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Medical tubing | materials feature


High performance fluoropolymers such as ETFE offer great promise for medical


tubing, but to take full advantage of them they must be processed carefully, says Steve Maxson


Top tips for extruding medical tubing from fluoropolymers


Fluoropolymers such as FEP, PFA, PVDF and ETFE are known for their excellent range of physical properties. As a consequence, they are promising materials to use in many medical applications, including micro tubing for intravenous (IV) catheters and insulin infusion, sin- gle lumen and multi-lumen tubing for endoscopy and cardiology devices, and heat shrink tubing used as a manufacturing aid for catheter reflow purposes. However, care must be taken when working with


these materials. Here, we have listed some important considerations


regarding extrusion equipment and safety related to processing fluoropolymers.


Corrosion protection Firstly, deal with corrosion. Components that come in contact with molten resin – such as the die head, tooling, adapter, breaker plate and screw – should be made from corrosion-resistant metals with high nickel content, such as Inconel 625 or Hastelloy C276. For protecting the extruder barrel, a tungsten carbide/ nickel-based bimetallic barrel liner such as X800 is recommended. For small medical extruders – 1.25in (32mm) or


smaller – the entire backing material of the bimetallic barrel is solid Inconel, while the liner material is made from X800.


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A typical fluoropolymer screw has a single-flight design with a 3:1 compression ratio. In some cases a low-shear Saxton-type distributive mixer is included at the end. Although Inconel and Hastelloy are corrosion- resistant, these speciality metals have low torsional strength compared to common screws – which are made of 4140 or 4340 heat-treated steel. For instance, Hastelloy 625 has a yield strength of around 52,000psi (359MPa) – about half that of 4140 steel at 28-32 Rc. Inconel 625 is a little stronger than Hastelloy C276 but less corrosion-resistant. Take care to avoid screw breakage when using an


Inconel or Hastelloy screw for high temperature, non-corrosive polymers such as Peek, Polysul- fone (PSU), and Polyetherimide (PEI). For example, a 1in (25mm) extruder with a 5HP motor geared for 100rpm screw speed is likely to snap a screw made from Has- telloy 625 during a cold start situation.





Take extra care when processing fluoropolymers


Fluoropolymers are ideal for medical


applications such as


catheter tubing


Steve Maxson, business development director for extrusion, Graham Engineering


” March 2017 | PIPE & PROFILE EXTRUSION 43


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