Thin-Wall TPU Injection Molding: How Flow, Gate Design, and Shear Shape Part Quality

An injection molding machine provides the pressure and velocity, but thin-wall TPU quality depends on the complete melt-delivery system, tool, and validated process window. Photo: 384/Wikimedia Commons, CC BY-SA 4.0. Contextual image; the photographed resin is not identified as TPU.

Quick answer

Thin-wall TPU parts are difficult because the melt must cross a narrow cavity before the flow front cools, while the nozzle, runner, and gate add pressure loss and shear heating. A robust design combines a short, balanced path; an appropriately sized and positioned gate; vents at the true end of fill; and controlled fill-to-pack transfer. The correct speed is the lowest practical velocity profile that fills consistently before freeze-off without burns, hesitation, excessive shear, or trapped air. Final settings must come from the selected grade, tool, and production-intent trials—not a universal flow ratio.

Why thin-wall TPU is a system problem

“Thin wall” is relative. A nominal wall can behave like a thin section when melt travels far, passes inserts, divides, or crosses a restriction. Evaluate flow length, local thickness, gate location, melt viscosity, temperature history, and available pressure together.

TPU is hygroscopic, and moisture plus heat can reduce molecular weight during processing. Its viscosity responds to temperature and shear, yet excessive shear or residence time can overheat or degrade the melt. Flexible parts may also distort during ejection when packing, draft, cooling, or ejector area is inadequate.

Start with a dry, traceable material baseline. GOAFLEX’s guide to TPU drying and moisture control explains why dryer setpoint alone does not prove that resin is ready to mold. Once moisture and handling are controlled, flow, gating, venting, packing, and cooling can be investigated without confusing hydrolysis with a tooling problem.

The entire melt path—from plasticating unit through nozzle, runner, gate, and cavity—affects pressure loss and thermal history. Illustration: Ariel Cornejo/Wikimedia Commons, CC BY-SA 4.0; resized to 1,280 px.

Flow length is a comparison tool, not a guaranteed limit

A flow-length-to-thickness ratio can help compare concepts made with the same resin and similar processing assumptions. However, it does not capture corners, ribs, texture, changes in thickness, multiple flow fronts, inserts, venting, gate loss, or the pressure capability of the molding cell. Published ratios from a different polymer—or even another TPU grade—should not become a drawing requirement.

For each gate, map the most restrictive route to the last area expected to fill and mark local minimum walls. Then ask:

• Does the front move continuously through thick-to-thin transitions?
• Where will fronts meet, and can that weld-line location be tolerated?
• Can air escape from the true last-to-fill zones?
• Is pressure consumed upstream before it reaches the cavity?

Simulation is valuable for comparing gate locations, pressure demand, shear-rate concentrations, air traps, and weld-line movement. It is still a model. Use grade-specific rheology when available, document assumptions, and confirm predictions with short shots, cavity-pressure data where justified, and molded-part tests.

Gate design controls more than fill

The gate is both a flow restriction and the connection through which packing pressure reaches the part. An undersized gate may generate a large pressure drop, shear heating, jetting, or early gate freeze. A very large gate can improve pressure transmission but leave a larger vestige, lengthen gate-seal time, complicate degating, or concentrate stress. The right choice balances fill, packing, appearance, ejection, automation, and tool service.

Covestro advises avoiding significant pressure drops and notes that TPU melt-delivery channels are commonly larger than for hard thermoplastics. BASF likewise recommends generously sized gates to transmit holding pressure. These are supplier directions—not universal dimensions.

Put the gate where it solves the real risk

Gating into or near a thicker, structurally stable region often supports progressive filling and packing from thick toward thin. A fan or film-style gate can spread flow into a broad, shallow section and reduce the concentrated high-velocity stream associated with a small edge gate. A direct, diaphragm, or ring gate may suit some symmetric parts. Pinpoint or tunnel gates can aid automatic separation but impose a restriction; their suitability depends on grade, part, gate geometry, and demolding behavior.

Avoid locating the gate solely where vestige is easiest to hide. Check whether that position creates a long final leg, sends the first melt directly across an open cavity, places a weld line on a loaded feature, or leaves a thin rib isolated from pack pressure.

Tool layout determines runner balance, flow direction, vent access, cooling, and ejection—not just the part outline. Photo: Wizard191/Wikimedia Commons, CC BY-SA 3.0. The pictured mold is contextual and is not identified as a TPU or thin-wall tool.

