Shear and mixing parts of extrusion lines

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Shear and mixing parts of extrusion systems are screw elements during extrusion , which are intended to bring about optimal homogenization and mixing of the melt.

motivation

The use of shearing and mixing elements in older extruders is only necessary in special cases due to their low output, e.g. for coloring the material. Due to the high throughputs of modern high-performance extruders, conventional screws are no longer sufficient in powder and granulate processing. For optimum quality of the melt and color distribution, shearing and mixing parts must therefore be attached to the screw shafts, which must be tailored to the properties of the material, the screw-cylinder system and the tool.

Shear parts

Pigments often have to be incorporated into the plastic by shearing.

Basically, shear is a fundamental part of mixing. Shear parts behave more "aggressively" to the material during homogenization than mixing parts and are always used when, for example, molecular chains have to be broken to form a connection with the added additives, or when large glass fiber components have to be crushed in the melt.

mixer

Can be divided into dynamic and static mixers.

Dynamic mixer

Dynamic mixers (as aggregates) ensure that low-viscosity liquids use moving parts (shear and mixing parts) for shear and expansion flows with a dispersing and distributing effect, and the mixing effect of an existing (turbulent) flow is further improved.

Static mixers

Static mixers (as elements) are only used if mechanical and thermal homogenization has not yet been achieved by a plasticizing screw with a dynamic mixing part (inhomogeneity of the material). Static mixers usually consist of guide bars, guide spirals or bundles of bores, which divide the flow into partial flows, stretch them and then continuously change the spatial arrangement of the flow. This significantly improves the homogeneity of the melt.

Static mixers usually consist of several composite mixing elements, which are installed in the flow channel with an offset of 90 ° to one another.

Disadvantages: higher costs, additional printing requirements, slight temperature increase

Mix

Mixing can be divided into distributive and dispersive mixing.

Distributive mixing (distributing)

Characteristic of distributive mixing is the increase in surface area, shear deformation and the rearrangement of particles, which are mutually compatible at the usual shear speeds that occur in the screw thread. One can also speak of a distribution here.

If solid agglomerates cannot be distributed during mixing, deformable agglomerates can, however, be dissolved and distributed. In order to be able to distribute such deformable agglomerates, it is necessary to work with mixing elements which have high shear for deforming the deformable agglomerates and which cause a subsequent mass flow division.

The decisive factor here is the geometry of the elements, in contrast to dispersive mixing, in which the shear stress plays a greater role with a corresponding residence time.

Dispersive mixing (dividing)

If substances that are not compatible with one another have to be mixed with one another, or if solid agglomerates have to be broken up or finely distributed, then we speak of dispersive mixing. Here particles have to be broken up in order to be able to mix with one another. This division is more complex than distributive mixing.

In the case of fillers and dyes, the binding forces between the atoms and molecules must be overcome, as the interfacial tension of these substances works against the deformation. The desired deformation is caused by an expansion and shear stress field. Here, the viscous particles are initially stretched until they finally break down into several small particles. High shear rates are required for dispersive mixing. Optimal cutting depends on the level of tension and the exposure time.

With dispersive mixing, shear parts (see baffles ) are used.

Optimization of a cylinder-screw system with a mixing part

requirements

When working with the basic screw without a mixer, the melting temperature must be below the maximum permissible temperature even at the highest speeds with the raw material used. This requirement is very important because a certain amount of energy is additionally introduced into the melt in the later installed mixing part.

When installing the basic screw, the drive of the extruder must still have power reserves in the desired speed range without a mixer. This requirement is very important because the amount of energy converted requires a higher power output from the drive. There must still be a speed reserve in order to enable the extruder to run even higher than the previously used speed range.

Annular gap gap mixing part and modified Leroy shear part

Both mixing elements can be easily optimized with simple rework by enlarging the gaps or reworking the shear gaps.

Quite good results when processing medium to high molecular weight PE of high density were achieved with the annular gap mixing part, the so-called Leroy torpedo, which was tested in a modified version, is also a mixing element with a stronger shear effect.

literature

• Johannes Wortberg, Dirk Kaczmarek: Extruders and extrusion systems. In: Friedrich Johannaber (Ed.): Plastic machine guides. 4th edition. Hanser, Munich / Vienna 2003, ISBN 978-3-446-22042-3 ( limited preview in the Google book search)