Extruder Net Output

Subtract the back flow from the drag flow and you have what actually leaves the die. Simple arithmetic, and its value is in what it becomes when you plot it.

Hold the screw speed constant and vary the head pressure. Drag flow does not change — it has no pressure in it. Pressure flow grows in direct proportion to $\Delta P$. So the net output falls as a straight line from the open-discharge value at zero pressure down to zero at the dead-head pressure. That line is the screw characteristic, and a family of them, one per screw speed, describes the machine completely. Raising the rpm slides the line up in parallel, because only the drag term moved.

Now put the die on the same axes. A die passes more melt when you push harder, so its own characteristic rises from the origin — roughly linearly for a Newtonian melt, and curving for a shear-thinning one. Where the two lines cross is the operating point, and the machine finds it by itself: at a pressure below the crossing the screw delivers more than the die can pass, so pressure rises; above it, the reverse. That intersection is the single most useful picture in extrusion, because it explains at a glance what happens when you change something. A more restrictive die rotates its line up and to the left, moving the operating point to higher pressure and lower output. A shallower screw makes the characteristic flatter, so the same die change costs less output — that is what "pressure-insensitive" means, drawn rather than asserted.

Where a healthy line usually sits: the pressure flow somewhere between about 10 % and 30 % of the drag flow. Below that band the screw is running nearly open-discharge, which sounds efficient and mixes badly — there is no pressure gradient to drive the cross-channel circulation that distributes colour and additives. Above it, the output is hostage to every change in melt temperature and every screen change, and the operator spends the shift chasing gauge.

There is a third flow this equation leaves out, and it grows with the machine's age: leakage over the flights. The clearance between the flight land and the barrel is small on a new screw — a fraction of a millimetre — and melt squeezes back through it under the same head pressure that drives the channel back flow. Wear opens that clearance, and because the leakage goes as the cube of it, a screw that has lost a few tenths of a millimetre has lost a great deal more output than the number suggests. Nothing on the control panel changes. The symptom is an extruder that used to make its rate and now does not, and the diagnostic is exactly this equation: calculate the drag flow, measure the real output, and watch the difference grow year over year. That is a genuine condition monitor and it costs nothing to run.