BASIC PRINCIPLE OF VACUUM-ASSIST CASTING
Vacuum-assist operates by means of a pressure differential. A vacuum pump creates a vacuum atmosphere within a receiver tank that will contain less matter than the ambient atmosphere. The volume of air that is present within the cavity and shot sleeve (Vc) will move to fill the lower pressure receiver tank at a theoretical maximum speed (vs) of 340 m/s. The exit flow rate of air (Rv) from the die is limited by the smallest area of the exit orifice (A) in the mold set. If the volumetric flow rate of air is assumed to be constant, then the time required to evacuate the cavity (te) can be approximated as:
When utilizing Valve-Less Vacuum Blocks, the pathway between the vacuum receiver tank and the die is governed by a solenoid valve. The solenoid valve is opened using a PLC, limit switch or timer when the pour hole of the shot sleeve is covered by the plunger. Valve-Less Vacuum Blocks allow the pathway to remain open throughout solidification to simplify process planning.
SIZING VACUUM PUMPS AND RECEIVER TANKS
Correctly sizing vacuum pumps and receiver tanks for specific applications is critical to proper function. Vacuum pumps generate the vacuum level within a defined volume that is the receiver tank. Vacuum level is measured in relation to ambient atmospheric pressure and reported in units such as Torr, inches of mercury (“Hg), Pascals (Pa) or millibar (mb).
The critical feature of the pump is the speed at which it can evacuate a volume of air down to a particular vacuum level as measured by average cubic feet per minute. If the pump is too small, it will not be able to achieve sufficient vacuum within the shot cycle. If the tank is too small, it will not provide a sufficient reservoir to evacuate air form the cavity. Midland Technologies can calculate the required pump and tank size for your specific casting application when you submit information using our Pump Sizing Form.
CHECKLIST WHEN UTILIZING VACUUM
- Vacuum pump is properly sized for the casting application
- Correct size and quantity of vacuum blocks provide sufficient area for evacuation
- Proper vacuum block runner design from the cavity to manage velocity
- Vacuum blocks are correctly installed in the die
- Proper filtration between the press and the vacuum pump
- Proper maintenance is performed on vacuum blocks, pump, filters and hoses
- Floor operators understand how to turn the vacuum process on and off