Key Causes of Blistering in Die Casting Explained

TL;DR

Blistering in die casting is a surface defect characterized by raised bubbles caused by the expansion of trapped gas just beneath the metal's skin. The primary cause is gas or air entrapment resulting from turbulent metal flow and inadequate mold venting. Other critical factors include excessive molten metal or die temperatures, improper application of die lubricants, and contaminants or physical imperfections within the aluminum alloy itself.

The Role of Gas and Air Entrapment in Blister Formation

The most fundamental cause of blistering in die casting is the entrapment of gas within the mold cavity during metal injection. Blisters are essentially a specific form of gas porosity, where the trapped gas is located just under the casting's surface. As the molten metal solidifies, this trapped gas is under immense pressure. When the part is ejected from the die, the external support is removed, and the still-soft metal skin can be pushed outward by the expanding gas, forming a distinct blister.

This gas can originate from several sources. The most common is air that is already present in the mold cavity and runner system before the shot. If the molten metal is injected too quickly or if the flow path is not optimized, it creates turbulence. This turbulent, chaotic flow folds over on itself, trapping pockets of air that cannot escape before the metal solidifies. As detailed in a technical analysis by CEX Casting, poor gate and runner design is a frequent culprit, failing to provide a smooth, laminar flow of metal into the die.

Inadequate venting is another critical factor. Vents are small channels designed to allow the air in the cavity to escape as the molten metal fills it. If these vents are blocked, too small, or poorly positioned, the air has nowhere to go and becomes trapped within the casting. The result is porosity and, if near the surface, blisters. Optimizing the venting system is a crucial step in preventing this type of defect.

To mitigate gas and air entrapment, several best practices should be implemented:

• Optimize Gating and Runner Design: Use mold flow simulation software to design a system that promotes smooth, non-turbulent filling of the die cavity.
• Ensure Adequate Venting: Design and maintain clean, effective vents and overflow gates to allow for the complete evacuation of air.
• Control Injection Speed: Adjust the shot profile, particularly the initial slow shot phase, to gently push air out of the cavity before the high-speed fill begins.
• Utilize Vacuum Assistance: For critical components, implementing a vacuum die casting process can actively remove air from the cavity before injection, virtually eliminating the risk of trapped-gas defects.

Process Parameters: How Temperature and Lubricants Cause Blisters

Beyond the physical trapping of air, operational process parameters play a significant role in creating the conditions for blisters to form. Temperature control and lubricant application are two of the most critical areas to manage. Excessively high temperatures, whether in the molten metal or the die itself, can exacerbate gas-related issues. According to an overview from Sunrise Metal, high temperatures can increase the vapor pressure within the molten alloy and cause die lubricants to break down, releasing gas that becomes trapped.

Die lubricants, or release agents, are necessary to prevent the casting from sticking to the mold, but their misuse is a major source of gas porosity and blisters. When too much lubricant is applied, or it's applied unevenly, the excess liquid can pool in the die. Upon contact with the hot molten metal, this excess lubricant instantly vaporizes, creating a large volume of gas that has no time to escape through the vents. As noted in a report by The Hill & Griffith Company, plunger lubricant is often the single biggest contributor, especially when extra lubricant is used to compensate for a worn plunger tip.

Moisture is another key contributor. Any residual moisture in the mold, from leaking water lines, dripping sprayers, or even the release agent itself, will turn to steam upon injection. This steam behaves just like any other trapped gas, creating pressure under the casting's skin that can lead to blisters. Therefore, maintaining a dry die environment is paramount.

To prevent blisters caused by process parameters, operators should adhere to the following corrective actions:

1. Maintain Strict Temperature Control: Ensure both the molten alloy and the die are kept within their specified temperature ranges to prevent overheating and excessive gas formation.
2. Apply Lubricant Sparingly and Evenly: Use automated spray systems to apply a minimal, consistent coating of a high-quality, low-residue release agent.
3. Allow for Evaporation Time: Ensure there is a sufficient delay after spraying for any water or solvent carriers in the lubricant to evaporate completely before the die is closed.
4. Conduct Regular Maintenance: Routinely check for and repair any leaking water or hydraulic lines, and ensure spray nozzles are not dripping.

Material and Physical Defects as Root Causes

The final category of causes relates to the integrity of the casting material and the presence of physical discontinuities within the metal flow. Blisters can originate from contaminants within the alloy itself. For instance, elements with low boiling points, such as lead or cadmium, can vaporize during the casting process or subsequent heat treatment, creating internal gas pressure. Similarly, aluminum alloys can absorb hydrogen during melting, which will attempt to escape during solidification, leading to porosity and blisters.

Physical defects introduced during the filling process are also highly detrimental. Research published in Engineering Failure Analysis highlights that cold flakes—semi-solidified pieces of metal that break off from the shot sleeve walls—are a primary cause of large blisters, especially in areas near the gating system. These flakes create discontinuities in the casting's microstructure. Gas present in these voids expands during heat treatment, forming significant surface blisters. Other similar defects include cold drops, cold shots, and oxide films, all of which disrupt the metal's homogeneity and act as initiation sites for blistering.

Preventing these material-related defects requires rigorous control over the entire process, from raw material handling to final production. Partnering with a supplier that demonstrates a strong commitment to quality control is essential. For example, manufacturers of high-performance automotive parts often rely on certified processes like IATF16949 and in-house quality control to ensure material integrity from start to finish, which is a critical practice for preventing such defects.

To better understand these distinct causes, the following table compares blisters originating from gas porosity versus those from physical or chemical defects:

Solutions include preheating and drying raw materials thoroughly, using high-purity alloys, and implementing effective degassing treatments with nitrogen or argon to remove dissolved hydrogen before casting.

Frequently Asked Questions About Die Casting Blisters

1. What is the main cause of blisters in die casting?

The primary cause of blisters is trapped gas, most often air from the mold cavity, which becomes entrapped due to turbulent molten metal flow and insufficient venting. This gas, located just beneath the casting surface, expands and pushes the soft metal skin outward, forming a bubble.

2. Can heat treatment cause blisters to appear on a die-cast part?

Yes, heat treatment is a common trigger for blister formation. A part may look perfect in its as-cast state, but if there is trapped gas or a physical discontinuity beneath the surface, the high temperatures of heat treatment will cause the gas to expand significantly, revealing the defect as a surface blister.

3. How do you differentiate between a blister and general porosity?

Blisters are a surface-level or near-surface defect, appearing as raised bulges on the casting's skin. General porosity, on the other hand, refers to voids that can be located anywhere within the casting, including deep inside the part. While both are caused by trapped gas, blisters are specifically those pores close enough to the surface to deform it.