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BATTERY GRID CASTING WITH CALCIUM LEAD ALLOY
Calcium Grid Casting may be new to many in the Automotive Battery Industry, but calcium grids have been cast in some Industrial Battery Plants for many years, so it is not altogether new or unknown.
The following comments are not intended to discuss or imply advantages or disadvantages of the different lead alloys, but to outline some factors to consider when they are used in casting battery grids.
First lets define calcium lead alloys as it is used in Automotive Batteries. The calcium alloys now in use consist of a corroding grade of lead, about .08 to .10% calcium and from .025% to 1.0% tin. (Which is a lead of higher purity than used in antimonial lead alloys.)
Comparing this alloy to a common lead antimony alloy of 4-1/2% to 5% antimony and about .25% tin, from a casting viewpoint, the 3 major differences in the two alloys are:
1. Hardness after casting.
2. Oxidization (drossing) rates at high temperatures.
3. Melting and solidification temperatures.
First a look at the problem of hardness after casting. The major differences between these two alloys is the hardness or stiffness of the grid immediately after ejection from the mold. The calcium alloy is much softer at this stage and therefore requires more careful handling by the grid casting machine and by the operator as he removes and stacks the trimmed grids.
The second major difference is the rate of oxidization at higher temperatures. The calcium alloy oxidizes or drosses more rapidly than antimony alloy. It therefore becomes more critical as to how we agitate the alloy by pumping or feeding, how the scrap is returned to the pot and how closely the temperature is controlled.
Antimony and calcium alloys react with one another in such a way that if accidentally mixed together in one lead pot, both the calcium and antimony will come out of the lead in the form of dross. Therefore it may be necessary to separate the calcium casting area to prevent contamination of the alloy.
The third major difference is that of melting and solidification temperatures. A common 4-1/2% lead antimony alloy when heated, is fully liquid at approximately 564°
F (296°
C). When cooled down, it will start to solidify at 564°
F (296°
C) but does not become fully solid until it is cooled to about 486°
F (252°
C). In the casting operation this alloy must be poured into the mold and kept about 564°
F (296°
C) until the cast is complete; then cooled about 78°
F (25.6°
C) to 486°
F (252°
C) before the grid can be ejected from the mold.
The calcium alloy when heated, is liquid at about 621°
F (327°
C). When cooled it solidifies at about 615°
F (324°
C) depending on the exact alloy composition. In the casting operation, this alloy must be poured into the mold and kept about 621°
F (327°
C) until the cast is complete, then cooled about 6°
F (-14.4°
C) to below 615°
F (324°
C) for the grid to solidify and be able to be ejected from the mold.
The calcium alloy is liquid at approximately 57°
F (14°
C) higher than the antimony alloy and it solidifies approximately 129°
F (54°
C) higher than the antimony alloy. Therefore to cast this alloy, it must be poured at a higher temperature and the mold temperature must be higher to keep it from solidification before the cast is complete.
As noted, the differences are: The calcium alloy runs hotter, it is softer when cast, is more difficult to handle and drosses much more rapidly. What does this mean to the casting operation? The calcium casting operation is very similar to the antimonial casting operation but the differences are important. The similarities make it possible to retrain your operators for calcium casting.
The main difference in casting are the heat controls, the grid handling system and the lead handling system.
Since heat controls are very important in calcium grid casting, the temperature controls on Wirtz grid casters are mounted for easy monitoring by the operator. One control for the ladle, two controls for electric heat input into the mold and one control for mold gate cooling.
Approximate operating temperatures with a typical calcium lead alloy:
| |
DEG. F |
DEG. C |
| Furnace Temp |
750°/800° |
399°/427° |
| Feedline Temp |
900°/950° |
482°/510° |
| Ladle Temp |
950°/1000° |
510°/537° |
| Mold Gate Temp |
390°/440° |
199°/227° |
| Upper Mold Zone |
400°/450° |
204°/232° |
| Lower Mold Zone |
400°/450° |
204°/232° |
The ladle is shielded with an inert natural gas shroud to reduce drossing in the ladle. The amount of as burned off is small and is contained within the ladle hood.
Even though the grids are very soft, they can be ejected and transferred to the trim die without distortion, but the handling system must be designed for gentle and reliable grid handling. For good grid ejection the molds require more ejector pins and the ejector mechanism must be more precise.
Since the alloy solidifies at about 615°
F the grids can be very hot when ejected from the mold and it is desirable to cool both the die entry plate and the trim die. In spite of these high temperatures, hot cracks are never a problem with this alloy. When the grids are ejected, they are either solid or liquid.
Trimming and stacking of the trimmed grids can be accomplished by conventional means, but the grid being very soft, the trim die must be more precise to trim clean. The grid stacker is more critical and must handle the grid more gently in order to prevent distortion.
Because the alloy is still soft, it can be planished or cold worked in the trim die. Cold working has not been found to be of any great significance but closer thickness control of the grid by planishing can be a real advantage in assembly of the battery.
Depending on the exact alloy used, the grids will age harden rapidly and after a few days, are strong enough to be handled and processed further. However, even after age hardening, the grids are very ductile and will not break easily. This makes them difficult or impossible to hand break after pasting.
The molten lead handling system recommended is an elevated lead pot which is open to the atmosphere. Lead is delivered from the pot to the ladle by gravity through an insulated feedline. This system generates very little dross and the loss of calcium from the pot to grid is very low. This system can be expanded with one furnace feeding several machines.
Care should be taken when loading scrap and alloy pigs into the furnace, so that agitation and resulting dross is minimized. Pigs should be loaded gently. It has been found advantageous to provide a barrier in the furnace to prevent the pigs from going to the bottom and disturbing the molten lead in the draw off potion of the furnace.
The conventional method of pumping lead by rotary pump from a floor pot and returning trim scrap to the same pot can also be used. However the agitation caused by the pump and the trim scrap will cause more dross.
The higher solidification temperature of the calcium alloy requires higher temperatures. With properly designed molds, the ladle pouring temperatures will be about 975°
F (524°
C) or about 50°
F (10C°
) hotter and the mold temperature will be about 425°
F (218°
C) or 45°
F (7°
C) hotter than used with antimonial alloy.
At the higher temperatures, it is necessary to add heat to the cold by lower and upper zone electric heaters and controlling the temperature of the mold cooling fluid by a mold cooling system, such as the Wirtz "Closed Cooling System" (C.C.S.).
Wirtz molds designed for calcium alloys incorporate venting, center ejector mechanism, provision for mold heating and cooling.
Grid weights can be comparable to those achieved in antimonial grids and grid thickness may be reduced below the .050 thickness commonly used in negatives.
More mold coat is generally required because of the higher temperatures that the mold is operated at. We recommend our W-10 Mold Coat for its improved insulating and adhesion characteristics under the higher temperature conditions. X-500 may also be used by itself or combined with W-10. Cork technique is more critical and requires more care, but with proper instruction and proper mold design, cork life of up to 6 to 8 hours, similar to antimonial casting is possible.
At casting speeds of 16 panels per minute, it is possible to cast approximately 8,000/10,000 single grids with one man operating one machine per 8 hour shift. With a 3 machine set up, it is possible for one man to operate the 3 machines and cast approximately 30,000 single grids per shift. Production rates and vary greatly, but generally calcium lead alloy can be cast at similar rates to antimonial alloy, and under some circumstances the rates may be higher.
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