Work done in the Extruder Processing Zone (EPZ) results in the desired quality of compounded material and levels of output in a co-rotating twin-screw extruder. In the EPZ, several actions are carried out on the material as it works its way through the extruder and exits from the die. Depending on the nature of work being carried out, these zones are called Intake, Melting, Atmospheric Venting, Mixing, Vacuum Venting and Metering. Proper configuration with the right choice of elements and barrels optimizes the performance of each zone. The functions of the various zones are:

• • Solids conveying in Intake
• • Softening of polymer in Melting
• • Degassing in Venting
• • Dispersion and distribution accompanied by kneading action in Mixing
• • Discharge control in Metering

Conveying screws, kneading blocks and other mixing elements are the working members in each zone. Making the right selection among numerous elements and configuring them in the right order needs an understanding of the functional characteristics of each element. This article attempts to aid understanding of the zones and explain the characteristics of elements.

Configuration of screw elements

The key to success in the EPZ of a corotating twin screw extruder lies with the exact design of the ‘Element and Barrel’ configuration. Elements work best in some combinations and some elements are more powerful than others. Firstly, the design has to deliver the correct amount of work on the product for melting and mixing. Secondly, the design should have the capacity to take the product ‘in’ and ‘out’ of the extruder. Finally, the design should allow gases or volatiles to escape without the product leaking out through vents. It can be imagined that there are ‘different zones’ (areas) inside the extruder performing a series of specific functions. Each zone passes on the material being processed to the next zone and until the final stage is reached. Extruder performance measured by energy consumption, quantity and quality of output, largely depends on the design of these processing zones. The effective selection of elements is the first step in design. The right length and combination of elements is the next step. The various zones will now be considered and the characteristics of the elements outlined, as well as their potential use and certain design principles.

Intake zone

The two popular methods of feeding an extruder are starve-feeding and forcefeeding. During force-feeding, a reserve of material is maintained in the hopper of the extruder and material is ‘positively displaced’ or ‘forced’ into the extruder.

Starve-feeding is the condition when an extruder is fed at a rate less than the capacity of the screw. The hopper remains empty and functions as a conduit to avoid material from spilling. Starve feeding is the more popular method for feeding due to a number of advantages, an important one being to split the feeding.

The modular design of the extruder screw assembly provides the option of a wide selection of screw elements for configuring in the ‘intake zone’. The popular twinscrew elements are Single Flight Element (SFE), Schubkanten Element (SKE), Special Schubkanten Element (SSKE), Normal Right-hand Screw element (RSE), Deep flight SK element (DSK), and the recently invented Single Flight ‘V’ shaped (SFV) element.

Free volume is the free space available for material in an extruder. This is obtained by removing the space occupied by screw elements inside the 8-shaped barrel. Conveying efficiency is the ability of the element to move the material forward in the extruder. It is 100% if all the material moves forward at the completion of each turn of rotation.

Compaction is the removal of air entrapped in the material. For example, knocking or vibrating a vessel containing a powder compacts the material (reduces the volume) while stirring does not. Breaking up results in a reduction in particle size or changes the particle morphology.

Melting zone

It is common to imagine melting as a phase transformation from solid to liquid. In the case of polymers, melting is generally associated with reaching the required melt viscosity or melt temperature. It is really the case that crystalline polymers undergo some amount of melting while amorphous polymers undergo glass transition. The work that can be carried out depends on the resistance to it. As the polymer melts, the resistance drops as the viscosity reduces. Strong compression aids in the melting of crystalline polymers, such as polyamides and polyolefins. Frictional heat generation by shearing forces is sufficient for increasing the temperature of amorphous polymers like atactic polystyrene and polycarbonate to the required process temperature which is usually above the glass transition temperature.

Mixing that accompanies melting is dispersive in nature. This is due to the high viscosity levels of the melt at the time of melting and as a result of which the high shear stresses opens an opportunity for elongation and break-up.

This melt mixing is achieved by kneading elements and by the amount of shearing between the tip of the kneading block and the barrel. The intensity of shear experienced by the material will vary depending on the gap it passes through as the kneading blocks complete a full rotation. In many cases, a hot zone is created due to intense action at certain points.

