No. 6555 Songze Avenue, Chonggu Town, Qingpu District, Shanghai, China
Watertightness: Which 3D Printing Process is Best for Pressure Testing?
Introduction: When Leaks Are Not an Option
Hi, I’m Barry Zeng, a manufacturing engineer at Shanghai Yunyan Prototype & Mould Manufacture Factory. One of the most demanding applications for 3D printed parts is pressure testing — fluid manifolds, pneumatic fittings, hydraulic components, and medical devices that must hold pressure without leaking. Not every 3D Printing Process can produce watertight parts. In this guide, I’ll compare SLA, SLS, MJF, FDM, and DMLS for watertightness. You’ll learn about porosity, layer adhesion, sealing techniques, and which 3D Printing Process is best for pressure testing up to 10 bar, 50 bar, or even 200 bar. I’ll also share a case study where we used SLA to print a 10 bar manifold that passed leak testing — and where FDM failed. By the end, you’ll know exactly which technology to choose for your pressure‑tight application.
Chapter 1: What Makes a 3D Printed Part Watertight?
A part is watertight if it holds fluid or gas without leaking under pressure. For 3D printed parts, two factors determine watertightness:
- Porosity: Microscopic voids between powder particles (SLS/MJF) or between layers (FDM).
- Layer adhesion: Weak bonds between layers can create leak paths.
No 3D Printing Process is perfectly watertight as‑printed. But some are close, and others can be post‑processed to become watertight. Let’s rank them.
Chapter 2: SLA – The Watertight Champion
SLA (stereolithography) cures liquid resin into solid plastic. There are no powder particles and no air gaps. As‑printed, SLA parts are nearly 100% dense. For 3D Printing Process watertightness, SLA is the best. Typical pressure rating as‑printed:
- Water pressure: 5–10 bar (safe).
- Air pressure: 2–5 bar (air molecules are smaller than water).
With post‑processing (clear coating or epoxy sealing), SLA parts can hold 50–100 bar. However, SLA resin is brittle — not suitable for high‑pressure cycling or impact.
Chapter 3: SLS and MJF – Powder Porosity Problems
SLS and MJF fuse nylon powder particles. Even with optimal settings, there are microscopic gaps between particles (porosity 1–5%). As‑printed, SLS/MJF parts are not watertight — they will leak under pressure. However, they can be sealed. Methods:
- Cyanoacrylate (super glue) sealing: Brush thin CA glue onto the surface. It wicks into pores and cures. Effective for water up to 10 bar.
- Epoxy sealing: Apply low‑viscosity epoxy, vacuum impregnation. Can hold 20–50 bar.
- Heat treatment: Re‑melting the surface (using a hot air gun) can close surface pores, but risks warping.
For the SLS 3D Printing Process, always plan for post‑sealing if watertightness is required.
Chapter 4: FDM – Layer Gaps and Leak Paths
FDM extrudes molten plastic layer by layer. There are inevitable micro‑gaps between layers and between extrusion lines. As‑printed, FDM parts are not watertight — they leak along layer lines. Watertightness can be improved with:
- Over‑extrusion: Increase flow rate (105–110%) to squish layers together. Reduces gaps but affects accuracy.
- Thicker layers: 0.2 mm layers have fewer interfaces than 0.1 mm layers.
- Coating: Epoxy or polyurethane coating seals surface pores.
Even with coatings, FDM is not recommended for pressure >2 bar. The layer lines create a spiral leak path that coatings may not fully seal.
Chapter 5: DMLS – Metal Porosity and High Pressure
DMLS (Direct Metal Laser Sintering) fuses metal powder. As‑printed, DMLS parts have 0.5–2% porosity — better than SLS but not perfectly dense. For high‑pressure applications (100–500 bar), as‑printed DMLS may leak. Solutions:
- Hot isostatic pressing (HIP): Heat + pressure closes internal pores. Achieves near‑100% density. HIP is expensive ($200–500 per batch).
- Surface sealing: For moderate pressure (20–50 bar), epoxy or metal spray can seal surface pores.
For the DMLS 3D Printing Process, specify HIP if you need absolute pressure tightness.
