Shrinking of bumps by drawing scintillating fibres through a hot conical tool

The LHCb SciFi tracker will be based on scintillating fibres with a nominal diameter of 250 $\mu$m. A small length fraction of these fibres shows millimetre-scale fluctuations of the diameter, also known as bumps and necks. In particular, bumps exceeding a diameter of about 350 $\mu$m are problematic as they can distort the winding pattern of the fibre mats over more extended regions. We present a method to reduce the diameter of large bumps to a diameter of 350 $\mu$m by locally heating and pulling the fibre through a conical tool. The method has been proven to work for bumps up to 450 – 500 $\mu$m diameter. Larger bumps need to be treated manually by a cut-and-glue technique which relies on UV-curing instant glue. The bump shrinking and cut-and-glue processes were integrated in a fibre diameter scanner at CERN. The central scanning and bump shrinking of all fibres is expected to minimise bump related issues at the four mat winding centres of the SciFi project.