GB2063738A - Metal mould for lead alloy casting - Google Patents

Abstract

A metal mould (10) for casting lead alloy products, in particular plate grids (1) for storage batteries, is provided on its inner surface with a layer of ceramic. The ceramic layer is preferred to have various thicknesses in accordance with the various thicknesses of the corresponding parts of the products to be cast, thus providing varied cooling rates of the different parts. The ceramic layer may also absorb gases generated during casting. The ceramic layer may be of ZrCaO3, 2MgO.Al2O3, MgO.SiO2 or TiO2 applied by plasma spraying with the use of masks. <IMAGE>

Description

SPECIFICATION Metal mould for lead alloy casting This invention pertains to an improvement on a lead alloy casting metal mould.

Plate grids to be used for a lead storage battery are usually produced by casting Pb-Sb alloy or Pb-Ca alloy in a casting metal mould. The plate grids have been continuously produced by the casting mould at a high rate of 10 sheets per minute. The metal mould is heated by a heater at a temperature of 1 60 C to 230"C. In order to easily remove a cast plate grid out of the mould and improve the heat insulating effect of the casting metal mould, a layer of cork powder which molten lead alloy contacts. The layer of cork powder is formed by combining cork with proper tacky producer and with water, and spraying them on the heated inner surface of the casting metal mould. Since the cork layer has a large heat insulating effect, it is one of the casting metal mould.Since the cork layer has a large heat insulating effect, it is one of the essentials to fully flow the molten metal into the cavities of the casting metal mould even though cast products such as plate grids to be produced have a small thickness and a complicated contour. This means that many casting defects tend to occur in the cast products unless the thickness of the cork layer is quite strictly controlled.

More particularly, referring to Fig. 1 of the accompanying drawings, a pair of plate grids 1 and 1 have been usually produced by one casting operation. The plate grids have lugs 2 which form terminals of the storage battery, and legs 3 by which the pair of plate grids are connected during their casting. The lugs 2 and 3 have a heat capacity higher than the other portions of the plate grids. Ideally, materials in the whole cavities of the casting metal mould should be solidified in a simultaneous manner. In the prior art, they have tried to be solidified nearly in an ideal manner by various means such as cooling means for the casting metal mould. In fact, this is a quite difficult problem. Thus, the lugs 2 and the legs 3 have been solidified later than the other portion of the plate grids.Casting defects tend to be produced in the portions of the plate grids which have been solidified later. If the plate grids are formed of Pb-Sb alloy, then the lugs 2 and the legs 3 have casting defects such as cracks and shrinkage porosities remarkably produced therein. The cork layer is required to be more thinly provided on the portions of the casting metal mould corresponding to the portions of the plate grids which tend to have such casting defects produced therein. Thus, in case the cork is sprayed on the mould surfaces, the thickness of the cork layer should be controlled by a given spraying pattern. However, since spraying cork highly depends on skilfulness of an operator, it is quite difficult to form the cork layer of required thickness.If the thickness of the cork layer even slightly exceeds a given value, then the lugs and the legs of the plate grids will have casting defects produced therein. Such casting defects of the plate grids cause the plate grids to be broken during the progress of manufacturing plates for a storage battery. Thus, quite many plate grids will be possibly disused.

As we examined the relation of the thickness of the cork layer to the temperature of the casting metal mould, it was found that 10 micro meter of the thickness of the cork layer corresponded to more than 1 0 C of variation in the temperature of the casting metal mould. The cork layer becomes removed out of the mould surface with casting time. If it is largely removed, then casting is no longer possible even though other casting conditions are controlled. This is because removal of the cork layer causes decrease in the heat insulating effect of the mould and, as a result, in flow of the molten lead alloy. Thus, the cork layer is required to be provided every 2000 casting operations. Therefore, cork spraying has been required twice per day.

