TPU Injection Molding: The Complete Thermoplastic Polyurethane Guide
Manufacturing Guide
TPU Injection Molding: The Complete Thermoplastic Polyurethane Guide
TPU injection molding is how industry makes parts that must bend, stretch, and absorb abuse without losing their shape: phone cases, shoe soles, cable jackets, gaskets, seals, casters, drone bumpers, and soft-touch grips. Thermoplastic polyurethane (TPU) sits between rubber and rigid plastic — it is an elastomer you can injection mold like a plastic, with abrasion resistance that outlasts rubber, rebound that returns energy, and hardness that tunes from jelly-soft (Shore 60 A) to engineering-stiff (75 D).
Because TPU is a thermoplastic, it melts, flows, and solidifies — no vulcanization, no two-part chemistry, no cold-runner tooling. That simplicity hides real process sensitivity: TPU is hygroscopic, its viscosity is temperature-and-shear dependent in ways that surprise ABS molders, and its 0.3–1.5 % shrinkage moves with hardness and process conditions. And a huge share of TPU work is TPU overmolding — bonding soft polyurethane onto rigid substrates — which is where the process gets genuinely difficult.
This guide is the complete reference from Molditquick (东莞国宏精密): process windows, grade selection, mold design, overmolding rules, tolerances, cost and lead times, applications, defect fixes, and how to buy tpu injection molding without tripping on the traps.
Table of Contents
- What is TPU injection molding?
- TPU properties: hardness, elasticity, abrasion
- TPU vs TPE vs silicone vs rubber
- TPU grades: polyester vs polyether, and modified compounds
- TPU process parameters: temperature, pressure, cycle
- Drying TPU — non-negotiable
- Mold design for TPU parts
- TPU overmolding: bonding soft to hard
- Tolerances and dimensional behavior
- Cost and lead times
- Applications: footwear, cables, seals, consumer electronics
- Common TPU defects and fixes
- DFM checklist for TPU parts
- Quality and testing for TPU programs
- Buying TPU molding: RFQ checklist
- FAQ
What is TPU injection molding?
TPU injection molding (also “tpu molding” or “thermoplastic polyurethane injection molding”) melts TPU pellets at 170–230 °C, injects the melt into a mold under pressure, and cools it into an elastomeric part. The cycle is identical to ABS or PP molding — screw plasticizing, fill, pack, cool, eject — which is exactly why manufacturers choose TPU over rubber: plastic-cycle economics with rubber-like properties.
The polymer itself is a block copolymer of hard segments (isocyanate + chain extender) and soft segments (polyol). The ratio of hard to soft segments sets the hardness and stiffness; this is why TPU spans Shore 60 A to 75 D within one material family. The same chemistry creates two property families:
- Polyester-based TPU — superior abrasion and tear resistance, good mechanicals, but hydrolyzes in hot humid environments. Best for footwear, wheels, industrial parts.
- Polyether-based TPU — better hydrolysis, low-temperature flexibility, microbial resistance. Best for cables, medical tubing, marine, and long-term outdoor use.
Grade selection between these two families is the first decision in any tpu injection molding program — and most quality problems trace back to a polyester grade placed in a hydrolyzing environment.
