How to Stop Casting Porosity

The work done in foundries requires attention to detail.

"Stop Casting Porosity" is a snarky slogan in San Francisco, inspired by a sign atop a foundry in Oakland. However, sculptors and foundry workers really do labor to "stop casting porosity." The simple declaration means simply that foundry workers should pay attention to details. Casting pores, which are air bubbles frozen in metal, do not usually impair the functionality of cast metal machine parts, but they can ruin decorative castings and statues. They happen often when new alloys are poured into complex forms.

Things You'll Need

• Well designed mold
• Properly prepared casting sand
• Correct metal

Instructions

1. Design the mold correctly. A leading cause of casting porosity is non-uniform cooling of the metal inside.

2. Mull, or mix, the casting sand correctly. Many casting defects are caused by sand that has been incorrectly prepared.

3. Pack the mold carefully and completely. A common cause of casting porosity is air trapped during filling because passages in the mold called sprues, runners and risers are improperly made. Molds may be packed mechanically or by hand.

4. Melt the correct amount of the correct metal. Impurities, like slag, are sometimes found in the cast metal, and cause gas bubbles.

5. Heat the metal to the correct temperature. Bronze heated to 1650 degrees is less likely to cause casting porosity than bronze that is overheated to 1850 degrees.

References

• Rastgar Group; Porosity in Castings; R. Monroe; 2005
• Metal Casting Design; Understanding Porosity; 2011
• Impco: Porosity Classification System Helps Determine Solutions to Casting Leaks
• CustomPartNet; Sand Casting; 2009