Pneumatic tire and molding die thereof

A sipe is provided to a land portion provided to a tread of a pneumatic tire. The sipe is formed of a wide portion having a groove width W1 and a narrow portion having a groove width W2 narrower than the groove width W1. The wide portion extends in a sipe depth direction D at both ends in a sipe length direction L and extends in the sipe length direction L at an upper end in the sipe depth direction D. The narrow portion is surrounded by the wide portion on three sides including both sides in the sipe length direction L and an opening side in the sipe depth direction D and extends to a sipe bottom. Consequently, chipping of rubber is suppressed by reducing pulling resistance of a sipe plate at the time of die releasing of a tire while suppressing breakage of the sipe plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2014-262729, filed on Dec. 25, 2014; the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a pneumatic tire and a molding die thereof.

2. Related Art

Snow tires (studless tires), for example, have incisions called sipes which are made in land portions, such as blocks and ribs. An edge effect by the sipes enables stable running on a road surface covered with snow and ice where a frictional coefficient is low. Whereas the sipes obtain the effect as above, as is shown inFIG. 9, a sipe101lowers rigidity of a land portion102and the land portion102undergoes deformation (collapses) to a greater extent when making contact with the ground. Such deformation may possibly reduce the edge effect contrary to the intention or lower resistance to irregular wear.

In order to suppress deformation of the land portions when making contact with the ground, increasing frictional resistance between a pair of opposing wall surfaces of a sipe by providing 20 to 300 μm irregularities to a pair of the wall surfaces (see JP-A-8-175115) or narrowing a groove width of a sipe has been proposed.

As a method of narrowing the groove width of a sipe, for example, JP-A-11-170818 discloses that a groove width of a sipe is made narrower in an intermediate portion in a sipe length direction than at both ends. JP-A-11-42913 discloses that a groove width of a sipe is set to 0.1 to 0.3 mm, which is narrower than a typical groove width, and also to provide a pillar-like space extending in a sipe depth direction to reinforce a sipe plate. JP-A-2-241806 and JP-A-2-303908 disclose to provide a sipe with a narrow portion or a knife cut portion having a narrow groove width and a wide portion having a wide groove width and surrounding the narrow portion or the knife cut portion at least partially.

When a sipe having a narrow groove width as above is provided, a sipe plate to shape the sipe becomes thinner, which raises a concern about breakage of the sipe plate over repetitive use for molding of a tire by vulcanization. In order to enhance a breakage suppressing effect on the sipe plate, it is effective to surround a thin plate portion of the sipe plate with a thick portion as are described in JP-A-2-241806 and JP-A-2-303908. However, when the thin plate portion is surrounded by providing a thick portion at a tip end of the sipe plate (that is, when a sipe is provided with a wide portion having a wide groove width at a sipe bottom and a narrow portion having a narrow groove width on top of the wide portion), problems as follows are raised. That is, in this case, resistance of the sipe plate is large when pulled out from the surface of tread rubber at the time of die releasing of a tire (that is, when a tire molded by vulcanization is released from the molding die), and large resistance causes chipping of rubber.

SUMMARY

In view of the foregoing, embodiments of the invention have an object to provide a pneumatic tire provided with a sipe having a partially narrowed groove width with the aim of suppressing deformation of a land portion when making contact with the ground, which is capable of suppressing chipping of rubber by reducing pulling resistance of a sipe plate at the time of die releasing of a tire while suppressing breakage of the sipe plate, and a molding die thereof.

According to the embodiments, a pneumatic tire includes a land portion provided to a tread, and a sipe provided to the land portion. The sipe includes a wide portion having a first groove width, which extends in a sipe depth direction at both ends in a sipe length direction and extends in a sipe length direction at an upper end in the sipe depth direction, and a narrow portion having a second groove width narrower than the first groove width, which is surrounded by the wide portion on three sides including both sides in the sipe length direction and an opening side in the sipe depth direction and extends to a sipe bottom. Regarding the terms, “wide portion” and “narrow portion”, “wide” and “narrow” are used to specify which one of the two portions has a groove width wider or narrower than the other. In other words, the wide portion has a wider groove width than the narrow portion and the narrow portion has a narrower groove width than the wide portion.

