Binder Ratio


The effect of the ratio of Part I to Part II resin for PUNB binders on porosity is shown in Table 1. The results designated "1998 version" refer to casting tests performed on current formulations.

Table 1. Effect of Binder Ratio on Porosity Formation



Binder ratios of 60:40 (Part I:Part II) provided sound test castings. As this ratio became balanced (50:50), trace amounts of porosity were found in a few test castings, but the majority made with balanced ratios were sound. In cases in which porosity was found, a substantial portion was surface porosity or semi-rounded holes (pockmarking). Because the binder ratio was unbalanced in favor of excess Part II (40:60 and 35:65), greater amounts of subsurface porosity formed.

The types of defects varied in intensity from nil to very severe. Although the recommended PUNB binder ratio varies between a 55:45 and 60:40, in actual practice, ratios favoring excess Part II often result from worn or defective binder pumps, air in binder lines, changes in binder viscosity from temperature and inefficient mixing. In the early 1970s, foundries would run binder ratios favoring excess polyisocyanates to facilitate the stripping of difficult cores or to increase fully cured core strengths.

New resin formulations showed little difference in casting performance compared to 1974 versions.

Table of contents
[Paper Header]
[Porosity Study]
[Binder Ratio] [Binder Level] [ Casting Temperature] [Section Size]
[Sand Effect] [Iron Oxide Additions] [ Binder Dispersion/Mixing]
[Metal Composition] [Core Age]
[Eliminating Porosity]
[Ti and Zr Additions] [In-the-Mold Additions] [ Core Washes]
[Core Post-Baking]
[Discussion]