Bio-based polymers have been the focus of applied research at the Steinbeis Research Center for Environmental Building and Building Materials for years. According to a nova Institute forecast for the production volumes of organic polymers, taking 2011 as a basis, by 2020 production will have doubled as a proportion of total production. Polymers include materials that have been reinforced with natural fibers. Most of the properties of these materials are already known, but until now little attention has been given to their machinability. This is now the topic of a project currently being carried out at the Steinbeis Research Center.
The properties that experts particularly value about engineering plastics include their low density (massive weight reductions in lightweight construction), the lack of wear, noise and vibration damping, resistance compared to corrosive media, good electrical insulation, and sometimes excellent friction properties. For example, in mechanical engineering they are used in applications ranging from cogs to glide bearings, coupling elements, bearing bushings and ship propellers. They also feature in fermenters, internal car fitments, aerospace technology and rotor blades on wind turbines.
These applications often require strong resistance to a variety of mechanical stresses and low thermal expansion properties. As a result, in many areas of mechanical engineering, the polymers have to be fiber-reinforced. The solid, stiff fibers absorb external forces while the synthetic matrix ensures that individual fibers are connected to one another and that the forces subjected to materials from the outside are transferred to the fibers. The matrix also protects the fibers in the material from external damage such as wear, radiation and humidity.
Natural fiber-reinforced plastics (NFRPs, which also include wood plastic compounds, or WPCs) have a justified existence in industry due to the scarcity of raw oil and their neutral carbon footprint. Aside from the aforementioned WPCs, which include wood fibers and powdered wood, a variety of manufacturers now also sell products with reinforced fibers like flax, hemp, cellulose (viscose) and lignin. Others are undergoing further development.
Despite their potential for lightweight construction, their stiffness and resistance, their excellent damping and crash properties, plus the ability to produce these materials from local renewable raw materials, industry is still slow to adopt them in applications. This is partly due to the sometimes significantly higher costs compared to petrol-based plastics and their lower strength versus glass and carbon fiber-reinforced plastics. A further stumbling block is the lack of sufficient experimentation into the processing of mostly extruded or injection-molded natural fiber plastics. Machining such materials is often unavoidable in producing premium quality parts that are ready for assembly. Made-to-measure drilling and milling technologies need to be developed for innovative natural fiber plastics, for each kind of fiber matrix composite. These milling processes have to be sensitive enough to protect the material structure. If they are not, it can result in mechanical or thermal damage to parts.
It was against this backdrop that the Steinbeis Center fitted a CNC processing center with comprehensive measuring devices. The aim was to carry out a variety of experiments in order to develop processing strategies with the lowest possible negative impact on components and highest possible productivity. Results of comparable experiments involving GFRPs and CFRPs were only partly applicable to the new testing because there are significant differences between the properties of natural fiber plastics and glass and carbon fibers. One example of this is the stress placed on tools when separating fibers.
The main focus of the research project was initially to understand mechanical factors affecting materials in the drilling and machining process. For example, the Steinbeis experts examined the reaction of the fiber matrix compound to different influences and conditions typically encountered in drilling and machine processes, and the selected processing technique. The main motivation was to produce parts with reduced forces aimed at minimizing damage. Aside from testing a broad variety of processing strategies, the Steinbeis experts measured forces exerted during the process and prepared comparative data. With the drilling processes, the focus lay primarily in possible delamination problems on the exit side of drilling and fraying. Here and on sharp edges along transitions between machining contours, it was found that with the right milling technique and cutting tools, it was possible to prevent fibers and the matrix separating off.
As expected, assuming there is plenty of room to feed in components, separating fibers is less problematic than encountered with CFRPs. From a milling point of view, the lower resistance of the natural fibers can be an advantage. This is also noticed – in comparison to CFRP processing – by the much less demanding requirement for sharp-edge cutting tools.
In terms of thermal factors when processing natural fiber plastics, it is not the tools that are the limitation but the material. This saves the need for users to acquire expensive cutting instruments, but it does require significant skill in choosing a processing technique that is not precarious for thermal reasons. This and related issues will be the focus of future work for the research team – which expressly welcomes collaboration with industrial enterprises and a specific application problem.
Prof. Dr.-Ing. habil. Hans-Volker Huth
Steinbeis Research Center Environmental Building and Building Materials (Berlin)
su0989@stw.de
Prof. Dr.-Ing. Reimund Klunder, Dr. rer. nat. Solveyg Gebhardt
HWR Berlin, FB 2, Mechanical Engineering
reimund.kluender@hwr-berlin.de