Maximizing the Potential of TPU Extrusion in Medical Tubing Applications

Tony Walder, Lubrizol Life Sciences

Whether for vascular catheters or implantable devices, medical tubing must meet tough standards for flexibility, strength, and biocompatibility. That’s why more manufacturers are turning to thermoplastic polyurethanes (TPUs) that strike the ideal balance between these key properties, making them an excellent choice for high-performance medical tubing. Unlocking the best that TPUs have to offer means optimizing the extrusion process. This article looks at why TPUs are a top pick, the common obstacles in extrusion, and the ways manufacturers can fine-tune their process to get the most out of different grades.

Why TPUs?

TPUs have become a key material for medical device manufacturers because they offer a balance between the flexibility of elastomers and the durability of engineered polymers. For example, TPUs excel in thin-walled applications, providing both strength and precision in medical devices.

The chemical resistance, biocompatibility, and fatigue resilience of TPUs make them a go-to choice for OEMs. The material softens in the body, which translates to greater comfort for patients without sacrificing strength. Beyond that, TPUs shine in the extrusion process because they can be tailored to the exact specifications needed for different uses.

Common Extrusion Challenges

Manufacturers understand that TPU extrusion isn’t without its hurdles. Keeping quality consistent and meeting specific product requirements requires careful attention, especially when managing moisture levels. Moisture sensitivity is the biggest challenge — it’s the most common and costly issue for OEMs.

Dryness Demands Diligence. Even when the air seems dry, TPUs can still take in moisture. Over time, that extra bit of moisture can cause the material to lose some of its durability. During extrusion, this can cause defects like bubbles or voids to appear. For medical tubing, where smooth surfaces and accuracy are critical, these flaws can render the product unusable.

Lubrizol, for example, emphasizes the need to dry TPUs down to a moisture content between 0.02 percent aromatic based and 0.05 percent aliphatic based before extrusion. It’s crucial to keep moisture levels extremely low to maintain the TPU’s stability throughout the extrusion process.

Standard drying techniques are insufficient — advanced systems are designed to precisely control moisture. Dehumidifying dryers with desiccant beds are the best option because they keep the humidity down and prevent the TPU from soaking up moisture again.

Constant monitoring is also vital. If TPU pellets aren’t properly dried, fluctuations in extrusion pressure can affect the material’s melt consistency. Inconsistent melts can lead to further defects, ranging from inconsistent wall thickness to irregularities in mechanical properties — all of which can compromise the end product.

Ideally, dehumidifying dryers will be combined with real-time moisture analyzers that offer instant feedback, so that quick changes can be made before any quality or production time is lost.

Extruder Configuration is Critical. The next step is making sure the extrusion equipment is properly calibrated. The length-to-diameter (L/D) ratio of the extruder is one important factor. Lubrizol engineers suggest an L/D ratio of at least 24:1 and preferred 30:1, which gives the TPU enough time to melt uniformly before reaching the die. A ratio that is too low can cause incomplete melting, leading to problems like gels, uneven wall thickness, and weaker tubing.

Maintaining proper screw clearance is also critical. If it’s too loose, the TPU pellets will not fully melt, and defects such as gels or fish eyes can appear. A barrier screw with a mixing section can solve this issue by separating the melted TPU from portions that haven’t fully melted, prior to the mixing section blending everything together to achieve a high-quality outcome.

Optimizing Screen Packs for Pressure Control. Screen packs help remove contaminants and ensure smooth material flow. But if they’re not selected or maintained carefully, they can create problems. Too much back pressure from an unsuitable screen pack can cause material degradation, reduce throughput, and compromise the quality of the tubing. A common mistake in extrusion is assuming that simply increasing the fineness of the screen pack will eliminate gel formation. In reality, this can make matters worse by trapping gels within the screen and disrupting the material flow.

It is best to use us a screen pack with a 250-mesh filter to find the right balance of filtration and back pressure. This configuration ensures a good balance between filtering contaminants and maintaining smooth flow. With the right screw design, it creates the ideal back pressure to keep the melt consistent and prevent material degradation.

In Pursuit of Optimal Performance

The path to successful TPU extrusion starts with picking the right grade. It is important to think about the device’s demands. Does it need to resist chemicals, handle long-term use, or have a certain level of flexibility? There are extensive TPU offerings that provide the flexibility to tailor the material to the specific requirements of many types of applications.

Material Selection Based on Application. For example, a catheter calls for a TPU that is more flexible, with a higher percentage of soft segments to avoid kinks. By comparison, a pacemaker lead requires a TPU with more hard segments to deliver the requisite mechanical strength. The goal is to get the right balance between flexibility and toughness, while still being safe for the body.

Balancing Durometer and Biocompatibility. In long-term medical applications, getting the balance just right between the TPU’s hardness and biocompatibility is essential. While softer TPUs provide great comfort and flexibility, they must be designed to endure ongoing exposure to fluids without degrading. For applications that involve blood contact, such as vascular catheters, polyether-based TPUs are usually the best option due to their durability and resistance to hydrolysis.

Customizing for Enhanced Performance. Suppliers typically adjust the proportion of hard and soft segments in TPUs to find the best balance of flexibility, stiffness, and strength. At times, additives might also be incorporated to improve chemical resistance for devices that are subject to sterilization or to reduce friction so that catheters feel smoother. Enhancing clarity for better imaging is another option. Customizations can go a long way toward ensuring that materials hold up and work well across a variety of tubing applications.

A Material Difference

Achieving the best results during TPU extrusion is about choosing the right material and dialing in the process by tackling routine challenges like moisture control, extruder setup, and pressure management. When these details are properly accounted for, OEMs can benefit from a smoother, more consistent extrusion that meets stringent medical device requirements. A materials expert can offer practical, tailored advice to help fine-tune the process.

This article was written by Anthony (Tony) Walder, Technical Fellow at The Lubrizol Corporation, Wickliffe, OH. For more information, visit here  .

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