Screw performance

The Buss AG continuous mixers 27) comprise a special screw performing an oscillatory motion. The screw has three slots per lead, whereas kneading bolts or teeth in the mixer casing are aligned with the slots in the screw as the shaft rotates. 

As reviewed concerning screw types, these dynamic mixers are used to improve screw performance. Static mixers are sometimes also inserted at the end of the plasticator. Proof of their success is shown by their extensive use worldwide. Each type of mixer offers its own advantages and limitations. Where practical they should not be located in a region where the melt viscosity is not too low. With some of these installations because they may have to operate at a lower speed to avoid problems such as surging, independently driven mixers can be used so machines can operate at optimum speed. Other benefits of independently driven mixers involving feeding capability and performance occur. For example, metering pumps can inject with precision liquid additives directly into the mixer. 

In typical extrusion operations, mixing devices are used in the screws. Many dynamic mixers, such as those included in Fig. 3-7 and in Chapter 2, are used to improve screw performance. Static mixers are sometimes inserted at the screw end (Fig. 3-9) or at the end of the barrel. There are also mixing... 

With all the varieties of materials available, and their individual grades with many different formulations, it has not yet been possible to design a screw in advance, based on the melt or the rheological/flow physical relationships involved in plastification and conveying. Trial and error, with an observant processor using reliable and reproducible controls, makes the screw perform to its maximum efficiency. 

This equation is used to calculate specific throughput of the extruder, which is often used as a measure of screw performance. 

Not all materials or products have the same flow characteristics. Some have plastic characteristics and flow easily. Others do not self-adhere and tend to separate when pumped or mechanically conveyed. As a result, the volumetric efficiency is directly affected by the properties of each product. This also impacts screw performance. 

Photomicrographs of sections covering a scale of 9 examples from good to poor mixing (1 = good, 9 = poor), were used for comparisons using a 30 mm 20D extruder over a range of output rates. The results show the shallower screw performed better than the... 

In the parison extrusion phase, the extruder die block valve is opened and the screw performs the action of a ram by moving forward in the axial direction without rotating. This forces the accumulated melt at the forward end of the screw through the parison head, where it is extruded at a relatively high flow rate. In practice, the flow rate is limited by the onset of shear phenomena such as sharkskin and melt fracture. 

H.F. Giles, J.R. Wagner, E.M. Mount III, Extrusion. William Andrew Publishing, Norwich, NY, (2004), p61 E. Steward, W. Kramer, Evaluating Extruder Screw Performance , 1988 SPE ANTEC Proceedings, (1988), pp 45-50... 

---