Improved Wear Protection
Hardmetal-coated plasticating screws are ideal for processing engineering resins with a high filler content. However, hardmetals (often called metal carbides) exhibit little toughness, so that they have difficulty withstanding the elastic deformation experienced by the screw’s steel and tend to crack. Moreover, hardmetals adhere only poorly to steel surfaces. This poses the risk of delamination of the coating. A research project at Engel has now succeeded in improving the mechanical properties of the hardmetal/steel combination to the point where the potential of hardmetals can be truly exploited to provide protection against wear in real-world applications.
The hardmetal powder is heated to about 1200°C in the coating system and accelerated to speeds of up to 2000 m/s. At these high speeds, the semi-liquid particles impinge on the blank and embed themselves in the roughened surface. The glowing-hot powder particles are quickly quenched from 1200 to about 150°C. In 30 to 60 cycles, a largely uniform layer is built up over the screw’s entire surface – outer face (OD) of the flights, edges of the flights, radius in the transition from flight to root and root diameter.
The coating is then annealed in an oven in a protective gas atmosphere or vacuum. During heating, the microstructure of the base material changes from cubic body-centered ferrite to cubic face-centered austenite. The interfacial layer between the coating and base material changes as well, increasing the adhesion. After a certain holding time, the steel is cooled in stages. This staged cooling process optimizes the properties of the coating and base material, and permits fabrication of a screw with almost no cracks of distortion.
The coating produced in this way is characterized above all by improved durability. Furthermore, adhesion is increased significantly by the annealing process. While standard coatings exhibit adhesive bond strengths between 90 and 110 MPa, the Onyx coating system achieves values of 250 to 350 MPa. This reduces the risk of crack formation and delamination noticeably. Coated screws are available in diameters from 25 to 70 mm.