machinery feature | Medical tubing German researchers


have devised a method to


make silicone rubber medical tubing using a heated die


IV and fluid delivery applications. Products ranging from 2mm to 10mm OD can be produced with an accuracy of +/- 50 microns at line speeds up to 100m/min. The company’s facility in Pawcatuck, Connecticut in the US also has a fully equipped medical tubing laboratory. Both labs support applications for alternate polymer,


microbore tubing, multi-lumen and catheter tubing, edotracheal and tracheotomy tubing, radio opaque tubing, bubble tube, taper tube, pipette tubing and multi-layer tubing, among others. Turn-key medical tubing systems support extruder outputs up to 700lbs (315kg) per hour and line speeds up to 800ft (240m) per minute for a range of materials.


Tooling for tubing Researchers from the Technical University of Munich in Germany have devised a method to make silicone rubber medical tubing using a heated die. The method is an alternative to curing the tubing outside the die – which insists that the viscosity of the uncured silicone is high enough to be dimensionally stable. “Even with high viscosities, achievable precision and


tolerances are limited due to this fact,” said the researchers, in a paper presented at the recent Antec conference in the US. The new process relies on a heated die to vulcanize


the extrudate inside the die. The work has derived a formula for the flow rate for an in-die-curing silicone extrusion, in order to describe this process mathematically – and be able to predict the capabilities of this technology. For the extrusion process, an extruder was modified


Davis-Standard has medical tubing lab lines in both the USA and China


for silicone extrusion. It was used with a standard three-zone screw, with screw speed controlled by a variable-frequency drive (VFD). Experiments were conducted at die temperature at (or above) 200°C, and VFD frequencies up to 2Hz – as these conditions caused fast enough curing.


“In-die curing is unusual for silicone extrusion, but


would enlarge the possibilities for extruded geometries and the use of special silicones,” said the researchers. Comparison of the measured and calculated values


showed good correlation – though the researchers admitted that the model had several limitations. “The model gives a small hint on the effects of


several parameters – and can be used for future estimations in LSR extrusion – but it will not replace experimental testing,” they concluded.


Graphic detail At the same time, researchers at the University of Minho in Portugal have used graphics processing units (GPUs) to improve the process of designing complex profile extrusion dies – including one for producing a medical catheter. The time needed for this type of calculation is


normally long, but the researchers parallelised the numerical code in the GPU using a simple programming approach, that required no complex memory manipula- tion. The code was used to design two real-life extru- sion dies – to make a wood-plastic composite decking profile, and a medical catheter. One of the main challenges in extrusion die design is


to achieve balanced flow at the flow channel outlet, said the researchers. Numerical codes can help to overcome this, by cutting the amount of trial and error that is normally required. These types of simulation tool can help the designer to improve die channel geometry – using either numerical-based trial and error proce- dures, or automatic algorithms that search for an optimised geometry. Medical catheters can comprise several channels


(‘lumens’) with different diameters that run along the length of the profile. The catheters have a constant cross-section, and are extruded from medical grade materials. The researchers made a five-channel catheter from


14 PIPE & PROFILE EXTRUSION | September 2016 www.pipeandprofile.com


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