There are many manufacturers but there are two main classes of positive-displacement pump
: piston pumps and piston-diaphragm pumps.
The Patriot from Magnum Venus Products, Clearwater, Fla., is a positive-displacement pump
for open and closed-mold processes that reportedly delivers [+ or -] 0.1% ratio control and [+ or -] 1% or better resin flow control.
A positive-displacement pump
delivers liquid additive to a nozzle, which sprays the additive over the solids as they pass through the cascade chamber just above the extruder throat.
Progressive-cavity, positive-displacement pump
produces accurate and constant flow of liquids with only one moving part.
Twin screws are typically fully intermeshing and can act like a positive-displacement pump
for sticky materials or like a valve to stem flooding of aerated materials.