Extrusion basics: Pumps and lumps, or life between the screw(s) and die

There are no toxic plastics. None of them. I’ve been starting my monthly column that way for a while, so I hope you’re already spreading this gospel to your families, friends and neighbors. To do this you have to know it, not just believe in it. The plastophobes believe the opposite; we have science on our side, but that scares people, too.

Gear pump for extrusion
Demo-scale pump with clear plastic housing. Image courtesy Allan Griff.

Now to extrusion technology. The pumps I’m talking about are gear pumps, devices that pick up the melt as it leaves the extruder after it has gone through screens and push it forward through the rest of the system—adapter and die, and maybe a static mixer (more on that later). 

They are called gear pumps because they consist of two intermeshing gears; typically the width across the gears is the same as the diameter. They turn in a tightly fitting chamber so that the entering melt gets caught in the teeth of the gears and is carried around between gears and chamber walls to the other side. Nothing goes through the nip of the gears. That nip is virtually sealed because one gear drives the other. Controlling the speed gives precise volumetric displacement, so that whatever the variation in entry, the output is constant in axial flow rate. No surging, no pulsing (from extruder sources). That is the main reason for their use. Their added cost can be justified by savings of material because aim thickness can be lower if the precision is better. Another benefit is reducing the load on the extruder, allowing high-resistance dies without high pressure and consequent high heat development in the screw(s).

The industry often refers to them as melt pumps (melt in, melt out) but I prefer the term gear pump, because an extruder is a melt pump, too, even though it’s pellets in, melt out. 

In operation, the inlet pressure to the pump is preset (e.g., 800 psi) and the screw speed in the extruder is controlled to maintain that pressure lest the pump be starved, which would damage the pump as its drive shaft is lubricated by the melt itself.

Buying a gear pump requires some idea of the pressures expected, as they are sold to match these needs: Light, medium or heavy, referring to the expected differentials across the pump and from inside to outside air. They are sized to match expected output, and the maker should therefore provide the free volume of the pump (cubic centimeters per turn for both gears). From this volume, we can calculate the weight per turn if we have a melt density.

That is the theoretical maximum output. In practice it is slightly less, as some melt is recirculating as shaft lubricant, and may be further reduced by backflow over adjacent teeth and along the gear sides if walls are too thin for the pressure differential (clamshelling).

As a rough rule, if you are getting 90% or more of the theoretical (free volume x rpm) that’s very good—go look for another problem to solve. If you are between 80 and 90%, that’s normal, but in the low 80s you aren’t getting the full benefits. If the ratio of production to theoretical is in the 70s, think about rebuilding or repairing, but first, find out why it’s so low. And if you are below 70%, you may be better off taking the pump out now.

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