materials feature | Medical tubing
Melt fracture, a common problem of fluoropolymers, can be solved by using larger tooling gaps
Heat treatment And then there is the temperature. Fluoropolymers have high melting points. For instance, PFA has a melting point at about 305°C (581°F). The barrel set points for FEP are in the 600-740°F range, so extruder barrel heaters should be made from cast bronze and rated up to 480°C (900°F). Due to the high operating temperatures, a special dual-layer barrel safety shroud should be used to protect the operator from burns. Fluoropolymers are also highly susceptible to melt
fracture, caused by excessive shear in the die tooling gaps as output rates are increased. This leads to a ‘shark skin’ appearance on the surface of tubing. The typical remedy is to use larger tooling gaps, which means making the tubing with a larger drawdown. While tooling drawdowns of 3:1 to 20:1 are common for many poly- mers, those for fluoropolymers can be 50:1 or greater.
Safety first Correct safety measures must also be considered when processing fluoropolymers, because the gases, vapours and fumes that are released during processing may be harmful to health, according to the Guide to the Safe
Handling of Fluoropolymer Resins, published by the Plastics Industry Association (formerly SPI). These vapours and fumes – which can also be
released when cleaning the die head – can cause flu-like symptoms, but typically pass within 48 hours. The corrosive gases should be captured immediately and removed with an exhaust ventilation system that includes extraction hoods, ducting and a fan. Extraction hoods should be placed above the die head and – for large-diameter tubing – an additional extraction hood over the area where the tubing is cut to length. Sealed packages of fluoropolymer resins should only be opened in well-ventilated areas. Consult the resin supplier’s MSDS for specific recommendations. Graham Engineering will be exhibiting at the
forthcoming MD&M West exhibition in the USA, where it will show a compact modular American Kuhne extruder with two barrel modules – including a high temperature corrosion resistant 1in barrel. ❙
www.grahamengineering.com
lSteve Maxson is business development director for extrusion at Graham Engineering in the US
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