Venting zone

Vents are required to remove air and moisture (or volatiles) continuously during or after the melting stage. Removal of moisture to prevent hydrolysis of condensation polymers such as nylon and PET is one of the most important requirements. Additionally, downstream of the venting zone, entrapped air may be pumped into the extruder from a sidefeeder that needs to be removed in order to increase the capacity of the extruder. The most important controlling factor affecting the functioning of this zone is the degree of fill (Dof ). Dof is the ratio of actual space (volume) occupied by material to the actual free space in that particular section of the extruder. When the Dof is high, material can escape through the vent port. The Dof in an extruder is controlled by factors such as extruder speed, feed rate, element design and element configuration. In the case of under-cut elements (SK type elements) actual free volume is increased by 20% compared to the regular constant clearance elements. The lead screw elements do not change the free volume but can still reduce the Dof. This is because the velocity of the melt in that zone is increased.

The behaviour of the material at the processing temperature can be slippery, viscous, watery and clumpy. A different flow characteristic is expected at an open vent depending on the material. The handling of these different materials involves specialized vent inserts in the barrel vent opening. Generally, there are three types of inserts used. The open type is used for watery material. The fully closed type is used for viscous material. A partially closed type is used for clumpy material. Slippery material and materials that show a transition in viscosity due to addition of additives and fillers may also be run with partially closed type inserts.

Mixing zone

Mixing is an essential function in an extruder. The goal of the mixing process in an extruder is to increase the uniformity of the composition. Mixing or the lack of it results from the work done in the extruder that causes ingredients (usually immiscible) to experience forces of shear, elongation or compression, bending, erosion and impact. Shear and extensional flow are the two common types of flow in a co-rotating twin-screw extruder. Extensional flow can occur as a result of building up of pressure or during its release.

Kneading

Mixing is complete only if ‘wetting’ is achieved and the term ‘kneading’ refers to the action in the extruder that causes wetting. Achieving a chemical union (wetting) of two or more components is an important objective of the compounding process. Complete wetting or 100% wet state can be defined as the state of a ‘foreign’ or an immiscible particle which is completely surrounded by the fluid molecules (melt) that have penetrated/bonded to it due to the forces of attraction. Wetting can be ‘natural’ but facilitated by the use of wetting agents like wax or it can be ‘forced’ by the use of a coupling agent since wetting depends upon the magnitude of affinity that exists between the particle and the melt. In the SEM image of glass fibre in polypropylene (PP) showing poor wetting (see Figure 2) it is clear that the individual fibres are completely surrounded by PP. However, this did not result in any kind of chemical union. However, in the pictures showing good wetting, the glass fibres are not only surrounded but also coupled with PP. A coupling agent is necessary since the glass fibre does not have natural affinity for PP.

A kneading action that comprises folding, pressing, and stretching as in the case of ‘dough kneading by hand’ can be performed in a co-rotating twin-screw extruder. However, due to limitations in understanding the nature of work carried out in an extruder, kneading action as a result of shearing action is the predominant way to create wetting. This type of action results in wasteful energy input without useful mixing. Use of Fractional Elements (US Patent 6783270) in the form of kneading blocks can result in higher wetting action due to inherent design advantages such as uniformity of shear and increased elongational mixing ability without sacrificing the cleaning action. If kneading blocks have to be avoided, continuous mixing elements with an Erdmenger profile or a Sakagami profile (SMAP elements) are highly effective in creating the right circumstances for wetting. The difference between the design that employs kneading blocks and continuous mixing elements is in the dispersive nature of the two types. Dispersion and distribution are two important terms that are constantly encountered when discussing mixing. Mixing is a broad term that comprises of both chemical as well as physical action. In terms of physical action, mixing can be broadly classified as dispersive or distributive.

Dispersive mixing

Stresses of shear and elongational origin bring about dispersion of the large agglomerated particles in the melt polymer matrix by separating individual units or grains or crystals. The lowering of the cohesive strength (strength of bond between the same type of particles) of the agglomerate (usually a pigment) is a factor resulting in better dispersion. Mixing at the time of melting improves dispersion as result of high shear stresses during the high viscosity phase of the material. However, with certain material such as TiO 2 and carbon black, this approach can result in a severe issue called reagglomeration. During the time of melting in an extruder, the material is subjected to high levels of pressure. The particles that have not been completely surrounded by the wetting agent or melt have a chance to bond together and ‘sinter’. One of the SEM pictures show TiO 2 particles of 200 nm (see Figure 3) fully dispersed in the polymer matrix. In the other picture, agglomerated particles of greater than 10 microns are seen although such particles were not present prior to the extrusion.