Chapter 6: Watertightness Comparison Table
| Technology | As‑Printed Watertight? | Max Pressure (sealed) | Sealing Method | Best Use |
|---|---|---|---|---|
| SLA | Yes (water up to 10 bar) | 50–100 bar | Clear coat / epoxy | Low‑pressure fluidics, manifolds |
| SLS/MJF | No | 10–50 bar | CA glue, epoxy impregnation | Sealed nylon parts |
| FDM | No | <2 bar | Epoxy coating | Non‑critical, very low pressure |
| DMLS (as‑printed) | No | 10–20 bar | HIP, epoxy | High pressure after HIP |
Chapter 7: Post‑Processing Sealing Techniques
If your chosen 3D Printing Process doesn’t produce watertight parts as‑printed, these sealing methods work:
- Vacuum impregnation: Place parts in a vacuum chamber, then flood with low‑viscosity epoxy. The epoxy fills pores. Cure, then remove excess. Best for SLS/MJF parts. Cost: $5–20 per part.
- Brush‑on cyanoacrylate: For small parts, brush thin CA glue. It wicks into pores instantly. Cure with accelerator. Good for low pressure.
- Spray clear coat: For SLA parts, a UV‑clear acrylic spray seals micro‑pores and improves watertightness.
- Electroplating (metal): For DMLS parts, electroplating with copper or nickel seals surface pores and adds pressure rating.
Chapter 8: Case Study – 10 Bar Manifold: SLA Wins, FDM Fails
A client needed a fluid manifold to hold 10 bar water pressure. They first tried FDM (ABS, 0.2 mm layers, epoxy coated). At 3 bar, water seeped through layer lines. Next, SLS (PA12, vacuum impregnated). Held 8 bar, but leaked at 9 bar. Finally, SLA (tough resin, clear coated). Held 12 bar with no leaks. The SLA 3D Printing Process was the only one that met the requirement. The client now uses SLA for all pressure‑tight prototypes.
Chapter 9: Testing Your Part for Leaks
Before putting a 3D printed part into service, test it:
- Soap and water: Pressurize the part (e.g., with a bicycle pump), spray soap solution. Bubbles indicate leaks.
- Pressure decay test: Pressurize the part, seal it, and monitor pressure drop over time.
- Dip test: Submerge the part in water and pressurize. Watch for bubbles.
For safety, test at 1.5× the working pressure.
Chapter 10: Summary – Watertightness Decision Checklist
- ☐ Need watertight as‑printed? → SLA.
- ☐ Need high pressure (50+ bar)? → SLA with coating or DMLS with HIP.
- ☐ Need nylon material and can seal? → SLS/MJF with vacuum impregnation.
- ☐ Very low pressure (<2 bar)? → FDM with epoxy coating.
- ☐ Always test your part before critical use.
Conclusion: Choose the Right Process for Pressure Testing
For pressure testing and watertight applications, not every 3D Printing Process is equal. SLA is the best as‑printed option for low to medium pressure. SLS/MJF can be sealed for higher pressure. DMLS with HIP is the choice for extreme pressure. We offer all these technologies and post‑processing sealing. Send me your CAD file and pressure requirements. I’ll recommend the best 3D Printing Process and sealing method — and provide a free DFM report and quote. Let’s make your part leak‑proof.
👇 Need a Watertight 3D Printed Part for Pressure Testing?
Send me your CAD file and pressure requirements. I’ll recommend the best technology (SLA, SLS, MJF, or DMLS) and sealing method — free DFM report and quote within 24 hours.
📞
Call Barry
Direct engineering line
(I answer watertightness questions)
+86 138 1894 4170
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Visit Our
Download “Watertight 3D Printing Guide”
(Sealing methods, pressure ratings)
Not sure if your part will hold pressure? Just say: “Barry, here’s my part and target pressure — which process should I use?” I’ll guide you.
💧 Watertight 3D Printing — Hold Pressure, Hold Trust 💧
P.S. Mention “watertight guide” when you email, and I’ll send you a pressure rating chart and a vacuum impregnation how‑to guide.
Barry Zeng
Senior Manufacturing Engineer, Shanghai Yunyan Prototype & Mould Manufacture Factory
(10+ years helping clients achieve watertight 3D printed parts for fluidics, pneumatics, and hydraulics. Let me help you stop leaks.)
