A wholesale buyer runs 50 sterling statement pendants on the shop centrifugal machine. The pieces come back with small round voids on the concave back, and the caster blames the investment powder. The real problem is machine choice: a centrifugal flings metal into a mold that still contains air, and on thin concave backs that air can get trapped. Switch the same tree to vacuum-pressure and the voids mostly disappear. The question every casting buyer eventually asks is which machine should be running their order.
How each machine pulls metal into the flask
Centrifugal casting spins the flask on an arm. Molten metal is thrown into the sprue by centripetal force and pushed into every detail of the investment. It is simple, fast, and handles heavy flasks; the spin also pushes dense metal to the outer edges and leaves lighter gasses toward the center, which can be good or bad depending on tree layout.
Vacuum-pressure casting does the opposite. The flask sits in a chamber, a vacuum pulls air out of the investment pores before the metal arrives, and positive pressure behind the melt pushes the metal in under 2 to 3 bar. The result is denser fill on thin details and far less trapped gas. Our defect log shows 925 silver porosity defects under 1 percent on vacuum-pressure runs, versus 3 to 5 percent on a poorly tuned centrifugal of the same tree.
Where each method wins
Vacuum-pressure wins on: rings with delicate gallery work, thin castings under 1 mm wall, pieces with fine filigree, and small-batch sterling or 14k runs where porosity scrapes margin. It also gives the most consistent surface, which means less finishing time before polishing compounds and finishing stages. It pairs directly with investment powder water ratio for vacuum; under-vacuum slurry must be denser than the centrifugal mix, or the mold breathes too slowly and traps gas anyway.
Centrifugal wins on: heavy flasks above 4 inches, platinum runs (which need the higher force to fill long sprues), and large pieces where a vacuum chamber simply cannot take the size. It is also more forgiving of temperature variation, because the spin drives metal in even if the pour is slightly cool.
What porosity failure looks like under each
Vacuum failures show up as incomplete fill (short shots) when the vacuum is weak or the investment is under-mixed. Casting porosity causes and remedies on vacuum runs almost always trace to slurry ratio or burnout hold: burnout flask temperature hold that is too short leaves wax residue, which turns to gas and blows voids in the vacuum chamber.
Centrifugal failures show up as round gas bubbles on the convex surface and incomplete detail on the far side of the tree. The fix is not more spin; it is re-balancing the tree so the heavy pieces sit near the outside and the light details climb the sprue.
Marking and alloy fit across both machines
Neither machine changes the alloy; both pour the same 14k, 18k, or sterling. What changes is density and surface. Vacuum-pressure castings hold cleaner detail on filigree and gallery wire, which means less filing before the bench file grit and surface prep stages. Centrifugal castings show a slightly rougher surface on the far side of the tree, which we compensate with a denser investment slurry. For pieces that must carry a stamped karat mark, we stamp after casting, not before; porosity under the stamp will blur it. We follow the 16 CFR Part 23 marking rules on every piece, whether it comes off the vacuum or the centrifugal line, and we photograph the stamp next to a loupe for the QC file.
FAQ: Vacuum vs centrifugal casting
Which casting method gives denser rings?
Vacuum-pressure. The vacuum pulls investment air out before the metal arrives, and positive pressure pushes the melt in. Porosity rates run under 1 percent on well-tuned vacuum runs, against 3 to 5 percent on centrifugal.
Can you switch between vacuum and centrifugal mid-order?
Yes, but the tree layout and investment slurry change. Centrifugal trees are sprued for spin; vacuum trees are sprued for vertical fill. We re-sprue before switching, which is priced as a setup change.
Why do my centrifugal castings have bubbles on the back?
Trapped air. The spin pushes metal in but does not pull air out. Move the heavy piece to the outer radius, thin the sprue on the far side, or switch the run to vacuum.
Is vacuum casting better for 14k gold?
Yes for rings and delicate pieces. For heavy 14k charm pendants above 15 g, centrifugal still wins on fill weight. We choose per piece weight and wall thickness, not per alloy.
Send over your piece weights and wall thickness. Our bench runs both vacuum-pressure and centrifugal on the same floor, and we assign each order to the machine that matches its porosity budget. Small-batch OEM custom manufacturing starts at 30 pieces per design; send the tree layout and we will recommend the machine.
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.
