Plaster core for manufacturing a tire mold

Abstract

 Making a plaster cast core (24) for casting a tire tread mold ring (44) from a mixture of plaster, and water pouted into a core mold where the quantity of water required for the process is limited by suitable means such as a superplasticizer added to the mixture and a predried insert (28) added to the core.

Description

Technical Field

[0001] This invention relates to the making of a plaster core which is then used in casting a tire tread mold ring.

Background of the Invention

[0002] In the manufacture of tire molds it has been the practice to make tire tread molds by pouring molten aluminum into a molding space between a plaster core and an outer cylindrical wall of a mold. The plaster core is molded from a tread ring which is formed by pouring rubber over a carved model so that the radially outer surface of the plaster core is configured to form the radially inner tread molding surface of the tire mold. One of the problems with this method is that the plaster core must be heated in an oven for an extended period of time to dry the plaster. During the handling process after heating, some of the projecting tread forming elements on the core may be broken off. These broken tread elements must be repaired by hand, requiring additional time and the services of a skilled mold maker. Also after the warm core is removed from the oven, shrinkage or thermal shock may form cracks in the core or during the metal casting process.

Summary of the Invention

[0003] It has been discovered that by forming the core with a mixture of plaster, water and a superplasticizer such as "Melment" the time required to dry the plaster core has been reduced appreciably. Also, the strength of the cured plaster has been substantially increased resulting in less breakage and requiring fewer repairs after heating in the oven. It is believe that this improvement is the result of the particles of gypsum in the plaster being coated with the superplasticizer which permits the particles to nest in closer relation and provides increased strength as well as requiring less water which reduces the drying time. Use of the superplasticizer is believed to minimize the effects of thermal shock cracking. Drying time may also be reduced by reducing the volume of plaster that must be dried after forming the tread by using a predried insert.

[0004] In accordance with one aspect of the invention there is provided a method of making a plaster core for a tire tread mold characterized by:

a. mixing a superplasticizer, gypsum cement and water to provide a plaster core mixture;

b. pouring the mixture into a plaster core mold to form the core;

c. removing the core from the core mold;

d. placing the core in an oven;

e. heating the core for a predetermined time until the mixture is dried; and,

f. removing the core from the oven.

[0005] In accordance with another aspect of the invention there is provided a plaster core for a tire mold formed by mixing gypsum and water with a superplasticizer to provide improved rheology, a reduction in the water required and a reduction in the drying time.

[0006] In accordance with a further aspect of the invention, there is provided a predried insert for positioning in the plaster core mold to reduce the volume of gypsum plaster which must be dried.

Brief Description of Drawings

[0007] 

Fig. 1 is a plan view of the plaster core mold with the tread ring segments in place for forming the tread on the radially outer surface of the core with the plaster poured around a predried insert ring.

Fig. 2 is a sectional view of the plaster core mold with the plaster poured into the mold taken along line 2-2 in Fig. 1.

Fig. 3 is a side elevation of the plaster core being dried in an oven with a side of the oven removed to show the core.

Fig. 4 is a sectional view of the tire tread mold with the aluminum poured around the plaster core to form the tread ring.

Fig. 5 is a sectional view of the tread mold ring before being sawed in half.

Fig. 6 is a sectional view like Fig. 2 showing the predried insert poured in a form prior to placing in an oven for the predrying process.

Fig. 7 is a plan view of the predried insert and form taken along line 7-7 in Fig. 6.

Detailed Description of the Invention

[0008] In the manufacture of tire treads, tire tread molds have been made by carving a model with the desired tread configuration and then pouring a rubber ring around the model. The rubber ring is then cut into segments 10 which are mounted in segmental retainers 12. As shown in Figs. 1 and 2, the segmental retainers 12 are assembled on a supporting plate 14 and held in place by clamps 16 mounted on the supporting plate 14. In accordance with this invention, a mixture by weight of 60.58% gypsum plaster, 39.06% water, 0.34% Melment F17G superplasticizer and 0.01% Hostapur foaming agent is prepared in a mixer which stirs the ingredients so as to include air in the mixture. The resulting plaster mixture 18 is poured into the tread forming space 20 to form the plaster cast or unicore 24.

