Casting metal is an ancient art. Since the birth of the Bronze Age, the process of metal casting has evolved through incremental improvements to technology and process. While these refinements have driven incremental changes, the core foundry process remains the same.
When molten steel is poured into a chemically bonded sand mold to produce a casting, heat moves rapidly from the liquid metal into the surface of the sand and produces steep thermal gradients. As a result, the sand grains will expand and the organic binder in the mold will pyrolyze, producing carbon monoxide and other gasses that can cause defects. Throughout history, refinements arose and the quality of metals improved. New additives were developed and perfected to bring new qualities to the castings. Colors and strengths improved, giving foundries the ability to cast ever-more-complex shapes. As smelting improved, so too did the quality of base metals and the alloys produced from careful addition of new materials. Sand additives are commonly used in molds and cores to reduce defects like veining, metal penetration and unacceptable surface finish.
The cost-effective additive to eliminate casting defects such as veining, penetration, pinholes, shrink, burn in, and lustrous carbon.
| Content | Percentage |
|---|---|
| Color | Black |
| Apparent Bulk Density | 175 lbs/ft3 |
| pH | 7.8 |
| Package | 2225 lb Jumbo Bag |
| Moisture | 1% max |
| Typical Particle Size Range |
+40 Mesh 5% 40 * 100 Mesh 46% 100 * 200 Mesh 31.5% -200 Mesh 17.5% |
| Content | Percentage |
|---|---|
| Fe3O4 [Magnetite Content] | 95 - 98% |
| SiO2 | 1.25 - 2.5% |
| Al2O3 | 0.15 - 0.35% |
| Mn | 0.15 - 0.25% |
| Fe content | 69 - 71% |
Veining: Veining defects result from the expansion and contraction in silica sand when it comes in contact with molten steel. Sand additives can modify these reactions, but they also can affect negatively mold/core strength, permeability and bench life. The challenge has been finding an additive with the best anti-veining qualities with minimal impact on other properties.
Metal Penetration: Also known as burn-in/burn-on in less severe cases, metal penetration occurs when liquid metal fills the small voids between the grains of a sand mold or core. This bonds a layer of sand to the surface of the casting, increasing cleaning room time and cost.
Penetration, which can be influenced by sand additives, can occur by both mechanical and chemical mechanisms. An additive with a particle size smaller than the sand grains will tend to fill voids and reduce mechanical penetration. Other additives, such as iron oxide and fluxing ESAs, may promote the formation of fayalite (Fe2SiO4) and increase chemical penetration.
Surface Finish: Surface roughness of a steel casting depends on the sand particles of the mold/core. Larger particles will produce a rougher surface than finer grains. The surface tension of the liquid metal and its ability to smooth over the small imperfections on the mold/core will impact the eventual finish. Sand additives will affect the surface finish similarly to metal penetrations. Fine material will tend to fill in imperfections on the mold/core surface. Carbonaceous additives may positively impact metal surface tension while oxides and fluxes may reduce surface tension.
Carbon Pickup: Steel castings can pick up surface carbon created by the decomposition of an organic binder. Carbon pickup can be controlled through a number of different process variables, including binder type and percentage. Sand additives also can have an effect. Additives like iron oxide that release oxygen at casting temperatures can remove some of the carbon, while others may promote carbon pickup if they contain carbon or if they reduce mold/core permeability and retard the escape of the carbonaceous gasses.
Gas Defects: Gas defects can occur in steel castings by two mechanisms. If gas pressure at the mold/metal interface is higher than metallostatic pressure, gas can push into the liquid metal to create blow defects. If the mold atmosphere contains gasses that are soluble in the liquid metal, the steel can absorb additional gas like hydrogen or nitrogen that may come out of solution during solidification.
