ENAMELED ALUMINIUM PAN SUPPORTS. A NEW APPLICATION
Louis J. Gazo, Charles A. Baldwin, William D. Faust
Appliance Division, Ferro Corporation, USA

Introduction
The current state-of-the-art of enameled pan supports could be considered cast iron and within the category of cast iron, electrophoretic deposition.
The reasons for the high quality associated with cast iron are the heat capacity of the metal, the thickness, and the final enameled appearance of the parts when produced well.
Other types of pan supports that are currently produced are cast iron with electrostatic dry powder enamel application as well as wire grates of various dimensions with wet spray and electrostatic powder application.
All of these products have specific markets and which span the range of cooking products produced worldwide. However, each of these products has limitations.

Pan Supports - Technical Needs
Pan supports are specifically needed for use on gas fired stove surfaces. They function to support the weight of various cooking utensils containing significant weight as well as to control the distance between the flame and the cooking utensil.
Exposure to the gas flame immediately subjects the metal-enamel composite to extremely rapid temperature excursions as well as high surface temperatures and oxidation.
Additionally, the gas flame heat causes static fatigue of the metal and enamel.
A pan support needs to be able to address the thermal shock, heat durability and esthetic needs of the producer.
Thermal shock is related to the ability of the pan support to minimize the structural degradation caused by sudden temperature shocks such as being exposed to water spillage during use as well as mechanical shocks when being loaded.
The heat durability of the pan support is equally important to maintain theappearance and anti-corrosion properties of the coated surface.
Finally, the esthetic appearance of the pan support should not significantly change in use over prolonged periods of time.

Pan Supports - Commercial Needs
Commercially, pan supports are important for the appearance of a range surface. They must well represent the quality of the stove and its apparent durability.
Performance of the pan support must extend over a significant portion of the useful life of the appliance without deterioration. To date, only a limited color pallet has been achieved for pan supports; black, gray and taupe.
These colors are adequate to maintain the esthetic appearance of the pan supports, but careful examination shows that they mask the gradual deterioration of the enamel surface due to heat exposure and wear over time.

Metal Characteristics
For over 100 years metals used for pan supports have been iron, due to the strength and high melting temperature of this metal. Iron and steel serve well as a support medium when properly designed. Thickness of the metal is an important characteristic of the heat transfer capacity.
Other lower melting point metals have not been used because the high temperature strength has not been considered sufficient to achieve the required durability and fatigue strength.
Aluminum metal has been used for stovetop applications such as burner caps for gas ranges.
In many instances, these pieces have been enameled but have not been load bearing components of the designed stove.

Aluminum Characteristics
Aluminum metal has a melting point of about 660 °Celsius.
Since the metal has a crystal structure either in sheet or cast form, the melting point is relatively sharp.
This would mean that high temperature strength would be lost precipitously if the melting point temperature were exceeded for even a short period of time.
However, the heat transfer characteristics of aluminum are significantly different and higher than that of iron and steel.
Reported values for the thermal conductivity of cast aluminum (Type 356) are 140 to 160 W/m K1 versus approximately 50 W/m K² for iron.
This greater thermal conductivity of aluminum allows for the possible design means to move the heat and maintain the structural integrity of the part made with aluminum.
Because aluminum alloys have been enameled commercially for about 40 years, alloy compositions that exhibit acceptable performance and produce acceptable esthetic properties are well known.

figure 1A - Prepared Cast Aluminum Pan Support

Design Considerations
The necessity of moving heat from one point to another is known and can be controlled. With conventional cast iron pan supports, the temperatures observed at the point of flame contact are normally 650 °Celsius to about 750 °Celsius with the flame temperature as high at 1010 °Celsius which exceed the melting point of all aluminum alloys for enameling.
These temperatures also exceed the fusion temperature of the enamel glass coatings. Moving the heat away from the point of impingement is a necessity and a virtue.
A cross-section of sufficient width and depth is needed to achieve this.
In practical terms, a cross-section of about 10 mm for the width and depth is needed at the narrowest point to allow a sufficient amount of heat flux to move from one point to another.
Measured pan support temperature of the aluminum alloy pan supports was approximately 400 °C.
This is well below the melting point of the aluminum alloys and fusion temperature of the enamels.
Repeated testing confirmed the good results.
The pan support cross-section needs to be robust enough to support the intended weight with some safety margin.
The enamel coating should be as smooth as possible. Excessive enamel coating thickness to cover a rough casting will contribute to spalling for both cast iron and cast aluminum. Coating thickness of about 75 to 100 microns will ordinarily produce good system performance and esthetic results.

