Abstract
In recent years, an increasing interest in sandmixes containing inorganic binders has been observed. These binders, including water-glass, are harmless for the environment, neutral for humans, and relatively cheap. In spite of numerous advantages, their wide application is restricted by poor knock-out properties and problems related to rebonding. Therefore, numerous researches aimed at eliminating the disadvantages of water-glass binders are directed, among others, to modifying the structure of hydrated sodium silicate or to applying new hardening techniques. An innovative method of rapid hardening by microwave heating, permitting the restriction of the quantity of binder used and thus improving knock-out properties, meets the expectations of present-day foundries. In this paper, available information is compiled on microwave hardening of water-glass containing sandmixes; furthermore, the costs of practical application of this technology are evaluated on the grounds of the authors' own research.
Highlights
Hardening of water-glass containing sandmixes consists in converting sol to gel, which can be chemically or physically initiated
The paper presents a comparison of four basic methods of hardening cores: traditional drying, microwave heating, hardening with CO2 and the ester process
From the viewpoint of foundry reason, the "economical" cores hardened in a microwave oven are characterised by the best properties, because: – they are characterised by the best bending, compression and tensile strengths already at 1.5 % of water-glass content [5], – low water-glass content guarantees good knock-out properties and easy reclamation of sandmix, – they have the best knock-out properties that are, acc. to the author of [14], decisive for wide application of this kind inorganic binders, – are characterised by low friability, ca. 15 times lower than in the case of CO2 hardening [5]
Summary
Hardening of water-glass containing sandmixes consists in converting sol to gel, which can be chemically or physically initiated. In the ARCHIVES of FOUNDRY ENGINEERING Volume 12, Issue 1/2012, 75-80 image of a binder bridge obtained by innovative microwave hardening (Fig. 1a), a homogeneous vitreous structure is visible, guaranteeing high strength. In comparison to traditional drying, the alternative method of microwave heating, based on highfrequency polarisation of water molecules, permits obtaining uniform, equal temperature increase in the whole volume and reducing dehydration [7]. It often happens that, depending on atmospheric conditions, this time does not guarantee complete removing water bound in the binder. This can result in problems manifesting themselves by casting defects [9,10]
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