Since the advent of the gravity casting and low pressure casting method, people recognize the significance of the metal coating. Choice and control to the application of the best metal paint, or paint a reasonable combination, as metal design, metal processing and The gating system, plays an important role in the production fine castings.
The fundamental role of the functions and features of the two coatings of metal coatings:
• protect the metal mold, to extend the life of the die;
• conducive casting mold release to prevent the accumulation of bonding and oxide;
• control by heat transfer to the metal mold casting alloys.
• improve the surface finish of castings.
Need to be controlled due to the production of good quality castings cast alloy filler metal mold cavity and the whole process in the solidification of the metal mold cavity, so the most important function of the metal coating is undoubtedly so that a metal mold having a heat insulating layer, and to control the heat flux from the cast alloy to The transfer of the metal cavity.
The thermal insulating properties of the coating mainly depends on three key factors:
(1) coating composition;
(2) coating thickness;
(3) coating porosity.
2.1 metal paint coating components is mainly based on water as the carrier, to join the high temperature binder and refractory filler insulation coating containing the adiabatic mineral mixes, paint filler should be enough refractoriness in the normal temperature of the aluminum alloy , to ensure that it does not undergo chemical changes, basically only the physical effect. metal casting in the choice of a paint or a paint component for the production of concrete castings, the need to consider the most critical coating properties: thermal conductivity, particle size, lubrication , to ensure that the cast is easily separable and smooth surface, and to ensure that the coating has a high thermal insulating properties in order to prevent casting misrun. the main component of the preparation of the insulation coating is alumina, talc, mica, diatomaceous earth and titanium oxide , these minerals having a high insulation resistance.
2.2 thickness of the coating
300μm thickness of the coating, adiabatic gravity and the development and application of the low-pressure casting coating is not more than 200μm coating to improve a lot, and because more than 300μm coating easily and metal type separation and peeling, and therefore the optimum thickness of the coating should be 150 ~ 250μm.
2.3 coating porosity
Coating depends mainly on the porosity of the formation with a metal mold cavity surface in contact with the evaporation speed of the carrier water, i.e. depending on the coating method and coating conditions. Usually the mold temperature is 170 ~ 200 ℃ coating can often be Most foundries use of paint or using artificial stirring, then pour off the use of part of the dilution at the production site to get the best results., many human factors, often lead to imbalance paint ingredients, sometimes binder imbalance uneven performance of insulation and coating off the metal. many foreign factory has been set up at the production site center mixing station, coatings to keep stirring diluted and has been central mixing station, thus ensuring that its composition is uniform, so that the coating uniform deposition on the model.
When coated at higher metal temperatures, due to the strong reaction between the coating of the carrier water and the high temperature metal, usually can be obtained a great porosity, then coating adiabatic naturally good, but the coating on the casting The type of adhesion and life are low, this is because of the high metal temperature will cause strong "rebound" so that the coating is difficult to adhere to the metal mold cavity surface sake.
Conversely, when the type of lower temperature, although the coated high life, but such coatings dense, poor thermal insulating, and low metal coating temperature will form the coating "stacked", when the metal mold is heated to the working temperature When will produce defective coating cracking and peeling