How To Select The Right Aluminum Furnace For Your Casting Operation

Given the various aluminum furnace styles and technologies that are available, choosing the one best suited for your casting operation can prove to be a daunting, but necessary task. Whether you are a sand foundry, die casting operation, permanent mould, or implementing any of the other aluminum casting technologies being employed, choosing the right melting furnace is paramount in determining your success, and your profitability.

The purpose of this article is to hopefully detail what type of furnace, or furnaces, have been predominately used in the various casting methods, and is solely based upon years of experience with the development, design, and sales of aluminum melting, holding, and handling equipment.

CONSIDERATIONS:
Several factors must be carefully considered prior to selecting the type of melting furnace that is best suited for a particular aluminum casting operation. In the following chart, there are listed 10 criteria that should be taken into consideration when selecting the ideal furnace for your operation.

Consider each of the 10 criteria and rate each one on a scale of 0 – 10 in terms of the importance to you, with 10 being the highest. Then add all of your ratings together to give you an “Ideal Furnace Rating”, with the maximum being 100.

Then analyze the specifications from the various furnace technologies / suppliers under each of the 10 criteria that you have established, and rate them, as you see fit. Add these ratings together for each potential technology / supplier, and compare these to your “Ideal Furnace Rating” that you have established.

The furnace that rates closest to your rating should be the one best suited, according to your requirements.

FURNACE SELECTION CRITERIA

Also, take into consideration the energy source, and their respective costs. The most commonly used energy source is natural gas. Electric resistance is used, in cases where there is a cost advantage, or where metal quality is of importance. For this discussion, we will attempt to disseminate the pros and cons of both, based solely on past experience.

Take into account what your overall goals are, the alloy, or alloys that you are casting, the required casting rate, and the pouring temperature, and relay this to your potential furnace suppliers. Then rate them according to the “Ideal Furnace Rating” that you have established.

FOUNDRIES:
This industry typically includes, sand casting, lost foam casting, lost wax, investment, etc. Other casting methods and technologies are under development, but for the purposes of this discussion, we will focus on the current commonly employed casting methods.

By far, stationary bale – out, or hydraulic tilting crucible furnaces are the most commonly used furnaces. This is due to their inherent flexibility that is required such as, frequent alloy changes, extended shutdowns, and their relatively small footprint for facilities that have limited available space. The heat source can be natural gas fired, oil fired, electric resistance heated, or electric induction.

In the case of larger capacity sand, or lost foam castings facilities, the use of gas fired, or electric resistance “globar”, wet bath reverberatory melters /, holders, gas fired dry hearth furnaces, and gas fired “Stack” melters are also utilized.

In foundries where the moulds are positioned on a rotary table, or continuous conveyor type, indexing casting system, automatic dosing furnaces, holding furnaces with metering molten metal pumps, and controlled pour, tilting furnaces, have been successfully applied. In the case of the automatic dosing furnaces, and/or holding furnaces, they are generally supplied with molten aluminum via a transfer ladle from a central melting furnace which can be either a gas fired, or electric wet bath reverb, gas fired dry hearth furnace, or in some cases, a gas fired Stack type melting furnace.

PERMANENT MOULD CASTING:
Experience shows, this industry leans towards gas fired dry hearth furnaces, and either gas fired, or electric resistance heated crucible furnaces. In cases where the metal quality is critical, or for electric motor rotor casting suppliers, the predominant choice is the electric resistance heated “globar” aluminum wet bath reverb furnace. This due to their high energy efficiency of 60 – 65%, the superior metal quality, and the low melt losses of less than 3%.

LOW PRESSURE CASTING:
With the casting method, the low-pressure casting machine normally includes its own pressurized holding furnace with a riser stalk, to feed the molten aluminum through the bottom of the die. These integral machine pressure furnaces are supplied with the molten aluminum from a melting furnace via a transfer ladle. The melting furnace can be gas dry hearth furnaces with manual tap blocks, gas, or electric tilting crucible furnaces, gas or electric resistance wet bath reverb furnaces. There also is a system available where the pressurized holding furnace is connected to a heated launder system with a molten metal metering valve and the central melting furnace.

HIGH PRESSURE DIE CASTING:
This industry by far, uses the most varying types of furnaces in the aluminum casting industry. They can be range from crucible furnaces, to central reverberatory melting furnaces, low energy electric, or gas fired holding furnaces, automatic dosing furnaces, and electrically heated distribution (launder) systems.

For small to medium sized, (1 – 4 diecasting machines), or captive shops casting parts for their own product line, (fence fittings, ornamental furniture, etc.) gas fired, or electric resistance crucible furnaces are the most commonly used. Particularly for jobbing shops where frequent alloy changes are required, crucible furnaces are the best choice.

