3D WERK

CAD DESIGN · FOR CASTING

DESIGN FOR CASTING GUIDE

Design it to cast,
not just to print.

A printer will happily print a wall, claw or gap that the flask cannot turn into good metal. These are the design rules that decide whether a pattern casts. The figures are common starting points published by casting services and suppliers. Check them with your caster, then prove them in your own alloy.

Before you start

This guide is general information to help you get the most from your WaxJet. It does not replace the Flashforge manuals, the safety data sheets for your materials or your own safety checks. Where they differ, follow the manufacturer's instructions and safety data sheets. Isopropyl alcohol (IPA) is highly flammable, and warm IPA gives off more vapour. Flashforge's manual uses a mix of PPG400, ethanol and IPA at 40 °C, never above 43 °C. Ethanol is also highly flammable. Keep it away from heat, sparks and open flames, heat it only with equipment suited to flammable liquids, use it in a well-ventilated area, and wear suitable PPE such as gloves and eye protection. Investment powder contains crystalline silica, and wax burnout and metal melting give off fumes, so use ventilation and the PPE your safety data sheets list. As a business, you are responsible for work health and safety in your workshop, including risk assessment and how you store, use and dispose of chemicals. See our Terms and Conditions.

THE NUMBERS AT A GLANCE

0.8 mm

most walls in gold, silver 0.6 mm with a gloss finish

1 mm min

ring band wall

0.5 mm

claws and small bezels, 0.8 mm for long wires

0.3 mm

min gap between parts, wider is safer

1 : 1 max

engraving depth to width

1.5 mm+

drain holes in hollow pieces, two or more, larger to clear support

Common starting points from the Materialise, Shapeways, Stuller and Formlabs design guidelines, mostly for gold, with Cooksongold printed-metal limits as a cross-check. Your caster's numbers come first, and Stuller revises its standards, so check its current edition.

Before you start

Ask your caster for their limits. The alloy, the investment and the casting machine all change them, and your caster's numbers overrule any chart, including this one.

Design for the metal. These limits are for finished castings. They are set by whether the investment survives, the metal fills and the piece stands up to wear, not by what the printer can print.

Know your metal. Materialise accepts 0.6 mm walls in gloss finished silver but asks for 0.8 mm in gold. Stuller's production standards want 0.8 mm between shank rails in platinum and palladium, against 0.6 mm for other metals.

Allow for finishing, then test. Polishing removes about 0.1 to 0.2 mm from each polished surface, so a wall polished on both sides loses up to double, and every feature must still meet the minimum after it is polished. Prove your numbers with a test plate (step 6).

Walls, wires and prongs

0.8 mm for most walls. Stuller and Materialise use 0.8 mm for structural and external walls in most gold pieces, and so does Cooksongold for printed metal. Materialise accepts 0.6 mm in silver with a gloss finish.

Ring bands 1 mm or more. Materialise asks for at least 1 mm, as a thin band can break in wear. Stuller's production standards start shank profiles at 1.5 mm wide and 1.7 mm thick.

Inner walls 0.6 mm. For walls and floors that run between structural walls (Stuller).

0.5 mm for small parts. Claws and small bezels need at least 0.5 mm (Cooksongold, printed metal). Stuller's minimum for metal to cast at all is 0.5 mm thick by 0.6 mm deep.

Long wires and prongs 0.8 mm. For rods and prongs in open, mesh-like designs (Materialise). Short details on a solid surface can go down to about 0.35 mm across and 0.4 mm high.

Go heavier where it matters. Anything that takes wear, holds a stone or gets polished hard needs more than the minimum.

Claws, 0.5 mm min,, sized to, the stone, Gallery and, other walls, 0.8 mm, Ring band, 1 mm min,, heavier, for wear

Stone seats and settings

Keep the stones in the file. Seat each one around the actual stone, or the calibrated size you will buy. Stuller only accepts customer stones within 0.1 mm of the stones in the CAD file.

Size prongs to the stone. A Formlabs castable resin test ring used 0.7 mm prongs on 2.5 mm side stones and 1 mm prongs on its 8 mm centre stone. Stuller sizes prongs in proportion to the stone, never under 0.45 mm.

Leave the setter metal to move. Stuller's standards overlap each prong 0.15 mm into the girdle, with the base of the domed tip level with the table. That leaves metal above the girdle to bend over the crown when the stone is set. Bezel walls at least 0.45 mm.

Clearance under the stone. About 0.2 to 0.3 mm between a diamond's pavilion and the gallery, and 0.3 to 0.5 mm for coloured stones, which often have bulging pavilions (Charlie Herner, Ganoksin). Keep the culet at least 0.5 mm clear of the finger (Stuller).

Stones 0.15 mm apart, girdle to girdle, when prong set (Stuller).

Design heavy where you polish. Finishing takes off about 0.1 to 0.15 mm (Materialise, gold) and up to 0.2 mm (Stuller). Add it to prongs, bezels and edges.

finger side, table, 1, 2, 3, 4, 1, Overlaps the girdle, 0.15 mm, tip at table, 2, Prong in proportion to, the stone, 0.45 mm min

Gaps, holes and engraving

Every gap becomes investment. It has to fill, then survive molten metal rushing past. Too thin, and it breaks or the parts fuse.

