An In-Depth Look at Extrusion - Part 2

Last month an overview of the extrusion process was given.  The components of the extruder and their functions were outlined, and a brief introduction of the main film-making processes were given.  Rather than start at the beginning of the extruder and discuss each component sequentially, this article will discuss the heart of the extrusion process.  The extruder screw, and the barrel which contains it, is the core of the extrusion process.  All other components of the extruder support the screw and barrel.  The first, and probably most critical, function of the extruder screw will be reviewed in depth, namely feeding, compacting and melting.

The purpose of the extruder of course is to convey, melt and pump plastic to the die.  The extruder is more or less indifferent to the downstream equipment.  This first step quite often determines the output, output stability and melt quality at the discharge end of the extruder. 

Single-screw extruders are generally divided into three sections, the feed, transition, and metering sections, as shown in Figure 1 below. The feed section brings plastic pellets (or powder) into the extruder barrel.  The volume of the first flight times the screw speed time the bulk density times 60 is the output per hour. The transition section is where melting theoretically begins, and contains both solid and liquid polymer.  In the metering section of the screw, melting is theoretically complete, and the molten polymer is simply pumped out of the extruder into the die.  Optional zones not shown in Figure 1 are a vent zone, a dispersive mixing zone and a distributive mixing zone.

Figure 1.  Extruder Screw Nomenclature[1]

As a reference for all future articles on this subject, Figure 1 is given showing commonly used extruder screw nomenclature.  This well-established schematic dates back to the Golden Age of Plastics from Tadmor & Klein, Bob Gregory, Bruce Maddox and others. 

A hopper rests on the barrel above the first flight, and the feed pocket brings the plastic pellets into the extruder barrel, conveying them downstream to the transition section.  The channel depth steadily decreases in the transition section, reducing channel volume, which in turn causes a restriction in the forward path of the pellets.  This restriction simultaneously compacts the pellets closely together until there is no free volume between them, to what is called “the solid bed”.  Tremendous pressure is creating during this compression section of the screw.  The solid bed moves as one mass, creating intense friction between the solid bed and barrel wall.  The coefficient of friction (COF) between the pellets/solid bed is largely responsible for the solids conveying angle in non-grooved, single stage, square-pitch screws.

The solids conveying angle can be thought of as the angle at which the solid bed transverses down the screw helix towards the discharge end of the extruder.  Two coefficient of friction conditions are shown in Figure 2 below.  This diagram represents what is called the Slip-Stick Model.  If the COF between the pellets and barrel is zero, and the COF between the pellets and screw is one, the pellets will slip on the barrel and stick to the screw, which results in zero output.  With the reverse situation, if the COF between the pellets and barrel is one, and the COF between the pellets and screw is zero, the pellets will stick on the barrel and slip on the screw, which results in a maximum solids conveying angle and maximum output.  The flow in the extruder barrel is helical, and cylindrical coordinates are native to this flow.  Additionally, the thermodynamic conditions in the extruder are non-steady state, non-isothermal.  The mathematics are cumbersome and beyond the scope of this article, but suffice it to say that if the pellets stick to the barrel, the screw will wipe it downstream as quickly as possible, resulting in maximum output, albeit poor melt quality. 

Figure 2.  Two forms of solids conveying in a single screw extruder[2]

Bob Gregory, formerly of Egan Machinery, then a consultant to the industry, proved in the 1960’s that the COF is a maximum at the DSC (differential scanning calorimetry) melting point of the polymer plus 15°C.  This is thus the ideal feed zone set point temperature.

With proper process conditions and screw design, the air between the pellets is forced backwards and out the hopper before melting begins.  Melting begins on the barrel surface.  When the melt film becomes thicker than the radial flight clearance, a melt pool is formed.  As the polymer moves downstream, the melt pool accumulates on the leading edge of the screw flight, growing with each revolution.  This is depicted in Figure 3 below.

Figure 3.  Melting Mechanism

The elimination of air, and thus oxygen, in the feed section prior to melt initiation is critical.  If a melt film forms before all the air is eliminated, then the air will be forced forward.  Air is relatively compressible, so variations in output increase with increased air intake.  Pressures at the end of the compression section in industrial extruders can reach 450 bar, which combined with high temperatures, shear rate, and oxygen, can degrade the polymer, causing gels, or worse.  This must be avoided to ensure quality extruded film.

If conditions are properly set, and the screw properly designed, melting begins at the end of the feed section.  Figure 4 below shows this phenomenon, with the melt film starting at the fourth flight.  These pictures were obtained from a “freeze-pull” experiment.  It is not an ideal method to determine the location of melting, but it is what was available in the 1990’s when this experiment was done.  Many thanks to my colleague and friend, the late Dragan Djordjevic of Er-We-Pa of Erkrath, Germany, for these photographs.  From this photograph, it can be easily seen that the solid bed is relatively compacted and melt film formed in the fourth flight.  In future articles we will see that the solid bed in this photograph is not completely formed, and air voids were conveyed downstream.

Figure 4.  Melt film formation as determined from a “Freeze-Pull” experiment

In summary, the coefficient of friction between the pellets and barrel is critical.  COF is maximized at the DSC melting point plus 15°C, which should be the feed zone temperature set point. Optimizing process conditions and screw design will maximize the solids conveying angle, and thus output, as well as melt quality and output stability.

Tom Bezigian holds a B.S. in Plastics Engineering from the University of Massachusetts–Lowell. He has been affiliated with the converting industry for more than 30 years and writes a column, Poly Ploys, for PFFC. It focuses on the field of polymers, laminations, and coatings with emphasis on R&D, quality assurance, manufacturing, marketing, operations, finance, and expert witness experience in the blown film, cast film, orienting, extrusion coating, and converting industries. He is the owner of PLC Technologies consultancy with over 30 years experience. Contact him at This email address is being protected from spambots. You need JavaScript enabled to view it..

[1]Plastics Components 2000, Spirex, p 6

[2] Bezigian, Extrusion Coating Manual, 4th Edition, TAPPI Press, 1999, p 38

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