TPU Tubing Extrusion Troubleshooting: How to Control Ovality, Wall Thickness, Surging, and Cooling
Quick answer
When TPU tubing moves out of specification, first determine whether the variation is around the circumference, along the tube, or developing downstream. Verify the exact grade and lot, resin moisture near the feed throat, actual melt conditions, pressure and motor-load stability, die centering, internal support air or vacuum, cooling-water conditions, line speed, guides, and puller pressure. Measure OD on more than one axis and wall thickness at several clock positions; OD alone cannot prove wall uniformity. Change one primary variable at a time and retain samples tied to time-synchronized process data.
There is no universal TPU drying, melt-temperature, vacuum, air-pressure, or cooling-water setting. Use the current processing guidance for the selected grade and validate the finished tube against its actual dimensional and end-use requirements.
Why TPU tubing defects can be difficult to isolate
A tubing dimension is the result of several connected systems:
- Material delivery and drying
- Feeding, melting, and pressure generation
- Die and mandrel flow balance
- Drawdown between the die and haul-off
- Internal air support or external vacuum, where used
- Cooling rate and mechanical support
- Puller, guides, cutting, and winding
- Measurement method and conditioning
A defect seen at the winder may have started at the hopper—or may have been created by the puller after the tube had left the cooling tank. TPU makes this distinction especially important because freshly extruded material can have relatively low shape stability. BASF’s Elastollan extrusion guide notes that newly extruded TPU is prone to distortion and requires effective cooling close to the die. Lubrizol likewise emphasizes rapid entry into the cooling stage, sufficient cooling length, and carefully controlled support for tubing.
Start by marking the machine direction, top of tube, die orientation, and sample time. Without those references, a “thin side” or “flat side” cannot be traced reliably.
For upstream moisture checks, see the GOAFLEX guide to TPU drying and moisture control. For related extrusion-pattern diagnosis, see TPU film extrusion troubleshooting.
Define the defect before changing the process
Measure ovality on multiple axes
At one cross-section, record the maximum and minimum outside diameters:
- Absolute ovality: (D_\text{max} - D_\text{min})
- Percentage ovality: report the same difference against the denominator required by the product specification—commonly nominal OD or measured mean OD.
Do not use “percent ovality” without stating the formula. Customer drawings, product standards, and internal control plans may use different denominators.
Measure at a defined distance from the die and after a defined conditioning period. A soft tube measured hot, immediately after the puller, can give a different result from the same tube after cooling and recovery.
Map wall thickness by clock position
Cut a square cross-section using a method that does not smear or stretch the wall. Record wall thickness at four positions at minimum—12, 3, 6, and 9 o’clock—and at eight or more positions when concentricity is critical.
Report:
- Minimum wall
- Maximum wall
- Mean wall
- Opposing-wall differences
- Clock position of the minimum wall
- Whether the thin position remains fixed relative to the die
A fixed thin side usually points toward centering, alignment, temperature, or flow balance. A thin side that rotates or wanders may indicate unstable support, tube twist, haul-off interaction, or measurement inconsistency.
A handheld caliper can compress soft TPU. Define the instrument, contact force, specimen temperature, and conditioning time. For thin or high-value tubing, consider a validated non-contact OD system and an appropriate wall-measurement method such as ultrasonic gauging or controlled cross-section microscopy.
ASTM D2122-22 and ISO 3126:2005 provide useful dimensional-measurement principles for thermoplastic pipe and fittings. Their applicability to a particular flexible TPU tube must be confirmed rather than assumed.
Separate variation around the tube from variation over time
Use three categories:
- Circumferential: one side is consistently thick, thin, or flat.
- Machine-direction: OD or wall rises and falls along the length.
- Random: isolated changes have no obvious position or period.
Then compare dimensions with time-synchronized trends for head pressure, motor load, screw speed, feeder events, line speed, support air or vacuum, and cooling conditions. This pattern-based approach is usually more informative than immediately changing barrel temperatures.