0.3 mm is the minimum gap (Materialise, Shapeways). Stuller's standards ask for 0.6 mm between shank rails, and 0.8 mm in platinum and palladium. Wider is safer.

Draft prongs near a wall. Formlabs suggests a 5° draft, which leaves more room for investment between prong and wall.

Holes 0.5 mm or bigger (Formlabs, Cooksongold, Stuller), and that small only in thin material.

Engrave no deeper than wide. 1:1 depth to width at most (Materialise, Shapeways). A deep, narrow line leaves a thin fin of investment.

Lines 0.3 to 0.35 mm wide or more. Stuller's standards: letters 0.35 mm wide, 0.3 mm apart, no more than 0.6 mm deep, with 0.6 mm of metal under recessed text. Cooksongold keeps recessed text to 0.5 mm deep.

Plain fonts. Shapeways prefers sans-serif. In Formlabs' tests, the smallest letters of an ornate script failed to cast.

Too tight: the thin, investment breaks, Room to fill and survive, 0.3 mm min, wider is safer

1 : 1 max, wider than deep, deep and narrow, d

Red: the pattern, which becomes metal. Speckled: investment.

Hollow pieces

Why hollow. Less metal in a large piece means lower cost and weight. In an illustrative CAD example, cutting the minimum wall of a 13 mm hollow bead from 0.75 to 0.5 mm took its gold weight from 5.91 g to 4.75 g, nearly 20% less.

Start the shell at 0.8 mm. That is the gold wall minimum at Materialise, and Formlabs' bangle test used 0.9 mm. Go thinner only after a test cast. Go thinner only after a test cast.

Two or more holes over 1.5 mm, spread out (Materialise). Shapeways says two at opposite ends work best. Make them part of the design, as decorative or stone setting holes (Bell, Formlabs).

Printed wax: let the support out. On a WaxJet a closed hollow prints full of white support wax. The holes let warm solvent in and dissolved support out, so go larger than 1.5 mm where you can and test one hollow print first. Check nothing is trapped, and let the solvent evaporate fully before investing.

Anchor the core. Through large enough holes, investment fills inside and out at once and the core stays tied to the mould (Bell). One 3.5 mm hole in a bangle leaves a large core on a plug too small to hold it when the metal arrives (Formlabs). Casters can laser weld the holes shut afterwards (Bell).

support, wax, IPA in, out, Printed: support fills it,, warm IPA flows through, core, Invested: the core is, tied to the mould, core, One small hole: the core, hangs on a thin plug

Help the metal feed

Metal shrinks as it freezes. Most gold and silver alloys lose several per cent of their volume from liquid to solid. Every section needs liquid metal feeding it while it freezes.

Thin sections freeze first. A thin section between the sprue and a heavy one freezes and cuts off the feed. Thick, thin, thick sections are a recipe for shrinkage porosity.

Keep sections even. Uniform cross sections are one of the best things a designer can do for fill and density. Where they must differ, sprue the heaviest part.

Take the mass out, not the strength. For a heavy top on a thin shank, Bell suggests hollowing the top from the finger side and thickening the shank.

Fillet inside corners. A sharp inside corner leaves a point of investment that can break off into the metal (Formlabs). Cooksongold suggests a 0.3 mm radius.

Add a sprue point to the model. A small post or pad on the thickest section, away from seats and detail, as in the Casting Tree guide.

thin shank, freezes first, porosity, even, sections, Heavy top, thin shank,, sprue on the thin part, Top hollowed, shank thicker,, sprue on the heavy part

sharp, radius, Sharp inside corner, Filleted, 0.3 mm or more

Prove it with a test plate

Make one small plate with steps of wall, pin, gap and engraving sizes around the numbers in this guide. Cast it in your own alloy with your caster's process and see what fills. Repeat for a new alloy or caster.

Published figures vary. In Austin's test, acrylic bars cast 0.71% and 0.86% smaller than their models, and Shapeways says gold may come out about 2.5% plus 0.25 mm smaller. A rubber mould adds more.

Measure shrinkage, do not guess it. Metal contracts as it cools, the investment expands and offsets much of that (Richard Austin, Ganoksin), and the pattern has its own tolerance (Flashforge quotes ±0.04 mm per 20 mm for the WaxJet). Stuller: the shrinkage you get depends on your process.

Your shrinkage, in %

(pattern minus cast) ÷ pattern × 100

Measure the same feature on the clean pattern and the finished casting, then scale the model. For rings, see the Ring Sizing guide.

Walls, Wires and prongs, Gaps, Engraving, 0.4, 0.5, 0.6, 0.8, 1.0, 0.4, 0.5, 0.6

Prepare the file

Model in millimetres. Set the units before you start. A millimetre file read as inches prints 25.4 times too big.

Closed and watertight. Every part is a closed volume, with no holes or gaps in the mesh.

One shell per part. Union overlapping pieces, such as prongs, head and shank, into one solid before export.

Normals out, no self-intersections. Run a mesh check and fix flipped normals, holes and intersecting triangles.

Fine enough facets. You cannot print a model smoother than its STL. You cannot print a model smoother than its STL. McNeel's STL guide suggests a tolerance far finer than the printer's precision (0.001 to 0.005 mm for jewellery on a Solidscape wax printer), so start fine and test print.

Export STL. The WaxJet software takes STL and SLC files (Flashforge).