Sep 2, 2026Technical Articles
How to Control Porosity Defects in Lost‑Foam Casting
Porosity is a frequent defect in lost‑foam casting. Learn its root causes and practical solutions. Sintered ceramic sand improves mold breathability to reduce gas‑hole defects for your castings.
How to Control Porosity Defects in Lost‑Foam Casting
Abstract
Porosity is one of the most common defects in lost‑foam casting. Trapped gas inside castings will degrade mechanical performance, reduce product qualification rate and cause hidden safety risks. This article introduces the main causes of porosity and practical countermeasures, and also mentions how sintered ceramic sand helps relieve such defects.
What is Porosity Defect in Lost‑Foam Casting
Porosity appears as smooth round or irregular cavities on casting surface or subsurface. These voids are formed when gas cannot escape before molten metal solidifies. Different from shrinkage cavities, gas‑caused pores normally have smooth inner walls.
Main Causes of Porosity Defects
1. Foam pattern material
When high‑temperature molten metal contacts EPS foam pattern, the foam will decompose and produce large‑volume gas. If foam density is too high, or pattern contains residual moisture, gas output will rise sharply, which greatly increases porosity risk.
2. Coating performance and drying status
Coating acts as the gas‑passing barrier between foam and molding sand.
- Low coating permeability: decomposition gas cannot pass through coating smoothly.
- Non‑uniform coating thickness or over‑thick coating on local positions.
- Incomplete drying of coated pattern: residual water vapor will generate extra gas during pouring.
3. Molding sand & sand system
- Excess fine powder inside sand will block sand gaps and reduce overall mold breathability.
- High sand moisture will produce steam when meeting high‑temperature liquid metal.
- Poor sand fluidity causes uneven sand compaction, resulting in partial poor ventilation.
Spherical sintered ceramic sand owns excellent flowability and stable grain distribution. It keeps consistent gap structure inside the mold, improves overall breathability, and lowers gas‑trapping risk compared with traditional silica sand.
4. Vacuum system parameters
Insufficient vacuum degree cannot extract decomposition gas out of mold in time. Excessive vacuum may lead to metal penetration and other side‑effects. Unstable vacuum during pouring is also a typical trigger for porosity.
5. Gating & pouring process
Unreasonable gating system lacks effective exhaust channels. Too‑fast or too‑slow pouring speed will disturb gas‑discharging sequence. Glue for bonding foam patterns also releases large amounts of gas when heated.
Practical Countermeasures to Reduce Porosity
- Optimize foam pattern Select low‑density EPS foam. Reduce glue dosage for pattern assembly. Make sure patterns are fully dried before coating.
- Optimize coating process Choose high‑permeability foundry coating. Control uniform coating thickness. Complete thorough drying after coating; avoid wet patterns going into molding box.
- Improve molding sand condition Remove fine dust inside sand continuously. Strictly control sand moisture. Adopt spherical sintered ceramic sand to maintain stable mold breathability.
- Adjust vacuum parameters Set proper and stable vacuum value according to casting weight and structure. Keep vacuum stable throughout the whole pouring process.
- Optimize gating system and pouring operation Design dedicated exhaust vents for thick‑wall and hot‑spot positions. Match proper pouring speed, avoid violent molten‑metal turbulence.
Conclusion
Lost‑foam casting porosity comes from multi‑factor superposition of foam, coating, sand, vacuum and pouring process. Systematic adjustment of every link is required. Adopting high‑quality spherical sintered ceramic sand can optimize mold breathability, effectively assist gas discharging and cut porosity failure rate.
For more details about our sintered ceramic sand for lost‑foam process, please visit our Ceramic Sand Product Page.