Sink marks are preventable in the aluminum die casting process by optimizing part design, precise process control, and proper cooling layout. Sink marks are shallow surface depressions formed by local shrinkage during solidification. They are common in thick-walled areas, near ribs, or at geometric changes due to uneven cooling. Common causes of sink marks include poor die casting mold design, inconsistent wall thickness, insufficient pressure holding, and thermal imbalance.
This article will delve into the root causes, design flaws, material factors, and prevention strategies of sink marks. It also covers its impact on different industries and key detection methods. Read on to learn how to effectively control and prevent this defect.
Causes of Sink Marks
Shrinkage Behavior
Sink marks are surface depressions formed by insufficient shrinkage of metal during the cooling and shrinkage of molten aluminum.
Thick-walled areas are more prone to sink marks due to slow cooling and large shrinkage.
Uneven mold temperature distribution will lead to local heat accumulation, amplifying the probability of this defect.
Product Design Issues
Uneven wall thickness will lead to inconsistent cooling speed, which will cause local shrinkage and sink marks.
Large ribs or bosses are prone to heat accumulation, slow cooling, and internal voids.
Geometric mutation areas can also accumulate heat, significantly increasing the risk of sink marks.
Mold Design Defects
Irrational gate position can lead to insufficient filling in some areas, resulting in sink marks.
Inadequate mold exhaust design can easily cause gas stagnation, especially in thick walls, forming voids.
Improper layout of cooling channels can slow local cooling, exacerbating thermal shrinkage and surface defects.
Inadequate Process Parameter Settings
Too low holding pressure or insufficient holding time will prevent the molten aluminum from being able to fill during shrinkage, resulting in sink marks.
Too high a mold or aluminum liquid temperature will prolong cooling time and increase the probability of defects.
Unstable injection speed can also lead to uneven feeding and surface depressions.
Sink Mark
Impact and Risks of Sink Marks
Appearance Defects
Sink marks destroy the surface flatness of die casting parts, especially after spraying, electroplating, or anodizing, resulting in uneven coating thickness, uneven reflection, or abnormal texture.
For appearance parts, this defect is difficult to cover up through later processes, which directly affects the appearance consistency of die casting products.
Strength and Precision Impact
Sink marks are not only a surface problem, but are often accompanied by internal shrinkage or cavitation, which reduces the strength and durability of aluminium alloy die casting components.
In addition, for parts with high dimensional accuracy requirements, such as mechanical assemblies or sealing parts, local deformation caused by sink marks can lead to assembly errors and even product function failure.
Increased Costs
sink marks lead to rework, refilling, grinding, or processing, increasing labor and time consumption. Defective parts may also trigger additional quality inspection processes and delay delivery.
To solve the source problem, companies often need to adjust molds or process parameters, interrupt the production line rhythm, and increase maintenance and trial production costs.
Prevention Strategy
Optimize Casting Design
Ensure consistent wall thickness to avoid cooling hysteresis caused by thick ribs or local accumulation.
The thickness of the reinforcing rib is controlled at 40-60% of the main wall thickness, taking into account both strength and heat dissipation.
The geometric transition uses rounded corners to reduce heat accumulation and stress concentration, reducing the risk of sink marks from the structural source.
Improved Mold Design
The gate should be arranged close to the thick area to ensure that the key area is fully refilled during pressure holding.
The cooling channel should be close to the heat-prone area to help balance the mold temperature.
By optimizing the gate position and cooling layout through mold flow and thermal analysis software, potential problems can be identified before trial molding.
Process Precision Control
Set high holding pressure and extend the holding time to ensure sufficient metal shrinkage.
A multi-stage injection program is adopted, with fast filling in the first stage and slow shrinkage in the second stage.
Equipped with a temperature closed-loop control system to keep the mold and melt temperature fluctuations within ±2°C.
Detection and Identification Methods
Manual Detection
Visual inspection under oblique light sources can quickly identify surface sink marks, and hand sliding can help find shallow defects. It is suitable for irregular contours and corners.
This method does not require equipment and is suitable for first-piece confirmation and production inspection. It is flexible and responsive.
Dimension Detection
Use a micrometer or depth gauge to measure key areas to determine whether the sink mark is out of tolerance.
The CMM three-coordinate measuring machine can scan the entire surface of the casting and output the sink mark position and deviation map.
This is suitable for complex structures and high-precision parts, which is helpful for defect analysis and process correction.
Nondestructive Testing
3D laser scanning compares CAD models to identify subtle sink marks, which is suitable for parts with high appearance requirements.
X-rays can detect internal shrinkage cavities and evaluate the structural defects behind the sink marks.
CT scanning provides tomographic images to accurately analyze the range and depth of sink marks to meet high-standard application requirements.
Advantages of CEX Casting in Sink Mark Prevention
Mold Simulation Optimization
CEX accurately identifies high-incidence areas of sink marks through simulations such as DFM and ProCAST in the early stage of the project.
Engineers optimize the gate position, shrinkage path, and cooling channel layout based on the simulation results.
This can ensure smooth molten aluminum filling and balanced thermal field distribution, thereby improving the stability and accuracy of mold molding from the source of design.
Comprehensive Defect Detection
Four types of detection equipment, including X-ray, three-coordinate, spectrometer, and helium detector, cover all-around monitoring from raw materials, dimensions, to internal defects.
Detection nodes are set up for each key process, so that problems can be identified and handled at an early stage, significantly reducing the risk of rework in the later stage.
Temperature Control and Pressure Holding
The temperature difference between the mold and the melt is controlled within ±2℃ through a multi-stage injection and temperature monitoring system to ensure thermal field stability.
With a holding pressure of 40–80 MPa and a holding time of 1.5–6 seconds, the sink mark defect caused by uneven cooling or insufficient shrinkage can be effectively suppressed.
One-Stop Manufacturer
With our mold manufacturing, aluminium die casting, cnc machining, and surface treatment production lines, all links implement standardized process specifications.
The production data is traceable throughout the process, and the delivery rhythm is stable, ensuring that batch high-pressure die castings have high consistency and repeatability.
Conclusion
The probability of sink mark formation during die casting can be greatly reduced through structural design optimization, reasonable layout of mold gates and cooling systems, and precise control of injection process parameters.
As a one-stop aluminum alloy die casting manufacturer, CEX Casting provides a full-process solution from mold design, material selection, to process control.
Contact us now to learn how we can effectively prevent key defects, such as sink marks, to improve the molding quality of your next die casting product.