MXPA98006553A - Process for the manufacture by dumping in mold of joints between free plates of porosity for ac lead batteries - Google Patents

Abstract

A method for producing porous cast-free mold unions for lead acid batteries, including the steps of cleaning a set of positive and negative plate ears by the combined action of an ultrasonic flux and vibration rinsing with water from the ears of the plates under ultrasonic vibration, and drying the ears of the plates by means of a pressurized gas stream. The gas stream is applied previously to the formation of the unions by casting in mold. The union by cast in mold is formed by introducing the ears of the plates in a liquid alloy of lead contained in a mold, and by solidification of the alloy in the mol

Description

"PROCESS FOR THE MANUFACTURE BY DUMPING IN MOLD OF JOINTS BETWEEN PLATES FREE OF POROSITY FOR ACID LEAD BATTERIES" DESCRIPTION TECHNICAL FIELD The present invention relates in a general way to the manufacture of batteries. More particularly, the present invention relates to processes for the manufacture by casting of plate-ear joints for lead acid batteries.

BACKGROUND A lead acid battery for energy storage is a device that converts chemical energy into electrical energy. The electrical energy is produced by the chemical reactions that take place between a liquid electrolyte and the active material contained in the negative and positive plates submerged in it. The electrolyte is sulfuric acid. The active material is lead dioxide for positive plates and spongy lead for negative plates. The chemical reactions begin and the electrical energy flows from the battery as soon as an external circuit is established between the positive and negative terminals.

The complete assembly for a lead acid battery consists of cell elements, intercell connectors, a container, a cover, vent plugs and terminal posts. The assembly that results when joining a group of positive plates with a group of negative plates, with their respective separators, is known as a cell element. The components of a cell element are: grids, active materials, separators and joints between plates. The main role of the grids, which are commonly made of a lead alloy, is to provide a support structure for the active materials of the plates, as well as to conduct the electric current to and from the active materials. The separators are thin sheets of a finely porous electrically insulating material, which are used to avoid the occurrence of short circuits by keeping the negative plates separated from the positive ones, but allowing the passage of charged electrolyte ions between the plates. Sheath-shaped spacers can be used around positive plates or negative plates. Usually these covers are closed on three sides and open at the top to allow the gases generated in the plates to escape. A cell element is formed by an alternate stacking of positive and negative plates. The plates are inserted into the covers before the construction of the cell elements. The ears of the negative plates of the cell elements are joined together by casting a liquid alloy of lead in a mold. The same is done to join together the positive plates of the elements of the cells.

Any number or size of plates can be used in the cell element, depending on the desired performance for the battery. The mold cast connection of each group of plates is used to connect these in series with the groups of plates of the adjacent cells. This is done by connecting the unions by casting the positive plates in a cell, with the mold cast connections of the negative plates of an adjacent cell. The steps that are followed for the manufacture of lead acid batteries are: emptying the grids and preparing the active material pastes; application of the pasta to the grids; curing of the paste; integration operations of the elements of the cell; process of joining the ears of the plates by mold casting; Subsequent battery assembly processes; addition of electrolyte to the battery; electrical formation of the battery, assembly and final test. At the beginning of the process, the groups of plates are placed in the loading station of the machine, the ears of the plates are turned downwards and the plates are vibrated to align them. Then the plates are transported to the next stage of the process, where the ears of the plates are brushed, using a rotating metal brush, to remove the layers of lead oxide and the paste of the active material. Subsequently, a liquid flux film is applied at room temperature on the freshly brushed surface of the ears of the plates, using a sponge impregnated with flux. Both operations of brushing and application of flux are carried out in a few seconds.

The next stage of processing involves the formation of unions by casting in mold, for which a metal mold is used 80 preheated and cooled by water. This metal mold has two small cavities for each group of plates loaded in the emptying machine. This allows the simultaneous manufacture of a union by cast in the mold for the ears of the positive plates of a group, and of another 85 union by casting in the mold for the ears of the negative plates of the same group, thus forming a cell element. To form the unions by casting in mold, the cavities of the mold are filled with a liquid alloy of lead. This is done either by gravity or by injection. Then, the ears of the plates impregnated with Flux is introduced into the liquid lead alloy, which is subsequently cooled and solidified in the mold. The liquid lead alloy is preheated before the casting operation to a temperature normally higher than the mold temperature. The final stage of processing is the discharge of cell elements 95 of the emptying machine. The flux helps to reduce lead oxides remaining on the surface of the ears of the plates after brushing. Subsequent to its application, the flux provides protection to the surface of the ears of the plates forming a film that prevents the 100 direct contact with the oxygen in the air until the ears of the plates are introduced into the liquid lead alloy.