It usually takes about 5 to 10 minutes to spray cork for every casting machine. Since the cork layer has a great effect on thermal characteristics of the casting metal mould and also flow and solidification time of the molten metal, cork spraying should be made carefully and strictly, which requires high skill of the operation. This means that even though a quite experienced operator sprays cork on the surface of the casting metal mould, unfavorable plate grids having fins are sometimes produced due to shortage in flow of molten metal. Furthermore, cork spraying causes work environment to be deteriorated. The cork is caused to stick even to articles other than the casting metal mould as well as human bodies. As noted from the foregoing, cork spraying causes many undesirable troubles.

Accordingly, it is a principal object of the invention to provide a lead alloy casting metal mould wherein cork spraying is not required to avoid undesirable troubles.

It is another object of the invention to provide a lead alloy casting metal mound wherein casting operating is made at a higher efficiency.

It is another object of the invention to provide a lead alloy casting metal mould adapted to manufacture satisfactory cast products having no casting defects.

In accordance with the invention, there is provided a lead alloy casting metal mould comprising a ceramic layer provided on an inner surface of said casting metal mould.

The above and other objects and features of the invention will be apparent from the embodiment taken with reference to the accompanying drawings in which; Figure 1 is a front view of a pair of plate grids for a lead-acid storage battery as one of examples of cast product: Figure 2 is a perspective view of a half of a plate grid casting metal mould constructed in accordance with the invention; Figure 3 is a cross sectional view of the mould half taken along the line Ill-Ill of Fig. 2; Figure 4 is a front view of a first main mask to be used for making the casting metal mould of the invention; Figure 5 is a front view of a first auxiliary mask to be used for making the casting metal mould of the invention; Figure 6 is a front view of a second main mask to be used for making the casting metal mould of the invention;; Figure 7 is a front view of a second auxiliary mask to be used for making the casting metal mould of the invention; and Figure 8 is a front view of a third mask to be used for making the casting metal mould of the invention.

Referring now to Figs. 2 and 3, there is shown one half 1 0A of a plate grid casting metal mould 10 made of cast iron. The mould half 1 0A comprises a gate 12, a passage 14 and four cavities 1 6, 18, 20 and 22. The cavity 1 6 serves to form a grid frame or parent bones 4 of plate grids 1 and 1 of Fig. 1. The cavities 1 8 and 20 serve to form lugs 2 and legs 3 of the plate grids 1 and 1. The cavity portion 22 serves to form subsidiary or grid bones 5 of the plate grids 1 and 1. The subsidiary or grid bone cavity 22 has a depth slightly larger than one second of other cavities 16, 1 8 and 20.Although another half of the casting metal mould is not shown in Fig. 1, it is identical to the mould half 1 or. The casting metal mould 10 further comprises a ceramic layer provided on the inner surface of the mould halves. The ceramic layer may be sprayed by melting powdered materials such as ZrCaO3, 2MgO AI203, MgO SiO2 and TiO2 in the plazma and then sputtering fine molten droplets onto the inner surface of the casting metal mould 10.Since the lugs 2, and legs 3 and the parent bones 4 of the plate grids 1 and 1 have a heat capacity larger than the subsidiary bones 5 of the plate grids 1 and 1 because they have a larger thickness so that that they are solidified later, portions of the ceramic layer corresponding to the lugs 2, the legs 3 and the parent bones 4 should have a thickness smaller than another portion of the ceramic layer corresponding to the subsidiary bones 5 of the plate grids 1 and 1. The ceramic layer has a thickness t, at the subsidiary bone cavity 22 and the passage 14 of the mould half, has a thickness t2 at the lug cavity 18, and has a thickness t3 at the abutting surface 24 of the mould halves. The thicknesses t1, t2 and t2 may range from 0.05 mm to 0.4 mm depending on the heat capacity of the portions of the plate grids.

