The Thermal Issue

In order of importance, the three critical control factors in the die casting process are as follows:

 

1.   Thermal –Die casting is inherently a thermal process—melting metal, injecting metal into a die, and then removing heat energy from the metal to achieve a solid state.  The productivity, quality, and dimensional accuracy of the casting process are all driven by thermal relationships.

 

2.   Injection – High pressure die casting is unique among casting processes in that the liquid metal is forcefully injected at high injection velocity.  Control of the injection process is important to the casting quality. 

 

3.   Timing – The casting machine cell cycling provides the basic construct of the timing of the process, which drives productivity. 

 

Injection and timing have been addressed and continue to be addressed through myriad tools, applications, and process improvements (e.g., shot-end monitoring systems).  As such, process variation due to these factors has been significantly reduced.  Further investment in technologies and process improvements in these areas results in continually diminishing benefits.

 

In contrast, the thermal aspect of the die casting process has been addressed only minimally by comparison.  A few of the reasons for this are as follows:

 

  • The scientific, mathematical, and engineering concepts involved in the thermal aspects of the die casting process are complex.
  • Thermocouples have significant problems with design integration, failure, and setup;  further, they measure only discrete points in the die where the thermocouples are located.
  • Simulation software is a valuable tool, but has its limitations issues  and, further, it provides no thermal control.
  • Thermal imaging provides informative data, but the data is only from static points in a continually changing process, and it also provides no actual thermal control.

 

In most die casting processes, thermal control is provided by a cooling system.  The typical process consists of start-up shots to heat the die, after which the cooling system is turned on, shown graphically as follows:

 

 

The net result of the above on the die casting process is significant process variation due to constantly varying thermal conditions.  This variation is exacerbated by the stop and start nature of most die casters’ processes due to down time events.  Most die casters run their process with a constant coolant flow.  Depending on how much bulk heat is in the die, this means that the amount of heat removed during each shot is inconsistent, producing variations in process results. 

 

Because of this lack of control over heat removal, any other attempts at process improvement have only limited effects.  Statistical studies performed by Die Therm at existing customers showed that 80-90% of existing process variation was thermal in nature, shown graphically as follows:

 

This further underscores the fact that investments in non-thermal process solutions only address a small portion of existing process variation.  Major improvements in process control can only be achieved through thermal control.