- THE MAGAZINE
Over the past several years, ceramic injection molding (CIM)—perhaps more commonly known as powder injection molding (PIM)—has begun gaining popularity as a way to fabricate ceramic powders into useful shapes. A descendant of traditional plastic injection molding, the technology combines the advantages of shape complexity with higher production rates.
The basic process consists of mixing a fine ceramic powder with a suitable binder to form a homogeneous mixture, followed by a granulation operation to form a feedstock. This feedstock is then molded at relatively low temperatures and pressures in conventional plastic injection molding machines. The molded "green" parts are then subjected to a suitable process to remove the binder, followed by a high temperature sintering process to obtain the desired mechanical and physical properties in the components. In some cases, it may be possible to combine the debinding and sintering phases into a single step. The main steps involved in the ceramic injection molding process are shown in Figure 1.
With ceramic injection molding, ceramic manufacturers can efficiently produce components with close tolerances and complex shapes.
The CIM ProcessPowder Preparation. Ceramic powders are generally produced by one or more of the following methods: precipitation and calcination, milling and grinding, chemical reaction, and/or plasma atomization. In addition to chemical impurities, the shape, size and size distribution of the powder particles will affect subsequent processing steps and the characteristics of the final product. Regular-shaped, submicron-size bimodal powders are desirable for better processing and high-quality final product characteristics. Irregular-shaped particles can increase strength after debinding but make sintering densification more difficult. It is therefore important to select a powder that has the appropriate characteristics and is compatible with both the available processing equipment and the desired final product properties.
Binder Addition. The main functions of a binder in ceramic injection molding are to provide a medium for uniformly packing the powder during molding and to hold the molded shape to the point at which the sintering process begins. During the debinding stage, the binder must be capable of being eliminated completely, without leaving any residues. The binder system is typically composed of a polymer, such as polyethylene, polypropylene, or other suitable material; a low-temperature melting material, such as wax, which can be extracted easily during the first phase of debinding; and a surfactant/lubricant that lowers the surface tension and hence promotes the wettability of the powder particles. Several binder systems are commercially available for applications in ceramic injection molding.
Since the quality of the final components depends on the uniformity of the feedstock, the feedstock must be prepared as a homogenous and contamination-free mix. Equipment that has been used successfully to produce high-quality feedstocks includes twin-roll mills, single-screw or twin-screw extruders, ball mills, z-blade and double planetary mixers, and twin-cam and plunger-type extruders.
Granulation/Pelletizing. Once a high-quality feedstock has been developed, it must be converted through granulation or pelletizing into sizes suitable for loading into molding machines. Granulation consists of forcing the feedstock through a rotary cutter, thus producing granules of irregular shape and size with a lower mixture homogeneity. When the feedstock is forced through a heated extruder, it produces a strand with a uniform cross section, which is cooled on exit and subsequently chopped using a rotary cutter--a process called pelletizing. Pelletizing produces pellets of uniform shape and size with better feedstock homogeneity. Granulation is primarily used for recycled materials from runners and sprues, while pelletizing is widely used for virgin stock.
Sintering. Sintering is the most critical step of the CIM process. In this stage, the components are heated to higher temperatures, and the material achieves its final properties and dimensions. The heating rate, sintering temperature and time at sintering temperature are crucial and can affect the properties of the final products. During high temperature sintering, pores are eliminated and densification occurs, causing the component to shrink to smaller dimensions. Sintering increases the hardness, strength, density and other engineering properties of the final components. The finished part retains the original shape and complexity of the molded part, and close tolerances can be achieved. If the size and shape of the original powder and all other processing variables are properly controlled, a predictable and consistent sintering shrinkage can be obtained. The tooling must be precisely oversized and compensated by the predicted amount to ensure that the sintered part shrinks to the required dimensions.
Secondary Processing. Whenever required, sintered components can also be subjected to secondary processing such as coining, tumbling, impregnation, etc.
CIM AdvantagesCompared to other conventional ceramic fabrication processes, CIM offers several advantages, including:
- Production of complex shapes in high quantities
- Production of higher and uniform density components with minimum distortion
- Production of net-shaped or near net-shaped components
- Better mechanical properties in the finished components
The proper selection of the ceramic-binder system, adequate control of the powder characteristics, and the development of suitable debinding cycles are crucial factors for successful applying the CIM technique in industrial environments. However, once these factors are controlled, injection molding is an economical way to produce components with complex shapes and superior technical properties.
CIM has already been successfully applied to produce various components such as ceramic ferrules, cutters, tooling and cores for investment casting, and other industrial products. As the understanding of the ceramic injection molding process continues to evolve, the market for these ceramic components has the potential to increase exponentially.
For further reading:- Mutsuddy, Beebhas C., and Ford, Renee G., Ceramic Injection Molding, Chapman and Hall, 1995.
- German, Randall M., and Bose, Animesh, Injection Molding of Metals and Ceramics, Metal Powder Industries Federation, 1997.