Various application methods can be used when processing refractory coatings. Medium sized cores and moulds produced in hand mould sectors are often coated by using flow-coating units. During this process, the pre adjusted coating suspension is pumped and flood onto the respective sand moulds, and following draining off the excess coating, leaves an evenly distributed, protective refractory layer. It has been observed that users often report different coating thicknesses from the upper to the lower area of moulds or cores after the process. This is due to a larger volume of material being delivered over the mould in the lower area supported by longer exposure of refractory fillers and additives to the sand surface in this specific area.
When dipping sand cores, a similar effect is visible. The lower area of the core experiences a higher hydrostatic pressure due to a deeper immersion, and in addition, the residence time of this area in the coating is sometimes considerably longer than that of the upper area. Since the layer thickness build-up can be seen as a function of immersion time and the pressure exerted by the suspension on the sand surfaces, it is not surprising that higher layer thicknesses are also found in the lower core areas during immersion than in the upper areas near the surface.
The coating development-Team of Hüttenes-Albertus has succeeded in counteracting this natural effect by developing new types of water-based coatings with modified rheological properties. The focus was to give a "pseudoplastic character" to the product, which causes the coating layer to increase its viscosity immediately after application and not continue to flow down the moulding material over a longer period creating run or tear marks.
Since this phenomenon occurs regardless of the area of the particular sand core or sand mould, it offers the ability to achieve more uniform layer thicknesses.
Another aspect of this new family of water-based coatings is the extended gloss time, which allows the operator more time for fettling of coated sand cores. From the beginning the coating properties here have been described as "very positive" by the users. During the matting of the coating layer, the effect of novel additives is leading to self-smoothening, drop-free and uniform surfaces.
The new developed water-based coatings from HA can be produced with increased viscosities and solids contents, which brings advantages regarding settling behaviour in flow-coating units and dip tanks. Likewise, a positive effect on the drying properties can be observed since less water has to be released into the ambient air.
At present, we can observe widespread customer satisfaction combined with defect-free castings. Further new developments are currently in progress in the HA Group laboratories, and we are looking forward to your evaluation.