WET TO DRY—REFRACTORY COATING CONTROL FOR PRECISION PUCB SANDS S. Ramrattan and M. Joyce
Western Michigan University, Kalamazoo, MI, USA Copyright © 2011 American Foundry Society Abstract
This research project was supported by the American Foundry Society (AFS) and the American Metalcasting Consortium (AMC). As the industry moves to tighter dimensional controls of castings, stricter coating controls are required. Baumé as a singular refractory measure does not suffice. An out-of-spec Baumé measure does not suggest the corrective wet chemistry adjustment. In addition, coating solids, surface tension, and viscosity must be monitored to determine the proper corrective action.
The effect of refractory coating thickness on thermal distortion was studied. Coatings were prepared using different levels of surfactant. Additionally, dip time was examined. Disc-shaped phenolic urethane coldbox binder (PUCB) sand specimens were robotically dipped into the refractory coating mixture and then dried. During these processes, wet and dry weights of the applied coating were measured. The coating thickness was measured and compared to permeability and elevated temperature testing.
To control the depth of coating penetration without altering coating solids, the level of surfactant in the coating was varied. Three different surfactant levels, 0.15, 0.25 and 0.35%, were studied. Increasing the level of surfactant decreased the surface tension of the coating, increased the low shear viscosity of the coating, and increased the thixotropy of the coating. The lower surface tension coatings wetted the PUCB sand samples more readily. The wet coat weight thickness was not influenced by dip time at the highest level of surfactant
Introduction
Refractory coated chemically bonded sand cores and molds are an important part of metal casting technology, and their behavior in contact with molten metal is of great interest. Test methods currently employed to measure heat-induced thermo-mechanical changes of coated sand composites have not specifically addressed the influence of changes in mold/metal interfacial temperatures. With today’s emphasis on near-net-shape, thin-wall castings, and with ever more stringent casting dimensional reproducibility requirements, there is a need to better understand the relationship between refractory coating thickness and thermal distortion of chemi- cally bonded binder systems. With this understanding, mea-
International Journal of Metalcasting/Spring 11
addition but was strongly influenced at the other two levels. At the highest surfactant level, complete wetting occurred at all dip times, resulting in equal wet coating weights. Wet coat weights and dry coat weights showed the same trends. Depth of penetration increased with surfactant level. The higher the surfactant level, the greater the influence of dip time on the depth of penetration. The results indicate a relationship between capillary pressure and surface tension forces. The lower the surface tension of the coating (higher surfactant level), the more readily the coating wets the sand specimen. The thickness of the refractory coating layer correlated well to permeability.
The distortion of the coated PUCB disc specimens was measured at 1000C (1832F). The thermal distortion curves (TDC) and mass changes are provided. The results from TDC for the different thicknesses of refractory coating on PUCB sand systems showed differences. The surface tension influenced the wetting and penetration of the coating. Coatings that wetted more readily had a thicker subsurface coating thickness and thinner proud layer coating thickness. The underlying sand distribution also affected coating thickness. The refractory coating prevented sand/binder losses but offered expansion at the hot surface/specimen interface. The level of distortion depended on coating thickness.
Keywords: refractory coating, chemically bonded sands, thermal distortion, coating thickness, coating control
sures can be developed for foundries to regulate coating thickness to minimize thermal distortion.
This study represents two phases of a research project guided and overseen by the American Foundry Society (AFS) 4F- Mold-Metal Interface Reaction Committee and the Ameri- can Metalcasting Consortium (AMC). Phase I was geared toward studying variations in wet coating properties (prior to application on sand) as the solids content was varied and relating solids content to the amount of dry coating deposit- ed onto a PUCB disc-shaped sand specimen.1
The deposited
coating consisted of a proud layer (coating above the surface of the sand specimen) and penetrated interlocking interfacial layer (coating penetration beneath the surface[Figure 1]).
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