claw vacuum pumps (Fig. 3) are now installed on each of the three extruders. NSW opted for this vacuum technology because it had already been used for many years in pneumatic material feeding to the extruders with positive results. Mink claw vacuum pumps are used in this process to generate the vacuum for suction conveying of the granulate. This has shown how reliable and economical Mink vacuum pumps are. The Mink MV sizes now used for hose extrusion operate completely redundantly and are controlled in such a way that they are operated alternately. Due to the length of the cables produced, an extrusion process can take up to one week. This means that during this time the vacuum is permanently maintained and the preset ultimate pressure is precisely maintained. Leaks during the feeding of the strands into the extrusion mould can cause higher or lower amounts of air leakage. The frequency-controlled drives of the Mink MV vacuum pumps ensure that the set point is maintained by changing the speed. Manual control is no longer necessary. Each individual vacuum pump is preceded by a separator that makes sure no entrained plastic particles can enter the vacuum pumps. The drive power of the Mink MV vacuum pumps is 2.1 kilowatts each, which is considerably lower than that of the liquid ring vacuum pumps with 7.5 kilowatts each previously used.
 


Fig. 3: Mink MV claw vacuum pump with integrated frequency controller for demand-dependent control of ultimate pressure and pumping speed. Source: Busch Dienste GmbH

The decision to install the new Mink MV vacuum pumps in May 2017 has proven to be the right one. The six Mink MV claw vacuum pumps operate completely trouble-free and efficiently. Maintenance is limited to emptying or cleaning the upstream separators and to an annual gear oil change on the vacuum pumps.