Melting/Forming/Joining

Maximum Efficiency, Economy in Aluminum Casting

November 1, 2013
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Saving energy is of key importance in the energy-intensive melting process. Making use of waste heat is one of the ways to save money.

Melting furnaces, such as that shown in Figure 1, make targeted use of the waste heat from the melting process in order to preheat the material to be melted. For this purpose, they have a special shaft geometry well-known under the EtaMax brand. An adapted burner technology combines the melting phases of preheating, heating and liquefying the aluminum.

    Whereas ingots and recycled material are melted in the lower part of the shaft, the waste-gas heat produced by the melting process simultaneously heats the raw materials in the upper part of the shaft. This effective principle of integrated heat recovery reduces the energy requirements of the actual melting process, achieving substantial fuel-consumption savings. In addition, the rapid melting process at the bottom of the shaft achieves low oxidation loss, high material yield and a good metal quality.

    The molten metal is then conducted into the holding bath without turbulences and with little dross. The efficiency of StrikoMelter technology is increased even more by the new PurEfficiency series. Its development was supported by the German Federal Ministry of Economics and Technology. In the development of this new series, engineers employed the latest numerical simulation tools. Using computational fluid dynamics (CFD), they visualized gas flows, temperatures and heat transitions, achieving an improvement of the flow conditions and a more homogeneous temperature distribution in the furnace (Fig. 2).

    For this purpose, various furnace-chamber geometries, burner configurations and insulating thicknesses are directly compared, and the optimum combination is determined. The result of these efforts is considerably improved efficiency in spite of unchanged dimensions. The new furnace series reduces energy consumption in comparison with conventional shaft furnaces by up to 15%, permanently lowering the costs of the cast parts. The actual energy consumption of 525 kWh per metric ton of material used puts StrikoMelter PurEfficiency in an efficiency class that is usually only achieved using regenerative burners. Depending on the material used, metal yields of up to 99.7% can be achieved.

 

Pneumatic Filling Device Increases Production Safety

To further increase the production safety and the quality of the liquid metal, a new filling device has been developed for Westomat dosing furnaces (Fig. 3). The innovative pneumatic system is self-contained, preventing the melt from reacting with the surrounding air. A riser tube ensures a constant flow rate and laminar flow conditions of the liquid aluminum right into the filling hopper of the dosing system. This makes it unnecessary to tilt casting ladles at great heights and minimizes the risk of accidents.

    The pressure supply of the pneumatic unit can be ensured via a plug-in hose connection to the Westomat pressure supply or by means of a compressor unit integrated into the lift truck. The new filling system consists of a thermally insulated ladle and has an insulated pressure-tight lid to which a filling tube is attached. Using a fork lift, the transport ladle is positioned in such a way that the filler neck of the filling tube protrudes into the filling hopper of the dosing furnace. The position of the ladle is therefore usually not much higher than it is during transport from the melting furnace to the Westomat.

    As a result of the virtually laminar filling process from a low height, this filling method minimizes quality losses in the melt. In addition, the system increases the safety standard since the ladle no longer has to be tilted. Optionally, an automatic overfill protection can also be installed for the furnace body. Using an operating console, the filling process is started manually, slowed down if necessary and then terminated. The result is a reproducible volume flow that can be set in such a way that the filling hopper of the dosing furnace is kept full at all times. If the melt drops below the minimum filling level in the filling system, the latter is vented automatically.

    The heating and refilling of the ladle itself and the venting of the melt within the ladle can be carried out with the help of a permanently installed treatment station in which the lid of the ladle is lifted and stored safe from accidental contact.

 

The Dosing Principle

The dosing process in systems belonging to the Westomat series is based on an overpressure in the interior of the furnace that guarantees a high-quality melt without oxide inclusions (Fig. 4). This gives the riser tube as well as the control system an important function. Since the introduction of the “DPC” and “ProDos XP” controls, a closed-control loop has regulated the dosing quantity to be dispensed. The melt is transferred to the die-casting machine via a temporary increase of the air pressure in the furnace chamber. The riser tube is constantly filled with metal up to the so-called “top-stop position” (i.e. until just before it overflows). From there, it supplies the die-casting machine with metal as required. This is made possible by a pneumatic unit fitted with an electroproportional valve.

    The clearly arranged touch screen of the control offers an overview of all process-relevant data and signals. In this way, the system can be adjusted at any time during the production process, and dosing imprecisions can be eliminated while production is in progress.

 

ProDos 3 – Hardware Without any Ifs or Buts

As a result of continuous further development and improvement, the new ProDos 3 control was launched into the market in the summer of 2013 (Fig. 5). To guarantee easy retrofitting to the new system, the hardware module is completely compatible – mechanically and electrically – with the current ProDos XP control unit and with the DPC control. 

