TPU processing and regrind: drying, defects, and fixes

Most TPU processing problems are one problem wearing different costumes: water. TPU is hygroscopic, and moisture in the melt does more than cosmetic damage; it hydrolyzes the polymer and takes mechanical properties with it. This guide covers the drying discipline that prevents most defects, the diagnostic table for the ones that show up anyway, and how to run regrind without paying for it later.

Drying: the step that decides part quality

TPU picks up moisture from ambient air within hours of a bag or gaylord being opened. Processing it wet flashes that water to steam in the melt and, worse, drives hydrolysis: chain scission that permanently lowers molecular weight. The part may even look acceptable while carrying tensile and tear values well below the datasheet.

The working rules:

• Dry every time, in a desiccant dryer. Typical conditions run in the 80 to 110°C range for 2 to 4 hours depending on grade and hardness. The TDS states the exact conditions and the target moisture; extrusion generally demands a drier resin than molding because the melt is unsupported at the die.
• Verify the dryer, not just the timer. A desiccant dryer with a weak dew point holds resin at temperature without drying it. If moisture defects persist after "drying," check the dew point first.
• Mind the hopper. Dried TPU re-absorbs moisture quickly. Keep hopper residence short, covered, or dry-air blanketed, and re-dry any material that sits open, including overnight machine hoppers.

Reading the common defects

• Splay, silver streaks, bubbles, foamy extrudate. Moisture, almost always. Re-dry to TDS conditions, confirm dew point, cover open hopper time. If it survives verified-dry material, look at melt temperature and residence time, since thermal degradation gasses too.
• Brittle parts or falling mechanicals. Hydrolysis from wet processing, an overheated melt, too long a residence time, or too much regrind. Each of these costs molecular weight, and the losses add up across them.
• Yellowing. Heat history. Lower the melt temperature toward the bottom of the grade's window, shorten residence, and check for dead spots in the flow path. On light or color-matched parts, tighten regrind discipline too.
• Die drool and melt fracture in extrusion. Usually a temperature-profile or shear problem: run the die at the right temperature, keep the melt inside the grade's window, and confirm the grade is an extrusion grade with the melt strength the profile needs. A molding grade run through a die drools.
• Extrudate sticky or blocking on the reel. Soft grades are inherently tacky, and winding before the material has fully cooled and set makes it worse. Add cooling capacity or line length before wind-up, and ask about anti-blocking grade options.
• Parts sticking in the mold. Common with soft grades. Cool the mold, extend the cycle slightly, review draft and polish, and ask whether a mold-release-optimized grade variant exists before reaching for spray release on every shot.
• Dimensions drifting after molding. TPU continues to set up for hours to days after molding as the hard segments organize. Measure and test after conditioning, and where the application allows, anneal per the TDS to stabilize properties sooner.

Regrind: what it tolerates and what it costs

TPU regrinds and reprocesses well by elastomer standards, which is one of its economic advantages. It still asks for discipline:

• Keep it clean and single-grade. Regrind streams that mix hardnesses, chemistries, or colors turn a controlled material into an uncontrolled one. Ester and ether grades do not belong in the same bin.
• Re-dry it like virgin. Ground TPU has more surface area than pellets and picks up moisture faster. Wet regrind is the fastest route to the whole defect list above.
• Cap the ratio to the part. Clean, dry, same-grade regrind is commonly run at modest let-down levels, with many processors staying at or below 20 to 30 percent. Every heat history costs some molecular weight, so tighten the cap on parts with demanding mechanical, appearance, or compliance requirements, and confirm the level with the grade's TDS and your own part testing.
• Exclude it where the spec says virgin. Food-contact and other compliance-documented applications may require virgin material; check before regrind enters the stream.

Set up for fewer surprises

Grade selection upstream prevents troubleshooting downstream: an extrusion profile needs an extrusion grade, a wet environment needs the right chemistry, and both are cheaper to fix on paper than on the line. The selection sequence is covered in the TPU buyer's guide, and the chemistry choice in polyester vs polyether TPU.

Frequently asked

Do I need to dry TPU before processing?

Yes, every time. TPU is hygroscopic and picks up moisture within hours of exposure. Processing wet TPU hydrolyzes the polymer, which shows up as splay, bubbles, and permanently degraded mechanicals. Typical conditions run 80 to 110°C for 2 to 4 hours in a desiccant dryer; the grade's TDS states the exact conditions.

Why does my TPU part have bubbles or silver streaks?

Moisture, almost always. Re-dry to the TDS conditions, confirm the dryer's dew point, and cover open hopper time. If the defect survives verified-dry material, look at melt temperature and residence time next.

How much regrind can I use with TPU?

Clean, dry, same-grade regrind is commonly run at modest levels, with many processors at or below 20 to 30 percent, but tolerance is grade- and part-dependent. Re-dry regrind like virgin, keep the stream single-grade, and confirm the level with the TDS and part testing.

Why is my extruded TPU sticky or blocking on the reel?

Soft grades are inherently tacky, and winding before full cooling makes it worse. Add cooling capacity or line distance before wind-up, and ask about anti-blocking options. Persistent tack can also point to an overheated melt.

Processing help comes with the material

Tell us the grade, the process, and the defect. We will work the diagnosis with you, and if the answer is a different grade, we will send the sample to prove it.