Lost PLA Casting: Turn 3D Prints into Metal

The first time I pulled a solid aluminum part out of a plaster mold — a part that started as a PLA print on my desk — I was completely hooked. Lost PLA casting bridges the gap between desktop 3D printing and real metalworking, and it is far more accessible than most people think.

Lost PLA casting is an adaptation of the ancient lost wax casting process. Instead of carving a wax model by hand, you 3D print one in PLA. The PLA burns out of the mold during firing, leaving a perfect cavity that you fill with molten metal. The result is a solid metal part that replicates your 3D model down to fine details.

How Lost PLA Casting Works

The process follows these broad steps:

1. 3D print your model in PLA with specific settings optimized for casting
2. Attach sprues and vents — channels for metal to flow in and air to escape
3. Invest the print — encase it in a plaster-like mold material (investment)
4. Burn out the PLA — heat the mold in a kiln to vaporize and ash the PLA
5. Pour molten metal into the empty cavity
6. Break away the mold and clean up the casting

Each step has its own tricks, and I have learned most of them the hard way through failed castings. Let me walk you through what actually works.

Step 1: Design and Print for Casting

Not every 3D model casts well. Casting has different rules than printing, and ignoring them leads to failed pours and frustrating results.

Design Rules for Castable Parts

• Minimum wall thickness: 2-3 mm. Molten metal does not flow into thin sections easily, especially aluminum. Walls thinner than 2 mm often result in incomplete fills.
• Avoid sharp internal corners. Metal shrinks as it cools, and sharp corners concentrate stress that causes cracking. Add fillets of at least 1 mm everywhere.
• Draft angles help. Slight tapering (1-3 degrees) on vertical surfaces helps the metal flow and the investment release.
• Think about where metal will flow. Thick sections should be at the bottom (near the sprue), and thin sections should be at the top. Metal flows downward with gravity.

Print Settings for Casting

These settings differ from normal printing because the PLA needs to burn out completely with minimal residue:

| Setting | Value | Why | |---|---|---| | Infill | 0% (hollow) or 5-10% | Less PLA means less material to burn out and less ash residue | | Walls | 2-3 perimeters | Enough structure to handle the mold but burns out cleanly | | Layer height | 0.1-0.15 mm | Finer layers mean better surface detail on the final casting | | Top/bottom layers | 2-3 | Minimal solid layers reduce PLA volume | | Supports | Avoid if possible | Support scars transfer to the casting |

I print my casting patterns with Hatchbox PLA — standard PLA works fine. Some casters prefer specialty casting PLA filaments that leave less ash, but standard PLA works for most projects.

For really detailed work like jewelry, eSUN Castable Resin on an SLA printer gives better results than FDM, but that is a different process.

Step 2: Build the Sprue System

The sprue system is how molten metal enters the mold and how gases escape. A bad sprue system is the number one cause of failed castings.

Components

• Sprue: The main channel where you pour metal in. Usually a tapered cylinder, thicker at the top (pouring cup) and narrower where it meets the part.
• Runners: Channels from the sprue to different sections of the part.
• Vents (risers): Channels from the top of the part upward, allowing air and gas to escape as metal fills the cavity.

I model my sprues in CAD and print them as part of the tree, but you can also attach wax rods to the PLA print manually. Freeman Sprue Wax works well for this — you can melt the ends with a soldering iron and stick them to the PLA print.

Sprue Sizing Guidelines

• Main sprue diameter: 6-10 mm for small parts, 10-15 mm for larger parts
• Runner diameter: 4-6 mm
• Vent diameter: 3-4 mm
• Attach sprues to the thickest section of the part — this ensures the thickest area (which solidifies last) has a direct metal supply

The VegOilGuy YouTube channel has excellent tutorials on sprue design for backyard casting that I learned a lot from when starting out.

Step 3: Invest the Print

Investment is the mold material that surrounds your PLA print. When the PLA burns out, the investment retains the exact shape of your part.

Investment Options

For jewelry and small parts: Use proper jewelry investment powder like Ransom & Randolph Plasticast. Mix with water according to the instructions, vacuum degas if possible, and pour into a flask around your sprued print.

For larger parts and aluminum casting: Plaster of Paris mixed with fine silica sand (50/50 by volume) works for basic casting. It is much cheaper than jewelry investment but less detailed and less durable. Some casters use a mixture of plaster, sand, and perlite.

Investing Process

1. Mount your sprued PLA print to the base of a casting flask (a steel cylinder) using wax or hot glue
2. Mix your investment — it should be the consistency of thin pancake batter
3. Pour slowly, letting the investment flow around the part without trapping air bubbles
4. Vibrate or tap the flask to release bubbles
5. Let it cure for at least 12-24 hours at room temperature

If you have access to a vacuum chamber, degassing the investment before pouring eliminates micro-bubbles that cause surface defects on the casting. A vacuum pump and chamber designed for resin work also works for investment degassing.

Step 4: Burn Out the PLA

This is where PLA differs most from wax, and it is the step that causes the most problems for beginners.

The Burnout Schedule

PLA does not melt and flow out like wax. It decomposes into gases and leaves behind a small amount of ash. The burnout schedule needs to be slow enough to avoid cracking the mold and hot enough to fully eliminate the PLA.