The ceramic layer may be formed by using masks as shown in Figs. 4 to 8. The first main mask 26 has openings 26a corresponding to the gate 12, the molten metal passage 14 and the subsidiary bone cavity 22 of the mould half 1 or. The first main mask 26 is applied to the inner surface of the mould half 1 or. The dotted line of Fig. 4 shows a configuration of the inner surface of the mould half 1 OA when the first main mask is applied to the mould half. This illustrates the relation of the mask to the inner surface of the mould half 1 or. Ceramics are sprayed on the exposed inner surface portions of the mould half 1 OA through the openings 26a in the first main mask 26.Since it is impossible to make the first main mask 26 with the whole subsidiary bone cavity 22 unmasked, the openings 26a is discontinued at connections 26b of the openings 26a. Therefore, the ceramic layer discontinues at the portions of the subsidiary bone cavity 22 corresponding to the connections 26b of the first main mask 26. After removing the first main mask 26 from the mould half 10A, the first auxiliary mask 28 shown in Fig. 5 is applied to the inner surface of the mould half 1 or. The first auxiliary mask 28 has openings 28a corresponding to the connections 26b of the first main mask 26. Ceramics are sprayed on the exposed inner surface portions of the mould half 1 OA through the openings 28a in the first auxiliary mask 28, after which it is uncovered.Thus, the first ceramic layer portion of thickness t, is formed on the inner surface of the mould half 1 0A at the passage 14 and the subsidiary bone cavity 22 thereof.

The second main mask 30 shown in Fig. 6 has openings 30a corresponding to the lug cavity 18, the leg cavity 20 and the parent bone cavity 1 6 of the mould half 1 or. The second main mask 30 is applied to the inner surface of the mould half 1 or. The dotted line of Fig. 6 also shows a configuration of the inner surface of the mould half to which the mask 30 is applied.

The openings 30a are discontinued at the connections 30b. Ceramics are sprayed on the exposed inner surface portions of the mould half 1 OA through the openings 30a in the second main mask 30. After removing the second main mask 30, the second auxiliary mask 32 is applied to the inner surface of the mould half 1 or. The second auxiliary mask 32 has openings 32a corresponding to the connections 30b of the second main mask 30. Ceramics are sprayed on the exposed inner surface portions of the mould half 1 OA through the openings 32a in the mask 32. This mask 32 is removed from the mould half 1 0A after spraying.Thus, the ceramic layer portions of thickness t2 are formed on the inner surface of the mould half 1 OA at the parent bone cavity 16, the lug cavity 1 8 and the leg cavity 20 thereof.

Finally, the third mask 34 is applied to the inner surface of the mould half 1 or. The third mask has a configuration corresponding to that of the cavities 1 6 to 22 and also to the gate 1 2 of the mould half 1 OA. Ceramics are sprayed on the exposed inner surface of the die half 1 OA at the abutting surface portion thereof through the third mask 34. Thus, the ceramic layer portion of thickness t3 is formed on the abutting surface of the mould half 1 OA.

Materials of the masks 26, 28, 30, 32 and 34 may be determined on the basis of the size required for the plate grids to be produced, but it may be properly selected unless materials of the casting metal mould and their expansion coefficient are largely changed. Since the casting metal mould is sometimes required to be preheated on spraying ceramics, the mask materials may be preferably of heat resistance, but they may be of iron or soft steel thin plate. The masks may be made by punching or etching. Particularly, the first main mask 26 having fine and complicated configuration, may be preferably made by etching.

It should be noted that the thus formed ceramic layer has a bonding strength sufficient to prevent it from being removed from the inner surface of casting metal mould 10. Therefore, the ceramic layer can be used substantially permanently. This avoids the necessity to respraying the ceramic layer during a casting run. Accordingly, the effectiveness in casting becomes higher with the result that the productivity is improved. Also, the cast products have a high quality without any casting defects because the ceramic layer is not required to be reformed and as a result it has predetermined thicknesses. In case that the cast products are plate grids, there will occur no trouble in assembling batteries. Furthermore, the working environment is never detoriorated because cork spraying is unnecessary.

The ceramic layer may have a function of absorbing gases generated when the molten metal is solidified. The absorption function of the ceramic layer is produced by controlling the condition of spraying ceramics and by properly choosing materials of ceramics. Since the ceramic layer is never wet with the molten metal, the cast products of plate grids can be easily removed out of the casting metal mould 10. Although there is a difference between the heat expansion coefficiences of the ceramic layer and the casting metal mould, if the operation is made at a temperature of less than 800"C, then the ceramic layer is never removed from the inner surface of the casting metal mould. This is assured by our heat fatigue test.

