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For a liquid-filled FDM part, start with a simple, even cavity, 3–4 perimeters, and a 0.2 mm layer height on a 0.4 mm nozzle. Keep layer height below roughly 60–65% of nozzle width, raise nozzle temperature by 5–10 °C within the filament maker’s range, and increase extrusion multiplier or line width by about 5–10%. Build a deliberate solid closure, pause at the open cavity layer, fill quickly without letting liquid enter the nozzle path, then print the closing layers. These are starting values, not a leak-proof guarantee.
Leak resistance depends on the complete system: cavity geometry, seam placement, layer bonding, material, printer calibration, liquid chemistry, and the consequences of failure. A water-tight test also does not establish food-contact, medical, pressure, or long-term chemical-storage suitability.
What makes an embedded-liquid FDM part difficult
An ordinary FDM print is made from adjacent roads of molten polymer. Small gaps can remain between lines, between layers, or at each perimeter’s start-and-stop seam. When a cavity is enclosed, liquid can find those paths even when the outside surface looks smooth.
Watertightness is a printing and design problem
Increasing infill alone is not a reliable solution. Prusa Research warns that thick walls which depend on infill can create internal voids where liquid accumulates. A simpler wall of consistent thickness is usually easier to seal and inspect. Strength in a shell is driven mainly by its perimeters rather than by sparse infill.
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The closure must be planned in CAD
An embedded cavity needs an opening during the fill operation and a top or cap that closes it afterward. Model that closure deliberately instead of hoping the slicer will bridge an accidental hole. The slicer preview must show continuous walls around the cavity and enough solid material above and below it.
Liquid safety is separate from leak testing
Prusa’s warning is explicit: “Watertight does not mean food safe!” A part that holds water for a short test may still contain additives, pigments, unbonded surfaces, or a coating that is unsuitable for food, medical use, pressure, heat, or long-term chemical contact.
Starting settings for a 0.4 mm nozzle
The following profile is a conservative place to begin. Tune one variable at a time on a small test coupon made with the same material and cavity geometry.
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| Function | Starting choice | Qualification |
|---|---|---|
| Perimeters | 3–4 | Prusa recommends at least three to four for a single wall; five or six can help when the application allows a thicker shell. |
| Layer height | 0.2 mm | A practical value for a 0.4 mm nozzle. For best watertightness, stay below about 60–65% of nozzle width; a separate general guideline keeps layer height below 80% of nozzle diameter. |
| Nozzle temperature | Normal preset plus 5–10 °C | Remain inside the filament manufacturer’s recommended range and check for stringing or dimensional drift. |
| Extrusion | Extrusion multiplier plus 5–10%, or line width plus 5–10% | Use one adjustment first, then check dimensional accuracy so a cavity does not shrink shut. |
| Infill | Prefer an even wall with no unnecessary internal infill | 100% infill does not guarantee a sealed part and can hide liquid-trapping voids. |
| Top and bottom | Enough solid material to close the cavity | At a lower layer height, more layers are required to achieve the same physical closure thickness. Confirm this in preview. |
| Spiral-vase mode | Only for suitable open containers | It produces one continuous outline with zero infill and no top solid layers, so it cannot close an embedded cavity by itself. |
| Pause | Insert a pause at the cavity opening | Prusa documents the M601 pause command; verify that your printer firmware and host interpret it correctly. |
| Base overlap | Test 25–35% XY overlap if the base still leaks | Prusa presents this as a possible test range after other causes have been addressed, not as a universal setting. |
Design the cavity before opening the slicer
Use an even, inspectable wall
Keep the cavity wall thickness consistent wherever possible. Sudden thick-to-thin transitions complicate extrusion and can create hidden pockets. A uniform wall also makes it easier to predict how many perimeters the slicer will generate. If the geometry requires a thick region, inspect the preview for infill islands and enclosed gaps rather than assuming the extra plastic is beneficial.
Give the fill operation a defined opening
Place the opening on a layer that is easy to reach with a syringe, pipette, or other filling tool. Leave enough clearance around the opening for the nozzle, the tool, and any insert. The liquid must remain below the nozzle path while the print is paused; a spill on a hot nozzle or moving axis can damage the part and the machine.
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Model the closure as a solid feature
Choose a top thickness that can be printed as continuous solid layers. The cavity roof should not depend on a long unsupported bridge or a few isolated lines. If the design uses a separate cap, provide a positive locating feature and a sealing strategy, then test the joint independently from the printed shell.
Allow for the liquid you will actually use
Design around the liquid’s volume, viscosity, temperature, and chemical behavior. Do not fill to the brim during the pause: leave room for handling and any expansion expected in service. If the liquid is hazardous, volatile, corrosive, or pressurized, an FDM shell should not be treated as a validated containment vessel without material and regulatory testing.
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Choose perimeters and layer height for bonding
Perimeters are the primary shell
Three or four perimeters give the slicer several overlapping roads around the cavity and reduce dependence on a single line. Five or six may be useful when a leak would have serious consequences or when the wall can be made thicker without creating an infill-dependent region. More perimeters consume space and material, so preserve the cavity dimensions in CAD rather than allowing the shell to encroach unpredictably.
