Aluminum, zinc, and magnesium die-casting alloys are all active metals. In a dry environment, they will slowly oxidize. Under higher temperature or humidity conditions, they are easily oxidized and corroded. This is determined by the characteristics of the alloy itself. The surface of the die-casting presents different colors from the base metal, uneven yellow and black dots, lines, patches, etc., which are called blackening. This phenomenon often occurs in the die-casting molding process and subsequent processing and storage processes.
Based on the characteristics of the blackening defects of the die castings, we can quickly determine which link has a problem:
Blackening of the surface of die castings like pine smoke
If the surface of the die casting shows blackening traces like pine smoke as shown in Figure 1, with irregular shapes and uneven colors, it is because, during the process of filling the mold cavity with alloy liquid, the release agent or punch oil is not entirely burned after encountering high-temperature alloy liquid. A large amount of oil smoke produced gathers in the mold cavity. Since this kind of waste gas cannot be completely and quickly discharged outside the mold cavity, the accumulated oil smoke pollutes the surface of the alloy liquid and mold cavity, causing the die-casting surface to show blackening traces like pine smoke.
Solutions: Adjust the concentration, formula, and injection amount of punch oil and mold release agent, optimize the mold pouring channel, and maintain the temperature balance of the mold.
Blackening of the surface of die castings like cloud shapes
If the surface of the die casting blackening traces in cloud shapes as shown in Figure 2, the black shape is irregular and the color depth is relatively uniform, it is because the surface of the mold cavity is polluted by the carbon black and oil smoke of the release agent. The carbon black on the surface of the mold is printed on the surface of the casting, causing each mold casting to have a black pattern with a relatively similar shape on the surface. Generally, the color of such blackening is relatively light and not as deep as pine smoke.
Solutions: Optimize the concentration and formula of the mold release agent and the SOP of the mold release operation.
Blackening of the surface of die castings in spots
As shown in Figure 3, the size of the black spots is different, the shape is approximately circular, and the color is relatively deep. This is caused by the droplets formed by the sediment or agglomerate produced by the residual release agent or punch oil. Because it is relatively viscous and not easily completely decomposed by high temperature, if it adheres to the surface of the mold or casting, it will leave a single black spot on the surface of the casting.
Solutions: Optimize the injection method of punch oil, choose better quality punch oil and mold release agent, minimize the use of punch oil and mold release agent, and regularly clean and maintain the mold.
Blackening of the surface of die castings due to oxidation
The surface of the die casting oxidized and blackened is shown in Figure 4. Figures 4A and 4B are both die castings with surfaces that have naturally oxidized. The effect of natural oxidation of the casting in Figure 4A after being placed in a cleaning workshop for 10 days without packaging protection, and Figure 4B is the natural oxidation effect of the casting placed in a packaging box for 35 days. The oxidation and blackening of Figure 4A is more obvious than that of Figure 4B because its surface is directly exposed to the outside, causing moisture and other substances in the air to easily come into contact with the surface of the casting, quickly oxidizing and blackening the entire casting. It can be seen that good packaging can prevent the surface of the casting from oxidizing and blackening.
Solutions: Store die castings in sealed boxes and keep the storage environment dry and clean.
Black dots appear on the surface of the die casting
As shown in Figure 5, this is the surface condition of the die casting after being placed in the die casting workshop for 10 days. The die casting is placed in the die casting workshop without protection. The air in the workshop is contaminated and oxidized by contact with mold release agents and oil smoke mist, resulting in black dots. The mold release agent and oil smoke mist intensify the surface oxidation of the die casting, and the oxidized black dots are relatively deep and cannot be removed by shot blasting.
Solutions: Die castings should not be stacked in the transfer area for a long time. Die castings temporarily placed in the transfer area need to be covered and protected to prevent contact with droplets and moisture in the air in the workshop.
Carbon deposits on the surface of the die casting
One of the carbon deposits formed on the surface of the die casting is shown in Figure 6. Carbon deposits are caused by carbon accumulation on the surface of the mold. They are relatively rough and look like traces of mold sticking, but do not have the characteristics of mold sticking. If the quality of the mold release agent or punch oil is not properly selected, or if the mold release agent is impure, the wax and organic oil in the mold release agent are easily burned. The residual carbon black after burning will accumulate and adhere to the surface of the mold, forming carbon deposits. Carbon deposits appear on the surface of the mold as a layer of deep black dirt. Carbon deposits usually appear as a rough surface similar to mold sticking, with a color similar to that of the adhered aluminum alloy. This is because a layer of aluminum alloy is adhered to the surface of the carbon deposit. Carbon deposits and aluminum alloy mold sticking will occur in succession and be based on each other.
Solutions: Choose better quality mold release agents and punch oils, adjust the concentration and ratio of mold release agents, and regularly maintain and clean molds.
From the above analysis, it can be seen that blackening defects mainly need to be investigated and prevented from three aspects:
- Punch oil and release agent
- Production management
- Mold optimization
