The use of liquid nitrogen cryopumps when vacuum metallizing plastic materials

Abstract

To provide a sufficiently low gas pressure (<5.10 −4 torr) for metal-finishing plastic components by vacuum evaporation, vacuum pumps of high mass throughput must be used because of the high degassing rates of most polymers. The pumping performance of a vacuum system used with volatile loads can be enhanced by condensing desorbed vapours on a cold trap mounted in the vessel. Such traps also pump non-condensable gas if it becomes entrained in a growing vapour condensate. In an early application of the technique the writers used a flask-type trap containing liquid nitrogen. The flask could be retracted into a vacuum enclosure thereby forming a Dewar flask for refrigerant storage. More recently a tubular trap (Meissner type) with continuous refrigerant circulation has been used with a device for rapidly defrosting the coil on atmospheric exposure. The combined performance of a liquid nitrogen cryopump and booster diffusion pump is considered in terms of the equilibrium pressures and residual gas mass-spectra obtained under given desorption conditions. The design and performance are reported of a vacuum metallizing plant with a vessel of 72 in. diameter × 72 in. in length which is exhausted by a liquid nitrogen cooled cryopump with a maximum ideal pumping speed for water vapour of 203,280 l./sec.