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- TPU is a flexible, abrasion-resistant thermoplastic elastomer processed at 180–230°C on standard injection molding machines.
- Shore A hardness spans 60A–98A, covering everything from soft grips to semi-rigid structural parts.
- Pre-drying at 80–100°C for 2–4 hours is mandatory — moisture above 0.02% causes splay and chain scission.
- TPU outperforms TPE in tensile strength (25–55 MPa) and abrasion resistance, making it ideal for demanding applications.
- Design for TPU requires generous draft angles (2–3° for soft grades) and gate diameters ≥1.5 mm.
What Is TPU Injection Molding?
TPU moldeo por inyección is melting thermoplastic polyurethane pellets and injecting them into a mold to make flexible, rubber-like parts. It runs on the same machines used for ABS, PP, or nylon.
In our factory, TPU has become one of the most requested elastomeric materials over the past five years. Customers come to us when they need parts that combine rubber-like flexibility with thermoplastic recyclability — think cable overmolds, wearable device skins, and industrial seals. The chemistry behind TPU consists of alternating hard segments (from diisocyanate and chain extender) and soft segments (from polyol), which create its signature combination of strength and elasticity at the molecular level.
TPU injection molding follows the same fundamental cycle as any thermoplastic: plasticizing, injection, packing, cooling, and ejection. However, TPU’s sensitivity to moisture and its unique flow behavior demand tighter process control than commodity plastics. The reward is a part with outstanding mechanical performance that no other thermoplastic fully replicates.
“TPU can be injection molded on the same machines used for ABS, PP, and nylon.”Verdadero
Standard reciprocating-screw injection molding machines process TPU effectively. The key differences are parameter settings (lower barrel temperatures of 180–230°C, slower injection speeds) and the mandatory pre-drying step — not the equipment itself.
“You need specialized rubber-processing equipment to mold TPU parts.”Falso
TPU is a thermoplastic elastomer, not a thermoset rubber. It melts and flows like a thermoplastic, so it can be processed on any standard injection molding machine without special rubber-processing attachments or vulcanization ovens.
What Are the Key Material Properties of TPU?
TPU’s key properties are Shore A hardness1 of 60A–98A, tensile strength of 25–55 MPa, and abrasion loss of 20–80 mm³. These are set at synthesis via hard/soft segment ratio.
| Propiedad | Typical Value | Significance |
|---|---|---|
| Shore Hardness | 60A–98A / 40D–80D | Soft grips to semi-rigid housings |
| Resistencia a la tracción | 25–55 MPa | Higher than most TPEs |
| Alargamiento a la rotura | 300–700% | Excellent flexibility |
| Operating Temp. | –40°C to +100°C | Outdoor and refrigerated use |
| Resistencia a la abrasión | 20–80 mm³ DIN loss | Superior to rubbers and TPEs |
| Absorción de agua | 0.5–2.0% (24 hr) | Mandatory pre-drying |
Polyester-based TPU grades offer superior oil resistance and abrasion performance, while polyether-based grades excel in hydrolysis resistance and low-temperature flexibility. For marine or humid-environment parts, we always specify polyether TPU. For automotive underhood components exposed to hydraulic fluid, polyester grades perform better.
What Are the Critical Process Parameters for TPU Injection Molding?
TPU process parameters are barrel temp 180–230°C, mold temp 20–50°C, slow injection speed, and 2–4 hour pre-drying at 80–100°C. Moisture below 0.02% is mandatory.
| Parámetro | Range | Notas |
|---|---|---|
| Pre-drying Temp. | 80–100°C | 2–4 hours; dehumidifying dryer |
| Barrel Temp. (Rear) | 170–190°C | Prevents premature melting |
| Barrel Temp. (Front) | 190–230°C | Abrasión |
| Temperatura del molde | 20–50°C | Higher = better surface |
| Presión de inyección | 50–120 MPa | Softer grades need less |
| Presión de mantenimiento | 60–80% of peak | Prevents sink marks |
| Tiempo de enfriamiento | 15–60 seconds | Flexible parts need longer |
| Velocidad del tornillo | 30–80 RPM | Slow prevents shear damage |
In our experience, TPU’s sensitivity to residence time is one of the trickiest parameters to manage. If TPU sits in the barrel at processing temperature for more than 5–10 minutes, it begins to degrade — you’ll see yellowing and a sharp drop in tensile properties. We always purge the machine between runs and minimize shot-to-cycle intervals during production startup.
What Are Common Defects in TPU Injection Molding and How Can You Prevent Them?
