Split dies for casting small segments of tire mold

Abstract: Split dies include an upper die (2) and a lower die (1) for casting small segments constituting tread pattern forming portions of a tire mold. The lower mold includes a telescopic die (7) having an upper surface forming a die bottom wall having a configuration corresponding to the tread pattern forming portions of the small segments. The lower mold further includes a main die (6) having a cavity (3) for receiving the telescopic die and having side walls surrounding the cavity to form die inner walls whose configuration corresponds to side walls of the small segments.

Description: This invention relates to split dies for casting small segments for constituting tread pattern forming portions of tire vulcanizing molds. Recently, in order to improve operating efficiency and effectiveness of maintenance of tire vulcanizing molds, small pieces or small segments having shapes corresponding to tread patterns are often arranged side by side in width directions of treads and circumferential directions of tires to form tread pattern forming portions (Japanese Patent Laid-open No. 61-108,512). As shown in Fig. 1 of the accompanying drawings, dies for casting small segments generally consist of a lower die 1 and an upper die 2 having a cavity 3 therebetween into which a molten metal such as aluminium is poured to form small segments cast in desired shapes. In manufacturing these dies, a bottom wall of the lower die 1 must be worked precisely to meet a tread pattern because tread pattern forming portions are formed at the bottom wall of the lower die 1. In other words, a portion of the die in a circle A in Fig. 1 corresponds to a groove of the tread, whose groove corner is particularly sharply formed. In the same manner, a portion of the die in a circle B corresponds to a side wall of the small segment which must be worked so as to be a sharply formed groove having a flat wall on one side forming a contact surface with a small segment in assembling. If a contact between two segments is inaccurate to provide clearances therebetween, rubber will penetrate into the clearances in vulcanization to permit fins occurring along the contacting surfaces, which would require a post-treatment similar to that in case of spews. For producing such dies, electrical discharging is generally used with electrodes identical in shape with tread pattern forming portions of the dies or separate electrodes for working portions corresponding to grooves and tread surfaces. Therefore, many processes are required for working the electrodes to obtain precise dimensions thereof. In addition, the requirement in manufacturing dies such as sharp working of groove corners above described could not be fulfilled. With the portion B of the dies shown in Fig. 1, moreover, there is a problem of gas venting being insufficient in casting so that side walls of small segments could not be sharply formed. As a result, contact between the small segments themselves is often inaccurate. The present invention aims to provide an improved die construction which fulfills all the requirements for dies for casting small segments of tire molds. The present invention provides a split die including upper and lower dies for casting small segments constituting tread pattern forming portions of a tire mold, characterized in that said lower die comprises a die insert portion having an upper surface forming a die bottom wall having a configuration corresponding to the tread pattern forming portions of the small segments, and a main die having a cavity for receiving said die insert portion and having side walls surrounding said cavity to form die inner walls whose configuration corresponds to side walls of the small segments, and a surface of the upper die on the side of the cavity has a curvature which is a curvature of an inner surface of a holder of a vulcanising mold for fixing the small segments plus an amount of shrinkage in casting. With the above arrangement of the invention, the lower die comprises the die insert portion and the main die so that these dies can be separately worked for manufacture. Particularly, as the die insert portion forms a die bottom wall for casting tread pattern forming portions of small segments, complicated working is generally needed. However, as the die bottom surface is located on an upper surface of the die insert portion and side walls belong to the main die, which would otherwise obstruct the working of the die bottom surface when the side walls belonged to the die insert portion, all the working is considerably easy and simple. Therefore, the working of the dies is effected sufficiently by milling or turning. When a molten metal is poured into the lower die formed by a combination of the die insert portion and the main die, gas venting is effected through contacting surfaces between the die insert portion and the main die, so that the molten metal can be sufficiently led to extreme ends of grooves. Therefore, sharp corners can be accomplished which correspond to side wall edges and bottoms of projections of small segments. If widths of the contacting surfaces serving the gas venting are too wide, there is a tendency for fins of metal therebetween to occur. On the other hand, in the event that the width is too narrow, the gas venting therethrough is insufficient. Accordingly, the widths are required to be adjusted according to materials to be cast and casting conditions. The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a sectional view illustrating dies of the prior art; Fig. 2a is a sectional view illustrating a split die according to the invention; Fig. 2b is a sectional view taken along the line IIb-IIb in Fig. 2a; Figs. 3a-3f are schematic views for explaining a method of producing the split dies according to the invention; and Fig. 4 is a schematic perspective view illustrating another method for producing the split dies according to the invention. Figs. 2a and 2b illustrate a preferred embodiment of a split die for die casting small segments of tire molds according to the invention; The die comprises a movable die 4 incorporating therein a lower die 1 and a stationary die 5 incorporating therein an upper die 2. As an alternative, the movable die 4 may incorporate therein an upper die 2 and the stationary die 5 may corporate therein a lower die 1, respectively, dependent upon a removing system of the respective dies. Moreover, the lower die 1 is formed by a main die 6 and a die insert portion 7, both of which are bolted to a supplementary plate 8. In the case that the die insert portion 7 is directly fixed to the main die 6, the supplementary plate 8 is not needed. The main die 6 forms a frame of the lower die 1 and has inner walls identical in shape with side walls of small segments. The side walls of the small segments have configurations enabling the small segments to be engaged with each other in a snugly fitted relationship like a jigsaw puzzle. The die insert portion 7 is formed to have a surface configuration identical (but slightly larger by shrinkage in casting in similar configuration with that of a tread pattern of a tire. Side surfaces of the die insert portion 7 correspond to inner walls of the main die 6 so that the die insert portion 7 can be fitted in the main die 6 without any clearances. A surface of the upper die 2 on the side of the cavity 3 has a curvature which is a curvature of back surfaces of small segments, that is inner surfaces of a holder of a vulcanizing mold for fixing the small segments plus an amount of shrinkage in casting. A method of producing the lower and upper dies 1 and 2 will be explained by referring to Figs. 3a-3f hereinafter. As shown in Fig. 3a, a die blank material 9 is first cut by wire cut electrical discharging along a curve to be given to the upper die 2 into two parts, one for the upper die 2 and the other being a blank 10 for the main die 6 of the lower die 1. The blank 10 for the main die 6 is then cut at its center by wire cut electrical discharging as shown in Fig. 3b into a configuration corresponding to side walls of small segments plus shrinkage amount in casting, whereby the main die 6 is completed by removal of the center from the blank 10. In order to produce the die insert portion 7 of the lower die 1, as shown in Fig. 3c a separately prepared blank 11 for the die insert portion 7 is worked by a lathe to form contours 12 and grooves 13. This process may be effected by milling if required. As shown in Fig. 3d, the blank 11 is further formed with grooves by milling, which are unable to be formed by the lathe, to complete the portion which is a die bottom wall for forming the tread pattern. As shown in Fig. 3e, the blank 11 of the die insert portion 7 subjected to the above processes is then cut with its circumference by the wire cut electrical discharging so as to be fitted within the inner walls of the main die 6 to obtain the final die insert portion 7. Finally, the die insert portion 7 is arranged in the main die 6 to complete the lower die 1 as shown in Fig. 3f. It is preferable that clearances between the main die 6 and the die insert portion 7 are less than 0.02 mm in order to avoid fins occurring between the die 6 and the die insert portion 7. The remaining portion of the blank 10 for the main die 6 may be used for the blank 11 of the die insert portion 7. In another method for producing the split dies, in working the blank 10 of the main die 6, a blank 10 for the main die 6 is first divided into three blanks 14, 15 and 16 and the blank 15 is then divided into three blanks 15A, 15B and 15C as shown in Fig. 4. Thereafter, the blanks 14, 16, 15A and 15C are combined to form a main die 6, while the blank 15B is used as a blank 11 of the die insert portion 7. The main die 6 need not be a unitary body as shown in Figs. 2a and 2b; the main die 6 may be a separable die in this manner. Costs and times were compared, which were required for manufacturing split dies for casting small segments according to the invention as above described and dies by electrical discharging with electrodes of the prior art. The costs and times for producing the split dies according to the invention were approximately one half of those according to the prior art. Moreover, small segments were cast by the use of the dies according to the invention and the prior art. Average radii of curvatures at ends of projections (in the circle A in Fig. 1) and side wall edges (in the circle B in Fig. 1) were 0.03 mm with the dies according to the invention, whereas those with the dies of the prior art were 0.12 mm. The sharp edges were clearly accomplished according to the invention. As can be seen from the above description, with the split die according to the invention the main die and the die insert portion constituting the lower dies can be individually treated or handled so that grooves and the like can be precisely worked and gas venting is possible through contacting surfaces between the main die and the die insert portion in casting. Therefore, corners at edges of respective small segments produced by the split dies according to the invention can be sharply formed. In addition, no rubber enters between contacting surfaces of such small segments incorporated in a vulcanizing mold, so that tires devoid of fins can be produced according to the invention. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the scope of the inventions as set out in the claims.

