas Porosity
Although shrinkage pores and gas pores form differently, they can interact. In thick zones, shrinkage cavities may trap gas or even draw in surrounding gas during cooling, creating combined defects.
How to Prevent and Fix Die Casting Porosity
1. Control Material Quality and Melt Cleanliness
- Use dry, clean alloy ingots
- Avoid moisture and contamination during melting
- Apply effective refining/degassing (fluxing or inert gas such as nitrogen) to reduce dissolved hydrogen
2. Select Low-Gas Coatings and Release Agents
Choose products with low volatilization, apply thin and evenly, and avoid wet residue on the die surface. Shorter spray time and better blow-drying reduce gas generation.
3. Design a Rational Gating System
Reduce air entrapment by:
- Using convergent runner cross-section principles
- Avoiding sharp turns that cause splashing
- Keeping flow smooth and stable
4. Optimize Overflow and Venting Layout
- Ensure last-filled zones have strong, open exhaust paths
- Add overflows near thick or difficult-to-fill areas
- Prevent vents from being sealed too early
5. Adjust Shot Speed to Avoid Vortex Entrapment
Keep filling in an orderly, stable flow state. Over-fast filling creates turbulence and “air rolling,” increasing porosity.
6. Control Pouring Temperature
Use the lowest melt temperature that still fills safely, reducing gas solubility and shrinkage-gas interaction.
7. Improve Mold Design for Chronic Porosity Zones
If pores repeat in fixed locations, tooling fixes are required:
- Add vent grooves between inserts
- Strengthen local exhaust capacity
- Introduce local squeezing/compaction where needed
8. Reduce Shrinkage Risk
Design with uniform wall thickness, add cores or cooling where needed, and avoid over-cooling thick sections that intensify shrinkage-gas coupling.
Real Production Case: Crankcase Oil Pan Porosity
A crankcase oil pan showed many pores after machining. Each pore was about 0.8–1.5 mm, with roughly 5–15 pores per part.
Investigation revealed two main causes:
- Slow-shot speed was set to 0.3 m/s, leaving gas in the shot sleeve insufficiently evacuated.
- Spray time was 3 s but blow-drying only 1 s, leaving moisture on the die surface and generating vapor during filling.
Fix implemented:
- Reduce slow-shot speed from 0.3 → 0.2 m/s
- Shorten spray time to 1 s
- Extend blow-dry time to 2 s for full die drying
Result: porosity was significantly improved without harming filling stability.
Quick Checklist to Reduce Porosity
- Keep alloy clean and properly degassed
- Avoid moisture in raw material, die, and release agent
- Ensure smooth, stable filling (no turbulence)
- Set the slow/fast shot switching point correctly
- Strengthen venting in last-fill and thick zones
- Control melt + die temperature in stable windows
- Fix recurring porosity with tooling upgrades
Work with a Partner Who Solves Porosity Systematically
Porosity control is a combined result of material cleanliness, gating/venting design, die temperature balance, and shot curve tuning. If you’re facing repeated air pores, we can help review your design and process.
At Cast Mold, we provide high pressure die casting and mold manufacturing services, including DFM/Moldflow validation, venting optimization, and stable HPDC parameter setup—so defects are fixed before they reach mass production.
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