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BATTERY GRID CASTING WITH LOW ANTIMONY LEAD ALLOY
During recent years there has been a great deal of interest in low antimony lead alloys for battery grid casting. Many Battery Manufacturers have asked for information concerning these types of alloys and this bulletin is intended as general information. As these alloys become more available, and more knowledge is developed through our research and development, further information will be issued.
LOW ANTIMONY ALLOY - DEFINITION
For this discussion, antimony alloy is an alloy with approximately 2 to 3% antimony and other alloying elements. (Such as tin, arsenic and a grain refining element)
If the antimony content of the grid metal is reduced to less than 4-1/2% by weight, without other additions, it is found that in a cast grid the strength will decrease. The tendency for coarse grains will increase and solidification, cracking and other casting defects will increase.
To solve this problem other elements can be added to the lead which act as "grain refiners". Experimental work is being done by many people, and several lead suppliers offer varying grades of Low Antimony Lead Alloys.
The relative low hardness and mechanical strength of low antimony alloys (as cast) will increase with aging.
The hardening can also be increased by the addition of other elements (such as arsenic) in the alloy and also by accelerating the cooling of the grid or by additional heat treatment.
THE FOLLOWING INFORMATION IS BASED ON A LOW ANTIMONY ALLOY WITH SUITABLE GRAIN REFINING ADDITIONS:
LEAD MELTING AND DELIVERY
It is possible and practical to change to a low antimony alloy without changing furnaces or lead delivery systems.
Most low antimony alloys will tend to dross or oxidize about the same as a normal 5% antimony lead. This means it can be pumped, trim scrap and pigs can be added to the furnace for melting without any special precautions (except as noted below), and the amount of dross generated will be about the same as with 5% antimony lead under the same heat conditions.
If the cast grids show a cracked condition and it is not caused by hot spots in the mold, it is possible some of the grain refiners in the alloy have come out of solution. This can be identified by a red or blue dross appearing in the surface of the lead pot. If this happens raise the pot temperature to 850°
(or slightly higher) and stir the dross back into the lead until the red and blue color disappears. The grain refining alloys in the lead may come out of solution if the temperature is too low. Therefore, be careful when solidifying and re-heating the lead. Do not dross the pot until the temperature is up to 850°
F., then stir the dross before skimming it off, this will prevent removal of the grain refining alloys.
GRID CASTING
As the antimony content drops, the melting and solidification temperatures of the lead alloy will increase. This means that grid mold temperatures and lead temperatures may have to be higher for production casting of grids.
Approximate operating temperatures with a typical Low Antimony Alloy with 2.75% antimony .35% tin and other related additives.
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Degrees F |
Degrees C |
| Furnace Temp |
850° / 900° |
454° / 482° |
| Feedline Temp |
900° / 950° |
482° / 510° |
| Ladle Temp |
900° / 950° |
482° / 510° |
| Mold Gate Temp |
300° / 340° |
149° / 171° |
| Lower Mold Zone Temp |
360° / 400° |
182° / 204° |
Generally the temperature adjustments can be done on most grid casting equipment without major modification of the equipment or molds. If the present casting equipment has automatic lead temperature controls, automatic electric mold heat and the mold is well vented, it is possible to change to low antimony by merely cleaning out the pot, adding the new alloy and start casting. If the casting equipment is older and does not have lead and mold temperature controls, it is still possible to merely change alloys and start casting if you have experienced operators.
GRID HANDLING DURING CASTING
A grid made from a low antimony alloy will be softer as cast which requires a little more sensitive handling of the grid by the machine from the mold to the trim die and to the stacker. Unless the machine is designed to handle this problem, probably more grids will be scrapped because of miss-trims and distorted grids.
Wirtz Grid Casting Machines that have Electric Mold Heat and Pallet Die Entry Mechanism will cast low antimony alloys without modification since casting temperatures can easily be varied and the handling system from the mold to the die and stacker are designed for handling thin soft grids.
Wirtz conversion kits are available for most older machines to provide improved grid handling and mold heat.
SUMMATION
Generally low antimony grids can be cast in present grid casting equipment. Mold coat techniques and casting speeds remain the same and subsequent operations such as pasting are not adversely effected. They do not present the problems in drossing, casting or handling that are present in calcium grid alloy.
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