TPU Injection Moulding: Practical Thermoplastic Polyurethane Guide
Thermoplastic polyurethane (TPU) is the flexible plastic to consider when a soft part also has to resist abrasion, tearing, oil, grease, and repeated movement. It is harder to process than many TPE grades, but the extra control pays off in wheels, seals, cable strain reliefs, protective parts, and medical or wearable components.
Nordmould can quote TPU parts from 100 pieces, using ether-based, ester-based, aliphatic, flame-retardant, food-contact, or medical-oriented grades when the project requirements justify them.
What are the mechanical and thermal properties of TPU?
TPU properties change with hardness, chemistry, additives, drying, and post-treatment. The values below are representative of injection-moulding grades; use the resin supplier's datasheet for the exact grade.
| Property | TPU Shore 75A | TPU Shore 87A | TPU Shore 95A | TPU Shore 55D |
|---|---|---|---|---|
| Tensile strength | 25-40 MPa | 30-50 MPa | 40-60 MPa | 40-60 MPa |
| Elongation at break | 500-800 % | 400-650 % | 300-550 % | 200-400 % |
| Tear resistance | 60-100 kN/m | 80-130 kN/m | 100-160 kN/m | 120-180 kN/m |
| Compression set (22-24 h / 70 °C) | 20-45 % | 15-35 % | 15-30 % | 10-25 % |
| Abrasion resistance (DIN / ISO 4649) | 40-90 mm³ | 25-70 mm³ | 20-60 mm³ | 15-50 mm³ |
| Heat deflection temp (0.45 MPa) | 55-70 °C | 65-80 °C | 75-90 °C | 85-105 °C |
| Continuous service temp | -40 to 80 °C | -40 to 90 °C | -40 to 100 °C | -40 to 110 °C |
| Water absorption (24 h) | 0.5-2.0 % | 0.4-1.5 % | 0.3-1.2 % | 0.2-0.8 % |
| Density | 1.10-1.22 g/cm³ | 1.12-1.24 g/cm³ | 1.14-1.25 g/cm³ | 1.18-1.28 g/cm³ |
Ether-based TPU gives better hydrolysis resistance, microbial resistance, and low-temperature flexibility. Ester-based TPU usually gives better oil, grease, abrasion, and tear performance, but it is more vulnerable in hot, wet service. Aliphatic TPU is used where non-yellowing UV stability matters. The right choice follows the service environment, not the material name.
What are typical applications of TPU injection moulding?
TPU is used where ordinary soft-touch TPE would wear out, split, swell, or take too much permanent set.
Industrial and automotive: Industrial wheels, rollers, conveyor scrapers, impeller liners, seals, bellows, dust boots, and hose connectors use TPU because it combines elastic recovery with abrasion and oil resistance. Ester TPU is common in dry wear or oily environments; ether TPU is safer where water, humidity, or hydrolysis is a risk.
Electronics and cable protection: Cable jackets, strain-relief boots, flexible conduit ends, rugged cases, and wearable housings use TPU for flex life and toughness. Flame-retardant grades are available for electronics, but the exact UL rating and wall thickness must be confirmed on the compound datasheet.
Medical and healthcare: Ether-based TPU grades are used for tubing, catheter components, wearable sensor housings, orthotic parts, wound-care components, and soft medical grips. Medical suitability depends on the grade's biological evaluation, sterilisation method, extractables/leachables risk, and the buyer's regulatory file.
Footwear: Outsoles, cleats, boot components, ski-boot parts, and abrasion guards use TPU for rebound, wear resistance, colourability, and low-temperature toughness.
Sports and safety equipment: Protective pads, shin guards, goggle frames, helmet details, and impact-absorbing brackets use harder TPU grades where a part must bend, take impact, and recover without cracking.
How is TPU processed in injection moulding?
