Porosity is one of the most common internal defects in aluminum die castings. It is formed by trapped gases within the die casting and appears as relatively regular, smooth-surfaced cavities. In high-pressure die casting (HPDC), proper aluminum die casting mold design is crucial for controlling gas entrapment. Mold design can optimize aluminum molds to minimize porosity. High pressure die casting mold design, high pressure die casting design, and high-pressure die casting design techniques, including customized solutions, produce high-quality aluminum die castings with minimal defects.
One of the Defects in High Pressure Die Casting Molds: Porosity
Gases released from the molten alloy, air trapped during the high pressure injection process, and gases entering the molten alloy from the decomposition of release agents or plunger lubricants can all form voids in the die casting; these voids are called pores. The inner surface of pores is usually smooth and full-bodied, appearing as circular or regular curved shapes. Depending on the cause of formation and location, the size of the pores varies, and their distribution can be concentrated or dispersed. Internal pores in the casting may appear in various forms, such as circular, full-bodied irregular circular, pear-shaped, or needle-shaped, and sometimes coexist with defects such as shrinkage cavities or oxide inclusions, forming complex irregular voids.
| Description | |
| Sources of porosity formation | Gases released from the molten alloy; gases entrapped during the die-casting process; gases produced from the decomposition of mold release agents and plunger lubricants that penetrate the molten alloy. |
| Appearance of porosity in die castings | Cavities |
| Characteristics of the internal surface of pores | Smooth, rounded |
| Pore shape | Circular, full irregular circular, pear-shaped, needle-shaped |
| Pore size | Varying in size |
| Pore distribution | Concentrated or dispersed |
| Combination of porosity with other defects | Gas pores appear together with shrinkage cavities and oxide inclusions, forming irregular cavities. |
What Causes Porosity in Aluminum Die Castings?
Porosity is one of the most frequent internal defects in HPDC of aluminum, which has significant effects on mechanical property, tightness, and surface quality of the casting. Though porosity manifests as tiny internal voids, their formation is often due to the interaction among factors. Aluminum die casting mold design and high pressure die casting mold design play a decisive role, and the mold directly controls metal flow, air evacuation, and solidification behavior.
In summary, the main causes of porosity appear to fall within five clearly demarcated areas: product and mold structure, casting design, alloy melting, release agents, and the process of casting itself.
Die Casting or Mold Structure Design Problems
① Poor gate design, resulting in unstable liquid flow in the runner and turbulence, leading to air entrapment.
② Unreasonable direction of liquid metal flow from the gate into the mold cavity, causing severe jetting and splashing.
③ The gate cross-sectional area is too small, resulting in excessively high metal filling speed, severe jetting, and excessive gas entrapment.
④ The metal liquid impacts the mold wall after passing through the gate, forming vortices and trapping air.
⑤ The jetting liquid flow prematurely blocks the vent, causing porosity in areas with poor venting or deep cavities.
⑥ Insufficient filling of the shot sleeve, requiring high low-speed injection, leading to gas entrapment.
⑦ The mold cavity is too deep, resulting in poor or insufficient venting in deep cavities, dead corners, and the middle of the cavity.
⑧ Incorrect vent location or insufficient cross-sectional area, resulting in poor venting capacity and high back pressure in the mold cavity.
Die Casting Design Problems
Features such as deep ribs, narrow grooves, or uneven wall thickness can create areas of uneven molten metal flow.
Unreasonable casting design can hinder smooth metal filling, often leading to gas entrapment or incomplete filling.
Alloy Melting Conditions
The gas present in the aluminum alloy solution is mostly hydrogen. The amount of hydrogen in the total gas present in the solution is more than 95%. In the process of melting the aluminum alloy solution, it is easy for the hydrogen to dissolve in the liquid state of the aluminum alloy. At higher temperatures, the solubility of the hydrogen gas will be high. At the melting point of the aluminum alloy solution, when it changes from solid state to liquid state, the solubility of the hydrogen gas will be 15 to 30 times more. The solubility of the hydrogen gas at the melting point of the aluminum alloy solution will be nearly 15 to 30 times higher when it is in the liquid state.
① Due to unclean molten metal or excessively high melting temperature, the amount of gas absorbed by the molten metal increases, and this gas is released during solidification and remains in the casting, usually as small pores below 0.5 mm. These small pores can coalesce with shrinkage cavities in the center of the casting to form larger voids.
② Unclean furnace charge and poor degassing and refining.
If dense small pinholes appear on the machined surface, it is often more related to the alloy melting factors.
Release Agents and Lubricants (Mold Sprays/Plunger Oil)
Volatile components in mold sprays or plunger oil vaporize during the high pressure injection process.
This additional gas can be directly introduced into the molten metal, producing fine or pinhole-shaped pores.
High-Pressure Die Casting Process Parameters
Excessively high injection speed can cause turbulence, trapping air inside the molten metal.
Insufficient holding pressure or improper filling sequence can prevent gas from escaping before solidification.
How to Prevent Porosity in Castings?
