Processing wood fibres in twin screw: interest of the numerical simulation

Processing wood fibres in a twin screw extruder is challenging as it requires a close control of the temperature and mixing efficiency:

  • temperature has to be limited for avoiding burning the wood fibres
  • mixing efficiency has to be important enough to disperse the fibres into the polymer matrix

For a first analysis of the process, temperature, pressure and residence time were chosen as interesting markers to be observed.

Woodforce fibres

Wood fibres by Woodforce

Twin screw profile

Twin Screw profile

But on the machine, those markers are not so easily measurable. It requires to place sensors at strategic places. Moreover those sensors provide only local information and not a global overview of the material behaviour.
In this context, simulation is a powerful tool that can predict the thermo-mechanical behaviour of the material all along the extruder.

In this study, the extrusion of a composite is validated by both experiments and simulation.
> The composite is a blend of a polypropylene matrix reinforced by wood fibers (supplied by Woodforce).
> The used machine is a Leistritz ZSE 27
> The process is performed at different screw speed (200 and 400 rpm) for a 30 kg/h throughput.
> For the experiments, pressure and temperature sensors are placed in the kneading blocks and at the die entrance and exit (on the hereafter picture, sensors are represented by yellow (for pressure) and pink (for temperature) circles).

Screw profile used for the experiments

Screw profile used for the experiments

For each experiment (1, 2, 3 & 4) the material flow is computed by Ludovic® and compared to the sensors measurements. Temperature at the die entrance, pressure in the first kneading blocks (placed after the introduction of wood fibers), as well as mean residence time are shown in the hereafter figure 2.

Article #3 figure 2

In all curves, numerical results are quite closed from the experiments. This is indeed the first step to check in using numerical tools. Thus, afterwards, optimization tasks can be performed, based on the reliability of those numerical results.

As a conclusion, this study validates the reliability of the Ludovic® results and confirms that it is a solid virtual tool used as a complementary analysis to the real experiments.

*based on a presentation “Comparison between experiments and simulations on extrusion cases: standard polymer and wood fibers“, presented by Sciences Computers Consultants and Fraunhöfer ICT at K 2013. Düsseldorf – Germany.

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