TPU needs more discipline than PP, ABS, or many styrenic TPEs. Moisture control, residence time, shear, and melt temperature have a direct effect on surface finish and mechanical strength.
| Processing parameter | Ether TPU (Shore 80A) | Ester TPU (Shore 95A) | Hard TPU (Shore 60D) |
|---|---|---|---|
| Melt temperature | 190-220 °C | 200-230 °C | 215-240 °C |
| Mould temperature | 20-50 °C | 30-60 °C | 30-70 °C |
| Injection pressure | 50-100 MPa | 60-110 MPa | 70-120 MPa |
| Shrinkage | 0.5-1.8 % | 0.5-1.5 % | 0.4-1.2 % |
| Recommended draft angle | ≥ 2° per side | ≥ 2° per side | ≥ 1° per side |
Drying is non-negotiable: TPU absorbs moisture quickly. Many suppliers call for drying to below 0.02 % moisture before processing. Typical drying for Shore A TPU is around 90-110 °C for 3-4 hours in dry air, while aliphatic and specialty grades may need lower temperatures. Use the supplier's drying table, not a generic setting.
Residence time: TPU can degrade when held hot for too long. A correctly sized barrel, short interruptions, and purging during stops reduce yellowing, bubbles, odour, splay, and loss of tensile strength. A 4-6 minute residence-time target is a useful starting point, but the safe window depends on grade and melt temperature.
Sink and warp: TPU shrinkage is moderate, but thick soft sections still cool slowly. Keep walls broadly uniform, use ribs instead of solid masses, and avoid sudden transitions. A practical wall range is 1.5-5.0 mm, with thicker sections checked for sink, voids, and long cycle time.
Gate design: pin, submarine, tunnel, and fan gates can all work. Transparent, medical, or cosmetic TPU often benefits from a fan or edge gate that reduces shear marks. Cold slugs and runner balance matter because chilled material can leave weak spots or cosmetic defects.
What grades and variants of TPU are available?
| Grade | Key feature | Typical use case |
|---|---|---|
| Ether-based, Shore 80A | Soft, hydrolysis-resistant | Medical seals, wet-service flexible parts |
| Ether-based, Shore 95A | Tough, balanced flexible grade | Cable strain reliefs, sports goods |
| Ester-based, Shore 85A | Oil, grease, and abrasion resistance | Automotive seals, industrial hose parts |
| Ester-based, Shore 55D | Semi-rigid, high strength | Wheels, brackets, impact parts |
| Flame-retardant TPU | Fire performance for specified wall thickness | Cable management, electronics housings |
| Medical-oriented TPU | Documentation package for healthcare use | Tubing, wearable sensors, device components |
| Transparent TPU | Clarity with flexibility | Screen protectors, windows, indicator parts |
| Aliphatic TPU | Better colour stability outdoors | Light-coloured outdoor and wearable parts |
Specialty TPU compounds, including electrically dissipative, glass-filled, carbon-filled, chemical-resistant, and bio-based grades, can be quoted where the design needs them. Availability and certification scope should be confirmed before locking the drawing.
What are the advantages and limitations of TPU?
Advantages:
- High abrasion resistance among injection-mouldable flexible plastics
- Higher tensile and tear strength than most soft-touch TPE grades
- Good oil, grease, and fuel resistance in suitable ester-based grades
- Useful low-temperature flexibility, often down to around -40 °C
- Broad hardness range, from soft Shore A grades to semi-rigid Shore D grades
- Transparent, flame-retardant, food-contact, medical-oriented, and aliphatic options are available
- Thermoplastic processing allows regrind use when quality requirements allow it
Limitations:
- Hygroscopic; poor drying causes splay, bubbles, and mechanical loss
- More expensive per kilogram than PP, ABS, or commodity TPE
- Ester-based grades can hydrolyse in hot, wet environments
- Processing window is narrower than many TPE grades
- Continuous service above 110-120 °C is limited for most grades
- Chemical adhesion in over-moulding is not automatic; mechanical locks are often needed
When should you choose TPU over alternative materials?
