When I talk about poppet extrusion, I am invariably talking about the head of the poppet flowing into the valve throat, like this.... This is a Delrin MRod poppet used in a 0.281" ID throat at 3000 psi, which is too much load....Note that three things have occurred.... the seat has pounded into the poppet head, flaring the material out behind that, and in addition the Delrin has extruded into the throat.... This does not occur with this poppet if the throat is reduced to 0.266", or if the pressure is reduced to below 2700 psi, or some combination that reduces the load in a similar way.... I have never pushed a PEEK poppet far enough to see a similar failure, even in my .457 Hayabusa at 3600 psi (.500" poppet OD, 0.422" throat ID)....The situation that Scott is describing, where the seat flattens or widens if it starts as a knife edge is a different form of distortion, caused when the loads exceed the compressive strength of the material.... If you start with a true knife edge seal (inside taper, outside taper, or a concave poppet on a flat body), then even small loads will flatten out that knife edge until the surface area of the seal is wide enough to support the loads.... The higher the compressive strength of the material, the narrower the required seat would end up, for a given force on the poppet when closed.... Hence, PEEK can use a narrower seat than Delrin, which would be in turn narrower than Teflon.... If you are really interested in how narrow a seat you can use with PEEK, in a given gun, simply make the poppet head slightly concave, running on a flat valve seat, fire a few rounds, disassemble and measure the seat margin.... Presto, you have your answer.... Regarding the 'hammering" of the poppet on closing, that would be a function of the closing velocity, the poppet mass, and the modulas of elasticity of the poppet/seat material.... It becomes a matter of how much mass you are stopping in how short a distance.... Compared to metal on metal (hammer on nail), however, even PEEK compresses a significant amount when the poppet hits the seat, slowing the poppet gradually to a stop.... gradually in this case being a few thou of compression.... The softer the material (lower modulas of elasticity), the further it compresses, and the LOWER the peak force during the brief time it is decelerating.... I have measured the compression on a Delrin poppet in a Disco at about 0.010" at 2000 psi, for example.... You can calculate the peak deceleration, and hence the peak force, during this part of the closing cycle (when the poppet is in contact with the seat and decelerating).... I have not done so, but I believe it to be less than the force on the poppet when closed.... If this were not the case, I think we would see most valve stems coming out of the poppet they are (usually) just pressed into, and (generally) we don't.... So, I don't have an exact answer for you, but in general terms, if you keep the compressive load on the poppet/seat material less than the compressive strength (related to but can be greater than the tensile strength), I sincerely doubt you will have any problems.... My Hayabusa .457, doing that calculation, works out to 12,500 psi, and that has not been an issue with PEEK.... Note, that if you use a tapered poppet or seat, or a concave one, if the load ends up too great, the seat will just widen slightly until a balance is reached.... Bob
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