Two-component plastic injection molding is a well-developed process to produce parts that combine properties of two different polymers, such as a hard core from polypropylene and a soft cover from thermoplastic elastomer.
Now researchers in Germany are trying to achieve the same effect with ceramic matched to metal or one type of ceramic married to another ceramic via the powder injection molding process.
"If we want to combine metals with ceramics, or different ceramics with one another, we need to find materials that will shrink to the same extent at the same temperatures," says Dr. Reinhard Lenk, a project manager at the Fraunhofer Institute for Ceramic Technologies and Systems in Dresden, Germany. Shrinkage rates are even more important in powder injection molding than in plastic molding because powder components also go through a sintering process to remove plastic binders.
Materials engineers simulated shrinkage behavior of a wide variety of materials before identifying candidate feedstocks with necessary mechanical properties.
In a specific project sponsored by the European Union called CarCIM, engineers produced prototype diesel spark plugs from two different ceramics. The new plugs are described as more durable, more corrosion resistant and cheaper than traditional metal igniter plugs. Fraunhofer says they will work well in new engines that operate at higher temperatures and are very efficient.
The two ceramic materials provide, respectively, insulation and electrical conduction. The conductive material is molded, inserted into another tool and over-molded with the insulating composition. Dr. Tassilo Moritz, who heads the CarCIM project, says the insert process is faster and cheaper than traditional assembly methods such as soldering.
The co-molded part is then sintered using kinetic models, says Moritz. The critical interface between the two materials has been studied and verified with optical and electron microscopy. Other testing methods have also been used to improve process parameters. Particle size distribution of the powder is a critical factor, according to the Fraunhofer investigators.
In a separate project, tests are under way on the co-molding of metal and ceramic powders for use in medical applications, such as minimally invasive forceps in which high-frequency current heats a tip to cauterize tissue. In currently used single-pole forceps, current flows from the forceps back through the patient's body.
In the new metal-ceramics forceps, the jaws operate as a plus and minus pole simultaneously. The conductive metal component is in the arm of the forceps and is over-molded with the ceramic insulating material. Another variation of the injection molding process is used to produce the forceps. A tool cavity is covered with a ceramic tape prior to injection of a metal powder. Solder metals such as titanium or silicon are mixed into the metal powder to facilitate adhesion with the ceramic tape.The forceps are still being tested to determine if the ceramic layer can withstand the frequent sterilization required in medical environments. Single-use two component forceps are also a possibility.