Here is the burnout schedule I use:

| Stage | Temperature | Hold Time | Purpose | |---|---|---|---| | Ramp 1 | Room temp to 150°C | 1 hour ramp | Drives off moisture | | Hold 1 | 150°C | 1 hour | Ensures moisture is gone | | Ramp 2 | 150°C to 350°C | 2 hour ramp | PLA begins decomposing | | Hold 2 | 350°C | 2 hours | Main PLA burnout phase | | Ramp 3 | 350°C to 730°C | 2 hour ramp | Burns remaining ash and residue | | Hold 3 | 730°C | 2-3 hours | Ensures complete burnout | | Cool down | 730°C to casting temp | Varies | Bring to appropriate casting temperature |

The casting temperature depends on your metal:

• Aluminum: Cast at 650-700°C mold temperature with metal at 700-750°C
• Bronze/Brass: Cast at 500-600°C mold temperature with metal at 950-1050°C
• Silver: Cast at 480-540°C mold temperature

According to Smooth-On's casting guide, the most common failure point is not holding at 730°C long enough. PLA ash is stubborn, and even a small amount of residue creates gas bubbles that ruin the casting surface.

Equipment for Burnout

You need a kiln or furnace capable of reaching at least 730°C with programmable temperature control. A pottery kiln works perfectly. The Paragon SC-2 kiln is popular among small-scale casters.

If you are doing backyard aluminum casting, some people use a propane-fired furnace for both burnout and melting. It works but is less controlled than a programmable kiln.

Step 5: Pour the Metal

This is the exciting part — and the most dangerous. Molten metal demands respect.

Safety Equipment (Mandatory)

• Face shield (not just safety glasses)
• Leather gloves rated for high heat
• Leather apron
• Long sleeves and pants (no synthetic materials — they melt onto skin)
• Leather boots (no sneakers)
• Work on dry concrete or dirt — never near anything flammable

Pouring Tips

• Heat the metal 50-100°C above its melting point to ensure good flow
• Pour steadily and continuously — stopping and starting creates cold shuts (visible lines where metal flows did not merge)
• Fill from one side and let the metal rise evenly
• Pour within seconds of removing the mold from the kiln — the mold cools quickly and cold molds cause incomplete fills

For aluminum, you can melt it in a steel crucible using a propane furnace. Aluminum melts at 660°C, which is achievable with a basic setup.

As detailed in the All3DP guide to lost PLA casting, aluminum is the best metal to start with because of its relatively low melting point and forgiving flow characteristics.

Step 6: Break Out and Clean Up

After the metal has solidified and cooled (give it at least 30 minutes), you can break away the investment.

For plaster-based investment, quenching in water while still warm (but solidified) causes the plaster to crack and fall away. For jewelry investment, it typically dissolves in water with some scrubbing.

Remove the sprues with a hacksaw or Dremel, then file, sand, and polish to your desired finish. Aluminum takes a beautiful brushed or polished finish. Bronze and brass can be patinated for an antique look.

Common Failures and How to Fix Them

Incomplete fill (part is missing sections)

• Metal or mold was too cold
• Sprue was too small
• Vents were inadequate (gas had nowhere to escape)
• Wall sections were too thin

Rough or porous surface

• PLA ash was not fully burned out (increase hold time at 730°C)
• Investment was not degassed (air bubbles transferred to the casting)
• Metal was not hot enough

Cracked mold

• Burnout ramp was too fast
• Mold was not fully cured before firing
• Thermal shock from rapid temperature changes

Shrinkage holes in thick sections

• Add risers (vents) over thick sections so they have a metal supply as the casting shrinks
• Redesign to make wall thickness more uniform

What Metals Work Best

| Metal | Melting Point | Difficulty | Best For | |---|---|---|---| | Aluminum | 660°C | Beginner | Structural parts, decorative items | | Brass | 930°C | Intermediate | Decorative items, hardware | | Bronze | 950°C | Intermediate | Art, sculptures, functional parts | | Silver | 961°C | Advanced | Jewelry | | Gold | 1064°C | Advanced | Jewelry |

I recommend starting with aluminum. It is cheap (you can melt scrap aluminum cans, though ingots give cleaner results), the melting point is manageable with a basic propane furnace, and it is very forgiving of beginner mistakes.

Cost Breakdown for Getting Started

You do not need thousands of dollars to start lost PLA casting. Here is a basic aluminum casting setup:

• 3D printer (you already have this): $0
• PLA filament for patterns: $5-10 per project
• Plaster and sand investment: $20 for enough to do many castings
• Propane furnace (DIY or purchased): $50-150
• Steel crucible: $20-40
• Safety gear: $50-80
• Propane: $5-10 per session

Total starting cost: around $150-300, and ongoing cost per casting is just a few dollars for plaster and propane.

Optimize Your Casting Patterns

Getting the print settings right for casting patterns is different from normal printing. If you want AI-optimized settings for thin-wall, low-infill prints that burn out cleanly, check out the AI Settings feature on 3DSearch. It can help you dial in the perfect layer height, wall count, and infill for your specific printer and casting application.

Final Thoughts

Lost PLA casting is one of the most satisfying extensions of 3D printing. It takes your designs from plastic to metal, opening up a world of possibilities — custom hardware, decorative art, functional metal parts, jewelry, and more. The learning curve is real, and your first few castings will probably have issues. But once you nail the process, you will have a capability that very few makers possess.

Start simple. Print a small medallion, invest it in plaster, burn it out, and pour aluminum. Once you have that first successful casting in your hands — heavy, shiny, and unmistakably metal — you will understand why people get obsessed with this hobby.