Enameling Aluminum Castings
Enameling of aluminum metal requires consideration of the alloy types and their processing. Alloys which have produced acceptable results for enameling have been identified as 4043, 356 and 43.
The methodology of casting is critical. Sand casting and permanent mold casting are recommended to control the porosity of product.
Die cast parts have not achieved success due to excessive porosity, which results in out gassing during the enameling operation and weakens the coating structure. Weakness causes spalling of the glass surface and/or generation ofexcessive gas blisters.

figure 1B - Prepared Cast Aluminum Grate, Close-up of Ring

Alloys that contain magnesium may be enameled if chromate treated or if a new Ferro additive is used. Chromate is undesirable due to environmental pollution concerns.

figure 2 - Coated Cast Aluminum Pan Support, Black Color

The composition of these alloys is shown in Table 1. Treatment of these alloys is primarily traditional cleaning involving alkaline etch cleaning as described in Table 2.

Pan Support Testing and Characterization
The concept of aluminum pan supports was initially tested with alloy 4043 made by sand casting. The mold was based on cast iron ware design.
The castings were enameled in the traditional wet spray manner. (Thickness of the casting required firing times commensurate with the thickness of the cross-section).
Firing temperature was conventional for aluminum substrates.

Testing of the pan supports included standard tests, such as thermal shock and prolonged heat exposure as well as loading with simulated and real cooking weights.
Initial test results of thermal shock showed that there was excellent resistance to shock, in fact the aluminum enamel coating could not be made to deteriorate.
Thermal shock of the parts was done by heating them to 343 °Celsius for 30 minutes followed by immediate immersion in water at 21 °Celsius.

The samples were dried and evaluated. This test was repeated for three cycles. Next, the heat exposure testing was carried out.
Short duration exposures (1 hour) were initiated with good success. Weekend long exposure (72 hours) with a heavy cast iron plate on a cast aluminum pan supports accidentally occurred.
No deterioration due to heat exposure was noted. Following these initial results, a formalized program of testing was started.
Side by side testing of aluminum grates with conventional cast iron and pressed steel pan supports was conducted with controlled experimentation.

Summary of Results
Upon completion of the formalized testing program, various types of aluminum enamels were evaluated.
Color does not pose a limitation on enameled pan supports. With proper design to control the heat flux, white, almond, and most other light colors may be produced. Special effects may also be possible.
Size poses less of a design obstacle for aluminum than for cast iron. Larger scale pan supports may be produced that minimize the number of parts on the top of a stove. The lower weight of the aluminum pan supports makes them easy to handle and less prone to damage the top of the stove (hob) due to accidental dropping.

Testing and Characterization
The unexpected performance of the aluminum metal pan supports is due to the high heat flux of aluminum that is reported to be 140 W/m K versus about 50 W/m K for iron.
The thermal conductance may be denoted by the following equation:

Substituting various values for metals, one may see that aluminum has about three times the thermal conductance of iron.
Flame temperatures of 650 °Celsius to 750 °Celsius are readily achieved.
However, the aluminum metal acts as a heat pump to quickly lower the tip temperature.
The temperature of the aluminum grates during heat test is approximately 400 °Celsius, which is about 200 °Celsius below the metal melting point.

figure 3 - Coated Cast Aluminum Pan Support, Gray Color

Conclusion
A new way has been found to extend a known technology in a desirable way. Aluminum pan supports offer new market opportunities pleasing to consumers. The new pan supports will enlarge the design range in size, color and feel.
Possibly less damage to hobs will be forthcoming with the lighter weight pan supports. As aluminum is a widely used metal that is easily recycled; this new application will further enhance the environmental appeal of porcelain enamel.
Various pan supports are illustrated, as well as a prepared aluminum enamel casting.

References
1. Metals Handbook, 9th Ed., Vol. 2, ASM © 1979, Table 17, Page 752.
2. Metals Handbook, 9th Ed., Vol. 2, ASM © 1979, Page 165.
3. Principles of Heat Transfer, 2nd Ed., Frank Kreith, International Textbook Company © 1967, Page 12.