Keep layer height within the nozzle’s practical range
For the common 0.4 mm nozzle, 0.2 mm is a practical starting point. The watertightness guidance from Prusa places the preferred ceiling below approximately 60–65% of nozzle width. A separate general layer/perimeter reference gives a broader upper limit below 80% of nozzle diameter; that is a ceiling, not a target for a leak-sensitive part. A smaller layer height can improve bonding opportunities, but it increases print time and the number of solid layers needed for a given closure thickness.
Inspect the seam
Every conventional perimeter has a start-and-stop point. If the seam lines up through the cavity wall, it can become a preferential leak path. Use the slicer preview to see where the seam is placed and avoid concentrating it at a critical corner when your slicer allows seam control. A change in seam position does not compensate for poor extrusion or weak layer bonding, so validate the result with a leak test.
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Set temperature and extrusion conservatively
Increase heat only inside the material’s safe range
Raising nozzle temperature by 5–10 °C above the normal preset can improve fusion between adjacent roads and layers. Make the change only within the filament maker’s recommended range. Watch for excessive stringing, sagging, discoloration, or a distorted cavity. Bed temperature, enclosure conditions, and cooling can alter the result, so record the complete profile rather than the nozzle value alone.
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An extrusion-multiplier increase of 5–10% is a starting adjustment; widening the line width by a similar percentage is an alternative. Do not apply both changes automatically. Excess flow can close the cavity, alter fit, or produce dimensions that no longer match the design. Measure a test feature after tuning and return to a calibrated multiplier for other parts if the adjustment was specific to this liquid-retaining geometry.
Preview the actual toolpaths
After changing flow or line width, step through the cavity layers in preview. Look for continuous contact between adjacent perimeters, gaps at corners, and a complete roof over the opening. The numerical setting is less important than the toolpath that it produces on your printer with your filament.
Decide whether spiral-vase mode fits
Spiral-vase, also called Spiralize Outer Contour in some slicers, prints a continuous single outline with no infill and no top solid layers. The uninterrupted path can remove the repeating perimeter seam that contributes to leaks in an open cup, bottle, or similar shape. UltiMaker’s polypropylene bottle example uses Spiralize Outer Contour and notes that a thicker wall can improve watertight reliability.
That mode cannot create a closed embedded cavity on its own: it intentionally omits the top. Use it only when the liquid container remains open or when a separately manufactured closure is designed and tested. For a cavity that must be filled mid-print and sealed by later layers, use ordinary perimeter and solid-top settings and inspect the closure in preview.
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Pause, fill, and resume safely
- Slice the complete model first. Identify the layer where the cavity is fully open and the next layers will form the roof. Confirm that the preview shows a pause at that transition, not inside a wall.
- Insert the pause command. Prusa documents
M601for pausing. Add it through the slicer’s layer-change or custom-G-code controls, then verify the generated G-code and your firmware’s expected pause behavior before committing a long print. - Prepare the liquid and tool. Measure the intended amount, use a container that will not shed particles, and keep the tool ready so the printer is paused for as little time as practical.
- Keep the nozzle path clear. Move or park the head only as your printer’s pause routine specifies. The liquid level and any insert must stay below the nozzle and away from moving axes.
- Fill without wetting the rim. A wet rim can prevent the next layer from bonding. Wipe any spill before resuming, and do not allow liquid to enter the hotend or electrical areas.
- Resume and observe the first closing layers. Stay with the printer while it prints over the opening. Stop if the roof drags, buckles, or leaves a visible gap; a failed closure is easier to diagnose immediately than after the part is removed.
- Let the part cool before testing. Handling a warm shell can distort the closure and create a leak that is not representative of the finished geometry.
Leak-test the finished part
Use a staged water test
For a non-hazardous part, begin with a small amount of water over a tray or absorbent surface. Inspect the base, vertical seams, roof, and any cap joint. Increase the fill level gradually rather than placing the part directly into service. Dry the exterior and the cavity completely before changing a setting so residual water does not confuse the next test.
Interpret what the test can and cannot prove
A short water test can reveal visible paths through the shell. It does not prove resistance to pressure, heat cycling, long-term diffusion, solvents, acids, oils, or biological contamination. Do not perform improvised pressure tests on a part that was not designed and validated as a pressure vessel.
Record the failure location
Mark the exact location and layer height of a leak. A leak at the base points toward bottom bonding or overlap; a vertical line suggests a perimeter gap or seam; a roof leak suggests an incomplete closure. This record lets you change one variable and compare a small coupon instead of repeatedly printing the full model.
