TPU Injection Molding: Complete Processing Guide for Engineers

What Is TPU Injection Molding?

TPU 射出成形 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.

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.

プロパティ	Typical Value	Significance
Shore Hardness	60A–98A / 40D–80D	Soft grips to semi-rigid housings
引張強度	25–55 MPa	Higher than most TPEs
破断伸度	300–700%	Excellent flexibility
Operating Temp.	–40°C to +100°C	Outdoor and refrigerated use
耐摩耗性	20–80 mm³ DIN loss	Superior to rubbers and TPEs
吸水	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.

パラメータ	Range	備考
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	6〜12 MPa
金型温度	20–50°C	Higher = better surface
射出圧力	50–120 MPa	Softer grades need less
保持圧力	60–80% of peak	Prevents sink marks
冷却時間	15–60 seconds	Flexible parts need longer
スクリュースピード	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.

欠陥	Root Cause	エネルギー消費量：
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
ショートショット	Insufficient fill pressure	Raise injection pressure 10–20%
フラッシュ	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.

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.

自動車： 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.

医療機器 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.

コンシューマー・エレクトロニクス 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.

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.

プロパティ	TPU	ティーピーイー	シリコーン	Rubber
引張強度	25–55 MPa	8–20 MPa	6–12 MPa	TPU射出成形：エンジニア向け完全ガイド
Abrasion	素晴らしい	グッド	貧しい	Very Good
Temp. Range	–40 to +100°C	–50 to +80°C	–60 to +200°C	–40 to +100°C
Recyclable	Yes	Yes	いいえ	いいえ
Pre-drying	Mandatory	Sometimes	いいえ	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.

壁の厚さ： 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.

ドラフトの角度 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.

アンダーカット： 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.

ゲートデザイン： 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.

オーバーモールディング： TPU is one of the most versatile materials for オーバーモールディング³ 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 射出成形金型 design principles, see our complete guide.

TPU Injection Molding: Frequently Asked Questions

TPU射出成形部品の最小注文数量はいくつですか？

普遍的な最低数量は存在しません——それは工具の償却に依存します。$3,000～5,000円のシンプルな金型の場合、500～2,000個の生産量で経済的になることがあります。高容量のTPU部品（年間100,000個以上）の場合、4～8キャビティの多キャビティ金型が標準的です。

リサイクルTPU（rTPU）は射出成形に使用できますか？

はい、ただし制限があります。リサイクルTPUは通常、引張強度が10〜20％低く、伸びも減少しています。非構造用途では、rTPUは良好に機能します。荷重部品の場合、10〜20％のrTPUをバージン樹脂と混合することが一般的な妥協策です。

TPU射出成形金型は通常どのくらい持続しますか？

TPU用の生産グレードP20鋼型は通常30万～50万ショットの寿命があります。TPUは研磨剤入り材料よりも金型鋼に対して優しいため、金型寿命は主にショット数によって決まります。

なぜ成形後にTPU部品が黄色くなるのですか？

黄変はほぼ常に熱分解によって引き起こされます。これは、バレル温度が高すぎる（240°C以上）、滞留時間が過剰、または水分による加水分解が原因です。まずバレルの温度プロファイルを確認し、生産前に機械をパージしてください。

TPUの典型的な収縮率はどれくらいですか？

TPUの収縮率は硬度によって0.8～2.0%の範囲です。柔らかいグレード（ショア60A～75A）は1.5～2.0%収縮し、硬いグレード（ショア90A～60D）は0.8～1.2%収縮します。金型寸法を決定する前に、必ず材料データシートを参照し、テストショットを実施してください。

TPUは食品接触用途に安全ですか？

一部のTPUグレードは21 CFR規制に基づく食品接触用途でFDA準拠ですが、特定のグレードは確認が必要です。全ての配合が食品用に安全とは限らず、着色剤や添加剤によってグレードの適格性が失われる場合があります。

ポリエステルTPUとポリエステルTPUの違いは何ですか？

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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1. 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). ↩

2. thermoplastic elastomers (TPEs): A class of polymers combining rubber-like elasticity with thermoplastic processability, including SEBS, SBS, TPV, and TPU families. ↩

3. オーバーモールディング 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. ↩