Every TPU and TPE grade has a processing window — a range of temperature, speed, and pressure inside which it fills cleanly and keeps its properties. Stay inside it and the parts are good. Wander outside and you get splay, bubbles, burns, short shots, or quietly degraded material. The useful thing about elastomer defects is that most of them are not mysterious: they trace back to a short list of material behaviors, and once you know those, the troubleshooting gets systematic.
This is the material-side view of the window — distinct from the buyer's defect checklist and from a generic molding-process course. For the broader process overview, see the TPU processing guide; for the single biggest root cause, drying TPU before molding.
Editorial note: This is an independent educational guide. Temperatures and settings are illustrative of published grades, not a specification. Always use the current grade data sheet and validate on your own equipment.
What "processing window" really means
The processing window is the overlap of all the conditions that have to be right at once: melt temperature high enough to fill but not so high it degrades, mold temperature that sets the part without freezing the flow front, injection speed and pressure that pack the cavity without flashing or burning, and residence time short enough that the polymer does not cook. A wide window is forgiving; a narrow one demands tight control.
For TPU, the window is narrowed by two things in particular: the material's moisture sensitivity and its sensitivity to heat and residence time. Both are material properties, which is why this is a materials conversation, not just a machine-setting one.
Melt and mold temperature ranges
Temperatures shift with hardness, backbone, and grade, but published TPU grades cluster in a recognizable band. Two representative examples:
| Grade example | Barrel zones | Melt temperature | Mold temperature |
|---|---|---|---|
| Soft polyester TPU (~47 Shore A) | ~175–200 °C | ~185–195 °C | ~20–40 °C |
| Aliphatic polyester TPU (~68 Shore A) | ~200–235 °C | ~210–230 °C | ~20–60 °C |
A few patterns hold across the family: TPU melt temperatures generally sit somewhere between about 175 and 235 °C, molds run relatively cool, and screw (circumferential) speed is kept modest — often below roughly 0.2 m/s — with back pressure commonly in the tens-to-low-hundreds of bar. The key habit is to confirm the melt comes out bubble- and foam-free; if it does not, the fix usually starts with drying, not with temperature.
Shear behavior: TPE flows, TPU resists
A defining material difference shows up the moment you try to fill a thin wall. Most TPEs are shear-responsive: at high shear rates their viscosity drops and they flow easily, which is exactly what you want for thin sections, long flow paths, and over-molding. Urethane-based materials are the well-known exception — TPU does not thin out under shear the same way.
The practical consequence: a thin-wall design that fills effortlessly in a styrenic TPE can short- shot in TPU at the same settings. With TPU you pay more attention to wall thickness, flow length, gate size, and fill speed, because the material will not bail you out with easy shear thinning.
Residence time and thermal degradation
TPU does not like sitting hot. Excess heat or long residence time in the barrel breaks the polymer down, which shows up as burnt spots, discoloration, and — more insidiously — strength loss in parts that still look acceptable. This is separate from moisture degradation; you can dry the resin perfectly and still degrade it by parking it in a hot barrel.
The defenses are practical: size the shot to the barrel so material is not over-residing, avoid drifting melt temperatures higher to "help" filling, and watch for color shift and burnt streaks as early warnings. If a part loses strength with no visible cause, residence time and melt temperature belong on the suspect list alongside moisture.
The material-side defect matrix
Standard TPU troubleshooting maps each defect to the parameters that move it. Some parameters are increased to solve a problem, some reduced, and some can go either way depending on the situation. A condensed version:
| Defect | Primary parameters to review | Material-side root cause to rule out first |
|---|---|---|
| Bubbles / blisters | Moisture, melt temperature, back pressure, residence time | Moisture (drying) |
| Splay marks | Moisture, melt temperature, injection speed | Moisture / contamination |
| Burnt spots | Melt temperature, injection speed, venting, residence time | Thermal degradation / trapped gas |
| Degradation (strength loss, discoloration) | Melt temperature, residence time, moisture | Heat + residence time, or moisture |
| Short shot | Injection speed, melt/mold temperature, shot size, gate size | Flow behavior (TPU does not shear-thin) |
| Flow lines | Injection speed, melt/mold temperature, gate size | Fill behavior and gating |
| Sink marks | Hold pressure/time, melt temperature, cooling time | Packing / wall thickness |
| Flashing | Clamping pressure, melt temperature, injection speed | Over-pressure / over-temperature |
| Distortion / shrinkage | Mold temperature, cooling time, hold pressure, melt temperature | Cooling and packing balance |
| Gloss / matt surface variation | Melt/mold temperature, injection speed, hold pressure | Surface replication conditions |
| Demolding problems | Mold temperature, cooling time, hold pressure, lubricant | Soft-grade release behavior |
The defects you will actually see
In practice, a small set of defects accounts for most TPU scrap, and they share root causes:
- Splay and bubbles — almost always moisture. Check the dryer before the press.