The cause of porosity in aluminum die castings may be due to the pocketing of gas within the liquid metal at the time of mold filling or solidification, or from shrinkage cavities caused by inefficient metal feeding. In high-pressure die casting, turbulences, irregular metal flow, as well as misdirected venting, increase the tendency of gas pocketing. Hydrogen absorbed within the metal, vaporization of the release agent, or the intake of air from the injection system may cause gas pocketing or generation of pores. The:size, shape, and number of pores are reliant upon the shape, thickness of wall, and rate of cooling.
The best means to achieve porosity-free castings is to rely on exact aluminum die casting mold designs. Good aluminum die casting molds need to exhibit effective metal flow, sufficient ventilation, as well as precise solidification to achieve porosity-free aluminum die casting molds. In Advanced High-Pressure Die Casting molds, special designs are used to achieve consistent filling. Skilled aluminum die casting mold manufacturers are capable of designing molds with minimum porosity by focusing on areas with turbulence during molding designs.
Ultimately, porosity control begins with the mold itself. Investing in high-quality aluminum die casting mold design and skilled manufacturing capabilities ensures castings achieve consistent density, structural integrity, and surface quality, transforming potential defects into reliable, high-performance components.
Main preventative measures:
① Select gate positions and runner shapes that facilitate gas evacuation from the cavity, avoiding premature closure of the overflow system on the parting line.
② The nozzle cross-sectional area of the sprue should be as large as possible compared to the gate cross-sectional area.
③ Increase the filling degree of the pressure chamber, using the smallest possible pressure chamber and quantitative pouring.
④ Increase the gate thickness to reduce the filling speed, while still ensuring good molding conditions.
⑤ Open overflow grooves and vents at the last filling point of the cavity, and ensure that the overflow grooves and vents are not sealed by molten metal.
⑥ Open vent plugs in deep cavities and use insert molding to increase venting.
⑦ Apply a thin and even coating, allowing it to burn out before filling, and use a coating with low gas evolution.
⑧ The furnace charge must be clean and dry, and the melting process must be strictly followed.
⑨ Reduce machining allowance.
⑩ Adjust the injection speed and the transition point of the fast injection speed. Lower the pouring temperature and increase the specific pressure.
FAQ
Q1: What are the main causes of porosity in aluminum die castings?
A: The main cause is trapped gas during the high pressure filling process, usually due to turbulence, insufficient venting, or improper aluminum die casting mold design.
Q2: Can mold design reduce porosity in high pressure die castings?
A: Yes. In professional die casting mold design, optimizing the gating, runner balancing, venting, and overflow systems can significantly reduce gas entrapment and porosity.
Q3: Is porosity in aluminum die castings acceptable?
A: A small number of pores that do not affect critical performance are acceptable, but pores in sealing or load-bearing areas must be strictly controlled through precise die-casting mold design.
Mold Design Related Causes and Solutions
Porosity may arise from design errors of the aluminum die-casting mold in HP aluminum die castings. Inadequate gate locations, imbalanced runners, insufficient venting, and inefficient overflow channels are some of the sources of air entrainment or imbalanced solidification of the molten aluminum casting, leading to porosity and shrinkage cavities. Complex shapes, corners, thick-to-thin walls, and dead-end runner designs can exacerbate turbulence during the injection of the molten aluminum casting material, making such regions prone to porosity. These are some of the most controllable sources of porosity, which emphasize the important role of aluminum die-casting mold design.
Optimal mold design requires careful layout of gates, runners, venting systems, and overflow channels in order to have optimal flow of metal in the mold. Advanced high-pressure die casting (HPDC) design techniques, such as vacuum-assisted filling and MAGMA flow simulation, allow designers to predict high-risk areas for porosity and make corrections before production. By integrating these solutions, experienced die-casting mold manufacturers can produce molds that reduce internal defects, improve casting density, and enhance surface quality.
For manufacturers seeking reliable solutions, Guangzhou Leidi Technology Co., Ltd., established in 1996, possesses extensive expertise in aluminum die-casting mold design and manufacturing.
Raidy Mold Manufacturerserves the automotive, motorcycle, and precision manufacturing industries, and its factory is equipped with high-precision CNC machining centers, wire EDM machines, electrical discharge machining equipment, coordinate measuring machines, laser scanners, and optical projectors. Combined with UG, Pro/E, and SolidWorks modeling software and MAGMA mold flow analysis, Raidy ensures that each mold is optimized for fluidity, cooling, and structural integrity.
With a team of over 200 highly skilled engineers and an ISO 9001 certified quality management system, Raidy provides molds that minimize porosity and guarantee consistency and high quality in aluminum die castings. Tell us your requirements to receive a mold design proposal.
Porosity is a type of internal defect in aluminum die casting, mainly on areas that are sealed or where high stress occurs. Effective control of porosity relies on accurate design of an aluminum die casting mold, optimized design of a high pressure die casting mold, and expertise from a professional manufacturer of die casting mold solutions. Utilizing advanced custom die-casting mold solutions and thorough flow and venting analysis, these defects can be minimized to ensure consistent casting quality. Contact us today to discuss your project and take advantage of Raidy’s custom mold design capability to manufacture defect-free aluminum die castings.
