This invention relates to valves, more particularly the invention relates to valves made primarily of plastic and which have contractible tubular valve elements.
Plastic valves are known which utilize collapsible plastic valve elements made of perfluoroalkoxy (PFA), see U.S. Pat. No. 5,297,773 to Timothy Collins et al. which is assigned to the owner of the instant invention and which is hereby incorporated by reference. The Collins valve utilizes multiple layers of PFA to form the collapsible valve element. Longitudinal edges of the sandwiched laminae are welded together and the ends of the collapsible portion of the valve element are welded to tubular end portions. The "normal" state of this collapsible valve element is in an open position and is closed by way of a camming member wedging against a clamping portion to pinch the collapsible portion of the valve element closed as desired. Although this valve is quite effective for fluid flows under pressurized conditions, if the fluid flow is in a negative or below atmospheric condition the collapsible portion of the valve member. Therefore, this valve does not have a positive distensible function. Valves such as shown in the Collins patent may be constructed exclusively of synthetic plastic materials.
U.S. Pat. No. 3,791,617 to Press discloses a valve with a contractible valve element formed of extruded PTFE tube. Bosses are machined on the extruded tube and are connected by way of metal connectors to the valve mechanism thereby providing a positive extensible function to the valve permitting it to operate with the fluid flow in a below atmospheric pressure condition. This valve element consists of a single lamina formed of PTFE. Moreover, a single tubular valve element of PTFE provides much more significant resistance to compressing and distending than the same thickness of multiple layers.
In the semiconductor processing industry, fluid flow conduits of PFA are the preferred material. A problem associated with PFA is that welding PFA components together is difficult. Contact welding can cause the PFA to stick to the weld elements and cause surface irregularities. Non-contact welding, such as illustrated in U.S. Pat. No. 4,929,293 to Osgar, which is owned by the assignee of this application, can leave an internal bead which is inimical to regulated fluid flow in a valve and is generally undesirable in many applications. Although PTFE is recognized as an inert substance, PTFE is difficult or impossible to mold into fittings and it is difficult or impossible to weld PTFE components to one another. Moreover, processors often prefer that all wetted surfaces in process system lines be made exclusively of PFA. PTFE is more readily machinable than PFA and has a higher flex life than PFA. U.S. Pat. Nos. 4,929,293 and 5,297,773 are hereby incorporated by reference.
Bosses on PFA contractible valve elements are difficult to manufacture and are prone to failure. Bosses formed on valve elements of PTFE, on the other hand, have been demonstrated to be quite reliable.
Therefore, it would be desirable to have a valve with a contractible valve element formed of multiple laminae that has wetted surfaces of PFA, that has a boss to provide positive distension of the valve element, that has fittings of PFA and that has a smooth beadless weld between the contractible valve element and the PFA valve fittings.