TPU properties: hardness, elasticity, abrasion
| Property | Typical value | Why it matters |
|---|---|---|
| Hardness range | Shore 60 A – 75 D | Tune from soft-touch to engineering-stiff |
| Abrasion resistance | Excellent (often > rubber) | Soles, seals, cables that survive dragging |
| Tear resistance | High (polyester grades highest) | Parts that peel and flex repeatedly |
| Elastic recovery | Excellent rebound | Seals, bumpers, shock absorbers |
| Tensile strength | 25–60 MPa (grade dependent) | Stronger than most elastomers |
| Elongation at break | 300–700 % | Flexing and stretching applications |
| Service temperature | −40 to +80 °C typical (polyether better cold) | Broad, but below silicone/rubber |
| Chemical resistance | Good vs oils, fuels (polyester); fair vs solvents | Automotive, industrial |
| Hydrolysis resistance | Polyether: good; polyester: limited | Grade choice = environment fit |
| Mold shrinkage | 0.3–1.5 % | Hardness- and process-dependent — see tolerances |
| Density | 1.10–1.25 g/cm³ | Heavier than PE/PP |
The hardness ladder in practice:
| Shore hardness | Feel | Typical parts |
|---|---|---|
| 60–75 A | Very soft, rubbery | Gaskets, soft grips, baby products |
| 80–90 A | Firm rubber | Phone cases, seals, casters, shoe soles |
| 95 A – 55 D | Semi-rigid | Cable jackets, bushings, rollers |
| 60–75 D | Rigid, plastic-like | Clips, gears, protective shells |
TPU vs TPE vs silicone vs rubber
| Criterion | TPU | TPE/TPV | LSR silicone | Vulcanized rubber |
|---|---|---|---|---|
| Molding process | Injection (plastic cycles) | Injection | Injection (cold runner) | Compression/transfer (slow) |
| Hardness range | 60 A – 75 D | 0 A – 70 D | 10–80 A | 30 A – 90 A |
| Abrasion resistance | Excellent | Good | Good | Good |
| Temperature range | −40 to +80 °C | −40 to +120 °C | −60 to +250 °C | −50 to +200 °C (grade) |
| Sterilization tolerance | Limited | Limited | Excellent | Good |
| Chemical resistance | Good (oils) | Fair-good | Excellent | Excellent |
| Recyclability | Yes (thermoplastic) | Yes | No (thermoset) | No |
| Relative cost | Medium | Low-medium | High | Medium |
| Best use | Abrasion + flex parts | Cost grips, gaskets | Extreme temp, medical, seals | Heavy-duty, high-durometer |
Selection rules: abrasion, tear, and rebound under mechanical abuse → TPU. Extreme temperature, sterilization, biocompatibility, permanent sealing → silicone (see our silicone injection molding guide). Cheapest soft-touch on a mild-environment product → TPE. If you are comparing elastomers for a specific part, the injection molding material comparison has the full matrix.
TPU grades: polyester vs polyether, and modified compounds
The two families:
| Polyester TPU | Polyether TPU | |
|---|---|---|
| Abrasion/tear | Best | Good |
| Hydrolysis resistance | Limited | Good |
| Low-temperature flexibility | Good | Better |
| Microbial/fungus resistance | Fair | Good |
| Typical uses | Footwear, wheels, industrial | Cables, medical, marine, outdoor |
Modified compounds a capable molder handles — First Mold’s decade of TPU experience spans glass-fiber-reinforced TPU, carbon-fiber TPU, antistatic TPU, flame-retardant TPU, and UV-stabilized TPU (First Mold TPU page):
- GF/CF-reinforced TPU — stiffer, lower creep for semi-structural parts.
- Flame-retardant TPU — cable and electronics compliance (UL ratings).
- UV-stabilized TPU — outdoor and automotive exposure.
- Antistatic/ESD TPU — electronics handling.
- Medical TPU grades — biocompatible, for tubing and device components.
Rule: never specify “TPU” alone — hardness, family (polyester/polyether), and any modification must be on the drawing, or you will get a material that fails in service.
TPU process parameters: temperature, pressure, cycle
Typical supplier-published windows (Covestro, Lubrizol, BASF guidance) as applied at Molditquick:
| Parameter | Typical range | Notes |
|---|---|---|
| Barrel temperature | 170–230 °C (soft grades lower, rigid higher) | Never exceed grade max — thermal degradation is irreversible |
| Mold temperature | 20–60 °C (overmolding 40–60 °C) | Soft grades can run cold molds; rigid grades want warmth |
| Drying | 100–110 °C for 2–4 h | Moisture < 0.05 % (ideally < 0.03 %); see section 6 |
| Injection pressure | 400–1,000 bar | TPU flows well when dry and hot |
| Hold pressure | Moderate, with time | Controls sink and final hardness feel |
| Injection speed | Medium | Too fast = shear heating + splay; too slow = short shot |
| Screw speed | Low-moderate | TPU degrades under high shear |
| Shrinkage | 0.3–1.5 % | Soft grades shrink more; rigid grades less |
| Cycle time | 25–70 s typical | Thin-wall cases 25–40 s |
| Regrind | 10–25 % typical max | Keep sprues/ runners dry and clean |
What catches ABS molders: TPU absorbs moisture fast, and its viscosity is highly shear-sensitive. A “standard” PP setup run on TPU produces splay, short shots, and black specks before the first hour. We set up TPU programs from a clean machine state, purge, and verify drying before first shots.
