the rate of shrinkage with volatile solvents, causing faster shrinkage. While higher binder levels may ini- tially slow the shrinkage, over time it is more likely to produce more total shrinkage. Several theories exist to explain mold and core binder shrinkage. T e fi rst involves the hardening
and curing of the binder. Organic mold and core binders are thermo- setting resins that harden and cure by cross-linking of polymer chains. As they react, an increase in density and a corresponding decrease in volume occurs. For the liquid resin only, the shrinkage can reach 10%. According to the theory, as the resin bridges contract, the sand grains are pulled closer together and the mold or core shrinks. With most binder systems, the
resin cures rapidly over a short time period. With coldbox cores, the reac- tion is caused by the introduction of catalyst gas into the corebox, and curing is nearly instantaneous. T e curing and cross-linking and result- ing shrinkage causes the core to decrease in size before ejection. One theory suggests if continued cross- linking and reaction occurs after the core is made, shrinkage will continue until the cure is complete. T e continuing strength develop- ment of the core over time suggests the curing process continues after the initial cure. Another theory suggests core
shrinkage is related to the solvents in the binder. Solvents may be used up to around 30% in a binder to reduce the viscosity of the resin, improve wetting of the sand grain surface, etc. When the binder cures, these solvents may be trapped in the resin. T e solvents are rela-
Fig. 1. Binder chemistries have a signifi cant impact on shrinkage. The graph shows the shrinkage curves for phenolic urethane coldbox cores (PUCB), phenolic urethane nobake cores (PUNB), furan nobake cores, inorganic warmbox cores and acrylic epoxy coldbox (AECB) cores.
tively volatile and evaporate over time. As the solvents evaporate from the resin bridges, a volume reduction of the resin bridge occurs, resulting in
shrinkage. T e more solvent in the binder, the more shrinkage occurs, and the more volatile the solvent, the faster the shrinkage occurs. It has been suggested that higher binder levels produce more shrinkage. With more binder, the thickness of the binder layer on the surface of the sand and the distance separating the sand grains would increase. A thick resin bridge would produce more shrinkage because it would be a higher percentage of the core volume. T e binder variables tested were
Fig. 2. Resin bridges separate the individual particles on this spherical, manmade aggregate.
binder levels, solvents, benchlife and binder chemistry. A binder made with no solvent had almost no shrinkage (Fig. 1). T is explains the minimal shrinkage from the acrylic epoxy binder that contains lower solvent levels than PUCB binders. T e furan nobake binder showed
October 2011 MODERN CASTING | 35
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