#technologie-innovationen 27.11.2025

Experience with innovative hot-air curing of binders

By Hartmut Polzin and Theo Kooyers

As early as GIFA 2003, various binder manufacturers presented developments in new thermosetting, inorganic binder systems – a process that continues to this day. These developments were based on the well-known fact that moulding materials bound with water glass can achieve significantly higher strengths in temperature-controlled core boxes than, for example, with conventional carbon dioxide gas treatment.

These high strengths were an essential prerequisite for inorganic binder systems, as one of the main drivers for this development came from the automotive industry – with the need for large-scale production of cores, some of which are highly complex and delicate, for vehicle components. This article reports on a hot-air curing process for the production of inorganically bonded cores, which does not require an actively heated core box mould.

State of the art

The current state of the art remains that a silicate-based binder system is usually mixed with the moulding sand and blown into a steel core box heated to 160–200 °C, in which a stable outer shell forms. By combining this with hot-air gas flushing, cycle times can be reduced to acceptable levels.

The main disadvantage of these processes is that, due to the high tooling and energy costs, they can practically only be used in large-scale series production (mostly in automotive foundries). Furthermore, their application is limited to the production of aluminium castings, as various technological properties (e.g. residual strength/crumbling behaviour) are inadequate.

Hot-air-curing inorganic cores

The development of the binder system presented here is intended in particular to offer iron and steel foundries – often customers or contract manufacturers with frequently changing product ranges and batch sizes – the opportunity to use inorganically bonded cores. A key factor here is the elimination of expensive, heated steel core boxes, which only pay for themselves in large-scale production runs.

The binder used is an alkali silicate or water glass-based system, which has been modified with a variety of oxides and contains no organic components whatsoever. The moulding material is cured exclusively by passing hot air at approx. 160 °C through it. It is a single-component system that can be used without additives at dosages of less than 2.5 per cent, and in most cases even less than 2 per cent.

Practical experience

Comparison of moulding compound strengths:

The investigations show that the strengths of the system are sufficient and suitable for practical applications when compared to other inorganic binders. Of particular interest is the fact that stable results are achieved even at very low binder contents. Furthermore, by using very dry hot air, satisfactory hardness results can be achieved even at lower temperatures.

It is generally assumed that strength also increases with rising temperatures. In fact, an increase in strength is only observed for binders B1 and B3; the strengths of the Peak system remain approximately the same. This is of interest when determining the optimum curing temperature in terms of cost and environmental protection. If the cores are stored in a damp environment, the advantage of a higher curing temperature becomes clearly apparent. Binder B3 exhibits the better values for specific strengths.

Influence of curing temperature
As the temperature of the curing air is a cost factor, tests were carried out in which the air temperature for curing was varied. The binder designated VC is the base system, whilst VC-HR and VC-CB are binders specifically designed for particular applications.

The following parameters were used in this series of experiments:

Curing temperature: 160 °C to 60 °C in five stages
Curing time: 60 s
Sand temperature: 25 °C
Quartz sand QQs 26
Relative humidity: 45 %

It has been shown that, with a few exceptions, usable results can be achieved up to a gas treatment temperature of 60 °C. Of particular interest here is the very low binder content of 1.75 %. The reason for this behaviour lies in the use of very dry air for curing the test specimens. This allows for satisfactory removal of water and water vapour from the core during curing, even at lower temperatures. These relationships are to be investigated in greater detail in the near future.

The strengths were determined immediately and after 1 to 3 days. The test conditions for the ‘1d wet’ values were 25 °C and 75 per cent relative humidity. For binders B1 to B3, the temperature refers to the core box temperature. The strengths achieved speak for themselves:

All binders show an increase in strength during the first few days of storage under normal conditions, which is attributable to progressive drying out.
Two of the comparison systems are more sensitive to storage in a damp environment.
The strengths of the Peak binder system are lower than those of the other systems, which is attributable to the lower binder content. These values could be increased if necessary by raising the binder content. However, the strengths shown here are sufficient for many applications.

Demoulding behaviour and residual strength

One of the ‘classic disadvantages’ of water-glass binder systems, as reported in the literature, is the high residual strength of cores or moulds after casting, combined with the high effort required for demoulding. Not least, this disadvantage was a key factor in the significant decline in the use of this process from the 1970s onwards.

The aim in the development of modern inorganic binder systems must therefore be, alongside other properties, to improve core removal behaviour in particular. As the binder system presented here was developed specifically for use in iron and steel casting, particular attention had to be paid to this aspect.

To this end, casting trials were carried out at an iron foundry, using both coated and uncoated test cores to produce a ventilated brake disc made of grey cast iron with lamellar graphite (core mass approx. 15 kg). Standard PUR cold-box cores were used for comparison.

Firstly, it should be noted that no significant differences were observed between coated and uncoated cores. Visual observation of the crumbling and core removal behaviour showed that the channels in the castings were filled with moulding sand adhering to the inorganic cores, which was not the case with the cold-box cores. However, following the standard sandblasting process, these sand deposits were also completely removed. The remaining cores in this series were fed into production as usual and gave no cause for complaint.

During the evaluation of the castings, one advantage of water-glass-based inorganic binder systems became apparent, which should not be underestimated: the virtual absence of veining. This moulding material expansion defect, which is particularly typical of the PUR cold-box process, occurs only to a very limited extent in water-glass-based binder systems due to the thermoplastic bonding present therein and, in certain casting areas, only to a very limited extent.

To substantiate this subjectively positive behaviour with figures, the demoulding behaviour should be assessed on the basis of the residual flexural strength: To this end, flexural beams were produced which, 24 hours after production, were exposed to the test temperature for 5 minutes and tested 2 hours after removal from the oven.

The test temperatures were 200, 400 and 800 °C.

The results confirm the positive properties already observed in the casting trials in this area. The test temperature of 400 °C is intended to represent the trend in the field of aluminium casting, whilst the temperature of 800 °C represents that of iron casting. It can therefore be concluded that the inorganic binder system presented exhibits decomposition and release behaviour similar to that of the PUR cold-box process.

Practical examples

To date, cores using the inorganic binder system described have been produced and successfully utilised in a wide range of foundries. The spectrum ranges from external cores for core packages, through the brake disc core mentioned earlier, to intricate valve cores. The field of non-ferrous casting, which has not been the focus of this article so far, has been impressively illustrated. Here too, the range of possible applications extends from simple cores, e.g. for intake manifolds, to highly complex cylinder head cores. The examples shown are used in the fields of copper and aluminium casting.

Summary

The inorganic binder system presented, based on water glass, is an alternative to the PUR cold-box process that can be used in iron and steel casting. The liquid single-component binder is dosed in smaller quantities than comparable binder systems: the quantities used to date range between 1.5 and 2.5 per cent, but can be increased if required.

The single-component system simplifies the addition of binder at the core shooting machine, and the desired strengths are ensured by comparatively higher specific strengths. The cores produced are cured using gas-blowing air heated to 160 °C. The core box mould is not heated. When selecting the core box material, care must be taken to use thermally stable plastics. Metal core boxes (aluminium or steel) offer advantages in terms of shorter cycle times.

If particularly dry air is used, curing can also take place at lower temperatures. The binder system can, of course, also be used in the warm-box or hot-box processes. The fact that Veining occurs only in exceptional cases with inorganically bound cores is certainly a welcome advantage in the core shop.

The residual strength and demoulding behaviour are very similar to those of the PUR cold-box process. In addition to the binders developed for iron and steel castings, variants for aluminium or copper castings are also available.

www.peak-giesserei.de