Case Study Of A Quality Complaint On Aluminum Die Casting Products
- The Importance Of Process Quality Capability Improvement To Ensure Stable Product Quality (Ⅳ) (Continued from the last issue)  ▌The impact of the proportion of mold release agent and its spraying amount on the black spots of castings
There are two points we should pay attention to when preparing the release agent. One is the selection of the release agent supplier. The other is the selection of good quality release agent that not only can prevent oxidation and corrosion on the surface of the die casting but also affect the quality of the casting parts when they are in use. Therefore, we should choose the release agent which has good mucositis prevention, high temperature resistance, low flammability and less oil fume.
The configuration of release agent is usually based on manufacturer suggested proportion and the production characteristics of the company. Generally, the proportion of release agent is about 1:120-150, with the proportion adjusted depending on the product structure. For example, for thin and deep mold cavities, it is easy to stick to the mold, so the spraying amount of the release agent could not be reduced but the concentration could be reduced by increasing the proportion of water to, for example, 1:140.
Pure water or soft water should be selected as the water used to prepare the release agent. Hard water should not be used because the metal ions in hard water will not only affect the stability, coating and release performance of the agent, but also cause the casting to be oxidized and corroded.
In die casting production, spraying release agent is usually used to rapidly demold and improve production efficiency. In the actual production, the die-casting machines of general manufacturers usually include fully automatic machines (automatic pouring, picking-up and spraying release agent) and semi-automatic machines (manual pouring, picking-up, spraying agent). The fully automatic die-casting machines are mainly used for mass production, while the semi-automatic die-casting machines are mainly used for small batch productions. In cases where the molds used in production are bad, workers often increase the amount of spray agent to reduce the difficulty of fetching parts.
If the agent is over-sprayed or the concentration too thick, the agent will accumulate in the mold cavity after spraying. The oil, grease, wax, organic silicone oil and other organic substances and moisture in the agent will produce a lot of fumes when they meet the high temperature alloy liquid. The excessive fumes not only seriously pollute the surface of the alloy liquid and the surface of the mold cavity, but also make the spray-filled alloy liquid oxidize rapidly, causing black spots and blackening on the surface and inside of the casting.
▌The impact of shot peening on the surface of castings having black spots. At present, the shot peening process at die casting companies mostly uses stainless steel shot, zinc shot and aluminum shot. The effect of shot peening with varying materials is different. Most companies choose stainless steel shot for shot peening, but the used shots are not regularly filtered and dust-treated. Therefore, the alumina dust will adhere to the surface of the steel shot which has been worn down due to long-term use. When shot peening strikes the casting parts, the steel shot will also strike the alumina powder on the surface of the casting, resulting in the overall color darkening of the casting surface.
In addition, the water and dust in the air will also be sprayed on the surface of the casting during shot peening. At the same time, the casting will heat up by the shot, and the rising temperature will accelerate the oxidation reaction on the surface of the casting, so the casting may soon turn black.
Therefore, we should wear gloves when operating the castings after shot-peening. If we touch the castings directly by hand, especially in summer, the surface of the castings will have hand marks which cause blackening. Because the die casting is in a high temperature state after demoulding, a layer of oxide film will form on the surface quickly. After shot peening, the oxide film disappears, and the casting surface is more prone to oxidation defects. In this case, the handling of the die casting parts after shot peening should be dustproof, moisture proof, waterproof, oil proof, sealed and packaged, and the surface treatment should be carried out quickly. (To be continued) |