Problems needing attention in the use of release agent
When the die-casting factory encounters the scrap of die-casting parts, it rarely analyzes the rationality of release agent selection and the correctness of spraying process. When a new release agent is used, if the traditional experience and operation habits cannot be accommodated, it will be denied immediately. Therefore, there is often such a puzzling situation: when casting the same kind of castings, one is comfortable with using a certain release agent, while the other complains that it is not easy to use. The reason is the lack of understanding of the characteristics of release agent and the neglect of the problems that should be paid special attention to. Attention should be paid to the following four aspects.
(1) Corresponding relationship between mold temperature and type of release agent
The basic components of the release agent are mineral oil and paraffin, and their ratio is different, and the applicable mold surface temperature is different. Taking the release agent for aluminum alloy as an example, according to the requirements of mold temperature, it is mainly divided into three categories: low temperature (below 200 ℃), medium temperature (200 ~ 250 ℃) and high temperature (above 250 ℃). From the formation characteristics of the film, it can be divided into the adhesion between the effective components and the mold cavity, the dryness of the film and the influence on the coating of castings. For example, non silicon mold release agent is widely used in castings with high coating requirements, but if the mold temperature is not well mastered, it will not give play to its specialty in this regard. Although the classification difference of mold temperature changes only 20% of the mold temperature range, which does not prevent the formation of film, it has a great impact on the demoulding property, surface quality and mold life of castings.
(2) Relationship between water mixing rate, mold temperature and adhesion
When spraying water-based release agent, only a fraction of it really adheres to the surface of the cavity, and most of it is scattered by the phenomenon of vapor film. The test shows that the effective solid content (mass fraction) shall not be less than 0 2 %～0. Under the condition of 3%, the greater the dilution rate and the more spraying amount, the more effective adhesion and the larger the adhesion area. This can explain why the water mixing rate is too low, but it will stick to the mold and produce waste products. At the same time, with the increase of mold temperature, the adhesion amount and adhesion area decrease. Experience shows that the ideal mold temperature should be lower than 250 ℃ after spraying, preferably 180 ~ 200 ℃. The film is formed within 1 s after spraying at a mold temperature below 270 ℃.
(3) Relationship between spraying process and film quality
In order to ensure the uniform formation of the film, the spraying process is very important. To atomize ultra-fine, uniform dispersion, strong adhesion. At the same time, the injection distance should be optimized. If the distance is too small, the mold release agent will rebound and cause loss due to the high injection flow rate; If the distance is too large, the mist release agent will fuse into large droplets, and the impact force when it falls may destroy the uniformity of the film. Because the mold temperature is high during spraying and low during film formation, condensation and dripping may occur. Therefore, spraying should not be used as a means to reduce the mold temperature, but the mold temperature difference from spraying to film formation should be reduced as much as possible. Experience shows that the ideal spraying distance is 100 ~ 200 mm, not more than 250 mm. For necessary spot spraying, such as core and sprue, it can be shortened to 70 mm. For large molds, it is better to adopt the device that can automatically track the running track of the nozzle.
(4) Water quality problems
The quality of dilution water for water-based release agent is easy to be ignored. Release agents such as the products of Italian levelit and marbo chemical manufacturing company, in addition to having a complete set of production process and quality inspection system, strictly control the purity, soft hardness, pH value and mixing water temperature of water quality. Because if unqualified water is mixed into the release agent, some of its characteristics will be lost. For example, when the number of bacteria in 1 m3 water reaches 105, the release agent may deteriorate, resulting in an increase in the scrap rate of some sensitive die castings. For another example, when water containing impurities or impure components is mixed into the original concentrate, the gas content in the die casting increases, which has an obvious impact on the production of castings with air tightness or weldability requirements. Generally, the original imported release agent shall be mixed with qualified water in time after opening the cover and used up within one week.