What is the manufacturing method of tungsten carbide workpieces?
Carbide workpieces can be formed by a variety of processes. Depending on the size of the workpiece, the level of shape complexity, and the production lot size, most cutting inserts are molded using a top and bottom pressure rigid mold. In order to maintain the consistency of the weight and size of the workpiece at each press, it is necessary to ensure that the amount of powder (mass and volume) flowing into the cavity is exactly the same. The fluidity of the powder is mainly controlled by the size distribution of the agglomerates and the characteristics of the organic binder. A molded workpiece (or “blank”) can be formed by applying a molding pressure of 10-80 ksi (kilopounds per square foot) to the powder loaded into the cavity.
Even at extremely high molding pressures, the hard tungsten carbide particles are not deformed or broken, and the organic binder is pressed into the gap between the tungsten carbide particles, thereby functioning to fix the particle position. The higher the pressure, the tighter the bond of the tungsten carbide particles and the greater the compaction density of the workpiece. The molding properties of the graded tungsten carbide powder may vary, depending on the amount of metal binder, the size and shape of the tungsten carbide particles, the extent to which the agglomerates are formed, and the composition and amount of organic binder. In order to provide quantitative information on the pressing characteristics of the grade of tungsten carbide powder, it is usually designed by the powder manufacturer to establish the correspondence between the molding density and the molding pressure. This information ensures that the supplied powder is in line with the toolmaker’s molding process.
Large-size carbide workpieces or carbide workpieces with high aspect ratios (such as end mills and drill bit shanks) are typically manufactured by uniformly pressing the tungsten carbide powder in a flexible bag. Although the production cycle of the equalization pressing method is longer than the molding method, the manufacturing cost of the tool is lower, so the method is more suitable for small batch production.
This process involves charging the powder into a bag and sealing the mouth of the bag, then placing the bag filled with the powder in a chamber and applying a pressure of 30-60 ksi by a hydraulic device for pressing. Pressed workpieces are typically machined to specific geometries prior to sintering. The size of the bag is increased to accommodate shrinkage of the workpiece during the compaction process and to provide sufficient allowance for the grinding process. Since the workpiece is processed after press forming, the requirements for consistency of the charge are not as strict as the molding method, but it is still desirable to ensure that the amount of powder per load is the same. If the loading density of the powder is too small, the powder loaded into the bag may be insufficient, resulting in a small workpiece size and having to be scrapped. If the loading density of the powder is too large, the powder loaded into the bag is too much, and the workpiece needs to be processed to remove more powder after press forming. Although the excess powder and scrapped parts can be recycled, this will reduce productivity.
Carbide workpieces can also be formed by extrusion or injection molding. The extrusion process is more suitable for mass production of axisymmetric shaped workpieces, while the injection molding process is commonly used for mass production of complex-shaped workpieces. In both molding processes, the grade of tungsten carbide powder is suspended in an organic binder that imparts uniformity to the tungsten carbide mixture like toothpaste. The mix is then either extruded through a hole or molded into a mold cavity. The characteristics of the grade of tungsten carbide powder determine the optimum ratio of powder to the binder in the mix and have an important effect on the flow of the mixture through the extrusion orifice or into the mold cavity.
After the workpiece is formed by molding, equalization pressing, extrusion or injection molding, the organic binder needs to be removed from the workpiece before the final sintering stage. Sintering removes the pores in the workpiece, making it completely (or substantially) dense. At the time of sintering, the metal bond in the press-formed workpiece becomes a liquid, but the workpiece can still maintain its shape under the combined action of capillary force and particle contact.
After sintering, the geometry of the workpiece remains the same, but the size shrinks. In order to obtain the required workpiece size after sintering, the shrinkage rate needs to be considered when designing the tool. When designing the grade of tungsten carbide powder used to make each tool, it must be ensured that it has the correct shrinkage when pressed under the appropriate pressure.
In almost all cases, the sintered workpiece which is also called as carbide blank needs to be post-sintered. The most basic treatment for cutting tools is sharpening the cutting edge. Many tools require grinding and geometry of their geometry after sintering. Some tools require grinding of the top and bottom; others require peripheral grinding (with or without sharpening the cutting edge). All carbide wear debris from grinding can be recycled.