Another choice to consider for a low to medium sized facility is “ in-cell “ systems where each cell consists of a melting and holding furnaces, gas fired, or electric resistance wet well reverb furnaces. With this concept you can dedicate single cells to a given alloy. Each cell would consist of a diecasting machine, melting / holding furnace, automatic ladler, and trim press, with the cells returns, (gates, risers, biscuits, trimmings) being returned to the melting / holding furnace, for remelt.

Larger, single alloy users predominately use gas fired central melting furnaces which can be wet well reverbs, dry hearths, and, stack type furnaces. Molten aluminum is often transferred to the machine holding furnaces via bull ladles, and in some cases through an electric resistance heated launder (trough) system which via gravity, continuously feeds the holding furnaces from the melter(s).

These large single alloy diecasting companies are for the most part, automotive suppliers of aluminum transmission cases, engine blocks, heads, and various suspension components. These facilities cast the same alloy at every cell, and due to the volumes of metal consumed, and as stated, they employ central gas fired wet well reverbs, or Stack type melting furnaces.

The molten aluminum is dispensed to the bull transfer ladles via manual tap-out blocks, pneumatic dispensing wells, or molten metal pumps. In most cases, the bull ladles are tilting tow motor operated, for distribution, as required, to the machine holding furnaces. The holding furnaces can either be, gas fired, or electric resistance, low energy furnaces. They have a molten metal receiving well, a hold zone, and a ladle well, sized to accommodate the automatic ladler which dispenses the molten metal to the shot sleeve of the high pressure die casting machine.

As mentioned earlier, another option is what is referred to as a “launder system”, which is an electric resistance heated, refractory lined, gravity fed, molten metal delivery system. With this concept, which has been adopted by a number of large captive die casting plants who basically cast one alloy, the system employs one or more, gas fired, central melting furnaces, which are the “core” of molten metal distribution to the machine holding furnaces through the direct connected launder which supplies at temperature metal.

Of late, the “European” technology of automatic “dosing” furnaces, and “Japanese” technology of Electric Immersion Tube aluminum holding furnaces, have been accepted by some North American die casting companies, and will be addressed in future articles.

The following is the rating of the various aluminum melting furnace technologies that have been detailed in this article, and are based upon a theoretical melting efficiency of 100 %, and the energy required to melt aluminum, of 487 BTU / lb. Also included is the holding efficiency ranking, with # 1 being the most efficient.

ELECTRIC RADIANT WET BATH REVERB
Melting Efficiency % ………….. 63 %
Melting Energy (BTU/lb) ……. 767
Btu/lb. Capacity …………………7.68
Holding Capacity ………………. 10,000 lbs.
Btu/Hr. …………………………….. 76,800
Holding Rank ……………………. 1

ELECTRIC RESISTANCE CRUCIBLE
Melting Efficiency % ………….. 57 %
Melting Energy (BTU/lb) …….. 853
Btu/lb. Capacity ………………… 26
Holding Capacity ……………….. 3000 lbs.
Btu/hr. ……………………………… 78,000
Holding Rank …………………….. 2

GAS FIRED STACK MELTER
Melting Efficiency % …………… 54 %
Melting Energy (BTU/lb) …….. 900
Btu/lb. Capacity ………………… 45
Holding Capacity ……………….. 2500 lbs
Btu/ Hr. …………………………….. 112,500
Holding Rank …………………….. 3

GAS FIRED DRY HEARTH
Melting Efficiency % …………… 27 %
Melting Energy (BTU/lb.) …….. 1800
Btu/lb. Capacity …………………. 75
Holding Capacity ………………… 3000 lbs.
Btu/ Hr. ……………………………… 225,000
Holding Rank ……………………… 5

GAS FIRED WET WELL REVERB
Melting Efficiency % ……………. 22%
Melting Energy (BTU/lb) ……… 2000
Btu/lb. Capacity …………………..75
Holding Capacity ………………… 10,000 lbs.
Btu/ Hr. ……………………………… 750,000
Holding Rank ……………………… 6

GAS FIRED CRUCIBLE
Melting Efficiency % ……………. 20 %
Melting Energy (BTU/lb.) …….. 2436
Btu/lb. Capacity …………………. 65
Holding Capacity ………………… 3000 lbs.
Btu / Hr………………………………. 195,000
Holding Rank ……………………… 4

NOTE: The holding capacity of the above furnaces is based upon a 1,000 lb / hr. melting rate.

SUMMARY:
The melting of aluminum is a “necessary evil” in the casting industry, and the selection of a new melting, holding furnace, or molten metal handling system, should be considered as carefully as that of a new die casting machine, or machining center. The most sophisticated machining center or die casting machine cannot correct a problem that occurred in the first step, the melting furnace, and the metal quality that it produces.

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ABOUT THE AUTHOR / DISCLAIMER:
This article was authored by Ed Lange who is an Aluminum Furnace Consultant elange2@cogeco.ca  The information presented in this article is based upon the author’s 40 plus years of experience in the development, design, and sales of non-ferrous melting, holding, and handling equipment. The author, does not in any way, endorse any particular furnace manufacturer, or their respective technologies.