Investment Casting with 3D-Printed Patterns: Lost-PLA and Resin Burnout for Small Metal Parts and Jewelry

Sand casting, already covered elsewhere on this site, is the right process for larger aluminum parts where a slightly rough surface finish and moderate dimensional accuracy are fine. Investment casting is the other end of the spectrum — a lost-pattern process capable of reproducing fine detail and thin sections that sand casting can't, at a smaller part scale, and it's the process actual jewelry casting, small brackets, and precision hobby parts have used for over a century (traditionally with hand-carved or lost-wax patterns). The genuinely useful update for a maker shop is that a 3D printer replaces the wax-carving step entirely: print a pattern in PLA or a castable resin, invest it in plaster or gypsum-bonded investment, burn the pattern out in a kiln, and pour molten metal into the void it leaves behind.

How Investment Casting Differs From Sand Casting

• Pattern is consumed, not reused. Sand casting pulls a reusable pattern out of green sand, leaving a cavity behind. Investment casting burns or melts the pattern out of a solid investment block, meaning every casting needs a fresh printed pattern — more filament/resin cost per part, but no limit on the geometry's complexity or overhangs since there's no pattern to pull out.
• Far finer detail and thinner sections. Because the mold is a solid block of fine investment material (not compacted sand grains), it captures detail down to the resolution of your 3D print itself — fine text, thin fins, intricate jewelry settings, and internal features that would be impossible to pull a sand pattern from at all.
• Smaller practical part size. Investment casting scales down to small precision parts extremely well (this is literally how cast jewelry is made) but scales up poorly compared to sand casting — the flask, oven/kiln capacity, and investment material cost all get expensive fast for large parts.
• Wider metal options at small scale. Jewelry-scale investment casting commonly uses bronze, silver, and even gold in addition to aluminum, since pour volumes are small enough to melt in a small crucible or even a jewelry-specific casting machine.

Pattern Material: PLA vs. Castable Resin

PLA (FDM)Castable Resin (MSLA) Surface finishVisible layer lines transfer to the casting unless sanded/smoothed firstMuch smoother — ideal for jewelry and fine detail work Burnout behaviorLeaves more ash residue; needs a slower, staged burnout ramp to avoid cracking the investment from rapid gas expansionFormulated specifically for clean, low-residue burnout — the standard choice for serious jewelry casting Cost per patternVery lowHigher (specialty resin, plus printer/wash/cure time) Best forLarger, simpler parts where surface finish will be filed/polished anywaySmall, detailed parts — rings, pendants, fine mechanical parts — where finish matters

Whichever material you choose, keep patterns solid or near-solid infill for anything under an inch or two — a hollow pattern can trap gas during burnout and crack the investment from the inside, ruining the mold. For larger patterns where full solid infill would waste too much material, a raised infill percentage (50%+) with no fully enclosed hollow cavities is the safer middle ground.

The Process

1. Print or design the pattern with sprues built in. Every casting needs a sprue (the channel molten metal flows down) and, for anything beyond the simplest shape, vents to let trapped air and gas escape as metal fills the cavity. Attach these directly in your CAD design before printing, angled to allow metal to flow into the lowest points of the part first and displace air upward and out.
2. Mix and pour the investment material. Gypsum-bonded investment powder (the standard for jewelry-scale work) is mixed with water to a specific ratio and vibrated under vacuum if you have a vacuum investing table, to eliminate air bubbles that would otherwise show up as unwanted nubs on the casting surface. Pour it around the pattern inside a casting flask, fully submerging the pattern with the sprue opening left at the top.
3. Let the investment set, typically an hour or more depending on the product, before beginning burnout — rushing this step risks cracking the still-green investment during the thermal ramp.
4. Burnout in a kiln. This is the step that replaces "melting out the wax" in traditional lost-wax casting, and it's the same principle for a 3D-printed pattern: a slow, staged temperature ramp (often several hours, ramping through intermediate holds around 300°F/150°C and again near 1000°F/540°C before a final hold at casting temperature) burns the PLA or resin pattern out completely, leaving a clean cavity in the now fully cured investment. Rushing this ramp is the single most common cause of cracked investment blocks — trapped gas and thermal shock from the pattern material expanding and combusting too quickly will crack the mold from the inside, sometimes invisibly until the pour reveals a flawed casting.
5. Melt and pour the metal — aluminum, bronze, or silver depending on your setup — at the appropriate temperature for that alloy, using the same propane furnace and crucible/tongs setup covered in this site's sand casting guide if you're working in aluminum, or a smaller dedicated melting furnace for jewelry-scale silver/bronze work.
6. Quench and break out the casting. Once solidified and cooled to a safe handling temperature, the investment block is broken away (it's a single-use mold, unlike sand casting's reusable pattern) and the casting is cut free of its sprue, then cleaned up.

Finishing

Cut the sprue off with a jeweler's saw or cutoff wheel, then work through progressively finer sanding and polishing — a rotary tool with polishing compounds for small jewelry-scale parts, or standard bench grinding and polishing wheels for larger cast parts. Investment-cast surfaces start noticeably smoother than sand-cast surfaces, so finishing time is typically much shorter for a comparable part.

Safety Notes

This process shares every hazard of the sand casting process already covered on this site — molten metal, splash risk, and kiln/furnace burns — with one addition specific to burnout: burning PLA and resin patterns inside a kiln releases fumes that must be vented outdoors or through dedicated exhaust, not run in an enclosed garage or basement without ventilation. Do this burnout step outdoors or in a well-ventilated space with the kiln's exhaust routed away from where you're standing, the same caution warranted for resin 3D printing's own curing and cleaning fumes. Full PPE for the pour itself — face shield rated for molten metal splash, leather gauntlet gloves, flame-resistant clothing, closed-toe boots — is non-negotiable, and a Class D fire extinguisher should be within reach for the metals you're actually casting (water on a molten metal fire makes things dramatically worse, not better).

This is a natural next step for anyone who's already built a 3D printer or resin printer and a small foundry setup for sand casting — same core metal-melting skills, applied to a process that trades scale for detail, and one that opens up genuinely fine jewelry and precision-part work that sand casting simply can't reach.