If the ceramic layer is of porous material, it may be preferably filled at the pores with a heat resisting layer of vinyl resin to prevent corroision of the ceramic layer which tends to occur due to lead dioxide in the pores of the ceramic layer.

One example of the invention will be described later. Two casting metal moulds were prepared having the same size and configuration. One of the casting moulds was covered on its inner surface with the ceramic layer according to the invention while the other casting mould was covered on its inner surface with the cork layer in a conventional manner. By producing two sets of plate grids of the same material and of the same size and configuration in the casting metal mould of the invention and the casting metal mould of the prior art, respectively, the casting defects of the plate grids were observed to compare them.

The ceramic layer was of ZrCaO3 and was formed by using a plazma spraying gun. The thickness of the ceramic layer varied at the portions of the inner surface of the casting metal mould. The thickness t, of the ceramic layer portions at the gate, the passage and the subsidiary bone cavity of the casting mould was 0.25 mm, the thickness t2 of the ceramic layer portions at the lug cavity, the leg cavity and parent bone cavity of the casting mould was 0.14 mm, and the thickness t3 of the ceramic layer portion at the abutting surface of the casting mould was 0.09 mm.

The cork layer was formed by an air compression type spray gun. The thickness t', of the cork layer portions at the gate, the passage and the subsidiary bone cavity of the casting mould was 0.1 8 mm, the thickness t'2 of the cork layer portions at the lug cavity and the leg cavity and the parent bone cavity was 0.10 mm, and the thickness t'3 of the cork layer portion at the abutting surface of the casting mould was 0.06 mm.

The casting moulds were preheated to the temperature of 160"C to 200"C and molten metal of Pb-Sb-As alloy flowed into the casting moulds at the temperature of 450"C.

Casting defects were estimated by cutting off the lugs, the legs and the parent bones at the depth of 0.2 to 0.3 mm from their surface, polishing them by means of abrasive paper to fully flatten them and thereafter detecting cracks on solidifying and shrinkage porosity by dyeing process. Table shows the result of the test. It will be noted from Table that there was substantial decrease in the casting defects of the plate grids produced by the casting mould of the invention having the ceramic layer provided. It was found that there was no occurence in discontinuation of the plate grids due to poor flow of the molten metal and also in trouble of the casting machine due to failure in removal of the plate grids out of the casting mould.In Table, casting defects "A" were linear defects of length of 1 mm or more than 1 mm, "B" were linear defects of length of less than 1 mm, "C" were dotted defects of diameter of 1 mm or more than 1 mm, and "D" were dotted defects of diameter of less than 1 mm. In the test, five pairs of plate grids per time were estimated and four times of estimation were made.

TABLE Examination Casting The The Prior Points Defects Invention A 0 5 Lug B 1 3 C 2 14 D 4 12 A O Leg B 1 5 C 2 D 0 4 A 0 2 Outer B O 4 parent bone C O D 3 9 A O 1 Inner B 0 3 parent bone C 2 3 D 3 11 A 0 2 Upper B 0 3 parent bone C 1 3 D 5 14 While an embodiment of the invention has been described and illustrated with reference to the accompanying drawings, it will be understood by those skilled in the art that it is by way of example, and that various changes and modifications may be made without departing the spirit and scope of the invention, which is intended to be defined only to the appended claims.

Claims (4)

1. A lead alloy casting metal mould having a ceramic layer provided on an inner surface of said casting metal mould.

2. A lead alloy casting mould as claimed in Claim í, wherein said ceramic layer has various thicknesses at portions of said inner surface depending on the thicknesses of portions of a product to be cast.

3. A lead alloy casting metal mould, substantias0ty as hereinbefore described with reference to the accompanying drawings.

4. Plate grids whenever cast in a mould as claimed in any preceding Claim.