In jewellery casting, sprue and tree design directly influences metal flow, defect rate, and overall productivity. Beginners and advanced technicians alike often ask how to choose diameters, angles, and positions that keep porosity away and deliver repeatable results. The goal is to create a feeding system that fills fast, stays laminar, and solidifies from extremities back to the button. This article explains the Top 10 Sprue and Tree Designs for Jewellery Casting Yield in clear, practical language, so you can apply each idea with confidence. Follow the guidance to improve first pass success, reduce rework, and protect fine details in every casting cycle.
#1 Tapered Central Sprue
A tapered central sprue is the backbone of a reliable tree. Keep the main sprue wider at the button and gradually narrower toward the top to encourage directional solidification. Use a gentle taper so the metal remains under positive feed pressure as it travels. Size the main sprue so its cross sectional area is at least equal to or slightly greater than the combined areas of the branch sprues that it must feed. This prevents starvation and porosity as pieces freeze. Maintain enough sprue height to deliver head pressure without creating unnecessary turbulence or long superheating times.
#2 Button and Base Geometry
Design the button as a heat reservoir that stays molten longer than the pieces. Choose a compact, slightly domed or cylindrical button with smooth fillets into the main sprue to avoid sharp corners that cause turbulence. Ensure the button volume comfortably exceeds the total shrinkage demand of the tree, especially when casting heavier signets or bangles. Keep the button centered on the flask base to balance heat flow and minimize distortion during burnout and casting. Add a small chamfer or radius where the button meets the base former to reduce stress concentration and make cut off safer after casting.
#3 Branch Diameters and Chokes
Set branch sprue diameters to maintain a gentle velocity and locate a controlled choke close to each piece. The choke should be the smallest section in the path so metal speeds up briefly at the entry, helping to push air out while limiting backflow. Blend all junctions with rounded fillets to prevent eddies. Avoid abrupt ninety degree gates because they trap air and create cold shuts. For thin items, use smaller branches with short, direct gates; for bulky items, increase branch size and move the choke slightly away so the piece receives sustained feed during early solidification.
#4 Angles and Piece Orientation
Angle branches between thirty and forty five degrees to the main sprue to smooth entry and reduce turbulence. Orient pieces so the heaviest section sits nearest the gate and the finest details are farther away. This encourages directional solidification and keeps shrinkage toward the sprue rather than in the jewellery. Position long, thin parts almost vertical so metal fills from the tip back to the gate, pushing air upward. Rotate settings so stones or hollows are protected from direct jet impact. Use consistent angles around the tree to distribute flow uniformly and prevent one side from freezing ahead of the other.
#5 Staggered Levels and Symmetry
Build the tree with staggered levels and radial symmetry. Alternate branch heights so each attachment has a clear metal path and good investment support. Keep similar mass pieces at similar elevations to even out solidification times. Balance the tree visually around the central axis to reduce uneven head pressure and to limit distortion during quenching. Avoid crowding many large items in one thermal zone because they will compete for feed and can cause shrinkage porosity. Leave enough clearance between items for investment strength and venting, particularly near delicate filigree that needs extra support during burnout and fill.
#6 Smooth Flow and Turbulence Control
Design every transition for smooth flow. Use gentle radii at the button to sprue junction, at branch takeoffs, and at the gate into the piece. A short trumpet shaped entry helps reduce separation and reoxidation. Avoid sudden expansions or dead legs that trap colder metal and dross. Keep branches as straight as practical and avoid long horizontal runs that lose heat quickly. Before investing, lightly smooth wax junctions to remove tool marks that can seed turbulence. Consistent smoothing results in laminar flow, cleaner surfaces, and fewer inclusions that would otherwise appear as specks or seams after polishing.
#7 Feeders and Risers for Heavy Sections
Use feeders or risers for heavy sections that cool slowly. A small reservoir attached opposite the gate can keep metal available as the piece contracts, pulling feed from the riser instead of forming voids. Neck the connection slightly so the riser stays molten longer and breaks away cleanly after casting. Place risers on signet faces, cufflink heads, or thick pendants that tend to shrink in the center. Balance risers around the tree to avoid overweighting one side. When properly sized, feeders reduce centerline porosity and improve polish, while adding only minimal cleanup time during finishing and quality control.
#8 Cluster Sizing and Flask Fit
Match cluster size to the flask so investment coverage and burnout are reliable. Keep at least ten to fifteen millimeters of investment between the tree and the flask wall for strength and ventilation. Avoid making the tree so tall that the top pieces sit in a cooler zone during pour. If needed, build two shorter tiers rather than one tall structure, and ensure each tier has clear vent paths. Check the mass ratio of metal to investment so thermal shock stays within safe limits. Right sized clusters cast more consistently, produce fewer cracks, and reduce the risk of breakout during vacuum or centrifugal fill.
#9 Alloy Specific Adjustments
Tailor sprue design to the alloy. Platinum needs larger sections and shorter, straighter paths because it is poured hotter and loses heat quickly. Gold alloys tolerate moderate sections, but high palladium or nickel content benefits from extra feed to counter shrinkage. Silver is very fluid yet oxidizes readily, so keep transitions smooth and avoid broad, flat reservoirs that expose metal to air. Brass prefers generous gates to fill thin charms cleanly. For all alloys, keep the button as the hottest, last freezing zone. Adjust diameters and lengths through small trials and record results to lock in your best practice.
#10 Standardization and Quality Control
Standardize your sprue and tree practice to make yield predictable. Use templates, calipers, and jigs to reproduce diameters and angles from batch to batch. Color code wax for main sprues, branches, and risers so teams follow the same pattern. Document flask size, tree weight, metal temperature, and gate positions alongside the defect rate and yield percentage. Review cut off time and quench timing together with tree geometry, because these variables interact. When you continuously refine and record the geometry that works, rework goes down, precious metal recovery improves, and customers receive consistent, high quality castings.