A production run of 40 rings came out with tiny round pits clustered on the top of every ring face, even though the wax patterns were flawless. The caster's first instinct was to blame the alloy. The root cause was the sprue and investment setup: the mold was not venting gas, and the sprues were too thin to feed metal as the piece shrank. Most casting defects are not metal failures. They are gating and burnout failures that appear on the finished surface. Understanding the basics helps buyers reject bad runs instead of accepting them as normal casting.
What sprues do
A sprue is the channel that carries molten metal from the pouring point into the mold cavity, and it doubles as a reservoir that feeds shrinkage as the part solidifies. Sprues that are too thin freeze before the part is done shrinking, and the metal pulls inward, creating shrinkage porosity at the heaviest cross-section, often the top of a ring face. Sprues placed at a thin point instead of a heavy section cause uneven flow and cold shuts. The art of gating is directing metal into the thickest area so the part solidifies directionally from the far end back toward the sprue. We gate each geometry individually; a simple band uses a different sprue layout than a carved pendant, and guessing is how defects get baked into the mold.
What investment does
Investment is the plaster-like refractory material that surrounds the wax pattern. It must be vacuum-mixed to avoid trapped air, poured in a controlled slurry, and left to set without cracks. During burnout, the wax vaporizes and the investment hardens into a ceramic shell. Two things go wrong here: trapped air in the mix creates rough, pitted surfaces on the casting; and under-fired or contaminated investment releases gas that gets trapped against the hot metal, leaving gas porosity, those round pits on the ring face. We control mix viscosity, vacuum time, and burnout temperature per alloy, and we inspect mold shells for cracks before pouring.
Defects are not random. Shrinkage voids sit at thick sections and look irregular. Gas porosity shows as small round pits scattered on surfaces. Cold shuts look like faint seams where metal did not fuse. Each points to a different fix: thicker sprues and a reservoir for shrinkage; better burnout and de-waxing for gas; faster pour or hotter metal for cold shuts. A shop that diagnoses by defect type rather than re-pouring blindly is a shop whose defect rate drops run over run.
How this connects to the rest of production
Gating decisions start in the CAD or wax stage. A 3D-printed wax pattern must have its sprue attachment planned so supports do not leave marks on visible surfaces. Alloy choice matters too: consistent, lot-locked grain flows and shrinks predictably, while a mixed remelt pours unevenly and amplifies the same gating problems. Defects that survive casting then show up in finishing, where pits refuse to buff out and require picking or, worse, re-casting.
Good shops track their defect rate per geometry. A healthy foundry runs under 3 to 5 percent defective on routine shapes; anything above that means gating or burnout needs review. For buyers, ask what the defect rate is on your geometry and how rejected pieces are handled. That number tells you whether you are paying for a controlled process or absorbing hidden re-casts.
Vent and sprue placement also decides where cleanup time goes. A sprue attached to a visible table leaves a nub that must be filed and re-polished; one attached to a hidden inner surface cleans up in seconds. We plan sprue attachment around the part's visible faces before printing, which is a CAD decision, not a last-minute choice at the bench. Every finished cast is also checked to fineness marking rules under 16 CFR Part 23 before it moves to finishing, because a porous or under-fine cast should never reach the polisher only to be scrapped at the last step.
Tracking defect type per lot is how a foundry improves run over run instead of treating every bad piece as bad luck.
FAQ
Why are there small pits on my cast jewelry?
Usually gas porosity from incomplete wax burnout or trapped air in the investment mix, not bad metal. It is fixed with adjusted burnout schedules and vacuum investment mixing.
Can polishing remove casting porosity?
Only shallow surface pits. Deep porosity requires picking the pit open, filling, or re-casting. Buffing over it just rounds the edge and leaves a dimple.
Is some casting defect normal?
A low rate, under 5 percent on routine shapes, is normal and expected. A high rate means gating, burnout, or alloy consistency needs correction, not acceptance.
Want a foundry that controls defects, not just re-casts them? HOLYCOME gates each geometry individually, burns out to a documented schedule, and reports defect rates per lot. Send your design for a custom manufacturing quote.
Frequently Asked Questions
How do I verify a factory before placing a bulk order?
Request a production video, business license, export records and a paid sample run. A factory that accepts a 50-piece test order is usually a real manufacturer.
What payment terms do you offer?
30% T/T deposit with the balance against the bill of lading. This protects both sides on first orders and can move to net terms after a track record.
What is your normal reject rate?
3–5% under 10x loupe inspection for cast and set pieces. Defective units are replaced free in the next shipment.
Where is the factory located?
Wuzhou, Guangxi, China—the core lab-grown and moissanite manufacturing cluster. Shipments go via Guangzhou or Shenzhen ports.