Distributive mixing

Distributive mixing is defined as the uniform spatial rearrangement of fibres or other dispersed materials in the base polymer matrix. In most applications, especially the ones with fibres, some form of dispersion and forced wetting of fibres brought about by kneading elements precedes distribution of fibres. If necessary, a distributive mixing zone with appropriate elements is configured towards the end of the EPZ and in some cases outside the extruder using a static mixer. The most common distributive mixing requirement is in the uniform distribution of different length fibres in the melt since fibres may be fully dispersed and wet without uniform spatial distribution. Otherwise, distributive mixing is commonly confused with kneading.

Vacuum zone

Vacuum vents in the barrels are provided in an extruder for the removal of gases effectively in a continuous manner at lower than atmospheric pressure. This degassing helps to preserve the quality of the melt that is affected by the presence of monomers, solvents, moisture and other volatiles. These generally separate from the melt only at those low to high vacuum conditions. Further, the elements used in this zone should expose the material by continuously thinning the melt for effective degassing. This task of smearing the melt on the barrel surface by thinning for degassing is efficiently exhibited by the long forwarding screw elements or SK type elements. Applications involve: [Principles of Extrusion, Dr. Chris Rauwendaal]

• Removal of monomers and oligomers in the production of polymers (PS, HDPE and PP, for example).
• Removal of residual carried fluid in emulsion and suspension polymerization (PS, PVC).
• Removal of solvent and unreacted monomers in solution polymerization (HDPE).
• Removal of volatile bonding agents particularly with glass fibre reinforced polymers.
• Removal of reaction products, such as water and methanol, from condensation polymerization.
• Removal of water from hygroscopic polymers (for example, ABS, PMMA, PA, PC, SAN, PU, polysulphone, CA and PPO)
• Removal of volatile components in compounding of polymers with additives and other ingredients.

A melt seal is the most vital requirement for removal of moisture and entrapped air from the melt during compounding at lower than atmospheric pressure. Suitable lengths of vent opening and a vent hood design that maximizes the pressure gradient inside the extruder further facilitate this process. Melt seal is achieved by simply ensuring that prior to the start of the vent zone and just after its end the melt completely fills up (degree of fill = 1). Usually a reverse lead element will ensure build up of a sufficient wall of material behind it. However, depending on the nature of work, adequate melt seal may have to be built up to avoid frequent breakage of the seal resulting in poor devolatilization. At least ‘1D’ length of the melt seal may be required while running high vacuum with thin melts.

Apart from this, the same issues encountered in the venting zone to prevent melt from rising and filling the vent opening are even more acute here. The best remedy for this situation is the use of a side-feeder (also called a side-stuffer) with a proper seal in the gearbox end of the screw to allow a vacuum pump to be connected to it.

Metering zone

This zone is also called the pumping zone of an extruder. The compounded polymer melt is transported towards the die by drag flow caused by the rotating action of the screws. Screws with higher degrees of fill with shorter leads are optimum for this zone for creating the pumping effect. The required pressure is based on the die design.

P= (2Lτ)/R

Where τ = µy° and y° = 4Q/(πR³ )

[ τ = Shear Stress (MPa-N/mm²);

L=Length of the zone (mm);

π=3.14159(Constant); R = Die radius (mm); Q = Volumetric flow rate (mm3/s); y° =Shear rate ( /s)]

This zone pumps the homogeneous compounded melt at constant temperature and pressure. The performance of the optimally designed screws is preserved only if the excessive cooling in this zone is avoided. Metering forms the final processing zone in an extruder. The function of this zone is to build the required pressure for filtration of foreign particles (if necessary) and at the die for a continuous streamlined output. Equipment placed after the metering zone would include screen changers, breaker plates, die heads and die plates.

Shear uniformity

Unlike a single screw extruder, a twin-screw extruder has shear planes in all three dimensions. Traditionally, shear in the radial plane and lateral planes has been ignored since they are unique to co-rotating twin-screw extruders. While the longitudinal shear (similar to single screw) can reach a value of 250/s, lateral and radial shears can reach 10 or 20 times this value. Importantly, not all parts of the melt will experience the intense localized shear action. The shear uniformity is a term used to characterize this. High uniformity will mean the shear rate in that entire section in all planes will be within a factor of four. Low uniformity will mean a variation of ten times or more.

Conclusion

A good compound is the result of the right chemistry coupled with the right extrusion technology. This is all about putting in the right kind of work - always a result of optimally configured screw design. An optimal configuration has the right elements in the right place for the right application and process parameters. Functional understanding of the potential characteristics of each element is the starting point for undertaking this task. Every characterization table forms the quick reference guide for formation of configuration for different applications.