    In terms of hardware, the ProDos 3 control is characterized by its modular concept and its significantly improved computing performance. For example, the new unit improves the reaction time by a factor of three, adapting the dosing weight to altered process parameters with a high degree of efficiency. Thanks to an integrated Profibus connection, the system also allows universal communication with the die-casting machine. Access to the control system of the die-casting machine allows the user to directly adapt the dosing quantity and the melt temperature of the Westomat to altered production conditions.

 

Maximum Process Reliability

Improving the process reliability was the principal aim pursued in the development of the control. Thus, the new system is even more resistant to electromagnetic disturbances than its predecessor. The ProDos 3 is also the first and only dosing furnace-control system worldwide to be equipped with a capacitive touch screen. In contrast to the usual technology, this no longer needs to be recalibrated, instead guaranteeing perfect functioning even after long years of operation under extreme temperature conditions. This new control will also have an integrated web server to allow remote diagnostics.

 

Extended Functionality

The expansion of the existing software will give the new ProDos 3 control a wide range of new functions (e.g., system setup will be quicker and easier in the future). In addition, biscuit correction has been completely integrated into the control unit, guaranteeing a high dosing precision (Fig. 6). High-precision dosing of the metal quantity requires the dosing weight to be adjusted several times a day. The newly developed patent-pending biscuit correction turns this into an automatic process. It constantly and automatically adjusts the dosing weight to the casting process so that the biscuit remains within the individually defined range of tolerance.

    The system achieves a permanent improvement of the process reliability and prevents the dosing weight from being incorrectly set. In addition, the biscuit correction automatically optimizes other empirical setting parameters of the dosing control. This includes the so-called furnace-chamber correction, which compensates for the reduction in the dosing quantity as the furnace empties. The refill correction is utilized during the filling of the dosing furnace. Dosing precisions of ±0.8% have already been achieved in foundry practice. In the future, biscuit correction will be integrated into the control system.

 

Increased Dosing Precision,  Improved Quality

In combination with additional options (such as an extended riser tube, including the riser-tube edge cleaning), biscuit correction improves the dosing accuracy by up to 35%. The elongated riser tube was developed for the special demands made on strength and expansion of structure castings. Structure-casting parts usually have a supporting function, which makes them safety- or crash-relevant. For this reason, it is important to have a high-quality metal with a low content of hydrogen or oxide contaminants.

    In the case of dosing furnaces using compressed gas, the metal flows out of the riser tube, through a transport channel and into the filling chamber of the die-cast machine. The usual channel lengths of between 800 and 2,000 mm (31.5 and 79 inches) encourage the formation of oxide skins that are rinsed, at least in part, into the filling chamber during subsequent dosing cycles and can contaminate the melt. With Westomat dosing furnaces, these effects can be largely avoided with the help of an extended riser tube. This riser tube has already been proven in foundry practice and achieves service lives comparable with those of conventional models. In addition to the standard lifting/tilting frames, there are also special lifting/tilting frames with an expanded lift and slew mount for adapting an extended riser tube to a casting cell. Thanks to a seal with a steel flange, the new riser tubes are extremely easy to assemble and have an improved permeability in comparison with their predecessors.

 

Riser-Tube Edge Cleaning

Depending on the alloy processed, adhesions of cooling melt can build up on the edge of the riser tube during foundry operation. These can be compensated via the biscuit correction. Nevertheless, the deposits have to be removed in order to guarantee precise dosing at all times. Up to now, these were removed manually in an additional work step by an employee of the foundry company. In order to rationalize this work step and improve dosing accuracy, a new pneumatic riser-tube edge-cleaning system is now offered. It is especially useful where riser tubes are hard to access (Fig. 7).

    The system, for which patents are pending, blows a short pulse of compressed air onto the edge of the riser tube and cuts off the aluminum thread. The integration of the control parameters into the ProDos control system guarantees the high-precision timing of the pulse. As the dosing tube is a wearing part of the furnace, the nozzles of the cleaning unit are designed for easy assembly and disassembly. The setting parameters of the compressed-air pulse depend on the dosing quantity and pressure. This system can be retrofitted with all existing systems.

 

Conclusion

StrikoWestofen Group’s latest technologies are significant developments en route to a “green” foundry with as little material and energy consumption as possible. In addition, significant improvements have been achieved in dosing precision and process reliability, and the process of transferring the melt within foundry operation has been optimized. New control hardware further expands the functionality of Westomat dosing furnaces. IH

 

For more information:  Contact Katharina Seidler, StrikoWestofen GmbH, Hohe Straße 14, 51643 Gummersbach, Germany; tel: +49 (0)22 61 – 70 91 108 ; e-mail: kse@strikowestofen.com OR Patrick Gälweiler, dako pr, Manforter Straße 133, 51373 Leverkusen, Germany Tel.: +49 (0) 2 14 / 20 69 10; e-mail: p.gaelweiler@dako-pr.de.

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