Flow marks and water lines primarily appear at the ends of castings, far from the gate, or in low-temperature zones. They are more prevalent during the hot die stage, in large structural components, or ultra-thin parts. During die casting, molten metal first enters the cavity, forming a thin, incomplete metal layer, which is then overlaid by subsequent molten metal. This process leaves marks on the casting surface aligned with the flow direction. These defects generally do not deteriorate over time and can be removed by polishing. However, with high-copper aluminum alloy raw materials in humid environments, black spots or mold spots may develop later.
Flow marks and water lines in die casting are closely tied to on-site processes, including temperature, speed, pressure, time, coatings (spray duration, dilution ratio, atomization, air blowing/drying), and mold design (gate design, venting). Key contributing factors are outlined below:
If the gate has an excessively small cross-sectional area or misaligned position, it disrupts molten metal filling—causing incomplete synchronous cavity filling or splashing during flow—which leaves marks.
When the aluminum alloy mold temperature is below 180°C before spraying, incoming molten metal experiences thermal shock, leading to uneven filling and defect formation—common during the hot die stage. This is exacerbated by prolonged spraying and incomplete air drying, resulting in dull, grayish-black castings. Attention should also be paid to molten aluminum temperature and composition (especially silicon content).
Too high: Molten metal may jet directly from the gate to the distant end, creating flow marks.
Too low: Inconsistent filling of aluminum liquid easily produces water lines and flow marks.
Over-application of coatings (release agents, plunger lubricants) darkens the molten alloy, causes oxidation, and leads to blackened surfaces. Coating accumulation—especially at casting ends—triggers flow marks and water lines. Compounded by incomplete air drying and dripping from spraying equipment during retraction, this further degrades appearance.
Inadequate pressure on molten metal prevents complete cavity filling, potentially causing cold shuts in severe cases.
Apply black oil and heat the mold to observe molten metal flow direction and filling sequence. Modify gate cross-sectional area, position, or filling direction to achieve synchronous filling.
Use a mold temperature gun to measure and adjust temperature within process specifications. Alternatively, modify cooling water duration or spraying time.
Bright spots on the casting surface—especially near the gate—indicate excessive speed. Adjust speed to optimize molten metal flow patterns during cavity filling.
If the surface blackens with localized coating buildup, reduce coating concentration to ensure thin, uniform coverage. Die casting release agents are not the more concentrated, the better; minimal effective use (preventing sticking) benefits both clean casting surfaces and equipment (reducing cleaning frequency).
If incomplete compaction and dull surfaces occur—especially opposite the gate—adjust injection pressure accordingly.
For more information about One of Die Casting Surface Defects (3): Analysis and Countermeasures for Flow Marks and Water Lines or die-casting auxiliary materials, and if you need customized die-casting release agent, please contact 15021483232 (same as Wechat)
