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对铸型表面既要求有一定的透气性,又要求密实和低孔隙度。这个固有矛盾,在很多情况下,我们是通过使用耐火的铸型涂料来解决的。这些涂料有光滑的不可渗透的表面,可以防止金属渗入到型砂中去。对低熔点合金铸造,涂料常是以很细的耐火氧化物、硅酸盐和炭质材料为基的。按理想来说,涂料在力学方面应该是稳定的,有一定的抗热冲击性,对液态金属是不活泼的,对型砂无污染。一种涂料既可做成浆料使用,也可做成干料使用。对前一种情况,耐火颗粒是悬浮在液体介质中的,在其中还加入其他物质以帮助悬浮和起粘结作用。作为涂料和填料的某些耐火材料见下表:
Requires a certain permeability of the mold surface, but also requires dense and low porosity. This inherent paradox, in many cases, is solved by using a refractory mold coating. These coatings have a smooth, impervious surface that prevents metal from penetrating into the molding sand. For low melting point alloys, coatings are often based on very fine refractory oxides, silicates and carbonaceous materials. Ideally, coatings should be mechanically stable, have some thermal shock resistance, be inert to liquid metal, and have no pollution to the molding sand. A paint can be used as a slurry, can also be used as dry material. In the former case, the refractory particles are suspended in the liquid medium, where other substances are added to aid in the suspension and bonding. Some refractory materials as coatings and fillers are listed below: