Die Casting Quality Control: The Ultimate Guide

Though die casting is a highly effective method to manufacture metal parts, incorrect control of die casting parameters can lead to many defects. If you are looking for a method to control the defects or trying to improve the quality of the die casting parts, this article will be useful for you. We will introduce you to the quality control method and inspection technology for die casting parts.

Common Die Casting Defects and Causes

The common die casting defects can be divided into 3 types according to their position, including surface defects, internal defects, and dimensional deviations.

Surface Defects

• Flow Marks

Flow marks refer to the patterns on the surface of the die casting parts. It commonly keeps consistent with the direction of molten metal. Commonly speaking, it can be optimized by some surface treatment.

Many factors may cause flow defects, such as low mold temperature, high filling speed, excessive use of release agents, and an unreasonable gate system.

• Cold Shuts

Cold shuts refer to the obvious irregular indentations on the surface of the die-cast part. It is usually in narrow and elongated shapes.

The formation of cold shuts usually relates to the temperature parameter, including pouring temperature, mold temperature, injection temperature, etc. If the temperature is too low, the molten metal will fail to fuse at the interface during the solidification.

• Gas Bubbles

Gas Bubbles are the bubbles formed on the surface of die casting parts.

During the process of die casting, especially in the injection stage, the gas is likely to be trapped by the molten metal. Or the gas will evolve from itself and leave a bubble on the surface of the die casting parts. The injection speed, mold temperature, and the gas content of molten metal can lead to gas bubbles.

• Net-like Burrs

The net-like burrs, sometimes shaped like hair, are typically caused by mold cracks. When the molten metal is forced into the cracks and cools down, it forms on the surface of die castings.

Sometimes the existence of net-like Bur is caused by high mold temperature and pouring temperature. So you should focus on the temperature parameter to avoid it.

• Cracks

Cracks are usually straight or wavy-shaped on the surface of the die-cast part. It will impact the mechanical properties of your products. 

The existence of cracks means the wall thickness of die castings has sudden changes, or the ejection process is unbalanced. Additionally, it may be related to the uneven stress of die casting parts or excessive mold retention time.

Internal Defects

• Porosity

Porosity is the holes formed by the gas inside die casting parts. Generally speaking, the structure of the product is deeply influenced by porosity. For die-cast parts with high air-tightness requirements, large gas holes can cause product leakage.

Porosity is related to the gas entrapped within the molten metal. During the die casting process. There are many factors of gas entrapment, such as improper mold design, incorrect melting temperature, excessive injection speed, etc.

• Shrinkage Cavities and Shrinkage Porosity

Shrinkage cavities and shrinkage porosity are common defects in die castings, which refer to internal holes. Those large and concentrated ones are called shrinkage cavities, while the small and dispersed ones are called shrinkage porosity. They are often formed during the solidification when the metal liquid compensation is not sufficient. So, if you want to reduce shrinkage cavities and shrinkage porosity in the die castings, you can reduce the shrinkage volume or increase the injection pressure.

• Inclusions

Inclusions inside die-cast parts can affect the mechanical properties of the final product. Inclusions are usually foreign substances, either metallic or non-metallic. The main causes of inclusions include: insufficient purity of the die-cast metal, formation of oxides during the smelting process, and poor mold venting.

Dimensional Deviations

When die castings are delivered to customers, part dimensions are a very important factor that directly affects whether the die castings can be used successfully. Die casting part dimensions must conform to the customer’s design drawings or 3D models and meet the corresponding tolerance standards. Dimension inaccuracy issues include warpage, distortion, and variations in thickness. 

Inaccurate dimensions can be caused by incorrect die casting mold design, mold wear, or problems with the metal filling process. Of course, inaccurate dimensions can also be caused by poor control of the shrinkage rate during the solidification process.

Key Factors Affecting Die Casting Quality

Material Control

The control of material quality is an important part of die casting quality control. Before the die casting process begins, the composition, chemical properties, and processing properties of the alloy used need to be tested. The testing of the material is to ensure that the material can meet the die casting processing requirements with sufficient purity and fluidity.

For example, ADC12 is composed of approximately 85% aluminum, 12% silicon, and 3% copper. It has a tensile strength of 290 MPa, a yield strength of 170 MPa, and an elongation of 3%.

The material you choose must have a relevant certification. If it has any problem, you need to trace back to its source. When checking the die casting material, you can verify it for compliance with standards.

Mold Design and Maintenance

The quality issue that die castings have may be caused by mold problems. Poor mold design leads to defects. For example, if the cavity is too narrow (usually <2mm), molten metal cannot fill it well. This creates unfilled areas in the casting. The mold’s parting surfaces matter too. In addition, if the mold is not properly preserved and cracked, the surface of the casting will be prone to develop net-like burrs.

Process Control

The die casting process is rapidly changing, and there are many parameters that need to be controlled by real-time monitoring. The capture of real-time data is realized through sensors, including injection pressure, speed, temperature, and cycle time. Take aluminum alloy as an example:

• Injection Pressure: The injection pressure of aluminum alloy die casting is between 3500Psi~4500Psi. By adjusting the hydraulic system of the die casting machine, as well as the optimization of the die structure and other ways to achieve.
• Speed: In the low speed stage, the speed is usually between 0.1-0.5m/s. In the high-speed stage, the speed ranges from 1 to 5m/s. For thin-walled die castings, the speed can be 6m/s.
• Temperature: The melting temperature of aluminum alloy is 650℃-710℃, while the temperature of the mold is usually 150℃-250℃.
• Cycle Time: Die casting cycle time varies with the size and complexity of the casting, usually within 30-60 seconds.

