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Using Coatings to add Pressure

Steinbeis works with manufacturing partner to develop rough surfaces on extrusion bushes

Extruders are devices used to pass solid, viscous, and liquid materials through tubes based on a coil principle. The materials are pushed through different temperature zones under high pressure, causing them to change from a solid state to a viscoelastic or thermoplastic state. The device used to convey the material is like a coil running down the inside of a cylinder. Depending on the material properties and the processing parameters, the feed section comes in different forms, with the main types being a smooth-bore extruder and a grooved feed extruder. As part of a research project, the Steinbeis Innovation Center for Intelligent Functional Materials, Welding and Joining Techniques, and Implementation decided it was time to optimize the technically more simple and less expensive smooth-bore extruder option to deliver more pressure.

View of the feed section with grooves

Smooth-bore extruders are used to move materials, or actually drag them, between forward-moving coils. The inner surfaces of the cylinders are smoothened in a honing process to minimize friction between the coil surfaces and the cylinder. This can impede movement, however, due to material slippage. This limits the efficiency of material movement, and, as a result, there is a limit to the amount of pressure that can be exerted in the section just before the material comes out. The problem is that sometimes an extrusion job needs more pressure where the material exits, so an extruder has to be used with grooved bushes. This has grooves on internal surfaces of the cylinder in the feed section that the material passes through. To work properly, more pressure may be needed, but by doing this it becomes possible to move materials that are more resistant to pressure and this improves flow. Because extruders with grooved bush cylinders are complex in technical terms, they require more energy and are expensive to purchase, so plastics processers prefer to use the more simple smoothbore extruders. The Steinbeis experts believed that for many extrusion tasks, a half-way house technology would probably be the best solution.

This was reason enough for the experts at the Dresden-based Steinbeis Innovation Center for Intelligent Functional Materials, Welding and Joining Techniques, and Implementation to embark on an optimization research project. The approach they adopted was to “roughen up” the inner surfaces of the cylinder in the smooth-bore extruder using a special coating. Working in collaboration with their research partner, Arenz GmbH from Meckenheim, they first investigated different technical approaches for the coating process. These different coating techniques led them to a variety of coating systems which could be adjusted by adapting processing parameters and the combination of powders used. The resulting coating systems were then tested, looking at coating properties, different degrees of roughness, and wear resistance.

To analyze parts welded to the extruders, the researchers at the Steinbeis Center also used 3D technology to provide images of the surfaces. Analyzing the surfaces subsequently made it possible to determine and compare the number of tungsten carbide particles exposed on surfaces. By examining the coatings added to the surfaces, it was possible to demonstrate that small particles of tungsten carbide were ideal for optimizing properties as required. It was important that there were not too many exposed particles to minimize the degree of mechanical wear in the coils. The best substance was found to be hardened tungsten carbide measuring between 63 and 170 μm.

View of the feed section on the Tribop extruder. The surface has been modified using laser dispersion.

The research partners built on these results by optimizing the coating systems and the corresponding coating processes. Using laser dispersion, they created a uniform rough layer that worked a bit like sandpaper and improved the performance of the extruder. Compared to the smooth-bore extruders previously in use, adding coatings meant there was less tendency for materials to wear.

After comparing the newly developed Tribop bushes with existing technology, the researchers were able to confirm that the project had addressed key parameters. Compared to smooth-bore bushes, the Tribop extruders raise throughput to 154%. The exit pressure with the Tribop bushes is also higher at 112%, confirmation that there is reduced material backflow. The Tribop bushes have a higher driving power and electricity consumption than with the smooth-bore bushes. In contrast to the grooved bushes, the required power reduces to 71%. It was clear from evaluations that by using the Tribop bushes, it is now possible to extend the capabilities of the smooth-bore extrusion machines. For Arenz, this opens the door to offering customers other innovative products positioned between smooth-bore extruders and grooved bush extruders.