18.104.22.168 COF flanges
COF flanges are used for large nominal diameters for ultra-high vacuum-compatible connections. They are bakeable, suitable for pressures up to approx. 1 · 10-12 hPa and not standardized, so that when combined with other-brand flanges, the compatibility must be tested. They are usually made of stainless steel with a low carbon content, if possible, in a forged quality.
A COF connection consists of a pair of flanges with different “male” – and “female” profiles, a wire seal and a sufficient number of screws, which provide the required high contact pressure (Figure 3-16). The flanges can be aligned with a hole spacing around the main hole axis. A welded wire ring from oxygen free copper OF (Oxygen Free) is used as a sealing material.
Figure 3.16: COF connection with copper wire seal and screws
During assembly, the seal is tightly inserted on the cutting edge of the “male” side. The sealing surfaces of the two profiles enclose the copper ring. A extrusion takes place on the cutting edges. The cold flow is limited by the vertical inner wall of the “male” flange, so that very high pressures are created in the boundary layer. Under the high pressure, the copper of the microstructure adjusts to the cutting edges and to fills small surface defects, which creates a metallic ultra-high vacuum-tight connection. Previously used copper wires cannot be reused. For pressures up to approx. 1 · 10-8 hPa, seals made of FKM can be used. They are usually not reusable, but allow faster assembly, for example, if flanges are mounted frequently, e.g. when leveling the system.
During assembly, it is important that the screws are initially tightened uniformly diametrically, to avoid any tension. Subsequently, the screws should be tightened in sequence, in several passes, step-by-step until the copper is connected to the sealing surfaces in an ultra-high vacuum-tight manner. When baking out, it must be ensured that the heating and cooling are smooth and not done too fast. Temperature differences on the flange connection lead to tensions, which may cause leaks.