[0009] Sipe forming blades 26 may be mounted in the rubber ring segments 10 so that they will be encased by the plaster mixture 18 and become part of the unicore 24.

[0010] As shown in Figs 6 and 7, a predried insert 28 of the same plaster mixture 18 may be made prior to forming the unicore 24 by pouring the plaster mixture 18 into a container 30 having an outer cylindrical wall 32 and an inner cylindrical wall 34 mounted on a base 36. This insert 28 is hardened and removed from the container then placed in an oven and predried for placing on the supporting plate 14 before the tread forming mixture is poured into the tread forming space 20 as shown in Figs. 1 and 2.

[0011] After the unicore 24 has been hardened sufficiently to be removed from the rubber ring segments 10, the clamps 16 are released and the segmental retainers 12 pulled away from the plaster cast, leaving the sipe forming blades 26 in the unicore 24, including the predried insert 28 which is then placed in an oven 38 shown schematically in Fig. 3. The unicore 24 is then dried for an appropriate time, which may be 24 to 48 hours depending upon size of cast unicore. The unicore 24 is then mounted in a tread ring mold 42 and an aluminum ring 44 is cast around the plaster cast a shown in Fig. 4. After the aluminum tread ring 44 and the mold are cooled, the unicore 24 is removed by breaking it away from the tread ring. The aluminum tread ring 44 is then machined to provide a tread ring suitable for placing in a tire mold.

[0012] In formulating the ingredients for the unicore 24, the recommended composition by weight is 60.58% plaster, 39.06% water, 0.34% superplasticizer, and 0.01% foaming agent. The percentage of superplasticizer may vary up to a maximum of 2%. It is believed that after the superplasticizer coats the gypsum particles that additional amounts will not increase the fluidity or reduce the water required by the mixture. It is important to limit the amount of water to provide optimum strength. After sufficient water is added to provide for adequate hydration of the plaster grains, and desired rheology is achieved, it is important not to add additional water. By adding the superplasticizer, the water gypsum plaster ratio can be lowered while maintaining desirable rheology. Also the gypsum density is increased which results in increased strength and less drying time. For example, by adding from 1% to 3% by weight of superplasticizer (Melment), the water to cement ratio was decreased from 0.9 to 0.6 and the water content was decreased by 23%. This has resulted in substantial decrease in drying time. Drying time has also been decreased by decreasing the volume tread forming space 20 with the predried insert 28 which results in decreasing the quantity of water required.

Claims

1. A method of making a plaster core (24) for a tire tread mold characterized by:

a. mixing a superplasticizer, gypsum cement and water to provide a plaster core mixture;

b. pouring said core mixture into a plaster core mold to form said core;

c. removing said core from said core mold after said core has hardened;

d. placing said core in an oven (38);

e. heating said core for a predetermined time until said mixture is dried; and,

f. removing said core from said oven.

2. The method of claim 1 further comprising limiting the quantity of water by mixing said superplasticizer with said gypsum cement and water.

3. The method of claim 2 further comprising said superplasticizer being Melment.

4. The method of claim 2 further comprising mixing a foaming agent in the plaster core mixture.

5. The method of claim 4 further comprising adding a pre-dried insert to said plaster core mold before pouring said mixture into said plaster core mold to reduce the volume of gypsum plaster which needs to be used.

6. The method of claim 1 further comprising limiting the quantity of water required for the core by placing a pre-dried insert (28) in said plaster core mold prior to pouring said mixture into said mold reducing the volume of the core which is dried.

7. The method of claim 6 further comprising said pre-dried insert (28) being placed in the center of said plaster core mold to reduce the thickness of said plaster core mold, which must be dried.

8. The method of claim 7 wherein said pre-dried insert (28) is cylindrical with a central opening for centering said insert in said mold.

9. A plaster core (24) for a tire tread mold comprising a mixture of gypsum plaster particles and water characterized by an addition of a superplasticizer to provide improved rheology and a reduction in water required and in drying time.

10. The plaster core (24) of claim 9 characterized by said core being cylindrical and having a pre-dried insertion (28) positioned at the center of said plaster core to reduce the thickness of said plaster core which needs to be dried.

Drawing