Cemented carbide die tools can be formed by a variety of processes. According to the size of the workpiece, the level of complexity of the shape and the production batch, most of the cutting inserts are molded with top pressure and bottom pressure rigid die tools. In each press, in order to maintain the consistency of the weight and size of the workpiece, 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. By applying 10-80ksi (thousand pounds per square foot) molding pressure on the powder loaded into the mold cavity, a molded workpiece (or "blank") can be formed.
Even under extremely high molding pressure, the hard tungsten carbide particles will not be deformed or broken, while the organic binder is pressed into the gaps between the tungsten carbide particles, thereby fixing the position of the particles. The higher the pressure, the tighter the bonding of tungsten carbide particles and the greater the compaction density of the workpiece. The molding characteristics of the grades of cemented carbide powder may vary, depending on the content of the metal binder, the size and shape of the tungsten carbide particles, the degree of agglomeration, and the composition and amount of the organic binder. In order to provide quantitative information about the compaction characteristics of grades of cemented carbide powder, the powder manufacturer usually designs and builds the corresponding relationship between the molding density and the molding pressure. This information ensures that the supplied powder is in harmony with the tool manufacturer’s molding process.
Tungsten steel sheet
Large-size cemented carbide workpieces or cemented carbide workpieces with high aspect ratios (such as end mills and drill shanks) are usually manufactured by uniformly pressing grades of cemented carbide powder in a flexible bag. Although the production cycle of the balanced pressing method is longer than that of the molding method, the manufacturing cost of the tool is lower, so this method is more suitable for small batch production.
This process method is to put the powder into a bag and seal the mouth of the bag, then place the bag filled with the powder in a chamber, and apply a pressure of 30-60 ksi through a hydraulic device for compression. The pressed workpiece is usually processed into a specific geometric shape before sintering. The size of the material bag is enlarged to adapt to the shrinkage of the workpiece during the compaction process and to provide sufficient margin for the grinding process. Since the workpiece needs to be processed after pressing, the requirements for the consistency of the filling are not as strict as the molding method, but it is still hoped to ensure that the amount of powder filled into the bag is the same every time. If the charging density of the powder is too small, it may lead to insufficient powder in the bag, resulting in the size of the workpiece being too small and having to be scrapped. If the charging density of the powder is too high, and there is too much powder in the bag, the workpiece needs to be processed to remove more powder after being pressed. Although the removed excess powder and scrapped workpieces can be recycled and reused, doing so will reduce production efficiency after all.
Cemented carbide workpieces can also be formed by extrusion die tools or injection die tools. The extrusion molding process is more suitable for mass production of axisymmetrically shaped workpieces, while the injection molding process is usually used for mass production of complex shaped workpieces. In these two molding processes, the grade cemented carbide powder is suspended in an organic binder, which gives the cemented carbide mixture a uniform consistency like toothpaste. Then, the mixture is either extruded through a hole or injected into a mold cavity to form. The characteristics of the grade cemented carbide powder determine the ratio of the powder to the binder in the mixture, and have an important influence on the fluidity of the mixture through the extrusion hole or into the mold cavity.
After the workpiece is formed by molding, isostatic pressing, extrusion or injection molding, the organic binder needs to be removed from the workpiece before the final sintering stage. Sintering can remove pores in the workpiece and make it completely (or substantially) dense. During sintering, the metal bond in the pressed workpiece becomes liquid, but under the combined action of capillary force and particle connection, the workpiece can still maintain its shape.
After sintering, the geometry of the workpiece remains unchanged, but the size will be reduced. In order to obtain the required workpiece size after sintering, the shrinkage rate needs to be considered when designing the tool. When designing the grades of cemented carbide powder used to manufacture each tool, it must be ensured that it has the correct shrinkage rate when compacted under the appropriate pressure.
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