English
TPU Injection Molding: Process, Materials, and Process Control Guide
Created on 02.02
TPU injection molding is widely used to produce flexible yet durable plastic components for demanding applications. Thanks to its broad hardness range, excellent elasticity, and strong resistance to wear and chemicals, TPU has become a preferred material in industries that require both performance and design flexibility.
This guide explains what TPU is, how TPU injection molding works, and how to optimize materials, machines, and processing parameters for consistent results.
What is TPU? Understanding Thermoplastic Polyurethane
Thermoplastic polyurethane, commonly referred to as TPU, is a unique class of polyurethane plastics with many properties, including elasticity, transparency, and resistance to oil, grease, and abrasion. Technically, it is a thermoplastic elastomer consisting of linear segmented block copolymers composed of hard and soft segments.
The Core Advantages of TPU
TPU is kind of like a mix between rubber and plastic. Here's what it's good at:
- Really Stretchy: It can stretch a lot and snap right back.
- Strong and Lasting: It can handle heavy loads and take a beating better than regular PVC.
- Resists Chemicals: It doesn't break down when it touches oils, grease, or solvents.
- Works in Different Temperatures: It stays bendy when it's cold and holds up okay when it gets pretty warm.
- Lots of Hardness Options: You can make TPU as soft as a rubber band or as hard as tough plastic.
TPU vs. Other Materials
Unlike thermoset rubber, which requires a lengthy curing process, TPU can be processed on conventional injection molding equipment. This makes it more efficient for mass production. Compared to standard plastics like PE or PP, TPU offers significantly better wear resistance and tactile "grip," which is why it is the go-to material for protective cases, automotive bushings, and footwear soles.
TPU Material Fundamentals for Injection Molding
Knowing the basics of the TPU pellets you're using is key before you start making anything.
Polyether vs. Polyester TPU
There are mainly two kinds of TPU:
- Polyester TPU: Great for resisting oil and wear. It's often picked for machine parts that touch fuel. But water can break it down.
- Polyether TPU: Stays bendy in the cold and resists water damage well. It works best when exposed to wetness or germs, like in medical tubes or outdoor gear.
Selecting the Right Hardness
TPU's hardness is measured with Shore A and Shore D scales. Shore A is for softer stuff (like 60A to 90A), and Shore D is for harder, more rigid stuff. Getting the right grade is super important because it changes how much the material shrinks and how much pressure you need to inject it.
How to Store and Prep the Material
TPU loves to soak up moisture from the air. Even a little water in the pellets can cause problems like bubbles, marks, and weak parts. Always keep TPU in airtight containers, and make sure to dry it out before you use it.
Can You Injection Mold Soft TPU? Hardness and Process Impact
A common question among manufacturers is: can you injection mold soft TPU? The answer is a definitive yes. TPU grades as soft as 60A or even lower are frequently used in injection molding. However, the softer the material, the more "fickle" the process becomes.
Soft vs. Hard TPU Processing
- Soft TPU (60A - 85A): These materials have a high friction coefficient. Soft TPUs usually stick to molds, making them hard to remove. Also, you need to keep the melt temperatures low to stop nozzle drooling.
- Hard TPUs (90A - 70D): These act more like regular engineering plastics. They flow better and cool faster, but you'll need to use higher injection pressures.
Challenges with Ultra-Soft TPU
Very soft TPUs can buckle in the injection molding machine's feeding area. To fix this, experts often use special screws with lower compression. They also make sure the mold surface is textured or coated so the part doesn't stick when it's being ejected.
Detailed Steps of the TPU Injection Molding Process
Achieving a high-quality finish requires a disciplined approach to the five stages of the molding cycle.
- Material Drying: This is the most critical step. TPU must be dried to a moisture content of less than 0.02%. Use a desiccant dryer at 80–110°C for at least 3 to 5 hours.
- Feeding and Melting: The dried pellets enter the hopper and are moved forward by the screw. As the pellets move through the heated zones of the barrel, they transition into a homogenous melt.
- Injection: The screw acts as a plunger, forcing the molten TPU into the mold cavity. High precision is required here to ensure the cavity is filled without causing "flash" (excess material).
- Cooling and Solidification: Once the cavity is full, the material must cool. TPU has a relatively slow crystallization rate compared to some other plastics, so cooling times may be slightly longer to ensure the part reaches sufficient hardness for ejection.
- Ejection and Post-Processing: The mold opens, and pins push the part out. Some TPU parts may require "annealing" (post-heating in an oven) to reach their final physical properties and dimensional stability.
Requirements for TPU Injection Molding Machine
TPU injection molding doesn't absolutely require special machines, but you do have to set up standard gear just right due to how TPU behaves. Start with the screw and barrel: a regular three-zone screw that has an L/D ratio of 15:1 to 20:1 works great, as long as it has a valve to keep material from flowing backward. Picking the right nozzle also matters. Open nozzles are generally recommended to prevent clogs and heat damage, which can occur with needle-valve nozzles.
Good results also depend on mold design. You need to think about how much TPU shrinks (usually between 0.5% and 2.0%) and make sure there's good venting to avoid air burns. Since TPU (mainly the softer kind) tends to stick, add draft angles of at least 5 to 6 degrees to help parts pop out easily.
Optimal Temperature Settings for TPU
Temperature control is the difference between a perfect part and a scrap pile. TPU is sensitive to thermal degradation, so the temperature profile should be carefully monitored.
Machine Zone | Soft TPU (60A-85A) | Hard TPU (90A-70D) |
Rear Zone (Feed) | 170 – 190°C | 190 – 210°C |
Middle Zone | 180 – 200°C | 200 – 220°C |
Front Zone (Nozzle) | 185 – 205°C | 210 – 230°C |
Melt Temperature | 190 – 210°C | 210 – 235°C |
Mold Temperature
The mold temperature should typically be kept between 15°C and 50°C. A colder mold will result in a faster cycle time but may increase internal stresses. A warmer mold improves surface gloss and reduces flow lines.
Additional Key Process Parameters
Beyond temperature, several other settings dictate the success of the TPU material injection molding run.
Parameter | Recommended Setting | Impact |
Injection Speed | Slow to Moderate | High speeds can cause shear heating and material burning. |
Injection Pressure | 20 – 100 MPa | Depends on part complexity and material hardness. |
Back Pressure | 0.3 – 1.5 MPa | Low back pressure prevents excessive shear and air entrapment. |
Screw Speed | 20 – 50 RPM | Keeps the melt uniform without overheating the material. |
Common Mistakes to Avoid
- Overheating: If the melt stays in the barrel too long at high temperatures, the TPU chains will break down, leading to brittle parts.
- Insufficient Drying: This leads to "silver streaks" on the surface and a significant loss of tensile strength.
- High Injection Pressure: Using too much pressure can cause the part to "over-pack," making it nearly impossible to remove from the mold without deformation.
Conclusion
TPU injection molding offers a versatile solution for creating durable, flexible, and high-performance components. By understanding the distinction between polyether and polyester types, ensuring rigorous drying protocols, and fine-tuning your temperature and pressure settings, you can produce high-quality parts with minimal waste.
Leading provider of industrial machinery and comprehensive manufacturing solutions since 1955.
Quick Links
Quick Links
Our Services
Our Services
Contact Us
Contact Us
Unit 04-05 16/F The Broadway
No.54-62 Lockhart Rd Wanchai .HK
Av. Sunset #415, Punta Hermosa, Lima Peru
+86 18767169649
+51 994 484 114
© 2025 Sountec Machinery. All rights reserved.
English
WhatsApp