Common failure modes and targeted fixes
| Symptom | Likely path | Next change |
|---|---|---|
| Pinholes along a wall | Perimeter or layer gaps caused by under-extrusion, low heat, or an overly large layer height | Increase perimeters first, then raise temperature or extrusion gradually within the material range. Reprint a small coupon and inspect dimensions. |
| Leak only through the bottom | Insufficient bonding or overlap at the first layers | After checking bed adhesion and other causes, test XY overlap in the 25–35% range described by Prusa. Change one parameter per iteration. |
| A straight leak at one vertical line | Perimeter start/stop seam aligned with the cavity | Change seam placement or orientation, then verify that the surrounding roads are fully fused. Spiral-vase mode is relevant only to suitable open geometries. |
| Liquid appears in the wall or never drains | Voids created by thick-wall infill or trapped internal pockets | Redesign for a simpler, even wall and remove unnecessary infill-dependent thickness. Inspect every cavity layer in preview. |
| Roof does not close | Too little solid top material, an unsupported span, or an interrupted pause/resume | Increase the physical closure thickness in CAD, add the required solid layers, and confirm the pause is at the correct layer. |
| First closing layer drags through the fill | Liquid or an insert is above the nozzle path, or the pause routine changed the head position | Lower the fill level, improve clearance, and test the pause sequence with an empty cavity before using liquid. |
| A repaired PP container still leaks | Repair material is incompatible, incomplete, or thermally damaged | UltiMaker notes that a small leak in a polypropylene container may be repaired with a soldering iron. Test the repaired area for chemical compatibility and repeat the water test before use. |
When a coating or repair is appropriate
A coating can close microscopic paths, but it changes the chemical and thermal behavior of the part. Verify adhesion to the exact filament, compatibility with the liquid, cure conditions, and whether the finished surface is approved for the intended contact. Do not assume that a coating makes an otherwise unsuitable filament food-safe or pressure-rated.
For polypropylene, UltiMaker describes soldering-iron repair as an option for one or two small leaks. Keep the iron localized: excessive heat can deform the cavity or introduce contamination. Treat the repair as a new material interface and test it under the same liquid, temperature, and duration expected in service.
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Material and application safety boundaries
Food and beverage contact
Food-contact suitability depends on the specific polymer, additives, pigments, processing history, surface finish, cleaning method, and regulatory requirements. A household water test is not certification. If the part will touch food or drink, obtain documentation for the exact filament and process or choose a validated manufactured container.
Medical, pressurized, and chemically aggressive liquids
Medical use, vacuum or pressure service, fuels, solvents, acids, and long-term storage require application-specific validation. Layered plastics can absorb liquids, stress-crack, or change dimensions over time. The consequences of a leak should determine whether an FDM print is appropriate at all.
Temperature and cleaning
Hot liquids and repeated washing can soften, warp, or fatigue a printed closure. Validate the complete temperature cycle and cleaning chemistry, not just the initial fill. If you cannot define those conditions, limit the part to a non-critical demonstration or dry-use object.
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Choose an approach by cavity type and risk
| Use case | Suitable print strategy | Main limitation |
|---|---|---|
| Open decorative cup or bottle | Consider spiral-vase mode, a continuous outline, and a thicker wall; test the base and sidewall. | No top is printed, so a separate closure must be validated if the container will be transported. |
| Embedded liquid capsule filled during printing | Use an even cavity, multiple perimeters, conservative layer height, tuned heat and flow, a programmed pause, and solid closing layers. | The pause, fill handling, and roof are additional failure points that must be tested together. |
| Liquid that must remain sterile, food-safe, pressurized, or chemically isolated | Use FDM only after material, process, and regulatory validation for the exact service. | Slicer settings cannot establish safety, pressure rating, or long-term compatibility. |
A repeatable tuning workflow
- Print a small coupon containing the same wall thickness, base, seam arrangement, opening, and closure as the real part.
- Begin with the 0.4 mm-nozzle profile in the table and record filament batch, nozzle, temperatures, layer height, flow, and cooling.
- Inspect the preview for gaps, infill pockets, seam concentration, and a complete solid roof.
- Run an empty-cavity print to verify the pause, head parking, and resume sequence.
- Run a water-filled test, document the leak location, and dry the part completely.
- Change only one factor—perimeters, heat, extrusion, seam, overlap, or closure thickness—then repeat the coupon.
- Stop tuning when the part meets the real application’s leak and compatibility requirements; do not convert a successful hobby test into an unverified safety claim.
Pre-print checklist
- The cavity wall is even and does not depend on hidden infill pockets.
- The opening and closure are modeled intentionally.
- The selected layer height is appropriate for the nozzle and the watertightness goal.
- Perimeters, temperature, and extrusion adjustments remain within the filament maker’s limits.
- The preview shows continuous walls and enough solid top and bottom material.
- The pause command is supported by the printer firmware and occurs at the intended layer.
- The liquid, fill tool, and machine are protected from spills.
- A staged leak test and a plan for recording failure locations are ready.
- The intended use has been screened for food, medical, pressure, temperature, and chemical risks.
Final recommendation
Treat liquid embedding as a controlled shell-and-closure process, not as a single magic slicer setting. Start with an even cavity, three or four perimeters, a conservative layer height, modest heat and flow increases, and a preview-verified solid closure. Pause with the documented command, fill below the nozzle path, test with water, and iterate from the observed leak location. If the application demands certified food contact, pressure containment, medical performance, or chemical durability, those requirements take precedence over any FDM profile.
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