Extrusion basics: Screw design essentials you learned a long time ago, but maybe forgot

I teach a one-day Intro to Extrusion seminar, and many of my participants are new to extrusion. Therefore, I explain the basic principles of screws for thermoplastics. That’s good for the newbies, but sometimes I get more experienced extrusion people thanking me for these basics, which they had never learned way back when. So here goes. I hope you all find it useful.

The screw is a conveyor. As it turns, it tries to screw itself backward out of the barrel, but a bearing keeps it from going out the back. Because every action has a reaction—remember Isaac Newton?—it pushes the other way, too, in the forward direction, and that’s what pushes the material out of the die.

Material needs to get soft to go through the die. Any thermoplastic will get soft and moldable (plastic) with heat (thermo). The feed is sometimes preheated (usually for drying), but it gets most of the heat from internal friction as it moves against the barrel walls and screw surfaces. The clearances from flights to barrel are where most heat is generated. Exceptions: Some twin screws, small machines, high-temp resins and PE coatings, where barrel heat is important, too.

Three-zone principle. The screw starts with a feed zone: Constant depth, takes up 15 to 30% of length. In the middle is the compression zone, walls close in on the melt/pellet mix, drive air backward and make up for slipping and rolling of pellets in the feed zone. This zone contains the “barrier” of barrier screws: A long double-channel section that separates the melt from pellets, so that pellets can rub against each other to generate more heat, rather than swim in the increasing volume of melt and just heat by conduction. Finally, at the output end is the metering (pumping) zone, channel depth constant again at 25 to 50% of feed depth, often with straining and mixing elements (Maddocks, pineapples, pins).

Channel depths, and not just their ratio, are critical. In small machines, the feed must be deep enough to allow smooth feeding (at least twice the particle size), but not so deep as to risk screw-shaft breakage. In the metering zone, shallower means better mixing and less output per turn, while deeper means the opposite, as well as more sensitivity to high pressure.

Length: The usual metric is the ratio of length to diameter, or L/D, also written as L:D. Today, 24:1 is standard, 20:1 is short (know the reason why) and 25 to 30 also are commonly seen. The longer the length, the more time to melt, which usually increases the output, but at a higher melt temperature. Longer lines have been built and are needed for vented extrusion, but otherwise the tendency is to go larger (cooler) instead of longer.

Vented barrels have a hole in the barrel to remove moisture and trapped air (as with powder feed). The screw gets very deep at that point to avoid pushing melt out of the vent, to which a vacuum is applied, and then gets shallow again to pump out the melt.

Pitch (angle) of the flights is often square: That is, the distance from one flight to the next is the same as diameter. This corresponds to a helix angle of 17.6° if the channel is “unwrapped.” This angle is increased in many barrier sections, and a few feed sections for light feeds.

Flight thickness is around 0.1 x diameter. Thicker means more area for heat development and less conveying per turn (both usually unwanted), while thinner results in more backward leakage (less pumping but more mixing).

Hollow screws. Many screws are bored full-length to allow passage of either water (helps mixing), oil (avoids tip degradation of rigid PVC) or even air (rare but cheaper). A few screws are bored only one-third down to prevent sticking to the root in the feed zone.

Radius of channel corners. Too small encourages stagnation and potential degradation; too large wastes channel volume. No one formula fits all: It depends on thermal stability of material, flow rate in channels, use of purges and metal-adhering process aids, and screw surface material.

Unusual variations include grooved barrels to increase inpush per turn (very common for HDPE, screws have little or no compression, mixing devices recommended), and channel-depth cycling in parallel channels to improve mixing and uniformity (wave screw).

Materials. Most screws are machinable steel with hardened flight surfaces, either via a welded-on cap around 0.040 to 0.080 in. (1 to 2 mm) thick or by nitriding the whole surface. The latter method is cheaper, but prevents alteration of the flight depth later; the useful life depends on the depth of nitride penetration. Chrome plating is common: The screw surface certainly looks better and is claimed to allow easier passage (less frictional heat) and less likely degradation. For abrasive and corrosive feeds, more expensive metals are available.

Computer simulation of screw performance is widely done, and is not new. I demonstrated it on a DEC computer (20 MB hard drive!) at my seminars back in 1987 to 1992, after which it became too complicated for an intro class. Today’s programs are good, but success depends on reliable viscosity data as a function of both temperature and shear rate. Would I make a screw based only on simulation? No. Would I make one based on my own experience alone? Not if I could help it. I’d want to combine the two, if the line was big enough and I had reliable viscosity data.

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 the fall, Griff will present his one-day practical seminar, "Introduction to Extrusion," in Chicago on Sept. 19, Los Angeles on Nov. 15 and Houston on Dec. 5. Topics include the ten (11) key principles of extrusion, plastics chemistry for non-chemists, a review of extrusion hardware, the limits to production rate, quality control of raw materials, simplified rheology, start-up and shut-down procedures and troubleshooting common extrusion problems. E-mail him at the address listed above for more information.