All concepts have their merits, and they are well
suited, however for different applications.
For mainstream extrusion billet production vertical casting is the first choice. Billet surfaces as smooth as a baby’s bum are achieved through the air slip mould technology. Consequently the residue at the extrusion press is minimized. The same is with rolling slabs where the skin zone is minimized by casting in a magnetic field.
But still the VDC process has some fundamental drawbacks compared to the HDC process:
- Higher costs of the VDC caster, especially due to greater building height required, necessity of overhead crane and foundation for the casting pit.
- The semi-continuous character of VDC-casting results in lower productivity.
- A great amount of set-up work per drop is required, which is rather labour intensive, whereas with the HDC, continuous production runs of 3 to 20 days are common.
- For T-bar production only one to two operators per shift are needed (two operators for cast start and stop).
- On VDC plants sawing is not integrated in the process, so that an additional sawing station plus operator is required. HDC casting employs an automatic flying saw, which cuts the T-bars to length without disturbing the casting process.
- Using the VDC process, some products, mainly those with small sectional areas, cannot be cast economically nor can the required quality be met.
Pouring of metal into open moulds causes dross:
Sows, pigs and ingots were traditionally produced employing the open mould technology. Although this technology was improved over many years, dross formation and inclusions are still unavoidable. Due to cascading, turbulence occurs when filling the mould. So a relatively big unprotected surface area is offered to the atmosphere for oxidation. The dross formation is mainly ruled by the metal temperature, pouring height and pouring rate.
The HDC process is almost free of dross formation. It results in savings due to avoided metal losses and in inclusion-free products.
On the HDC the metal flows smoothly, protected by an undisturbed oxide layer via launder and tundish to the mould. Thus there is no chance for oxides and other impurities to get into the product. The HDC cast T-bars and foundry ingots are chilled at least ten times faster than sows and pigs. This ensures a fine and uniform grain structure as well as a uniform analysis throughout the cast product. A further step ahead in the production of remelt products in terms of quality is the combination of the HDC process together with an Inline Degasser and Ceramic Foam Filter (CFF). Both items are needed to obtain T-bars and foundry ingot free from porosity and inclusions.
The advantages of HDC-cast foundry alloy ingots over open mould cast ingots are numerous:
- low hydrogen, extremely low oxide inclusions
- fine uniform grain structure
- consistency and uniformity of alloying elements’ content and distribution
- no gravity segregation
- no cracks and shrink holes and no water inclusions
- consistent dimension, straightness, weight
- smooth surface, easy for stacking and strapping, compact bundles