Updated: Feb 25
A Quick Guide to Understanding Your Control Options
Constant Suction Pressure Mode – The Single-Screw Workhorse
Melt pumps (Gear pumps) have been used in plastic extrusion as far back as the 1950s, originally running in open-loop operation where operators manually managed the process. It wasn't until the mid-1980s that closed-loop control systems began to be integrated with melt pumps, transforming them from a useful tool into a precision instrument.
Today, Melt pumps with closed-loop control are one of the extrusion industry's most trusted solutions for stabilizing and improving output — and for most single-screw extruders, constant suction pressure mode is the only mode they'll ever need
Here's how it works: the controller continuously monitors pressure at the inlet (suction side) of the pump and adjusts extruder screw speed to hold that pressure steady at the setpoint. The pump takes over the job of pressurizing the die, freeing the extruder screw from having to build and maintain high head pressure on its own.
That shift in responsibility delivers a number of solid benefits.
Because the screw is no longer working against high discharge pressure, throughput increases by 10–15%. The screw becomes more efficient at what it's actually designed to do — melting and conveying material — rather than fighting backpressure.
On top of that, for every 1,000 psi reduction in head pressure, melt temperature drops roughly 6–7°F, giving you a cooler, more consistent melt with less thermal degradation.
Thrust bearings and screws also last significantly longer because the mechanical load on the extruder is reduced.
Output consistency improves dramatically as well. A well-designed extruder screw running without a pump might hold output variation to around ±5%. With a gear pump running in constant suction pressure mode, that variation drops to ±0.5% to ±1% — a tenfold improvement. For operations running precision profile, sheet, or film, that level of stability translates directly into tighter dimensional tolerances, less scrap, and more consistent product quality.
One advantage that often gets overlooked is resin flexibility. Because the Melt pump is handling pressurization and the controller is managing suction pressure, the extruder screw no longer needs to be precisely matched to a specific resin's flow characteristics. This allows operators to run multiple resins with different viscosities and melt properties through the same screw design without swapping hardware or rebuilding the process. For lines running several different materials, that flexibility adds up quickly in reduced downtime and faster changeovers.
One limitation worth understanding: constant suction pressure mode stabilizes the extruder side of the system, but discharge pressure at the die can still drift over time due to shifts in melt properties or gradual pump gear wear. For most single-screw applications this is manageable, but it's worth monitoring.
For single-screw lines, this mode is simple, effective, and proven. It was the mode for years in my single-screw world, until I entered the Twin Screw Extrusion world.
I initially assumed that constant pressure mode would apply to twin screw systems, but it took me some time to realize this isn't the case.
The Melt Pump mode Primer from PSI Polymer Systems Inc. was instrumental in helping me understand the other melt pump control modes and when they should be applied. I've included it here for your reference.
