Application
Valve and Airlock Selection for Plastics Processing
Answer in brief
In plastics the valve is where pellets get cut and where contamination enters.
A sheared pellet becomes a fine or a streamer, and both end up in the extruder as a defect rather than as product.
- Match rotor clearance and tip design to the pellet, not to a catalogue standard.
- Ground everything, because dry polymer charges strongly.
- Treat wear debris as contamination, not as maintenance.
Reviewed October 21, 2026 · Updated October 21, 2026
The problem
Pellets caught between a rotor tip and the housing are sheared, and the fragments stay in the stream.
Dry pellets also charge strongly, so the valve region is a credible ignition source in a dust-bearing line and a place where material clings rather than discharges.
Desired outcome
A valve that passes pellets without cutting them, holds the pressure boundary of the conveying line and adds no metal or polymer debris to the product.
Process approach
Rotary valves for pellets use bevelled or relieved rotor tips so that a pellet is pushed out of the shear zone rather than cut.
Diverter valves routing between silos or between virgin and recycled streams need a junction that leaves no pocket where the previous grade can remain.
Where powder grades such as PVC resin share the line, the aerated behavior of the powder and the pellet duty have to be covered by the same device.
- Pellet-cut-resistant rotor geometry and the right number of pockets.
- Grounded and bonded components throughout the line.
- Contact materials that do not shed into the product.
Selection factors
- Pellet size, shape and hardness, including regrind if it is handled.
- Permitted contamination level, which is strict in food-contact and medical grades.
- Conveying pressure and the leakage the system can absorb.
- Number of grade changes and the cleaning method between them.
- Static control measures in the whole line, not only at the valve.
Common failure modes
- Pellets sheared at the rotor tip, producing fines and streamers
- Static build-up causing material to cling and discharge unevenly
- Residual grade left in a diverter junction after a changeover
- Wear debris from the housing appearing as specks in the product
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