Quick answer: TPU can be extrusion blow molded, but only with a grade and process window that can hold a stable parison long enough for the mold to close and the part to inflate. Do not select the material by hardness alone. Validate melt strength, moisture control, thermal history, parison programming, wall distribution, pinch-off quality, cooling, and the finished part’s actual flex, pressure or vacuum, chemical, abrasion, and temperature duty cycle.
A hollow part can look complete and still be weak where the wall has drawn down, at the pinch-off, around a tight radius, or at the root of a bellows fold. The practical development question is therefore not simply “Can TPU be blow molded?” It is “Can this grade, on this machine and tool, repeatedly produce the required geometry and survive the real service environment?”
Where blow-molded TPU fits
Extrusion blow molding is an established TPU processing route. A heated tube of polymer, the parison, is extruded between mold halves; the mold closes, pinches the ends, and air inflates the material against the cavity before cooling and ejection. The method can suit flexible hollow components that need combinations of toughness, abrasion resistance, elastic recovery, and complex geometry.
Supplier application literature identifies uses such as automotive boots, bellows, industrial hollow parts, consumer goods, and sports or recreation products. That does not mean every TPU grade is suitable. Blow-molding formulations normally need rheology and melt strength appropriate to parison formation, while the required chemistry and hardness still depend on the application.
TPU may be worth screening when the part needs:
- flexible hollow geometry with molded folds, transitions, or attachment features;
- abrasion and cut resistance beyond what a commodity flexible plastic can provide;
- elastic recovery across repeated articulation;
- a thermoplastic process and the possibility of welding or secondary assembly;
- application-specific resistance to oils, grease, water, cleaners, or weathering.
Another material may fit better when the duty cycle requires properties the selected TPU cannot substantiate, when the required processing window is incompatible with the available line, or when cost, permeation, compression behavior, temperature, or chemical exposure dominates the decision. Compare TPU with rubber and other TPE families using the complete part specification—not a single tensile, hardness, or abrasion value.
Start with grade suitability, not nominal hardness
Melt strength and parison behavior
A blow-molding grade must exit the die smoothly, resist excessive sag and drawdown, and remain able to inflate and reproduce the cavity. Two materials with similar Shore hardness can behave very differently because molecular architecture, melt viscosity, temperature sensitivity, and additives influence parison stability.
Ask the material supplier whether the proposed grade is intended or has supporting experience for extrusion blow molding. A conventional injection-molding grade should not be assumed interchangeable merely because it melts at a similar temperature.
Chemistry and service environment
Polyether- and polyester-based TPUs can differ in hydrolysis behavior, oil and chemical response, low-temperature flexibility, mechanical performance, and weathering. The correct direction depends on the actual fluid, concentration, temperature, contact duration, strain, and aging conditions. Use supplier data to screen candidates, then test molded parts in the real environment.
Moisture and thermal history
TPU is moisture-sensitive during melt processing. Inadequate drying or wet material transfer can contribute to bubbles, splay, unstable output, surface defects, reduced melt strength, and loss of properties. Excessive melt temperature, residence time, shear, or repeated reprocessing can also reduce stability.
Record drying equipment, set point, dew point or equivalent dryer performance, residence time, material exposure after drying, regrind ratio, melt temperatures, head pressure, output, and stoppages. A dry resin specification is useful only when the handling system keeps the resin dry until it reaches the feed throat.
Control the parison before correcting the finished part
The parison records the combined effect of the resin, dryer, extruder, die head, temperature profile, output, programming, and cycle timing. If its behavior changes, the finished wall distribution usually changes too.
Parison sag and drawdown
A hot parison can lengthen and thin under its own weight before mold closure. Longer drop time, higher melt temperature, lower melt strength, unstable output, and heavy lower geometry can intensify the problem. Do not compensate blindly with more material. First determine whether the sag is repeatable and whether it tracks melt temperature, cycle time, or lot changes.
Record parison length versus time, visible diameter profile, drop time, melt temperature, head pressure, and part weight. Short, controlled trials that change one factor at a time are more informative than simultaneously adjusting temperature, speed, and die gap.
