10 ways to improve melting: achieving perfection may be too much to ask from your aluminum melt department, but adhering to the quality procedures and tips listed here can help you close the gap.
"In this day and age, I feel that the melting/molding departments really need to emphasize strict adherence to procedures," said Jason Place, Ohio Aluminum Industries Inc., Cleveland.
"Not doing so will make a good job bad."
1 Put a Lid on It
While there may be very little that metalcasters can do to control the market price of utilities, a great deal of control can be exercised over how efficiently the utilities are managed.
According to Mike Nelson, customer service manager of Thermtronix Corp., Adelanto, Calif., cost studies show that 15% of the heat lost from most crucible-style furnaces escapes from the exposed molten metal surface (Fig. 1). You can improve your furnace efficiency by keeping an insulated lid on it. If your furnace already has an insulated lid, then insist that operating personnel use it. If you don't have insulated lids, acquire one for each furnace.
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Once your lids are in place, record the recent history of your gas cost and production data and compare it to previous months on your utility bills. Nelson said the 15% savings will be easy to spot when it's time to write the check.
Furnace lids and covers may be as simple as single piece units that cover the exposed surface area whenever metal is not being charged into or ladled out of the furnace. More energy efficient lids may consist of sections that can be individually removed as required for charging or ladling. This minimizes heat loss during normal operations while still allowing for complete removal of the lid when cleaning or maintenance is required.
Energy-saving, insulated, sectional manual service lids (Fig. 2-3) may be purchased complete and ready for operation for as little as $700. Or, you can put your maintenance staff to work on designing and fabricating suitable lids for your furnaces. Just be sure to use high quality, lightweight, energy efficient materials and practice good safety.
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2 Keep the Good Always Good land the Bad Always Bad
According to Place, even bad melts can be reviewed and investigated if they are continuous. For instance, if you typically add your modifying element (either strontium or sodium) to the melt at 0.015 to 0.025%, then try to keep it them. The amount of magnesium in the aluminum heat treatable alloys also should be held consistent.
"We have a few jobs in our green sand casting facility that will reveal porosity if the strontium is above 0.014," Place said. "Of course, the specification will permit much higher concentrations, but the process will not."
3 Flux Regularly
Routinely fluxing your furnace will help remove oxides from your melt. Aluminum combines very quickly with oxygen to form oxides when it is at a high temperature. As the temperature increases, so does the quantity of the oxides. Because oxides and aluminum alloys have specific gravities that are close together, agitation in the molten metal will allow the oxides and the aluminum to mix easily together. If the oxides aren't removed from the metal, they will become unwanted inclusions in the casting.
Different kinds of fluxes are available to help prevent gas pickup, reduce dross formation, remove solid nonmetallic inclusions, remove entrapped gasses and recover metal from drosses. The type of flux is determined by the aluminum alloy used because different alloys have different oxidation rates. Melting and pouring temperature also can affect flux compositions.
4 Hold Your Aluminum in Liquid for Off Shifts
Metalcasters can combat rising energy costs in the melting department by avoiding re-melting charge material where possible.
"One thing I see all too often in metalcasting facilities is bringing down a batch or crucible of metal and letting it solidly only to bring it up to a liquid eight hours later," Place said. "In many cases, it would be advantageous to the melter to hold the metal in the liquid for off shifts."
The phase transformation that occurs during the melting of solidified metal is often more costly than keeping the material warm enough to remain liquid, according to Place.
Other factors, such as size of the reverb/pot, temperature and length of holding, may affect this comparison, but considering holding the melt versus shutting it down could reveal a possibility for significant savings.
5 Banish the Heel According to Dan Groteke, QC Designs, St. Joseph, Mich., one way to keep detrimental oxides from finding their way into your melt is to avoid putting the heel back into the furnace when you're pouring from a ladle. "There are oxides in the melt surface that remain as the heel for pouring," Groteke said. "Every time you put that heel into the furnace, you're adding oxides."
