Advances in ac drive technology have helped a Leeds machine builder win valuable orders from customers in Korea and Japan. Fibre Extrusion Technology (FET) has built two machines to manufacture a high tech suture material for the medical industry, called synthetic absorbable PGA. Sutures made from this material are biodegrade within the human body, whereas previous synthetic sutures, based on mono-filament nylon, were not dissolvable, making them unsuitable for internal stitching. PGA also offers tensile strength, pliability in use, knot strength and a smooth finish. Critically, synthetic suture material must also have a consistent diameter (denier) - and control of denier during manufacture is FET’s speciality.
PGA is manufactured via a melt spinning process, originally developed for the manufacture of polypropylene textile fibres. Following polymerisation, it is extruded through special heated dies. Consistent denier is dependent on the melt pumps, the speed of which must be very accurately controlled. Original versions of the machines used synchronous or synchronous reluctance motors, driven by standard inverters. Now, the more accurate vector control of modern drives has enabled standard 1.5kW induction motors, fitted with encoder feedback, to be adopted instead.
Passing from the die, the fibre is air quenched to solidify and stabilise it before it passes to a series of four pairs of heated draw rolls, each pair of rolls being driven by a single inverter drive. Here again, high speed accuracy and stability is critical, as the accuracy of draw-ratio between roll pairs imparts molecular orientation and governs the material denier. There is a final relaxation to condition the yarn before it passes to a wind-up section where the fibre is ‘laid on’ to a bobbin in an accurately controlled pattern to ensure a stable pack. Managing director, Richard Slack takes up the story.
“We selected the new Danfoss AutomationDrive FC302 for a number of reasons. In addition to accurate vector control, there are a number of features that simplified their adoption and gave us confidence. Not least of these is the the machine set-up remotely. Danfoss is also well represented worldwide so service considerations for Korea and Japan were fully met. From the first evolution of these machines almost 20 years ago, we’ve always relied upon our drive suppliers not only to supply first class products but also to provide a high level of consultation during the development process. Danfoss again met that need in every regard.”