Wall distribution and parison programming
Uniform shot weight does not guarantee uniform walls. Inflation stretches the parison differently around shoulders, corners, deep draws, large diameters, and folds. Programmed die-gap control may place more material where the geometry will stretch most, but the program must be validated with wall mapping rather than appearance alone.
Create a fixed measurement map covering:
- top, middle, and bottom of the part;
- the thick and thin sides around the circumference;
- shoulders, radii, and attachment transitions;
- bellows peaks, valleys, and fold roots;
- both pinch-off regions and any welded or trimmed area.
Inflation, venting, and mold closure
Air pressure, blow timing, venting, mold-close timing, and surface temperature influence cavity reproduction and trapped-air marks. More pressure is not automatically better. It can intensify thinning or stress at a vulnerable feature. Verify that the parison is centered, the cavity vents are functioning, and inflation occurs at a repeatable point in the cycle.
Treat the pinch-off as a structural feature
The pinch-off closes the hollow part and creates a weld-like region. Its geometry, land condition, mold alignment, temperature, material distribution, and trimming method can control leakage and fatigue life.
Inspect the pinch-off for incomplete fusion, trapped contamination, excessive thinning, sharp trim damage, asymmetry, and local stiffness changes. A clean external appearance does not prove the internal bond or wall profile is acceptable.
Bellows and boots need additional attention at fold roots and attachment interfaces. These zones combine curvature, thickness change, repeated strain, and possible contact with clamps or mating parts. Avoid treating the nominal wall value as the only geometry requirement; minimum local wall and transition quality are usually more meaningful.
Diagnostic matrix: symptom, evidence, and first checks
| Symptom | Evidence to collect | First checks | Avoid |
|---|---|---|---|
| Parison sags or necks before closure | Video with timestamps, parison length profile, melt temperature, drop time | Grade melt strength, temperature profile, cycle timing, output stability | Adding shot weight before identifying the time/temperature cause |
| Bubbles or foamy streaks | Drying log, moisture result if available, purge appearance, stoppage history | Dryer performance, wet-air exposure, residence time, overheating or contamination | Assuming every bubble is only a resin defect |
| Thin corners or one thin side | Indexed wall map, cut sections, parison centering, mold orientation | Die centering, programming, geometry stretch, inflation timing, mold alignment | Accepting average wall thickness without minimum-location data |
| Weak or leaking pinch-off | Leak location, cross-section, trim condition, close timing | Pinch geometry, contamination, temperature, wall allocation, alignment | Raising blow pressure without inspecting the seal geometry |
| Distortion after ejection | Part temperature, cooling time, fixture condition, dimensional trend | Cooling balance, ejection temperature, residual stress, wall asymmetry | Approving dimensions immediately after molding only |
| Cracks at bellows folds | Crack origin, cycle count, local wall, temperature and chemical exposure | Radius, fold strain, wall transition, material aging, process damage | Predicting life from tensile strength alone |
| Sticking or surface damage | Mold-surface location, release practice, ejection marks | Surface finish, cooling, draft, local pressure, approved processing aids | Using an unapproved release agent that affects bonding or service |
Build a finished-part validation plan
Coupon data helps screen materials, but the final decision belongs to the molded geometry. Define acceptance criteria before the trial, identify the most vulnerable locations, and test after production-equivalent conditioning.
A useful plan may include:
- Dimensional and wall map: part weight, minimum and maximum wall, concentricity or symmetry, attachment dimensions, and recovery after a defined conditioning time.
- Leak or pressure/vacuum testing: use the actual direction, level, duration, fittings, and temperature. Locate failures rather than recording pass/fail alone.
- Flex and articulation: reproduce travel, bend radius, compression/extension, frequency, dwell, temperature, and cycle sequence. Inspect fold roots, pinch-offs, and interfaces.
- Chemical exposure: use named fluids, concentration, temperature, duration, stress state, and replenishment schedule. Track mass, dimensions, hardness, appearance, leakage, and retained function as relevant.
- Abrasion and contact: reproduce contact pressure, counterface, contamination, motion, and vulnerable wall location.
