Drying TPU Before Molding: Why It Matters and How

Of all the things that go wrong with TPU on a molding floor, the single most common root cause is moisture. TPU pulls water out of the air faster than most people expect, and once that water is trapped in the melt it shows up as splay, bubbles, weak parts, and unstable processing. The frustrating part is that it usually looks like a process problem — so teams chase melt temperature, injection speed, and gates for a day before someone checks the dryer.

This is a single-point deep dive into TPU drying. For the full processing picture across molding, extrusion, and blow molding, see the TPU processing guide.

Editorial note: This is an independent educational guide. Drying times, temperatures, and moisture targets vary by grade. The values here are illustrative of published TPU grades — always follow the current data sheet for the grade you are running.

Why drying is the number-one TPU issue

TPU is hygroscopic, meaning it actively absorbs moisture from the surrounding air rather than just carrying surface dampness. Dry TPU left open on a bench starts taking on water within minutes to hours, depending on humidity. That is a fundamentally different situation from a material that only needs a surface wipe-down — the water gets into the pellet, and the only reliable way back out is controlled heat and dry air.

Because the absorption is built into the chemistry, drying is not an optional polishing step. It is part of the process. Skip it, shortcut it, or let an open bag sit out, and the defects follow predictably.

What moisture does in the barrel

When wet TPU is plasticized, the trapped water flashes to steam and reacts with the hot polymer. Two things happen, and both are bad:

• Physical defects. The steam foams the melt. That produces gas bubbles and blisters in the part, foaming at the nozzle, and silvery splay streaks on the surface as gas drags along the flow front.
• Chemical degradation. Water at melt temperature breaks polymer chains (hydrolysis in the barrel). This lowers molecular weight, which quietly costs you tensile strength, tear strength, and abrasion resistance — even when the part looks acceptable.

That second point is the dangerous one. A bubbly part gets scrapped at the press. A degraded-but-pretty part can ship and fail in the field, which is far more expensive. Thorough pre-drying is considered essential to reach a TPU's full abrasion and mechanical performance for exactly this reason.

The target: under 0.02% moisture

The widely published moisture target for TPU before processing is below 0.02% by weight. Many grade data sheets state it in those exact terms. It is a small number — 200 parts per million — which is part of why eyeballing it never works and why under-drying is so common.

If you have a moisture analyzer, this is the number to verify against, not the dryer's clock alone. Drying time is a guideline; residual moisture is the actual acceptance criterion.

How fast TPU picks up water

Moisture pickup accelerates with humidity and temperature. Published absorption curves for both polyester- and polyether-based TPU show the granulate climbing well past the 0.02% limit in a matter of hours when left in a warm, humid atmosphere. The practical consequence: a hopper or bag left open on a humid afternoon can put already-dried material back out of spec before it ever reaches the screw.

This is why the discipline around the dryer matters as much as the dryer itself. Drying correctly and then exposing the pellets to room air defeats the whole exercise.

Typical drying conditions by grade

Drying conditions shift with backbone, hardness, fillers, and the supplier's own recommendation. The following are representative of published TPU grades — use them to set expectations, not as a substitute for the data sheet:

A pattern worth internalizing: softer grades generally dry at the lower end of the range, while rigid and glass-reinforced grades take the higher temperatures. But the safe practice is always the same — read the data sheet, dry to under 0.02%, and verify rather than assume.

Which dryer, and why dehumidified wins

Both conventional circulating hot-air dryers and dehumidified-air (desiccant) dryers are listed as suitable for TPU. In practice, the dehumidified-air dryer is the more reliable choice, and on a humid day it is the only one that consistently hits the target.

The reason is dew point. A hot-air dryer heats the air but does nothing about the moisture already in it; if the ambient air is humid, you are blowing warm, damp air through hygroscopic pellets and fighting yourself. A desiccant dryer drops the dew point of the drying air first, so the air actually wants to pull water out of the resin. For TPU specifically, that difference is the line between "dried" and "looks dried."

Reading splay: the moisture fingerprint

Splay — the silvery or cloudy streaks that fan out from the gate along the flow direction — is the classic moisture signature in TPU. When you see it, moisture is the first and most likely cause, well ahead of melt temperature or injection speed.

Standard troubleshooting practice treats splay as primarily a moisture-and-contamination problem, adjusted by melt temperature and injection speed only after the moisture content has been ruled out. The honest workflow on the floor is:

1. See splay → check residual moisture and the dryer first.
2. Confirm the dryer is at temperature, on time, and dehumidifying.
3. Confirm material was not left exposed and the hopper is covered.
4. Only then start moving melt temperature, injection speed, and back pressure.

Other moisture tells back up the diagnosis: gas bubbles or blisters inside the part, foaming at the nozzle, and — in extrusion — throughput variation and die drooling. If several of these show up together, stop adjusting the press and fix the drying.

Shop-floor habits that prevent trouble

Most TPU moisture problems are prevented by handling discipline, not by exotic equipment:

• Keep packaging sealed; reseal bags immediately after taking material.
• Expose granulate to room air for as little time as possible.
• Keep the feed hopper covered during the run.
• Bring cold material up to room temperature before opening the bag, so condensation does not form on the cold pellets.
• Dry additives and colorants along with the granules — they carry moisture too.
• Re-dry material after a bag has been opened and left, or after repeated openings.
• Confirm the melt runs bubble- and foam-free; if not, raise drying before touching the press.

None of these are exotic. They are the small habits that separate a clean TPU run from a day lost to chasing splay.

Pre-molding drying checklist

• Grade data sheet pulled, drying temperature and time confirmed.
• Dehumidified-air dryer available and at the correct dew point.
• Moisture target (typically < 0.02%) verified, not just timed.
• Additives and colorants dried with the resin.
• Hopper covered; material exposure minimized.
• Cold material warmed to room temperature before opening.
• First-shot melt confirmed bubble- and foam-free.
• Splay, bubbles, and nozzle foaming watched as moisture indicators.

Bottom line

TPU drying is not a precaution, it is part of the process. The material is hygroscopic, the target is roughly under 0.02% moisture, and the typical conditions run a couple of hours somewhere between 80 and 120 °C depending on the grade — best done in a dehumidified-air dryer. Get it right and TPU runs cleanly and keeps its strength and abrasion resistance. Get it wrong and you get splay, bubbles, and quietly weakened parts. When something looks off, check the dryer before you touch the press.

For grade-specific drying and processing recommendations, BASF's Elastollan processing documentation is a solid reference.

Related: TPU Processing Guide → · Polyester vs Polyether TPU →