Because the specific gravities for aluminum and its oxide are so close, aluminum oxide will settle in molten aluminum, which can be seen in the amount of aluminum oxide sludge removed from the floor of a reverberatory furnace during cleaning. The drosses that float do so because they are filled with air and gas.
Aluminum oxides can form quickly; before you can reach to the other side of even a small ladle, the surface is filled with oxide formation. So, the oxide surface, or skin, should be protected from being broken and forming more oxides.
6 Filter It
A filtering crucible is helpful in ensuring clean metal goes into the casting, reducing the amount of oxide inclusions in your castings. Place the removable, changeable filter at the bottom of your crucible before lowering it into your molten aluminum. As the liquid metal flows into the crucible from the bottom, the filter will prevent dross from entering. It's important to remember to preheat the filter and crucible before introducing it to the molten metal to avoid a dangerous combustion.
7 Use Vermiculite to Prevent Heat Loss
Heat loss occurs in wells that are exposed to the environment, such as the dip well, charge wells and gassing/processing wells. Lining your furnace walls with vermiculite, which is available throughout the U.S., can help reduce heat loss and conserve energy.
Vermiculite is a highly insulative mineral that is used as a castable refractory. Place said that when vermiculite was first discovered as a material to use in aluminum melting, he performed his own test to verify its effectiveness.
He first placed a sheet of plain white paper over the charging well of an offline reverb (holding at 1,250F). which had a layer of ceramic wool material found in most casting facilities. According to Place, the paper turned brown in about two minutes. Next, he spread a 2-in. layer of vermiculite on the well and performed the same test.
"The paper never turned even a shade darker than white by the next morning," Place said.
8 Use a Charging Cart
The use of a charging cart. or its equivalent, reduces the amount of time the furnace is open, shortens the time needed for cool down, and reduces oxide growth.
A charging cart is loaded with charge material while the furnace door is closed. When the furnace is ready to be charged, the entire load of charge material in the cart can be forced into the open furnace using hydraulic levers. This allows the entire load of charge material to be added to the furnace in one shot, rather than loading it piece by piece, shortening the amount of time the furnace door is open.
9 Control the Variables in Your RPT
Hydrogen gas can be picked up from the surrounding atmosphere and from products of combustion or exhaust gases during melting. The presence of hydrogen in your melt can lead to detrimental porosity in the casting.
The reduced pressure test (RPT) to evaluate the hydrogen gas levels in molten aluminum is a desirable test for metalcasting facilities because it in determining if the molten aluminum needs to be degassed. This analysis still has its flaws, however. "If your RPT is to be used as a process control, then it has to be. in fact, controlled." Place said.
Two variables to control are the vacuum and the temperature. The vacuum should be at the same force at all times. Place suggests checking the plumbing for blockage if the unit seems to fall short of the target.
Also, each metalcasting facility pulls its samples at different vacuum levels, so the amount of vacuum you pull is not as important as consistently pulling the same amount every time. Pulling a wide range, such as 25-28 inches of mercury, will be detrimental to achieving a reliable reading.
"The RPT result from a metalcaster pulling at 25 in., will be much higher than a metalcaster pulling at 28 in. of mercury," Place said. "Be this value because it is reflected in every casting made."
Finally, Place said the temperature to which the RPT equipment is preheated can affect the RPT's reading, so keep this process consistent. "If one furnace tender or laboratory person preheats the ladle until it's cherry red. and the other doesn't, then the readings mean nothing."
10 Clean it Up Avoid contaminating your metal by keeping the area around the melt deck free from grit, sand and dirt. Sweep to the floor any residual sediment that falls from the ladle onto the melt deck. This sediment can quickly and easily sift into the molten aluminum if the effort isn't taken to keep it clear. The sediment sent to the floor can then be swept up during your regular cleaning rounds.
For More Information
"2 Means to Melt Aluminum," A MODERN CASTING Staff Report, MODERN CASTING, September 2005, p. 31-34.
"The Best of Aluminum Melt Shops," A MODERN CASTING Staff Report, MODERN CASTING, September 2004, p. 30-34.
Shannon Kruse, Associate Editor
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|Date:||Sep 1, 2006|
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