Extrusion basics: The relationship of inpush to output, part 2

Can a screw bite off more than it can chew? You bet, as they say in Vegas, but what does that mean? It means that the first flights try to push forward more than the last flights want to pump out. Of course, in a continuous operation what goes in, must go out, so a compromise is reached: The inpush is held back, while the output is given a boost from behind. The displacement in the feed zone of one turn of the screw, which I call “perfect bite,” can be estimated by the following formula:

Perfect bite in lb/hr = 6.13 (D-h)DNhρ, where

D = diameter, inches

h = channel depth in feed zone, inches

N = screw rpm

ρ = bulk density, g/cc

The dimensions are easy enough to use, although sometimes we don’t know the channel depth in the feed zone, and it may be inconvenient (especially in a big machine) to pull the screw and measure it. (That’s a good thing to do when it is being cleaned or otherwise idle.)

The bulk density can be determined with a cup and a scale, by dividing the weight of plastic in the cup to the weight of water at the same level in the same cup. If you want a quick fix for pellets, take about 2/3 of the solid density.

When you solve this equation, you get astronomically high results. For example, a 2.5-in. screw with a 0.450-in. feed depth, running at 80 rpm on polyethylene pellets, bulk density 0.63, gives a PB of 713 lb/hr! That’s far more than anyone can expect from that machine, but that’s why it’s called “perfect” bite. Nothing is perfect, and here it isn’t even close. The pellets don’t all stick to the barrel wall (remember this), nor do they all slip freely on the screw root; they roll around on each other, too. (See part 1 of "Extrusion basics: The relationship of inpush to output," where I explained my coffee can test).

We need to apply a “reality factor,” which we can estimate from past experience and knowledge of the material. High-friction pellets, such as polyurethanes and some elastomers, can have factors as high as 0.40. I use 0.35 for PPVC, 0.30 for HIPS and UPVC, 0.25 for LDPE, 0.20 for PP and 0.15 for HDPE, if I have no other data to help me.

Grooved barrels, which are common for HDPE, make up for that low reality factor, but that’s for another “Extrusion Basics” article.

Going back to pellets sticking to the barrel wall, here is where we have some control via rear barrel temperature (RBT). This is the extrusion tech’s aspirin tablet: If you are not sure what is causing a problem, change RBT one way or the other. Bob Gregory did a landmark experiment (ANTEC 1983), where he reported the coefficient of friction for ABS dragged over a steel plate, varying both the temperature of the plate and the plastic. He clearly showed that there was a sharp increase in material sticking to the barrel wall (the steel plate) as the wall temperature passed through the melting range, but that it later peaked and dropped as the plastic melted to a low viscosity on contact (ice-skater’s effect). So, there was an optimum wall temperature where sticking (hence inpush) was the highest.

However, if the output end can’t pump out what goes in, a pressure peak develops somewhere around the entry to the metering zone (the “tollbooth”). This means leakage over the flights is in the forward direction from the peak to the screw tip, and that’s why flight wear there may not matter and may even be helpful. You can tell if you have such a pressure peak if the output is near or even above the drag flow; computer calculations can support this conclusion, as long as the viscosity/shear/rate/temperature data are reliable.

One more sign: The high pressure at the “tollbooth” leads to high temperature there, and the screw may turn an iridescent blue, or some other color based on the trace metals in it (the “blue screw syndrome”). If your screw is blue, it isn’t necessarily bad for the screw; steel melts around 2600 to 2800°F, far above anything generated in a barrel. The greater concern is the overheating and degradation of the plastic, so measure your melt temperatures and keep your cool.

Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years, as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or email him at [email protected].