Last updated: 2026-09-05 · 5 min read

TPU Injection Molding — Why Does Thermoplastic Polyurethane Need Drying Below 0.02% Moisture and How Do You Process It Right?

Thermoplastic polyurethane (TPU) is a block copolymer with hard segments (MDI + chain extender) and soft segments (polyol) that gives it the toughness of rubber with the processability of thermoplastics. Melt temperature is 190–230°C, Shore hardness 60A–74D, tensile strength 30–60 MPa and elongation 300–600%. TPU is the default for cable jacketing, medical tubing, conveyor belts and overmolded grips — but it is extremely hygroscopic and must be dried below 0.02% moisture. Resin cost $5–12/kg.

Key Facts

  • TPU absorbs 0.3–0.6% moisture at 23°C/50% RH — among the highest of any engineering resin. At processing temperatures above 190°C, this moisture causes hydrolysis: the polymer chains break, molecular weight drops and the part loses tensile strength, elongation and abrasion resistance. A TPU molded from wet resin can have 40–60% lower tear strength than the same resin molded dry. Drying at 90–100°C for 3–4 hours in a desiccant dryer (dew point -40°C) brings moisture below 0.02% — the threshold for acceptable mechanical properties. See the resin drying guide for dew-point targets across all hygroscopic resins.
  • TPU has excellent abrasion resistance — the DIN abrasion volume loss is 20–40 mm³ (vs. 100–150 mm³ for PVC and 200–300 mm³ for nylon). This makes TPU the default material for conveyor belt covers, ski boot shells, caster wheels and industrial screen panels that see continuous sliding wear. The abrasion resistance comes from the hard-segment domains acting as physical crosslinks — they resist cutting and tearing while the soft segments provide elasticity.
  • TPU is available in three polyol chemistries: polyester-based (best mechanical strength and oil resistance, but poor hydrolysis resistance), polyether-based (best hydrolysis and microbial resistance, but lower oil resistance) and polycarbonate-based (best UV and hydrolysis resistance, highest cost). The chemistry determines whether the TPU survives in humid environments (polyether), oil contact (polyester) or outdoor UV exposure (polycarbonate).

How Does TPU Compare to Silicone, PVC and TPE Across Key Parameters?

Parameter TPU Silicone PVC (plasticized)
Tensile strength 30–60 MPa 6–12 MPa 10–25 MPa
Elongation 300–600% 400–800% 200–400%
Abrasion (DIN mm³) 20–40 150–250 100–150
Continuous use 80–100°C 180–200°C 60–80°C
Processing Injection moldable Compression/LSR Injection/extrusion
Resin cost $5–12/kg $15–30/kg $2–4/kg

Hydrolysis (in TPU and other condensation polymers) — chain scission caused by water molecules breaking ester or urethane bonds at elevated temperature. The part looks dimensionally correct but has 40–60% lower mechanical properties. Prevented by drying to <0.02% moisture before processing. See the hygroscopic resin drying guide for TPU-specific protocols.

TPU wins on abrasion resistance and tensile strength vs. silicone and PVC. Silicone wins on temperature resistance (200°C vs. 100°C). PVC wins on cost but cannot match TPU’s elasticity, tear strength or chemical resistance. For cable jacketing, medical tubing and wear parts at ambient to 80°C, TPU is the standard choice. See the TPU vs TPE comparison for when each elastomer class is the better fit.

What Three Processing Rules Determine Whether a TPU Part Succeeds or Fails?

  1. Drying is non-negotiable — 90–100°C for 3–4 hours, dew point -40°C, moisture below 0.02%. TPU’s ester and urethane bonds hydrolyze at processing temperatures in the presence of moisture. The molecular weight drops during molding — the part looks correct but has 40–60% lower tear strength and elongation. A desiccant dryer with dew point -40°C is required; compressed air dryers cannot achieve this level. After drying, process within 1 hour or keep in a sealed hopper with nitrogen purge. Re-dry if the resin sits for more than 2 hours in open air.
  2. Melt temperature must stay below 240°C — above this, TPU degrades within minutes. The hard segments (MDI-based) begin to thermally decompose above 240°C — the urethane bond breaks, releasing CO₂ and forming bubbles, black specks and a burnt odor. For polyester-based TPU, keep melt at 190–220°C. For polyether-based TPU, keep melt at 200–230°C. For polycarbonate-based TPU, keep melt at 210–240°C. Use the lowest melt temperature that fills the cavity — if short shots occur, increase injection speed before increasing melt temperature.
  3. Mold temperature affects surface quality — 20–40°C for general parts, 40–60°C for optical or high-gloss surfaces. At low mold temperatures (20–30°C), TPU solidifies quickly and the surface replicates the mold texture well — suitable for grips, cables and matte-finish parts. At higher mold temperatures (40–60°C), the melt stays fluid longer, flow marks and weld lines are reduced and the surface is glossier — required for medical tubing, transparent parts and cosmetic surfaces. The higher mold temperature also improves inter-layer adhesion, which increases tear strength by 10–20%.

