Compounding

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Compounding (from English : to compound = "to put together") is a term from plastics technology that describes the admixture of aggregates ( fillers , additives , etc.) in order to achieve desired property profiles. The compounding takes place mainly in extruders (mainly co-rotating twin screw extruders , but also counter rotating twin screw extruders, as well as planetary roller extruders and co-kneaders ) and includes the process operations conveying , melting , dispersing , mixing , degassing , pressure build-up and extrusion (ejecting). Compounding in its broadest sense can also be referred to as preparation in German .

Thanks to significant gains in knowledge in this area, it has been possible since the end of the 20th century to produce plastic molding compounds with a range of properties “made to measure”.

Compounding

The processing turns the raw material into a plastic molding compound with the best possible properties for processing and application. This happens in particular through:

  • Change in particle size
  • Incorporation of additives
  • Removal of components

Change in particle size

Raw materials in the form of powder, lumpy resins, fibers or plastic waste (recycled material, regrind) in irregular form are processed into granules that are suitable for feeding processing machines (injection molding machines, etc.).

Incorporation of additives

Additives are added to the raw material as powder, granulate, liquid or paste and mixed with it as homogeneously as possible. If substances such as fibers are added to the polymer that are still recognizable as a separate component after processing, it is called a composite material .

Drying and degassing

Before further processing, moisture and low molecular weight components should be removed from the molding compound ( degassing ).

Processing method

Mix

The mixing process can be carried out in the viscous or in the solid phase. Mixing in the solid phase is about the even distribution of the particles, since the additives are already in a comminuted form. Mixing in the solid phase is often premixing before subsequent mixing in the melt state, which can also be referred to as "viscous mixing".

Viscous mixing can serve the following purposes:

  • Melting the polymer and additives
  • Dividing the solid agglomerates (agglomerates are clumps)
  • Wet the additives with polymer melt
  • even distribution of the components
  • Separation of unwanted components (air, moisture, solvents, residual monomers ...)

The heat required for viscous mixing is mainly generated by shear and friction .

Distributive mixing

Distributive mixing means the uniform distribution of all particles in the molding compound.

Distributive mixing
Dispersive mixing

Dispersive mixing means the distribution and comminution of the components to be mixed.

Dispersive mixing 1
Discontinuous mixer
Heating and cooling mixers

In order to improve the absorption and diffusion processes from the additive to the granulate, the plastic must be mixed at a higher temperature. A heating and cooling mixer system is used here. The material is mixed in the heating mixer and then flows into the cooling mixer, where it is temporarily stored. In this way, so-called "dry blends" are produced that have better flow properties than the molding compound that is still heated and that could clump in the feed of an extruder .

Continuous mixer

This type of mixer is mainly used in viscous mixing.

Planetary roller extruder

The planetary roller extruder is suitable for processing sensitive materials that require precise temperature control. In addition, high filler grades of up to 80% can be achieved.

Kneader

A particularly effective mixing takes place in the co-kneader, a special single-screw extruder that performs both a rotary and a translational (linear) movement. It is characterized by the fact that shear can be applied at moderate shear rates and regardless of the size. It is mainly used for compounding

  • Shear and temperature sensitive polymers and additives
  • Highly filled compounds (up to 92%)
  • High viscosity compounds
  • Highly abrasive compounds

and their combinations are used. The so-called kneading bolts or kneading teeth used in the mixing chamber are another unique feature. In the continuously drilled state, they can serve as injection points, which enable direct mixing in the melt pool without smearing and thus specifically control reactive extrusion processes. In the hollow drilled state (blind hole) they serve as a precise product temperature measuring point. It is so precise that many users use it as a measure for online monitoring of the process.

Co-rotating twin screw extruder / compounding extruder

The task of an extruder is to draw in the plastic mass supplied to it, to compress it, to plasticize and homogenize it while supplying energy, and to supply it to a shaping tool under pressure.

The main area of ​​application of a co-rotating twin screw extruder is in plastics processing. However, it is also used in plastics production and processing.

Twin screw extruders and compounding extruders perform the following work steps:

Drag flow in the screw gap of a co-rotating DSE

Twin-screw extruders with a pair of co-rotating screws are well suited for processing (compounding) plastics.

A co-rotating twin screw extruder is divided into several process zones. These zones are linked to one another and cannot be viewed independently of one another.

Granulate

Since most processors need the plastic in the form of granules, granulation plays an important role. A basic distinction is made between hot and cold cut, depending on the processing, different grain shapes result:

  • Pearls or lentil grain shape for hot stamping
  • Cylinders or cube shapes for cold cutting
Classification of the granulation
Hot cut

In the case of hot cutting, the extrusion strand is chopped up directly after the nozzle by a rotating knife overflowing with water. The water prevents the granules from sticking together and cools the material. Most of the time water is used for cooling, but air can also be used. The selection of the right coolant depends on the material. The disadvantage of water cooling is that the granulate has to be dried afterwards.

Cold cut

The strands are first pulled through a water bath and then cut into the desired length by a rotating knife roller (granulator) in the solid state.

Process measurement technology

The extruder machines used for this purpose are equipped with measuring probes to control and regulate the compounding . For process monitoring and control, at least the measurement of the melt temperature and the melt pressure is required.

In addition, there are a number of other measuring methods for process and quality control in the melt:

literature

  • T. Hochrein, I. Alig: Process measurement technology in plastics processing. Vogel, Würzburg, 2011, ISBN 978-3-8343-3117-5 .

Web links

Process measurement technology in plastics processing: http://www.skz.de/de/forschung/geschaeftsfelder/schwerpunkt-prozessmesstechnik/index.html

Individual evidence

  1. ^ T. Hochrein, I. Alig: Process measurement technology in plastics processing. Vogel, Würzburg 2011, ISBN 978-3-8343-3117-5 .
  2. ↑ T. Hochrein, J. Botos, K. Kretschmer, P. Heidemeyer, B. Ulmer, T. Zentgraf, M. Bastian: Faster and closer to the process. In: plastics. No. 9, 2012, pp. 76-80.
  3. ↑ G. Schober et al .: Degree of dipsersion monitoring by ultrasonic transmission technique and excitation of the transducer's harmonics. Lecture. 29th International Conference of the Polymer Processing Society, Nuremberg 2013.