For this reason, closed, preferable hermetically sealed launder or transfer systems which minimise turbulence during transfer are preferable. Care should also be taken to ensure solutions chosen are adequately insulated and feature controllable heating to ensure melt temperature is maintained at a constant level. This eliminates any need for potential reheating which, in addition to wasting energy, can also result in further oxidation and reduction in usable yield.
Metal loss - the enemy of cost-efficiency
There are many ways in which aluminium foundries can reduce costs. Reducing energy consumption is certainly a significant opportunity for savings. This topic is covered in detail here with our special energy savings guide.
Another significant cost driver, especially considering the cost of non-ferrous raw materials and related logistics/handling, is metal loss. When considered over the service life of a melting furnace these two factors combined are responsible for up to 95 percent of the overall costs. Depending on aluminium alloy, a metal loss of just 1% of an annual melting output of 5,000 metric tons could equate to an average financial loss of 70,000 EUR.
Industry leaders agree - metal loss is the enemy of cost-efficiency and improving yield should be an important objective. This remains the single biggest opportunity for die casters to reduce waste and maximise return on investment (ROI).
To help seize this opportunity, here are some of the most common causes of metal loss together with tips and technologies that can help tackle them.
Avoiding aluminium oxidation: the path to real savings
Metal loss during melting, transfer and dosing is almost always linked to the oxidation of aluminium. Aluminium and aluminium alloys oxidise relatively quickly compared with other metals in both solid and molten states – though oxidation rate significantly increases with temperature, meaning molten aluminium is particularly susceptible.
Aluminium oxides form as a direct result of exposure to air and can be impacted by excess heat. Oxides also vary in type – some leading to greater levels of dross, while others are more likely to negatively impact general metal quality which will send scrap rates soaring.
This may sound a complicated puzzle to solve. However, it is important to remember that solutions exist that are specifically designed to minimize opportunity for oxidation, and also to limit any subsequent disturbance of oxides so that they do not mix into the melt and can be more easily removed – all of which helps increase your usable yield. This is why prevention and control starting with the furnace design is a logical place to start.
3 melting furnace features to maximize metal yield
1) Shaft design that does more and loses less
Overheating (otherwise known as “super-heating”) is a major contributor to oxidation and melt loss, so minimizing the time melt is subjected to high temperature heating is beneficial. This is one of the reasons combined shaft or ‘stack’ or “tower” melting furnaces are often preferable for aluminium die casting operations.
Aluminium shaft melting furnaces combine preheating, heating and liquefaction in one melting shaft - cold state material added at the top of the shaft is gently warmed via convection as it descends, maximizing heat transfer but minimizing temperatures required. On reaching the shaft base the material is then heated up to its melting point very quickly that then minimizes the time material spends in close proximity to the melting burners which helps optimize metal quality. Rapid liquefaction in what is the hottest zone of the melting chamber is possible in less time which significantly reduces risk of oxide formation therefore improving yield.Solution spotlight: StrikoMelter’s shaft height and the retraction of the shaft above the melting chamber, means metal is preheated to such an extent that its time in the melting chamber - where it is liquefied by the strong melting burners - is very short. Less time for oxidation to take place = low metal loss.
2) Twin is a win for metal yield
A twin chamber design consisting of a melting bridge and a separate holding bath – with separate, distinct burner systems - has been proven to limit oxide and dross formation while also minimizing risk of suspended, insoluble contaminants, therefore guaranteeing a high yield of high quality tapped metal. This is therefore a key design feature in StrikoMelter. What’s more, due to the consistent separation of melting and holding areas, a high metal quality can be produced even if the charge material is of low quality.
Tip: Make sure the charge material, especially the return material, is as clean and dry as possible to ensure best melting conditions right from the start. Moisture and dirt can cause oxidation, which in turn can lead to metal loss.
3) Clean and cover to max your metal
Effective, regular cleaning of the furnace to remove dross is another important way metal loss can be significantly reduced.
Access all areas: Good access to the surfaces to be cleaned is essential. It must be possible to separate the metal from the dross easily on the melting surface.
The heat is on: Daily ‘free melting’ on full power is also necessary to ensure dross removal. Covering the shaft during this process retains heat (also preventing energy wastage) and allows better separation of the metal from the dross.
Did you know: StrikoMelter’s specially designed doors and the different, adjusted inclination angles of the melting table, optimize cleaning conditions. StrikoMelter also features a heat-resistant shaft cover, the hot gas baffle, which can be closed during the free melting process. This keeps the heat in for optimal dross/metal separation - the aluminum alloy flows into the holding chamber and the dry dross can be removed from the melting chamber.