Claims: A split die including upper and lower dies (2,1) for casting small segments constituting tread pattern forming portions of a tire mold, characterized in that said lower die comprises a die insert portion (7) having an upper surface forming a die bottom wall having a configuration corresponding to the tread pattern forming portions of the small segments, and a main die (6) having a cavity (3) for receiving said die insert portion and having side walls surrounding said cavity to form die inner walls whose configuration corresponds to side walls of the small segments, and a surface of the upper die (2) on the side of the cavity (3) has a curvature which is a curvature of an inner surface of a holder of a vulcanising mold for fixing the small segments plus an amount of shrinkage in casting. A split die as claimed in claim 1, characterized in that side walls of the die insert portion correspond in configuration to inner walls of the main die. A split die as claimed in claim 1 or 2, characterized in that the main die and the die insert portion are fixed to a supplementary plate (8).

Inventors: KAMATA SACHIKO C O BRIDGESTONE, KATSUMATA SHIROU C O BRIDGESTO, OHTA KAZUHIKO C O BRIDGESTONE, TANAKA HIDEAKI C O BRIDGESTONE, KAMATA, SACHIKO, C/O BRIDGESTONE CORPORATION, KATSUMATA, SHIROU, C/O BRIDGESTONE CORPORATION, OHTA, KAZUHIKO, C/O BRIDGESTONE CORPORATION, TANAKA, HIDEAKI, C/O BRIDGESTONE CORPORATION
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1990
Main classification: B29C33
IPC codes: B29C33, B22C9