How to avoid bubbles in 3D printing

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  Bubbles in 3D printing: symptom, cause, fix
• 2
  Where bubbles in 3D printing actually come from
• 3
  Filament moisture and melt-zone voids
• 4
  Trapped air and outgassing in resin
• 5
  When bubbles are a process limit, not a setting
• 6
  Print, machine or cast: picking the right process
• 7
  How to avoid bubbles in 3D printing: 7 steps

A bubble is gas that was not supposed to be inside the part. In FDM the gas is mostly water vapor. PLA and PETG pick up moisture from air, and a spool left out for a week can hold enough to foam at the nozzle. At 200-250 °C that water turns to steam at roughly 1,600 times its liquid volume, so even 0.2% moisture shows up as popping and voids.

In resin printing the gas is air, not water. The tank holds liquid resin, the build plate pulls the part up, and fresh resin has to flow back under the cured layer before the next exposure. If the lift is fast and the layer is large, resin cannot refill in time. You get a pocket that cures empty. That is why the same model prints clean at 30 mm height and fails at 120 mm.

There is a third source that gets blamed on both: gas dissolved in the polymer. ABS and PC release volatiles when the melt sits too long at high temperature. Pellets and some filaments also carry process gas from manufacturing. That gas has to escape through the nozzle, and if it cannot, it becomes a void inside the bead.

So the first real question is not which setting to change. It is which gas you are fighting. Water needs heat and time. Air needs slower motion and a rest interval. Dissolved volatiles need a cooler, shorter melt. Pick the wrong one and you will reprint the same failure three times.

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  WaterDry the spool before touching any slicer value.
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  AirSlow the lift, add rest time, warm the vat.
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  VolatilesLower nozzle temperature, shorten dwell time.

Dry the filament first, always. PLA dries in 4 hours at 45-50 °C. PETG needs 5-6 hours at 65 °C. Nylon and PC need 8-12 hours at 70-80 °C. Use a dryer with a lid, not an oven that swings 20 °C. Weigh the spool before and after. A 1 kg spool that loses 3-5 g has given up real water.

Keep it dry while printing. A dry box at 15-20% relative humidity is enough for a 20-hour print. If you cannot control that, print the wet-prone material in one session and put it straight back. Every hour in room air adds back a little, and long prints show it near the end.

Then look at the melt zone. Nozzle temperature that is too low for the flow rate leaves unmelted polymer that tears and traps gas. As a starting rule, if you print 0.2 mm layers at 80 mm/s with a 0.4 mm nozzle, stay at the top of the material window, not the bottom. Raise 5-10 °C rather than pushing the flow harder.

A worn nozzle or a leaking hot end pulls air into the melt. Check the nozzle bore with a 0.4 mm pin. If it is oval after 200 hours, replace it. Reseat the nozzle against the heat break while hot, at 240-250 °C for PETG, and confirm no filament oozes from the threads. A 5 mm cut on the PTFE tube removes the deformed end that causes the same leak.

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  DryPLA 4 h / 45-50 °C; Nylon 8-12 h / 70-80 °C.
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  StoreDry box at 15-20% RH during the print.
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  Hot endReplace an oval nozzle, reseat while hot.

Resin bubbles form in two places: inside the tank during the print, and inside the part during post-processing. The first is a flow problem. The second is a curing problem. They look different. Flow voids are flat pockets against a layer, often on the side facing the build plate. Cure bubbles are round and sit near the surface or in a thick section.

Fix the flow side with slower motion and rest. Reduce lift speed by 30-40% and add a 1-2 second rest after the lift. Lower the retract speed as well; resin is viscous, around 200-400 mPa·s at 25 °C, and it does not fill a 0.05 mm gap in 0.2 seconds. Warm the vat to 28-30 °C if the printer allows it, which drops viscosity and halves the fill time.

For thick or solid parts, hollow them with two drain holes of 2.5-3 mm. Resin trapped inside a closed shell outgasses for hours and shows up as bubbles on the outer skin after the final cure. Angle the part so the holes sit at the lowest point during printing and can drain when you lift it off the plate.

Post-cure is where a clean print picks up defects. Let the part rest 10-30 minutes before curing so absorbed gas can leave. Wash in two baths, 3 minutes each, and use clean IPA for the second one. Then cure at the resin maker's window, usually 60 °C for 30-60 minutes. Curing a wet part traps solvent that boils into bubbles.

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  HollowTwo 2.5-3 mm drain holes, lowest point at print start.
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  Rest10-30 min before cure, cover from light.
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  WashTwo IPA baths, second one clean.

Some geometries will never print clean. A 100 mm solid block in clear resin has no path for gas to leave and no path for light to cure the core. A tall thin wall in FDM flexes as the nozzle passes and opens gaps between beads that look like bubbles. Changing retraction will not close them.

Clear and translucent parts are the hardest case. Any void of 0.1 mm shows through, even though the same void is invisible in an opaque grey resin. If the part must be optically clear and the surface must hold Ra 0.8-1.6 μm, printing is usually the wrong process. Machining a billet gives a solid section with no voids at all.

Thick sections over 8 mm in FDM also trap gas by design. The infill cannot vent, and the top skin seals the part before the inside cools. You can print in two halves and bond them, but the joint becomes the new weak point. For a load-bearing part, that trade is rarely worth it.

We see this often on prototype housings and manifolds. The print is used to check fit, then the same geometry is cut from 6061 or 316L for the functional test. Printing and machining answer different questions, and mixing them up wastes a week.

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  Solid clear partsNo gas path and no cure path; machine instead.
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  Tall thin wallsFlex and bead gaps, not trapped air.
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  Sections over 8 mmVent by splitting, but the joint is weaker.

If the part is a fit check and the surface does not matter, fix the print. Drying and flow tuning cost almost nothing and take an afternoon. That is the right call for a bracket, a jig or a cover that will be handled twice and then replaced.

If the part carries load, seals a fluid, or has to be optically clear, move to CNC. A machined part has no internal voids because it comes from solid stock. We hold ±0.005 mm on critical features and Ra 0.8-1.6 μm on sealing faces without a sealing coat. That removes the whole bubble question from the drawing.

If you need 50-500 units with a smooth surface and a tooling budget, vacuum casting or die casting may beat both. A silicone mold from a clean master gives parts with no layer lines and no trapped gas if the pour is degassed. For metal, die casting with proper venting handles thin walls that would trap air in a print.

The practical rule: use printing to learn the geometry, then pick the production process on function. A printed prototype that shows bubbles is telling you something about the print, not about the design. Do not redesign the part around a process defect.

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  Fit check onlyDry, tune flow, reprint.
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  Load or sealMachine from solid; no voids by nature.
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  50-500 unitsVacuum cast or die cast with venting.