Block copolymer (TPU structure) — a polymer built from alternating hard segments (MDI + chain extender) and soft segments (polyol). The hard domains act as physical crosslinks that resist cutting and tearing; the soft domains provide elasticity and elongation. Changing the hard-to-soft ratio shifts the Shore hardness from 60A to 74D without changing the base chemistry.

TPU: block copolymer, melt 190–230°C, Shore 60A–74D, tensile 30–60 MPa, elongation 300–600%. DIN abrasion 20–40 mm³ (3–5× better than PVC). Must dry to <0.02% moisture (90–100°C/3–4h, dew point -40°C). Polyester (oil resistant), polyether (hydrolysis resistant), polycarbonate (UV resistant). Resin $5–12/kg.

  • Desiccant dryer at dew point −40°C — 90–100°C for 3–4h; moisture must be <0.02% before processing. Wet resin = 40–60% tear strength loss.
  • Process within 1 hour of drying — or keep in sealed hopper with nitrogen purge. Re-dry if resin sits >2h in open air.
  • Melt temperature below 240°C — polyester TPU: 190–220°C; polyether: 200–230°C; polycarbonate: 210–240°C. Above 240°C = yellowing, bubbles, CO₂ release.
  • Polyol chemistry matched to environment — polyester (oil/fuel), polyether (hot water/microbial), polycarbonate (UV/outdoor).
  • Substrate pre-heated to ≥60°C for overmolding — nylon gives strongest bond (10–15 N/mm); PP the weakest (6–10 N/mm).

Frequently Asked Questions

Can TPU be overmolded onto PP, ABS or nylon substrates?

Yes — TPU overmolds well onto PP, ABS, PC and nylon (PA6, PA66) because the urethane groups form hydrogen bonds with the substrate surface. The key is substrate surface preparation: the substrate must be at least 60°C when the TPU is injected (pre-heated in the mold or immediately after the first shot). Peel strength for TPU on PP is 6–10 N/mm, on ABS 8–12 N/mm and on nylon 10–15 N/mm. For medical tubing overmolded onto rigid connectors, nylon substrates give the strongest bond.

For substrates that are difficult to bond (POM, PBT, PPS), use a tie-layer adhesive or add mechanical interlocks (through-holes, undercuts) to the substrate. The TPE overmolding article covers the general principles — TPU follows the same rules but with higher peel strength due to the hydrogen bonding mechanism. See the TPE overmolding bond strength guide for substrate-specific peel-test data.

Why does my TPU part turn yellow after molding?

Two causes: thermal degradation or aromatic MDI yellowing. If the melt temperature exceeded 240°C or the barrel residence time was above 5 minutes, the TPU has thermally degraded — the yellowing is accompanied by black specks, bubbles and reduced mechanical properties. The fix: lower the melt temperature, reduce residence time and verify the heater band calibration.

The second cause is inherent to aromatic TPU — the MDI (methylene diphenyl diisocyanate) hard segment contains aromatic rings that absorb UV light and turn yellow over time. This is cosmetic only — the mechanical properties are not affected. For applications where yellowing is unacceptable (medical devices, transparent parts), switch to aliphatic TPU (HDI-based, hexamethylene diisocyanate) — it costs 2–3× more but remains water-clear under UV exposure.

What is the difference between polyester, polyether and polycarbonate TPU?

The polyol chemistry determines the TPU’s resistance to environmental degradation. Polyester TPU has the highest mechanical strength (tensile 50–60 MPa) and best oil/fuel resistance — it is the default for hydraulic seals, fuel lines and industrial belts. However, it hydrolyzes in hot water above 60°C and is susceptible to microbial attack in tropical climates.

Polyether TPU has lower mechanical strength (tensile 30–45 MPa) but excellent hydrolysis resistance (survives 80°C water for 1,000+ hours) and microbial resistance — it is the default for medical tubing, swimwear components and outdoor cable jacketing. Polycarbonate TPU combines the best of both — good mechanical strength, excellent hydrolysis resistance and UV stability — at 2–3× the cost of polyester TPU. It is used for automotive exterior trim and architectural profiles.

Key Takeaways

  • TPU: tensile 30–60 MPa, elongation 300–600%, DIN abrasion 20–40 mm³.
  • Dry to <0.02% moisture: 90–100°C/3–4h, dew point -40°C.
  • Melt 190–230°C (below 240°C to avoid degradation).
  • Polyester (oil), polyether (hydrolysis), polycarbonate (UV). $5–12/kg.

Send the part drawing with the environmental exposure (oil, water, UV), the hardness requirement and the substrate material if overmolding — the quotation specifies the TPU chemistry, drying protocol and processing parameters needed to hit the target performance.

Sources: Covestro — Desmopan TPU Technical Data Sheets and Processing Guidelines · BASF — Elastollan TPU Product Range and Polyol Chemistry Comparison · Plastics Technology — TPU Injection Molding Parameters and Drying Requirements · Society of Plastics Engineers (SPE) — Thermoplastic Polyurethane Processing Guidelines · MatWeb — TPU (Thermoplastic Polyurethane) Material Property Database

— Arthur, Tooling Engineer at aCatMould