Spark Plug Insulator and Method of Making the Same
A manufacturing method, firing tray, and microwave kiln for spark plug insulators. Using microwave energy and particularly structured time-temperature profiles may allow for efficient sintering of the spark plug insulator ceramic material. The method may use a combination of radiant heat energy heating and microwave energy heating to facilitate the sintering process.
1 . A method of manufacturing a spark plug, comprising the steps of:
loading an unfired spark plug insulator into a kiln, wherein the unfired spark plug insulator has a density and at least partially comprises a ceramic material;
heating the unfired spark plug insulator with radiant heat energy when the temperature of the unfired spark plug insulator is below a minimum absorption temperature, wherein the ceramic material is at least partially transparent to microwave energy at the minimum absorption temperature;
heating the unfired spark plug insulator with microwave energy until the temperature of the unfired spark plug insulator reaches a maximum absorption temperature, wherein the unfired spark plug insulator becomes a fired spark plug insulator upon reaching the maximum absorption temperature, wherein the fired spark plug insulator has a density; and
unloading the fired spark plug insulator from the kiln, wherein the density of the fired spark plug insulator is higher than the density of the unfired spark plug insulator.
2 . The method as defined in claim 1 , further comprising the step of:
cooling the fired spark plug insulator in a thermally controlled environment until the temperature of the fired spark plug insulator reaches a thermal shock avoidance temperature.
3 . The method as defined in claim 2 , wherein the minimum absorption temperature is in a range from 560° C. to 900° C., inclusive, the maximum absorption temperature is in a range from 1300° C. to 1700° C., inclusive, and the thermal shock avoidance temperature is 1000° C.
4 . The method as defined in claim 2 , wherein the kiln includes a pre-heating zone for heating the unfired spark plug insulator with radiant heat energy, a sintering zone for heating the unfired spark plug insulator with microwave energy, and an exit zone for cooling the fired spark plug insulator.
5 . The method as defined in claim 4 , wherein the kiln includes a susceptor in the pre-heating zone for absorbing microwave energy and providing radiant heat energy.
6 . The method as defined in claim 1 , further comprising the step of inserting the fired spark plug insulator into a metallic shell having an internal bore, wherein an intermediate portion of the fired spark plug insulator is at least partially housed within the internal bore of the metallic shell.
7 . The method as defined in claim 1 , wherein the ceramic material of the unfired spark plug insulator comprises 90 wt % alumina, the density of the unfired spark plug insulator is in a range of 2.02 g/cc to 2.28 g/cc, inclusive, and the density of the fired spark plug insulator is in a range of 2.63 g/cc to 3.55 g/cc, inclusive.
8 . The method as defined in claim 1 , wherein the ceramic material of the unfired spark plug insulator comprises 96 wt % alumina, the density of the unfired spark plug insulator is in a range of 1.83 g/cc to 2.09 g/cc, inclusive, and the density of the fired spark plug insulator is in a range of 2.64 g/cc to 3.75 g/cc, inclusive.
9 . The method as defined in claim 1 , wherein the ceramic material of the unfired spark plug insulator comprises 99 wt % alumina, the density of the unfired spark plug insulator is in a range of 2.02 g/cc to 2.30 g/cc, inclusive, and the density of the fired spark plug insulator is in a range of 2.63 g/cc to 3.9 g/cc.
10 . The method as defined in claim 1 , wherein the step of heating the unfired spark plug insulator with radiant heat energy includes heating the unfired spark plug insulator at a heating rate of about 50° C. per minute.
11 . The method as defined in claim 1 , wherein the microwave energy has a frequency of about 2.45 GHz and the minimum absorption temperature is about 650° C.
12 . The method as defined in claim 1 , wherein the microwave energy has a frequency of about 915 MHz and the minimum absorption temperature is about 560° C.
13 . A method of manufacturing a spark plug, comprising the steps of:
loading an unfired spark plug insulator into a kiln, wherein the unfired spark plug insulator comprises a ceramic material that is at least partially transparent to microwave energy, and the kiln comprises a pre-heating zone, a sintering zone, and an exit zone;
heating the unfired spark plug insulator with radiant heat energy in the pre-heating zone of the kiln;
moving the unfired spark plug insulator to the sintering zone of the kiln;
heating the unfired spark plug insulator with microwave energy in the sintering zone of the kiln until the unfired spark plug insulator becomes a fired spark plug insulator, wherein the unfired spark plug insulator has a density and the fired spark plug insulator has a density, and a ratio of the density of the unfired spark plug insulator to the density of the fired spark plug insulator is in a range of about 1:1.65 to about 1:1.92, inclusive;
cooling the fired spark plug insulator in the exit zone of the kiln; and
unloading the fired spark plug insulator from the kiln.
14 . A spark plug insulator firing tray, comprising:
an exterior housing having a bottom wall and a side wall, wherein the exterior housing is at least partially transparent to microwaves and is comprised of a low mass refractory material that is coated with a layer of ceramic material, wherein the layer of ceramic material is denser than the low mass refractory material; and
a spark plug insulator support, wherein the spark plug insulator support is configured to maintain at least one spark plug insulator in a vertical position during a spark plug insulator firing process.
15 . The spark plug insulator firing tray as defined in claim 14 , wherein the spark plug insulator support is an insert placed within the exterior housing, wherein the insert is comprised of a susceptor material.
16 . The spark plug insulator firing tray as defined in claim 14 , wherein the low mass refractory material comprises mullite.
17 . The spark plug insulator firing tray as defined in claim 14 , wherein the layer of ceramic material that coats the low mass refractory material comprises mullite.
18 . The spark plug insulator firing tray as defined in claim 14 , wherein the spark plug insulator support comprises an integrally cast dimple pattern in the bottom wall of the exterior housing.
19 . The spark plug insulator firing tray as defined in claim 14 , wherein the spark plug insulator support comprises a network of hollow recesses.
20 . The spark plug insulator firing tray as defined in claim 14 , further comprising a susceptor for absorbing microwave energy and providing radiant heat energy.