Choose TPU over TPE when abrasion, tear strength, oil resistance, or tensile strength is a primary requirement. TPU is usually the better choice for wheels, scraper lips, cable boots, protective covers, and seals exposed to grease or repeated flexing.
Choose TPU over silicone when the part can run within TPU's temperature range and needs thermoplastic processing, colour, abrasion resistance, or integration into a rigid-plastic assembly. Silicone is still stronger for sustained heat above TPU's range, very soft seals, and many implant or high-temperature medical applications.
Choose ether TPU over ester TPU when the part is exposed to water, humidity, steam cleaning, or long wet service. Choose ester TPU for dry wear, oil, grease, and many industrial or automotive applications.
Choose harder TPU (Shore 50D-80D) over flexible POM or semi-rigid PP when the part must be stiff enough to locate in an assembly but still absorb impact and return elastically after deflection.
Recyclability and sustainability
TPU is thermoplastic and can be reprocessed when it has not degraded or been contaminated. Some supplier guidance allows regrind levels up to about 30 %, but cosmetic, medical, food-contact, and critical mechanical parts often require lower limits or virgin-only material. A conservative starting point for non-critical parts is 10-15 % clean, dried regrind of the same grade.
Post-consumer TPU recycling is technically possible but less mature than PP or HDPE recycling because TPU parts are often small, coloured, bonded to other materials, or hard to identify. Chemical recycling routes for polyurethane are developing and used in some industrial streams, but they should not be assumed available for a specific moulded-part waste stream. Bio-based TPU grades with renewable polyol content are commercially available for projects that can accept their certification and performance limits.
Send Nordmould your STEP file, target hardness, wear conditions, fluid exposure, and any food-contact or medical requirements. A free DFM review can cover hardness, ether versus ester chemistry, drying risk, draft, gate strategy, and the quotation route.
Frequently asked questions
What makes TPU different from other flexible injection-moulding materials?
TPU combines rubber-like flexibility with higher tensile strength, tear resistance, abrasion resistance, and oil resistance than most soft-touch TPE grades. TPE is usually selected for grip and feel; TPU is selected when the part sees friction, repeated flexing, oily contact, or higher mechanical load.
Does TPU need to be dried before injection moulding?
Yes. TPU is hygroscopic and should be dried before processing. Many TPU suppliers specify drying to below 0.02% moisture, commonly around 90-110 °C for 3-4 hours for Shore A grades, with grade-specific exceptions. Wet TPU can foam, splay, lose molecular weight, and mould into weaker parts.
What Shore hardness grades of TPU are available?
TPU is available from roughly Shore 60A to Shore 80D. The common injection-moulding range is around Shore 70A-95A, with Shore D grades used for semi-rigid wheels, brackets, and protective components. The final hardness should be selected from a specific resin datasheet and prototype test.
Can TPU be over-moulded onto rigid plastics?
Yes, but the bond must be designed. TPU can be over-moulded onto substrates such as ABS, PC, and nylon when the grade is compatible. POM and some low-energy plastics are difficult and usually need mechanical interlocks. Nordmould can review bond strategy during DFM, with final adhesion confirmed on the chosen resin pair.
Is TPU suitable for food contact or medical applications?
Food-contact and medical-oriented TPU grades are available, but compliance is grade-specific. Medical projects normally require documented ISO 10993 or equivalent biological evaluation for the final use. Food-contact projects need the correct FDA, EU 10/2011, or other market-specific declaration from the resin supplier.
What is the minimum order for TPU parts at Nordmould?
TPU injection moulding is available from 100 pieces. Tooling starts from €3,000. TPU resin is more expensive than PP or ABS, so the business case usually depends on avoiding separate rubber parts, improving wear life, or simplifying assembly.
How does TPU compare with PU cast (polyurethane casting) for prototyping?
Cast PU is useful for prototypes and very small batches without hard tooling. It is a two-component thermoset process, so the parts are not identical to injection-moulded TPU. Bridge tooling can make production-representative TPU parts once quantities justify tooling, typically from 100 pieces.