Drying TPU — non-negotiable
TPU is one of the most hygroscopic thermoplastics on the market. It pulls moisture from the air aggressively, and at 170–230 °C that moisture hydrolyzes the polymer chains — producing splay (silver streaks), brittle parts, and voids. The published spec: dry at 100–110 °C for 2–4 hours to residual moisture below 0.05 % (we target <0.03 % for clear and thin-wall parts).
Shop rules:
- Dehumidifying dryers with dew-point control; TPU in an open hopper degrades visibly within hours in humid climates (relevant in South China summer).
- Drying time from full-load temperature.
- Do not overdry beyond the datasheet window — over-dried TPU can yellow or lose plasticizer balance on some grades; follow the grade sheet.
- Regrind must be re-dried like virgin; damp regrind is the classic source of intermittent splay.
If your molder cannot state their drying procedure, that is the answer to your question about who to avoid.
Mold design for TPU parts
Steel, finish, and construction
- Prototype tools: P20/718H — fine for Class 105–104 volumes (RJC Mold).
- Production tools: P20/718H up to moderate volumes; H13 or S136 for high-volume and abrasive reinforced compounds.
- Cavity finish: gloss part surfaces need polished cavities; soft TPU can stick to rough surfaces — mirror or textured finishes both release well when the steel is right.
- Mold classes follow the standard ladder: Class 105 prototype (<500 shots, 10–18 days) through Class 102/101 for mass production (RJC Mold, Xometry).
Geometry
- Uniform wall 1–4 mm; soft TPU parts (phone cases, gaskets) commonly run 1–2 mm.
- Draft 0.5–1° minimum, 1–2° for texture — soft TPU peels, but firm grades need draft.
- Radii everywhere — sharp corners concentrate flex stress and tear.
- Thick sections are expensive: TPU’s shrinkage and softness make thick walls sink and take long to cool; design ribs and gussets instead of growing wall.
- Gates: pin-point and submarine gates work for cases and seals; fan gates for large soft panels; gate location matters most in overmolding (section 8).
- Venting at 0.01–0.03 mm — TPU outgasses; trapped air causes burn marks on soft parts.
TPU overmolding: bonding soft to hard
TPU overmolding is the fastest-growing TPU application — soft TPU molded onto rigid substrates for grip, cushioning, sealing, and protection: phone cases onto PC frames, tool handles onto nylon bodies, toothbrush grips, watch bands, cable strain-relief boots, automotive interior soft-touch parts (First Mold TPU page).
Bonding rules that decide success:
- Chemical compatibility first. TPU bonds well to PC, ABS, PC/ABS, PA (nylon), and PBT/PET when the substrate is dry and clean. It does not bond to PE/PP without surface treatment.
- Clean, dry substrate. Mold release, dust, or moisture on the substrate kills adhesion — the #1 overmolding failure. Substrates are wiped/plasma-treated where needed.
- Substrate temperature. The first-shot part must be warm enough (often 40–60 °C) when the TPU hits it, or the skin freezes before bonding. Two-shot (2K) machines keep the substrate hot between shots; insert molding reheats it.
- Mechanical reinforcement. Even with chemical bonds, add undercuts, grooves, or through-holes where peel loading is high — bond + lock beats bond alone.
