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Problems in the use of mold release agent
When the die casting parts are rejected in the die casting factory, it is seldom analyzed from the rationality of mold release agent selection and the correctness of spraying process. When a new mold release agent is used, if the traditional experience and operation habits cannot be accommodated, it will be denied immediately. So there is often such a confusing situation: die casting the same kind of casting, one is handy for using a certain mold release agent, while the other is not good at complaining. The reason lies in the lack of understanding of the characteristics of mold release agent and the neglect of the problems that should be paid special attention to. The following four aspects should be noted.
(1) Relationship between mold temperature and type of mold release agent
The basic components of mold release agent are mineral oil and paraffin. The suitable surface temperature of mold is different with different proportion. Taking the mold release agent for aluminum alloy as an example, according to the requirements of mold temperature, it can be divided into three categories: low temperature (below 200 ℃), medium temperature (200-250 ℃), high temperature (above 250 ℃). According to the characteristics of the formation of the film, it can be divided into the adhesion between the effective component and the cavity, the dryness of the film and the influence on the coating of the casting. For example, non silicon mold release agent is widely used in castings with high coating requirements, but if the mold temperature is not well controlled, it will not play its advantages in this respect. Although the classification difference of die temperature only changes in 20% of the range of die temperature, which does not prevent the formation of skin film, it has a great influence on the demoulding property, surface quality and die life of castings.
(2) The relationship between water ratio, mold temperature and adhesion
Only a few of the water-based release agents adhere to the surface of the mold cavity, most of them are dispersed by the phenomenon of vapor film. The test results show that under the condition that the effective solid content (mass fraction) is not less than 0.2% ～ 0.3%, the larger the dilution ratio is, the more the spray quantity is, the more the effective adhesion quantity is, and the larger the adhesion area is. In this way, we can explain why the mixing rate is too low, but it will stick mold and waste products. At the same time, with the increase of mold temperature, the adhesion and adhesion area decrease. Experience shows that the ideal mold temperature should be less than 250 ℃ after spraying, and 180-200 ℃ is the best. The film is formed within 1 s when the temperature is lower than 270 ℃.
(3) Relationship between spray technology and film quality
In order to ensure the uniform formation of skin film, spray technology 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 release agent will rebound and lose due to the high spraying velocity; if the distance is too large, the spray release agent will fuse into a large droplet, and the impact force when it falls may damage the uniformity of the skin film. Due to the high mold temperature during spraying and the low mold temperature during the formation of skin film, the phenomenon of condensation 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 minimized 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 the large-scale die, the device which can automatically track the running track of the nozzle is better.
(4) Water quality problems
The quality of dilution water of water-based release agent is easy to be ignored. Release agents such as Italian levelit and marbo chemical manufacturing company's products, in addition to having a complete set of production process and quality detection system, have strict control over the purity, soft hardness, pH value, mixing water temperature, etc. of water quality. Because if unqualified water is mixed in the release agent, it will lose its proper characteristics. For example, when the number of bacteria in 1m3 water reaches 105, the mold release agent may deteriorate, resulting in the increase of scrap rate of some sensitive die castings. For example, when water containing impurities or impure ingredients is added into the original concentrate, the gas content in the die casting increases, which has obvious influence 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.