TPU: haze, matte patches or white marks — bad material or a processing problem?

A TPU part comes out cloudy, dull, streaky or with white marks, and the first reaction is usually the same: the material must be bad. Sometimes it is. More often, several very different causes have produced a very similar-looking defect.

The problem

Milky or hazy areas, a matte instead of glossy surface, flow streaks, white stress marks or other surface faults on TPU. The visual result looks like a material defect — so a good lot gets rejected while the real cause stays in the process.

Why it happens

TPU is sensitive and hygroscopic, and many different factors create overlapping fault patterns: moisture and insufficient drying, an incorrect temperature profile, excessive shear, contamination or material carry-over, an incompatible masterbatch or carrier, or a TPU chemistry (polyester- vs polyether-based, different hardness) that simply does not suit the part. Because the symptoms overlap, swapping the material first is usually the most expensive guess.

How to solve it — isolate, don't swap

Work the fault pattern in order, changing one thing at a time:

1. 1Drying — confirm correct residual moisture, dryer temperature and time for this TPU before anything else.
2. 2Temperature profile — check melt and zone temperatures against the grade's window; too hot degrades, too cold streaks.
3. 3Residence time — long dwell at temperature degrades TPU; check shot size against barrel capacity.
4. 4Tool & shear — gate design, injection speed and sharp transitions can shear-mark TPU surfaces.
5. 5Masterbatch / carrier — verify the colour carrier is compatible with TPU; a wrong carrier alone causes haze and matteness.
6. 6Material — only once the above are ruled out, question the TPU type or lot.

Understand the fault pattern first; then choose the right TPU type or lot for the job.