How to Completely Eliminate Internal Voids, Surface Bubbles and Latent Micropores in TPU Thick‑Sheet Extrusion (For Supercritical Foaming Pre‑cursor Sheets)

Key Premise: Any tiny pores in the pre‑cursor sheet will develop into large bubbles, cell rupture, uneven cell distribution and blistering scrap during supercritical autoclave foaming. TPU pre‑cursor sheets must achieve zero internal voids, no entrapped air and no volatile‑induced pores. Pores mainly originate from four sources: moisture / low‑molecular volatiles in raw materials, air entrapment at feeding section, gas entrainment during melt conveying, and gas precipitation upon cooling. Practical process control measures are provided below by process sequence.

1. Raw Material Section: Pores Induced by Moisture Hydrolysis & Volatiles (Most Common Cause)

TPU absorbs moisture readily; water generates CO₂ under high temperature and forms bubbles inside melt.

1. Mandatory Drying Specification

• Polyester TPU: 80～85 ℃, 2.5‑4 h; Polyether TPU: 85～90 ℃, 3‑4 h
• Target moisture content: ≤300 ppm, preferably controlled within 200 ppm.
• Prohibitions: Open hopper feeding; drying air shall be dehumidified instead of ordinary hot air; dried pellets shall be conveyed via insulated hoppers to prevent re‑moisture absorption.

2. Re‑grind Material Control Crushed re‑grind edges absorb moisture faster due to larger specific surface area. Re‑grind ratio ≤20%; re‑grind material must be re‑dried. Screen crushed material to remove dust which entrains air.
3. Additives Nucleating agents, talcum powder and color pigments are moisture‑prone; pre‑dry powder additives in advance.

2. Feeding Section: Prevent Air Entrapment from Solid Pellets (Easily Overlooked for Thick‑Sheet Extrusion)

Air trapped among loose pellets may be encapsulated by forward‑flowing melt and form elongated internal voids if not vented in time.

1. Adopt forced feeding (loss‑in‑weight feeder preferred) to avoid bridging and pulsating feeding from gravity free‑fall.
2. Keep moderate low temperature at extruder feed zone: Maintain pellets in solid state for compaction; air is forced backward out of hopper by screw compression. Excessively high feed‑zone temperature causes premature melting, seals pellet gaps and traps air inside melt.
3. Match screw feed‑zone channel depth and compression ratio for TPU: Apply moderate compression ratio for gradual compression and reserve sufficient venting window.

3. Extruder Main Unit: Vacuum Degassing System — Eliminate Gases inside Melt (Core Configuration)

Drying only removes free water. Low‑molecular oligomers, process‑generated small molecules and residual monomers of TPU can only be removed by vacuum venting.

Configuration Requirements

1. At least one vacuum venting zone; dual‑vent configuration for premium lines.
2. Vacuum degree: ≥‑0.095 MPa. Equip vacuum pump with water‑gas separator plus condenser to avoid back‑flow contamination from water vapor and oil mist.
3. Vent port design: Melt shall generate continuous surge flow to renew melt surface for sufficient gas escape. Avoid melt sealing the vent port. Apply deep screw channel at vent section to maintain low melt pressure.

Taboo: Excessively high melt pressure at vent zone → gases cannot separate out, latent pores are carried into pre‑cursor sheets.

4. Optimize temperature profile to suppress thermal degradation Small‑molecule gases are generated by over‑heated TPU degradation: Avoid local overheating along barrel zones; melt temperature controlled within 190～220 ℃ (adjust per hardness grade). Adopt low‑shear screw design to reduce shear‑induced temperature rise. Shear overheating is a hidden gas source.

4. Melt Conveying & Die Section: Avoid Secondary Air Entrapment and Pressure Fluctuation

1. Highly polish melt pipelines and die interior without dead corners to prevent degraded gas‑generating stagnant material; perform regular purging.
2. Install static mixer upstream of die Homogenize melt temperature and pressure, eliminate local low‑pressure zones where dissolved gases precipitate into bubbles.
3. Maintain stable and sufficient melt back‑pressure Establish continuous stable back‑pressure throughout extrusion. Severe pressure fluctuation triggers instantaneous local low pressure and precipitates dissolved gas. Avoid excessive back‑pressure which causes shear‑induced thermal degradation.
4. Prevent partial die lip blockage and carbonized build‑up; fallen degraded deposits bring entrapped air.

5. Three‑Roll Calendering Section: Surface Bubbles & Skin Blisters

Bubble defects may occur after melt exits die, not only generated inside extruder:

1. Uniform die discharge across full width to avoid air entrapment by turbulent local melt flow.
2. Complete melt lamination onto rolls; no air shall be trapped between melt and roll surface. Adjust die horizontal level, three‑roll gap and haul‑off speed matching.
3. Implement gradient roll temperature: Do not set the first cooling roll too cold. Sudden quenching forms solid skin prematurely and locks internal gases inside sheet as subsurface pores. Apply gradual cooling to provide escape window for internal gases before skin solidification.

6. Practical Troubleshooting Sequence when Pores Occur

1. Verify raw‑material moisture content and confirm drying performance.
2. Inspect vacuum system: vacuum degree, pipeline leakage, melt blockage at vent port.
3. Lower feed‑zone temperature to improve solid conveying and backward venting.
4. Match screw speed and haul‑off speed to stabilize melt pressure.
5. Reduce local over‑heating to mitigate thermal degradation.
6. Optimize three‑roll temperature and lamination condition to eliminate entrapped air.

7. Extra Critical Requirements for Supercritical‑Foaming‑Grade TPU Pre‑cursor Sheets

Minor subsurface pores are acceptable for general extrusion, but not for foaming pre‑cursor sheets. Even micron‑scale latent pores act as preferential cell nucleation sites inside autoclave, resulting in uncontrolled foaming ratio, mixed big/small cells and sheet hole defects.

• On‑line inspection: No visible voids on cross‑section of cut sheets. If available, perform boiling water test: soak pre‑cursor sheet in 90 ℃ hot water for 30 min; no surface blistering indicates no dissolved gas or micro‑pores inside.
• Shut‑down specification: Purge melt completely for long‑term shutdown to prevent accumulated degraded small molecules inside barrel, otherwise pores persist after restart.

8. Rapid Root‑Cause Identification for Typical Pore Defects

1. Elongated through‑thickness voids: Air entrapment at feeding, insufficient venting
2. Fine dispersed micro‑bubbles: Excessive moisture / insufficient vacuum, volatile precipitation
3. Sporadic irregular large bubbles: Local melt over‑heating and degradation
4. Subsurface skin‑only bubbles: Air entrapment at three‑roll stack, premature skin solidification
5. Periodic intermittent bubbles: Unstable feeding, screw pulsation, intermittent vacuum‑line leakage