Extrusion: Definition, Type, Equipment, Design & Process Techniques

Extrusion Equipment and Processing Techniques

Extrusion, as we know, is today is more than 100 years old. Early extruders were used to extrude rubber, an elastic material. As material development progressed, synthetic viscoelastic materials were developed for extrusion.

These materials behaved differently, and the extruder was modified to accommodate these new materials. Original rubber screws utilized constant depth/variable pitch to compress the rubber material, versus screws for polymers, used variable depth/constant pitch to provide the compression necessary to generate pressure.

The machine used for extrusion is not surprising called an extruder. Extruders are complex machines which consist of the following components:

A motor
A drive system to control the motor
A gearbox
An extruder barrel
An extruder screw
A system for heating and cooling various extruder components
A feed system to introduce plastic pellets into the extruder barrel
A filtration system, including break plate and screen changer
A die
Instrumentation such a temperature, pressure, and thickness measurement
Various ancillary equipment such as coextrusion, ozonizing, corona treating

Components of Extruder Machine

Single Screw Extrusion Process

The configuration of an extrusion line depends on whether there is a substrate or not. The figure below shows the essential components of a blown film line, including:

Extruder
Die assembly
Automatic air ring, and
Bubble cage

Alpha Marathon 77-layer Nanofilm Blown Film Line

The rotating tower assembly, thickness gauge, corona treater, and winder are not shown.

Blown Film Process

In the blown film process, the extruder or extruders melt plastic pellets and pump the hot, viscous polymer melt into the die, which forms a circular film. As the film exits the die, it is called by an air ring and transported upwards to the collapsing tower, and then onto the winder. Blown film line speeds are typically limited to a few hundred feet per minute (100-200 m/min).

Cast Film Lines

Cast film lines differ from blown film lines in that the die is flat instead of round, the web is cooled with a chill roll instead of an air ring, and the web is transported horizontally instead of vertically. The two processes yield films with vastly different properties due to the cooling methods and resins used.

The figure below shows a complete small cast film/extrusion coating line:

The SAM, NA Pilot Laboratory in Phoenix, New York

Included in this line are all the components normally associated with an extrusion coating line, namely:

An unwind stand
Corona treater
Liquid primer
Flame treating
Coating/casting section
3-extruder system with coextrusion feedblock
Beta-gauge, and
Winder

The nip roll on this laboratory line can be substituted with an air knife and vacuum box to convert from an extrusion coating configuration to a cast film configuration. Line speeds on this type of equipment frequently exceed 365 m/min (1200 ft/min) and can be as fast as 760 m/min (2500 ft/min).

Flat sheet extrusion dies are not limited to thin coatings, but can also produce thick sheet for subsequent thermofoming, for example as shown in figure below.

Extrusion of a 6.35 mm (0.250”) thick 3-layer coextruded ABS sheet out of a
4 meter (10 foot) wide multi-manifold die into a 3-roll calendar stack
The line speed in this process is about 1 meter per minute (3 feet/minute)

The extruder itself, as well as process conditions essentially, can remain the same, as varying end-product thicknesses are governing by take-off speed at a given extruder output and width.

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Extruder Screw and its Purpose

As mentioned above, the extruder has several components, which are depicted in the figure below. All these pieces fit together to yield a salable product.

(a) Schematic Illustration of a Typical Screw-Extruder
(b) Geometry of an Extruder Screw

Complex shapes can be extruded with relatively simple and inexpensive dies.

A Summary of the Process

The extruder motor turns the gears in the gearbox, which is coupled to the extruder screw. Once the extruder reaches operating temperatures, the screw turns and brings the plastic pellets supplied by the hopper into the barrel. The pellets are conveyed forward by the pushing action of the screw flight. The volume of the first flight times the bulk density of the pellets times the RPM gives the output per minute.

As the pellets move forward, they encounter a restriction at compression (or melt) section of the screw. In this section, the channel depth (H) of the screw flight diminishes, causing pressure to rapidly increase in this area. The pressure forces the pellet against the barrel wall, where friction between the pellet and the barrel wall increases the temperature of the pellet, causing it to melt.

The frictional forces acting on the pellet determine what is called the solids conveying angle, which ultimately determines extruder output. It is critical that all air between the solid plastic pellets be eliminated before melting begins. Failing to do so will convey air forward, which will contribute to the formation of gels, a generally undesirable defect in the finished product.

As the plastic moves downstream, the average temperature increases and more and more pellets become molten. High shear sections may be added into the screw design to ensure complete melting prior to exiting the extruder barrel.

There is a tremendous energy transfer from the screw to the pellets in the compression zone, and pressure within that zone can easily reach 400 bar (~6,000 psi). The energy to pump and the melted plastic pellets ultimately come from the extruder motor which is transmitted through the gearbox and the extruder screw. Once the plastic leaves the compression section, pressure generation ceases, and pressure consumption begins. Enough pressure must be generated to convey the viscous molten plastic out of the extruder, through the filtration system, piping, coextrusion adapters and die so that a uniform flow exits the die along its circumference or width, depending on geometry. It is the job of the screw designer to design a screw that will provide the required output and melt temperature.

The Purpose of the Extruder

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.

Let’s review the first, and probably most critical, function of the extruder screw, 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 below.

