Other
Types of Non return Valves
Spring
(or Poppet) Valves
A spring (or poppet) valve uses a rod type mechanism with an enlarged
head to close off the valve. The pressure of the melt forces the head
away from the opening allowing the melt to flow through. The rod is
attached to a spring at its base which pulls it back, closing the valve,
as soon the screw stops and the pressure of the incoming melt is halted.
This method closes the valve before (and independently) of the injection
thrust.
Historically, these valves have been unreliable in shut-off consistency.
Newer spring valves claim to have improved upon past problems, but have
not yet gained mainstream acceptance. While excellent performance potential
exists in this type of design, long term reliability has not yet been
proven.
Smear Valve
A smear valve is essentially a solid cone shaped valve that is used to
process extremely viscous materials such as rigid PVC or thermoset
plastics. As the material leaves the screw it moves between the surface
of the valve and the end of the barrel into the shot cavity. There
is no shut-off mechanism in this type of valve because the viscosity
(or thickness) of the material itself prevents it from flowing back
into the screw.
Choosing
the correct steel
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