106
THERMAL BALANCING AND POWDER DIE LUBRICANT PROCESSES
noncontact method of measuring die temperature. Some infrared cameras are capable of examining entire die faces for easy analysis
of temperature variation. Closed-loop coolant temperature control units are also commercially available to better control heat flow
out of the die.
Thermal balancing offers component producers several benefits, including
1. a reduced potential for solder, 2. elimination of thermal shock and heat checking at hot spots,
3. reduced cycle time from shorter spray cycles, and 4. less lubricant usage.
Although thermal balancing is a way to control several potential problems encountered in high integrity die casting processes, com-
mitment to upfront engineering is a necessity.
6.6 MINIMIZATION OF THERMAL CYCLING
EFFECTS WITH POWDER LUBRICANTS
Powder lubricants are one method of minimizing the magnitude of thermal cycling in high integrity die casting processes. Instead
of applying a lubricant with a liquid carrier, the lubricant is ap- plied dry. This eliminates the thermal shock caused by spraying
a room temperature liquid on the die face. Powder lubricant meth- ods, however, rely entirely on cooling lines to remove heat from
the die surface. Thermal balancing is a necessity when using pow- der lubricants.
Successful application of powder lubricants to the die surface is not intuitively obvious. Traditional open die spray methods pro-
duce unacceptable levels of particulate matter in the air. To min- imize this environmental issue, experiments have been performed
in which the powder lubricant is electrostatically charged. Al- though electrostatic methods reduce particulate matter in the air,
overspray is a major issue. The best results have been obtained with a novel closed die process.
Application of a powder die lubricant with a closed die requires the use of a vacuum and modification of the shot sleeve for a
6.6 THERMAL CYCLING EFFECTS WITH POWDER LUBRICANTS
107
Vacuum Line Vacuum Valve
AirPowder Feed Line
Nozzle
Figure 6.4 Shot sleeve and tool design for use with closed die powder lubricant
application.
powder feed nozzle, as shown in Figure 6.4. The same system used in vacuum die casting can be utilized with powder lubricant
application, although vacuum valve placement may not be com- mon.
Figure 6.5 is an illustration of the powder lubricant casting cycle. Initially, the plunger is positioned in the shot sleeve a
such that the pour hole is closed. A vacuum is applied to the closed die cavity while a controlled quantity of powder lubricant
is metered b into the shot sleeve. The vacuum pulls the lubricant into the die cavity c, creating a thin layer of lubricant on the
surface. The plunger is retracted d opening the pour hole. Liquid metal is metered into the shot sleeve e and then injected f
through a runner system into a die cavity g under high pressure. High pressures are maintained on the alloy during solidification.
After complete solidification, the die opens h and the component is ejected.
Although powder die lubricants are commercially available, their use is not widespread. Some companies have reported ex-
tended die life, shorter cycle times, and improved plant cleanli- ness. The increase in productivity is attributed to the elimination
of spray automation.
