Co-Molded Ceramic and Metal Parts Boost Part Properties

Two-component plasticinjection molding is a well-developed process to produce parts that combineproperties of two different polymers, such as a hard core from polypropyleneand a soft cover from thermoplastic elastomer.

Now researchers in Germany aretrying to achieve the same effect with ceramic matched to metal or one type ofceramic married to another ceramic via the powder injection molding process.

"If we want to combine metalswith ceramics, or different ceramics with one another, we need to findmaterials that will shrink to the same extent at the same temperatures," saysDr. Reinhard Lenk, a project manager at the Fraunhofer Institutefor Ceramic Technologies and Systems in Dresden, Germany.  Shrinkage rates are even more important inpowder injection molding than in plastic molding because powder components alsogo through a sintering process to remove plastic binders.  

Materials engineerssimulated shrinkage behavior of a wide variety of materials before identifyingcandidate feedstocks with necessary mechanical properties.

In a specific projectsponsored by the European Union called CarCIM, engineers produced prototypediesel spark plugs from two different ceramics. The new plugs are described asmore durable, more corrosion resistant and cheaper than traditional metaligniter plugs. Fraunhofer says they will work well in  new engines that operate at higher temperaturesand are very efficient.

The two ceramic materialsprovide, respectively, insulation and electrical conduction. The conductivematerial is molded, inserted into another tool and over-molded with theinsulating composition. Dr. Tassilo Moritz, who heads the CarCIM project, saysthe insert process is faster and cheaper than traditional assembly methods suchas soldering.

The co-molded part is thensintered using kinetic models, says Moritz. The critical interface between thetwo materials has been studied and verified with optical and electronmicroscopy. Other testing methods have also been used to improve processparameters. Particle size distribution of the powder is a critical factor,according to the Fraunhofer investigators.

In a separate project, testsare under way on the co-molding of metal and ceramic powders for use in medicalapplications, such as minimally invasive forceps in which high-frequencycurrent 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-ceramicsforceps, the jaws operate as a plus and minus pole simultaneously. Theconductive metal component is in the arm of the forceps and is over-molded withthe ceramic insulating material. Another variation of the injection moldingprocess is used to produce the forceps. A tool cavity is covered with a ceramictape prior to injection of a metal powder. Solder metals such as titanium orsilicon are mixed into the metal powder to facilitate adhesion with the ceramictape.

Theforceps are still being tested to determine if the ceramic layer can withstandthe frequent sterilization required in medical environments. Single-use twocomponent forceps are also a possibility.