The heat flow from the melt plays an important role in the quality of die castings. In fact, a die temperature that is not optimal for casting is the greatest primary source of error in the process. The use of temperature control units should therefore be the first key measure in process optimization. This article explains how the units work and what benefits they bring.
The thermal conditions in the mould should be well understood, because mastering them has a decisive influence on the quality of the die cast parts.
Heat balance of the die casting die
During a cycle, exactly the amount of heat is released from the die to the environment that is introduced by the melt until demoulding. The amount of heat absorbed by the heat transfer medium is dissipated via the temperature control unit. Additional heat dissipation takes place through radiation, heat transfer to the air and heat conduction into the clamping platens.
The task of die temperature control
The die temperature is important for the heat dissipation from the melt, for die filling and for the solidification of the die cast part. Various factors such as melt temperature and filling time also have an influence on part quality. However, experience shows that a very high proportion of scrap is primarily due to unsuitable die temperatures. To prevent this, the temperature control unit
- heats the die to operating temperature
- keeps the die at operating temperature
If the temperature control is ideally adjusted, this literally pays off through optimum cycle times, a long service life of the mould and a consistently high quality of the die cast parts.
Mastering the thermal processes in the die casting process is a prerequisite for consistently high part quality. If the die temperature is not correctly adjusted, this has an immediate effect on the part quality. Some typical examples from practice:
Die temperature too high
If the parts are difficult to demould because they are deformed or stick to the die, this is often due to an excessively high die temperature. More release agent is often sprayed as a remedy. However, this raises additional problems: The formation of pores in the cast part increases. At the same time, the release agent burns on the mould surface, so that the release properties are lost.
Longer cycles and heavy die wear are also typical of excessively high mould temperatures. Shrinkage cavities and poor dimensional accuracy result in unnecessary scrap, which is reflected in poor productivity.
Die temperature too low
Die walls that are too cold also impair the lubricating effect of the spray agent. Using more of it does not help, as the substance is generally not properly adsorbed by the die surface. This also results in poor dimensional accuracy and, above all, incomplete mould filling. This effect is particularly strong after production interruptions.
Thermal shocks during material feed shorten the life of the die, which results in increased maintenance costs. Quality losses can also occur due to premature solidification of the melt, streaking or material overlap.
Remedy: Adjusting temperature control
Such effects can occur with all alloys. However, they are differently pronounced depending on the alloy. A temperature control technology that is optimally adapted to the process reduces such cost-intensive effects to a minimum.
A temperature control system consists of the following four components
- temperature control channel system in the die
- temperature control unit
- heat transfer medium
- connections between unit and die
In order for them to work together optimally, various conditions must be met.
The die itself requires a sufficient temperature control channel surface as well as channel cross-sections that are dimensioned as large as possible. This is necessary to keep the pressure losses in the die low. A standard pump in the temperature control unit is usually sufficient. Any additional costs in die making are more than compensated for by shorter cycle times and better part quality.
The temperature control unit must keep the die temperature within narrow limits by providing optimum regulation. For this purpose, heating, cooling and pump capacity must be adjusted to the respective requirements.
The heat transfer medium should have good heat transfer properties in order to dissipate large amounts of heat in a short time. Operation with water is preferred, as it ensures a much better heat transfer compared to oil. When working with oil, the generous dimensioning of the temperature control channels plays an important role in order to compensate for this factor.
The connecting lines between the unit and the die need the right cross-section and good thermal insulation. The cross-section of the supply lines should correspond to the connection for outlet and inlet flow. If not, the pressure will drop and the supply line will not be able to provide the amount of heat transfer medium that is actually required in the die. This would unnecessarily reduce productivity and part quality.
When selecting the temperature control unit, the following criteria determine the performance profile of the temperature control system:
|processed material||die temperature|
|mould temperature||heat transfer medium (water/oil)|
|die weight & heating time||heating capacity|
|&quantity processed per time unit||cooling capacity|
|temperature gradient over the die||flow rate|
|pressure and flow conditions in the die||delivery pressure of the pump|
Temperature control in the process
The temperatures in the die vary locally and change periodically with the casting cycle. It is very important for the quality of the die cast part that the level of the periodically changing temperature remains constant over the entire cavity.
The temperature control system keeps the preheating temperature of the mould constant during production while limiting the maximum surface temperature. It ensures that the differential band of these two temperatures does not drift up or down during production. Three control modes are available for this purpose:
- Control of the medium temperature
This method is the most commonly used. The control accuracy is sufficient in many cases.
- Control of the die temperature
A temperature sensor is installed for this purpose. This method is necessary if the die temperature has to be kept within particularly narrow limits and fully automatic operation is required.
- Cascade control
This method combines the control modes described above and further improves the temperature control in the mould.
Correctly designed temperature control units offer numerous advantages for production. Used in a targeted manner, they help to run the production process economically at various levels.
Increased die life
Uniform heat distribution ensures considerably longer die service life. Stress cracks caused by local overheating are avoided; cores do not overheat either. Maintenance intervals are extended so that maintenance costs and downtimes are reduced to a minimum.
Lower production costs
Other advantages are shorter heating times and lower release agent consumption. When using heat transfer oil, there is no calcification or corrosion of the temperature control channels.
Fast and targeted heating before the start of production ensures short preparation times and minimum scrap at start-up. After interruptions, the process quickly runs stable again, as the die is kept warm. Automatic temperature control ensures fully automatic operation if required and makes the control independent of the experience of the operating personnel.
Correctly used, temperature control units guarantee high quality castings. Constant and high dimensional accuracy are standard as well as reduced shrinkage cracks and a clean surface of the castings. Temperature control units therefore save significant costs due to less scrap, less maintenance and less repair costs.