The most common TPU defects are splay marks (moisture), mold sticking (high tack), short shots (low pressure), and yellowing (thermal degradation). Each has a specific root cause and a practical fix.
| Defecto | Root Cause | Prevention |
|---|---|---|
| Splay / Silver Streaks | Moisture in material | Dry at 90°C for 3–4 hours |
| Part Sticking | High elasticity, low draft | Increase draft to 2–3° |
| Bubbles / Voids | Thermal degradation | Reduce barrel temp; improve venting |
| Disparo corto | Insufficient fill pressure | Raise injection pressure 10–20% |
| Flash | Overpacking; worn parting line | Reduce holding pressure |
| Yellowing | Excessive temp or residence | Keep barrel below 240°C; purge |
We’ve found that part sticking is the single biggest production bottleneck for soft TPU grades. In one project for a wearable fitness device, we were running Shore 75A TPU with a complex undercut geometry. The fix wasn’t more release agent — it was increasing the draft angle from 1° to 2.5° on the sidewalls and redesigning the ejector pin locations. Cycle time dropped from 45 seconds to 28 seconds after the mold modification.
“TPU must be dried at 80–100°C for 2–4 hours before every production run.”Verdadero
Pre-drying is non-negotiable. Moisture content above 0.02% causes hydrolytic chain scission during the melt phase, producing splay marks, surface bubbles, and significant reduction in tensile strength.
“If TPU pellets are stored in a sealed bag, they don’t need drying.”Falso
TPU is highly hygroscopic — it can absorb enough moisture in 30–60 minutes of open-air exposure to cause splay marks and chain scission. Pre-drying is mandatory regardless of storage conditions.
What Industries Rely on TPU Injection Molded Parts?
TPU is widely used across automotive, footwear, medical, consumer electronics, and industrial applications. Its combination of flexibility, abrasion resistance, and chemical tolerance makes it the go-to elastomer for demanding environments.
Automóvil: Door seals, gearshift boots, cable grommets, and vibration-damping mounts. Polyester-based TPU grades offer the fuel and oil resistance required for underhood applications. We’ve produced TPU cable harness guides for EV battery packs where dimensional stability at –30°C was a hard requirement.
Footwear: Midsoles, outsoles, and toe caps use TPU for exceptional abrasion resistance (20–40 mm³ DIN loss) and flex fatigue endurance. Injection molded TPU outsoles withstand millions of flex cycles without cracking.
Productos sanitarios: Medical-grade TPU formulations are used for surgical tubing, catheter components, and wearable sensor housings. Polyether TPU’s hydrolysis resistance and biocompatibility (ISO 10993) make it suitable for body-contact applications. We produce flexible TPU housings for continuous glucose monitoring devices.
Electrónica de consumo: Phone cases, cable overmolds, earphone tips, and smartwatch bands. TPU’s chemical resistance to sweat, cleaning solvents, and UV light makes it durable in daily consumer environments.
Industrial: Hydraulic hose couplings, conveyor components, roll covers, and protective boots from harder TPU grades (Shore 90A–60D). Abrasion and tear resistance exceeds natural rubber by 3–5× in high-speed conveyor applications.
“TPU can accommodate minor undercuts by flexing during demolding.”Verdadero
TPU’s elasticity allows it to stretch over minor undercuts without damage, reducing tooling complexity by eliminating sliders or lifters.
“TPU parts are too expensive for high-volume consumer products.”Falso
TPU costs more per kilogram than commodity TPE, but its longer mold life (300K+ shots), lower scrap rate, and runner recyclability often make it cheaper per-part in volumes above 10,000 units.
How Does TPU Compare to TPE, Silicone, and Rubber?
TPU is stronger and more abrasion-resistant than standard thermoplastic elastomers (TPEs)2, with tensile strength of 25–55 MPa versus 8–20 MPa for TPE. It also offers recyclability that vulcanized rubber and LSR cannot match.
| Propiedad | TPU | TPE | Silicona | Rubber |
|---|---|---|---|---|
| Resistencia a la tracción | 25–55 MPa | 8–20 MPa | 6–12 MPa | 10–30 MPa |
| Abrasion | Excelente | Bien | Pobre | Very Good |
| Moldeo por Inyección de TPU: Guía Completa para Ingenieros | –40 to +100°C | –50 to +80°C | –60 to +200°C | –40 to +100°C |
| Recyclable | Yes | Yes | No | No |
| Pre-drying | Mandatory | Sometimes | No | N/A |
We’ve run direct comparison projects for clients choosing between TPU and TPE for phone case applications. TPE (SEBS-based) costs 20–30% less per kilogram and processes more easily, but drop test results were consistently 15–25% worse with TPE versus a comparable Shore 85A TPU grade. When mechanical performance under repeated impact is critical, TPU wins.
What Design Guidelines Should You Follow for TPU Parts?
TPU parts are best designed with 1.5–4.0 mm walls, 2–3° draft for soft grades, gate diameter ≥1.5 mm, and gradual transitions. TPU flexes enough to demold 3–5% undercuts without sliders.
