81/1

AWB – an environment friendly core production technology Th. Steinhäuser *, A. Wolff * * Institut für angewandte Materialtechnik, Universität Duisburg - Essen, Germany. Abstract The AWB core binder system is based on sodium silicate binders. The hardening of the cores is done by dehydration, there is no chemical reaction. This makes the hardening process reversible and reclamation simple. Dehydration is done by a combination of heated core box, vacuum and microwave treatment. The binder system has been widely tested at the University Duisburg – Essen and at the labs of Hydro Aluminium. A great number of cylinder heads in aluminium gravity die-casting have been made to prove suitability for serial production. Key words AWB, inorganic core binder,

81/2

Introduction Because of rising interest in environmental issues within the foundry industry during the last decade, there has been a lot of development especially in core binder systems. One direction was the reduction of emissions on organic binders. The other way was the development of inorganic core binders with competitive performance [1]. Patented already in 1997, the AWB process is one of the first new inorganic binder systems [2]. Due to now eight years of development it has become an efficient, practical, suitable system that can be used in serial coremaking with only minor modifications to the existing equiment. Experimental The basis of the technology is a modified sodium silicate. The AWB-binders are of a distinctly lower viscosity than conventional sodium silicate core binders and are therefore much easier to mix and homogenise with sand. This also serves to improve core shooting characteristics of the sand-binder mixtures which are in the range of cold-box mixtures. With AWB even thin-walled and complicated cores such as water jackets for cylinder heads can be produced. Sodium silicate binders are based on the chemical formula Na2OxxSiO2xH2O. During conventional hardening by gassing with CO2, the strength is generated through precipitation of Na as Na2CO3 by changing the “module”, i.e. the Na2O/SiO2 ratio. This procedure has decisive disadvantages:

• The chemical reaction is irreversible. • The “solvent” water largely remains within the core. • Only low strengths can be attained. • Na2CO3 and SiO2 can form a glassy phase at higher temperatures

which makes decoring difficult • Reclamation of such core sands is difficult or impossible.

In the AWB-process, hardening is done only by the removal of water (Fig.1). This has the following advantages:

• Physical drying reaction is reversible. • AWB-cores are nearly free of water and thus can be stored for a

longer period. • Due to their dry state, cores emit much less core gas compared

with organic binders. • Distinctly high core strengths are attainable. • Easy decoring possible due to lack of formation of sodium

carbonate. • Reclamation easy, remaining binder can be reactivated by addition

of water.

81/3

Hardening starts in the heated core box at temperatures of approx. 140 – 200 °C with a connected vacuum to absorb water vapour. After approx. 10 – 60 seconds – depending of core weight and geometry – a solid shell has been formed so that the core has sufficient handling strength and can be removed from the core box. Final dehydration takes place in a microwave at low power. This step is very important because any water remaining in the core may solve binderbridges and reduce storability. After cooling to room temperature, the core has reached its final high strength and can be used for casting. Results Characteristics of cores produced with the AWB-process as compared to cores produced with other binders have been evaluated in several extensive test series performed both at University Duisburg – Essen and Hydro Aluminium’s R&D in Bonn. Cold-box cores with a binder content of 0.4 / 0.4 % (resin and activator) and 0.8 / 0.8 % were used as benchmarks. Standard bending test pieces size 180 x 22.5 x 22.5 mm were used at a constant shooting pressure, silica sand H32 was used. The tests showed that in order to reach the same strength as with the above reference mixture, 1.5 and 2.5 % respectively of AWB-binder is required [3]. To produce strong cores in short cycle times it is important to keep the hardening time in the heated core box as short as possible. Fig. 2 shows the influence of the hardening time on the bending strength without microwave treatment. In practice, usually 100 N/cm2 is good enough to handle the core without problems. After microwave treatment, the core will have reached its full strength and will keep it over storage time. In Fig. 3 you can see, that the shorter the hardening time in the core box, the higher the strength after the microwave treatment. Particularly when casting aluminium alloys, the gas surge of vaporised binder material can lead to massive problems. The gas evolution was measured during casting in accordance with the COGAS®-system. Fig. 4 shows the gas emission during casting compared to those of a Hot-Box and a Cold-Box core. The results show that AWB drastically minimises the risk of gas-induced casting defects. In fact, it can be assumed that the measurable gas evolution is attributable to the expansion of the pore volume within the core sample. In order to document the suitability for serial production at Hydro Aluminium Mandl & Berger GmbH in Austria, slightly modified Hot-Box core tools were used to produce all cores for an automobile cylinder head, e.g. waterjackets, oil-galleries, inlet, outlet and cover cores. Cycle times of the core production corresponded to those of the current serial production. The connected vacuum helped to reduce the core shooting pressure by approximately 0.5 – 1 bar. Hardening time in the

81/4

microwave (1.5 kW) took 1 – 3 minutes, depending upon the core weight. This microwave time is independent from the cycle time of the core blower and can be performed either in batches or by a run-through microwave. Fig. 5 shows the process flow chart for the cylinder head production. It was proven that an uninterrupted core production with AWB on a standard core shooting machine is possible with the quality of the cores being as good as those of the current serial production with organic binders. The results of extensive tests of aluminium gravity die casting at Hydro Aluminium Mandl & Berger with AWB-cores proved that the process:

• Does not produce emissions other than water vapour. • Does not cause any condensate deposits within the die. • Allows production of defect-free castings. • Decoring can be done with standard equipment.

Recently there have been made successful tests with core package castings (CPS) for engine blocks at the Hydro Aluminium foundry in Dillingen [4]. AWB has also shown very good results after several trials of University Duisburg-Essen in iron, steel, stainless steel and other metal alloy castings. Fig. 6 + 7 show different castings produces with AWB. First tests to determine the compatibility of AWB bonded core sand with bentonite bonded systems have shown no negative effects on the compactibility and the wet tensile strength when adding up to 25% of AWB core sand. These tests have not yet been finalised, a definite report on their outcome will be published at a later stage. Conclusions The AWB-process is a new inorganic process, suitable for serial production. Based on the purely physical bonding, the sand can be regenerated without any problems. Core strength and cycle times are comparable to those of organically bonded cores. The production of filigree and complicated cores creates no problems. Serial production of aluminium cylinder heads and test production of cast iron, stainless steel, steel and non-ferrous metals have been very successful. The inorganic AWB-binder does not cause health-injurious emissions, neither during core production nor during casting. References 1. Anorganische Binder – Durchbruch oder Hoffnung, VDG-

Fachtagung, Wuppertal/Germany 14.11. 2002. 2. EP 0 917 499 Optimierte Kernherstellung, Prof. Dr.-Ing. Thomas

Steinhäuser, 26.07.1997 3. Wolff A., Steinhäuser T., AWB – ein umweltverträgliches

Kernherstellungsverfahren, Giesserei 91 (2004) Heft 6, Seite 80 - 84

81/5

4. Gosch R., Inorganic core manufacturing technology for engine blocks in core package process CPS, WFO/VDG Conference, Hannover/Germany, 23.-24.11.2005

5. Figures

81/6

81/7

81/8