Dimensional Accuracy and Process Optimization with Inorganic Binders

This is the second of two parts. Read Part 1, Advances Show Progress, Potential for Inorganic Binder Systems

Many positive influences of inorganic series core production on metalcasting operations have already been established: Odorless coremaking, odor-reduced casting, significantly less cleaning of machines and tools, and the resulting higher output quantity and productive. All these advantages are in addition to the benefits to the casting process, such as faster solidification made possible by lowering of ingot mold temperatures.

In particular, the absence of combustion residue gives component developers new freedoms that they did not have previously with organic-based binding agents. One impressive example is the new central feed principle, which is used at BMW's Landshut plant for crankcases of future engine generations^{[1, 2]}. Inorganic cores are used as central feeders here, thus minimizing the risk of sooting ventilation ducts in the low-pressure permanent mold. This concept is not feasible with organic cores.

The DAS distribution of the three concepts is shown in Figure 9 (first generation top left; second generation top right; third generation with central feeder bottom middle.) It is apparent that the new central-feeder concept leads to DAS advantages in all component areas. The warmest point (thermal center, binding of the feeder) and therefore the point with the highest local DAS is in the area of the lower dead center of the piston, a point that is not subject to excessive thermal or mechanical stress. The tension rod area also solidifies very quickly and can be influenced externally via the permanent mold. The tendency toward leaking after mechanical processing falls dramatically, and the sealing rates are miniscule.

Using modern inorganic binders in iron casting (core hardening with a hot tool and hot air, not CO₂) is not yet widespread. This is probably because – in contrast to light-alloy permanent mold casting – the process sequence and the sand system are regarded as more complex, the casting temperatures are around twice as high, and therefore the requirements for thermal resistance much higher.

In addition, in the case of inorganic binders, the cold-box method is superior to the largely physical (drying) process in terms of productivity in many areas, particularly if the core geometries become bigger and bulkier.

Even so, inorganics have considerable potential, particularly in iron casting. In particular, problematic parts that need to be worked on with special sands or additives in combination with a coating against veining are predestined for use of inorganic binders since they show a much lower tendency toward veining – or indeed none at all – compared with organic systems.

Figure 10 shows test castings from step cores in GJL, 1,458°C. The casting of a cold-box system with an additive and the one with an inorganic system are shown.

One half of each of the cores was coated. It is clear that the coated and uncoated sides of the inorganic core are completed better and have fewer penetrations right through to the final stage, i.e., the stage with the highest thermal stress. This figure also reflects initial experience from foundry operations, where inorganic cores can be used successfully in a targeted manner to reduce veining, penetrations and gas. There is no doubt that even more positive news can be expected from this area of application in future.

Optimizing Moisture, Coating Resistance

Moisture stability has always been the Achilles' heel of inorganic cores because of the nature of the chemistry. Binding agents are based on silicates that are dissolved in water, known as water glass. Thus, water is the solvent in the system.

In addition, the hardening reaction is largely reversible (balanced reaction.) This means that when large amounts of energy and water are present (e.g., in the case of high air humidity and high temperatures), the back reaction takes place and the cross-linking of the silicates is reversed, resulting in the cores losing their strength and breaking down. This can be prevented by removing water from this balance, i.e., through storage in a dry place. Since the latter is not always easily possible in practice, additives (known as promoters) are used to delay the back reaction significantly, thereby allowing the cores to be handled in a process-consistent manner, even after "normal" storage. However, inorganic cores remain hydrophilic.

An even greater challenge is coating the cores with a water-based coating because the water acts on the core directly, and in concentrated form. Applying water coating on the cold core is not critical at first but it becomes critical by the time the coating is to be dried in the oven. Then, the process shown in Figure 11 takes place.

Before coating, the cold core has a strength level of 460 N/cm². The core is coated and starts its "furnace journey." Because of the high temperatures (150°C) and the water present, the relative air humidity rises rapidly, which makes the core increasingly weak and causes the strength level to fall from 295 to 120 N/cm².