- Wall control. TPU walls 0.5–3 mm typical; a thick TPU lobe over a thin substrate edge warps and peels.
- Process staging. Fill the TPU cavity gently (it is soft and low-pressure), hold to pack, and avoid gate placement on bond-critical faces.
For the full matrix of compatible material pairs and design rules, see our overmolding guide and two-shot vs overmolding guide. Molditquick runs TPU overmolding as insert molding and two-shot on our Sodick presses, with in-house tooling for both approaches.
Tolerances and dimensional behavior
TPU’s shrinkage band is wide — 0.3–1.5 % — because it is hardness- and process-dependent:
- Soft grades (60–80 A) shrink more and behave like rubber: tolerances of ±0.1–0.2 mm are realistic, with generous allowances on flexible spans.
- Rigid grades (55–75 D) shrink near the low end and can hold ±0.05–0.1 mm on critical features.
- Flexible geometry moves: thin, long TPU parts sag and flex under their own weight during measurement — tolerance and inspection must account for it (fixtures, not free-hand calipers).
| Feature | Realistic tolerance |
|---|---|
| General dimensions (soft grades) | ±0.2 mm |
| General dimensions (rigid grades) | ±0.1 mm |
| Critical features (rigid, validated) | ±0.05 mm |
| Mold cavity machining | ±0.02 mm; cores ±0.005 mm |
Reference values for context: Protolabs machining tolerance ±0.003 in (0.08 mm) + resin ±0.002 in/in (Protolabs); First Mold standard ±0.1 mm / precision ±0.05 mm (First Mold); Zetar ±0.1–0.2 mm general / ±0.05 mm critical (Zetar); tolerance tables per DIN 16901 / ISO 20457 (Boyan).
Dimensional drift is real: TPU continues to relax after ejection, and humidity affects soft grades. Critical programs get first-article CMM reports plus re-check after 24 h settling — a step most quotes skip.
Cost and lead times
Cost drivers for TPU parts:
- Resin — TPU costs more than PP/ABS/TPE but less than silicone and most engineering polymers; polyether grades cost more than polyester.
- Drying and machine cleanliness — TPU programs need dedicated drying and purge discipline; scrap from splay is a real yield cost.
- Cycle time — thin-wall cases run fast (25–40 s); thick soft parts run slower.
- Overmolding — two-shot or insert-molding adds a second process; tooling and cycle both increase.
- Cavitation — phone cases and seals tool at 4–16 cavities; high cavitation cuts per-part cost sharply.
Lead-time reference bands: rapid prototype tooling 7–12 days, production tooling 25–30 days (automotive to 45) (First Mold); prototype tools 10–18 days, production 4–6 weeks standard (RJC Mold, Zetar); platform rapid production ~7 days, no MOQ (Protolabs). First Mold quotes TPU delivery “as little as one week” for standard runs (First Mold TPU page).
Volume reality: footwear and phone-case programs run 50K–millions/year; prototype and small-batch runs start at hundreds of parts. Molditquick covers both on the same Sodick lines (18 + 3 machines), with in-house wire EDM (9+4) and 20 Aida presses for metal-insert TPU work — one 10,000 m² facility, one supplier from DFM to global shipping.
Applications: footwear, cables, seals, consumer electronics
Footwear & sports — shoe soles, midsoles, cleats, spikes, insoles, sports equipment grips, yoga mat corners, wheel chocks. TPU’s abrasion resistance and rebound make it the performance-sports workhorse.
Cables & wire — cable jackets, connectors, strain-relief boots, cable ties, charging-cable housings. Polyether TPU’s hydrolysis resistance and flexibility win here (First Mold TPU page).
Seals & gaskets — O-rings, gaskets, dust seals, window seals, headlamp gaskets, suspension bushings, brake cable covers, door panels (First Mold TPU page).
Consumer electronics — phone cases, tablet cases, smartwatch straps, keyboard covers, remote-control soft keys, charger covers, earbud tips (First Mold TPU page).