Single-screw Extruder¹

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 are a vent zone, a dispersive mixing zone, and a distributive mixing zone.

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.

Relation of COF between the Pellets and Barrel

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 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 solid conveying angle and maximum output.

Two Forms of Solids Conveying in a Single Screw Extruder²

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.

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.

Melt Mechanism and Root Causes of Defects

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 below shows this phenomenon, with the melt film starting at the fourth flight.

Melting Mechanism

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.

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.

Melt Quality & The Root Causes of Defects in Melt

The most important question on all extrusion engineer’s agenda related to melt quality and the root causes defects seen in the melt is:

“Do you see specs or inclusions when extruding polymers? For example, PET. I would like to know if you have any idea of where inclusions come from, e.g. unmelted polymer, degraded polymer, cyclic oligomers etc)?

Specs and inclusions are commonly seen in extruded films, but they are not desirable and quite often they are not acceptable in the finished product. Quite often these “inclusions” are bubbles, gels, light brown or black specs, streaks or occasionally foreign contamination.

Bubbles are due to moisture in or on the pellets, degradation within the process, or air entrapment due to improper feeding conditions. They can be microscopic in size; in which case you would observe an overall haze in the melt film. The melt curtain should be crystal clear.

See the figure below for comparison:

Comparison b/w Hazy and Crystal Clear Melt Curtain

The picture on the left has thousands of microbubbles in it while the picture on the right is clear, as should be all unpigmented melt curtains.

Identify the Cause of Bubbles

Larger bubbles will cause visible voids, which as always unacceptable to the customer, and could cause web-breaks and waste in the extrusion process. These are sometimes seen on start-up if the start-up procedure is performed improperly.

Look for Moisture Content in the Pellets

The source of the bubbles must be determined. Moisture is always the first place to look. Assuming moisture content in the pellets is within specification (that is, dry resin, adequate drying, etc.), then look to overheating for the resin. Moisture in or on the pellets is particularly destructive in PET extrusion due to the fact that PET is a condensation polymer, and that water will de-polymerize polyester resin. Dry PET resins according to manufacturer’s recommendations.

Keep a Check over Temperature

Most thermocouples read the metal temperature, not melt temperature. Because of the efficiency of water cooling, water-cooled barrels will mask what is actually going on inside the barrel at that section. Air-cooled barrels are less efficient than water-cooled barrels, and thus overheating in a particular section are more easily observed with air-cooled barrels.

An old “trick-of-the-trade” is to turn off barrel heating and cooling once the extruder is up to full speed and temperatures have stabilized, and then observe where the barrel temperatures equilibrate. These temperatures are the ideal settings for that particular resin and output.

Longer Residence Time in Extruder

Another possibility is that the residence time is too long, and the resin is simply degrading because of excessive time in the extruder. The normal residence time in most commercial extruders is 5-8 minutes. Many extruder operators reduce screw speed to just a few RPM while setting up the machine of shutting down the machine.

If the residence time is 8 minutes at 100 RPM, then the residence time is almost 4.5 hours at 3 RPM. This is ample time for most resins to fully degrade. The issue now becomes eliminating the degraded polymer, gels, black and brown specs, etc. This can only be done with an extensive high-speed purge or by disassembling the extruder and cleaning out the entire system, then staring up with the proper conditions, which means 30 RPM minimum so that residence time is not too excessive.

Entrapped Air

If the bubbles are less frequent and more random in nature, the possibility exists that the feed zone temperature is too high, causing premature melting and thus entrapment of air. The ideal Zone 1 temperature of the extruder is the DSC melting point plus 15°C. This was proven by Bob Gregory in the 1960’s. The COF is maximized at DSC +15°C, which prevents premature melting and maximizes the solids conveying angle of the solid bed. Entrapped air in the extruder is not good, as conditions half-way down the extruder are typical 450+ Bar and nearly 300°C, which are essentially reactor conditions. The presence of oxygen with polymer at elevated temperature and pressure can create free radicals and thus create gels, light-brown colored carbon, and/or carbon specs. See the figure below:

Entrapped Air in Extruder

If this condition exists, double check all thermocouples, temperature settings, and start-up/shutdown conditions. If the melt curtain is smoking, it is simply too hot. Speed up the extruder to purge this overheated material out. If you find that the material or certain temperature zones overheat further at higher speeds, then the screw design is at fault.

A Quick Note on Screw Design

When a new screw design is needed, the designer will ask for the resin to be extruded, the desired melt temperature, and the desired output. Screw design technology is quite sophisticated today, and the designer usually does a good job of providing the user with a workable screw design. However, deviations from the target melt temperature, output, and resin will alter screw performance.

Unmelted resin is not typically seen today since the use of one or more “mixing” sections is common on most extruder screws. However, old-fashioned single-stage screws with no mixing head operated at high speeds can pump “unmelts” out of the extruder, as seen in the photo below. This is actually a photograph of thousands of unmelted particles in a polyester melt curtain.

Find Suitable Polymers that Undergo Extrusion

View a wide range of extruded polymer grades available in the market today, analyze technical data of each product, get technical assistance or request samples.

References:

Plastics Components 2000, Spirex, p 6
Bezigian, Extrusion Coating Manual, 4th Edition, TAPPI Press, 1999, p 38

Extrusion - Equipment, Processing Techniques & Troubleshooting