Thermoplastic Polyurethane (TPU) Injection Molding: Process, Mold Design And Defect Solutions

Table of Contents

1. Introduction

2. What is Thermoplastic Polyurethane(TPU)?

3. Key Properties of Thermoplastic Polyurethane(TPU)

4. TPU Injection Molding Process

5. TPU Mold Design Consideration

6. Common TPU Injection Molding Defects and Solutions

7. Applications and Case Studies of Thermoplastic Polyurethane(TPU)

8. Conclusion

1. Introduction

Thermoplastic polyurethane (TPU) is a versatile and high-performance elastomer widely used in industries such as automotive, consumer electronics, medical, footwear, and sports equipment. Thanks to its excellent flexibility, abrasion resistance, and mechanical strength, TPU is an ideal material for injection molding applications.

This article explores the key aspects of TPU injection molding, focusing on three crucial areas: injection molding process, mold design considerations, and defect troubleshooting. We’ll also include comparative tables and practical tips to support engineers and designers in achieving better molding outcomes.

2. What is Thermoplastic Polyurethane(TPU)?

TPU (Thermoplastic Polyurethane) is a kind of thermoplastic elastomer, which is generated by the reaction of isocyanate, chain extender and polyol.

2.1 TPU Classification

TPU can be divided into three main categories:

Type	Characteristics	Typical Applications
Polyester-based TPU	Excellent wear resistance and mechanical strength	Shoe soles, industrial belts
Polyether-based TPU	Superior hydrolysis resistance and low-temperature flexibility	Medical tubing, seals
Polycaprolactone-based TPU	Balanced performance, better processability	Consumer electronics, cables

2.2 Advantages of TPU

• High elasticity and flexibility

• Excellent wear, impact, and tear resistance

• Oil, grease, and chemical resistance

• Good low-temperature performance

• Transparent and colorable

3. Key Properties of Thermoplastic Polyurethane(TPU)

TPU is characterized by its unique combination of elasticity and toughness. It offers the following performance advantages:

Property	Description
Hardness Range	Typically 60A to 85D, adjustable depending on application
Tensile Strength	High, between 30–55 MPa depending on the formulation
Abrasion Resistance	Excellent, suitable for high-wear applications
Flexibility	Retains flexibility across a wide range of temperatures
Chemical Resistance	Good resistance to oils, greases, and various solvents

These characteristics make TPU suitable for complex geometries, overmolding, and components requiring impact resistance and durability.

4. TPU Injection Molding Process

Injection molding is the most efficient and widely used processing method for TPU. Due to its thermoplastic properties, TPU can be reheated, melted, and molded multiple times.

4.1 Material Preparation

• Drying: TPU is hygroscopic and must be thoroughly dried before molding. Typical drying parameters are:

• Temperature: 80–110°C

• Time: 2–4 hours

• Recommended moisture content: <0.03%

• Drying Equipment: Use a dehumidifying dryer or vacuum oven.

4.2 Injection Parameters

Parameter	Recommended Range
Barrel Temperature	190–230°C (depending on TPU grade)
Mold Temperature	30–60°C
Injection Speed	Medium to high
Back Pressure	Low to medium (2–10 bar)
Holding Pressure	50–80% of injection pressure

4.3 Special Considerations

• Screw Design: A general-purpose screw with a compression ratio of 2.5–3.0 is suitable.

• Cooling Time: TPU cools slowly; mold design must ensure uniform cooling.

• Release Agent: In some cases, a mold release agent may help improve demolding.

TPU's unique elasticity and shrinkage behavior require special attention in mold design to ensure optimal part quality and mold longevity.

5. TPU Mold Design Considerations

5.1 Gate and Runner Design

• Preferred gate type: Pin-point gate or edge gate

• Gate size: Slightly larger than rigid plastics to reduce shear

• Runner: Should be round for better melt flow and easier ejection

5.2 Mold Venting

TPU produces gas during injection; poor venting will lead to burn marks or incomplete filling.

Recommended vent depth: 0.02–0.03 mm

Vent locations: parting lines, ribs, and flow ends

5.3 Cooling System Optimization

Although TPU has lower molding temperatures than some engineering plastics, precise cooling is still critical.

Use conformal cooling channels where possible

Ensure uniform cooling to reduce internal stress and shrinkage

5.4 Demolding and Ejection

TPU parts may stick due to their flexibility. To ensure smooth ejection:

• Use polished mold surfaces (Ra ≤ 0.2μm)

• Apply suitable draft angles (minimum 1°–2°)

• Use ejector pins or stripper plates evenly distributed

6. Common TPU Injection Molding Defects and Solutions

Due to TPU’s soft, elastic nature, it is more prone to specific molding defects. Below is a guide to help troubleshoot and resolve common issues:

6.1 Bubbles and Voids

Causes: Moisture in material, trapped gas, or excessive injection speed.

Solutions:

Ensure material is thoroughly dried.

Improve mold venting.

Reduce injection speed slightly.

6.2 Sink Marks

Causes: Inadequate holding pressure or improper cooling.

Solutions:

Increase holding time and pressure.

Optimize cooling system layout.

6.3 Short Shots

Causes: Low injection pressure or poor gate design.

Solutions:

Increase injection pressure or speed.

Reassess runner and gate dimensions.

6.4 Flashing

Causes: Excessive injection pressure, poorly fitting parting lines.

Solutions:

Adjust clamping force.

Inspect mold alignment and tolerances.

7. Applications and Case Studies of Thermoplastic Polyurethane(TPU)

TPU's versatility makes it suitable for various high-performance and aesthetic parts. Below are typical industry applications:

Industry	Application	Features
Automotive	CVJ boots, gaskets, bushings	Oil-resistant, durable
Electronics	Cable jackets, connectors, mobile phone cases	Flexible, abrasion-resistant
Medical	Catheters, seals, surgical tubing	Biocompatible, soft
Consumer Goods	Watch straps, sports gear, wheels	Wear-resistant, colorful

Case Example: Mobile Phone Protective Cover

Material: Polyether-based TPU

Features: Anti-shock, anti-yellowing

Mold: Hot runner system with 4 cavities

Injection cycle: 30s

Benefit: High transparency and flexibility maintained after 100,000 flex cycles

Case Example: Sports Footwear Sole

Material: Polyester-based TPU

Features: High abrasion resistance and flexibility

Process optimization: Short cooling time with uniform hardness

Mold strategy: Multi-cavity cold runner with vacuum venting

8. Conclusion

TPU injection molding combines the benefits of flexibility, resilience, and process efficiency, offering unmatched design possibilities for technical and consumer parts. With proper material handling, mold design, and process control, manufacturers can achieve high-quality, durable TPU parts at scale.

For those seeking reliable solutions in TPU injection molding, Alpine Mold brings decades of expertise in custom mold manufacturing and engineering support tailored for TPU and other engineering thermoplastics.