Vacuum is indispensable for diecasting aluminum components that have to be heat-treated or welded. In the production process, vacuum is applied during casting to minimize air bubble formation in the solidifying metal. Trapped air bubbles are undesirable because they expand during heat treatment or welding, causing blistering on the surface of the diecastings.
Two German companies, Pfeiffer Vacuum Technology A.G. and the consulting firm, Glimo N.V., have developed Vacu2, a multi-step process that reportedly eliminates the shortcomings of existing techniques with respect to achievable vacuum, process reliability, and process control.
The diagram illustrates the standard version of the new process. During diecasting, two independent but coordinated vacuum steps are generated. In the first step, a vacuum system is connected with the die cavity via the shot sleeve. After this system has been isolated from the shot sleeve, the second vacuum step is initiated by connecting a second system with the die cavity via one of the conventional vent valves.
Under the two-step process, the volumes and outlet pressures can be separately matched and set for each step of the process. The complete independence of the two vacuum circuits from one another eliminates the linkage between the parameters, which increases exponentially the theoretically achievable vacuum. Because the second vacuum step begins with a pre-evacuated die cavity, significantly lower pressures can be achieved with only a fraction of the comparable total system volume that would be required under a single-step process.
The vacuum system volume, initial pressure, and connection cross sections for the first step are designed so that an absolute pressure of some 50 mbar can be achieved in the mold cavity within as little as 0.5-1.0 second. At such speeds, the leakage rate is hardly a factor.
The starting pressure in the second step is far lower than in conventional processes. Because nearly the entire volume of air has been removed during the first step, volume and available time play only a secondary role in influencing the second step.
Variations in process conditions result in only minor changes in the vacuum achieved. An impact analysis was able to demonstrate that changes in one step are attenuated by the other step. As a result of these altered interdependencies, vacuum casting is now fundamentally possible in connection with dies that have higher leakage rates, with slide-equipped dies, less elaborately sealed dies, or dies with very large volumes.
In terms of process control, a system has been developed to monitor and document the main parameter of vacuum, but also changes in other process-governing variables such as leakage and conductivity. The control system includes PC-based process monitoring. Only with a powerful computer was it possible to integrate the sophisticated algorithms into the process control system in order to be able to determine both the vacuum achieved as well as variances in leakage and flow rates for every shot on a near-real-time basis.
During the casting operation, the actual ultimate pressure in the mold cavity, along with changes in leakage rate and conductivity, is displayed. All monitored parameters are documented for each shot. To simplify operation, the process parameters at the beginning of the casting process are suggested when the process control system is set up via a recipes and recipe administration function.
A pilot plant was used to conduct tests on diecasting machines ranging in size from 7,000 to 35,000 kN. Tests were conducted on both sealed and unsealed dies, as well as on dies with multiple slides. The developers report that all application combinations produced excellent results with respect to the vacuum achieved and the quality of the workpieces. They recommend that sealing the dies to a certain degree, which benefits both workpiece quality and process stability. Depending upon the application in question, absolute pressures of 20 -100 mbar were achieved in the die.