Espesor de pared: Very thin walls (6 mm) produce long cooling times and potential voids. Uniform wall thickness is especially important with TPU — sudden changes cause differential shrinkage that creates warpage.
Ángulos de tiro: This is where we see the most design revisions. Shore 60A–75A grades need 2–3° minimum, and 3–5° for textured surfaces. TPU grips the mold core during cooling, so generous draft is essential for reliable ejection.
Socavones: Unlike rigid plastics, TPU can accommodate minor undercuts (3–5% of part diameter) by flexing during demolding. This reduces tooling complexity — many features that would require sliders in ABS can be directly demolded with TPU.
Diseño de la puerta: Submarine gates are popular for TPU because they self-degate. Gate diameter must be 1.5–2.5 mm minimum — undersized gates cause excessive shear. Avoid pin gates smaller than 1.0 mm with soft grades.
Sobremoldeado: TPU is one of the most versatile materials for sobremoldeo3 onto rigid substrates (ABS, PC, PC/ABS, nylon). We’ve successfully overmolded Shore 85A TPU onto PC/ABS housings, achieving bond strength exceeding 5 MPa in peel tests. For more on molde de inyección design principles, see our complete guide.
TPU Injection Molding: Frequently Asked Questions
¿Cuál es la cantidad mínima de pedido para piezas moldeadas por inyección de TPU?
No hay un mínimo universal — depende de la amortización de las herramientas. Para un molde simple que cuesta $3,000–5,000, cantidades de 500–2,000 piezas pueden ser económicas. Para piezas de TPU de alto volumen (100,000+ unidades/año), moldes multicavidad con 4–8 cavidades son estándar.
¿Puedo usar TPU reciclado (rTPU) para moldeo por inyección?
Sí, pero con limitaciones. El TPU reciclado generalmente tiene una resistencia a la tracción 10–20% menor y una elongación reducida. Para aplicaciones no estructurales, el rTPU funciona bien. Para piezas que soportan carga, mezclar un 10–20% de rTPU con resina virgen es un compromiso común.
¿Cuánto tiempo suele durar un molde de inyección de TPU?
Un molde de acero P20 de grado de producción para TPU suele durar entre 300.000 y 500.000 inyecciones. El TPU es más suave con el acero del molde que los materiales rellenos de abrasivos, por lo que la vida útil del molde depende principalmente del número de inyecciones.
¿Por qué mi pieza de TPU se vuelve amarilla después del moldeo?
El amarillamiento casi siempre es causado por degradación térmica — temperatura del cañón demasiado alta (por encima de 240°C), tiempo de residencia excesivo o hidrólisis inducida por humedad. Verifique primero su perfil de temperatura del cañón y purgue la máquina antes de la producción.
¿Cuál es la tasa de contracción típica para el TPU?
La contracción del TPU varía de 0,8 a 2,0% dependiendo de la dureza. Los grados más blandos (Shore 60A–75A) se contraen 1,5–2,0%, mientras que los grados más duros (Shore 90A–60D) se contraen 0,8–1,2%. Consulte siempre la hoja de datos del material y realice disparos de prueba antes de finalizar las dimensiones del molde.
¿Es el TPU seguro para aplicaciones de contacto con alimentos?
Algunos grados de TPU cumplen con las normas de la FDA para contacto con alimentos bajo las regulaciones 21 CFR, pero el grado específico debe verificarse. No todas las formulaciones son seguras para alimentos: los colorantes y aditivos pueden descalificar un grado.
¿Cuál es la diferencia entre el TPU de poliéster y el TPU de poliéter?
Polyester TPU offers better oil resistance and abrasion performance; polyether TPU provides superior hydrolysis resistance and low-temperature flexibility. For marine or humid environments, specify polyether. For automotive underhood applications, choose polyester.
Get Expert TPU Injection Molding Support
In our 20 years of running TPU tools, we’ve learned that the biggest cost driver isn’t material price — it’s getting the drying, draft angles, and gate design right the first time. At ZetarMold, our 8 senior engineers run mold flow simulation on every TPU project before steel is cut, catching fill balance and demolding issues that would otherwise surface during T1 sampling.
With 45 injection molding machines (90T–1850T), 400+ material options, and ISO 9001/13485 certified quality systems, we have the equipment and experience to handle TPU projects from prototype through volume production. Contact us to discuss your TPU project — our English-speaking engineering team responds within 24 hours.
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Shore A hardness: A standardized scale (ASTM D2240) that measures the resistance of soft materials to indentation by a spring-loaded probe, covering the range from very soft (0A) to nearly rigid (100A). ↩
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thermoplastic elastomers (TPEs): A class of polymers combining rubber-like elasticity with thermoplastic processability, including SEBS, SBS, TPV, and TPU families. ↩
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sobremoldeo A two-step injection molding process where a second material is molded over a pre-formed substrate, commonly used with TPU for soft-touch grips and seals. ↩
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