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Introduction
In recent years there has been a drive to increase the re-use of foundry sands to limit the cost of new sand purchases and to minimise both the cost of dumping used sand and to limit environmental impact. Traditional foundry sands systems are often based on silica sand, with speciality sands, such as chromite, being used in areas of the mould where higher refractoriness or thermal stability is required. However these additions of speciality sands (specifically chromite) can have a detrimental effect on the performance of the reclaimed sand reducing its refractoriness and increasing the potential of sand fusion and metal penetration, and should be effectively separated before re-use of both the reclaimed silica and chromite components. [1]
Additionally the physical characteristics of silica sand such as grain shape, angularity and porosity can limit the effectiveness of mechanical reclamation processes by preventing the easy removal of the binder without damaging or fracturing the sand grain. High intensity scrubbing of the sand to effectively remove residual binder can lead to a reduction in the average grain size of the reclaimed sand. This will then require increased binder additions to subsequently re-bond the processed sand and increase dust levels that can, if not controlled, contribute to airborne particulates and a respirable silica dust hazard. [2]
The use of synthetic sands offer the opportunity to eliminate the use of both silica and special refractory sands, and provide good yield during the reclamation process:
- Superior refractory properties compared with silica sand
- Low thermal expansion
- High strength, resistant to breakage during reclamation
- Spherical shape, enabling easier removal of residual binder
Synthetic sands have a different composition and form compared with both silica and chromite sand, and it follows that the performance of the mould coating may also differ in terms of application properties and subsequent casting surface finish.
This article considers the characteristics of synthetic sands when compared with both silica, zircon and chromite sand and assesses the application performance of the mould coating.
The fundamental characteristics of synthetic sands
The characteristics of commercially used silica, zircon, chromite and synthetic sands are shown in table 1. On determining the composition of the synthetic sands by means of X-ray diffraction they were all found to be alumina silicates. Furthermore, as shown in figure 1, all of the synthetic sands have a distinct spherical form.
The heat conducting properties of synthetic sands
To measure the heat conducting properties of the different sands, test pieces were placed on a heat source as shown in figure 2, and temperature at a set point within the test piece was recorded over time using a thermocouple and data recorder. The measured results at a number of fixed time points are shown in table 2.
When comparing the recorded temperature within the test pieces after they had been located on the heat source for 5 minutes, the temperature was lower with all of the synthetic sands than with silica sand and that the highest temperature was achieved with the zircon sand. From these results it was concluded that the thermal conductivities of the synthetic sands were lower than that of silica sand and significantly lower than that of zircon sand.
Refractory performance of synthetic sands
Casting tests were carried out with uncoated moulds in order to evaluate the refractory performance of the synthetic sands. A furan mould was constructed to produce a hexagonal cross-sectioned casting with each side measuring 350mm wide by 720mm high, and incorporating two cylindrical cores in each side, see figure 3. The cores that formed each side were made of different base sands including silica, zircon, chromite and synthetic sands. The casting was poured using standard carbon steel at 1650°C, and the resultant casting weighed 110kg.
The casting results are shown in figure 4. The casting surface adjacent to the silica sand exhibits burn-on, both on the hexagonal face and within the cylindrical indentation, whereas adjacent to the zircon sand the casting surface was smooth. For the synthetic sands it was observed that both samples 1 & 2 showed some burn-on in the cylindrical indentation and on the hexagonal face. The third sample showed a clean defect free surface. The chromite sand had a similar performance to the synthetic sand samples 1 & 2, with a rougher surface finish than either zircon or synthetic sand 3 and some minor burn-on defects.
Application of mould coatings to synthetic sand moulds
To establish whether the application performance of a mould coating could be adversely affected by the mould substrate, the differences in applied layer thickness and the penetration of the coating into the mould surface was determined for each substrate. A standard zircon-based coating with an ethanol solvent was used (ISOMOL* 310PE).
The coating was adjusted to 60 Baume and applied to a standard test-piece by dipping, and dried by a subsequent ignition of the solvent. The results are shown table 3.
The coating penetration depth into the mould surface was significantly lower for the synthetic sands when compared with silica sand, whilst layer thickness was equivalent in all these cases. The zircon sand had a significantly lower penetration depth, which also resulted in an increased average layer thickness. Comparing the synthetic sands, there was a marked variation in the penetration depths with synthetic sand 3 being significantly lower than the other two samples; this can be attributed to the slightly lower AFS number, the particle size distribution, form and composition.
Conclusions
The investigation of widely used commercial synthetic sands has shown that the bonded sand has a lower thermal conductivity than silica, zircon or chromite sand providing a more insulating moulding material. The refractoriness of the synthetic sand under casting conditions is significantly better than that of silica sand, but the indications are that it does not perform as well as zircon sand and the performance can be dependent on the particular grade of material. Standard coatings can be applied to synthetic sand moulds without problems and similar layer thicknesses are achieved without modification to the coating dilution. However, penetration into the mould surface can be less than with conventional sand types, and is effected further by the specific grade of synthetic sand.
According to these results it is possible that when using synthetic sands that the solidification of the cast material will be retarded and that hot spots may be formed in areas not previously observed with conventional sand types. Moreover, since the penetration of the coating into the mould surface is reduced it is possible that defects such as burn-on may arise in parts where they have not occurred previously.
Consequently when a switchover from conventional sand is made it is recommended that a coating should be selected that can compensate for the different properties of synthetic sands when compared with conventional sands. By establishing the correct synthetic sand, binder and coating combination it is possible to utilise a single sand type, eliminating the use of expensive speciality sands for mould facing. This will allow for an optimised sand reclamation process that minimises new sand purchases and dumping costs, whilst additionally eliminating the formation of silica dust that may contribute to a respirable hazard.
References
[1] Influence de la presence de chromite – Poyet P and Chevriot R – Fondrie 1980 Vol.35 p93-103
[2] OSHA Fact Sheet – Crystalline Silica Exposure (Health Hazard Information)