- Temperature and aging: include realistic hot/cold cycles, humidity or water exposure, and recovery periods. Test assembled parts when clamps, inserts, or neighboring materials change strain.
- Process capability: repeat across lots, shifts, start-ups, and expected machine settings. A single successful sample is not evidence of a stable production window.
For adjacent test planning, see GOAFLEX guides on TPU tear strength and flex fatigue, chemical-resistance validation, service temperature, and air-retention diagnostics.
Common mistakes that delay scale-up
- Selecting a grade by hardness and tensile strength without confirming blow-molding suitability.
- Changing resin, die gap, melt temperature, output, blow timing, and cooling in the same trial.
- Measuring only part weight or average wall thickness.
- Approving appearance before sectioning the pinch-off, corners, shoulders, and fold roots.
- Testing chemicals on unstressed plaques when the field part is flexed or clamped.
- Using a generic cycle count without reproducing actual travel, temperature, dwell, and pressure.
- Ignoring moisture pickup between the dryer and machine.
- Treating a supplier’s typical value as a finished-part guarantee.
- Assuming a visually identical regrind blend has the same melt strength and fatigue behavior.
What to send GOAFLEX for grade and trial review
Send enough information to separate a material question from a machine, tool, geometry, or service-condition problem:
- part drawing, 3D geometry, photos, nominal wall, and minimum critical wall locations;
- current resin or material family, hardness, chemistry, color/additive package, and regrind target;
- annual volume, part weight, scrap rate, and current failure mode;
- machine, screw, accumulator or head type, die size, parison programming capability, and mold details;
- drying, handling, temperature, output, pressure, cycle, inflation, cooling, and trimming records;
- wall-thickness map, cut sections, dimensional trends, leak locations, and failed samples;
- service fluid, concentration, temperature range, pressure or vacuum, movement, abrasion, weathering, and expected life;
- required regulatory, color, bonding, marking, or documentation constraints.
GOAFLEX can use this package to discuss a defensible starting material direction and controlled trial plan. Grade availability and blow-molding suitability must be confirmed before sampling. Email hello@goaflex.com with the drawing, process window, duty cycle, and failed-part evidence.
Frequently asked questions
Can any TPU grade be blow molded?
No. Blow molding is an established TPU process, but the selected grade must have suitable melt strength and process behavior for the machine, die, parison length, and geometry. Confirm supplier guidance and validate on the actual line.
Why is one side of the part thinner?
Common contributors include die mis-centering, asymmetric parison temperature or flow, mold alignment, uneven drawdown, gravity, inflation timing, and geometry-dependent stretch. Use an indexed wall map and parison observation before changing several settings.
Is higher melt temperature a good fix for poor inflation?
Not automatically. Higher temperature may improve flow but can increase sag, drawdown, sticking, residence-time damage, bubbles, or dimensional instability. Work inside grade-specific guidance and monitor the full process response.
Which part areas usually deserve the closest inspection?
Pinch-offs, shoulders, tight radii, thinned corners, bellows fold roots, attachment transitions, trimmed edges, and interfaces that see clamps or repeated movement.
Sources and further reading
- Covestro — Processing of thermoplastic polyurethanes — confirms injection molding, extrusion, and blow molding as established TPU processing routes; it does not establish suitability of every grade.
- BASF — Elastollan TPU Processing Recommendations — supplier processing guidance for drying, thermal history, extrusion, and specialized processing; use the current grade-specific data sheet for production settings.
- Lubrizol — Blow Molding — application context for hollow TPU parts including boots, bellows, automotive, industrial, consumer, and recreation uses.
- GOAFLEX — TPU Drying and Moisture Control — practical drying and handling diagnostics.
Image credits
- Vladimir Srajber, plastic bottle production line, Pexels source, Pexels License.
- Katharina-Charlotte May, automated industrial machinery, Pexels source, Pexels License.
- Michael Orshan, digital caliper measurement, Pexels source, Pexels License.
- Alex Domínguez, mechanic using a caliper, Pexels source, Pexels License.
- Pexels License.
All four photographs are contextual. They do not prove GOAFLEX ownership, endorsement, exact TPU chemistry, grade performance, or that the depicted equipment and parts use TPU.
