A check valve comprises a valve seat and a valve body pushed onto the valve seat. Thus, the check valve allows the passage of only a fluid which forces the body away from the seat. Any fluid that flows in the opposite direction applies back pressure to the body, pressing the body onto the seat, and is therefore not allowed to pass through the check valve.

In the case of a check valve wherein the valve body is pressed onto the valve seat under its own weight, the body is readily moved away from the seat when it is applied with a pressure slightly greater than its weight is applied to it. CHECK VALVESThe valve body is more easily moved away from the valve seat when the check valve is positioned slantwise, in which case the check valve fails to perform its function completely. Therefore, most check valves have a valve spring located at the back of the body and forcing the body onto the valve seat, thus holding the body in contact with the seat.

The operation pressure required to move the valve body from the valve seat can be set to any desired value, merely by using a valve spring having an appropriate spring constant. Hence, it is easy to design check valves having different operation pressures, by using valve springs of different constants.

Generally, compression coil springs made of metal are used as valve springs in check valves.Check Valves The valve springs made of metal are likely to be corroded since they remain in contact with the fluid flowing through the check valves. Consequently, the kinds of fluids are limited, whose flow a check valve having a metal spring can control.

Accordingly, check valves have been invented, wherein the valve body and the valve spring are molded in a unit made of plastics, thus dispensing with a metal valve spring. Check Valves As is disclosed in, for example, Japanese Utility Model Provisional Publication No. 137763/81, a conventional check valve has a valve spring which is a hollow cylinder made of plastics and integral with a valve body. When the valve spring is plastically deformed, and its cross section changes to an elliptical one, it pushes the valve body onto the valve seat. As is disclosed in, for example, Japanese Utility Model Publication No. 31965/82, another check valve has a plurality of corrugated leaf springs made of plastics. Since these check valves have no metal springs which are corroded with some fluids, they can be used to control the flow of various kinds of fluids. Further, since the check valve requires no springs independent of the valve bodies, they comprise less components and can be assembled more easily than otherwise.

However, to mold a body-spring unit having a hollow cylinder or a plurality of leaf springs, a relatively complex metal mold is required, which is no doubt expensive. Therefore, the check valve having the body-spring unit can hardly be manufactured at low cost.

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