A distinctive feature of the cell elements produced using the conventional mold casting process just described is the presence of a substantial amount of porosity 105 located mainly at the interface between the surface of the ears of the plates and the joints by mold casting, which is easily visible in longitudinal or transverse sections of said joints. This results in accelerated corrosion in the pores, which can significantly shorten the life of the batteries. Currently, the 110 porosity in cast-in-mold unions is universally present in automotive lead acid batteries, being one of the main manufacturing defects found in this industry. However, as a result of the introduction of higher quality standards by the manufacturers of 115 cars around the world, it is necessary to develop a process capable of producing unions by casting in mold free of porosity. Recently some attempts have been made to find a solution to the problem of porosity formation in cast-in-mold unions. For example, Cook and collaborators published in the 120"The Battery Man" magazine, pp. 12-22, April 1996, the results of an investigation carried out on the effect of emptying parameters such as temperature of the liquid and mold lead, immersion speed of the ears and time between filling of the mold and the entrance of the ears of the plates in the liquid lead, on the quality of the 125 unions by casting in mold, using different lead alloys for the plates and for the joints.

These researchers found that the depth of penetration of the ears of the plates into the liquid lead, the thickness of the ears and the temperature of the mold are critical factors. Other The important discovery was that the type of lead alloy used for the plates and for the mold cast joints is also a relevant factor. However, these researchers were unable to develop a process that is truly capable of producing mold-free mold-free porosity joints. Another antecedent that can be considered related to the present invention is the American patent number 3,912,544, whose author is Anthony Sabatino, and which was assigned to the company Gould Inc. The objective of the invention described in said patent is to improve the unions by emptying in mold of the ears of the plates. Nevertheless, The problem of porosity in the joints of the ears of the plates is not considered by the inventor.

DESCRIPTION OF THE TECHNICAL PROBLEM AND SOLUTION AT THE SAME 145 In order to determine the causes of formation of porosity in the joints by casting using the conventional process, an extensive research program was carried out. This investigation led to the development of the present invention. The effect of 150 several parameters on the quality of the unions by cast in cast.

The parameters considered were: temperature of the mold and the liquid lead, chemical composition of the ears of the plates and of the liquid lead, thickness of the ears of the plates, type of flux (including tests without the use of fluxes), drying of the flux on 155 the surface of the ears of the plates (natural drying or drying by use of cold or hot air streams at various flows and pressures), preheating of the ears of the plates, shape of the application of the flux (with or without preheating the flux, and with or without the simultaneous application of ultrasonic vibration), duration of the 160 application of the flux, method of cleaning the surfaces of the ears of the plates (without cleaning, brushing, brushing and scraping), elimination of the products left by the chemical reactions that take place between the flux and the contaminants found on the surface of the ears of the plates (by removal with towels 165 paper, using air streams at various flows and pressures, or by rinsing with water, with or without preheating it, and with or without the simultaneous application of ultrasonic vibration during rinsing with water), duration of rinsing with water and use of different types of union by casting in mold. It was discovered that the cause of the formation of the porosity commonly observed in the unions obtained by casting in mold, using the conventional process, is the generation of a considerable volume of gases in the liquid metal during the contact time between it. and the ears of the plates. 175 These gases are generated due to the presence of various contaminants on the surface of the ears of the plates. The main gas generating agents that were identified are: residual dirt (mainly lead oxides, active material paste, grease and oil), moisture, flux, as well as waste products. 180 reactions formed by the chemical interaction that takes place between the flux and surface contaminants. Due to the conditions of emptying conventionally used, and the continuous evolution of gases throughout the process, these gases can not be completely eliminated during cooling and solidification of the liquid metal 185 in the mold. Therefore, a considerable amount of gas bubbles are trapped in the solidified bonds, which appear as pores when these junctions are sectioned. It was found that the individual action of each of the mentioned gas generating agents was sufficient cause for the 190 formation of porosity in the joints by mold casting. Therefore, it was concluded that porous cast free unions can not be produced reproducibly by using the conventional process. The reasons for this are explained below. In the conventional process, the brushing of the ears of the plates is carried out in a direction parallel to the surface thereof. This appreciably reduces the efficiency of the brushing operation, which results in the presence of a considerable amount of dirt remaining after said operation.