TPU tubing diagnostics matrix
| Observed pattern | Likely mechanisms to investigate | First checks | Controlled corrective direction |
|---|---|---|---|
| Ovality is already present before the first guide | Low melt strength, excessive melt temperature, long unsupported air gap, insufficient internal air support, unstable vacuum, die/mandrel misalignment | Observe the tube immediately after the die; record actual melt temperature, air gap, support pressure/vacuum, and die alignment | Move effective cooling/support closer where the equipment permits; stabilize support air or vacuum; verify the grade-specific melt window; correct alignment |
| Tube is round before the bath but oval after cooling | Uneven cooling, unsuitable guide geometry, tube floating or dragging, asymmetric water flow, inadequate residence time | Measure at bath entry, mid-bath, and exit; record inlet/outlet water temperature and flow; inspect rollers and spray pattern | Balance water flow and support; use guides matched to the tube; increase effective cooling capacity or residence time without exceeding grade guidance |
| Tube is round after the bath but flat after the puller | Excessive belt or roll pressure, narrow contact area, hot core at haul-off, puller misalignment | Compare OD before and after puller; measure tube temperature at both points; inspect belt gap and tracking | Reduce or redistribute puller compression; improve cooling before haul-off; align and maintain belts or rolls |
| One wall remains thin at the same clock position | Die/mandrel not centered, bent or misaligned pin, fixed die deposit, heater-zone imbalance, nonuniform flow path | Mark die orientation on samples; map wall by clock position; inspect centering hardware, temperatures, and buildup | Recenter only under approved procedures; verify heater and thermocouple function; clean and inspect tooling after safe shutdown |
| Thin side wanders around the circumference | Tube twist, unstable support air/vacuum, guide or puller interaction, mandrel movement, inconsistent measurement orientation | Mark a longitudinal reference line; trend support pressure and puller behavior; check tooling retention | Stabilize the support system; remove twist and lateral loading; inspect mechanical looseness |
| OD varies periodically and wall changes in the same cycle | Extruder output surging, feeder refill or bridging, pressure-control cycle, drive or heater cycle | Trend head pressure, motor current, feeder events, screw speed, output, and OD on one time base | Correct feed-throat or feeding instability; verify controls and heaters; reduce output only as a diagnostic trial; inspect screw suitability and wear |
| OD cycles while wall stays relatively stable | Puller-speed variation, internal air/vacuum cycle, measurement vibration, sizing interaction | Trend actual line speed and support pressure with OD; inspect encoder and gauge mounting | Stabilize downstream control and measurement before changing melt conditions |
| Wall varies while OD remains acceptable | Eccentric ID, die centering or internal support problem; OD control masking wall movement | Measure ID or wall around the full circumference; do not rely on OD alone | Correct centering and internal-shape control; review tooling balance and support |
| Slow dimensional drift through the run | Dryer drift, screen loading, die buildup, temperature drift, water-temperature rise, puller heating | Compare start/middle/end samples; trend moisture, head pressure, heater output, water inlet/outlet temperature, and puller condition | Restore the drifting system; avoid compensating continuously with unrelated settings |
| Bubbles, roughness, and unstable output occur together | Excess moisture, volatile contamination, overheating, excessive residence, incompatible secondary stream | Measure resin moisture near the throat; isolate masterbatch/regrind; check actual melt temperature and residence history | Restore the grade-specific dry-material path; run a known-dry control; isolate suspect secondary streams |
| Tube sags between die and bath | Melt too fluid for the unsupported span, excessive span length, inadequate cooling position, unsuitable grade/output combination | Measure actual melt temperature and die-to-cooling distance; observe sag at different rates | Shorten unsupported distance where safe; use supplier-approved process adjustments; confirm grade suitability |
| Surface is damaged at sizing or guides | TPU tack or friction, insufficient water lubrication, rough or contaminated contact surfaces, excessive vacuum | Inspect contact marks and guide surfaces; confirm water film and vacuum level | Improve lubrication/support; clean or change contact materials; avoid forcing soft TPU through unsuitable rigid sizing hardware |
| Output is stable but dimensions oscillate at higher drawdown | Draw resonance or downstream speed/control interaction | Compare mass output, line speed, and dimensions; repeat at a modestly reduced draw ratio | Reduce drawdown where possible; review die sizing and grade-specific melt conditions |
| Dimensions shift after adding regrind or color | Moisture, bulk-density variation, feed segregation, incompatible carrier, viscosity or thermal-history change | Run a clean virgin control; verify blend ratio, component identity, drying, particle size, and feed consistency | Isolate the secondary stream and requalify it; do not assume the base resin is responsible |
This matrix identifies diagnostic directions, not automatic setting changes. A correction that helps one mechanism can worsen another.
