How is TPU Processed for Injection Molding?

Thermoplastic Polyurethane¹ (TPU) is a fantastic material, flexible and tough. But processing it can be a real headache. Get the details wrong, and you'll face frustrating defects and wasted costs on what should have been a simple job.

Processing TPU for injection molding² requires meticulous pre-drying of the material³, precise temperature control⁴ (typically 190-220°C), and specific machine settings. Success depends on using a high-torque screw⁵, ensuring the mold is properly vented, and carefully managing injection pressure⁶s and speeds.

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I've worked with TPU on countless projects, and I can tell you it's one of the most versatile polymer⁷s out there. Yes, TPU is very commonly injection molded to make everything from phone cases to caster wheels. However, I've also seen many projects fail simply because the team didn't respect the material's unique demands. It's a polymer⁷ that requires your full attention from start to finish, beginning with the machine itself.

What are the Key Machine and Temperature Settings for TPU?

Using the wrong machine setup for TPU is a common mistake. It can lead to unevenly melted plastic and weak, inconsistent parts, costing you a lot of time and money in trial and error.

For TPU, you need a machine with a high-torque, three-section screw. Set the barrel temperature between 190-220°C and the mold temperature⁸ between 20-40°C. Accurate temperature control⁴ across the board is critical to prevent degradation and ensure a smooth process.

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Let's dive deeper into the specifics, because this is where a good part starts. Your machine's setup is fundamental. For the screw, a standard three-section design works best, with a compression ratio of about 2:1. The most important thing here is that your machine must have enough torque. TPU is quite viscous when it melts, and a weak machine will struggle to plasticize it evenly. The good news is that TPU is not corrosive, so you don't need to invest in special alloy-plated screws.

Temperature control is the next critical piece. You need to be precise.

Temperature Zone	Recommended Range (°C)	Notes
Melt Temperature	190°C - 220°C	Harder grades can go up to 240°C.
Mold Temperature	20°C - 40°C	Standard for most grades.
Modified/GF TPU	40°C - 60°C	Higher temp improves surface finish9.

The melt temperature in the barrel gets the TPU ready, while the mold temperature⁸ affects the part's final properties and cycle time¹⁰. For glass-fiber-reinforced TPU, a warmer mold helps the resin flow better and bond to the fibers. For very thick parts, you might lower the mold temperature⁸ slightly to help it solidify faster, cutting down your cycle time¹⁰. Always start with the manufacturer's datasheet, but be prepared to adjust.

How Do You Control Injection Pressure and Speed for TPU?

Incorrect pressure and speed are the primary culprits for common molding defects. It's frustrating to get the material and temperature right, only to have the part fail due to flash or short shots.

Set the main injection pressure⁶ between 100 and 1200 bar, with the holding pressure¹¹ at about 50% of that value. Use a slow injection speed for thick-walled parts and a fast speed for thin-walled parts to ensure the mold fills correctly.

Let's talk about the pressure and speed relationship, as it's a constant balancing act. The initial injection pressure⁶ is what pushes the molten TPU into the mold cavity. After the mold is mostly full, you switch to holding pressure¹¹. This lower pressure is essential to pack out the part and compensate for material shrinkage as it cools, preventing¹² sink marks. A good starting point for holding pressure¹¹ is half of your main injection pressure⁶. You should also use a small amount of back pressure, just 1-2.5% of the injection pressure⁶, to ensure the melt is uniform and free of trapped air before it's even injected.

Injection speed depends entirely on the geometry of your part.

Part Type	Injection Speed	Why?
Thick-Walled Parts	Slow	Prevents turbulence and trapped air bubbles (voids). Ensures even cooling.
Thin-Walled Parts	Fast	Fills the entire cavity quickly before the material freezes off in thin sections.

I remember a project involving a thin, flexible gasket. We struggled with incomplete parts until we increased the injection speed significantly. It felt counterintuitive to push a viscous material so fast, but it was the only way to fill the mold before the TPU cooled. Getting these parameters right is crucial for a stable process.

How Should Your Mold be Designed for TPU?

You can have the perfect machine settings, but a poorly designed mold will always fail with TPU. Its soft, flexible nature creates unique challenges with filling and ejection that can trap even experienced designers.

A good TPU mold requires runners and gates that are 25-50% larger than those for rigid plastics. It must also have excellent venting¹² to let air escape. For easy demolding, use a minimum draft angle¹³ of 5 degrees and large-surface-area ejector pins.

If you want to mold TPU successfully, you have to design the mold specifically for it. First, let's talk about getting the material in. Because TPU is thick and doesn't flow as easily as hard plastics, the runners and gates—the channels that lead the plastic into the part cavity—need to be bigger. Think of it like using a wider pipe for thick syrup. This reduces the pressure needed and prevents excess shear heating. Next, venting¹² is absolutely critical. As plastic rushes in, air needs a way to get out. If it's trapped, it gets compressed, superheats, and burns the TPU, leaving black marks on your part. We design shallow channels (0.02-0.05mm deep) along the mold's parting line to let air escape but stop the plastic. Finally, getting the soft part out can be tricky. TPU likes to cling to the mold. You need a draft angle¹³ of at least 5 degrees on all vertical walls, and even more for very soft grades. Use large ejector pins so the force is spread out, pushing the part out gently without tearing it or leaving marks.

How Do You Handle TPU After Molding?

The job isn't done when the part comes out of the machine. Improper handling of TPU scrap, colorants, and other additives can ruin your consistency and profitability on the very next run.

Post-molding, handle TPU with care. Clean, dry scrap can be reground and reused at up to a 30% ratio. All colorants and additives¹⁴ must also be thoroughly dried. If using hot runners, precise temperature control⁴ is vital to prevent material degradation.

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What you do after the part is made is just as important. You will inevitably have scrap from the runners. This material can be reused, but with strict rules. It must be clean and, most importantly, completely dry. You can typically mix up to 30% of this reground material with fresh, virgin pellets. Any more than that, and you risk altering the final properties of your part. The same rule applies to any additives you use for color or function.

• TPU Masterbatch: 1.0 - 4.0%
• Color Paste: 0.5 - 1.0%
• Dry Pigments: 0.2 - 0.5%

Every single one of these must be perfectly dry before use. If you use a hot runner system, which keeps the plastic molten all the way to the gate, temperature control⁴ must be flawless to avoid cooking the TPU.

But out of all these points, I have to repeat one thing: DRYING IS EVERYTHING. Moisture is the number one enemy of TPU. Any hint of water in your pellets, regrind¹⁵, or colorants will turn to steam in the hot barrel, causing splay marks, bubbles, and brittle parts. It is the first and most important step. If you get nothing else right, get the drying right.

Conclusion

Successful TPU processing comes down to mastering the fundamentals: meticulous drying, precise machine and temperature control⁴, and a mold designed specifically for a flexible material. Get these basics right to succeed.

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