A bench jeweler in Guangzhou pulled a lost-wax ring tree out of the kiln and found three castings short: two had hot tears at the shoulder, one had a void where the stone seat should be. The mold had been poured at the correct temperature, the burnout schedule was fine, and the wax models looked clean. The problem was the sprue: a single 1.2 mm round runner feeding a 4.2 g ring, too thin and too far from the heaviest mass. Spruing is where a casting order is won or lost before the metal ever melts.

For wholesale production, sprue design directly controls scrap rate. A jobbing shop running 18k yellow gold at 1,080 C can lose 8-12% of castings to porosity if runners are undersized, while a shop that matches runner cross-section to the heaviest section typically holds scrap under 4%. The difference shows up in your per-piece cost on reorders. This guide covers the numbers behind runner sizing, attachment angle, and venting so you can read a factory sprue tree and ask the right questions.

Why runner size follows the heaviest metal section

The sprue's job is to feed the shrinking alloy as it solidifies. Gold contracts roughly 4-5% by volume on solidification, and silver about 5%. The thickest part of the ring, usually the shank cross-section or a cathedral shoulder, solidifies last and needs a liquid-metal supply until it does. A runner smaller than that section acts as a neck that freezes first and starves the heavy area, creating shrinkage porosity that polishing will not hide.

Bench rule: the sprue attached to a piece should be at least as thick as the heaviest wall it feeds, and often 1.5x. On a 4.2 g solitaire shank that measures 1.8 mm at the thickest point, a 2.5-3.0 mm round or 2.0 x 3.0 mm oval runner is the starting point. A 1.2 mm sprue, the common shortcut on thin pendants, is correct only for pieces under 1 g with uniform walls.

Attachment angle and the thermal node

How the runner meets the wax matters as much as its size. A sharp 90-degree joint creates a thermal node: the junction cools slower than the surrounding metal and pulls from the ring instead of the other way around, flipping the feeding direction. Most casters attach the sprue at a 45-degree angle to the shank or to a point that will be cut away later, such as the bottom of a ring post, so the hot spot lives on the scrap sprue, not on the jewelry.

Placement also decides where the sprue stump remains. On a solitaire, attaching to the bottom of the shank leaves a small stump to file; attaching near the gallery risks digging into the seat. Wholesale factories plan the attachment so the stump lands on a surface that will be polished or set, not on a visible high point.

Venting and the pour cup

Investment mold gases have to escape as metal enters. A well-vented tree uses a short, wide pour basin (sprue base) that holds a reservoir of liquid metal above the mold, plus thin wax vents (0.5-0.8 mm) on pieces that trap air, such as enclosed bracelets or hollow-form pieces. Without vents, air pressure stops the metal from reaching fine detail and leaves a back-pressure void at the farthest point of the model.

The pour cup itself should be 15-20 mm in diameter and deep enough to cover the main sprue by at least 10 mm. A shallow cup lets the reservoir freeze too early and ends the feeding push before the heavy sections solidify. On a 20-piece tree in sterling silver, this single detail is the difference between a first-pour yield of 85% and 95%.

Reading the factory tree before you approve

When a supplier sends photos of a wax tree before burnout, check three things: runner thickness relative to the heaviest piece, angle of attachment, and whether the heaviest piece sits lowest in the mold (closest to the pour cup) so gravity feeds it. If the heaviest rings hang at the top of the tree, they are the first to starve. For more on how casting defects show up later in production, see our notes on visual inspection points and preventing quality disputes.

How thick should a sprue be for a silver ring?

At least as thick as the heaviest shank section, typically 2.5-3.0 mm round for a 4-5 g solitaire. A 1.2 mm sprue is too thin and causes shrinkage porosity.

Why do my castings have voids near the stone seat?

Usually the sprue attached too close to the seat and the seat is the heaviest, last-solidifying area without a direct feed. Move the runner to the shank bottom and size it to the seat mass.

Does gold need a thicker sprue than silver?

Gold has a wider solidification range and contracts about 4-5%, so runners are often 1.2-1.5x the heaviest section; silver sets faster and tolerates slightly smaller feeders, but the same angle rules apply.

How many pieces can hang on one sprue tree?

For 4-5 g rings, 15-25 pieces per 100-120 mm flask is typical. Crowding beyond 30 pieces creates mold wall interference and uneven feeding.

If you are comparing casting partners or reviewing a first sample run, the sprue tree is the earlie

What a sprue tree costs in scrap and remelt

Undersized runners do not just fail castings; they force the factory to cut, remelt, and re-pour the bad pieces. On a 20-piece tree with three voids, that is 15% of the lot going back through burnout and pour. Gold remelt carries a 2-4% loss each time, and the labor of re-investing a second tree is double the first. A factory that prices aggressively on casting may quietly absorb this loss for the first order, then raise the price or thin the runners on the reorder. Asking for the sprue photo on the first sample is what keeps the runner schedule honest on repeat runs.

st signal of how a factory manages yield. We cast 925 silver and 10k/14k/18k gold with documented runner schedules per piece weight, and share tree photos on approval. Review our MOQ tiers or use the first-order audit checklist to line up your next production lot.