According to the embodiments, a molding die of a pneumatic tire includes a sipe plate to shape a sipe in a land portion provided to a tread. The sipe plate includes a frame-like portion formed of a lower frame portion extending along a root portion to a molding die main body and side frame portions extending in a height direction from both ends of the lower frame portion, and a thin plate portion having a thickness thinner than a thickness of the frame-like portion, which is surrounded by the frame-like portion on three sides except for a tip end side corresponding to a sipe bottom and extends to a tip end.

According to the embodiments, chipping of rubber can be suppressed by reducing pulling resistance of a sipe plate while suppressing breakage of the sipe plate in a pneumatic tire provided with a sipe having a partially narrowed groove width with the aim of suppressing deformation of a land portion when the tire makes contact with the ground.

DETAILED DESCRIPTION

Hereinafter; embodiments of the invention will be described according to the drawings.

First Embodiment

A pneumatic tire according to a first embodiment is formed of a pair of bead portions and side wall portions on the right and left, and a tread portion10provided between the side wall portions on the right and left so as to connect the both side wall portions along outer edges in a radial direction, but the illustration is omitted herein. The pneumatic tire can adopt a typical tire structure except for a tread pattern.

As is shown inFIG. 1, a plurality of main grooves (grooves in a circumferential direction)12extending in a tire circumferential direction and a plurality of traverse grooves (grooves in a width direction)14extending in a direction (tire width direction) intersecting with the main grooves12are provided to a surface of the tread portion10. The main grooves12and the traverse grooves14together define blocks16as a plurality of land portions. Each block16is provided with a sipe18extending in a direction intersecting with the tire circumferential direction. Herein, the sipe18is an incision that does not open to a block edge at both ends in a sipe length direction (that is, an incision that terminates within the block16without opening to the main grooves12), and is therefore referred to as a closed sipe. InFIG. 1, CL denotes a tire equator.

As is shown inFIG. 2in enlargement, the sipe18is a linear sipe extending parallel to the tire width direction and three sipes18are provided to each block16. Also, as is shown inFIG. 3B, the sipe18is of a linear shape that extends in a tire radial direction in cross section across the width, in a sipe depth direction D. Only one or at least two sipes18spaced apart in the tire circumferential direction may be provided to each block16. Alternatively, the sipe18may extend at an angle with respect to the tire width direction. Further, the sipe18is not limited to the linear sipe when viewed in a plane as shown inFIG. 2, and may be a wave-like sipe (that is, a sipe having a wave-like opening) when viewed in a plane.

The sipe18is formed of a wide portion20having a groove width (referred to also as a sipe width) W1of 0.6 to 1.5 mm, which is a typical sipe width, and a narrow portion22having a groove width W2narrower than the groove width W1of the wide portion20. The groove width W1of the wide portion20may be 0.7 to 1.3 mm.

As are shown inFIG. 2andFIGS. 3A and 3B, the wide portion20is a frame-like portion extending fully in the sipe depth direction D at both ends in a sipe length direction L and also extending fully in the sipe length direction L at an upper end in the sipe depth direction D (a portion located along a sipe opening24in the sipe depth direction D). In other words, the wide portion20is formed of an opening-side portion20A extending in the sipe length direction L at the upper end in the sipe depth direction D and side-edge portions20B and20B on the right and left extending in the sipe depth direction D at the both ends in the sipe length direction L.

Meanwhile, the narrow portion22is a portion surrounded by the wide portion20on three sides, namely, both sides in the sipe length direction L and an opening side in the sipe depth direction D, and extends to a sipe bottom26in the sipe depth direction D. The narrow portion22is a rectangular region provided in a center region in the sipe length direction L, and situated next to the opening-side portion20A via an upper side22A on the side of the sipe opening24and situated next to the side-edge portions20B and20B on the right and left via two lateral sides22B and22B on the right and left on the both sides in the sipe length direction L. On the other hand, a bottom side22C of the narrow portion22is not situated next to the wide portion20and coincides with the sipe bottom26.

