Plasticizing with Extremely Short Extruders
The so-called high-intensity plasticizer, which is characterised by its extremely short length, represents an inexpensive concept for processing thermoplastic resins. The feed, compression and metering (discharge) zones are arranged coaxially; the resin is plasticized solely by means of friction. Initial investigations show good results when processing a variety of semi-crystalline and amorphous thermoplastics.
Single-screw extruders for processig thermoplastic resins are usually equipped with electrically heated barrels. The L/D ratio is generally between 20:1 and 36:1. In the February issue of Kunststoffe, Prof. Dr.-Ing. Helmut Potente and his colleagues at the Institute for Plastics Technology (KTP) introduce an alternative concept. This so-called high-intensity plasticizer is distinguished by its very short length – the L/D ratio is 2:1.
The processing unit is designed in such a way that plasticizing occurs solely by means of friction , i.e., conversion of the drive energy into frictional energy. The inside surface of the barrel is helically grooved over its entire length, with the groove depth decreasing in the direction of extrusion. In contrast, the multi-flighted screw has a constant root diameter but a flight depth that increases in the direction of extrusion. There is a gap between the flights on the screw and those forming the grooves inside the barrel (Fig.1).
In the simplest case, extruders are divided into the feed, compression and metering (discharge) zones. In extruders to date, these zones have always been arranged axially behind one another. In contrast hereto, these zones are arranged coaxially inside one another in the high-intensity plasticizer (Fig. 2).
In trials, the team of researchers at the KTP determined that solids conveying takes place primarily in the grooves on the inside surface of the barrel, while conveying of the melt produced during plasticizing occurs largely in the channels formed by the flights on the screw. This is facilitated by the corresponding changes in screw flight depth and groove depth. As a result of the different conveying rates for the solids transported in the barrel grooves and screw channels, a velocity gradient exits in the gap between the screw and barrel that permits conversion of the frictional work into heat, thus permitting plastification of the processed material.
In the initial investigations at the KTP, both amorphous and semi-crystalline resins were processed on the high-intensity plasticizer in order to experience different enthalpies of fusion and energy conversions. It was possible to process all of the selected resins (PP, PE, PS, SAN and PC) on the high-intensity plasticizer.
The energy required to melt the solid resin is introduced to the material via shear and friction. This subjects the material to a very severe load that gives rise to the risk of material degradation. Comparative measurements between processed specimens and virgin material indicate, however, that only very limited degradation occurs during processing. The researchers attribute this to the very short residence times.
In future research projects, the principle of operation will be investigated theoretically in order to gain a better understanding of the interplay among the numerous geometric parameters as well as the material and process parameters. On the basis of these results, the researchers at the KTP plan to optimise important criteria such as throughput and pressure buildup capability.