07.07.2022

Extruder Shafts: Grinding is more than a finishing operation.

Although grinding is one of the oldest manufacturing processes in the world, for a very long-time grinding has led a shadowy existence compared to the other machining production processes, but this is increasingly changing.

From finishing to high-performance machining

We have often heard it said in the past that grinding is a nice finishing operation. Perhaps that's because the touch points with sandpaper and angle grinders from the hardware store are familiar to virtually every do-it-yourselfer. Granted, the touch points with hardware store products are significantly higher than with 5-axis high-tech sanders. It is therefore quite understandable that awareness of the performance of modern sanding processes has not yet become widespread.

But to anticipate: It is not only the prominent metal-cutting manufacturing processes that achieve a large amount of material removal in a short time. Modern grinding processes are in no way inferior and often pay off several times over. Not only is the machining time comparable but grinding also brings significantly lower tool costs. But even more attractive is the fact that complex workpieces can actually be produced completely on one machine. A particularly interesting example of the fundamental advantages of grinding, can be found in the tools of extrusion lines.

Extrusion technology: A demanding field for grinding

Extrusion is a highly complex process in which the extruder, as a conveying device, continuously presses viscous mass under pressure through a shaping opening. The basic principle itself can be explained - simplified, of course - using the children's toy Play-Doh as an example. The plasticine is pressed through a mold, and thanks to the shaping attachments, very different elements emerge that are conducive to the creativity of the little ones. 

The products that can be made by extrusion are encountered several times a day. Among other things, chewing gum, hoses, foils, door seals, V-belts and toothed belts, pencils, noodles, peanut flips, ceramic products, bicycle rims, meat substitutes, PVC, breakfast cereals, millet balls, onion rings, garbage cans and pastries are made using this basic principle. And the list can be extended at will.

Extruder screws: High performance through precise grinding

A decisive momentum in the extrusion lines themselves used for production are the extruder screws. In addition to pistons, single screw extruders, twin screw extruders, multi-screw extruders as well as cascade and planetary roller extruders are used there. Depending on the product to be manufactured, the requirements regarding to the geometry of the shafts are very different. 

This can be illustrated particularly well by the example of barrier screws used for mixing. Here, an optimum barrier screw is characterized by a high melting capacity and a reduction in the melting temperature. In plastics processing, the barrier screw is the most important type of screw and is used as a standard in high-performance extrusion for the production of pipes, blown films and blow molds, among other things. The separation of solids and melt in the melting zone by an additional barrier flight is characteristic of a barrier screw. Depending on the specific application, different geometries are required to ensure the highest plasticizing performance.

The following figure shows the simplified structure of the functional zones of a barrier screw: feed zone, plasticizing zone, barrier zone, mixing section and shear section.

Schematic representation of a barrier screw with the 4 functional zones.

Mixing and shear parts are of particular importance in extrusion. Basically, the mixing parts increase the thermal and mechanical homogeneity of the melt by splitting and recombining the melt. Uniform distribution is achieved by the flow-dividing elements through which the flow must pass. In contrast, shear parts increase the homogeneity of the melt by selectively introducing a shear load. Basically, mixing and shear parts are attached to the end of the screw, and several geometry elements can be used in series.

The figure below shows different common mixing and cutting part geometries.

Mixing geometry: Pins in the screw channel.

Mixing geometry: Pineapple mixing section.

Mixing geometry: Ring with bore.

Cutting part: Cylindrical cutting part.

Cutting part: Helical cutting part.

Cutting part: Maddock cutting part.

Cutting part: Troester cutting part.

In addition to the mixing and shearing sections, the tip is also of particular importance in the design of the shafts. This is part of the non-return valve (RSP), which forms the heart of the plasticizing unit. During the injection process of the melt into the mold, the non-return valve prevents the material from flowing back into the screw flights. As these components are subjected to extreme loads, the materials and alloys are also correspondingly demanding.

 

High-performance plasticizing technology: an interplay of geometry and material

A wide variety of geometries, demanding materials and high-quality material designs on sometimes very long shafts predestine complete production on high-tech grinding machines from the Multigrind® series.

Up to now, such workpieces have often been produced on several different machines. In such production layouts, lathes, milling machines, whirling machines and polishing machines are used, among others. In contrast to complete production on a Multigrind® machine, these production strategies require significantly more time both for setup and for the actual machining.

One reason for the operation of such complex production layouts can be seen in the different materials and alloys (e.g. hardened steel and welded-on contours) required for highly efficient extruders. In contrast to the aforementioned manufacturing processes, machining with a high-tech grinding machine from the Multigrind® series makes it possible to produce the workpiece in one clamping. This reduces working time, set-up times and workpiece costs. The fact that Multigrind® hardware and software have this potential is demonstrated by the various turnkey solutions for machining turbine components made of Inconel, components for the optical industry made of sapphire glass, and precision tools made of carbide and prostheses made of special alloys.  

The author

Zita Bader

Zita Bader works in the marketing and communication department of Haas Schleifmaschinen GmbH.

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