Homogenizing Section Clarified
Measuring Technique Provides New Insight into the Compounding Process
Rotation, shear, heat and pressure – this much is clear: These are the factors required to compound plastics on twin screw extruders. From a scientific point of view, however, the question remained until recently as to which mechanisms exist during incipient fusion along with the energy input into the melt resulting. Scientists from the plastics division at Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt, Germany, have been successful in gaining an insight into these processes by using new types of measuring techniques.
According to institute information, this knowledge will enable industrial companies to design homogenizing sections in a way that will better fit the specific material and process, in the future. It says that it is now possible to diminish energy input into the plastic material to the absolute minimum and make the entire process much more efficient, while maintaining the same level of process reliability. The optimized process causes less damage to the polymer, mechanically and thermally, thus leading to better mechanical properties and chemical resistance. In addition, emissions from the process are reduced.
Plastic Deformation Becomes Visible
In order to systematically investigate energy input into the homogenizing section of the co-rotating twin screw extruder, Fraunhofer LBF developed a new instrument to visualize the cross-section of the plasticizing section. For this purpose, the scientists apply a high speed camera. With its resolution of 2000 individual images per second, the camera enabled the engineers, for the first time, to document and evaluate movement, deformation and incipient fusion of plastics pellets.
The images were linked to high-resolution torque measurement. This way mechanical energy input can be spatially attributed to any visualized state, and theoretical temperature increase be calculated.
Polypropylene: Two-Phase Deformation
Their new view of the homogenizing section made it possible for the LBF scientists to see, for example, the plastic deformation of a polypropylene pellet and document it. They found out that the pellets are caused to flow by massive plastic deformation, and initially plasticize locally within a fraction of a second.
At the beginning, the pellets are jammed between the active flank and the barrel wall. Then follows deformation, which can be divided in two phases: First, the pellets are compressed and pressed into the free volume. Second, a large amount of energy is introduced into this pre-compacted volume by means of more plastic deformation taking place. At approx. 1200 rotations per minute screw speed, these processes only require approx. five milliseconds.
Quantification by Torque Measurement
In addition to plastic deformation in the intermeshing zone, compression takes place in front of the active flank. LBF scientists were able to show that, in addition to material properties, the major factors to determine homogenizing are geometrical aspects, such as size and shape of the pellets, as well as the free volume in the kneading block. Quantification is done by high-resolution torque measurement.
In addition to quantification of the various mechanisms, another challenge to the Fraunhofer LBF Institute is to develop a model on this basis that is feasible for practical application. The Institute closely works together with Kunststofftechnik Paderborn, KTP, to solve this question.
Find out more at K 2016, Hall 7, Fraunhofer Booth SC01, Düsseldorf, 19-26 Oct 2016
Press Release from Fraunhofer Institute for Structural Durability and System Reliability LBF