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.
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