A tree comes out of the burnout kiln, the flasks are quenched, and half the pendants on the upper tier are short-fed: crisp detail on the top of each piece, a rough blob where the sprue meets the bail. The caster shrugs, adds more sprues, and the next tree comes out with the same defect plus a new one, heavy finning at the sprue joint. Sprue design is not about how many sprues you add; it is about where the metal enters the piece and how fast it can fill the mold before it freezes. The investment molding reference on Wikipedia covers the gating physics at a process level; below is how the bench applies it.
Sprue diameter, angle, and the thickest point rule
The rule taught in trade school is still right: attach the sprue to the thickest, heaviest part of the piece, never to a thin edge. Metal flows through the path of least resistance and freezes last at the heaviest mass; attach the sprue there and the gate feeds the piece until it is solid. Attach a sprue to a thin bail and the bail freezes first, starving the pendant below. For sterling pendants, sprue diameter is 2.5 to 3.0 mm; for chunky 14K signet rings, 3.5 to 4.0 mm. The sprue should meet the piece at a 45-degree angle, never straight on; a straight sprue erodes the mold wall at the joint and leaves the finning the caster is fighting.
Tree layout: mass balance matters more than count
A tree should be radially balanced: heavy pieces on the lower, hotter tier, light pieces on the upper tier. A 2-inch flask (50 mm) comfortably carries 15 to 25 rings or 30 to 45 small pendants, depending on sprue count. Overpack the flask and the outside pieces freeze before the inside fills; under-pack and the tree tilts during burnout. Our notes on how many pieces fit on a casting tree cover the tier math, and sprue and investment defects tie each failure pattern to a layout choice. The short-fed pendants at the top of the tree? They were on the wrong tier; heavy pendants belong low, where the metal arrives hotter.
Vents, buttons, and the pour cup
Add a small vent sprue (1.0 mm diameter) on any openwork piece that traps air during fill, a filigree cuff, an open bangle, or a cage pendant. The vent lets air escape but does not carry metal. Put a pour button (a 12 mm ball) at the top of the main sprue; it acts as a reservoir that feeds the tree as the metal shrinks. Without a button, the last pieces on the tree starve. The pour cup at the base of the flask should hold about 30% more metal than the tree weight, which is how you guarantee the button stays liquid until the tree solidifies.
Common sprue mistakes and what they look like
Short-fed tops = heavy pieces on the upper tier. Rough finning at the sprue joint = sprue attached straight-on instead of at 45 degrees, or the sprue was too thin and eroded. Porosity inside the piece = sprue attached to a thin edge, so the heaviest part fed on trapped gas. Metal beads on the highest detail = no vent on an openwork piece. Each of these is a sprue problem, not an alloy problem; the grain you see at shakeout is the mold telling you where the gate was wrong. If you keep seeing porosity that polishing cannot remove, look at the tree layout before you blame the alloy.
FAQ: Sprue design for jewelry casting trees
How thick should a sprue be for a 5g silver pendant?
2.5 mm diameter minimum. A 10g piece wants 3.0 mm. The sprue should always be thicker than the section it feeds.
Why do I get fins at the sprue joint?
The sprue met the piece at 90 degrees instead of 45, or the wax sprue was not welded cleanly to the model, leaving a gap.
How many pendants fit on a 3-inch flask?
About 40 to 60 small pendants, 25 to 35 larger ones. See tree piece count for the tier layout.
Should every piece have its own sprue?
Yes. Branch sprues (a single main sprue feeding multiple pieces) save wax but increase short-feed risk on complex shapes.
Send us your wax tree design or a CAD model; our casting room will flag sprue angle, tier balance, and vent placement before the flask goes in the kiln. Trees are photographed pre-burnout, and small-batch custom manufacturing starts at one tree. The clean tree is the cheapest tree; we will quote it that way. We also run a pre-burnout weight check: the tree weight plus the pour cup button should total about 15% more metal than the finished pieces, which is the feed reservoir you need to avoid shrinkage porosity at the heaviest point of each casting.
Frequently Asked Questions
What care instructions should I give end customers?
Remove jewelry before swimming, cleaning or applying perfume. Store dry in a sealed bag. Clean with mild soap and a soft brush, then dry fully.
How often does rhodium plating need renewal?
Every 12–18 months under daily wear. Offer a re-plating service to your customers to build repeat visits.
Does chlorine damage sterling silver?
Yes. Chlorine and saltwater accelerate tarnish and can pit the surface. Rinse and dry immediately after any contact.
What causes green skin marks?
Copper or nickel in the base alloy reacting with sweat. Switch to 316L, titanium or sterling, and specify nickel-free plating.
