Ceramic injection molding (CIM) has been the focus of much effort as a manufacturing method capable of producing very complex ceramic components in high volumes. However, the debinding stage of the process has limited its widespread use, and, thus, its true manufacturing potential. Debinding is the limiting factor in the CIM process.
To date, there has been no single debinding method that allows manufacturers to take full advantage of CIM's vast potential. An ideal CIM process is one in which molded components can be placed on trays, stacked efficiently and placed in a simple, cost-effective furnace to carry out debinding and sintering without the need for packing medias, solvents or expensive furnace equipment requirements. Today, use of CIM components is limited to very few applications. CIM parts usually are no larger than a golf ball and have as thin a cross-sectional area as possible. Larger parts can be made for use in special applications, where long debinding times, high labor rates and expensive binders and debinding equipment can be justified. However, the most widespread use of CIM is for relatively small components that are easy and cost effective to debind.
A collaborative effort that combines 18 years experience of Springboard CIM with Solar Atmospheres Inc.'s (Souderton, Pa.) 30+ years experience resolved this technical hurdle. The solution involves a practical, efficient debinding process that could open the door of CIM to many companies that never would have considered it before. The process allows manufacturing complex components having a wide range of sizes to exacting tolerances without the need for high labor costs, long debinding times, cost-prohibitive equipment and potentially harmful packing medias and solvents.
Ceramic forming methods
Engineering ceramic materials, with intrinsic resistance to heat, corrosion and wear, and electrically insulating properties, are commonly used in more extreme applications where metal and plastic materials are less likely to survive. But the properties that make ceramics attractive in extreme applications also make them more expensive to manufacture. For example, manufacturing costs can be quite high for a complex three-dimensional (3-D) ceramic component in a moderate to high volume (e.g., >10,000 units/mo) application that also requires close dimensional tolerances (e.g., +/-0.001 in./in.). The parts are very expensive due to the high cost of machining hard materials.
Current forming methods to produce complex shape components include slip casting, cold isostatic pressing (CIP), die pressing, extrusion and CIM. CIM is the only one that offers the potential to produce complex 3-D shaped components in a rapid, automated and cost-efficient manner. Four CIM process stages are compounding, injection molding, debinding and sintering.
Compounding the raw materials (ceramic powders and binders) provides a homogeneous dispersion of the individual components. The selection of the ceramic powder depends on the end product application and associated material requirements. For example, fine-grain (submicron) silicon-nitride ceramic materials are used to fabricate ceramic turbine blades, while coarse-grain materials (75 to 100 um) are used for refractory fused-silica casting cores used in the investment casting of superalloy turbine blades. Because ceramic powders initially are agglomerated, one of the primary goals of the compounding stage is complete deagglomeration of the powders.
Deagglomeration is essential for two reasons. First, ceramic agglomerates function as minute porous sponges that trap the binder. The binder serves to carry the ceramic particles into the mold during the injection molding stage. If agglomerates exist in the feedstock (compounded material), the material does not have optimum flow characteristics necessary to mold complex shape components.
Second, agglomerates function as flaw centers in final sintered components, thereby reducing product strength and reliability. The strength of a ceramic component is determined by its largest flaw, or "weakest link." Springboard carefully engineers both the ceramic-binder compatibility and the mixing process to ensure the highest degree of material dispersion. Figure 1 shows the difference between an agglomerated and deagglomerated system. In this case, a chemical means was used to deagglomerate silicon powder used to make reaction-bonded silicon-nitride parts. Careful engineering of both the chemistry and mixing process ensures the highest quality microstructure, which, in turn, produces a final product having the highest degree of reliability.
The choice of binder chemistry for the CIM process has been a subject of great interest since CIM's inception in the 1940s. The purpose of the binder phase is to form a liquid medium to carry the ceramic powders into the mold during the injection stage. The criteria for the binder phase include compatibility and nonreactivity with the ceramic particles, proper flow characteristics and the ability to burn out cleanly and easily during the binder-removal stage. Two of the most critical aspects of the CIM process are the choice of binder and method of binder removal.
Compounded material is pelletized to form the feedstock for the injection molding machine. In most cases, a thermoplastic binder is used for the CIM process. A reciprocating screw is used to convey and melt (via shear heating) the CIM feedstock to form a "shot," which is defined as the amount of material necessary to fill the injection mold, sprue and runner system. After generating the shot, the screw serves as a ram to force the molten material into a cooled mold, upon which the material solidifies and takes the shape of the mold. After the mold is filled, the pressure inside causes the material to become packed. During this packing stage, the material continues to cool, shrink and solidify, to a point at which it is strong enough to be ejected from the mold, and the cycle is repeated. Injection-molding process parameters include temperature (both mold and material), injection pressure and rate and cooling time.