In addition, the metal brush used can easily cause a considerable plastic deformation of the surface of the ears of the plates, which results in the encapsulation of the dirt, instead of the elimination of the same. On the other hand, both the residual flux and the reaction products formed by the chemical interaction that When it occurs between the surface contaminants of the ears of the plates, they can generate a considerable volume of gases during their heating in contact with the liquid lead alloy. In fact, it was found that this is the main cause of porosity formation in the mold casting process, especially 210 under the conditions of rapid cooling existing in the metal molds cooled by water, already mentioned. In this way, the possibility of dispensing with the use of the flux was considered. However, when no flux was used, a lack of adhesion was observed between the ears of the plates and the joints by casting in 215 mold. Therefore, it was concluded that the flux is really necessary to clean and activate the surface of the ears of the plates, and can not be eliminated from the process of union by casting in mold. However, in order to obtain porous mold free unions, the application of the flux must be optimized. In the present invention, the cleaning and activation process given to the surface of the ears of the plates prior to their introduction into the liquid metal for the formation of the unions by casting in mold, has been modified. This has been done by using a treatment of the ears of the plates in three stages. 225 This process includes cleaning the ears of the plates by the combined action of a flux and ultrasonic vibration, followed by rinsing with water from the ears of the plates under ultrasonic vibration and finally; by drying the ears of the plates by means of a pressurized gas stream. 230 The operation of brushing the surface of the ears of the plates has been eliminated. The present invention is a process for the manufacture of porous mold-free cast connections. The process includes the steps of: cleaning the surface of the ears of the plates by means of the 235 combined action of a flux and ultrasonic vibration; rinse with water under ultrasonic vibration; and dried by a pressurized gas stream. This process eliminates the presence of lead oxides, fats, oils, active material pastes, fluxes, residual reaction products and moisture on the surface of the ears of the 240 plates. The gases that come in contact with the liquid lead during the union by casting in the mold are eliminated. As a result, the porosity of the cast-in-mold joints is eliminated.

DESCRIPTION OF FIGURES Figure 1 is an isometric view showing the major components of an automotive lead acid battery. Figure 2 shows an isometric view of a cast-in-mold joint for a set of negative and positive plate ears as referred to in the present invention. Figure 3 is a photomicrograph at a magnification of 3.5X where a cross section of a cast-in-mold joint manufactured according to the conventional process is shown. Figure 4 represents a flow chart showing the steps of the mold casting process previously used. Figure 5 represents a flow chart showing the processing steps required to obtain free porosity joints by mold casting according to the present invention. Figure 6 is a photomicrograph comparable to Figure 3 showing a cross-section of the mold casting obtained according to the present invention.

BEST METHOD OF CARRYING OUT THE INVENTION Referring to figure 1, it shows the main components of a lead acid battery that uses cast-in-mold unions, to which reference is made in the present invention. The main components of a lead acid battery (10) include a pair of terminal posts and a sealed cover to a container. The container is divided into several cells and a group element is located in each one of them. Each group element includes a set of positive plates (11) and negative plates (12). These plates in turn include support grids, each of which has an integrated ear (13) and (14), a layer of active material adhered to the grids, and a cast-in-mold connection (15) that joins the ears (13) and (14) of the plates (11) and (12). The positive plates (11) and the negative plates (12) are kept separated by the separation sleeves (16). Figure 2 shows an isometric view in greater detail, with respect to Figure 1, of a cast-in-mold joint (15) joining a set of ears of the negative plates (13) and a set of ears of the positive plates ( 14) of a group element. The particular chemical composition and the dimensions of the ears of the plates and of the mold cast connections shown in Figures 1 and 2 can be varied according to the required application.

As described in the background of the invention, the previous method used for the manufacture of unions by casting in mold produces pieces that have a high porosity, as shown in the photograph of figure 3. The previous method used to manufacture unions by mold casting, which is shown in Figure 4, includes the steps of assembling and aligning groups of a set of negative and positive plates, brushing the ears of the plates, applying flux on the surface of the ears of the plates. the brushed plates and, finally, union of the ears of the plates by the process of casting in mold, which consists in the introduction of the ears of the plates in a liquid alloy of lead contained in a metallic mold. Cast-in-mold unions are formed by subsequent solidification of the liquid lead alloy in the mold. By microscopic observation it was found that the surfaces of the ears of the plates, just before their introduction into the liquid metal, generally have micropores filled with oil, active material and other residues remaining from the previous stages of processing. These residues are partially covered by the lead alloy from which the plates are made, which has been smeared on the surface of the ears due to the plastic deformation suffered by said surface during the brushing step referred to in Figure 4. On the other hand part, the brushing operation parallel to the ears of the plates is an inefficient method to completely remove the residues present on the surface of said ears.

