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MODERN CORKING TECHNOLOGY
All cork used in the Battery Industry is a product of nature. There is no cork or insulating compound commercially manufactured used in the Industry today.
All cork comes from a Cork Oak Tree. The bark of the tree is removed every seven years and this bark is the cork you are using in your battery plants.
The importance of this information is that, since cork is a product of nature and the bark will vary from tree to tree in weight and color, it will also vary as to quality.
If nothing else is remembered in this report-purchase a high quality cork from a reliable supplier that meets your casting needs.
How do you determine what your casting needs are?
Remember that Thermal Insulation is defined as a method of isolating, or separating, one conducting body form another by means of non-conductors to prevent, or reduce, the transfer of heat.
In the Battery Industry, cork has been used as the basic non-conductor. However, cork was not chosen just for its insulating properties because the insulation that is required for grid casting has several other requirements that make it unique. Factors that must be considered besides insulation are surface texture, venting elasticity, and binding properties.
Surface texture is important because a smooth casting is desirable; but, even more important, is the friction generated when the surface texture of the mold coat is rough.
This friction will slow down the filling of the mold, can generate turbulence, accelerate cork wear, and even cause porosity in the casting because of the air induced into to alloy from turbulence. A close relationship between surface texture and venting seems apparent and must be considered before attempting to alter a mold coat to a mirror or glass textured surface.
The venting of the insulating compound has been found to be increasingly important in the past few years. The air that the lead displaces must be removed from the mold cavity, along with as much of the air entrapped into the alloy stream by turbulence as possible.
If the insulating compound does not have a slightly textured surface, the mold will close too tight and could even seal air tight. This textured surface can, and will vent air and can even displace air across the surface of the mold.
Elasticity and binding properties are also important considerations and are associated with how the grid fills and the length of running time.
The insulating properties of the mold coat must be high enough to give the alloy the time needed to fill the casting, but must be bound together and to the mold surface tight enough to give a long wearing life.
As the binding properties are increased, the elasticity and insulating values of the compound decrease. Thus, the terms of a hot or cold cork come into consideration. A hot cork being a highly insulating cork with a shorter working life, and a cold cork having higher binding and faster heat transfer rates.
The insulating compound must have a certain amount of elasticity since the surface temperature of the coating fluctuates with each cycle. This fluctuation will cause a brittle cork to crack and chip off the mold while a compound with a higher elasticity rate would survive the temperature fluctuations.
Therefore, it can be seen that in buying a mold coat, many factors must be considered. The mold coat must insulate enough to fill the grid, yet be cold enough to chill the casting at high speeds, stick as hard as possible, and still vent the casting.
Cork suppliers in the Battery Industry should have considered these requirements and developed a mold coat that will suit the needs of the average Battery Manufacturer.
Although the exact mold coat formulas will vary from different suppliers in the Battery Industry, several basic ingredients are used.
A. Cork, of course, is the main ingredient
B. Bentonite clay
C. Sodium silicate
D. Casein glue
E. Distilled or mineral free water
F. H2SO4 - Sulfuric acid
G. Vermiculite
H. Dag dispersions
You, the Battery Manufacturer, should try all the cork products available. Mix according to the supplier’s directions and decide upon a basic mold coat that most closely fits your casting requirements. Once a basic formula has been accepted, it should not be varied from day to day. The mixing times should not even vary, because, varying the mixing time will change the insulating properties of the mold coat.
A standard cork mixing time table might be as follows:
A. Add bentonite to cork
B. Add cork to 180° F. distilled water
C. Mix 10 minutes
D. Let stand over night, or until solution is at room temperature
E. Add sodium silicate
F. Mix 10 minutes
It is important to realize that changing the mixing times, or adding the sodium silicate before the mold coat has cooled, will vary the insulating properties of the mold coat.
If after using a supplier’s mixing instructions, the mold coat does not meet your exact requirements, the following is generally true.
To increase the insulating properties of cork, (1) add more cork, (2) cut down on the bentonite, (3) cut down on the sodium silicate. To make a colder or tighter binding cork, (1) add more bentonite, however, remember that bentonite will increase in volume several times when added to water, (2) add more sodium silicate, remembering that the more sodium silicate added, the harder and more brittle the mold coat becomes.
It is important to consider some aspects of the mold since the condition and/or the preparation of the mold before corking could affect the mold coat.
The leader pins on the stationary half of the mold should be lightly greased each time the mold is corked. They should be greased with a non-residue forming, high temperature grease. The leader pins should also be covered with two pieces of one inch diameter pipe anytime cork is applied to the stationary half of the mold. Never put an excessive amount of grease on the leader pins because, if the grease gets on the mold, the cork will not stick.
