Types of Non-Return Valves: Spring and Smear 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 Valves

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.

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