Extrusion basics: To optimize output, run smarter, not faster

My last three extrusion articles in PlasticsToday dealt with inpush, getting the material into the extruder (see links at the end of this article). Now I'll focus on outpush, or output as it's more commonly called.

I'm referring to pounds or kilos per hour, the rate at which the material is coming out of the die. This is not something you read on the data-display screen, but rather something you figure out from line rate and dimensions, or by weighing a timed sample. It's not always easy to get—production people, for example, often think in terms of rolls per shift, which is, after all, what they are doing, rather than any specific rate of production at any one moment. But it is this specific value that is needed to follow the efficiency of the screw (lb or kg/turn) or the efficiency of a gear pump (compare to displacement x melt density).

You can follow the deterioration of a screw or a pump by following output per rpm. If it stays constant over a long period, you don’t have to worry about screw or barrel wear, as the line is “rear-driven” by its inpush, and wear doesn’t matter much. If output/rpm is dropping, then it may be possible to run the screw faster to make up for it. Only when that causes overheating of the melt, and that overheating causes serious problems, must we consider rebuilding or replacing the screw.

Before this, make sure that it’s the screw that’s worn, not the barrel, which might expand and creep due to high pressure inside. It’s embarrassing to buy a new screw and find that it still doesn’t run efficiently because of leakage over flights at the site of the barrel expansion.

The output of an extruder in lb/hr can be estimated by the drag flow: 2.64 x D²Nhρ. Diameter (D) and channel depth (h) are in inches, screw speed (N) is in rpm, and melt density (ρ) is in gm/cc.

This is modified in two ways:

1. Reduced by the resistance to everything after the screw: The screens and contamination on the screens, passages from extruder to die, and the die itself. If there is a gear pump in line, the resistance is just the sum of the screening section and pump inlet pressure setting. Melt temperature has an effect here, as the hotter the melt, the lower the viscosity and the smaller the reduction. Also, the feed end (inpush) has to be able to keep up with the output end; if it can’t, that becomes the limiting factor to production rate.

Internal cooling of a screw also may be a factor. This is occasionally done to improve mixing, but it also reduces output, which may then be restored with higher screw speed.

2. Increased by the overbite in the rear due to compact feed, deep feed channels, less slippery feed and optimal rear-barrel temperature.

Thus, the actual output may be above or below the drag flow. There is no ideal here, and we take what we get based on the two aforementioned factors. Moreover, the extruder may not be the real limit to the production rate, as there are other factors such as cooling and sizing the product that may control it. And before we are too driven by the “faster, faster” image in the form of a production manager who wants it now, we need to remember that faster extrusion may reduce thickness precision and, thus, risk product failure and/or waste resin via the need to raise thickness aim, and the generation of more scrap and trim. Economies of scale do help, as faster operation means less cost per lb for labor and machine payoff, but the material-saving aspect will be dominant.

Finally, don’t forget the need to store the product and sell it. If we can’t sell more, why run faster and make more?

And, as promised at the top of this article, everything you ever wanted to know about inpush is right here: Part 1, part 2 and part 3.

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 e-mail him at [email protected].

Griff will conduct a one-day seminar, Introduction to Extrusion, at the Amherst campus of the University of Massachusetts on May 18. For more information and to register to attend, go to Griff’s website.