Owing to the configuration as above, the sipe18is formed in such a manner so as to have the constant groove width W1by the wide portion20at the position of the opening24which corresponds to a surface of tread. At the both ends in the length direction L, the sipe18is formed in such a manner so as to have the constant groove width W1from the opening24to the sipe bottom26by the wide portion20. Meanwhile, at the center in the length direction L, the sipe18is formed in such a manner that the sipe18has the groove width W1at the position of the opening24by the wide portion20whereas the groove width is made narrower by the narrow portion22having the groove width W2from some midpoint in the depth direction D for the sipe18to have the constant narrow groove width W2to the sipe bottom26. A depth position of the upper side22A (that is, a distance dl from the opening24to the upper side22A), which is a boundary between the wide portion20and the narrow portion22, is not particularly limited, and may be, for example, 1 to 3 mm. A dimension p1of the narrow portion22in the sipe length direction L preferably accounts for 40% or more, and more preferably, 50 to 80% of a full length p0of the sipe18with the aim of enhancing a collapse suppressing effect on the block16when making contact with the ground.

As has been described, the groove width W2of the narrow portion22(an interval between a pair of opposing wall surfaces28and30of the narrow portion22) is narrower than the groove width W1of the wide portion20(W2<W1). The groove width W2is preferably 0.6 mm or less, more preferably 0.4 mm or less, and further preferably 0.3 mm or less. The smaller a value of the groove width W2becomes, the greater will be an attraction effect by water described later. Hence, the groove width W2has no particular lower limit and may be, for example, 0.1 mm or greater.

It is preferable for the narrow portion22that an arithmetic mean roughness Ra on a pair of the opposing wall surfaces (groove wall surfaces)28and30is set to 1.6 μm or less. In other words, the wall surfaces28and30of the narrow portion22herein are made smooth or made into a mirror-smooth state in comparison with typical sipe wall surfaces in the related art. The arithmetic mean roughness Ra on the wall surfaces28and30is preferably 1.3 μm or less, and more preferably 1.0 μm or less. The smaller a value of the arithmetic mean roughness Ra becomes, the smoother are the wall surfaces28and30and the greater will be the attraction effect by water described below. Hence, the arithmetic mean roughness Ra has no particular lower limit and may be, for example, 0.1 μm or greater, or 0.5 μm or greater.

Herein, the arithmetic mean roughness Ra is defined according to JIS B0601:2013. More specifically, the arithmetic mean roughness Ra is a value obtained by extracting a reference length alone from a roughness curve in a direction of a mean line, adding up an absolute value of a deviation from the mean line to a measured curve of the extracted portion, and computing an average of a total.

The arithmetic mean roughness Ra on a pair of opposing wall surfaces of the wide portion20is not particularly limited, and the arithmetic mean roughness Ra may be, for example, 2.5 μm or greater, which is a surface roughness of typical sipe wall surfaces.

FIG. 4shows a major portion of a tire molding die (molding metal die)50provided with a sipe plate52which is a metal plate to shape the sipe18in the block16. InFIG. 4, numeral54denotes a rib to shape the main grooves12and the traverse grooves14, which is shown schematically in a partially cut-out state.

The sipe plate52is of a shape conforming to the sipe shape as described above and formed of a frame-like portion56having a thickness T1of 0.6 to 1.5 mm, which is a thickness of a typical sipe plate, and a thin plate portion58having a thickness T2thinner than the thickness T1of the frame-like portion56.

The frame-like portion56is formed of a lower frame portion62extending along a root portion to a molding die main body60, and a pair of side frame portions64and64on the right and left extending in a height direction from both ends of the lower frame portion60. The lower frame portion62is formed fully in the root portion (that is, in a full width of the sipe plate52) in a width direction of the sipe plate52, which corresponds to the sipe length direction L. The side frame portions64are formed fully in height of the sipe plate52.

The thin plate portion58is surrounded by the frame-like portion56on three sides except for a side of a tip end66which corresponds to the sipe bottom26, and extends to the tip end66in the height direction of the sipe plate52. The thin plate portion58is a rectangular portion provided to the sipe plate52at a center region in the width direction, and situated next to the lower frame portion62on the side of the root portion and situated next to the side frame portions64and64on the right and left on the both sides in the width direction. It should be noted, however, that the tip end66is not surrounded by the frame-like portion56.

As with the narrow portion22of the sipe18, the thickness12of the thin plate portion58is preferably 0.6 mm or less, more preferably 0.4 mm or less, and further preferably 0.3 mm or less. The lower limit may be, for example, 0.1 mm or greater.