- Burnt spots and discoloration — heat, residence time, or trapped gas (venting).
- Quiet strength loss — degradation from moisture or excess heat, even when the part looks fine.
- Short shots in thin walls — TPU's lack of shear thinning, not necessarily low temperature.
Three of those four are material behaviors, not machine faults. That is the central point of troubleshooting elastomers from the material side: the press is often innocent.
Gate and flow-related defects
Short shots, flow lines, and surface defects are often as much about how the cavity is filled as about the material. Gate size and location, fill speed, and venting all change where the flow front knits and where gas gets trapped. Because TPU does not shear-thin like other elastomers, gating that would be marginal for a styrenic TPE can become a real flow problem in TPU — which is why gate design deserves attention early. For the tooling-side detail behind these flow and weld issues, PlasticsTechnologyAlliance.com has a useful reference on injection molding gate design.
One more material note for multi-component and over-molded parts: TPU bonds well to compatible plastics in multi-component molding without additives, but polyolefins such as PP and PE are generally not suitable partners — a mismatch there shows up as a failed bond, not a fillable defect.
A material-first troubleshooting workflow
- Confirm the material. Is it dried to spec, uncontaminated, and the right grade? Is the melt bubble- and foam-free?
- Rule out moisture for any splay, bubble, or blister before touching temperature.
- Check residence time and melt temperature for burns, discoloration, or unexplained strength loss.
- Address flow (speed, gate, wall) for short shots, remembering TPU does not shear-thin.
- Balance packing and cooling for sink, distortion, and shrinkage.
- Only then fine-tune the remaining press parameters.
Working in that order keeps you from chasing a moisture defect with temperature changes for half a shift — the most common time-waster on an elastomer line.
Bottom line
The TPU and TPE processing window is defined as much by the material as by the machine. TPU runs in a roughly 175–235 °C melt band with cool molds, it does not shear-thin the way other TPEs do, and it is sensitive to both moisture and excess heat or residence time. Most defects map cleanly to those behaviors: splay and bubbles to moisture, burns and strength loss to heat and residence time, short shots to flow. Troubleshoot from the material side first — confirm drying, grade, and residence time — and the defect list shrinks fast.
For grade-specific processing parameters and troubleshooting guidance, BASF's Elastollan processing documentation is a thorough reference.
FAQ
What is the processing window for TPU?
It is the range of melt temperature, mold temperature, speed, and pressure within which a grade fills and solidifies without defects or degradation. Published TPU grades commonly run melt temperatures roughly between 175 and 235 °C with relatively cool molds, but the exact window is grade-specific.
Why does TPU degrade in the barrel?
Two main causes: residual moisture, which hydrolyzes the polymer at melt temperature, and excessive heat or residence time, which thermally breaks down the polymer. Both lower molecular weight and cost strength, so drying and barrel residence time both have to be controlled.
What causes splay in TPU parts?
Splay in TPU is most often a moisture problem. Adjusting melt temperature and injection speed can help, but residual moisture and contamination should be ruled out first because they are the usual root cause.
Do TPU and TPE flow the same way?
No. Most TPEs are shear-responsive and become low in viscosity at high shear rates, which helps them fill thin walls. Urethane-based materials are the common exception and do not show that same easy shear thinning, so TPU needs more attention to fill thin sections.
Can TPU be combined with any plastic in multi-component molding?
No. TPU can bond well to compatible plastics in multi-component molding without additives, but polyolefin-based plastics such as PP and PE are generally not suitable partners for TPU.
What is the first thing to check when a TPU defect appears?
Confirm the material is properly dried and uncontaminated and that the melt runs bubble- and foam-free. A large share of TPU defects trace back to moisture, so checking drying first usually saves time before adjusting the press.
Related: TPU Processing Guide → · Drying TPU Before Molding →