How to control ovality
Check where the tube first becomes oval
Measure or observe the tube at four locations:
- At the die exit
- Immediately before cooling
- At the cooling exit
- Before and after the puller
This “location test” is more useful than one final inspection. If the tube is already oval before water contact, investigate melt strength, unsupported span, tooling, and internal support. If it becomes oval in the tank, investigate water flow, guides, vacuum, and cooling time. If it becomes oval after the tank, investigate residual heat and mechanical compression.
Stabilize internal support carefully
Depending on the grade, dimensions, and line design, a tubing process may use controlled internal air, low external vacuum, free extrusion, or a modified calibration arrangement. BASF recommends supporting air to prevent collapse and notes that vacuum can assist hollow-shape stability. Lubrizol cautions that conventional mechanical sizing is not normally suitable for softer TPU melts, although modified vacuum systems with precooling have been used successfully.
The practical rule is to avoid treating vacuum or air pressure as a standalone number. Record:
- Regulator setpoint and actual pressure near the die
- Pressure stability over time
- Leaks or restrictions
- Tube-end sealing arrangement
- Vacuum level and tank-water behavior
- Response to output and line-speed changes
Too little support may allow collapse; too much can enlarge or destabilize the tube. Establish limits through controlled trials with the selected grade and tooling.
Prevent downstream flattening
Guide rolls should fit the tube without concentrating load. Puller belts should provide enough traction without reshaping a tube that is still warm internally. Surface-temperature readings alone may not reveal a hot core, especially with a thick wall or high line speed.
If ovality increases with speed, calculate whether the available tank length, water flow, and heat removal still provide adequate cooling residence. Do not simply lower the water setpoint: colder water is not always the only—or best—answer, and supplier guidance can differ by TPU chemistry and grade.
How to correct uneven wall thickness
Center the die and mandrel using a wall map
Make small, documented centering changes only after recording the baseline and confirming the safe procedure. After each adjustment:
- Allow enough residence time for the change to reach the sampling point.
- Mark sample orientation.
- Measure the same clock positions.
- Compare minimum wall, maximum wall, and OD.
- Confirm that the correction remains stable after steady state.
A die adjustment can improve one location while moving the thin area elsewhere. Stop only when the complete circumference—not one measurement—meets the acceptance criteria.
Check temperature balance, not just setpoints
A displayed die-zone temperature does not prove uniform metal or melt temperature. Failed heaters, poorly seated thermocouples, external drafts, cooling-water proximity, or deposits can create asymmetric flow.
Useful checks include:
- Heater-current or output comparison
- Independent surface-temperature survey where safe
- Actual melt-temperature measurement by an approved method
- Die and adapter residence/dead-spot review
- Deposit location after safe disassembly
Avoid “chasing” eccentric wall by making multiple heater and centering changes together.
Separate output variation from cross-sectional imbalance
If every wall position becomes thicker and thinner together, investigate mass output and line speed. If one side stays thick while the opposite side stays thin, investigate centering and flow balance. If OD remains controlled while wall distribution moves, the outer-shape control may be hiding an eccentric ID.
How to diagnose extrusion surging
Surging is a repeating change in output. It may appear as cycles in head pressure, motor current, OD, wall thickness, or line load. Lubrizol’s TPU extrusion guide lists potential causes including moisture, feed-throat bridging, inappropriate rear-zone conditions, excessive screw speed, tooling pressure, drive or controller problems, and screw design. It also emphasizes feed-throat cooling as a critical check.
Build one synchronized trend
Trend these signals on the same time axis where possible:
- Feeder rate, refill event, and hopper level
- Screw speed
- Motor current or torque
- Head or adapter pressure
- Actual barrel and die temperatures
- Heater output
- Melt temperature
- Internal air pressure or vacuum
- Puller speed
- OD or wall gauge result
- Cooling-water inlet and outlet temperature
The phase relationship matters. If pressure and motor load cycle before OD changes, the source is likely upstream of the die. If pressure remains stable while OD follows puller speed, start downstream.
Inspect material delivery first
Check for:
- Pellet bridging or softening at the feed throat
- Poor feed-throat cooling
- Fines or irregular regrind
- Blender segregation
- Conveying interruptions
- Hopper-dryer refill cycles
- Wet masterbatch, additives, or regrind
- Condensation or ambient moisture pickup
TPU moisture can create unstable output, but not every surge is a drying problem. Verify moisture analytically rather than repeatedly increasing dryer temperature.
