Blister defects in aluminum alloy die casting refer to the formation of bubbles on or inside the casting surface, usually after post-processing, spraying, or heat treatment. This type of defect can reduce structural strength, damage surface finish, and increase scrap rate. To effectively prevent blisters, it is necessary to start from mold design, process control, and material handling.
This article will explore the root causes, effects, detection methods, prevention measures, and CEX Casting‘s preventive measures for Blister defects. Read on to learn how to prevent blisters and achieve defect-free aluminium die casting components.
Root Causes of Blister Defects
Gas Entrainment
Air is trapped in the mold cavity during metal injection, which is the main cause of the Blister. It is often caused by poor gate design or venting.
These gases expand in subsequent heat treatment or coating processes to form visible or hidden bulges.
Improper Use of Release Agents
Excessive or unevenly sprayed release agents tend to vaporize and mix into the metal at high temperatures, expanding to form bubbles during solidification or processing stages.
Moisture Contamination
The residual moisture in the mold, tool, or raw material is quickly converted into steam when the aluminum liquid is injected and trapped inside the casting. It expands during heat treatment and causes blisters.
Uneven Mold Temperature
Uneven mold heating can easily produce local cold spots, causing the aluminum liquid to solidify prematurely and trap gas.
This thermal imbalance directly induces bubbles under subsequent stress.
Alloy Quality Issues
Aluminum easily absorbs hydrogen during smelting, and high hydrogen content is one of the main causes of blistering.
Using inferior recycled materials or raw materials with too many impurities may also decompose and produce gas during solidification.
Improper Cooling or Solidification Control
If the cooling is too fast or uneven, the gas is trapped inside the casting before it escapes and forms bubbles.
Poor thermal management can also cause shrinkage and pores, especially in thick-walled areas.
Blisters Defects
The Impact of Blister Defects
Appearance Defects
Surface blisters will ruin the appearance of the product and make it unusable for end-user parts such as automotive trim or electronic housings.
These die casting products require extremely high visual quality, and once Blister occurs, they need to be reworked or scrapped, which is very costly.
Decrease in Structural Strength
Blisters are essentially cavities inside the material, which will reduce the bearing capacity and fatigue life.
In structural parts or load-bearing parts, blisters will cause cracking or even early failure, which is unacceptable for high-reliability fields such as automobiles and aerospace.
Poor Coating
Blisters will destroy the adhesion of subsequent spraying and electroplating, causing cracking, peeling, or blistering of the coating.
Gas expansion will also reduce the corrosion resistance of the coating, thereby limiting the application of castings in harsh environments.
Detection Methods for Blister Defects
Visual Inspection
Some blisters are visible on the surface of the casting after processing, showing as bulges or deformations.
Although visual inspection is fast and economical, it cannot detect potential internal blisters and needs to be used in combination with other methods.
Nondestructive Testing (NDT)
X-ray, ultrasonic, and airtightness tests can detect internal blister defects without damaging the product, which is suitable for high-pressure die casting products with high precision or high safety requirements.
Metallographic Analysis
Through slice observation and microscopic analysis, the formation process and type of blister defects can be confirmed, and at the same time, they can be distinguished from defects such as shrinkage and inclusions, which helps to verify the effect of process improvement.
How to Prevent Blister Defects
Pouring and Mold Design Optimization
Use mold flow analysis to find out where gas is easily trapped. Optimize the layout of runners, exhaust grooves, and overflow grooves to allow air to be discharged smoothly.
An auxiliary exhaust or vacuum structure can be added to the die casting mold.
Process Parameter Control
Control the injection speed and pressure to prevent air from being trapped too quickly or sealing too slowly. The mold temperature must be stable to avoid local overcooking.
Check the equipment parameters regularly to ensure that they match the die casting process.
Mold Preheating and Maintenance
The mold needs to be preheated to a uniform temperature, such as above 180°C.
Clean the mold surface and exhaust channel before each shift to remove carbon deposits and moisture.
Mold maintenance should be systematized, and key parts should be maintained regularly.
Mold Release Agent Control
Use a low-residue, fast-drying mold release agent. Use robot spraying to control the spray volume and apply a thin, uniform coating to prevent evaporation and blistering.
The spray gun parameters should be fixed and recorded, and continuously checked during production.
Raw Material Management and Degassing
The aluminum ingots need to be thoroughly dried before entering the furnace to avoid moisture absorption.
Nitrogen or argon is used for degassing during smelting to reduce the hydrogen content. It is forbidden to use recycled materials of unknown origin or uncleaned.
Vacuum Die Casting Application
Before the aluminum liquid is injected, the vacuum is evacuated, and the pressure drops below 0.9 bar.
Gas entrapment can be almost completely avoided, which is particularly suitable for high-density products.
It is recommended for high-end scenarios such as automotive housings and airtight parts.
CEX Casting’s Measures to Prevent Blisters
Optimize Flow Design
Every project starts with mold flow simulation to ensure that the metal flows smoothly without eddy currents, effectively exhausts, and prevents the source of blisters in aluminium alloy die casting.
Mold Heating and Drying Control
We strictly control the mold temperature and preheat evenly. All molds and tooling are thoroughly dehydrated before use to eliminate the risk of water vapor Blister.
Automated Mold Release Agent Spraying
The automated robotic system for mold release agent spraying ensures precise application with minimal usage and uniform coverage, preventing localized excess residue of gases.
Degassing Treatment
We use rotary degassers and inert gas treatment during the smelting process to effectively remove hydrogen from the aluminum liquid and eliminate the risk of blistering.
Nondestructive Testing Before Shipping
Before leaving the factory, the products are tested by X-ray and helium leak detection to confirm that there are no bubbles inside, ensuring that every product received by the customer is qualified and reliable.
Conclusion
Blister defects in aluminum alloy die casting can be completely avoided through reasonable design, precise control, and standardized operation.
Preventing blisters can significantly improve the strength, surface quality, and cost-effectiveness of castings.
As an experienced aluminum alloy die casting manufacturer, CEX Casting provides high-quality custom aluminum die castings without Blister defects.
Contact us now to learn how to optimize your next aluminium die casting project.