It is preferable for the thin plate portion58that the arithmetic mean roughness Ra on a pair of side surfaces68and70that shape a pair of the wall surfaces28and30of the narrow portion22is set to 1.8 μm or less. Generally, a surface roughness on sipe wall surfaces transferred from both of two side surfaces of the sipe plate is lower than a surface roughness on the side surfaces of the sipe plate. Hence, by setting the arithmetic mean roughness Ra on the side surfaces68and70of the thin plate portion58to 1.8 μm or less, the arithmetic mean roughness Ra on the wall surfaces28and30of the narrow portion22can be set to 1.6 μm or less. The arithmetic mean roughness Ra on the side surfaces68and70of the thin plate portion58is preferably 1.6 μm or less, and more preferably 1.3 μm or less. The lower limit may be, for example, 0.1 μm or greater, or 0.5 μm or greater. The arithmetic mean roughness Ra on a pair of wall surfaces of the frame-like portion56is not particularly limited, and the arithmetic mean roughness Ra may be, for example, 2.5 μm or greater, which is a surface roughness of a typical sipe plate.

For configurations other than the sipe plate52, the tire molding die50can adopt a structure of a typical tire molding die. In this embodiment, a plurality of sipe plates52are planted in a die surface of the tire molding die50at positions corresponding to the sipes18. The pneumatic tire described as above can be manufactured by subjecting an unvulcanized green tire to molding by vulcanization in an ordinary manner using the tire molding die50. In a fabrication sequence of a tire molding die, it is general to apply sand blasting after the sipe plates are attached to the die surface in order to make the entire die surface smooth. Hence, surfaces of the side surfaces68and70of the thin plate portions58of the sipe plates52are also roughened unless some measures are taken. For this reason, it may be configured in this embodiment in such a manner that sand blasting is applied while the side surfaces68and70of the thin plate portion58are covered with a masking sheet and the masking sheet is removed after the sand blasting. Alternatively, it may be configured in such a manner that sand blasting is applied without using a masking sheet first and then the side surfaces68and70of the thin plate portion58are polished so as to have the predetermined arithmetic mean roughness Ra.

According to this embodiment described above, by providing the narrow portion22, breakage of the sipe plate52can be suppressed during the manufacturing of a tire and chipping of rubber can be suppressed by reducing pulling resistance of the sipe plate52while suppressing deformation of the block16when the tire makes contact with the ground.

More specifically, by providing the sipe18with the narrow portion22having a narrower groove width and higher surface smoothness than a sipe in the related art, water is trapped inside the narrow portion22as is shown inFIG. 5when the block16makes contact with a road surface S containing moisture, such as a road surface covered with snow and ice, which allows the opposing wall surfaces28and30to adhere to each other. In other words, the opposing wall surfaces28and30of the narrow portion22have a narrow interval W2in between and smooth surfaces without irregularities. Hence, the wall surfaces28and30are in an adhesion state via a water film F without having air remained in between. In this instance, an attraction effect or a pressure bonding effect is thought to be obtained by a difference of a water pressure and atmospheric pressure between the wall surfaces28and30as is shown inFIG. 5. In contrast, a typical sipe in the related art has a relatively wide groove width of about 0.6 to 1.5 mm and the arithmetic mean roughness Ra of 2.5 μm or greater as a surface roughness on the wall surfaces. Regarding a surface roughness, even when surfaces of the sipe plate are mirror surfaces before the attachment to the die surface, it is unavoidable for the surfaces to be roughened to a certain degree by sand blasting applied after the attachment. Consequently, the wall surfaces of a sipe shaped by the sipe plate in the related art may appear to be flat, but actually have the arithmetic mean roughness Ra of 2.5 μm or greater. A relatively wide sipe having rough surfaces on the wall surfaces as above cannot exert the attraction effect by water as described above. According to this embodiment, however, because an adhesion effect between the wall surfaces28and30can be obtained by providing the narrow portion22as above, rigidity of the block16can be increased. Consequently, deformation of the block16when making contact with the ground can be suppressed regardless of the sipe18provided to the block16. Hence, not only can resistance to irregular wear be enhanced, but also an edge effect can be exerted and therefore ice performance can be enhanced.