Treat runner balance and gate balance separately

Equal runner lengths do not guarantee equal filling: channel diameter, turns, gates, temperature, and cavity resistance also matter. Compare progression by cavity, favor smooth balanced paths, control hot-runner zones, and minimize dead spots.

Shear can help flow—and can damage the process window

TPU viscosity generally decreases as temperature and shear rate increase. This shear-thinning behavior is one reason a cavity may fill at production velocity even when a slow purge appears viscous. It does not mean that maximum velocity is safest. High local shear at a small gate, sharp turn, or restricted nozzle can raise melt temperature beyond the barrel setpoint. That can produce discoloration, odor, streaks, deposits, or loss of properties if the thermal and residence-time limits of the grade are exceeded.

Conversely, filling too slowly can allow the skin layer to thicken before the flow reaches the end of a long thin section. The result may be a short shot, hesitation mark, weak flow-front meeting area, or excessive pressure demand. Supplier guidance therefore appears different at first glance: Covestro notes that thin walls generally need faster filling, while BASF advises keeping speed as low as practical but dependent on gate, wall, and flow distance. The compatible interpretation is to develop a part-specific velocity profile: move fast enough through thin, long-flow regions to avoid freeze-off, then reduce local velocity where the geometry would otherwise jet, trap air, or overheat.

Venting must match the real end of fill

As TPU enters the cavity, air must leave. Trapped air can resist the melt, cause incomplete fill, or heat under rapid compression and create dark burn marks. Vents belong at the last-to-fill edges, around trapped pockets, and near weld-line zones identified by simulation and short-shot studies. Ejector pins, inserts, and parting-line features may support venting if designed and maintained for that purpose.

Vent depth is grade- and tool-specific: it must release gas without allowing unacceptable flash. More injection speed is not a reliable cure for poor venting. If burn marks move or intensify as velocity rises, verify the air-trap location, vent cleanliness, clamp condition, and actual fill sequence before changing material temperature.

Filling, packing, and cooling need separate decisions

Use velocity control for filling, then transfer consistently to pressure control before a spike or flash event. A systematic short-shot study—from partially filled to nearly full—reveals hesitation, imbalance, jetting, weld lines, and last-to-fill regions while other settings remain stable.

Packing should compensate for shrinkage while the gate remains open, not force material into a constrained cavity. In a gate-seal study, increase hold time in steps and weigh conditioned parts until weight stabilizes. Then optimize hold pressure against dimensions, sink, flash, gate appearance, and ejection. Thin walls may freeze while a thick boss or gate region still responds to packing.

Cooling and demolding complete the process. A part that looks full may still distort because its hot strength is insufficient, cooling is uneven, or ejection load is concentrated. Optimize for accepted parts per machine hour, not the shortest displayed cycle. See GOAFLEX’s focused article on TPU solidification, demolding, and cycle time for a deeper validation approach.

Process and validation matrix

Stage	Control or study	Evidence to record	Acceptance focus
Material preparation	Supplier-specified drying; closed transfer and covered hopper	Lot, dryer type, time, temperature, dew point where available, verified moisture when required	Stable appearance, viscosity response, and mechanical baseline
Tool review	Flow-path map, local wall audit, gate/runner/vent/cooling review	CAD revision, measured gate and vent condition, cavity IDs	No avoidable restriction, air trap, dead spot, or unbalanced path
Fill study	Short shots and stepped velocity profile	Fill time, transfer position, pressure, short-shot photos and weights	Continuous, balanced flow without jetting, burns, hesitation, or flash
Pack study	Hold-pressure and gate-seal-time ladder	Part weight versus hold time, gate appearance, dimensions	Stable weight and dimensions without local overpack or ejection damage
Thermal study	Melt check plus mold-temperature and cooling-time window	Actual melt temperature, cavity/mold surface temperatures, cycle timing	Repeatable surface, demolding, shrinkage, and warpage
Capability run	Production-intent run across cavities and normal interruptions	Reject codes, cavity data, dimensional trend, start/stop response	Agreed capability and accepted-parts-per-hour target
Part validation	Conditioned dimensional, mechanical, assembly, and service tests	Test method, conditioning, sample count, failure mode	Meets drawing and application requirements—not appearance alone

