Moisture is what holds a green sand mold together, and missing the right amount in either direction causes defects. Too much water turns to steam at the pour and gives gas porosity, scabs and buckles, metal penetration and oversized castings; too little leaves dry, friable sand that breaks at the pattern draw and ends up as sand inclusions.
Water is not the binder in no-bake resin sand, so there moisture in the sand or picked up from humid air weakens the mold and adds gas at the pour.
Why Green Sand Needs Moisture
The water activates the bentonite clay in green sand. Wetted clay turns plastic, coats the sand grains and binds them together, and the bond lets a green mold hold its shape from pattern draw to pour.
What Happens When Casting Sand Is Too Wet
Excess water turns to steam when the metal reaches the mold face, and the steam, together with the softer mold the extra water makes, produces four kinds of defect: gas porosity, expansion defects, metal penetration and dimensional error.
Excess water lowers green strength, mold hardness and permeability, but it raises dry and hot strength, so the mold is soft when rammed and hard to break down after the pour.
Gas Porosity and Blowholes
Excess moisture causes blowholes because the water flashes to steam faster than the mold can let it out. The gas is trapped against the metal, which cannot push it aside, and leaves holes at or just below the casting surface.
Whether the steam escapes depends on the mold’s permeability and venting, and a wet, hard-rammed mold has the least permeability.
Steam also breaks down at the metal face into hydrogen, which iron and steel dissolve. As the casting solidifies, the dissolved gas comes back out as tiny bubbles that show as pinholes rather than open blowholes.
Rattails, Buckles and Scabs
Excess moisture causes these expansion defects by creating a wet, weak layer just behind the mold face. At the pour, water in the hot face boils off and the steam moves back into the cooler sand, where it condenses at about 100 °C. That condensation zone keeps only a fraction of the sand’s normal green strength.
Meanwhile the dried face layer expands as the sand grains heat and comes under compression. When the weak wet zone behind it can no longer hold it, the face layer deforms or splits away and metal follows it:
- Rattails: irregular lines on the casting surface where the face deformed during filling.
- Buckles: indentations in the casting where the expanding sand pushed into the cavity.
- Scabs: a rough, thin layer of metal partly separated from the casting by a thin layer of sand.
Metal Penetration
Excess moisture opens a path for penetration. The water fills the spaces between the sand grains, and when it boils off at the pour it leaves those spaces open for the liquid metal to push into under its own pressure, where it solidifies with sand locked in it.
In iron there is a chemical route as well. Oxygen from the steam oxidizes iron at the mold face, and the iron oxide reacts with the silica sand to form fayalite, an iron silicate that bonds the sand to the casting.
Mold Wall Movement and Dimensional Error
Wet sand rams to a softer mold, and a soft mold wall gives way under the pressure of the liquid metal. The cavity grows, so the casting comes out oversized and can fall outside its drawing tolerances, and the extra volume the metal has to feed raises the chance of shrink.
What Happens When Casting Sand Is Too Dry
Sand that is too dry loses its bond and turns friable, so the mold breaks at the edges and tears when the pattern is drawn.
A short spell of air drying makes the sand surface brittle, so deep pockets will not draw cleanly and loose sand drops into the cavity, where it becomes sand inclusions in the casting. Loose grains are also easy for the metal stream to wash out of the face, which gives cuts, washes and erosion scabs.
How Much Moisture Green Sand Should Have
Green sand typically carries about 2 to 5% water at around 10% bentonite, but there is no single correct figure: where a given system sits in that band depends on how much clay and additive it contains.
In lab mixes, green strength peaks at a water-to-clay ratio of about 0.4, so a sand with more clay needs more water to reach the same condition. Dead clay, fines and combustibles such as seacoal soak up water without adding bond, and they push a system’s moisture requirement up.
Foundries control water additions by compactability alongside the moisture reading. Compactability is the percentage a loose sample shrinks in height when rammed (AFS 2220-00-S). Each foundry sets its own compactability target for its molding line and adds water to hold it. Moisture itself is measured by drying a sample with forced hot air (AFS 2218-00-S).
Sand also dries between the muller and the mold, so a reading taken at the muller can overstate the moisture in the mold.
What Moisture Does to No-Bake Resin Sand
In no-bake resin sand, water is not part of the bond, so moisture weakens the mold. Wet sand slows the cure and leaves a weaker mold, and moisture a cured mold picks up from humid air turns to extra gas at the pour.
For acid-cured binders such as furan and phenolic no-bake, the published limit is 0.25% moisture in the base sand.
A cured mold left in humid air absorbs water at its face. In a published test at 75 to 80% relative humidity, furan sand on reclaimed sand took up almost three times as much moisture in 12 hours as furan sand on calcined silica, and lost the most stiffness. A no-bake line running mostly reclaimed sand carries the larger risk when molds stand in humid weather.
That absorbed water comes off early in the pour, on top of the binder’s own gas, and raises the gas pressure at the mold face. One study of humid-stored cores linked the extra gas to blisters in the casting.