Porosity, blowholes, and sand holes are the most common surface defects in jewelry manufacturing, especially in precious metal vacuum casting, gypsum mold precision casting, and wax-set casting. Jewelry requires ultra-high surface finish and fine detail retention, so even small holes will damage appearance, reduce density, weaken mechanical strength, and cause cracking or stone loss in later processing. For high-value materials like gold, silver, platinum, these defects directly increase scrap rate and production cost.
In this article, we explain the real causes of surface porosity and pinholes, and share practical solutions based on more than 20 years of experience in precious metal casting equipment and technology from Yihui Casting.
Main Causes of Porosity & Sand Holes
1. Over-high Casting Temperature
When pouring temperature is too high, molten metal absorbs more gas and expands violently. Strong thermal shock also damages the mold, leading to gas bubbles and surface holes.
Rapid heating traps moisture and gas inside the mold. Unburnt mold residues vaporize during casting and form blowholes.
Repeatedly remelted scrap metal contains more impurities and dissolved gas. High old gold ratio results in poor fluidity and heavy porosity.
If the investment flask goes into the oven too soon after mixing, internal gas and moisture cannot escape naturally, causing sudden gas burst while casting.
Use recommended casting temperature for each metal. Stable temperature improves flowability, reduces gas pick-up, and minimizes bubbles.
Set staged heating curve to remove moisture and organic binder slowly and completely. Avoid quick temperature rise to protect mold integrity.
Control recycled metal proportion to maintain purity and low gas content. This greatly improves surface smoothness and density.
Allow enough resting time to release internal stress and air. A stable mold structure ensures cleaner casting results.
Surface porosity and sand holes are avoidable with standardized process control. By optimizing temperature, heating, material ratio, and mold preparation, you can greatly improve yield, reduce waste, and produce high-quality jewelry castings consistently.
With professional equipment and strict technical management, Yihui Casting helps manufacturers achieve stable, efficient, and high-standard precious metal casting production.
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