Defects and diagnostics: read the pattern before turning a knob

Symptom	Mechanisms to check first	Useful diagnostic
Short shot at the same far edge	Freeze-off, excessive flow length, restricted gate/runner, inadequate venting	Short-shot sequence; compare cavity and machine pressure; inspect vents
Burn mark at end of fill	Compressed trapped air, blocked vent, excessive local velocity	Reduce velocity locally; clean/add venting; confirm air-trap location
Jetting or snake-like surface line	High-velocity stream from gate into open cavity; poor impingement	Slow initial profile; redirect/enlarge/change gate; compare gate concepts
Flash near gate or parting line	Local overpressure, early transfer error, tool mismatch or damage	Check transfer repeatability, clamp/tool condition, and cavity pressure if available
Sink or dimensional drift in thicker feature	Insufficient pressure transmission before gate seal, cooling imbalance	Gate-seal and hold-pressure studies; part-weight trend; cooling review
Splay, bubbles, or silver streaks	Moisture, volatiles, trapped gas, overheating, contamination	Verify moisture and handling; purge; check actual melt temperature and residence time
Dark streaks or deposits	Excessive thermal/shear history, dead spots, long residence, contamination	Purge inspection; reduce restriction/residence; audit nozzle and hot-runner zones
Warpage or ejector marks	Uneven packing/cooling, premature ejection, concentrated ejection force	Cavity-temperature map; cooling-time ladder; inspect draft and ejector area

Gate geometry can change whether a melt stream jets across the cavity or attaches to a surface. Image: Nalbarian/Wikimedia Commons, CC BY-SA 4.0. This educational experiment is not TPU-specific; validate the mechanism in the intended tool and resin.

Common mistakes that make thin-wall trials harder

• Treating pressure as the first cure. Pressure cannot recover a flow front that has already frozen or evacuate a sealed air pocket.
• Copying a flow ratio from another resin. Grade rheology, gate loss, temperature history, and geometry are different.
• Using a tiny gate to minimize the vestige. The apparent cosmetic gain can consume pressure, create shear heating, and isolate the cavity from packing.
• Changing drying, melt temperature, speed, and hold pressure together. The trial may make a good part but cannot reveal why.
• Judging only the first cavity or first hour. Represent every cavity, start-up, interruptions, and normal residence-time variation.
• Accepting appearance as validation. Thin-wall parts can look complete while missing dimensional, tear, fatigue, assembly, or environmental requirements.

What to send GOAFLEX for a thin-wall TPU review

Send the following to hello@goaflex.com:

• 3D CAD and a drawing showing nominal and local minimum wall, tolerances, texture, draft, ribs, bosses, and undercuts;
• expected gate location/type, runner or hot-runner concept, number of cavities, vent strategy, and any tool restrictions;
• longest flow routes from each proposed gate and any existing simulation report, including material model and assumptions;
• molding-machine screw diameter, maximum pressure basis, shot utilization, nozzle, clamp, and process-data export if available;
• current TPU grade, hardness, color/additives, lot, drying and transfer method, regrind policy, and regulatory needs;
• short-shot photos, defect photos by cavity, actual cycle data, part weights, dimensions, and rejected-part pattern;
• assembly method, mating materials, service temperature, chemicals, loads, flex cycles, appearance standard, and test plan.

GOAFLEX can review the brief and identify candidate TPU directions for testing. Suitability and performance require confirmation in the production-intent tool and application. For data screening, see How to Read a TPU Technical Data Sheet.

Planning a thin-wall TPU part or troubleshooting an unstable fill? Email CAD, wall/flow details, tool concept, current grade, and trial evidence to hello@goaflex.com for a focused material-and-process review.

References

1. Covestro, Processing of TPU by Injection Molding. PDF.
2. Lubrizol Engineered Polymers, TPU Injection Molding Processing Guide. PDF.
3. BASF, Elastollan® Thermoplastic Polyurethane Elastomers (TPU): Processing Recommendations. PDF.
4. ISO 294-1:2017, Plastics—Injection moulding of test specimens of thermoplastic materials—Part 1: General principles, and moulding of multipurpose and bar test specimens. ISO catalog record.
5. ASTM D955, Standard Test Method of Measuring Shrinkage from Mold Dimensions of Thermoplastics. ASTM catalog record.

Image credits

All four images are existing web assets from Wikimedia Commons and permit commercial reuse under the stated Creative Commons licenses; full source URLs, modification notes, and cautions are recorded in image_manifest.md. Contextual photographs and illustrations do not prove GOAFLEX ownership, endorsement, the pictured polymer chemistry, or performance of any TPU grade.