Efficient spraying in high pressure die casting

The potential for savings in a high pressure die casting (HPDC) process is becoming increasingly limited. Significant improvements are achieved by companies in thermoregulation and die spraying, which are closely interlocked. Here, not only resources but also costs can be saved.

Nowadays, all foundries can implement effective spraying, but is this spraying efficient? Effectiveness shows the ability to achieve the set goal, while efficiency shows the ability to achieve the set goal with the least number of resources.

Precision and repeatability are crucial

As far as die spraying is concerned, new release agents (oils or concentrates) are available on the market that change the spraying philosophy: Whereas in the past spraying was used to cool the die surface, today spraying is only used to create the release film between the die and the casting.

In this case, the differentiator is the technology used to apply these products: To achieve maximum results, the release agents must be micro-dosed. The precision and repeatability of micro-spraying is critical and can only be achieved with appropriate technologies.

Spraying technology is geared to the goals

Wollin's spraying technology can reach the target in different ways that adapt to the different needs of the foundry, whether long-term production or frequent tool changes.

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The objectives are multiple:

reduce cycle time
reduce release agent, water and air consumption
extend die life and increase availability
improve quality of castings
reduce scarp

Experience shows that a conversion of casting processes takes one to two years and ties up capacities in process technology. Those who take this on will be rewarded with more stable processes, a reduction in the scrap rate and better casting surfaces.

Where efficiency potentials are buried...

The die casting process has been established for many years and itself offers little potential for savings. It is the periphery that continues to offer opportunities for optimization. In addition to furnaces, die tempering and the spraying process should be explicitly mentioned here.

Multi-circuit temperature control units, in which each individual cooling channel can be controlled, the use of jet cooling systems as well as 3D printing in production create new possibilities. Even more approaches can be found when companies incorporate more temperature-resistant release agents for the spraying process. This is greatly improved if it only has to ensure good release properties and good demouldability.

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Mould temperature control — <div>Die tempering</div>

Spraying without cooling

Water- and oil-based multi-channel die temperature control systems enable a casting process that is much more thermally stable than in the past. This means that the temperature difference of the die - between the filling phase and the solidification phase - is significantly lower. Thanks to temperature control channels close to the die and jet cooling systems for squeezers and hotspots, the die no longer needs to be cooled by the spraying process.

The significantly lower cooling of the die saves energy, as the die has lost much less heat energy after the part has been removed and sprayed. This does not have to be supplied again.

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New release agents allow greater temperature spread

The new water- or oil-based release agents that have been developed in recent years allow a significantly greater temperature spread of up to 400 degrees Celsius. Depending on the application, wax-containing, wax-free or even wax-reduced release agents are available. What they all have in common are very good release properties.

Wax-reduced release agents also offer improved demoulding lubrication, a significant improvement in surface quality, low machine contamination, clean surfaces and reduced scrap rates.

Mould temperature before and after spraying — <div>Die temperature before and after Spraying</div>

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The ideal spraying process

Ideally, a spraying machine with a high payload that can support a mask spraying tool adapted to the die is used for micro-spraying. Separate pressure control for blowing and spraying air is just as advantageous for a good result as a booster station for control air pressures of up to eight bar.

When the spraying tool is moved into the die, the two die halves are blown out at high pressure. The masking tool, which is adapted to the die and equipped with DDV nozzles, ensures an even application of release agent during spraying. Patented dosing nozzles are used for micro-spraying, which are available with different dosing volumes so that a suitable release film is achieved for every part of the die.

The advantages of good release agents

Today's release agent concentrates have excellent flow properties, so that even areas of the die that are difficult to reach receive sufficient release agent application. The release agent is applied in one impulse: The spray nozzle in the chamber sprays the release agent into the die in a very finely atomized form under high pressure.

The air pressure must not be set too high so that the release agent does not evaporate ineffectively. Contrary to initial assumptions, ionization of the release agent and die is not necessary, but in many cases even a hindrance. Due to the course of the field lines, most of the release agent is deposited in the front area of the die.

Due to the fine atomization of the release agent, there is practically no Leidenfrost effect during micro-spraying. Another advantage of applying only the amount of release agent required for the release effect is that only very small amounts of substances are measured in the air.

Afterblowing after spraying is unnecessary, as no excess water was sprayed. The formation of cracks due to the thermal shock of conventional spraying is avoided, which results in a significant extension of the die service life.

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No water, but more quality

The lack of water during spraying enables the release agent to be distributed evenly in the die, which contributes to a significant improvement in part quality. The surface and the microstructure become much more homogeneous, practically no blowholes are formed, and porosity is reduced to a minimum.

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Reducing the carbon footprint in the die casting process

Today, the awareness of environmentally compatible and resource-saving production is greater than ever. The aluminium industry already has a major advantage in this respect thanks to the very good recyclability of die-cast parts.

However, the classic casting process has a not inconsiderable environmental impact, which is also reflected in the costs. Worth mentioning here are the energy costs due to the cooling and reheating of the die, as well as the enormous water and compressed air consumption during spraying.

With micro-spraying, on the other hand, only very small amounts of the release agent are evaporated, no fresh water is needed and, crucially, no waste water is produced. The consumption of energy-intensive compressed air is reduced enormously. This makes the die-casting process much better in terms of its carbon footprint.

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Representation of the environmental and cost aspects

Environmentally friendly processes save costs

The environmentally friendly process of micro-spraying creates considerable cost advantages with only slightly higher investment costs for the dies and their temperature control. The energy savings in die temperature control often achieve a return on investment in just a few months. The generation of compressed air is very expensive; a saving of 70 to 80 per cent offers enormous financial relief.

The costs for fresh water treatment are eliminated, the release agent mixing in the foundry is no longer necessary. In addition to cost savings, the risk of bacterial contamination is considerably minimized. The expensive disposal of waste water is completely eliminated. New foundries are already being designed without the equipment for waste water disposal.

By eliminating the need for afterblowing during the spraying process, up to ten seconds or more of cycle time can be saved. The extension of the die service life also ensures considerable savings for the maintenance or renewal of the die.

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