The knockout pins should also be lubricated once a day. Although grease would work on the pins, grease should be kept away from the surface of the mold. Bee’s was should be used on the knockout pins.
The mold should be clean before corking. The cork will not stick to a dirty or greasy mold. The mold should be brushed with a wire brush until the old cork is removed from the surface of both halves of the mold.
If the surface has any oil, grease, or is contaminated with something else, or if the cork just does not seem to stick, the mold must be washed.
A good washing agent is an Oakite solution. The Oakite solution is very strong and care must be exercised when using it. Rubber gloves and a face mask should be worn when mixing or applying. For best results, the following procedure should be used:
1. Mix 1\2 cup powered Oakite with one quart of water. NOTE: The Oakite must be added to water; DO NOT ADD WATER TO OAKITE!!!!
2. Heat the mold to at least 300° F.
3. Wire brush the mold surface.
4. Use a bristle fiber scrub brush, dip in Oakite solution and brush mixture on face of mold to clean surface. Brush entire surface several times. (The gate area must be completely clean)
5. Using clean water, rinse the mold surface.
6. Wire brush the surface to remove any powdered Oakite left on mold.
Washing the mold is done only if oil or grease is spilled on the casting surface or if the cork will not stick properly. The mold does not have to be washed before each corking operation.
Two important notes that deserve mentioning are sodium silicate build-up in the wires of the mold and water cooling lines liming or rusting up. If either of these two conditions occur, the casting operator will swear up and down that the cork has changed.
If the mold will not fill properly, wash the surface with an Oakite solution to remove a possible sodium silicate build-up in the wires. This build-up cannot be seen by looking at the mold.
If the casting operator complains about the cork being too hot, check for limed water cooling lines. In checking for limed lines, run a drill though the lines. Visual inspection of the water lines will not work because a .003 of an inch build-up will affect the cooling of the grid.
This brings up another subject of a mold cooling system, but that is a different subject for another time and place.
It is important that the casting operators develop the proper spraying techniques.
The proper spraying technique could be compared to spray painting a car. The basic requirement is that a smooth even layer be applied to the surface.
The mold coat should be diluted with enough water to facilitate spraying. The spray gun should be held close enough to the mold to eliminate spraying the entire room and to prevent partial drying of the mold coat before contracting the mold surface. A distance of about six inches is a good starting position.
A gun air pressure of 80 P.S.I. will give the mold coat a high impact force on the mold. This high impact force will help the mold coat adhere to the mold surface and give a more uniform heat transfer rate.
The spray gun should be adjusted to give a spray pattern of about one inch wide by four inches long at a spraying distance of about six inches.
The spraying pattern should consist of both horizontal and vertical movements of the spray gun, parallel to the mold surface.
The gun should also be inclined on the horizontal passes, to a 20° angle up and a 20° angle down, or an included angle of 40°. That is to say a typical layer of cork should consist of overlapping horizontal passes of the spray gun inclined at 20° up. The next layer of cork should consist of the same number of overlapping passes with the spray gun inclined at a 20° down angle. The vertical layers should be applied in the same manner with a 20° angle to the right and 20° angle to the left, or again, an inclined angle of 40°. The speed the gun passes across the mold is determined by the evaporation rate of the mold coat. The gun should move fast enough to prevent splattering, bubbling, or a high concentration of cork in one area. The layer of cork should be even and smooth.
The basic requirements is to make sure every wire is sprayed with the same amount of cork, giving a uniform cooling rate to the wire section of the mold.
Several different spray techniques are used in the Industry today; spray-on scrape, spray-scrape, and spray-wipe-scrape-spray. While the techniques may seem basically the same, several distinct advantages and disadvantages go along with each technique.
The most elementary or basic spray technique is the spray-no scrape. This spray-no scrape technique consists of applying several layers of cork to each half of the mold. Basically the only advantage is the ease of application. The disadvantages are numerous: (1) poor casting thickness control, (2) poor flash control, (3) venting of the grid is not consistent, (4) slower casting speeds.
The poor thickness control is due to the cork wearing out of the frames or from wiping cork out of certain areas of the lug and frame to prevent hot spots. The surface cork can also compact from the continuous closing of the mold, thus, changing thickness and changing the venting action of the mold.
The spray-scrape-spray corking technique has more advantages and consists of spraying both surfaces of the mold. Scraping the surfaces with a wide putty knife, a bar, or brushing with a file card, and spraying the surfaces with a light layer of cork.
The advantages of this spray-scrape-spray technique are more numerous: (1) better casting thickness control, (2) better vent control, (3) better flash control, (4) touch up spray procedures are easier to apply.
