HPDC offers a range of advantages, such as higher production rates and good surface finish; as a result, is the process of choice for many of the new, lightweight parts needed by the growing e-mobility and 5G markets. However, it is also not without its challenges. One significant limitation is the ability to produce complex internal cavity shapes. In order to overcome that obstacle, it is necessary to develop disposable cores that must be able to tolerate the high pressures, temperatures and speeds involved in the HPDC process.
A new type of sand core, developed by the Foseco Foundry R&D Centre, provides a solution to these challenges. These cores are made with an innovative Water-Soluble Binder and sealant using standard sand core production equipment. They therefore offer a more cost-effective and sustainable option for HPDC of complex, hollow shapes at high volume and are equally suitable for use in liquid HPDC or also in semi-solid (Rheocasting) process.
For high-pressure casting processes, a sealant may also be necessary to avoid penetration of the liquid metal into the pores of the sand, which results in unacceptable roughness (encapsulation of the sand grains) of the casting surface. Different techniques can be used to apply a sealant, such as dipping or spraying.
MECHANICAL STRENGTH
Figure 2 shows the mechanical strength and sample weight of sand cores made from H33-type quartz sand, as a function of the amount of the additive. The liquid binder was set at 6.0wt% of the sand. Depending on the casting process and the related requirements, the exact strength values can be selected
As can be concluded from Figure 2, the strength values of samples without the additive were relatively low (low compaction (low sample weight)); the average value was about 100N/ cm². However, the addition of only a small amount of the additive, in this case 2.0wt%, resulted in a significant improvement in the mechanical properties: strength values were around 700N/cm². A further increase of the concentration resulted in strength values higher than 1200N/cm² (high compaction (high sample weight)).
MANUFACTURING SAND CORES
All sand cores are produced using a standard core shooter equipped with a hot box system. For the new HPDCsuitable cores, the sand mixture is prepared using the liquid binder and additive (powder). It is then automatically injected at high speed into a specially designed core box using compressed air, and cured
WATER-SOLUBILITY OF THE BINDER
Irrespective of the mechanical strength of the sand cores, the water solubility of the binder was excellent with full dissolution feasible in less than 5 seconds (Figure 3). It is interesting to note that the new Water-Soluble Binders showed excellent solubility after the casting trials in multiple processes from liquid HPDC, Rheocasting, Gravity and LPDC processes, indicating that the application temperature of such a type of binder is at least 750°C. This makes these cores very promising candidates for slow and fast solidifying casting processes.
SURFACE ROUGHNESS
Surface roughness is one of the most important characteristics of the casting pieces after removal of core residue from the hollow part. In HPDC, use of a sealant is indispensable, as an unsealed core will result in a casting with unacceptable surface roughness. This is caused by the penetration of liquid metal into the pores of the cores and consequent encapsulation of the sand grains into the surface of the casting. Figure 4 (left) shows the inner surface of a HPDC casting from an unsealed sand core; the use of an incorrect sealant type can also result in similar surface appearance (Figure 4 – centre). Casting with an optimised sealant, however, achieves a smooth and sand-free inner surface (Figure 4 – right).
Surface roughness of these castings was also measured by using a Keyence non-contact profilometer: In this case, the surface of a casting made with an unsealed sand core and one made with the optimised sealant were measured. The value, Sa, is the extension of Ra (the arithmetical mean height of a line) to a surface, and expresses, as an absolute value, the difference in height of each point compared to the arithmetical mean of the surface. The unsealed core showed a relatively high roughness of Sa = 123µm (Figure 5); this confirmed the visual observation shown in Figure 4 (left). The application of a well-developed sealant resulted in a significant improvement; the surface was much smoother with an average Sa of just 14 µm (Figure 6).
MAIN ADVANTAGES
Laboratory tests, as well as testing trials in the field, have demonstrated the strong potential of the new WSB system to meet a wide range of customer requirements, showing very promising results not only for liquid HPDC, but also gravity die casting and Rheocasting for aluminium.
CONCLUSIONS
The new WSB systems developed by the Foseco Foundry R&D Centre in Enschede, The Netherlands, have demonstrated their high strength in various applications. Even in severe processing conditions, such as HPDC, with the use of an appropriate sealant, these innovative sand cores can withstand high pressures and high temperatures, whilst facilitating easy core removal from internal cavities by flushing water, leaving a smooth and sand-free surface.
ACKNOWLEDGEMENTS
The authors wish to express their gratitude to the complete R&D Mould and Core team of the Foseco R&D Centre in Enschede, The Netherlands. Special thanks to Rafael Wattimena and Marco Huusken for their extensive experiments and measurements
