OPTIMIZATION OF DIE VENTING DESIGN THROUGH ANALYTICAL CALCULATIONS

Air entrapment during diecasting die filling is a major problem that causes most rejects, after quality inspection, due to unattended and dangerous gas porosity, especially in high performance and structural parts. In the past years, many technologies were developed in order to reduce air entrapment during filling; among them, optimal results were mainly obtained by active (vacuum) and passive (chill vents) air evacuation systems. Nevertheless, only small knowledge on how to optimize dies design for those technologies is available for the die maker. In the same way, at now, many casting simulation software lacks in this specific field and today ventings, vacuum channels and chill vents are designed according to proprietary formulas and, mainly, experience. In order to understand what happens to air inside a die cavity before and during metal injection, an analytical model was developed to calculate and forecast air pressure and flow through vents. This model takes into account, in a very detailed way, all the phenomena that occurs during injection, with particular relevance to process parameters (injection speeds and strokes, temperatures, vacuum pressures…) as well as flow-related phenomena that can impact on air escape, like premature vents clogging. This mathematical model was then validated and optimized through extensive casting trials in different venting simulation (with vacuum, only vents, chill vents, no air escape…) on different casting shapes and with different process parameters. The model allows to the die designer to optimize vents and related operating process parameters, simulate and compare different air evacuation solutions and forecast the mean gas porosity fraction in the casting with a calculation time of only few seconds. Calculations were useful to demonstrate the influence of process parameters on air trapping and the main result was that venting (with or without vacuum) is ineffective without an optimal setup of casting parameters and a correct air escape area calculation.