Cycle time is dictated by the thickest part cross section. For a typical part having a thickness of 0.125 to 0.250 in. (3 to 6 mm), the cycle time is approximately one minute. The material is in the cooling phase for approximately 80 to 90% of that time. Increasing the number of cavities in the mold and using multiple molds in one machine can increase the throughput dramatically. In the plastics industry for example, it is common to use molds with 8, 16, 32 and even 64 cavities. Using a rotary machine with 8 molds having 16 cavities each, 7,680 parts per hour can be injection molded (16 x 60 shots/hr x 8 molds). It is this type of manufacturing throughput that makes the CIM process extremely useful for high-volume applications.
The binder must be removed from injection-molded parts before sintering. The most common method of debinding CIM components is pyrolysis in a powder bed. The green (as-molded) parts are carefully placed in an open ceramic box (sagger), which contains ceramic powder (packing media). The packing media has characteristics that allow the CIM component to be supported during the debinding process. The powder also serves as a wicking media to facilitate binder removal. Wicking together with a slow heating rate removes the binder from the parts.
The mechanisms that occur during debinding, including melting, boiling, wicking and pyrolysis, are complex. The cycle time to complete this stage depends on an number of variables including the type of binder used, the wicking power of the packing media, the size of the CIM component and the heating rate. A typical debinding schedule may involve numerous ramps and soaks at select temperatures to achieve proper debinding. The degree of debinding also is a function of the factors mentioned above. In practice, manufacturers try to remove the optimum amount of binder so the material does not become too fragile to handle.
For components having varying cross-sectional areas, thin sections contain less binder than thicker sections. In these cases, a delicate balance must be met so the thinner areas are not too fragile and the thicker sections don't contain too much binder. If this occurs, the brown part (in which binder has been removed) will either break during handling or result in bubbles and/or lamination during sintering. Assuming an optimum cycle has been developed, each brown component must be carefully excavated from the sagger, cleansed of residual powder and prepared for sintering. This time-consuming debinding process is extremely labor intensive and inefficient with respect to furnace-packing density. Also, it often results in high reject rates due to operator mishandling. However, this practice is still widely used in the CIM industry.
The second most common method of debinding is one in which the molded CIM components are immersed in a solvent bath, which serves to dissolve the soluble binder component. In these systems, the binder usually consists of two distinct phases: one that is soluble in the solvent and another that is not. The solvent can be organic or water based, but in either case, processing the solvent after debinding the components can be problematic, and often calls for special environmental precautions.
A third method of debinding is vacuum debinding. In this method, debinding and sintering can be carried out in one furnace. Debinding occurs through boiling-point suppression and evaporation of the binders at a temperature below that which would occur at atmospheric pressure. The advantage of this process is that it can be conducted without the need for packing media, solvents and extensive labor. It also uses the optimum loading density of the debinding/sintering oven. The primary disadvantage is the expensive design and construction of a vacuum tight furnace and the collection and isolation of the debinding products.
Possibly the simplest debinding method is drying. Binders based on agar and water have been developed for CIM. In this case, the part is simply dried using conventional drying methods. After the water is removed, a small percentage of the agar remains resulting in extremely rigid components, which are easily machined. The process is relatively simple, nontoxic and often inexpensive. Unfortunately, it results in CIM components lacking green strength and rigidity, so complex 3-D green components require special handling equipment. Another factor to consider is the storage and environmental control of the feedstock material. Because these binders are primarily based on water, the humidity in which the material is being processed and stored must be carefully controlled to maintain the proper binder level. Finally, these materials can be quite aggressive on molding machine components such as screws, barrels and check rings.
A final debinding method is based on sublimation. In this case, freeze driers are used to sublime high vapor-pressure organic binders at or near liquid-nitrogen temperatures. The components are injected into molds that also are maintained at or near liquid-nitrogen temperatures. The parts are then quickly and carefully transferred to freezers and stored. After enough components have been molded, racks of parts are placed in a freeze drier, which controls the vacuum level and temperature to carefully remove the binder via sublimation. Advantages of this process are the ability to debind relatively large cross-sectional area components and the efficient use of furnace-packing density. Disadvantages include special-equipment requirements to handle and process extremely cold components and the need to handle and recycle the sublimed binder phase.
Solving the debinding dilemma
Springboard CIM and Solar Atmospheres collaborated to develop a process to debind and sinter CIM components of virtually any size, shape and complexity. This can be done using inexpensive binder materials to injection mold components in a practical manufacturing environment with relatively inexpensive debinding and sintering equipment. In the process, the molded components are placed on trays in an inexpensive inert atmosphere debind/sintering furnace. The debinding process was designed so approximately 60 to 70% of the binder is removed from the component using a simple ramp/soak temperature cycle. After the binder has been removed, the components can be directly sintered in the same furnace. This occurs without the need for special binder traps, because the binders are pyrolyzed in situ during the sintering operation in an air atmosphere.
Figures 2-6 show some of the components that are possible through this method. Like in any debinding process, thicker cross sections lead to longer debinding schedules. However, the debinding/sintering timeframes of this new process make it a viable option for manufacturers considering parts of larger size and a higher degree of complexity.
The process offers distinct advantages over existing CIM processing systems, including the capability to injection mold complex hollow shapes, foamed microstructures, and components that contain multiple materials in one component. This capability provides unique, practical solutions to increasingly stringent material and product design requirements.