Machine molding process
1, The process of solidifying powder into the desired shape is called molding. The most commonly used method is pressure forming (die pressing), which involves compressing materials using molds or other tools. In addition to technical classification, molding can also be classified based on the degree of completion of the formed products. Near-net forming is a method of shaping complex finished products into shapes close to their original form, minimizing mechanical processing as much as possible. Net forming is a method for shaping finished products.
2. Mold forming
The method of pressure forming using molds is called die pressing. This is the most commonly used sintering method. The process of powder die pressing can be roughly divided into three steps: powder filling, pressurization and block removal.
Applying high pressure during the pressing process can increase the density of the powder compact, thereby enhancing its strength. Generally, applying a pressure of approximately 100 MPa is sufficient to achieve an adequate compact density. For common sintered powders, the compact density is 50-60%. For metal powders, when pressure is applied, the powder particles undergo plastic deformation, which enables the attainment of a higher density.
To enhance the strength of the powder compact, additives such as binders are sometimes added to the powder. Specifically, these additives are organic substances, such as polyvinyl alcohol (PVA, also known as polyvinyl butyral) and polyvinyl butyral (PVB). The granular powder itself contains this adhesive. Additionally, lubricants can be added to reduce the friction between the mold and the powder particles as well as among the powder particles themselves. Commonly used lubricants include stearic acid and zinc stearate. The granular powder mixed with the lubricant and binder is then placed in the mold and pressed into shape.
Another method is to make a mold with rubber or soft plastic and then apply hydrostatic pressure to the liquid or gas medium to form it. This method is called the rubber pressing method, or cold isostatic pressing (CIP). The rubber compression molding method can achieve near-net molding, and due to the application of hydrostatic pressure above 100MPa, a relatively homogeneous and high-density molded body can be formed.
3. Other molding methods
Injection molding is a method of mixing powder with thermoplastic polymers and then pressing them into a mold at high temperatures. It is commonly used for large-scale production of small products. For metals, it is called Metal Injection Molding (MIM), and for ceramics, it is called Ceramic Injection Molding (CIM). Whether it is metal powder injection molding or ceramic injection molding, it can achieve near-net-shape forming of small and complex shapes.
Slip Casting is a method of forming by pouring a slurry composed of liquids such as water and powders into a porous mold. For oxide ceramics, a slurry is usually made by mixing water with the powders; for non-oxide ceramics or metals, organic solvents such as ethanol are sometimes used. The key point is how to effectively disperse the powder particles in the slurry without causing agglomeration. The disadvantage is that the forming speed is very slow. This forming method can also achieve linear near-net-shape forming and is widely used in ceramics. In the early days, this method was used for the manufacturing of traditional ceramics such as tableware, tiles, and decorative items; now, it has been applied in the forming of engineering ceramics and metal parts.
Tape Casting (also known as Doctor Blade Method) is one of the application methods of slurry casting. It involves forming the slurry into a plate shape on a plastic sheet, and then shaping it into a thin plate-like structure.
Gel Casting is an improved method of casting. In gel casting, monomers are added to the slurry, and after being placed in the mold, the monomers undergo polymerization to achieve gelation. This molding method can achieve the molding of complex shapes, has a shorter molding time, and the density within the molded body is uniform. It is widely used in ceramics and has recently been reported to be applied in the metal field as well. Figure 4 shows the photo of porous ceramics after sintering using gel casting. Even for complex shapes with structures such as large grooves, it can be accurately molded, which is one of its advantages.
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