Medical & devices — catheters, surgical tubing, respiratory masks, orthopedic braces, insulin pump components, drug-delivery systems, protective covers (First Mold TPU page).
Industrial — casters, rollers, gear covers, vibration dampers, tool grips, protective coatings, flexible connectors, industrial mats (First Mold TPU page).
Automotive & EV — steering-wheel covers, window seals, door panels, suspension bushings, headlamp gaskets — soft-touch interiors and durable seals (First Mold TPU page).
Common TPU defects and fixes
| Defect | Appearance | Root cause | Fix |
|---|---|---|---|
| Splay / silver streaks | White streaks near gate | Moisture (hygroscopic TPU) | Dry 100–110 °C/2–4 h to <0.05 %; re-dry regrind |
| Black specks / burn marks | Dark spots | Degraded melt, shear heating, trapped air | Purge, lower temps/screw speed, improve venting |
| Short shot | Incomplete fill | Cold mold, low melt temp, blocked vent | Raise temps, slow-start fill, vent |
| Sink marks | Depressions on thick sections | Shrinkage + under-packing | Hold longer, thin sections, move gate |
| Warpage | Twist after ejection | Uneven cooling, soft grade | Balance cooling, uniform wall, colder mold for rigid grades |
| Flash | Fins at parting | Low clamp vs pressure | Raise clamp, clean parting, lower pressure |
| Sticking / ejection marks | Part clings, pin marks | Soft grade sticking to cavity | Draft, polish cavity, larger ejectors, mold release (non-medical) |
| Layering / delamination in overmolding | TPU peels from substrate | Dirty/cold substrate, incompatible pair | Clean/plasma substrate, preheat, verify compatibility |
| Tear on demolding | Torn thin walls | Over-aggressive ejection, sharp edges | Radii, slower ejection, high-tear grade |
| Yellowing | Color shift over time | UV exposure, over-drying | UV-stabilized grade, respect drying window |
| Inconsistent hardness feel | Soft/hard variation | Moisture, ratio of regrind, melt temp drift | Control drying, limit regrind %, hold melt temp |
| Dimensional drift | Size change after molding | Relaxation, humidity | 24-h settling check, tolerance per hardness |
DFM checklist for TPU parts
- Hardness and family specified (Shore A/D, polyester vs polyether).
- Wall 1–4 mm uniform; thick sections avoided (sink and cost).
- Radii ≥0.5× wall — TPU tears at sharp corners under flex.
- Draft 0.5–2° depending on hardness and texture.
- Gates placed for balanced fill; overmolding gates off bond-critical faces.
- Venting plan stated on the RFQ.
- For overmolding: substrate material, cleanliness, preheat, and bond reinforcement (undercuts/grooves) designed in.
- Tolerances realistic for hardness (soft grades ±0.2 mm, rigid ±0.05–0.1 mm).
- Environmental spec: UV, temperature, chemicals, hydrolysis risk (drives polyether vs polyester).
- Regrind policy (max 10–25 %, re-dried).
- Testing requirements: hardness, tear, abrasion, adhesion (overmold) per AQL.
Full checklist: DFM checklist guide.
Quality and testing for TPU programs
Elastomer quality is invisible to a caliper — testing is the difference between a phone case that lasts and one that splits:
- Hardness test (Shore A/D durometer) per lot — catches moisture and ratio drift.
- Tear and tensile on critical seals and straps.
- Abrasion where wear is the spec (soles, cables).
- Adhesion/pull tests on overmolded parts.
- Dimensional CMM first articles plus 24-h settling re-check.
- Lot traceability — resin lot, machine, parameters, and shift logged per lot (required for medical TPU under ISO 13485).
Molditquick runs IATF 16949, ISO 13485, and ISO 9001 systems with in-house CMM and elastomer test equipment — the same facility that builds the tool tests the part, on 21 Sodick machines with 280 people behind the program.
Buying TPU molding: RFQ checklist
- Specify hardness + family — Shore A/D and polyester vs polyether; “TPU” alone is not a spec.