The disadvantages are: (1) a finer knowledge of spray techniques is required and (2) greater mold care is needed.
Better cast thickness control is achieved because the surface cork is light and cannot compact under continuous mold closing. The only change in casting thickness that can result is from cork wear in the frame areas or where cork is wiped from certain hot areas, and not wiped out of others. The light surface cork causes less vent changes and also casts a cleaner grid with less flash.
The two disadvantages are not as important when considered against the advantages, but this spray-scrape-spray technique does require a finer knowledge of spraying since the weight of the grid will be determined by the amount of cork sprayed on the mold before scraping. Not enough cork being applied before the scraping operation will result in an overweight grid. The operator must also exercise caution in scraping the mold because damage could result to the mold if the operator is careless. The file card can cut down on injuries to the molds.
The final spray technique is an improved spray-scrape-spray technique and requires a wiping operation during the spraying process. The wiping operation consists of wiping all the frame and lug areas clean of cork, during the spraying operation, and re-spraying with a smaller amount of cork. The advantages and disadvantages are basically the same as the spray-scrape-spray method, except better thickness control is achieved and higher casting speeds are possible.
Better thickness control and a more uniform casting thickness is achieved because the same amount of cork is in all areas of the frame and scratching in certain areas is eliminated. Higher casting speeds can be achieved because hot spots are cut down and the casting cycle can be speeded up. It is important to remember that two different casting conditions are present in the mold when casting speeds are increased.
(1) Cooling is the prime concern in the frame, lug, and gate sections of the mold and
(2) insulation is the prime concern in the wire sections of the mold. These two conditions result because of the different cross sectional areas of the frames and wires. The wires being smaller, need more insulation than the frames.
Another big advantage of a spray technique with a scraping operation is the touch-up procedures that can be applied to give excellent weight and flash control, along with increased production. The type of touch-up needed will depend on the condition of cork on the stationary and the moving half of the mold. This can be broken down further to the condition of the gate and wire sections of each half of the mold.
The gate will be the first section to show wear. The cork may be completely gone in several areas. If this is the case, the gate section, all the way down to the top frame of the grid, should be wire brushed with horizontal strokes. This brushing is to remove the cork left on the gate, since the cork may have worn off unevenly. The cork should be brushed off both halves of the mold in the gate area.
The cork should then be re-applied using horizontal strokes. A layer of cork about the same thickness as originally applied should be used. The gate touch-up will need practice because the cork must be put on evenly and in the gate section only. No cork should be in the wire section, since this could cause a flashing situation or hot spots in the top of the horizontal wires.
When the gate area is sprayed, the top of the gate or the angled part should also be sprayed. The gate section should then be wiped with a clean piece of felt and the top frame wiped with a wooden stick. This procedure covers a gate touch-up.
Another touch-up that can save time is a light spray over the whole face of the mold after the old cork is scraped off the surface of the mold with a putty knife or a file card. The cork then applied to the surface of the mold is about the same as was sprayed on after the mold was scraped in the original spray-up. The bottom of the mold should then be cleaned off with a wire brush or putty knife up to the bottom frame.
This touch-up may be needed on one half only. The moving half usually receives most of the cork wear and only the moving half may need corking.
Another touch-up consists of a gate touch-up along with the following: brushing vertically with a wire brush to remove the burnt cork from inside the wires. The burnt cork is a darker color than the normal cork. Remember, only a light stroke is needed.
The mold should then be sprayed with about three layers of cork and the frame and lug areas wiped free of cork with a wooden stick. Another layer of cork is then applied to the mold surface.
The entire mold surface should then be scraped with a putty knife or file card. A final two layers of cork and the touch-up is complete except for cleaning the bottom of the mold with a wire brush and wiping the gate area with a piece of felt.
It is important to wipe the gate since the cork sprayed in this area must be smooth. In the past, many people have used acetylene smoke to smooth the gate area; however, acetylene smoke is greasy and should not be used unless absolutely necessary. In most cases, wiping the gate area with a piece of hard felt, or a clean rag, will polish the gate and eliminate the need for acetylene smoke.
Other small touch-ups consist of spraying small amounts of cork into the lug areas, when holes appear in the center of the lug, and spraying cork on the mold to fill in a spot where the cork has pulled off.
If the spray-no scrape procedure is used, the outside edges of the mold may need additional cork from time to time to hold the mold open and to insure proper venting of the mold.
In conclusion, while this report has touched on the basic corking technology used in the Battery Industry today, a successful and productive casting operation involves more than just cork. Cork, although important, does not make a casting operation. It takes an integrated casting system with enough controls and information on important temperatures along with the proper corking technology to produce a successful casting operation.
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