Modelling the Impregnation of a Pressure-Tight Casting

Abstract

AbstractPressure tightness is important for many die-cast aluminium castings, but the interconnected porosity formed in the aluminium alloy high-pressure die castings (HPDC) can form a network connecting several surfaces of the casting, especially as a result of machining after casting. The resulting potential leakage path can be imagined as a series of discontinuities varying in size over wide limits, where voids of different volumes are connected by channels of smaller cross section. Although vacuum impregnation is often used to ensure pressure-tightness is a robust technology, cavity systems through the casting can still cause leakage after impregnation. This study addresses the issues of the impregnation of discontinuities with a complex geometry. The elementary part of a complex cavity geometry consists of a cavity and a capillary channel. We estimated the limited permeability of the capillary section using a mathematical model based on the Hagen–Poiseuille equation. The calculation results prove that the degree of filling of the cavity with a capillary section is basically a function of the geometrical characteristics of the cavity, primarily the radius of the capillary. If the cavity system also contains a section with a diameter of less than 28 μm, impregnation does not necessarily ensure the gas tightness of the casting. Depending on the radius of the capillary, the impregnation of a cavity of a given volume and connected to the surface through a capillary section of a given length can produce three different results. Above a certain capillary radius, filling is complete, below a certain capillary radius only the capillary itself will contain resin, and between the two limit values, the partial impregnation of the cavity is expected but the capillary section is not filled.