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THERMAL BALANCING AND POWDER DIE LUBRICANT PROCESSES Prone to Soldering
Martensite
Bulk Die Steel Transition
Transition to Bulk Die Steel
Heat-affected Zone AnnealedSoft
Martensite
Figure 6.3 Die surface cross section illustrating microstructural weaknesses in
a welded die surface.
well as flash. For components with stringent surface requirements, dies are often designed for easy maintenance. Die cavities are
often replaced when heat checking occurs. This leaves the major- ity of the die untouched. Even severe heat checking of the runner
system does not affect component functionality. The most costly solution to resolve heat checking is the complete replacement of
a die.
To combat soldering, many strategies may be used. In some cases, a slower cycle time may eliminate the problem, allowing
water lines to cool the die. More often, component producers look to liquid die surface lubricants. Longer spray times may be used
to apply additional lubricant across the entire die surface. This may overlubricate the die. An alternative is to spray additional
lubricant on select areas of the die prone to soldering. Component metal chemistry may also be adjusted. Additions of iron can re-
duce the soldering potential of an alloy. However, iron additions have a negative effect on mechanical properties. In many cases,
once soldering occurs, it cannot be totally eliminated. Re-
6.5 THERMAL MANAGEMENT OF DIE CASTING PROCESS TOOLING
105
placement of die cavity inserts may be the only solution in these cases.
6.4 REPERCUSSION OF HEAT CHECKING AND
SOLDERING CONTAINMENT ACTIONS
Containment of heat checking and soldering problems does have repercussions. The most obvious is cost. The replacement of die
inserts is costly. Beyond the cost of the tooling, resources must be expended to measure and qualify tooling for dimensional ac-
curacy and functionability. Longer spray times increase cycle times. This reduces productivity and increases materials cost
through the use of additional lubricant.
In some cases, the containment actions for combating soldering induce other defects. Spraying hot spots on the die surface to
resolve soldering problems induces additional thermal cycling and heat checking. Additional lubrication on the die surface may com-
bust and form porosity in components during processing.
The root cause of heat checking is thermal cycling. The root cause of soldering is variation in die surface temperature during
processing. All of the strategies suggested contain the problems of heat checking and soldering. None address the root cause.
6.5 THERMAL MANAGEMENT OF HIGH INTEGRITY
DIE CASTING PROCESS TOOLING
Although most component producers work to optimize die cooling to eliminate hot spots and accelerate solidification, few work to
minimize variation in die surface temperature. Minimization of die surface temperature variation, or thermal balancing, can re-
solve several processing issues.
Cooling lines typically account for 90 of all heat removal from a die during processing. Numerous tools are available to aid
in thermal balancing and die surface temperature control. Com- puter models have evolved to a point where cooling line placement
can be optimized before dies are built.
2
Infrared cameras are a
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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
