VACUUM DIE CASTING DEFINED

30 VACUUM DIE CASTING the liquid alloy. 2 The vacuum die casting process minimizes gas entrapment by removing gases from the cavity generated by two of these mechanisms. Both air in the die cavity and gases gener- ated by the decomposition of lubricants can be removed using the vacuum die casting process. In conventional die casting, gases are typically vented from the die. However, the amount of gas that must vent from the dies is much greater than that of just the die cavity. All gases in the runner system must be vented as well as any volume of the shot sleeve not filled with metal. When examining the volume of gas that must be evacuated from the die combined with the short cycle times of conventional die casting, one finds that it is virtually impossible for all gases to exit the die before die fill is complete, as presented in the following sample calculation. Example Calculation 3.2 Using the conventional die casting process, a component is man- ufactured with a four-cavity die in a commercially available alu- minum alloy. Each casting weighs 0.5 kg. The runner system with all four components attached weighs 4 kg and includes a 5.0-cm- thick biscuit that is 7.5 cm in diameter. The length of the shot sleeve is 100 cm, and the length from the cover die end to the edge of the pore hole is 75 cm. Within the die are eight vents that are 3 cm in length and 0.1 mm in width. Given that the density of aluminum is 2.7 gcm 3 , answer the following questions: 1. Assuming no porosity, what is the volume of the runner system with all four components attached? 2. What is the volume of the shot sleeve from the pour hole to the die? 3. What is the volume of the entire system, including the four component cavities, the runner system, and the shot sleeve? 4. What percent of the shot sleeve volume is filled with metal for each shot? 5. What percent of the system volume is filled with metal for each shot? 6. How much gas must be managed in this example? 7. For a 1-sec shot, how fast must the gases flow through the vents?