Treat a melt pump as control equipment, not a cure-all
A properly selected melt pump may reduce pressure and output variation delivered to the die. It will not correct wet resin, intermittent feeding, bridging, contamination, poor melting, or a worn or unsuitable screw. If a pump improves dimensions, the upstream disturbance should still be identified.
Cooling: control heat removal without distorting the tube
Record the whole cooling system
“Water temperature” is incomplete. Record:
- Water inlet and outlet temperatures
- Flow rate or verified flow indication
- Tank level
- Bath or spray length
- Die-to-water distance
- Line speed and calculated residence time
- Spray-nozzle condition and pattern
- Guide and submersion arrangement
- Vacuum level, if applicable
- Tube temperature before and after the puller
- Ambient conditions during sensitive runs
BASF states that TPU generally requires effective cooling close to the die and may require a longer bath than other thermoplastics, with the needed length depending on grade, shape, section, and haul-off speed. Lubrizol describes immersion, spray, or a combination and notes that sufficient immersion time is needed to avoid distortion.
Do not confuse stronger cooling with uniform cooling
A cold tank with poor circulation can still produce warm zones. One-sided spray, trapped warm water, or inconsistent submersion can create asymmetric shrinkage and ovality. Compare inlet and outlet temperatures and inspect the flow pattern under normal production conditions.
Where staged cooling is considered, validate surface quality, dimensions, blocking, residual stress, and downstream handling. The best arrangement is grade- and geometry-dependent.
A controlled troubleshooting sequence
- Freeze the baseline. Record grade, lot, blend, output, screw speed, temperatures, pressure, line speed, support conditions, cooling, and dimensions.
- Confirm the measurement system. Check calibration, repeatability, contact force, sample temperature, orientation, and conditioning.
- Locate the defect. Compare the tube at die exit, cooling entry, cooling exit, and puller.
- Classify the pattern. Circumferential, periodic, slow drift, or random.
- Verify the dry-material path. Include every concentrate, additive, and regrind stream.
- Trend upstream and downstream signals together.
- Change one primary variable. Record the exact time and wait for process residence plus measurement delay.
- Retain marked samples. Keep both the acceptable control and defective condition.
- Return to baseline if the trial fails.
- Confirm the correction over time. A short good section does not establish capability.
Common mistakes
- Measuring OD on only one axis.
- Approving wall uniformity from OD alone.
- Measuring soft, hot TPU with uncontrolled caliper pressure.
- Reporting ovality percentage without defining the formula.
- Adjusting die centering before confirming sample orientation.
- Changing melt temperature, line speed, vacuum, and water temperature simultaneously.
- Assuming every output fluctuation is a resin-lot problem.
- Relying on dryer setpoints without measuring material moisture.
- Drying virgin TPU while leaving masterbatch or regrind unverified.
- Increasing vacuum or internal air without checking stability and leaks.
- Lowering water temperature without checking flow uniformity and residence time.
- Ignoring deformation created by guides, puller belts, winding tension, or hot handling.
- Using one supplier’s moisture or temperature recommendation for another supplier’s grade.
- Treating a melt pump as a substitute for diagnosing unstable feeding.
- Cleaning or adjusting hot, pressurized tooling without approved shutdown, depressurization, lockout, and PPE procedures.
For reprocessed material, consult the GOAFLEX TPU regrind validation guide.
Validation checklist before releasing the tube
Material and process
- [ ] Exact TPU grade, lot, color, and additive package are recorded.
- [ ] Current supplier processing guidance has been reviewed.
- [ ] Moisture is measured using a defined method and sampling point.
- [ ] Masterbatch, additives, and regrind are included in moisture control.
- [ ] Actual melt temperature—not only barrel setpoints—is documented.
- [ ] Screw speed, motor load, head pressure, output, and line speed are stable.
- [ ] Internal air or vacuum is measured and alarm limits are defined.
- [ ] Cooling inlet/outlet temperature, flow, residence, and guide setup are recorded.
- [ ] Puller alignment, contact pressure, and speed are verified.
Dimensions
- [ ] OD is measured on at least two axes.
- [ ] Ovality formula and denominator are stated.
- [ ] Wall thickness is mapped at defined clock positions.
- [ ] Minimum wall, maximum wall, and opposing-wall differences meet the drawing.
- [ ] Sampling distance, specimen temperature, and conditioning time are controlled.
