Graded coatings for the inner walls of screw barrels
In plastic extrusion and injection moulding, the raw materials and additives are fed into a heated barrel where a screw head turns to mix and force the materials into a mould. The screw-barrel system is affected the most by wear and corrosion problems. This is due to the friction between the plastic materials and the screw with the inner walls of the barrel. Conventional coatings are uniform throughout but both mechanical and chemical stresses vary along the length of the barrel. Researchers working on the EU-funded project DEBACOAT (Development of high-performance barrels with innovative gradient coatings) went beyond the state of the art and developed screw and barrel coatings featuring a compositional gradient along their length. Industrial partners relied on a knowledge-based methodology rather than the current trial-and-error methods to select the most suitable powder materials and processing techniques. Based on systematic scientific investigations, they gained a better understanding of the relationship between materials and processes to obtain high-quality coatings. Powder materials were characterised according to chemical composition, microstructure, mechanical and chemical corrosion properties as well as behaviour during processing. The benefits of graded coating for bimetallic barrels include higher performance and increased process efficiency with limited impact on the original product concepts. The team identified environmental concerns to be addressed with regard to powder processing. These included energy consumption, waste production, release of particles in the working environment, and lack of recycling strategies. Significant actions were devoted to transferring the acquired knowledge to the industrial field. The estimated 20 % decrease in the wear and corrosion rate in laboratory samples should extend service life by 20 % and reduce the environmental impact of the manufacturing chain by an equal amount. The industrial project partners used EU funding to improve flexibility, reliability and repeatability in their production process. DEBACOAT graded coatings for screw barrels promises to enhance the competitive position of small- and medium-sized enterprises in the coatings industry as well as the plastics processing sector. This technology is expected to reduce the environmental impact of extrusion and injection moulding by decreasing energy consumption and waste destined for landfills.
Keywords
Screw barrels, screw extrusion, screw injection moulding, plastic-based products, graded coatings
