ELEMENTS OF VACUUM DIE CASTING

36 VACUUM DIE CASTING Oil Mist Eliminator Exhaust Gas Ballast Inlet Inlet Screen Anti- Suckback Valve Vane Rotor Oil Return Line Oil Return Valve Exhaust Valve Spin-On Oil Filter Main Oil Feed Line Figure 3.4 Illustration of a rotary vane vacuum pump. Courtesy of Busch, Inc. pump cylinder is an eccentric rotor that pulls the vacuum. As the rotor turns, gases are trapped and compressed between several vanes and the walls of the pump cylinder. The compressed gases are discharged into the exhaust box. The gases then pass through an oil eliminator that extracts oil vapors from the exhaust gases prior to discharging them to the environment. Numerous vacuum pumps are commercially available, as shown in Figure 3.5. Often the systems are portable, making for easy placement within the die casting facility. In vacuum die casting, a shut-off valve is needed to prevent liquid metal from entering the vacuum pump. A runner is used to 3.4 VACUUM DIE CASTING MANUFACTURING EQUIPMENT 37 Figure 3.5 Examples of portable vacuum systems for use in vacuum die cast- ing. Courtesy of Fondarex Corporation. connect the vacuum shut-off valve to the die cavities. The gate connecting this runner to the cavity should be located at the last location in the die cavity to fill, as discussed in Section 3.2. Vac- uum shut-off valves fall into two distinct categories: static and dynamic. Static vacuum shut-off valves have no moving parts and utilize a thermal gradient to protect the vacuum system. The most com- mon type of static vacuum shut-off valve is a corrugated chill block such as the one shown in Figure 3.6. Static vacuum shut- off valves are basically oversized vents chilled with multiple cool- ing lines connected to a vacuum pump. Although gases may pass through the vent, the chill solidifies liquid metal, filling the die 38 VACUUM DIE CASTING Multiple Cooling Lines Corrugated Chill Block Figure 3.6 Schematic of a corrugated chill-block-type vacuum shut-off valve. before it reaches the vacuum system. The use of a corrugated geometry forces metal to change direction numerous times as it passes through the vent. Each turn of the metal slows the fill front and aids in transferring heat into the chill. This promotes solidi- fication and protects the vacuum pump. Static-type shut-off valves are low in cost and are easy to main- tain, as they contain no moving parts. Moreover, the vacuum pump may remain engaged and running throughout the fill, thus maxi- mizing the amount of gases extracted from the die. Static vacuum shut-off valves have several shortcomings. Al- though static shut-off valves may be easy to maintain in the die room, maintenance during production is an issue. The valve must remain clear. Flash build-up within the chill block can impede gas flow. Furthermore, a typical static vacuum shut-off vent has a width of 5–10 cm and a gap of 0.5 mm. Although a vacuum system may be capable of pulling a strong vacuum, the static valve is a bottleneck, stifling gas flow out of the die. This is illustrated in Figure 3.7. Experimental tests showed nearly a 0.5-sec lag be- tween the vacuum pump and die cavity when using a static chill- block-type vacuum shut-off valve. Dynamic shut-off valves offer less resistance to gas flow than static types due to their larger cross sections. Numerous designs 3.4 VACUUM DIE CASTING MANUFACTURING EQUIPMENT 39 Figure 3.7 Experimental test data showing the pressure lag when using a cor- rugated chill-block shut-off valve 1200 cm 3 volume with 0.4 cm 2 X section. Courtesy of IdraPrince. have been developed, including mechanical valves and actuated valves. Mechanical valves are the simplest of all dynamic vacuum shut- off valves, as pictured in Figure 3.8. When the die is closed, the mechanical valve is opened. Once the pour hole is closed by the plunger tip, the vacuum is applied and evacuation of the die cavity and runner system begins. As the liquid metal fills the die, cavity pressure is used to close the shut-off valve. Mechanical valves remain open throughout cavity fill and close only after a preset cavity pressure is achieved. Closure of the mechanical valves typ- ically occurs in 8–10 msec. Actuated valves offer the most control in vacuum die casting as they are linked to electronic controllers. Actuated valves can either be electrical or hydraulic, as shown in Figure 3.9. The elec- tronic control system monitors the position of the plunger. Once the pour hole is closed by the plunger tip, the electronic control system opens the valve, applying a vacuum to the die cavity. Metal enters the die and the valve is shut by the electronic controller at a preprogrammed time before metal reaches the valve. Actuated valves remain open throughout cavity fill and can be closed before metal reaches them. These valves have much larger cross-sectional areas and offer little impedance to the evacuation of gases in the 40 VACUUM DIE CASTING Figure 3.8 Example of a mechanical vacuum shut-off valve. Courtesy of Fon- darex Corporation. die cavity. This is illustrated in Figure 3.10. Electrical valves typ- ically close in 8–10 msec; however, hydraulic valves have a much slower response time, closing in 120–150 msec. As with any die casting process, shot control is essential. Often the shot control systems currently available on conventional die casting machines may be used to control a vacuum system.

3.5 APPLYING VACUUM DIE CASTING

Vacuum die casting builds upon conventional die casting practices by minimizing the effects of a major contributor to porosity. The cycle time and economics of vacuum die casting are equivalent to conventional die casting. The only economic penalty in using vac- uum die casting is the capital cost of the vacuum system and its operation. These additional costs, however, are minor in compar- ison to increased component integrity.

3.5 APPLYING VACUUM DIE CASTING