According to this embodiment, the narrow portion22is surrounded by the wide portion20, and the frame-like portion56provided on the three sides of the sipe plate52that shapes the narrow portion22and the wide portion20functions as a blade reinforcement frame to reinforce the thin plate portion58that shapes the narrow portion22. Hence, breakage, such as breaking, of the sipe plate52can be suppressed.

According to this embodiment, the wide portion20is not provided to the narrow portion22on the side of the sipe bottom26. Hence, the thick frame-like portion56is not present on the side of the tip end66of the thin plate portion58in the sipe plate52that shapes the wide portion20and the narrow portion22. Consequently, resistance of the sipe plate52is small when pulled out from the surface of tread rubber at the time of die releasing of a tire and chipping of tread rubber can be suppressed. In addition, according to the present embodiment, because smoothness of the thin plate portion58of the sipe plate52is high, pulling resistance of the sipe plate52can be reduced.

Second Embodiment

Configurations of sipes and sipe plates according to a second embodiment will be described according toFIG. 6throughFIG. 8. A sipe18A of the second embodiment is different from the counterpart in the first embodiment above in that at least one concave groove32extending in a sipe width direction D is provided to at least one of a pair of opposing wall surfaces28and30of a narrow portion22.

More specifically, the concave groove32is provided to one wall surface28alone herein, and two concave grooves32spaced apart in a sipe length direction L are provided to the one wall surface28. The concave groove32is a fine groove having a width W3(a dimension of the concave groove32in the sipe length direction L) equal to or less than a width W2of the narrow portion22(W3≤W2). A recess depth K1of the concave groove32shown inFIG. 6when viewed in a plane is set to be equal to or less than the width W2of the narrow portion22(K1≤W2). As are shown inFIG. 7AandFIG. 7B, the concave grooves32are provided to the narrow portion22fully in a depth direction D and formed linearly from an upper side22A to a sipe bottom26of the narrow portion22.

FIG. 8shows a sipe plate52A to shape the sipe18A in a block16. In order to shape the concave grooves32, the sipe plate52A is configured in such a manner that at least one convex ridge72extending in the sipe depth direction D is provided to at least one of a pair of side surfaces68and70of a thin plate portion58. Herein, the convex ridge72is provided to one side surface68alone, and two convex ridges72spaced apart in the sipe length direction L are provided to the one side surface68. As with the shape of the concave groove32, a shape of the convex ridge72has a width equal to or less than a thickness T2of the thin plate portion58, and a protrusion height when viewed in a cross section is set to be equal to or less than the thickness T2. The convex ridges72are provided to the thin plate portion58fully in a height direction, and formed linearly from a lower end of the thin plate portion58to a tip end66of the sipe plate52A.

According to the second embodiment, in addition to the function and the effect obtained in the first embodiment above, a function and an effect as follows can be obtained. That is, by providing the concave grooves32to the wall surface28of the narrow portion22of the sipe18A, introduction of water into the narrow portion22can be accelerated. The concave grooves32thus serve to draw water into the narrow portion22and an adhesion action between the wall surfaces28and30described above can be exerted at an early stage by providing the concave grooves32. Other configurations, functions, and effects of the second embodiment are the same as those of the first embodiment above, and a description is omitted herein.

Other Embodiments

In the embodiments above, the sipes18and18A are closed sipes. However, the embodiments above may be applied also to a one-end open sipe that opens to the main groove at one end and terminates within the block at the other end, or a both-end open sipe that opens to the main grooves at the both ends. It is preferable to apply the embodiments above to closed sipes because deformation of the blocks16can be suppressed more effectively.

The embodiments above have described a case where the sipes18or18A are provided to the blocks16as the land portions. However, the land portion to which the sipe18or18A as above is to be provided not limited to a block and may be a rib continuing in the tire circumferential direction. The sipe configuration as above may be applied to all the land portions within a tread pattern or may be applied to only a part of the land portions within the tread pattern. For example, all the sipes within the tread pattern may have the wide portions and the narrow portions or some of all the sipes may have the wide portions and the narrow portions. In short, it is sufficient that at least one sipe including the wide portion and the narrow portion is provided to at least one land portion within the tread pattern.

