In aluminum die casting, drag marks are linear scratches or grooves that form when ejecting a part. They are caused by friction between the casting and the die casting mold surface. This defect reduces surface quality and impairs mechanical properties. Drag marks can be effectively prevented by optimizing draft angles, polishing the die surface, properly arranging ejector pins, and maintaining a stable die temperature.
This article will explore the root causes, effects, prevention measures, detection methods, and practical prevention measures of drag mark defects at CEX Casting. Read on to learn how to prevent drag marks and produce defect-free aluminium die casting components.
Root Causes of Drag Mark Defects
Design Causes
Insufficient Draft Angle
Insufficient draft angle causes the part to rub against the mold sidewall during demolding, especially on vertical surfaces, resulting in obvious drag marks on the surface of aluminum die casting parts.
Sharp Corners and Deep Cavities
Sharp transitions and deep cavities can trap aluminum liquid during solidification, making it more difficult to release the part, increasing friction, and causing drag marks on die casting products.
Complex Structures
Parts with complex structures or undercuts require special ejection strategies. If the reasonable ejection path is not considered in the design stage, it will lead to uneven ejection, resulting in drag marks.
Mold and Tooling Problems
Rough Cavity Surface
Mold wear, corrosion, or inadequate polishing will cause a rough cavity surface, increase friction, and scratch parts during demolding.
Lack of Protective Coating
Uncoated molds are more likely to adhere to aluminum liquid, resulting in drag marks during ejection.
Ejector Arrangement
Insufficient number of ejectors, improper position, or severe wear will lead to uneven ejection force, resulting in drag marks on the surface of aluminium alloy die casting parts.
Process and Operation Factors
Uneven Use of Release Agent
Using poor quality or uneven spraying of the release agent will cause local sticking of mold and form drag marks.
High Mold Temperature
Overheating of the mold will cause aluminum liquid to adhere to the mold surface locally, especially in sharp corners or thick wall areas, and tearing or drag marks will occur during ejection.
Unbalanced Ejection Force
Uneven distribution of ejection force will lead to excessive local force, resulting in drag marks, cracks, and even part deformation.
Drag Marks
Impacts of Drag Marks
Degradation of Surface Quality
Drag marks significantly affect the appearance of die castings, especially when used for display or appearance parts.
Functional Defects
In high-precision parts, drag marks may damage sealing surfaces, interfere with fit clearances, or affect flatness, leading to leakage, misalignment, or assembly difficulties, and increasing the difficulty of subsequent processing.
Reduced Structural Performance
Drag marks form microcracks, which are potential causes of fatigue cracks. Under cyclic loads, these defects can cause premature failure of components and reduce their structural reliability.
Economic Losses
The high scrap rate caused by drag mark defects increases the cost of rework, scrapping, and production delays of castings.
Effective Measures to Prevent Drag Marks
Mold Design Optimization
Reasonable Draft Angle Design
All vertical surfaces should be designed with a draft angle of 1°~3° to reduce demolding resistance and protect the mold and product surface.
Rounded Corner Transition Design
Using rounded corners instead of sharp corners can promote metal flow and smooth demolding, reduce stress concentration, and prevent aluminum liquid from accumulating at corners.
Simplified Structure
Avoid designing cavities that are too deep or complex in structure. If the structure cannot be simplified, core pulling, sliders, or exhaust structures should be designed to assist demolding.
Mold Surface Treatment
Mirror Polishing
The smoother the surface of the mold cavity, the less friction, and the less likely it is to stick to the mold and cause scratches. The mold cavity can be cleaned and polished regularly.
Advanced Surface Coatings
Coatings such as nitriding, PVD, TiCN, etc., can significantly reduce the adhesion between the mold and the aluminum liquid, reduce the risk of drag marks, improve the demolding effect, and extend the life of the mold.
Optimization of Ejector System
Ejector Arrangement
Ejectors should be reasonably distributed in areas such as side walls and reinforcing ribs where the force is concentrated to ensure uniform force during ejection and prevent local scratches, indentations, or deformation.
Precision Alignment
The ejector needs to be aligned with the mold cavity with high precision, and the ejection direction is consistent to avoid scratches caused by offset or tilting, especially in areas with high surface requirements.
High-Strength Ejector Materials
Select ejector materials with high hardness and strong wear resistance, such as H13 or SKD61, and check and replace them regularly to prevent scratches on the casting surface due to wear and deformation.
Process Parameter Control
Stable Mold Temperature
Maintaining the mold temperature within a reasonable range can prevent aluminum liquid from sticking to the mold and avoid demolding difficulties caused by local overheating.
Uniform Spraying of Release Agent
The automatic spraying equipment can achieve quantitative and uniform coverage of the mold surface, which can effectively reduce local mold sticking, reduce the friction between the aluminum part and the mold, and prevent drag marks caused by uneven demolding resistance.
Injection Parameter Optimization
Adjust the injection speed and pressure to ensure that the aluminum liquid fills the mold smoothly, reduces internal stress concentration, and avoids sticking, strain, or dimensional deformation caused by rapid cooling or excessive impact.
Drag Marks Detection Methods
Visual Inspection
Visually inspect the side wall, ejection direction, and ejector area of the aluminum die casting to find obvious drag marks on the surface.
Dye Penetrant Testing
For die castings with high surface grade requirements, dye penetrant testing can be used to identify subtle drag marks that cannot be found by the naked eye.
CEX Casting’s Measures to Prevent Drag Marks
In-House Mold Development
CEX Casting provides internal mold design and manufacturing services, and the mold quality is guaranteed.
We customize each mold according to customer drawings to ensure that all vertical surfaces have reasonable draft angles to reduce demolding resistance.
CEX In-House Mold Development
Mold Maintenance
Each mold is mirror-polished by the internal team before production, and micro-defects are removed.
And according to ISO9001 operating procedures, the mold is regularly maintained to keep the cavity smooth.
Scientific Ejector Pin Layout
CEX analyzes and determines the ejector pin position based on 3D simulation software to achieve balanced ejection force and avoid surface drag marks caused by excessive local pressure.
Mold Temperature Control and Automatic Spraying of Release Agent
CEX uses a digital system to accurately control the mold temperature to prevent local overheating from causing mold sticking.
At the same time, automatic spraying equipment is used to evenly cover the release agent to reduce friction resistance and effectively avoid drag marks during the demolding stage.
Conclusion
Drag marks can damage the surface of aluminum die castings, weaken mechanical strength, and lead to high scrap rates in the high-pressure die casting process.
The causes are mostly poor draft angle design, rough mold surface, and uneven ejection force.
As an aluminum alloy die casting supplier with 29 years of experience, CEX Casting provides one-stop in-house solutions from mold design to surface treatment to present you with high-quality, defect-free aluminum die castings.
Contact us now to find out how we can make your next aluminum die casting project a reality.