- [ ] Gauge resolution and repeatability are suitable for the tolerance.
- [ ] Measurements before and after the puller show no downstream deformation.
- [ ] Capability is confirmed across startup, steady state, and relevant production duration.
End-use validation
- [ ] Finished tubing is tested with the intended fittings.
- [ ] Proof, burst, leakage, creep, bend, kink, and retention tests are defined as applicable.
- [ ] Chemical, humidity, temperature, and aging exposures reflect actual service.
- [ ] Regulatory or customer-specific requirements are reviewed.
- [ ] Any processing change is evaluated for mechanical and aging effects, not dimensions alone.
For chemistry selection before trials, see Polyether vs. polyester TPU for tubing and hose.
What to send GOAFLEX
To shorten the troubleshooting cycle, send:
- Application and service environment
- TPU grade, hardness, chemistry, color, lot, and supplier data sheet
- Tube drawing with OD, ID, wall, ovality, concentricity, and tolerance definitions
- Required end-use tests and acceptance limits
- Extruder size, L/D, screw design, output, and screen-pack arrangement
- Die and mandrel drawing, land length, die gap, and centering method
- Dryer type; set and actual temperatures; dew point; airflow; residence time; and measured moisture
- Percentage and identity of every masterbatch, additive, and regrind stream
- Barrel, adapter, die, and measured melt temperatures
- Screw speed, motor load, head pressure, and time trends
- Die-to-water distance, tank or spray length, water inlet/outlet temperatures, flow, and vacuum
- Internal air-pressure arrangement and trend
- Puller type, belt or roll gap, speed, and tube temperature at haul-off
- OD and wall maps with clock positions
- Photos and marked samples showing machine direction, top, die orientation, and sample time
- A good sample and a defective sample from identified production conditions
- A brief change log showing which trials improved or worsened the defect
Need help selecting or troubleshooting a TPU extrusion grade? Contact GOAFLEX at hello@goaflex.com with the tube drawing, service requirements, current resin, and process record.
References and further reading
- Lubrizol — TPU Extrusion Processing Guide. Supplier guidance covering drying, equipment, surging, tube/profile extrusion, cooling, and troubleshooting. Product-specific technical data should take precedence.
- BASF — Elastollan Processing: Extrusion. Supplier guidance on screw and die design, cooling, calibration, supporting air, vacuum, and dimensional stability.
- BASF — Elastollan Processing Recommendations. Broader supplier guidance on moisture control, extrusion parameters, cooling, and tubing.
- Covestro — Processing of TPU by Extrusion and Special Processes. Supplier guidance on pretreatment, extrusion equipment, melt conditions, die design, tubing, and calibration. Its numeric recommendations apply to Covestro’s referenced products.
- ASTM D2122-22 — Standard Test Method for Determining Dimensions of Thermoplastic Pipe and Fittings. Active ASTM method covering diameter, wall thickness, out-of-roundness, length, and straightness for its stated scope.
- ISO 3126:2005 — Plastics piping systems—Plastics components—Determination of dimensions. Measurement-method reference for plastics pipes and fittings; confirm applicability to the product.
- GOAFLEX — TPU Drying and Moisture Control.
- GOAFLEX — TPU Film Extrusion Troubleshooting.
- GOAFLEX — Polyether vs. Polyester TPU for Tubing and Hose.
- GOAFLEX — Using TPU Regrind in Production.
Technical note: Supplier guides describe their own TPU portfolios and equipment assumptions. Their numeric moisture, temperature, speed, and tooling recommendations should not be presented as universal GOAFLEX specifications. Final settings and acceptance criteria require trials on the selected grade, line, geometry, and application.
Image credits
- Bundles of colored plastic tubing: A S M Jobaer, Wikimedia Commons, CC BY-SA 4.0. Cropped/resized.
- Pipe extrusion line: Ikiwaner, Wikimedia Commons, CC BY-SA 3.0. Cropped/resized.
- Tube thickness measured by a Vernier caliper: Hiroshi Ishii, Wikimedia Commons, CC BY 2.0. Cropped/resized.
- Plastic pellets: C. Rieck / SuSanA Secretariat, Wikimedia Commons, CC BY 2.0. Cropped/resized.
These search-sourced images provide processing, measurement, material-handling, or finished-product context. They do not prove GOAFLEX ownership, creator endorsement, exact TPU chemistry, grade identity, or product performance.