The embodiments above are capable of improving ice performance and are therefore suitably applied, for example, to snow tires (studless tires or winter tires). Use of tires is not particularly limited, and tires can be tires for passenger cars or heavy load tires for trucks and buses.

A dimension, such as a width, of the sipes of the embodiments above is a dimension in a regular state under no load when a tire is attached to a regular rim and inflated to a regular internal pressure. There are several standard systems including standards for tires and the regular rim is a rim specified for each tire according to a standard included in the corresponding standard system. For example, the regular rim is a standard rim according to JATMA, a “design rim” according to TRA, and a “measuring rim” according to ETRTO. Likewise, the regular internal pressure is an air pressure specified for each tire according to a standard included in the standard system. The regular internal pressure is a maximum air pressure according to JATMA, a maxima value set forth in the table of “tire load limits at various cold inflation pressures” according to TRA, and an “inflation pressure” according to ETRTO.

EXAMPLES

In order to confirm the effects of the embodiments above, examples and comparative examples of heavy load pneumatic radial tires of a block pattern (size: 11R22.5 16P.R.) were prepared. The sipe configurations of the respective tires are set forth in Table 1 below and the other tire configurations are the same. Examples 1, 3 and 4 are cases having the sipe configuration without the concave grooves according to the first embodiment above shown inFIG. 2andFIGS. 3A and 3B. Example 2 is a case having the sipe configuration with the concave grooves according to the second embodiment above shown inFIG. 6andFIGS. 7A and 7B. Comparative Examples 1 and 2 are cases having a constant groove width across the sipe (having no variance in groove width). Comparative Example 3 is a case shown inFIG. 10in which the wide portion20is provided to the sipe bottom26by surrounding the entire circumference of the narrow portion22of the sipe with the wide portion20.

The arithmetic roughness Ra set forth in Table 1 below was measured in accordance with JIS B0601:2013 using a stylus surface roughness meter “E-35A” available from Tokyo Seimitsu Co., Ltd.

Each tire was evaluated in terms of resistance to irregular wear, durability of the sipe plate at the time of molding of a tire by vulcanization (durability of the sipe plate), and ease of pulling of the sipe plate (ease of removal from die) at the time of die releasing of a tire. Evaluation methods are as follows.

That is, resistance to irregular wear was evaluated in the following manner. The tire was attached to a rim (22.5×7.50) and inflated to an internal pressure of 700 kPa. The tire was then attached to a drive shaft of a heavy truck having a vehicle total weight of 20 tons. The truck was run on a paved dry road and a road covered with snow and ice for predetermined travel distances (about 7000 km and about 25000 km) under a load condition of 80% of a maximum load. An amount of step-like wear, X (seeFIG. 11), between one block and the following block in the tire circumferential direction was measured. In each travel distance, the resistance to irregular wear is represented as an index relative to a value of an amount of step-like wear in Comparative Example 1 which is taken as 100. The smaller the index number becomes, the more satisfactory is the resistance to irregular wear.

Durability of the sipe plate was evaluated by checking the number of breakages in the sipe plate after the die was used 3000 times for molding of tires by vulcanization.

Ease of removal from die was evaluated by checking the number of tires on which chipping of rubber occurred in sipe portions among 30 tires molded by vulcanization.

The results are set forth in Table I below. In comparison with Comparative Example 1 having wide sipes with rough surfaces, Comparative Example 2 having sipes with a narrower groove width can improve resistance to irregular wear. However, durability of the sipe plate is poor. On the other hand, in comparison with Comparative Example 1, an effect of improving resistance to irregular wear can be acknowledged in Comparative Example 3 having the sipes provided with the wide portion and the narrow portion. However, ease of pulling of the sipe plate is poor and chipping of rubber is found in the sipe portion. In contrast, according to Examples 1 through 4, resistance to irregular wear can be improved without deteriorating durability of the sipe plate and ease of pulling of the sipe plate. In particular, resistance to irregular wear can be improved markedly in Examples 1, 2, and 4 in which the wall surfaces of the narrow portions are made smooth in comparison with Example 3 in which the wall surfaces are not made smooth, not to mention Comparative Example 1. Also, a comparison between Example 1 and Example 2 reveals that resistance to irregular wear can be improved further by providing the concave grooves to the narrow portion.