When the turnaround point is reached, i.e. the maximum air humidity falls again, the drying process of the coating continues and the core reaches its minimum strength, probably the most critical point in the furnace drying process. It is now determined whether the core withstands the stress, deforms or even breaks. If it gets through this critical phase, at the end of its furnace journey, the core will have a highly respectable final strength of as much as 260 N/cm² when hot, and as much as 360 N/cm² when cold. Therefore, the final strength of a coated core may be quite high. The main factors are the drying process and the temporarily air humidity levels in conjunction with the high temperatures in the furnace.

Consequently, the major chemical challenge is transferring a water-soluble system – as this is what inorganic binders are – to a moisture-resistant state as much as possible after hardening.

In this respect, Figure 12 shows the result of the latest research, namely the moisture level of coated cores in two binder systems in relation to the dwell time in the drying furnace. The standard system shows the strength pattern portrayed here, with a minimum strength of approximately 90 N/cm². Although the second system essentially has a somewhat lower initial strength level, it only drops to a figure of approximately 250 N/cm² during furnace drying. This means that in relative terms, the cores produced with this optimized binder system lose a maximum of 30% of their initial strength, while the standard system loses approximately 80% of its strength. 
It also can be seen again here that the final strength rises back to a very acceptable level in both cases, i.e. after complete drying and cooling, provided that the cores come through furnace drying intact.

The optimized binder system is currently undergoing testing by the customer and, if the results are confirmed, this could broaden the process scope of inorganic binder systems even further, either in the use of these binder systems under non-optimal climatic conditions or in use with water coatings, which could particularly benefit the introduction of inorganic binders in iron casting.

Putting Rumors to Rest

Inorganic binders are subject to more rumors than almost any other aspect of metalcasting. What can they actually do, and what can't they do? The growing interest and increasing number of users clearly show that this technology is now an established part of aluminum permanent mold casting, at least. The cost savings in terms of maintenance and cleaning of the systems, as well as the resulting higher productivity in the casting process, are critical factors in this success.

At the same time, new development stages of the binders are closing the gap on organic systems: Better casting surfaces, higher thermal stability and optimization of disintegration after casting have been significant optimization steps of the last generation of inorganic binders. And there are also signs of progress in improving the storage stability of the naturally moisture-sensitive inorganic cores.

At the same time, it is clear that the use of inorganic cores does not have to be limited to light-alloy permanent mold casting as inorganics offer huge potential in prevention of classic casting defects (such as veining.) "Nothing is impossible" is therefore a very fitting phrase for the core of development in the inorganic sector: Much of what has been achieved with inorganic binders to date would have seemed impossible to many people in the past. As a result of intensive research in this field, it can be assumed that so many hurdles that seem restrictive at the moment will be cleared in future.

Each of the authors — Jens Mueller, global product line manager; Heinz Deters, head of laboratory; Martin Oberleiter, chemical-technical assistant; Henning Zupan, lab assistant; Hannes Lincke, manager, R&D; Ronja Resch, lab assistant; Joerg Koerschgen, manager, application technology; and Axel Kasperowski, manager, pilot foundry — is affiliated with ASK Chemicals GmbH in Hilden, Germany. Contact [email protected] or via phone. +49 211 711 030.

REFERENCES
[1] "Zylinderkopffertigung der Zukunft – Ökologie, Ökonomie und Werkstoffoptimierung im Einklang" (Tomorrow's Cylinder Head Production - Ecology, Economy and Material Enhancement Brought in Line), Emmerich Weissenbek, Thomas Kautz, Jörg Brotzki, Jens Müller, MTZ06/2011, Volume 72, 484–489

[2] "Anorganische Innovation für die neuen Diesel-Spitzenmotorisierungen im BMW M 550xd: Konstruktion und Gießtechnik des Alu-Kurbelgehäuses" (Inorganic Innovation for the New Top-of-the- Range Diesel Engines in the BMW M550xd: Design and Casting technology of the Aluminum Crankcase), Emmerich Weissenbek, Bernhard Zabern, Andreas Fent, Johann Stastny, Christian Högl, Giesserei-Praxis 5/2013, 175–181