Statistical Process Control (SPC)

Statistical Process Control (SPC) is a quality management method that uses statistics to monitor and improve the die casting process. By tracking key production data like pressure, speed, temperature, and cycle time in real time, SPC helps detect issues early and prevent defects. It uses control charts to display important trends, such as average values, variation limits, and defect rates, ensuring consistent product quality.

Die Casting Parts Inspection Method for Selection

Visual Inspection

Visual inspection checks for visible flaws on the surface of die-cast parts. These flaws include cracks, flow marks, and bubbles. Nearly all products go through this check to make sure they look perfect before reaching clients. This method is easy and affordable. It usually uses basic tools like magnifying glasses or microscopes.

The operator also uses instruments like a height gauge to measure the heights of die castings, ensuring parts’ precision and consistency.

Non-Destructive Testing

Non-destructive testing is a key part of quality control in die casting. It checks for problems inside or on the surface of cast parts, without breaking or damaging them. Common NDT methods include: X-ray inspection, ultrasonic testing, eddy current testing, and penetrant testing.

• X-ray Inspection

X-ray inspection helps find problems inside metal parts, like porosity, shrinkage cavities, and foreign materials. The best thing about X-ray inspection is that you can see hidden defects clearly. However, the operator must be very careful to protect themselves when using the X-ray machine, for the process cannot avoid radiation.

X-ray inspection works great for detecting engine blocks and transmission housings for porosity.

• Ultrasonic Testing

Ultrasonic testing works by sending high-frequency sound waves through metal parts. When these sound waves hit a defect like a crack, they bounce back. This makes it useful for finding hidden cracks inside die-cast parts and measuring wall thickness. The advantage of ultrasonic testing is the depth of detection. This test method requires a skilled operator to get accurate results.

• Eddy Current Testing

Eddy current testing works by using a magnetic field to create small electrical currents in metal parts. When there’s a crack or defect near the surface, these currents change. That’s why it’s great for finding surface cracks in conductive metals like aluminum and magnesium. Eddy current testing is very fast and works well with automated systems.

• Penetrant Testing

Penetrant testing uses special colored or fluorescent liquids to find surface cracks in metal parts. The liquid seeps into any surface flaws, then a developer makes them visible. This method is generally used to detect surface cracks, porosity, and other defects in die-cast parts. It is affordable and simple to use, but is limited to surface defects.

Destructive Testing 

• Metallographic Analysis

Metallographic analysis is a method used to analyze the internal structure of metal materials by microscope, including grain size, porosity, and the presence of any defects or inclusions. 

Grain Size: If the grains are too large, obvious grain boundaries can be seen, which reduces the strength and toughness of the die-cast part.

Porosity: The porosity can be seen as circular or elliptical black holes during metallographic analysis, which can easily reduce mechanical properties and cause cracks.

Inclusions: If there are inclusions inside the die-cast part, particles with a different color from the metal can be seen during metallographic analysis.

• Mechanical Property Testing

Mechanical properties are one of the key indicators for measuring the quality of die-cast parts. That’s why testing mechanical properties is a critical step in quality control.

Common mechanical property testing includes tensile testing, impact testing, and hardness testing.

Tensile Testing

Tensile strength (UTS): The maximum stress a material can withstand before breaking (e.g., typical value for ADC12 aluminum alloy ≥ 310 MPa)

Yield strength (YS): The stress at which 0.2% plastic deformation occurs (approximately 170 MPa for ADC12)

Elongation: The plastic deformation capacity at break (approximately 3.5% for ADC12)

Impact Testing

Impact testing is primarily used to check the toughness of die-cast parts under dynamic loads. It also shows if they can handle cold temperatures without cracking. This test is especially important for safety, such as automotive steering knuckles, battery pack housings, and aircraft engine mounts.

Hardness Testing

Parts that carry weight or need to resist wear must pass hardness tests to work properly. For example:

Engine blocks made of ADC12 aluminum need 80-100 HBW. This is tested using a 500 kgf load with a 10mm ball to make sure they can handle wear.

Hydraulic pump housings made of AlSi17Cu4Mg alloy need 80-120 HV. This is tested with a 0.5 kgf load for 10-15 seconds to ensure they work reliably under high pressure.

Dimension Inspection 

The dimension of the completed die-cast parts is required to meet the tolerance standard. It ensures a tight fit during assembly. For the automotive and aerospace industry, the accuracy of the die castings’ dimensions is especially important. We typically use coordinate measuring machines and 3D optical scanning to do the test.

• Coordinate Measuring Machine（CMM）

A coordinate measuring machine(CMM) is a device that measures the shape of the die-cast parts by sensing discrete points on their surface. It is usually equipped with mechanical sensors and laser sensors. In addition to moving the probe position along the XYZ axes, the angle of the probe can be adjusted to achieve comprehensive measurement. 

CMM can be operated by operators or controlled by a computer. Through programming, it is possible to measure any size of die-cast part with high precision. But the measuring speed is relatively slow.

• 3D Optical Scanning

3D optical scanning is commonly used to measure the dimensions of large, heavy die-cast parts, such as outer housings. The data can be captured without touching the die casting parts and analyzed to compare the accuracy against the original CAD design. In the generated 3D model, parts without problems are displayed in green, while those falling outside the tolerance range are displayed in red. You can use it to do further analysis, indicating where the parts are undersized or slightly oversized.

Conclusions

In conclusion, quality control in die casting spans the entire production process and employs diverse inspection methods. It encompasses both raw material controls and post-production part inspections, covering surface defects and internal integrity evaluations. If you want to learn more about die casting quality control, please feel free to contact us. Sunrise has 10+ years of experience in die casting manufacturing. We will offer you the best answer.