Extrusion basics: The relationship of inpush to output, part 3

In parts 1 and 2 of this series, I showed the effect of particle shape and lubricity (the Maxwell House coffee can test), explained “perfect bite” and its relation to the real world and stressed the importance of rear-barrel temperature. In this, the third and final installment on the importance of inpush, I’ll tie up some loose ends: The ice-skater’s effect, grooved barrels, sticking to the screw root, second-zone surge and feeding the four fluffy Fs.

Ice-skater’s effect. If the rear barrel gets hot enough, the incoming pellets won’t stick like they should but, instead, will melt on contact and slide around on that surface, reducing inpush. This is called the ice-skater’s effect because the melting point of ice decreases with pressure, so that the ice melts under the skater’s weight and, thus, lubricates the sliding blade. That explains the speed at which the racers go and the force that hockey players have when they slam into each other.

Grooved barrels. This concept was developed in Germany in the 1970s to get a lower melt temperature—hence, faster output—for HDPE blown film. The grooves (see photo with ¼ barrel cut away) direct the flow downstream, allowing a much lower rpm for the same production rate; a lower speed reduces overheating downstream in the mixing/metering zone of the screw. To do this, the first (feed) zone must be cooled enough to prevent melt from sticking in the grooves; it is typically a separate barrel section flanged to the main barrel. Unique screws have been developed for this process, including mixing assists in the last zone and/or afterward, as channels in that zone are deeper and mixing is consequently less intense. The concept works remarkably well for low-friction HDPE pellets. It does help with other resins, but to a lesser degree, and, therefore, is seldom used.

Sticking to the screw root in a single-screw system will block free movement of particles in the feed zone, as there is nothing to scrape or pull them off once they are stuck. It’s very much resin-dependent, with some materials more susceptible than others. A common cause is stopping with the barrel full. Remedies include purge compounds, chunks of the same material and mechanical poking with notched, flexible heat-resistant strips. Steel strapping works well, but has been criticized for potentially scratching the root surface. Aluminum and brass will avoid this, but are not normally present in a factory unless bought for that purpose. The ultimate remedy is cooling the screw root with water (sometimes oil).

Second-zone surge is a pulsation of output that originates in the second zone, or wherever the feed zone ends and channel depth starts to decrease. Melt is forced up into the spaces between the pellets, and the resulting domains stick to the root, temporarily blocking flow until continued pressure and conduction loosens and disperses them, increasing rate again. Think chunky peanut butter with a lot of chunks and less butter. The cycle will repeat itself every 30 to 90 seconds, and can be followed on the motor ammeter as well as a direct measure of linear mass flow. This is a nasty problem to resolve, as it may require more heat in the first zone or even preheated feed; sometimes only a slower rate will gain the steady output necessary for uniform thickness. This behavior is often sensitive to screw design; barrier screws have a good reputation for discouraging second-zone surging, but before you go off changing screws, try the other remedies first. If they don’t work, make sure any proposed design has been tested on your material and maybe also by computer simulation. The ultimate remedy here is a gear pump.

The four fluffy Fs are film, foam, filaments and fibers. None of them feed well in a conventional hopper, so when they are (re)used as feed they need to be pushed into the screw or otherwise densified. Most materials will feed well if their bulk density is at least half their solid density (this also varies with particle shape, as flat flakes won’t flow as well as more uniform particles). There are hoppers with screws and stuffers, and other systems with no central hopper at all but, rather, separate feeders (by weight or volume) that drop into a common chute. There are also machines that just densify fluff, but the best solution may be to reprocess it through another extrusion. Advantages include another filtration step and a really uniform, dense product, while disadvantages include possible discoloration and molecular breakdown, plus the cost of the additional operation. The breakdown (degradation) may be avoided by using additives, if it affects the finished product enough to justify the added cost. PVC is most likely to need this, while HDPE is essentially immune because of its crosslinking rate.

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].

In association with the Society of Plastics Engineers, Griff will present a one-hour webinar, Plastics Chemistry for Non-Chemists, on April 26. Click here to register. He is also scheduled to 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.