- State the environment — UV, humidity, temperature, oils, sterilization: family selection depends on it.
- For overmolding, specify the substrate and the bond requirement (peel test?).
- Ask about drying — procedure, dew point, and regrind policy in the quote.
- Get a DFM on gate, vent, and overmold bond design before tooling.
- Agree tolerances and inspection — hardness testing, tear, dimensional reports, 24-h settling.
- Verify certifications — ISO 9001 minimum; IATF 16949 for automotive; ISO 13485 for medical.
- Compare quotes like-for-like — cavity count, mold steel/class, cycle estimate, yield assumption.
FAQ
1. What is TPU injection molding? Injection molding thermoplastic polyurethane at 170–230 °C into molds — an elastomer with plastic-cycle economics, used for phone cases, shoe soles, cables, seals, and soft-touch parts.
2. What temperature is TPU injection molding? Barrel 170–230 °C (soft grades cooler, rigid grades hotter), mold 20–60 °C, drying 100–110 °C for 2–4 hours to <0.05 % moisture.
3. What is the shrinkage of TPU in injection molding? 0.3–1.5 %, depending on hardness and process — soft grades shrink more, rigid grades less; tolerances should follow hardness (see section 9).
4. What is the hardness range of TPU? Shore 60 A (jelly-soft) to 75 D (rigid plastic-like) — the widest tunable range of any common elastomer.
5. Can TPU be injection molded? Yes — it is the most moldable high-performance elastomer, running on standard injection machines with correct drying and temperatures.
6. What is TPU overmolding? Molding soft TPU onto a rigid substrate (PC, ABS, nylon, PBT) in a second shot — for grips, seals, and cushioning. Bond requires clean, dry, warm substrates and compatible pairs. See our overmolding guide.
7. TPU vs TPE — what’s the difference? TPU is a specific polyurethane elastomer with superior abrasion/tear/rebound; TPE is a broader family (often cheaper) with lower mechanical performance. Choose TPU when abrasion and flex life matter.
8. TPU vs silicone — which is better? TPU: plastic-moldable, recyclable, abrasion champion, −40 to +80 °C. Silicone: −60 to +250 °C, sterilizable, biocompatible, permanent-sealing, but costlier and non-recyclable. See our silicone guide.
9. What is TPU used for in injection molding? Footwear soles, cable jackets and strain reliefs, seals and gaskets, phone cases and smartwatch straps, medical tubing, casters, rollers, and automotive soft-touch parts (First Mold TPU page).
10. What tolerances can TPU injection molding hold? Rigid grades ±0.05–0.1 mm on critical features, ±0.1 mm general; soft grades ±0.2 mm with flexible-span allowances — with CMM first-article verification.
11. Why does my TPU part have silver streaks? Moisture. TPU is strongly hygroscopic; dry at 100–110 °C/2–4 h to <0.05 % and re-dry regrind. Splay is almost always a drying failure.
12. Is TPU recyclable? Yes — TPU is a thermoplastic; sprues and runners can be reground (typically 10–25 % max ratio, re-dried). This is a key advantage over thermoset rubber and silicone.
13. Does TPU injection molding need MOQ? No — prototype and low-volume runs start at hundreds of parts; mass production scales to millions (Protolabs). See our low-volume injection molding service.
14. How long does TPU injection molding take? Rapid prototype tooling 7–15 days, production tooling 4–6 weeks, first articles days after tool completion; production lead 2–4 weeks after approval; standard runs quoted as fast as one week (First Mold TPU page).
15. What are the best TPU injection molding manufacturers? Screen on: drying discipline, overmolding experience, testing capability (hardness/tear/adhesion), and certifications (ISO 9001 / IATF 16949 / ISO 13485). Molditquick qualifies on all — send your model to our injection molding service for a DFM review and costed quotation.
Building something soft and strong? Send your model and hardness spec to our injection molding service — free DFM review with material family recommendation, overmold bond check, and quotation. Compare elastomers in the plastic material selection guide and the injection molding material comparison, or browse the materials library.