SYNTHETIC IRON OXIDE SPHERE OX

Due to the many variables involved in the casting process, it is difficult to control all the variables that can lead to a defect in the part. The best way to combat defects is to design a strategy in the core/mould manufacturing strategy that limits the influence these variables can have on the part.

⇒ LET’S CREATE A STRATEGY

1.  To begin with, the foundry must monitor and control the casting temperatures and time, and the type of metal. Once the casting temperature is under control, control of other variable “processes” can be addressed.

2. Secondly, the addition of SphereOx to the system is recommended. SphereOx is manufactured by a very unique and controlled process under precise chemical specifications and with high particle size uniformity. SphereOx is a spherical shaped particle with a core composed of pure FeO, which is encapsulated by outer layers of swept Fe₂O₃ gas and a veining preventive element, Fe₃O₄.

As mentioned above, silica sand expands at an average of approximately 5%. When molten metal enters a mould, thermal expansion takes place and often causes the bond between the sand particles to fracture, causing veining.

Years ago, sand additives were introduced to soften the core to allow expansion and prevent fracture. Most of the existing additives and iron oxides on the market are angular in shape and require more resin to achieve the tensile strengths, with SphereOx this is not necessary.

⇒ THE DIFFERENCES WITH SPHERE OX

Unlike angular shaped particles, when resin is burned into spherical shaped SphereOx particles, these not only promote the free passage of harmful gases that have been generated in the metal/mould phase to escape through the mould, but also the SphereOX will absorb some of the gas generated. This is even more remarkable, when SphereOx is used in conjunction with a mostly spherical silica grain, 20-30% less resin is required to achieve proper adhesion between the sand and resin. Even with angular sand, using the SphereOx requires 10% less resin. Less resin to burn equals less harmful gases and VOCs generated. The end result is moulded parts with no pores or shiny carbon defects. Keeping resin levels to a minimum has many benefits, both economically and ecologically.

⇒ THE MALE MOULD

Mould/core dimensional stability is another critical factor for defect-free parts. In many cases foundries use high-priced zirconium sand as a replacement for silica sand when dimensional stability is critical. SphereOx has a higher specific gravity than zirconium sand (5.2 vs. 4.7), as well as a higher thermal conductivity – the result is higher cooling power without any expansion.

Therefore, many foundries use a more economical blend of SphereOx and silica sand as a substitute for zirconium sand when dimensional stability is critical. Zirconium sand is increasingly limited and overpriced at around €2,000/tn compared to the SphereOx and silica sand blend, which averages less than €200/tn.

Thermal distortion is a method of measuring the propensity of a core, in an iron or steel casting, to fracture upon melting. Thermal distortion data is evaluated (table below) on 3 different samples, and shows the undulations of each sample as well as the thermal distortion over time. At 40 seconds, the “silica sand” curve breaks down and can no longer resist thermal expansion forces (would produce a penetration), however, the male samples having a “5% SphereOx/silica sand” and the “zirconium sand” continue to respond, and allow thermal expansion and contraction, which ends in solidification. This test provides evidence that it is possible to substitute a mixture of SphereOx and silica sand for the more expensive zirconium sand, and achieve very similar results.

Historically, foundries require core tensile strengths to determine the percentage of resin needed to make a core that is strong enough for immediate handling and subsequent transport to the casting line without breakage. The core must be strong enough to resist thermal distortion under the pressures generated by the incoming molten metal.

In many cases, the casting process uses sand recovered through a mechanical sand recovery system and then a percentage of this regenerated sand is mixed with new sand at a cost saving. A mechanical sand recovery system reduces metal particles and impurities retained in the sand used, but often does not significantly reduce the retained resin. A subsequent LOI (Loss on Ignition) will indicate the percentage of resin remaining in the recovered sand. The LOI value should be added to the percentage of resin that is added from new resin. LOI values of more than 1.50% are considered high, with a higher propensity for gas related defects. Studies by many foundries using SphereOx indicate that SphereOx actually produces a GOI (Gain on Ignition) of approximately 10%, demonstrating its ability as a gas scrubber to prevent gas defects.

SphereOx has a neutral pH (7.1), making it compatible with most resin and binder systems.

⇒ THE EVIDENCE

In addition, recent studies on a massive, large section steel part provided by an EPMA (Electron Probe Micro-Analyzer) show that the SphereOx particles used in the contact sand, at elevated temperatures, penetrate the core of the part and create a much more effective barrier that prevents metal penetration. The EPMA data verifies that there is no sign of burn-in in the samples where SphereOx was used as an additive in the contact sand, however, samples where SphereOx is not used as an additive exhibit high burn-in defects. It was also observed during this study that SphereOx particles improve the chemical homogeneity of the part core, and also improve the surface finish of the part.

It is important to be consistent and control the amount (percentage) of resin and additive that is mixed with the sand when making a mould or core. Because SphereOx is spherical, has a constant size, and has no dust or impurities, it flows well and flow rates are easy to control. Traditional iron oxides and other commercially available additives are fine-grained and angular in shape. These materials do not flow easily and tend to reduce feed systems, resulting in non-uniform distribution levels from batch to batch. Non-uniform distribution causes process variations and a much higher probability of casting defects. In almost all cases when SphereOX is used to replace other additives, a decrease in binder percentage of 10-20% is observed for optimum results.