We’ve developed a range of videos, guidelines and specifications to help customers clean and keep their furnaces in top condition for maximum metal yield, while innovations such as the Part Load Efficiency Control also keep metal losses low and productivity high by enhancing melting efficiency.
Time to rethink metal transfer?
Metal transfer from melting furnace to holding/dosing solutions and die casting machines, is a foundry process prone to metal loss.
Open ladle systems with holding furnaces present the biggest threat to yield, not only due to greater risk of atmospheric exposure but also because of potential spills – a safety concern in its own right – and melt turbulence.
Less processes, less loss
Each individual process which takes place between melting and casting introduces a potential opportunity for metal loss.
For this reason, adopting ‘combination technologies’ which blend multiple functions can prove beneficial when seeking to increase yield. Many are also highly adaptable and can integrate with aluminium casting cells of different size and layout.
Multi-purpose melters: multi-purpose melters are a prime example and can often be tailored to integrate with aluminium casting cells of different sizes and set ups. Particularly suitable for aluminium foundries short on space, 2-in-1 or 3-in-1 melting furnaces present an attractive option for cutting out separate stage processes that can negatively impact metal yield. For example, dual chamber melting and holding shaft furnaces are increasingly available with integral metal treatment functionality (impeller degassing) and bale out pockets.
Treat and transfer: if a multi-purpose melter is not an option, or simply not ideally suited to your specific production set-up, what about metal treatment and metal transfer in one unit? Closed, heated, transfer solutions like StrikoWestofen’s Schnorkle can also be used, while not in transit, to treat metal using an impeller unit. This saves time, energy, and ensures delivery of a high yield of high quality to the dosing or holding furnace.
Automation to avoid the unnecessary: it is worth remembering that cutting out processes can also mean eliminating unnecessary or untimely actions – a task for which digital solutions are ideally suited. An automated dosing furnace supply is a good example. Using fill level sensors to automatically determine optimum metal refilling ensures:
- The right melt is transferred only when needed – potentially reducing risk of oxidation if open transfer systems are employed.
- Better use of limited resources – prioritize the next furnace to fill based on current production demand and “time until empty” data.
- Additionally, it means dosing furnaces are never under or overfilled, both of which are factors that could potentially affect metal quality and dross formation leading to reduced yield.
Dosing that won’t drain resources
Dosing furnaces – closed holding furnaces which support direct and timely delivery of molten aluminium to casting lines – are widely recognised for their favourable metal yield in comparison to traditional, stand-alone holding furnaces. In fact, figures from practical experience show they can reduce metal loss by as much as 80%.
Even with dosing furnaces, however, there are specific features die-casters should look out for to ensure their furnace is delivering the metal yield it should.
Be precise: reheating melt is a common source of metal loss, meaning heat distribution and temperature controls for dosing furnaces need to be even and precise respectively. Die casters should be able to ensure their melt temperature is maintained within a tolerance of ±2° C. Metal can also be wasted if dosing accuracy drops. To ensure precision, die-casters should look out for software solutions that facilitate real time monitoring of pressure sensor data and correction of dosing volumes, for instance based on casting machine biscuit data.
Below the bath: the surface of any dosing furnace holding bath will feature an oxide layer. Some dosing furnace pump systems operate by allowing liquid aluminium to flow over an edge into the pump for transfer to the die casting machine. Even though the metal flowing over the edge is taken from below the bath level, it is exposed to air in this process and will therefore rupture the oxide layer that has formed on top of the bath. This rupturing can cause oxides to mix into the melt reducing usable yield and increasing the likelihood of defective castings. Instead, die casters should always look for dosing furnaces which employ riser tubes which take metal from below the bath surface line.
Keep it short: finally, it is important to opt for solutions where the transfer launder is as short as possible. This limits environmental exposure and therefore is a final measure – prior to casting – for reducing risk of oxidation. Shorter transfer launders also reduce the risk of possible adherences and remnant build up.
Keep it clean: Look out for dosing furnaces with integrated or optional metal treatment systems which improve the overall metallurgical quality of the melt and help serve as a final contaminant filter before casting. Integrated porous plugs, for example, allow inert gas (typically Argon or Nitrogen) to be dispersed into the melt via extremely fine bubbles which remove hydrogen and other impurities from the liquid metal to enhance melt quality – lowering the risk of production scrap further down the line.