After the operation of brushing the ears of the plates, using the process indicated in figure 4, the flux is applied on the surface of said ears to reduce the lead oxides. However, through industrial observations and developed experimental work, it was found that during the subsequent casting process, as indicated in figure 4, residues of lead oxides and other contaminants such as grease, oil, paste of active material , flux, reaction products and moisture, are still present on the surface of the ears of the plates. On the other hand, the experimental work developed, in which the effect of factors such as temperature of the mold and of the liquid lead, thickness of the ears of the plates, chemical composition of the ears of the plates and of the liquid lead, type of flux and form of application of the same, different methods of cleaning the surface of the ears of the plates, and different types of union by casting in mold, also showed that the best way to avoid the formation of porosity in the joints by casting In mold consists in the complete elimination of the surface of the ears of the plates of the residues of lead oxides, grease, oil, paste of active material, flux, reaction products and humidity, which is not fulfilled, as already mentioned, in the method conventionally used for the manufacture of unions by casting in mold. In order to completely eliminate the causes responsible for the formation of the porosity commonly observed in mold cast connections produced using the conventional process, the method of the present invention was developed.

This method is shown in Figure 5. This method comprises the 75 following stages: once a set of positive and negative plates have been previously loaded with the active material and have been grouped and aligned, a deep cleaning of the surface of the ears of the plates is carried out by the combined action of an appropriate flux and ultrasonic vibration. He The objective of this stage is to reduce the lead oxides of the surface of the ears of the plates, as well as to completely eliminate from these any residues of grease, oil and paste of active material. At this stage, even the micropores located on the surface of the ears of the plates are cleaned. Depending on the type of flux used, This stage can be carried out at different temperatures and for different periods of time, using different frequencies and powers for ultrasonic vibration. As an illustrative example, the application of an alkaline flux at 65 ° C, using a frequency of 20 to 40 Khz and an ultrasonic power of 200 to 500 wats, during 90 10 to 20 seconds, will result in desirable cleaning conditions for the ears of 6 to 12 groups of plates formed by a set of positive and negative plates. According to this invention, the second stage of the process comprises rinsing with water from the ears of the plates under 95 ultrasonic vibration. The objective of this stage is the complete elimination of the flux residues and the reaction products derived from it, even from the interior of the micropores located on the surface of the ears of the plates.

Depending on the rinsing time required, they can be used 100 different frequencies and powers for ultrasonic vibration. As an illustrative example, the use of ultrasonic frequencies of 20 to 40 Khz, at ultrasonic agitation powers of 200 to 500 wats, for 10 to 20 seconds, will result in desirable rinsing conditions for the ears of 6 to 12 groups of shaped plates by a set of 105 positive and negative plates. The third stage of the process, according to the present invention, comprises the drying of the surface of the ears of the plates with a gas stream, which has the purpose of avoiding the re-oxidation of the ears prior to the formation of the unions by emptying in 110 mold. This stage must be carried out in the shortest possible time using dry, clean, oil-free and high pressure gases. As an illustrative example, cleaning the ears of the plates with dry, clean, oil-free air at pressures greater than 80 psi, for at least 5 seconds, will result in desirable drying conditions for 115 the ears of 6 to 12 groups of plates formed by a set of positive and negative plates. After the drying process of the ears of the plates according to the present invention, the ears of a set of negative and positive plates are introduced into a liquid lead alloy 120 contained in an appropriate metal mold, cooled by water. Cast-in-mold unions are formed by the subsequent solidification of the liquid lead alloy in the mold.

The photomicrograph of figure 6 shows a cross-section, at a magnification of 3.5X, of a cast-in-mold connection obtained from 125 according to the present invention. The best way to apply the process of the present invention is to subject the surface of the ears of the plates to each and every one of the cleaning and activation steps described, each of them performing in the specific order that has been indicated . Therefore, as it has been seen, the present invention allows the manufacture of unions by mold casting, free of porosity, for lead acid batteries.

Claims (1)

1. CLAIMS A method for producing porous cast mold-free unions for lead acid batteries, comprising the steps of: cleaning a set of plate ears belonging to a set of groups of plates previously assembled and aligned, by the combined action of a flux and ultrasonic vibration; rinse with water from the ears of the plates under ultrasonic vibration; and drying the ears of the plates by a gas stream prior to the formation of the unions by casting in mold. The method of claim 1, further comprising the step of forming the cast-in-mold joint. The method as described in claim 2, further comprising the steps of: inserting the ears of the plates into a liquid lead alloy contained in a mold; and solidification of the liquid lead alloy in the mold.