Lost PLA Casting: From Printed Pattern to Finished Aluminum

Lost PLA casting lets you turn a 3D-printed plastic pattern into a solid aluminum part, using a burnout furnace, a flask, and molten metal. You start with a PLA print, invest it in refractory slurry, burn out the plastic, and pour aluminum into the resulting cavity. This article walks through each stage, the equipment you need, common pitfalls, and how the process compares to other casting methods.

Formula

The calculator above simply converts the burnout time you enter into a confirmation that the cycle is complete. There is no complex math—the key is that the burnout schedule must reach at least 600°C (1112°F) and hold for several hours to fully gasify the PLA. The formula is:

Burnout complete = burnout time (hours)

Where t is the total time in hours that the flask spends in the furnace at or above 600°C. The metal temperature m is shown for reference—aluminum typically pours at 700–750°C (1292–1382°F).

Worked Example

Using the default values: you set a burnout time of 8 hours. After 8 hours at temperature, the PLA is fully burned out, leaving a clean cavity. The calculator confirms that the burnout is complete. For a typical small part (e.g., a 5 cm cube), 8 hours is sufficient; larger parts may need 12–16 hours.

Background and Why It Matters

Lost PLA casting is a modern twist on the ancient lost-wax process. Instead of carving wax, you 3D-print a pattern in PLA (polylactic acid), a biodegradable thermoplastic. The printed pattern is then coated in a ceramic shell (investment), and the plastic is burned out in a furnace, leaving a negative mold. Molten aluminum is poured in, and after cooling, the shell is broken away to reveal a metal part. This method is popular with makers and small foundries because it allows complex geometries—like internal channels or intricate details—that are difficult to machine or cast with traditional patterns.

How It Works: Step-by-Step

1. Print the pattern in PLA with a low infill (10–20%) and no supports if possible. Solid or high-infill prints take longer to burn out and may crack the shell.
2. Attach a sprue (a wax or PLA rod) to the pattern to create a channel for metal to flow in and air to escape.
3. Invest the pattern by coating it with a refractory slurry (e.g., plaster/silica mixture) in several layers, allowing each to dry. This forms a heat-resistant shell.
4. Burn out the PLA in a kiln or furnace. Ramp up slowly (e.g., 100°C per hour) to avoid thermal shock, then hold at 600–700°C for several hours until the plastic is completely gone.
5. Pour the aluminum at 700–750°C into the preheated mold. Preheating the shell reduces thermal shock and improves metal flow.
6. Cool and break out the part. After cooling, the shell is chipped away, and the sprue is cut off.

Choosing the Right PLA and Print Settings

Not all PLA is created equal. Standard PLA is the most common and burns out cleanly, but some specialty PLAs (like those with metal or wood fillers) may leave ash that contaminates the casting. For best results, use plain, unfilled PLA. Print settings matter: use a low infill (10–20%) and a thin wall (1–2 perimeters) to reduce the amount of plastic that must burn out. Also, avoid supports—they leave rough surfaces that are hard to invest and may trap air. If supports are unavoidable, design them to be easily removable or use a soluble support material.

Common Problems and How to Avoid Them

• Shell cracking during burnout—caused by rapid heating or thick sections of PLA expanding. Solution: slow ramp rate (50–100°C/hr) and keep infill low.
• Incomplete burnout—leaving ash that creates porosity in the aluminum. Solution: hold at 700°C for at least 4–6 hours, and ensure adequate airflow in the furnace.
• Metal not filling the mold—due to cold shell or thin sections. Solution: preheat the shell to 200–300°C before pouring, and increase pouring temperature slightly.
• Surface roughness—caused by the layer lines of the print. Solution: sand the PLA pattern smooth before investing, or use a chemical smoothing (e.g., acetone vapor for ABS, but for PLA use fine sandpaper or a filler primer).

Comparing Lost PLA to Other Methods

Lost PLA sits between traditional lost-wax and sand casting. Compared to lost-wax, it is faster for small batches because you don’t need a metal die to make wax patterns—just a 3D printer. However, the surface finish is typically rougher due to layer lines. Compared to sand casting, lost PLA allows much more complex internal geometry, but it is more expensive per part and requires a burnout furnace. For one-off parts or small runs, lost PLA is often the most practical choice for aluminum.

Costs and Equipment Essentials

The core equipment includes a 3D printer (already owned by most makers), a burnout furnace (a small electric kiln works), refractory investment material (plaster-silica or commercial casting investment), and a crucible and torch or furnace for melting aluminum. The consumables—investment, PLA filament, and aluminum—are relatively inexpensive, but the furnace and safety gear (face shield, gloves, leather apron) are the main upfront costs. A small tabletop furnace can be purchased or built from a kiln, and many makers use a propane furnace for melting aluminum. Expect to spend a few hundred dollars on the furnace and safety gear if you don’t already have them.

Safety First: Handling Molten Metal and Furnaces

Working with molten aluminum and a burnout furnace carries serious risks. Always wear a face shield, heavy leather gloves, and a leather apron. Work in a well-ventilated area, as burning PLA releases fumes (though generally not toxic, it is best to avoid inhalation). Never pour molten metal into a cold or wet mold—moisture can cause steam explosions. Keep a dry chemical fire extinguisher nearby, and never use water on a metal fire. Also, ensure your furnace is rated for the temperatures you need; a standard pottery kiln may not reach 700°C, so check specifications.

Frequently Asked Questions

Can I use any 3D printer for lost PLA?

Yes, any FDM printer that can print PLA works. The key is to use a low infill and thin walls to make burnout easier. Resin printers (SLA) are not suitable because resin does not burn out cleanly.

How long does the whole process take?

From print to finished part, it typically takes 1–2 days. Printing takes a few hours, investing and drying takes 12–24 hours, burnout takes 8–12 hours, and cooling and finishing takes a few hours.

Can I cast other metals besides aluminum?

Yes, but the burnout temperature and pouring temperature must be adjusted. Bronze and brass have higher melting points (around 900–1000°C), so you need a higher-temperature furnace and a more refractory investment. Aluminum is the easiest for beginners.

Why did my mold crack during burnout?

Cracking is usually due to rapid heating or too much plastic expanding. Slow down your ramp rate (e.g., 50°C per hour) and use a lower infill. Also, make sure the investment is completely dry before heating.

Do I need a vacuum casting machine?

No, lost PLA works with gravity pouring for most parts. Vacuum or centrifugal casting helps fill very thin sections, but for typical hobby parts, gravity is sufficient.

Can I reuse the investment?

No, the shell is destroyed when you break it away. However, you can recycle the broken investment as a filler in new mixes, but it may weaken the shell. Most casters discard it.

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in t | Burnout time | hours | 8 | 0
in m | Metal temperature | °C | 720 | 0
out burnout | Burnout complete | - | t | 0