Yes, TPU absolutely needs to be dried before printing. TPU is hygroscopic, meaning it actively pulls moisture from the air and traps it inside the filament. Even a spool left out for a few days in a humid room can absorb enough water to ruin your prints. This applies to every TPU formulation regardless of Shore hardness, whether you’re printing with soft 85A or firmer 95A.
Why TPU Absorbs Moisture
TPU’s chemical structure is naturally hydrophilic. The polymer chains contain segments that attract and hold water molecules, which is why conventional polyurethane films exhibit rapid water absorption even in moderate humidity. This isn’t a defect or a sign of cheap filament. It’s a fundamental property of the material’s chemistry.
The problem isn’t just that TPU gets wet on the surface. Moisture migrates into the interior of the filament over time. When that wet filament hits the hotend at printing temperatures, the trapped water instantly turns to steam. That steam disrupts the flow of melted plastic, creating a cascade of print quality issues.
How to Tell Your TPU Is Wet
The most obvious sign is sound. If you hear popping, crackling, hissing, or sizzling during extrusion, that’s steam bursting through the nozzle. This is the single easiest diagnostic: dry filament extrudes quietly.
Visual signs are just as telling:
- Stringing and blobs: Expanding steam pushes melted plastic out unevenly, leaving thin hairs between print features, blobs on corners, and drips on bridges.
- Rough or pitted surfaces: Steam escaping from the plastic leaves behind bubbles, pinholes, and a sandy or cratered texture across the surface.
- Weak layer bonding: Moisture disrupts the bond between layers, resulting in parts that delaminate more easily under stress.
If your TPU prints were coming out fine last week and suddenly look rough or stringy, moisture is the most likely culprit before you start adjusting slicer settings.
What Moisture Does to Print Strength
Beyond cosmetic defects, moisture compromises the mechanical properties of your finished parts. Research on 3D printed elastomeric polyurethane found that initial stiffness dropped measurably after water exposure, going from 6.8 MPa in dry samples to 6.3 MPa in immersed ones. That may sound modest, but for functional parts like gaskets, phone cases, or flexible hinges, even small reductions in stiffness change how the part performs under load.
The steam bubbles trapped between layers also act as weak points. A part that looks solid on the outside can have a porous internal structure riddled with micro-voids, making it far less durable than the same geometry printed with properly dried filament.
Does Shore Hardness Matter?
Not when it comes to moisture absorption. Both 85A and 95A TPU absorb moisture at similar rates because Shore hardness doesn’t change the underlying chemistry. However, softer 85A filament is often more sensitive to visible moisture defects in practice. Because softer TPU is typically printed at slower speeds with thinner walls, there’s more time for steam to form bubbles and disrupt each layer. So while you need to dry both, you may notice the consequences of wet filament faster with 85A.
How to Dry TPU
The standard recommendation for TPU 95A is 70°C (158°F) for 7 hours. This is long enough to drive moisture out of the filament’s interior, not just the surface. Shorter drying times may improve things slightly but won’t fully solve the problem, especially if the spool has been sitting out for weeks.
Dedicated Filament Dryers
A purpose-built filament dryer is the most reliable option. Good units hold temperature within 1 to 2°C of the set point and maintain low humidity inside the chamber. Many also let you feed filament directly from the dryer to your printer, keeping the spool dry throughout a long print. If you regularly print moisture-sensitive materials like TPU or nylon, a dedicated dryer pays for itself in saved filament and fewer failed prints.
Kitchen Ovens and Food Dehydrators
A kitchen oven can work, but temperature accuracy is a real concern. Many ovens swing 10 to 15°C above or below the set temperature, and TPU starts to soften and deform well before it reaches printing temperature. One common experience from the 3D printing community: people assume their oven is holding 70°C only to discover with an external thermometer that the actual temperature is wildly off. If you go this route, verify the temperature with an independent thermometer placed inside the oven, away from the heating element.
Cheap food dehydrators have the same accuracy problem. Some blow air at 230°C near the heater while the chamber itself never gets close to the target temperature. The result is uneven drying at best and warped filament at worst. If you use a dehydrator, again, verify its actual chamber temperature before trusting it with a spool of TPU.
Storing TPU to Prevent Moisture Buildup
Drying filament is only half the solution. If you put a freshly dried spool back on an open shelf, it will reabsorb moisture within days. The goal for long-term storage is keeping relative humidity below 50%, though the sweet spot is closer to 10 to 30%.
The simplest approach is a resealable bag or airtight container with desiccant packets inside. Vacuum-sealed bags work even better because they remove most of the air (and its moisture) from around the spool. You don’t need to chase 0% humidity. Extremely low levels near 0 to 1% don’t provide meaningful additional protection over a well-sealed container sitting at 15 to 20% relative humidity.
For frequent printers, a dry box that feeds filament through a sealed port to the extruder keeps the spool protected during the entire print. This is especially useful for large or multi-day TPU prints where the spool would otherwise sit exposed for hours.
A Practical Drying Routine
If you’re new to TPU, the simplest workflow is: dry every new spool before its first use, even if it arrived sealed. Factory-sealed bags aren’t always perfectly airtight, and some filament absorbs moisture during manufacturing or shipping. After that initial dry, store the spool in an airtight container with desiccant whenever it’s not on the printer. Re-dry any spool that’s been left out for more than a day or two in a humid environment, or immediately if you start hearing pops during extrusion.
TPU is a forgiving material in many ways. It’s flexible, durable, and great for functional parts. But it punishes you quickly if you skip drying. A seven-hour session in a dryer before your print starts is a small investment compared to scrapping a 12-hour print because of bubbles and stringing halfway through.