When the die casting production plant encounters the waste of die casting parts, it is seldom analyzed from the rationality of the release agent selection and the correctness of the spraying process. When a new release agent is used, if the traditional experience and operating habits cannot be accommodated, it is immediately denied. So it often appears such a puzzling situation: die casting of the same kind of casting, one of the use of a release agent is very good, and the other one is not good. The reason is because of the lack of understanding of the characteristics of the release agent, ignoring the problems that should be paid special attention to. The following four aspects should be noted.
(1) The corresponding relationship between mold temperature and type of release agent
The basic composition of the release agent is mineral oil and paraffin, and its ratio is different, and the applicable mold surface temperature is different. Taking aluminum alloy release agent as an example, according to mold temperature requirements, it is currently mainly divided into three categories: low temperature (below 200 ° C), medium temperature (200 ~ 250 ° C), and high temperature (above 250 ° C). From the perspective of the formation characteristics of the skin film, it can be divided into the adhesion of the effective component and the cavity, the dryness of the skin film and the influence on the coating of the casting. For example, non-silicon release agents are widely used in castings with high coating requirements, but if the mold temperature is not well mastered, it cannot play out its advantages in this respect. Although the difference in mold temperature classification is only 20% of the mold temperature range, it does not cause the skin film to form, but has a greater impact on the release of castings, surface quality and mold life.
(2) The relationship between water filling rate, mold temperature and adhesion
When spraying water-based release agent, only a fraction of it really sticks to the surface of the mold cavity, and most of it is dispersed by vapor film phenomenon. The test shows that under the condition that the effective solid substance content (mass fraction) is not less than 0.2% ~ 0.3%, the greater the dilution rate, the more the spraying amount, the more the effective adhesion amount, and the larger the adhesion area. This can explain why the water rate is too low but will stick to the mold and waste products. At the same time, with the increase of mold temperature, the amount of adhesion and the area of adhesion decrease. Experience shows that the ideal mold temperature should be lower than 250 ℃ after spraying, and 180 ~ 200 ℃ is better. The skin film is formed within 1 s after spraying at a mold temperature below 270 ℃.
(3) The relationship between the spraying process and the quality of the film
In order to ensure the uniform formation of the skin film, the spray process is very important. To atomize ultrafine, uniform dispersion, strong adhesion. At the same time, the spray distance should be optimized. If the distance is too small, the release agent will rebound and lose due to the high injection velocity. If the distance is too large, the mist release agent will fuse into a large droplet, and the impact force when falling may destroy the uniformity of the skin film. Due to the high mold temperature during injection and the low mold temperature during the formation of the skin film, condensation may occur during the process, so spraying should not be used as a means to reduce the mold temperature, but should minimize the mold temperature difference from injection to film formation. Experience shows that the ideal spray distance is 100 ~ 200 mm, not greater than 250 mm, for necessary point spray, such as cores and sprue, can be shortened to 70 mm. For large molds, it is better to use a device that can automatically track the trajectory of the nozzle.
(4) Water quality problems
The dilution water quality of water-based release agent is easy to be neglected. In addition to a complete production process and quality testing system, the purity of water quality, hardness, pH value, water temperature and so on are strictly controlled. Because, if the release agent is mixed with unqualified water, it will partially lose its due characteristics. For example, when the number of bacteria in 1 m3 water reaches 105, the release agent may deteriorate, resulting in an increase in the reject rate of some sensitive die casting parts. For example, when the original concentrate is mixed with water containing impurities or impure components, the gas content in the die casting increases, which has a significant impact on the production of castings with air tightness or weldability requirements. In general, the original imported release agent should be promptly added with qualified water after opening the lid and used up within a week.
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